SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES • NUMBER 30

Field and Laboratory Investigations
of Antarctic Meteorites Collected
by United States Expeditions, 1985–1987

Ursula B. Marvin
and Glenn J. MacPherson

EDITORS

ABSTRACT

Marvin, Ursula B., and Glenn J. MacPherson, editors. Field and Laboratory Investigations of Antarctic Meteorites Collected by United States Expeditions, 1985–1987. Smithsonian Contributions to the Earth Sciences, number 30, 116 pages, frontispiece, 38 figures, 9 tables, 1992.—This monograph describes the meteorite collecting activities of the United States Antarctic Search for Meteorites (ANSMET) expeditions during the 1984–1985, 1985–1986, and 1986–1987 field seasons. Descriptions and classifications are given of most specimens collected during those expeditions with the exceptions of the types 4, 5, and 6 ordinary chondrites, whose properties are tabulated. Two articles are included that summarize data on the terrestrial ages and thermoluminescence properties of Antarctic meteorites. The Appendix lists all ANSMET specimens classified as of June 1987, in numerical order for each locality and by meteorite class.

OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution’s annual report, Smithsonian Year. SERIES COVER DESIGN: Volcanic eruption at the island of Krakatau in 1883.

Library of Congress Cataloging-in-Publication Data

Field and laboratory investigations of Antarctic meteorites collected by United States expeditions, 1985–1987 / Ursula

B. Marvin and Glenn J. MacPherson, editors.

p. cm. - (Smithsonian contributions to the earth sciences ; no. 30)

Includes bibliographic references

1. Meteorites—Catalogs and collections—Antarctic regions. 2. Astronomy—Research—Antarctic regions. 3. Scientific expeditions—United States. I. Marvin, Ursula B. II. MacPherson, Glenn J. III. Series.

QE1.S227   no. 30   [QB755.5.A6]   550 s–dc20   [523.5’09989]   92–10135

art The paper used in this publication meets the minimum requirements of the American National Standard for Permanence of Paper for Printed Library Materials Z39.48—1984.

Contents

1. EDITORS’ INTRODUCTION, by Ursula B. Marvin and Glenn J. MacPherson

2. THE 1984–1985 ANTARCTIC SEARCH FOR METEORITES (ANSMET) FIELD PROGRAM, by Scott A. Sandford

3. THE 1985–1986 AND 1986–1987 FIELD SEASONS, by William A. Cassidy

4. DESCRIPTIONS OF STONY METEORITES, by Brian Mason, Glenn J. MacPherson, Roberta Score, Rene Martinez, Cecilia Satterwhite, Carol Schwarz, and James L. Gooding

5. DESCRIPTIONS OF IRON METEORITES, by Roy S. Clarke, Jr.

6. TERRESTRIAL AGES OF VICTORIA LAND METEORITES DERIVED FROM COSMIC-RAY-PRODUCED RADIONUCLIDES, by John Evans, John Wacker, and James Reeves

7. NATURAL THERMOLUMINESCENCE LEVELS AND THE RECOVERY LOCATION OF ANTARCTIC METEORITES, by Fouad A. Hasan, Roberta Score, Benjamin M. Myers, Hazel Sears, William A. Cassidy, and Derek W.G. Sears

APPENDIX: Tables of ANSMET Meteorites

FRONTISPIECE.—Two of the most interesting meteorites collected during the field seasons described in this volume were also among the smallest. LEW86010 (top) was initially believed to be a terrestrial igneous rock, until examination of a thin section showed it to be the second example of one of the very rarest meteorite types—an angrite. The photo shows the entire specimen prior to sampling. ALH85085 (bottom, shown in a “group photo” with the other ALH8508x specimens) was also not recognized as being unusual until thin section examination and electron microprobe analyses revealed its unique characteristics.

Field and Laboratory Investigations
of Antarctic Meteorites Collected
by United States Expeditions, 1985–1987

1. Editors’ Introduction

Ursula B. Marvin and Glenn J. MacPherson

Ursula B. Marvin, Smithsonian Astrophysical Observatory, Mail Stop 52, 60 Garden Street, Cambridge, Massachusetts 02138. Glenn J. MacPherson, Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560.

This is the fifth publication in the Smithsonian Contributions to the Earth Sciences series to present the results of the yearly United States Antarctic Search for Meteorites (ANSMET) expeditions to Antarctica. This issue describes the 1984–1985, 1985–1986, and 1986–1987 field seasons (Figure 1-1). Descriptions and classifications are given of most of the meteorites collected during those expeditions with the exceptions of types 4, 5, and 6 ordinary chondrites, whose properties are tabulated in the Appendix. Two articles are included that summarize data on the terrestrial ages and thermoluminescence properties of Antarctic meteorites. Appendix Table A lists all meteorites classified through June 1987 in numerical order for each locality; Appendix Table B lists specimens in consecutive order by meteorite class; Appendix Table C summarizes the total numbers of meteorites collected by ANSMET through 1986, by type.

The numbers and main categories of meteorite specimens collected in the 1984–1985, 1985–1986, and 1986–1987 seasons are listed in Table 1-1. Total numbers and aggregate weights of specimens of each meteorite class are given in Appendix Table B. The aggregate weights are of interest because of the inherent intractibility of the pairing problem. We are unlikely ever to obtain secure counts of the number of falls of each meteorite class represented on the Antarctic stranding surfaces, and so we cannot compare numbers of Antarctic falls with those in the rest of the world. We can, however, obtain aggregate weights and, allowing for the vagaries of discovery, gain a general idea of how relative proportions of meteorite classes in the Antarctic collections compare with those found elsewhere. The system for naming Antarctic meteorites was changed in 1982 by the dropping of a letter (A, for example) to designate the collecting expedition. When the system was originally adopted, some members of the Nomenclature Committee looked forward optimistically to a time when two or more expeditions, from different countries or organizations, might visit an area such as the Allan Hills during the same season. They forsaw the need for a letter (A, B, C) to identify each one. Hence, letters and numbers in a name such as ALHA76001 were chosen to indicate place, expedition, year, and specimen number: ALH (Allan Hills), A (Expedition A), 76 (1976), 001 (Specimen 1). In practice, however, meteoriticists viewed the expedition number as incomprehensible and unnecessary. It has been dropped for all meteorites collected after the 1981 season; thus, the first Allan Hills specimen of 1982 was ALH82001.

The ANSMET program is governed by an interagency agreement between the National Science Foundation, the Smithsonian Institution, and the National Aeronautics and Space Administration. At the request of the scientific community, procedures (based on those used for lunar samples) were adopted for collecting specimens by sterile techniques and keeping them frozen until they are processed in nitrogen-filled cabinets at the Johnson Space Center at Houston. Details of the field and laboratory procedures are outlined in the first

FIGURE 1-1.—Outline map of Antarctica showing sites of meteorite finds.

publication in this series (Marvin and Mason, 1980). In order to distribute research samples quickly and widely, all newly classified specimens are described in the Antarctic Meteorite Newsletters; these are mailed, on request, to investigators throughout the world. Any scientist wishing to obtain samples may submit a request, describing the proposed research and the numbers, weights, and types of samples required, to the Meteorite Working Group, a committee with a rotating membership responsible for monitoring the program and allocating samples. Requests for the Antarctic Meteorite Newsletter or for research materials should be addressed to the Secretary, Meteorite Working Group, NASA Johnson Space Center, Code SN2, Houston, Texas 77058.

The Antarctic Meteorite Working Group meets twice each year, usually in April and October. Each issue of the newsletter publishes dates of meetings and deadlines for requests. Sample requests are welcome from all qualified scientists and are considered on the basis of their merit regardless of whether a scientist is funded for meteorite research. The allocation of Antarctic meteorite samples does not in any way commit a funding agency to support the proposed research.

For references on Antarctic meteorites, see the earlier publications in this series (Marvin and Mason, editors, 1980, 1982, 1984; Marvin and MacPherson, editors, 1989) and the computerized lists of publications from the Antarctic Meteorite Bibliography, which may be obtained on request from the Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, Texas 77058–1113. The Bibliography references articles in

TABLE 1-1.—Numbers of classified meteorite specimens collected in the 1984–1985,1985–1986, and 1986–1987 field seasons.

Meteorite Type
Locality
1984
1985
1986
Total

Achondrites
   Allan Hills
22
2

24

   Elephant Moraine
1

1

   Lewis Cliff

9
5
14

       TOTAL
23
11
5
39

Enstatite Chondrites
   Allan Hills
8
2

10

       TOTAL
8
2
0
10

Unique Chondrites
   Allan Hills

2

2

   Lewis Cliff

1

1

       TOTAL
0
3
0
3

Ordinary
Chondrites
   Allan Hills
204
139
13
356

   Bowden Neve

1

1

   Dominion Range

11

11

   Elephant Moraine
7

3
10

   Geologists Range

2

2

   Grosvenor Mountains

17

17

   Lewis Cliff

152
491
643

   Miller Range

1

1

   Rekling Peak

6
6

       TOTAL
211
323
513
1047

Carbonaceous
Chondrites
   Allan Hills
27
13

40

   Lewis Cliff

5

5

   MacAlpine Hills

7
7

   Grosvenor Mountains

1

1

       TOTAL
27
19
7
53

Stony-Irons
   Lewis Cliff

1
1

   Queen Alexandra Range

1
1

       TOTAL
0
0
2
2

Irons
   Allan Hills
2

2

   Elephant Moraine
1

1

   Grosvenor Mountains

1

1

   Lewis Cliff

1
4
5

       TOTAL
3
2
4
9

Meteoritics, in the annual Proceedings of the Lunar and Planetary Science Conferences at Houston, the Symposiums on Antarctic Meteorites held by the National Institute of Polar Research in Tokyo, and numerous other sources.

Libraries of polished thin sections are maintained in Washington, Houston, and Tokyo for the use of visitors who wish to make petrographic examinations. To obtain meteorite samples collected by parties sponsored by the Japanese Antarctic Research Expeditions, or to use the thin section library in Tokyo, contact Dr. Keizo Yanai, Curator, at the National Institute of Polar Research, 9–10 Kaga 1-chome, Itabashi-ku, Tokyo 173, Japan. To use the thin section library at the Johnson Space Center at Houston, contact the Secretary of the Meteorite Working Group at the address given above. To use the thin section library at the National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, contact Roy S. Clarke, Jr., Curator.

Literature Cited

Marvin, Ursula B., and Brian Mason, editors

1980.    Catalog of Antarctic Meteorites, 1977–1978. Smithsonian Contributions to the Earth Sciences, 23: 50 pages.

1982. Catalog of Meteorites from Victoria Land, Antarctica, 1978–1980. Smithsonian Contributions to the Earth Sciences, 24: 97 pages.

1984.    Field and Laboratory Investigations of Meteorites from Victoria Land, Antarctica. Smithsonian Contributions to the Earth Sciences, 26: 138 pages.

Marvin, Ursula B., and Glenn J. MacPherson, editors

1989.    Field and Laboratory Investigations of Meteorites from Victoria Land and the Thiel Mountains Region, Antarctica, 1982–1983 and 1983–1984. Smithsonian Contributions to the Earth Sciences, 28: 146 pages.

2. The 1984–1985 Antarctic Search for Meteorites (ANSMET) Field Program

Scott A. Sandford

Scott A. Sandford, NASA/Ames Research Center, Moffett Field, California 94035.

The purpose of the 1984–1985 ANSMET (Antarctic Search for Meteorites) expedition was to recover meteorites from the Main, Near Western, Middle Western, and Far Western icefields in the Allan Hills area and to carry out a reconnaissance of other nearby blue icefields. A brief summary of the locations visited is provided in Table 2-1. Figures 2-1 and 2-2 contain maps of these locations and the routes taken between them.

Expedition members included leader WA. Cassidy (University of Pittsburgh, Pittsburgh, Pennsylvania), Catherine King-Frazier (James Madison University, Harrisonburg, Virginia), John Schutt (University of Pittsburgh, Pittsburgh, Pennsylvania), Roberta Score (National Aeronautics and Space Administration/Johnson Space Center, Houston, Texas), Carl Thompson (Canterbury, New Zealand), Robert Walker (Washington University, St. Louis, Missouri), and the author (then at Washington University, St. Louis, Missouri).

The expedition proper began in late November 1984 when all the members except for Cassidy gathered at McMurdo Station on Ross Island; Cassidy joined the party at a later date. The first week was spent at McMurdo Station, undergoing survival training (for two days, on the lower slopes of Mt. Erebus) and gathering and preparing the expedition’s gear for transportation to the Allan Hills area.

During the 6 weeks of the expedition the party lived entirely in Scott tents, two persons to a tent. Typical “non-storm” day temperatures were in the −5° to −10° F range with windchill factors generally falling between −10° and −40° F. Since it was the austral summer, the sun never set during the entire expedition. As a result, work days tended to be long, weather permitting. The entire day was usually spent away from camp. Cooking was done over portable, single burner gas stoves. Water was obtained by melting snow and ice. Lunches were generally very simple affairs, because most food froze solid once it was taken from the tents (although the author did determine that thinly sliced pastrami, when put in a plastic bag and taped against the engine block of a snowmobile, would remain partially thawed until lunchtime). Typical lunches consisted of chocolate bars, raisins, and similar items.

After a delay of several days due to bad weather, the party was transported to the Allan Hills on December 8 using UH-1N Huey helicopters with an intermediate refueling stop at Marble Point on the Antarctic mainland (Figure 2-1). The group was accompanied by a two man photography team from public television station WQED (Pittsburgh) who were filming sequences for a documentary. Some of this footage was subsequently shown in the seven part film series PLANET EARTH. The film crew left on 10 December.

The first camp was made between the Allan Hills Main Icefield and the Near Western Icefield (Figure 2-2). The camp remained in its first location for 8 days, during which the two nearest icefields were searched for meteorites (Table 2-1, Figure 2-2). Approximately 80 meteorite fragments were found during this period. The collection procedure consisted of search, collection, and surveying. During the search phase the snowmobiles were driven en echelon (similar to the pattern used by the fighter plane squadrons of WWII) in order to cover the maximum possible area in each sweep. This line formation swept back and forth over the icefield until a meteorite was spotted. Once found, meteorites were photographed, assigned a sample number, and enclosed in several successive sample bags (Figure 2-3). The position of the meteorite was then determined relative to survey reference points. All the meteorites were kept frozen until they arrived at the curatorial facility at the Johnson Space Center in Houston.

On December 17 the camp was moved to a new site at the Middle Western Icefield (Figure 2-2). The Middle Western

FIGURE 2-1.—A map of the continent of Antarctica. Major sites visited by expedition members during the 1984–1985 field season are marked on the map. The positions of many of these sites are also listed in Table 2-1. The boxed portion represents the extent of the area enclosed in the map in Figure 2-2.

TABLE 2-1.—Antarctic locations mentioned in the text

Icefield | Location | Dates and camps
McMurdo Station | (77°30'S, 166°40'E) |
Near Western Icefield | (76°44'S, 158°49'E) | 8–17 Dec 1984 (Camp 1)
Middle Western Icefield | (76°50'S, 158°26'E) | 17–21 Dec 1984 (Camps 2 and 4)
Far Western Icefield | (76°54'S, 157°01'E) | 21 Dec 1984-6 Jan 1985 (Camp 3)
Elephant Morraine | (76°ll'S,157'10'E) | 8–10 Jan 1985
Allan Hills Main Icefield | (76°41'S, 159°17'E) | 9–15 Jan 1985
Battlement Nunataks | (76°32'S, 159°21'E) | 13 Jan 1985
Trinity Nunataks | (76°26'S, 160°38'E) | Not Reached
Odell Glacier Camp | (76°47'S, 159°35'E) | 15–19 Jan 1985 (Camp 7)
Carapace Nunatak | (76°53'S, 159°24'E) | 16 Jan 1985
Natural Rock Arch | (76°45.5'S, 159°57.7'E) | Discovered 17 Jan 1985
Geologists Range | (82°30'S, 155°30'E) | Aerial Recon 25 Jan 1985
Anderson Hills | (84°30'S, 84°00'W) | Aerial Recon 25 Jan 1985

Icefield was searched from December 18 to December 21 and approximately 30 additional meteorites were found. All traverses were made using snowmobiles to pull trains consisting of 1 to 3 Nansen or Knudsen sleds. Traverses at this location were made with the snowmobiles in single file to minimize risks associated with crevasses. On December 21, the camp was moved to the Far Western Icefield (Figure 2-2). The southeastern arm of the icefield was extensively searched and about 120 meteorites were found. The northwestern arm of the icefield was quickly surveyed but not thoroughly searched. Christmas Eve was celebrated by cramming 6 people in one Scott tent and sharing a meal of ham, lobster, shrimp, and yams.

On January 6 the party returned to the Middle Western Icefield and made an additional search for meteorites. On the 8th, Schutt and Sandford made a trip to Elephant Morraine (Figure 2-2) in order to meet with two geologists (Gunther Faure and Karen Taylor of Ohio State University, Columbus, Ohio) and to escort them back to the Allan Hills area. The rest of the party proceeded on to the Allan Hills Main Icefield.

FIGURE 2-2.—A map of the Allan Hills area. Major traverses made during the 1984–1985 field season are shown as dashed lines and the camp sites are given as numbered dots. The positions of many of the features on the map can be found in Table 2-1.

While at Elephant Morraine, Sandford and Schutt found an additional 9 meteorites, including one of the season’s few iron meteorites.

The Elephant Moraine party then proceeded to the Allan Hills Main Icefield where the entire expedition was reunited on January 10. Bill Cassidy joined the party on January 12. The Main Icefield camp was maintained until January 15. During this time the party searched the nearby icefield and visited the blue icefield associated with the Battlement Nunataks; no meteorites were found there.

While at the Allan Hills Main Icefield, an experiment was set up to determine the extent to which wind moves rocks and meteorites on the ice. Rocks spanning a range of sizes (about 1 to 10 cm in diameter) were placed on the icefield in two lines

FIGURE 2-3.—The author with the largest meteorite found during the 1984–1985 field season. This meteorite is an ordinary chondrite.

perpendicular to the prevailing wind direction. One line of rocks was placed directly on the ice while the other line of rocks was buried just below the surface of the ice. Both lines were examined during the next year to see how various sized rocks were excavated and moved by the wind. After one year the buried rocks were found to have been entirely exposed by wind ablation of the ice. While the larger rocks had been moved very little, some of the middle-sized rocks had moved downwind by distances in excess of 100 meters. Many of the smaller rocks were never found again.

On the 15th, camp was broken and a traverse to the Trinity Nunatak area was begun. Unfortunately, this traverse had to be abandoned since a recent snowfall had covered up many of the crevasses along the way and travelling conditions were hazardous. The party therefore proceeded instead up the Odell Glacier and established camp at its head. Additional reconnaissance trips from this camp were made to the Odell Glacier and the blue ice near the Carapace Nunatak (see Figure 2-2). It was during one of these day trips that the author discovered the natural rock arch shown in Figure 2-4. As far as the author has been able to ascertain, this is the only known rock arch in Antarctica.

TABLE 2-2.—Summary of identified meteorites found.

    Meteorite class | Number found
Ordinary Chondrites |
   H | 112
   L | 63
   LL | 15
Enstatite Chondrites |
   E3 | 1
   E4 | 7
Carbonaceous Chondrites |
   C2 | 24
   C3V | 2
   C4 | 2
Irons | 3
Ureilites | 1
Diogenites | 1
Aubrites | 18
Unique Achondrites | 3

The entire party was airlifted out of the field and returned to McMurdo Station on January 19. Overall, the expedition enjoyed generally good weather, and most of the time was spent successfully hunting for meteorites. A total of 274

FIGURE 2-4.—This natural rock arch lies on the western slopes of the Coombs Hills overlooking the Odell Glacier in southern Victoria Land. The arch is at approximately 76°45.5’S, 159°57.7’E and is the only formation of its type known to exist on the southern continent. The arch is estimated to be about 15 meters in height and width.

meteorite fragments were found, ranging in size from about 0.5 × 0.5 × 0.1 cm (0.4 g) to one stone measuring 32 × 22 × 18 cm (16,000 g) (see Figure 2-3). Most of these samples have now been examined and a summary of the breakdown by type is given in Table 2-2. Of special interest are a 17 × 9.5 × 6.5 cm diogenite and 3 unique achondrites.

At this point, most of the expedition personnel returned to the United States. Three of the party members (Cassidy, Score, and Sandford) remained and on January 25 participated in an airborne reconnaissance (using a C-130 Hercules aircraft) of potential meteorite-bearing icefields near the Geologists Range and the Anderson Hills in the Patuxent Range (see Figure 2-1). The latter site looks promising and future expeditions to this area are being planned. Additional reconnaissance of icefields near the Argentina Range had to be cancelled when an exploratory “ski-drag” landing in the Anderson Hills damaged the landing gear of the aircraft and it was forced to return to McMurdo Station.

On January 28 an additional flight was made to the Amundsen-Scott Station at the south pole. This trip was made in order to change the sample surface of an electrostatic dust collector which is in continuous operation at the pole. The collection surface was returned to the University of Pittsburgh for examination for extraterrestrial dust particles.

ACKNOWLEDGMENTS.—I would like to thank co-expedition member Robbie Score for providing the latest summary of the meteorite types found in the 1984–1985 field season, and for spurring my memory concerning various details of the trip. My thanks go also to Mike Zolensky for updating me on the status of the rock movement experiment at the Alan Hills Main Icefield.

3. The 1985–1986 and 1986–1987 Field Seasons

William A. Cassidy

William A. Cassidy, Department of Geology, University of Pittsburgh, Pittsburgh, Pennsylvania 15260.

The 1985–1986 Field Season in the Allan Hills Region and Reckling Peak

Four members of the 1985–1986 Antarctic Search for Meteorites (ANSMET) field party went to the Allan Hills, a proven source of meteorites in Southern Victoria Land, and also made an exploratory trip to Reckling Peak: J. Schutt (University of Pittsburgh), Ludolph Schultz (Max Planck Institute, Mainz, Federal Republic of Germany), Michael Zolensky (NASA/Johnson Space Center), and Ernst Zinner (Washington University). The geography of this area, and ANSMET expeditions to it, have been described in detail previously (e.g., Cassidy, 1989), so only a summary of the results during the 1986–1986 season are presented here.

Starting in 1976 the Allan Hills yielded meteorites in 10 consecutive austral summers, and by the start of the 1985–1986 field season only about 40% of the available blue ice area in the Far Western Icefield remained to be systematically searched. This was done over the course of approximately 6 weeks from 7 December 1985 on, during which over 140 meteorites were recovered. A reconnaissance visit to another nearby small icefield yielded only one specimen. Two reconnaissance searches for meteorites on blue icefields near Reckling Peak during this season produced no meteorite finds. Finally, at the end of the season, the field party returned to the Allan Hills Main Icefield and during the course of random searches and traverses there 17 more meteorites were recovered.

The 1985–1986 Field Season near Beardmore Glacier

The other four members of the 1985–1986 field party were stationed at Beardmore Camp: Peter Englert (San Jose State University), Twyla Thomas (United States National Museum), Carl Thompson (Methven, New Zealand), and the author. From this base we investigated possible occurrences of meteorites near Lewis Cliff, Otway Massif, Grosvenor Mountains, Dominion Range, Geologists Range, and Milan Ridge (Figure 3-1). Only sparse meteorite occurrences were discovered at Otway Massif and other sites in the Grosvenor Mountains. A potentially more productive location lies adjacent to Dominion Range. A few specimens were recovered at Geologists Range and one at Milan Ridge. Many were found on the surface of an ice tongue immediately to the east of Lewis Cliff; this area was exceptionally productive.

On December 11 we made a 69-mile round trip on snowmobiles out of Beardmore Camp and discovered our first meteorites: 5 specimens lying on ice about one kilometer east of the ice tongue below Lewis Cliff. We returned to this site, and to the ice tongue itself, three times between December 12 and December 16 and collected 13 additional specimens.

On December 20, we were put in by helicopter at Otway Massif, where we occupied the site called Camp 1 (Figures 3-1 and 3-2). GRO85203 (Figure 3-2) marks a site where we left at least 20 large and small meteorite fragments undisturbed on the ice. They all appeared to belong to the same fall. If we had collected them the total mass would have added unduly to the cargo we had to transport.

On December 23, we abandoned Camp 1 and made the 36-mile traverse to Camp 2, near Mt. Raymond (Figure 3-3). This picturesque campsite is located in a shallow, somewhat protected valley between rounded, dunelike snow masses. We remained at Camp 2 from December 23 to December 28.

On December 28 we traversed 40 miles to Camp 3, above the Scott Icefalls (Figures 3-1 and 3-4), where a large area of exposed ice between our camp and the Icefalls is visible on aerial photos. The ice, unfortunately, was covered by a thin layer of snow. Little searching was done because of the snow cover and semi-whiteout conditions; moreover, crevasses are common in this area and some were difficult to identify under the existing conditions. Nonetheless, we found four specimens

FIGURE 3-1.—Map showing locations in the vicinity of Dominion Range and the Grosvenor Mountains that were visited during the 1985–1986 field season.

FIGURE 3-2.—Enlarged map of the region around Otway Massif. Meteorite recovery locations at Otway Massif, Mt. Raymond (Figure 3-3), and south of Scott Icefalls (Figure 3-4) are labelled GRO (for Grosvenor Mountains).

(shown on Figure 3-4). We plan to return to this site in the future in the hope of finding the ice swept clean.

On December 31 we made the traverse to Camp 4 (Figure 3-5), a 52-mile trip, and remained there until January 4. Our reconnaissance searches and recoveries are detailed in Figures 3-4 and 3-5. We were picked up by helicopter at Camp 4 on January 4 and returned to the Beardmore Camp.

The period between December 20 and January 4 was notable for the large proportion of fresh-appearing meteorites that we collected. Many had fusion crusts intact or nearly so. Many were larger than those we had been accustomed to finding. It will be interesting to determine whether or not these individuals have younger-than-average terrestrial ages.

On January 7 we established a new base of operations near Coalsack Bluff at the site of the old “Hard Times” camp, where paleontologists processed the first vertebrate fossils found in Antarctica. The fossil site was discovered in November, 1969, and the camp was occupied during that summer. Traces of the old camp remain: the generator shack was full of snow to its roof but had a wind scoop on two sides; a number of double

FIGURE 3-3.—Map of the ML Raymond area of the Grosvenor Mountains.

two-by-fours, nailed together and sticking up out of the snow, still had electrical wiring stapled to them; and the tops of 55-gallon drums were exposed at the current snow surface. Large cans of mandarin oranges and grated cheese had been left in a cache, and were still edible. This is a convenient campsite, because when storms threaten a party can quickly drive the 28 miles to the Beardmore Camp which never seems to have bad weather in the summer. Throughout that season the heavy windstorms stopped north of Mt. Sirius.

We used a scheduled helicopter flight on January 12 to make a reconaissance survey along the Ice Plateau/Transantarctic Mountains boundary line, as far north as the Geologists Range. Meteorites associated with this site have the symbol GEO on the map (Figure 3-6). We found three specimens on a small ice patch west of Mt. Summerson, and collected two of them in a 20 mph wind with blowing snow. We later recovered one fragment on the ice above the valley next to Milan Ridge (symbol MIL, for Miller Range; Figure 3-7).

On January 16 a four-person party from the U.S. Geological Survey, led by Gary Parasso, arrived at the Lewis Cliff Ice Tongue from the Beardmore Camp and established a baseline for us. They put in two stations on rocky hilltops that describe a line running approximately north-south, parallel to the eastern side of the upper part of the ice tongue. All mapping of this area will be done with reference to these two points.

We systematically searched the entire upper level of the ice tongue, collecting numerous specimens and mapping their locations. Recovered specimens at the Lewis Cliff site were typically small, with very few completely fusion-encrusted individuals. Many of the specimens were severely weathered, which made it difficult to distinguish them from the ferruginous Ferrar dolerites that are scattered liberally across the ice surface. Weathered coal fragments and pieces of grey shaly sandstone, both common on the ice surface, presented additional difficulties because they tend to resemble carbonaceous chondrites. These terrestrial rocks made systematic searching highly tedious and added pleasure to the rare discovery of an exceptionally large meteorite specimen. Two

FIGURE 3-4.—Map of the region near Camp 3 where four meteorites were found. At upper left is the Mt. Ward area, where a number of meteorites were found during the traverse to Camp 4.

FIGURE 3-5.—Map of the Dominion Range (DOM) in the vicinity of Camp 4.

FIGURE 3-6.—Map of the Geologists Range where two meteorites (GEO) were recovered during a reconnaissance survey in the 1985–1986 field season.

such finds were LEW85320 (110 kg) and LEW85319 (11 kg), both H5 chondrites found on the slope between the upper level and the lower level of the ice tongue. These specimens may represent a paired fall.

The meteorite specimens on the upper ice tongue are very heterogeneously distributed. They occupy a narrow zone at the southern (upstream) end and a much wider zone at the northern end, just above the slope down to the lower level. These distributions are real, and the boundary between the zones is sharp, but the cause is not obvious.

A symmetrically patterned suite of ice samples was collected for δ¹⁸O determinations by Peter Englert. Unfortunately, the samples were lost in transit to, or at, McMurdo Station.

In summary, the 1985–1986 field season was very successful and the Lewis Cliff Ice Tongue was established as a major new source of Antarctic meteorites.

The 1986–1987 Field Season

Members of the 1986–1987 party were Christian Koeberl (University of Vienna), Louis Lindner (National Institute for Nuclear Physics and High Energy Physics, Amsterdam), Austin Mardon (Texas A&M University), John Schutt (University of Pittsburgh), Keizo Yanai (National Institute of Polar Research, Tokyo), and the author. The entire field season was

FIGURE 3-7.—Map of the Miller Range (MIL), showing the location where a meteorite was found near Milan Ridge during the 1985–1986 field season.

spent at the Lewis Cliff Ice Tongue because of the importance of this newly discovered stranding surface. We were in the field from December 6 to January 21. We concentrated mainly on foot searches in the southern part of the lower ice tongue, just below the step, where relatively high densities of small meteorite fragments were found. We collected and mapped all of our finds.

We also began finding meteorites along the southern edge of an extensive moraine that lies east of the ice tongue. Fragments occurred among the rocks as far as 20 meters into the moraine, making the search procedure particularly difficult; this was the first time any of us had had to search on our hands and knees. Most of the specimens we collected are weathered fragments of ordinary chondrites with minor patches of fusion crust; most of them probably are paired. We named this area Meteorite Moraine.

Ordinary chondrites dominate the accumulations on the lower ice tongue as well as in Meteorite Moraine. Interesting distributions of specimens with differing thermoluminescence properties are described by Hasan et al. (Article 7).

We carried out an ice sampling program in which we duplicated Englert’s pattern of the previous season and also sampled at 10-meter intervals along a line crossing the boundary of the meteorite distribution field on the upper ice tongue. On a reconnaissance survey we discovered a significant accumulation of meteorites on exposed ice around Goodwin Nunataks, about 50 kilometers south (i.e., upstream) of the Lewis Cliff Ice Tongue. That accumulation was left in place for a future field season when we can put in a camp nearby.

The total number of probable meteorite specimens from Lewis Cliff Ice Tongue and Meteorite Moraine is now over 700.

Stratigraphic Features at the Lewis Cliff Ice Tongue

Lewis Cliff Ice Tongue is a promising site for establishing an ice-stratigraphic series that would provide a new reference framework within which to evaluate the observed distribution of meteorites on the stranding surface. The main time markers are dust bands composed of volcanic ash that dip into the ice. It appears that an essentially horizontal series of tephra deposits have been tilted and exposed at the surface by wind ablation. We have been mapping and sampling these bands.

Other interesting features at the ice tongue are bed-load dust bands and regelated ice. Ice at the bed of a glacier or moving ice sheet can be melted for short periods and refrozen. The melted stage lasts long enough for extensive, but not complete, degassing to occur. As a result, the regelated (recrystallized) ice is mostly bubble-free; the remaining bubbles often form linear trains suggestive of streaming. Dust and rocks, the latter often rounded and glacially striated, can be entrained in regelated ice and carried slowly downstream parallel to the bed of the glacier. If the ice reaches a barrier it may buckle or shear, and in either case regelated ice, with its load of sediment, then can be thrust to the surface. If only the finer particles reach the surface, they form dust bands composed of basal rock detritus instead of tephra. If larger rocks are brought to a surface where ablation is taking place, a mass of unsorted glacial debris soon litters the ice on either side of the dust band. This sedimentary cover effectively slows the ablation process and the result, after a period of time, is an ice ridge strewn with a thin veneer of boulders and cobbles with a planar core of glacial debris. This is a so-called ice-core moraine. Ice-core moraines are structural features that can yield stratigraphic information because they preserve a record of forces that have acted to distort the original simple stratigraphic sequence.

Much less well understood at the Lewis Cliff Ice Tongue are bands of evaporite-type mineral deposits that apparently extend in a planar array along levels stratigraphically higher than the interface between regelated ice and unmelted ice. They seem to be parallel and close to this boundary, but detached from it. They may be related to the regelation process but the nature of this relation is not clear. In any case, although they may not represent time horizons, the evaporite deposits probably give stratigraphic data, at least over the short range.

With the available stratigraphic and structural clues at the Lewis Cliff Ice Tongue, it should be possible to establish a relative stratigraphic sequence across the meteorite stranding surface. If we then can determine absolute ages for some of the marker horizons we will have what amounts to an ice core laid out horizontally at the surface. Such an expanse of ice, where dated dust bands can be mapped in context with distributions of meteorite terrestrial ages, will provide an excellent testing ground for the different models of the meteorite stranding mechanism.

ACKNOWLEDGMENTS.—During the 1986–1987 field season, Gunter Faure of Ohio State University led a party to the Allan Hills, Reckling Moraine, and Elephant Moraine, principally to study moraines associated with meteorite stranding surfaces. During the course of their field work, they came across 22 meteorites (plus 2 possible meteorites), collected them in a careful manner, and turned them over to our group at the end of the season. We are very grateful for this action on their part. Members of their group were G. Faure, D. Buchanan, E. Hagen, and M. Strobel.

We also acknowledge with thanks the efforts of the U.S.G.S. group that surveyed our mapping baseline. The members of that field team were G. Parasso, E. Eckel, M. Hower, and G. Sandul. We are indebted to all the dedicated professionals listed above. This work was supported by NSF Grant DPP 83-14496.

Literature Cited

Cassidy, W.A.

1989.    The 1982–1983 Antarctic Search for Meteorites (ANSMET) Field Program. In U.B. Marvin and G.J. MacPherson, editors, Field and Laboratory Investigations of Meteorites from Victoria Land and the Thiel Mountains Region, Antarctica, 1982–1983 and 1983–1984. Smithsonian Contributions to the Earth Sciences, 28:5–8.

4. Descriptions of Stony Meteorites

Brian Mason, Glenn J. MacPherson, Roberta Score, Rene Martinez,
Cecilia Satterwhite, Carol Schwarz, and James L. Gooding

Brian Mason and Glenn J. MacPherson, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560. Roberta Score, Rene Martinez, Cecilia Satterwhite, Carol Schwarz, and James L. Gooding, National Aeronautics and Space Administration, Johnson Space Center, Houston, Texas 77058.

This article gives descriptions of the achondrites, mesosiderites, and the more unusual chondrites (unique, carbonaceous, enstatite, and type 3 ordinary chondrites) collected during the 1984–1985, 1985–1986, and 1986–1987 field seasons. Summary data for the other ordinary chondrites are included in the Appendix. Within the carbonaceous and enstatite chondrites, the specimens are grouped according to the Van Schmus-Wood (1967) classification, and the descriptions follow the order of increasing petrographic type. The descriptions are based largely on those published in the Antarctic Meteorite Newsletter, with additional information as available. The letter-number designation concurs with the guidelines recommended by the Committee on Nomenclature of the Meteoritical Society, and carries the following information: location, e.g., ALH, Allan Hills; field season, e.g., 84, 1984–1985 field season; xxx, digits indicating sequential number of the specimen. The original weight of the specimen is given to the nearest 0.1 gram.

Chondrites

UNCLASSIFIED OR UNIQUE

FIGURES 4-1, 4-2, 4-3

ALH85085 (11.9 g).—This small stone is entirely covered with an iridescent brown fusion crust. The interior is black with abundant small (˜1 mm) white inclusions. Some oxidation is present.

Detailed petrographic studies of this unusual meteorite have been published by Scott (1988), Weisberg et al. (1988), and Grossman et al. (1988); the following description is taken from these three papers. ALH85085 consists of ˜5%–10% chondrules (mostly of the radial pyroxene and cryptocrystalline varieties), −5% dark lithic clasts, 20%–23% metal, and ˜60%–70% silicate fragments. There is little or no true matrix. Electron microscope examination reveals the presence of numerous (100 or more per thin section) but tiny (<120 μm) and volumetrically-minor refractory inclusions. Weisberg et al. (1988) and Scott (1988) found rare grains of osbornite (TiN). Silicate phases in chondrules and fragments are very iron poor: the olivine composition is Fa0–40, with most Fa0–5 and only rare grains as Fe-rich as Fa₅₀; low calcium pyroxene is Fs0–25. Many lithic clasts appear to be C1 fragments or C2 matrix lumps rich in sulfide, magnetite framboids, platelets and spheroids, and a presumably hydrous matrix that gives low microprobe analysis totals; also present are “reduced” clasts that are rich in tiny metal grains.

Estimates of bulk composition from broad- or rastered-beam electron microprobe analyses show that ALH85085 has roughly chondritic ratios of refractory lithophile elements, is greatly depleted in volatile elements such as Na, Mn, and S, and is greatly enriched in siderophile elements such as Fe and Ni. Grossman et al. (1988) and Weisberg et al. (1988) note that ALH85085 has close affinities with the Renazzo-type carbonaceous chondrites, although only the former authors advocate classifying it as such; Weisberg et al. (1988) and Scott (1988) regard ALH85085 as a new variety of chondrite. The oxygen isotope composition of bulk ALH85085 (Clayton and Mayeda, 1988) is similar to those of bulk Renazzo and Al Rais. Wasson and Kallemeyn (1990) give trace and minor element data for

FIGURE 4-1.—Photomicrograph of the unique chondrite ALH85085. Micro-chondrules, chondrule fragments, and mineral grains are the main features visible in this photo. The largest chondrule is cryptocrystalline; the largest fragment is a piece of a porphyritic chondrule. The dark background is not matrix but, rather, abundant metal plus underexposed silicate grains. Width of field is 1.1 mm.

ALH85085 and argue that it is not a true chondrite at all but, rather, predominantly the product of an impact ejecta cloud. Grady and Pillinger (1989) found ALH85085 to have the highest enrichment of ¹⁵N ever measured in any whole-rock meteorite.

ALH85151 (13.9 g).—Traces of dull black fusion crust remain on an exterior surface that is otherwise dull brownish-gray and heavily pitted; clasts and inclusions are present in relief. One notable clast is black with small white inclusions.

FIGURE 4-2.—Photomicrograph of the chondrite ALH85151. A mixture of chondrules, chondrule fragments, dark clasts, and mineral grains are dispersed throughout the dark matrix. Opaque grains are almost exclusively sulfide, with sparse chromite; metal is absent. Width of field is 2.3 mm.

FIGURE 4-3.—Photomicrograph of the unique chondrite LEW85332. Abundant chondrules of a wide range of sizes, chondrule fragments, iron-nickel metal, sulfide, and mineral grains are scattered in the darker matrix. The tiny chondrule at lower left center is roughly 80 μm in diameter. At left is a small, irregularly-shaped, rimmed, hibonite-bearing refractory inclusion. Width of field is 2.3 mm.

The interior is medium-gray with white inclusions; oxidation is extensive in some areas. A thin section shows numerous chondrules (up to 1.2 mm across) and chondrule fragments in a fine-grained matrix. Most of the chondrules consist mainly of porphyritic and barred olivine; pyroxene is rare. Minor amounts of nickel-iron and sulfide are present, mainly as very small grains scattered through the matrix. Microprobe analyses give the following compositions: olivine, Fa0.4–41, with a mean of Fa₃₄; pyroxene, Fs6–30.

Weisberg et al. (1989) report oxygen isotope data for ALH85151 that show it belongs to a previously unknown chondrite group that includes the meteorite Carlisle Lakes. Rubin and Kallemeyn (1989) give detailed petrologic and bulk chemical data that also demonstrate the unusual nature of this chondrite; those authors note the similarity of ALH85151 to Carlisle Lakes and, possibly, Y-75302.

LEW85332 (113.7 g).—Dark fusion crust covers all but one surface of this meteorite. The interior has a dark gray fine-grained matrix with a few light-colored inclusions. A thin section shows an aggregate of small chondrules (up to 1.2 mm across, but most are less than 0.5 mm), chondrule fragments, and irregular granular masses set in a translucent yellow-brown matrix. Chondrules are mainly porphyritic olivine and radial pyroxene/cryptocrystalline. Minor amounts of nickel-iron and sulfide are present, as small grains scattered through the matrix, and locally concentrated around chondrule rims. Including microprobe data from Rubin and Kallemeyn (1990), olivine has a wide composition range, Fa2–37, with a mean of Fa₉; less abundant pyroxene has a composition range Fs1–30.

Rubin and Kallemeyn (1990) report petrologic and bulk

FIGURE 4-4.—Photomicrographs of C2 chondrites: a, ALH84029; b, ALH84033; c, GRO85202. The photos show an assortment of chondrules, inclusions, and mineral grains dispersed within abundant dark matrix; in ALH84029 the visible grains are mostly phyllosilicate, in ALH84033 most are olivine, and in GRO85202 both phyllosilicates and olivine are present The matrices in all three consist of green-brown phyllosilicate. Width of field in each photo is 2.3 mm.

chemical data for LEW85332 that show it is probably a carbonaceous chondrite of a unique type.

CLASS C2

FIGURE 4-4

ALH84029, 84030, 84031, 84032, 84034, 84035, 84040, 84041, 84042, 84043, 84044, 84045, 84047, 84048, 84049, 84051, 85004, 85012 (total weight 542.5 g).—These are all small fragments, some with fusion crust, the largest of which (ALH84044) weighs 147.4 g. They were found close together on the SE edge of the Allan Hills Far Western Icefield, and this, together with their unique mineralogy (almost entirely phyllosilicate), indicates that they are likely paired (probably also with ALH83100, ALH83102, and ALH83106). The major component is a brown to black phyllosilicate matrix that encloses green to pale brown phyllosilicate pseudomorphs of chondrules, crystals, and inclusions. Calcite is abundant. Sporadic primary olivine crystals (composition Fa0–2) are preserved, although a few as iron-rich as Fa₃₇ were found. Chondrules and inclusions range up to a little over 1 mm in diameter, and are completely altered to phyllosilicates. Chromite, pentlandite, and (?) magnetite are accessory. Gibson (1987) measured carbon abundances in ALH83100 (1.948%) and ALH84029 (1.863%). McSween (1987) determined matrix compositions in ALH83100 and ALH84034 and agreed that they are probably paired.

ALH84033, 84036, 84039, 84046, 84050, 84053, 84054, 84191 (total weight 139.3 g).—These small fragments (the largest, ALH84033, weighs 60.4 g), some with fusion crust, were collected at widely scattered locations on the Allan Hills Far Western Icefield; they are so similar in texture and mineralogy that they are tentatively paired. Unlike the ALH84029 group, these C2 carbonaceous chondrites contain abundant preserved primary phases. Chondrules up to nearly 2 mm in diameter, along with abundant and varied inclusion types, crystals, and crystal fragments, are dispersed in a black matrix that is reddish brown on thin edges. Olivine is uniformly Fa0–2 among the grains analysed. Rare pyroxene is nearly pure MgSiO₃. Troilite, pentlandite, nickel-iron, chromite, and (?) magnetite are accessory phases. Many refractory inclusions are present, containing spinel, perovskite, and in a few cases blue-pleochroic hibonite.

Gibson (1987) determined the carbon content in ALH84033 to be 1.671%.

ALH85005, 85007, 85008, 85009, 85010, 85013, 85106 (total weight 326.5 g).—These specimens are very similar in texture and mineralogy and are tentatively paired. Most are fragments, some with fusion crust. The largest, ALH85013, is a completely crusted stone weighing 130.4 g. ALH85008, ALH85009, ALH85010, and ALH85011 were found close together at the NW end of Allan Hills Far Western Icefield. In thin section the meteorites are seen to consist largely of black opaque matrix, through which are scattered small (up to 0.2 mm) mineral grains and sparse chondrules and chondrule fragments. The chondrules and most of the mineral grains consist of olivine, usually close to Mg₂SiO₄ in composition but some are more iron-rich. Pyroxene is less common, and is close to MgSiO₃ in composition. A few grains of calcite were noted.

GRO85202 (27.2 g).—Thin fusion crust covers two sides of this stone; it is highly fractured, and disintegrated on chipping. A thin section shows a dark brown to black matrix with numerous mineral grains and aggregates and rare small chondrules. Most of the mineral grains and aggregates consist of an isotropic to weakly birefringent serpentine-like mineral. A few grains of olivine near Mg₂SiO₄ in composition were analysed; some grains of calcite were noted.

LEW85306, 85307, 85309, 85311, 85312 (total weight 293.5 g).—These specimens are very similar in texture and mineral compositions, and are tentatively paired. The largest, LEW85311, weighs 199.5 g. Frothy black fusion crust is present as patches on these stones. A thin evaporite deposit is present on LEW85309. The interiors show abundant light-colored clasts and chondrules set in a black fine-grained matrix. Some reddish-brown oxidation was noted. Thin sections show numerous mineral grains and aggregates and a few small (maximum diameter 0.6 mm) chondrules in a brown to black matrix. Most of the mineral grains are olivine, usually near Mg₂SiO₄ in composition, but some are more iron-rich. Pyroxene is less abundant, and is near MgSiO₃ in composition.

LEW86004, 86005, 86007, 86008, 86009 (total weight 20.5 g).—These specimens are so similar that they are tentatively paired. Some fusion crust remains on each of these stones. White evaporite deposit speckles the crust-free surfaces of LEW86005 and LEW86007. A brownish weathering rind extends 1–2 mm into the interior of LEW86004, LEW86008, and LEW86009. Unweathered interior matrix is jet black and contains abundant rounded and irregular inclusions. Thin sections show numerous small colorless grains (mainly olivine), irregular aggregates, and rare chondrules in an opaque to translucent brown matrix. Microprobe analyses of the olivines show a composition range of Fa0–54 with a marked peak at Fa0–1. A few grains of clinoenstatite were analysed, having a composition range of Fs0–7.

CLASS C3

FIGURE 4-5

ALH84028 (735.9 g), 84037 (3.0 g).—These two meteorites are so similar that they can confidently be paired. Bubbly black fusion crust covers about 50% of ALH84028; the interior is gray with numerous 1–2 mm sized lighter inclusions, and shows a few oxidation halos. A thin section shows a variety of chondrules up to about 2 mm in diameter, together with clasts and inclusions up to about 4 mm in maximum dimension, in a pristine matrix consisting of abundant minute olivine plates with composition Fa45–50, troilite, and awaruite. The chondrules are very well preserved, and many contain devitrified glass. The olivine in chondrules, and larger matrix grains, has a wide range of composition, Fa0–30, but most are Fa0–10. Pyroxene grains having a composition close to Wo₁En₉₇ were analysed, although a wider range of compositions like that of olivine is presumably present. Fine-grained and “coarse-grained” refractory inclusions are present; one example found is a type A, irregular in shape, that contains gehlenitic melilite, spinel, and very Ti-rich fassaitic pyroxene. This is a C3V carbonaceous chondrite.

ALH85003 (50.1 g).—Thick patchy fusion crust covers approximately 70% of this meteorite. The interior is light gray and chondrules or clasts are not distinguishable in the granular matrix. A 1 mm thick weathering rind and small patches of rust are present. A thin section shows an aggregate of small chondrules (up to 0.9 mm diameter, but most are less than 0.6 mm), chondrule fragments, and irregular aggregates set in a translucent yellow-brown matrix. Chondrules are mainly granular or porphyritic olivine. Minor amounts of nickel-iron are present, as small grains scattered throughout the section. Microprobe analyses of olivine show a wide composition range, Fa1–56, with a mean of Fa₁₇; only a few grains of pyroxene were found, having a composition range of Fs0.5–23. This meteorite is a C3 chondrite of the Ornans subtype; it is possibly paired with ALH82101 (they were found 2.5 km apart at the northern margin of the Allan Hills Far Western Icefield).

ALH85006 (49.0 g).—Fusion crust is present on only one surface of this coherent stone. The interior is made up of chondrules up to 2 mm in diameter and irregular white inclusions up to 3 mm long. A thin section shows a variety of chondrules (up to 2.5 mm across), chondrule fragments, and irregular clasts in a dark brown to black matrix. Fine-grained opaques are dispersed throughout the matrix, and rim some of the chondrules. The matrix consists largely of fine-grained iron-rich (Fa45–47) olivine. Olivine in the chondrules and mineral fragments is usually near Mg₂SiO₄ in composition, but more iron-rich grains are also present. Pyroxene is much less

FIGURE 4-5.—Photomicrographs of C3 chondrites: a, ALH84028 [C3V]; b, ALH85006 [C3V]; c, ALH85003 [C30]. Large chondrules, chondrule fragments, mineral fragments, and irregularly-shaped inclusions are dispersed within the abundant dark matrices. The chondrules in ALH85003 are smaller and more numerous than in the two C3V meteorites shown. Width of field in each photo is 2.3 mm.

abundant than olivine, and is close to MgSiO₃ in composition. The meteorite is a C3V chondrite.

LEW86006 (0.8g).—Forty percent of this small stone is covered with dull black fusion crust; dark gray matrix with some irregular white inclusions makes up the interior. The very small thin section (5 × 3 mm) shows several chondrules, up to 1.5 mm across, and some irregular granular aggregates in a very fine-grained translucent brown matrix. The chondrules consist mainly of olivine, some with polysynthetically-twinned clinopyroxene. Microprobe analyses give olivine compositions in the range Fa0–27, with a mean of Fa₆; pyroxene compositions range from Fs₀ to Fs₅. The matrix appears to consist largely of fine-grained iron-rich olivine (Fa40–50). The meteorite is tentatively identified as a C3V chondrite.

CLASS C4

FIGURE 4-6

ALH84038 (12.3 g).—This fragment has black to reddish-brown fusion crust on all but one surface; the interior is dark gray and fine-grained with no features visible. Thin section examination shows the meteorite to consist largely of finely granular olivine (grains up to 0.1 mm) with rare chondrules and chondrule fragments, and a little opaque material. Microprobe analyses, including data from Kallemeyn et al. (1991), give the following compositions: olivine, Fa25–30 (one grain was found with a composition of Fa₃₉), with a mean of Fa₂₉; pyroxene (Fs26–28); and plagioclase (An22–82). The minor element and trace element data of Kallemeyn et al. (1991) support the original proposal that this meteorite might be paired with ALH82135. Kallemeyn et al. (1991) have proposed a new group of carbonaceous chondrites, CK (after Karoonda), within which ALH84038 is classified by them as CK4.

ALH85002 (437.7 g).—Most of the surface is covered with reddish-brown polygonally-fractured fusion crust. The interior is light gray with dark rounded inclusions as large as 1 mm and white irregular inclusions up to 3 mm long. Thin section examination reveals mostly fine-grained olivine (grains up to

FIGURE 4-6.—Photomicrographs of C4 chondrites: a, ALH85002. Chondrules, chondrule fragments, magnetite, and sulfide are dispersed in the granular olivine-rich matrix. b, LEW86258. Chondrules, chondrule fragments, mineral grains, and some small irregular inclusions are visible in the granular olivine-rich matrix. Width of field in each photo is 2.3 mm.

0.1 mm), with a little pyroxene, plagioclase, and opaques (largely magnetite). A few chondrules, made up of coarser-grained olivine, are present. Microprobe analyses, including data from Kallemeyn et al. (1991), give the following compositions: olivine, Fa₃₀; pyroxene, Fs23–29; plagioclase, An30–66. Kallemeyn et al. (1991) give trace and minor element data for ALH85002, and they classify it as CK4.

LEW86258 (24.1 g).—This stone is gray and friable with black chondrules protruding from weathered surfaces; about 40% of the surface is covered with polygonally-fractured black fusion crust. A thin section shows a few chondrules in a matrix consisting largely of fine-grained olivine (grains up to 0.1 mm), with a little pyroxene, plagioclase, and opaques (almost entirely magnetite, with a little nickel-iron). Most chondrules are about 1 mm in diameter and consist of granular olivine; one large (2.5 mm) chondrule was found that consists of radiating plagioclase laths up to 0.8 mm long in a matrix of granular olivine and diopside. Microprobe analyses, including data from Kallemeyn et al. (1991), give the following compositions: olivine, Fa₃₂; low-Ca pyroxene, Fs₂₆; diopside, Wo₄₂Fs₅ with 4.8% Al₂O₃, 1.9% TiO₂; plagioclase, An31–83. Kallemeyn et al. (1991) give trace and minor element data for LEW86258, and they classify it as probable type CK4 (weathered).

ENSTATITE CHONDRITES

FIGURE 4-7

ALH84170 (39.2 g).—Fifty percent of this fragment is covered by extremely weathered fusion crust; the interior has a black matrix with numerous white to gray rounded and irregular inclusions. Seen in thin section, chondrules (0.3–2 mm) are abundant and consist of radiating or granular pyroxene, some with olivine. The matrix is made up of chondrule fragments and mineral grains, with a considerable amount of opaques (nickel-iron and sulfides). Weathering is extensive, with brown limonitic staining throughout. Microprobe analyses show many grains of olivine and pyroxene close to Mg₂SiO₄ and MgSiO₃ in composition, but some contain a considerable amount of iron. The nickel-iron contains 2.2%–3.0% Si. This meteorite is classified as type EH3.

ALH84188, 84200, 84206, 84220, 84235, 84250, 84254, 85159 (total weight 64.1 g).—Most of these small fragments, some of which are extremely weathered, were collected in a limited area of the Allan Hills Near Western Icefield. In thin section the chondrules and chondrule fragments are abundant but mostly small (ranging up to 0.6 mm across with only a few larger ones); they consist of fine-grained to coarsely granular pyroxene. The matrix consists of small pyroxene grains and opaques (mostly nickel-iron with some sulfides). Weathering is indicated by brown limonitic staining throughout the sections. Microprobe analyses give the following pyroxene compositions: Fs06–4, with a mean of Fs₁₈. The nickel-iron contains 2.3% Si. These fragments are classified as type E4.

ALH85119 (20.6 g).—This stone, found near the NW end of the Allan Hills Far Western Icefield, is completely covered with frothy black fusion crust; evaporite deposit is present underneath the fusion crust in some areas. The interior is charcoal gray with dark chondrules; oxidation is extensive in some areas. A thin section shows a compact aggregate of chondrules, chondrule fragments, and irregular clasts up to 3 mm across, together with about 25% nickel-iron in grains up to 1.5 mm and minor troilite. A blue ˜isotropic phase—possibly ringwoodite—is present as isolated sporadic grains in the matrix and as an interstitial phase (replacing glass?) within a few chondrules. Pyroxene is the predominant silicate; minor olivine occurs as small grains poikilitically enclosed in pyroxene. Many of the pyroxene crystals are clouded with very

FIGURE 4-7.—Photomicrographs of enstatite chondrites: a, ALH84170 [EH3]; b, ALH84188 [E4]; c, ALH85119 [E4?]. Numerous chondrules, chondrule fragments, and mineral grains are visible in the dark matrices of each meteorite. All three show deformed and broken chondrules. Width of field in each photo is 2.3 mm.

fine-grained opaque material; in addition, many pyroxenes are intensely deformed and even granulated, suggesting—taken together with a clear directional fabric visible in the section and the possible presence of ringwoodite—that this meteorite has been severely shocked. Coarsely crystalline graphite occurs within some metal grains. Pyroxene compositions are Fs0.3–12, mean Fs2.4. The nickel-iron contains 0.4%–0.6% Si. This meteorite has been tentatively classified as an E4 chondrite in the Antarctic Meteorite Newsletter, but the silicon content of the metal is low for EH.

CLASS H3

FIGURE 4-8

ALH85121 (55.3 g).—Frothy brown and black fusion crust covers the top surface of the “mushroom cap” shaped stone, and thick (2 mm) viscous ropy fusion crust covers the lower (B) surface. The interior is moderately to heavily weathered, with a weathering rind up to 5 mm thick; numerous clasts and chondrules are present. A thin section shows abundant chondrules and chondrule fragments up to 3 mm across, in a matrix of fine-grained olivine and pyroxene with moderate amounts of nickel-iron and troilite (locally rimming chondrules). Chondrule types include granular and porphyritic olivine and olivine-pyroxene, and radiating or cryptocrystalline pyroxene. Weathering is extensive, with limonitic staining and small areas of red-brown limonite throughout. Microprobe analyses show olivine and pyroxene with a considerable range in composition: olivine, Fa9–20 (the coefficient of variation, c.v., for FeO is 16); pyroxene, Fs3–31. The meteorite is classified as an H3 chondrite (estimated H3.8).

LEW85383 (18.5 g).—This angular stone is completely covered with polished black fusion crust. The interior is heavily oxidized, masking any structure present. The section shows abundant chondrules and chondrule fragments, up to 1.5 mm across, in a minor amount of finely granular matrix containing considerable nickel-iron and lesser troilite. A variety of

FIGURE 4-8.—Photomicrograph of the H3 chondrite ALH85121. Chondrules, chondrule fragments, and mineral grains are visible in a dark matrix. Part of the “matrix” is in fact dark limonite staining. Width of field is 2.3 mm.

chondrule types is present, mostly porphyritic and granular olivine and olivine-pyroxene, but some cryptocrystalline and radiating pyroxene chondrules also occur. Weathering is extensive, with brown limonitic staining throughout the section. Microprobe analyses show olivine and pyroxene of variable composition: olivine, Fa6–23 with a mean of Fa₁₇ (c.v. FeO is 26); pyroxene, Fs2–18. The meteorite is classified as an H3 chondrite (estimated H3.7).

LEW86102 (21.8g), 86105 (6.4 g).—Shiny smooth brown/black fusion crust completely covers both of these specimens. The interiors consist of black matrix with abundant small weathered chondrules. Oxidation is scattered throughout the interiors with LEW86105 being the more weathered of the two samples.

Thin section examination shows that the two specimens are very similar and probably paired. Chondrules and chondrule fragments are abundant, ranging up to 2.1 mm across; most are porphyritic or granular olivine and olivine-pyroxene, but a few radiating or cryptocrystalline pyroxene chondrules were noted. They are surrounded by a dark finely granular groundmass containing some metal and a little sulfide, often as rims to chondrules; the metal is extensively altered to limonite. Microprobe analyses show olivine and pyroxene of highly variable composition; olivine, Fa1–50 (c.v. FeO is 70); pyroxene, Fs1–41. The meteorites are classified as H3 chondrites (estimated H3.3).

CLASS L3

FIGURE 4-9

ALH84120 (129.0 g), 85045 (145.0 g).—Thirty percent of ALH84120 is covered with fusion crust; fractured brown fusion crust covers most of ALH85045. The interiors of these unequilibrated chondrites are light to medium gray and show abundant chondrules (up to 3 mm across) and irregular inclusions. Sections show an aggregate of chondrules (0.3–2.4 mm across) and chondrule fragments in a fine-grained matrix of olivine and pyroxene with minor amounts of nickel-iron and troilite. A variety of chondrule types is present; one barred chondrule was found with transparent pale brown glass between the olivine bars. Most of the pyroxene is polysynthetically twinned clinobronzite. Minor weathering is indicated by brown limonitic staining around metal grains. Microprobe analyses gave the following compositions: olivine, Fa22–26, mean Fa₂₃ (c.v. FeO is 8); pyroxene, Fs6–22. These L3 chondrites (estimated L3.9) are very similar and may be paired.

ALH84205 (25.2 g).—Fusion crust covers 60% of this stone. The interior is made up of light to medium gray matrix with abundant light and dark chondrules and inclusions. In thin section chondrules and chondrule fragments are abundant, ranging up to 2.1 mm across; most are granular olivine and olivine-pyroxene, but barred olivine and cryptocrystalline pyroxene chondrules are also present. Many have narrow black rims. Nickel-iron and troilite are present in small amounts. Microprobe analyses give the following compositions: olivine, Fa12–33, with a mean of Fa₁₉ (c.v. FeO is 27); pyroxene, Fs4–19. The meteorite is classified as an L3.7 chondrite.

ALH85062 (167.3 g), 85155 (18.5 g).—These stones are partly covered with dull black fusion crust. The interiors show a gray matrix with abundant inclusions. Sections show a close-packed aggregate of chondrules and chondrule fragments (up to 2.1 mm across) in a dark finely granular matrix, which includes minor amounts of nickel-iron and troilite. A variety of chondrule types is present, including granular and porphyritic olivine and fine-grained pyroxene. Both olivine and pyroxene show a wide range in composition: olivine, Fa1–28, mean Fa16 (c.v. FeO is 40); pyroxene, Fs3–20. These meteorites are very similar and may be paired. They are classified as L3.5–3.6, and may belong to the ALHA77011 pairing group.

ALH85070 (12.9 g).—Fusion crust covers about 75% of this stone. The interior is light-gray and has abundant inclusions. Oxidation is light. The section shows abundant chondrules and chondrule fragments, up to 2 mm across, in a fine-grained matrix containing a few coarser grains of nickel-iron and troilite. A variety of chondrule types is present including granular and porphyritic olivine and olivine-pyroxene, barred olivine, and crypto-crystalline pyroxene. Much of the pyroxene is polysynthetically-twinned clinobronzite. Minor weathering is indicated by rusty halos around metal grains. Microprobe analyses show olivine and pyroxene of variable composition: olivine, Fa4–25 with a mean of Fa₁₈ (c.v. FeO is 38); pyroxene, Fs1–29. The meteorite is classified as an L3 chondrite (estimated L3.6).

LEW85339 (28.8 g).—Polygonally fractured fusion crust completely covers this small specimen. The interior is light gray with chondrules and other inclusions measuring up to ˜3 mm in diameter. One dark haloed inclusion (metallic?) is ˜7

FIGURE 4-9.—Photomicrographs of L3 chondrites: a, ALH84120; b, ALH84205; c, LEW86018; d, LEW86127. Abundant chondrules, chondrule fragments, and mineral grains are visible in the photos; only a little matrix is actually present in any of these three meteorites, the dark regions in fact being mostly limonite weathering. Width of field in each photo is 2.3 mm.

mm across. In thin section, chondrules and chondrule fragments up to 2.1 mm across are closely-packed in a minimum amount of dark matrix that contains some troilite and a little nickel-iron. A variety of chondrule types is present, including granular and porphyritic olivine and olivine-pyroxene, barred olivine, and fine-grained radiating pyroxene. Considerable weathering is indicated by brown limonitic staining throughout the section. Microprobe analyses show olivine and pyroxene of variable composition: olivine, Fa1–30, with a mean of Fa₁₄ (c.v. FeO is 59); pyroxene, Fs3–13. The meteorite is classified as an L3 chondrite (estimated L3.4).

LEW85396 (60.2 g), 85401 (3.9 g), 86018 (502.0 g), 86022 (351.7 g).—These stones are very similar and are possibly paired. They all retain some fusion crust, but are extensively weathered. Interiors are brown and show numerous chondrules and inclusions up to 5 mm long. Thin sections show a close-packed mass of chondrules (up to 2.9 mm across), chondrule fragments, and irregular granular aggregates in a small amount of opaque matrix which includes minor amounts of nickel-iron and troilite. Most chondrules consist of granular or porphyritic olivine, some with polysynthetically-twinned clinopyroxene. Both olivine and pyroxene show a wide range in composition: olivine, Fa1–34; pyroxene, Fs1–31. The meteorites are classified as L3.5 chondrites.

LEW85434 (19.4 g), 85437 (9.4 g).—Both specimens have red-brown interiors covered with weathered fusion crust. Light colored chondrules are visible in LEW85437.

Thin section examination shows that these two specimens are very similar and probably paired. Chondrules and chondrule fragments, up to 1.8 mm across, are closely-packed in a small amount of dark matrix containing a little troilite and nickel-iron. Chondrule types include granular and porphyritic olivine and olivine-pyroxene, barred olivine, and fine-grained radiating pyroxene. Weathering is extensive, with brown limonitic staining pervading the sections. Olivine and pyroxene show a wide range in composition: olivine, Fa1–23 with a mean of Fa₁₀ (c.v. FeO is 76); pyroxene, Fs2–11. The meteorite is classed as an L3 chondrite (estimated L3.4).

LEW85452 (9.2 g).—Black fusion crust completely covers LEW85452. Macroscopically, the interior structure has been obscured by oxidation. The section shows a close-packed aggregate of chondrules and chondrule fragments, up to 2.4 mm across, in a finely granular matrix containing some troilite and nickel-iron. Chondrule types include granular and porphyritic olivine and olivine-pyroxene, and fine-grained to cryptocrystalline pyroxene. Olivine and pyroxene show a wide range in composition: olivine, Fa5–23 with a mean of Fa₁₅ (c.v. FeO is 35); pyroxene, Fs2–18. The meteorite is classed as an L3 chondrite (estimated L3.6).

LEW86021 (325.8 g).—This rounded stone is mostly covered with iridescent fusion crust; evaporite deposit is present on some surfaces. The interior is red-brown and heavily weathered. A section shows a close-packed mass of chondrules (up to 2.7 mm across), chondrule fragments, and irregular crystalline aggregates, together with minor amounts of nickel-iron and troilite. A variety of chondrule types is present, including granular and porphyritic olivine and olivine-pyroxene, barred olivine, and radiating pyroxene. Weathering is extensive, with limonitic staining and areas of brown limonite throughout. Microprobe analyses give the following compositions: olivine, Fa18–28, mean Fa₂₁ (c.v. FeO is 10); pyroxene, Fs4–17. The meteorite is classified as an L3.9 chondrite.

LEW86127, 86134, 86144, 86158, 86207, 86246, 86270, 86307, 86367, 86408, 86417, 86436, 86505 (total weight 150.9 g).—All these small stones show light and dark chondrules and angular fragments in a darker, very coherent matrix. They are very similar in all respects and may be paired. Sections show a close-packed aggregate of chondrules and chondrule fragments, up to 3 mm across, in a minimum amount of fine-grained dark matrix that contains a little nickel-iron and troilite. Chondrule types include granular and porphyritic olivine and olivine-pyroxene, barred olivine, and radiating and cryptocrystalline pyroxene. Weathering is extensive, with brown limonitic staining throughout. Composition ranges are: olivine, Fa1–30; pyroxene, Fs1–24. The meteorites are classified as L3.5 chondrites.

LEW86549 (50.1 g).—A gray interior is overlain by a black polygonally-fractured fusion crust that covers about 50% of the surface of this specimen. The section consists mainly of chondrules and chondrule fragments, with a minor amount of dark finely-granular matrix containing a little nickel-iron and troilite. The chondrules are relatively small, the largest found being 0.9 mm across; most are granular olivine and olivine-pyroxene, but barred olivine and radiating pyroxene chondrules are also present. Weathering is extensive, with brown limonitic staining throughout the section. Olivine and pyroxene are variable in composition: olivine, Fa5–20, with a mean of Fa15 (c.v. FeO is 28); pyroxene, Fs1–29. This meteorite is classified as an L3 chondrite (estimated L3.7).

RKP86700 (424.1 g).—Brown fusion crust spotted with oxidation halos covers 90% of this stone. The interior is dark gray with black inclusions up to 2 mm across; oxidation is heavy along fractures. A thin section shows a close-packed mass of chondrules (0.6–2.4 mm across) and irregular aggregates. Some chondrules have dark rims consisting largely of fine-grained troilite. The sparse matrix is fine-grained, with a small amount of coarser nickel-iron and troilite scattered throughout. A notable variety of chondrule types is present; many are granular or porphyritic olivine and olivine-pyroxene with transparent to turbid interstitial glass. The pyroxene is polysynthetically-twinned clinobronzite. Brown limonitic staining pervades the section. Composition ranges are as follows: olivine, Fa17–27, mean Fa₂₃ (c.v. FeO is 9.3); pyroxene, Fs14–23. The meteorite is an L3 chondrite (estimated L3.9); it resembles RKPA80256, and may be paired with it.

CLASS LL3

FIGURE 4-10

ALH84086 (234.0 g).—Fusion crust covers most of this chondrite. Abundant inclusions, both chondrules and clasts (one 0.7 × 0.9 cm), are present in the medium gray matrix. A section shows a close-packed aggregate of chondrules, chondrule fragments, and irregular inclusions up to 3 mm across, with a few grains of nickel-iron and sulfide and hardly any matrix. A considerable variety of chondrules is present, the commonest being porphyritic or granular olivine with or without polysynthetically twinned clinobronzite. Some chondrules have intergranular transparent pale brown glass; in others the glass is turbid and partly devitrified. Microprobe analyses, including data from Rubin (1990), show a moderate range in olivine composition (Fa25–29) with a mean of Fa₂₈, and a wider range in pyroxene (Fs17–26). The meteorite was originally classified as an LL3.9 chondrite; Rubin (1990) has suggested LL3.8.

ALH84126 (41.2 g).—This fragment retains four small patches of fusion crust. Numerous chondrules and inclusions show in relief on the brown weathered surface. A thin section shows a close-packed aggregate of chondrules, chondrule fragments, and angular clasts ranging up to 3 mm across. Many chondrules have dark rims. A variety of chondrule types is present, including porphyritic olivine, granular olivine and olivine-pyroxene, and radiating pyroxene. A few grains of troilite and nickel-iron are present. Mineral compositions are: olivine, Fa7–31, mean Fa₁₆ (c.v. FeO is 46); pyroxene, Fs3–24, mean Fs₉. The meteorite is classified as an LL3.5 chondrite.

FIGURE 4-10.—Photomicrographs of LL3 chondrites: a, ALH84126; b, ALH84086. Numerous chondrules, chondrule fragments, and mineral grains are visible in very sparse dark matrices. Much of what appears to be dark matrix in ALH84126 is actually limonite staining. Opaque (iron-nickel metal and troilite) grains are visible in the photo of ALH84086. Width of field in each photo is 2.3 mm.

Achondrites

BRACHINA-LIKE AND UNGROUPED

FIGURE 4-11

ALH 84025 (4.6 g).—This small meteorite is largely covered with thick fusion crust. In composition it is essentially a dunite, consisting of large (up to 1.5 mm) polygonal olivine crystals that are uniformly Fo67–68 in composition, with lesser diopside (Wo₄₄En₄₆) and sparse polygonal chromite grains. Crisscrossing the meteorite are veins of troilite, within which are tiny globules of Ni-rich (about 30% Ni) metal. In many cases these sulfide veins are no more than trails of tiny sulfide grains that outline crystal boundaries and define (presumably) healed fractures within crystals. Only the larger and more continuous veins contain metal. Neither the olivine nor the pyroxene show significant undulatory extinction. A well-defined fusion crust, 0.6 mm thick, encloses much of the area of the thin section, reflecting the small overall size of this meteorite. A very few fractures show slight limonitic staining, indicating only minor terrestrial weathering. This specimen most closely resembles the unique achondrite Brachina in texture and mineralogy, but unlike Brachina it is more coarsely crystalline and contains no plagioclase.

Prinz et al. (1986) made a comprehensive study of the mineralogy of ALH84025 and confirmed its close relationship to Brachina. Warren and Kallemeyn (1987) compared ALH84025 to Brachina and Chassigny, noting that in some respects it bears little resemblance to either. It has far lower contents of light REE, and also shows a small (barely significant) positive Eu anomaly, which neither Brachina nor Chassigny has. They interpret the texture and bulk composition of this meteorite to reflect an origin as an igneous cumulate. Ott et al. (1987) have measured the noble gases in ALH84025 and conclude that it is like Brachina and unlike Chassigny.

ALH84190 (7.9 g).—This small fragment probably fits on a corner of a larger meteorite that is angular and has an ablation flange. The section shows an aggregate of anhedral to subhedral grains, 0.06–0.5 mm across, of olivine and pyroxene, with about 15% of disseminated nickel-iron and minor amounts of plagioclase and troilite. The proportion of pyroxene to olivine is estimated as 4:1. Weathering is extensive, with veinlets and small areas of brown limonite throughout the section. Microprobe analyses gave the following compositions: olivine, Fa₄; plagioclase, An₁₉; pyroxene, Wo₃Fs₆ (slightly variable, with one grain of diopside, Wo₄₃Fs₃, analysed). This ungrouped achondrite is essentially identical with ALHA81187; in mineral compositions and texture these meteorites closely resemble inclusions in iron meteorites, such as in Campo del Cielo (Wlotzka and Jarosewich, 1977).

EET84302 (59.6 g).—The exterior of this specimen is mostly covered with thin fusion crust. The section shows an anhedral granular aggregate (grain size 0.1–0.4 mm) consisting largely of olivine and orthopyroxene, with minor amounts of plagioclase, diopside, nickel-iron, and troilite. Weathering is extensive, with limonitic staining throughout the section. Microprobe analyses gave the following compositions: olivine, Fa₅; orthopyroxene, Wo₂Fs₈; diopside, Wb₄₂Fs₃; plagioclase, An₂₃.

ANGRITE

FIGURE 4-12

LEW86010 (6.9 g).—Greenish brown crystals are visible in relief on the polished black exterior surface of this tiny stone; some flow-like features are also visible. In thin section the rock

FIGURE 4-11.—Photomicrographs of Brachina-like and ungrouped achondrites: a, ALH84025; b, ALH84190; c, EET84302. ALH84025 (Brachina-like) shows mostly olivine and chromite (opaque). In ALH84190 olivine, pyroxene, and metal and sulfide (both opaque) are visible in the photo. EET84302 contains olivine, abundant metal and sulfide, pyroxene, plagioclase, and extensive limonitic staining. Width of field in each photo is 2.3 mm.

consists of a granular aggregate of subequal amounts of plagioclase, pyroxene, and olivine, with minor amounts of troilite, whitlockite, hercynite spinel, and rare merrilite. A striking feature is the strongly zoned, reddish-purple pleochroism of the pyroxene.

The pyroxene is a fassaite, with 10% Al₂O₃ and 2% TiO₂. The olivine is Fa₆₃ in composition and contains l%–3% CaO; present within the olivines are optically-visible exsolution lamellae of kirschsteinite. The plagioclase is An₁₀₀ in composition.

Correlated studies of the petrologic and trace element characteristics have been reported by McKay, Lindstrom, Le, and Yang (1988), McKay, Lindstrom, Yang, and Wagstaff (1988), and Crozaz and Lundberg (1988); Rb-Sr, Pb-Pb, and Ca and Ti isotopic properties are reported by Lugmair et al. (1989).

McKay and co-workers conclude that there is little doubt that LEW86010 is an igneous rock. The trace element studies of Crozaz and Lundberg (1988) support such a conclusion; moreover, the lower REE abundances and higher Fe/Mg in LEW86010 relative to Angra Dos Reis (ADOR) makes it unlikely that these two rocks are co-magmatic.

Lugmair et al. (1989) found that the initial ⁸⁷Sr/⁸⁶Sr for LEW86010 is slightly higher than that of ADOR and slightly lower than that of the cumulate eucrite, Moore County. The Pb isotopic composition of LEW86010 proved to be severely contaminated by modern terrestrial lead; however, measurements on plagioclase yielded model ages of 4.536 b.y. relative to primordial lead. The aluminous composition of LEW86010 led to a proposal (Prinz et al., 1988) that it might somehow be related to Ca-Al-rich inclusions such as those found in carbonaceous chondrites; Lugmair et al. (1989) measured Ca and Ti isotopic ratios in the angrite and found no anomalies.

On the basis of zoning in the pyroxene and olivine, McKay, Lindstrom, Le, and Yang (1988) and McKay, Lindstrom, Yang, and Wagstaff (1988) concluded that LEW86010 cooled more rapidly than did ADOR.

FIGURE 4-12.—Photomicrograph of the angrite LEW86010. Plagioclase (clear), olivine (clear with high relief), fassaitic pyroxene (grey), and sparse troilite are visible. Width of field is 2.3 mm.

AUBRITES

FIGURE 4-13

ALH84007-84024 (total weight 3.6 kg).—These 18 specimens were collected in the Middle Western Icefield over a distance of some 6 km. The largest specimen, ALH84007, weighs 705.6 g. All are fragments, some with minor amounts of yellowish to black fusion crust, and are almost certainly pieces of a single meteorite, along with ALH83009 and ALH83015. Macroscopically they are complex breccias of large white clasts up to 2 cm across, some dark inclusions up to 5 mm across, and occasional metal grains with rusty halos. Thin sections show that the meteorite is made up almost entirely of large (up to ˜4 mm) intensely shocked enstatite crystals (essentially pure MgSiO₃; FeO less than 0.2%). The dusty brown matrix consists of granulated enstatite with minor forsterite (FeO 0.1%), diopside (Wo41–44En56–59), and plagioclase (An₉Or₃). Diopside occurs both as independent grains in the matrix and as rounded inclusions within enstatite. Sparse kamacite grains, up to 3 mm across, are invariably associated with a complex assemblage of phases that includes troilite, schreibersite, daubreelite, and alabandite.

Schwarz et al. (1986) have examined the chemistry of mineral separates from ALH84007, ALH84008, and ALH84011. The “enstatite” separates have V-shaped REE patterns with positive Eu anomalies, and appear fairly uniform in comparison with previous data on aubrites. Ryder and Murali (1987) have continued the work on these meteorites, and find that the REE patterns are influenced by several sulfide phases, which are the principal carriers of these elements. Random microprobe analyses of 55 enstatite grains in ALH84011 give the following: Al₂O₃ is fairly uniform, mostly in the range 0.06%–0.08%; CaO 0.22%–0.30%; FeO variable,

FIGURE 4-13.—Photomicrograph of the aubrite ALH84007. Most of the photo is occupied by large deformed enstatite crystals, with scattered opaque sulfides. Width of field is 2.3 mm.

with most less than 0.03%; Na₂O and TiO₂ both less than 0.01%; Cr₂O₃ less than 0.015%.

DIOGENITES

FIGURE 4-14

ALH84001 (1930.9 g).—Much of this rectangular shaped meteorite is covered with dull black fusion crust. Remnants of flow marks are visible on two surfaces. Areas not covered by fusion crust have a greenish-gray color and a blocky texture. Cleavage planes are obvious on some large crystals and the stone has a shocked appearance. Small areas of oxidation are present, and small fractures are numerous.

A thin section shows orthopyroxene (Wo₃Fs₂₇) crystals up to 5 mm long forming a polygonal-granular mosaic. Despite the fact that the pyroxene contains 1.5% CaO, no exsolution lamellae were observed. Veins of intensely granulated pyroxene cross cut the section. Other minerals include minor chromite and irregular patches of maskelynite (An35–39Or3–4). The section does not show iron-oxide staining but does contain patches of brown Fe-rich carbonate, (Fe₂₉Mg₆₀Ca₁₁)CO₃. Although this diogenite contains granulated areas, it does not appear to be a breccia.

ALH85015 (3.2 g).—This small fragment is partly covered with black fusion crust, shiny in some places and dull in others. Part of the area devoid of fusion crust is highly polished. A weathering rind extends 2 mm into the interior, which is medium gray in color with white and dark clasts. The thin section consists almost entirely of orthopyroxene clasts, up to 3 mm across, in a groundmass of comminuted orthopyroxene, accessory plagioclase and opaques, and traces of olivine. The pyroxene is fairly uniform in composition, Fs₂₅, with CaO 0.8%–1.5%, MnO 0.45%–0.67%, Al₂O₃ 0.32%–0.66%, and

FIGURE 4-14.—Photomicrographs of diogenites: a, ALH84001. The photo shows the largely undeformed character of this meteorite, except for some narrow granulated zones crossing the section. Clear maskelynite and opaque chromite are accessory to the predominant orthopyroxene. b, ALH85015. In contrast with ALH84001, this meteorite is highly brecciated. Orthopyroxene-rich clasts are prominent in a comminuted matrix of pyroxene, plagioclase, minor olivine, and opaques. Width of field in each photo is 1 cm.

TiO₂ 0.05%–0.17%. The plagioclase composition is An84–95. One grain of olivine, Fa₃₉, was analysed.

EUCRITES

FIGURE 4-15

ALH85001 (212.3 g).—This meteorite is an oriented stone covered by a shiny black fusion crust with thick flow lines. Areas devoid of fusion crust have weathered to a brownish-gray color. A discontinuous weathering rind, up to 4 mm thick, was exposed when the stone was chipped. The interior is made up of abundant laths of chalky plagioclase in a light gray matrix. The thin section shows angular fragments of orthopyroxene and plagioclase, up to 2.4 mm across, in a comminuted groundmass of these minerals. Some of the pyroxene has lamellae and blebs of exsolved augite. A large gabbroic clast, 6 mm across with individual grains up to 3 mm, was found in one section. Trace amounts of nickel-iron and troilite are present in the groundmass. Microprobe analyses show that pyroxene compositions are remarkably uniform, Wo₂Fs₃₂, with a few more calcic grains, up to Wo₈ (possibly incipient augite exsolution). Plagioclase composition is also fairly uniform, An92–94. The meteorite is a monomict eucrite with unusually magnesian pyroxene, similar to that in the Binda eucrite.

Mittlefehldt and Lindstrom (1988) have provided REE data on this meteorite.

LEW85300 (210.3 g), 85302 (114.5 g), 85303 (408.0 g).—These three specimens of a complex polymict eucrite are clearly paired. Thin shiny fusion crust with flow marks coat most of the top of LEW85300; the bottom surface has some fusion crust but most of this face is a fracture surface which appears to have been moderately polished. Fusion crust appears as dull patches on LEW85302 and LEW85303. Several large semi-rounded polymineralic clasts (as large as 2 × 2 cm) have sharply defined edges and are set in a black matrix.

Thin sections show numerous clasts with diverse textures and mineralogy set in a dark matrix with numerous comminuted grains of pyroxene and plagioclase. Some clasts have gabbroic or basaltic textures, others are breccias, and one is identified as a C3 chondrite. Microprobe analyses of section LEW85300,14 give pyroxene compositions clustering around Wo₃Fs₆₀, ranging to Wo₄₃Fs₂₆, with a mean (n = 15) of Wo₁₂Fs₅₂; two grains were found having the composition Wo₃Fs₃₃. The plagioclase composition is An84–93.

The C3 chondrite clast consists largely of fine-grained olivine, with a composition of Fa1–44; one grain each of clinoenstatite (Fs₅₉) and spinel (FeO 0.8%) were found.

A detailed account of clasts in these meteorites has been provided by Kozul and Hewins (1988). Mittlefehldt and Lindstrom (1988) have provided REE and other trace element data on some clasts from these meteorites.

LEW85305 (40.8 g).—This cuboidal stone is completely covered with shiny fusion crust showing flow lines. It has been described in detail by Delaney (1987), as follows: “Lewis Cliff” 85305 is a granular fine grained eucrite … containing pigeonite and augite with homogeneous plagioclase (An88.9Or0.63) in roughly equal proportions. A SiO₂ polymorph is the most abundant minor phase with troilite, metal and intimately intergrown ilmenite and Ti-bearing chromite. The pyroxene has a relict subophitic texture with a grain size of about 500–1000 μm, but the present pyroxene is very fine grained equant grains (50–100 μm) developed by granulation and recrystallizion of the earlier texture. Plagioclase does not show the same comminution effects. Both the augite and the pigeonite have well-defined exsolution lamellae and are unzoned. Unlike that in most eucrites, the plagioclase in 85305 … is also unzoned. The SiO₂ mineral in 85305 occurs in a distinctive textural

FIGURE 4-15.—Photomicrographs of eucrites: a, ALH85001. The photo shows a large gabbroic clast (upper left) enclosed in a granulated matrix of pyroxene (grey) and feldspar (light). b, LEW86001. The photo shows diverse-textured clasts in a granulated matrix. c, LEW85300. The photo shows several clast types enclosed in the dark granulated matrix. d, LEW85305. Fine- to coarse-grained pyroxene (grey) and plagioclase are prominent in the photo of this unbrecciated eucrite. Width of field in a is 1 cm; in all others, 2.3 mm.

setting as fairly large poikilitic grains with abundant small inclusions of both pyroxenes and tends to form secondary veins or “pools” through the thin section."

In its texture and the lack of compositional zoning in its constituent phases, LEW85305 resembles the Ibitira eucrite (Steele and Smith, 1976); however, the mineral compositions are different.

Mittlefehldt and Lindstrom (1988) report that this meteorite has a relatively flat REE distribution pattern at about 7–9 × ordinary chondrites.

LEW85353 (24.5 g).—This small stone has retained most of its thin fusion crust. Weathering has removed some crust and left a pitted surface. The interior is light gray and has a basaltic texture. A thin section shows a fine-grained aggregate (grains 0.1–0.4 mm) of pale brown pyroxene and colorless plagioclase, with a few opaque grains. Minor weathering is indicated by brown limonitic staining around the opaques. Plagioclase is fairly uniform in composition, averaging An₈₈. Pyroxene is largely hypersthene with a mean composition of Wo₂Fs₆₀; accessory augite (Wo₄₈Fs₂₂) has exsolution lamellae of hypersthene.

LEW86001 (290.6 g).—Dull black fusion crust covers 80% of this stone. Large vugs, typical of Antarctic eucrites, are abundant. Interior surfaces have a light gray color and show numerous clasts up to 0.5 cm across; some oxidation is present. A thin section shows a groundmass of comminuted pyroxene and plagioclase (grains up to 0.5 mm), with numerous clasts up to 3 mm. The clasts consist of pyroxene and plagioclase, with a variety of textures from fine-grained granular to ophitic to gabbroic. A small amount of opaques is present. Microprobe analyses show pyroxene compositions clustered around Wo₂Fs₅₆ but ranging to Wo₄₄Fs₂₂, with a fairly uniform En content. The plagioclase composition is An76–88.

Mittlefehldt and Lindstrom (1988) report that this meteorite is relatively enriched in REE (10–20 × ordinary chondrites), and shows a LREE enriched pattern with a negative Eu anomaly.

LEW86002 (32.6 g).—This oblong stone is mostly covered with thin black fusion crust. Thin section examination reveals an ophitic intergrowth of pyroxene and plagioclase (plagioclase laths up to 0.7 mm long). Pyroxene ranges in composition from Wo₃Fs₆₁ to Wo₃₇Fs₃₁, with fairly uniform En content. The plagioclase composition is An₉₀.

LEW86003 (1.6 g).—This small stone is mostly covered with dull black fusion crust. Ninety percent of the interior is light gray in color with minor black inclusions; the remainder is jet black with minor inclusions and is more oxidized than the lighter material. The contact between the two areas is sharp. Most of the thin section is an ophitic intergrowth of pyroxene and plagioclase, the plagioclase laths being up to 0.9 mm long. One end of the section is sharply bounded by an area of subophitic to gabbroic clasts, up to 1.8 mm across, in a dark brown glassy matrix. Mineral compositions are similar throughout. Pyroxene compositions range from Wo₂Fs₆₄ to Wo₄₃Fs₂₇, with fairly uniform En contents. Plagioclase compositions are An87–91.

HOWARDITES

FIGURE 4-16

LEW85313 (191.2 g).—Dull fusion crust covers most of this meteorite except where large pieces have been plucked out, giving the appearance of a piece of Swiss cheese. A brownish-gray weathering rind extends from less than 1 mm to greater than 1 cm into the interior. The massive gray matrix contains rounded and irregular inclusions ranging in color from white to black. Some oxidation halos are present. This meteorite was originally classified as a diogenite on the basis of a non-representative section (LEW85313,5), which was a large orthopyroxene clast. Berkley (1988) has shown that it is actually a howardite. It is a microbreccia of pyroxene clasts (orthopyroxene with minor clinopyroxene) and eucrite clasts, up to 0.9 mm across, in a comminuted groundmass of pyroxene grains, with minor plagioclase and accessory opaques (troilite and nickel-iron). Microprobe analyses give the following pyroxene composition range: Wo1–10Fs15–64En34–85, with a mean of Wo₈Fs₃₃ (one grain of ferroaugite, Wo₃₇Fs₃₈, was analyzed). The plagioclase composition range is An78–94. Two grains of a silica polymorph, probably tridymite, were found during reconnaiscance examination.

LEW85441 (10.9 g).—Half of the surface of this meteorite is

FIGURE 4-16.—Photomicrographs of howardites: a, LEW85441; b, LEW85313. Both photographs show heterogeneous aggregates of basaltic, gabbroic, and diogenetic clasts in comminuted matrices. Width of field in each photo is 2.3 mm.

covered with dull frothy fusion crust. Areas devoid of fusion crust are brownish-gray and show numerous clasts. Some oxidation halos are present. The section has a cataclastic texture, with angular fragments of pyroxene and plagioclase up to 2 mm across in a comminuted groundmass of the same minerals. The pyroxene is mainly hypersthene, but ranges widely in composition: Wo1–10Fs25–48En34–74, with a mean of Wo₃Fs₂₈. Plagioclase composition is An92–96. This meteorite is very similar to LEW85313 and is possibly paired with it.

UREILITES

FIGURE 4-17

ALH84136 (83.5 g).—This specimen is entirely covered with flaky black fusion crust. The interior is dark gray and

FIGURE 4-17.—Photomicrographs of ureilites: a, LEW85238. Olivine and pyroxene crystals are surrounded by dark, mostly carbonaceous material that also includes minor metal and troilite. b, LEW86216. The rounded olivine clasts have been transformed into fine-grained olivine mosaics, enclosed in a dark granulated matrix. Width of field in each photo is 2.3 mm.

granular with crystals as large as 2 mm in a red-brown matrix. A thin section shows an aggregate of anhedral to subhedral grains (0.6–2.4 mm across) of olivine and pyroxene, with about 10% of opaque material, in part disseminated throughout and in part concentrated along grain boundaries. Olivine grains are gray due to submicroscopic opaque inclusions, whereas pyroxene grains are clear but are extremely fractured. Micro-probe analyses give the following compositions: olivine, somewhat variable, Fa0–5, mean Fa₃; pyroxene, essentially uniform, Wo₅Fs₃; one grain of endiopside, Wo₃₄Fs2.5, was analysed. This meteorite is so similar in all respects to ALH82106 and ALH82130 that it can be confidently paired with them; they were all found within 2–3 km of each other on the Allan Hills Far Western Icefield. This pairing is also supported by oxygen isotope data (Clayton and Mayeda, 1988).

LEW85328 (106.8 g).—The meteorite is covered by black fusion crust, frothy in places and with well-defined ablation marks. A thin section shows an aggregate of anhedral to subhedral grains (0.3–2.4 mm across) of olivine and pyroxene, in the approximate proportions of 2 : 1. Individual grains are rimmed by carbonaceous material. Trace amounts of finely-divided nickel-iron and troilite are present, mainly along grain boundaries. Microprobe analyses indicate that the olivine is uniform in composition, (Fa₂₀), with notably high CaO content (0.3%). The pyroxene is pigeonite with a composition of Wo₉Fs₁₇. This meteorite is notably unshocked compared to most ureilites.

LEW85440 (43.8 g).—Thin black fusion crust with greenish-gray streaks cover 60% of this stone. A thin section shows an equigranular aggregate of rounded to subhedral olivine and pyroxene grains, 0.3–0.6 mm across. The grains are rimmed with black carbonaceous material, which also contains trace amounts of nickel-iron (partly weathered to brown limonite) and troilite. Microprobe analyses show olivine and pyroxene of uniform composition: olivine Fa₉ (CaO 0.3%); pyroxene, Wo₅Fs₈. Takeda et al. (1988) have made a detailed study of this meteorite and report one grain of augite, Wo₃₆Fs₅. Clayton and Mayeda (1988) have determined the oxygen isotopic composition.

LEW86216 (6.5 g).—This small stone has black frothy fusion crust on most surfaces. It is extremely weathered, and much of the thin section is obscured by brown limonite. It shows rounded olivine grains, up to 3 mm across, in a finer-grained, probably comminuted matrix of olivine with minor pyroxene. The probable presence of accessory diamond is indicated by difficulties in cutting and polishing, and by brightly fluorescent particles in an electron beam. The meteorite is heavily shocked, the large olivines being converted into a mosaic of tiny grains averaging 0.05 mm across. Microprobe analyses show an olivine composition range of Fa12–18, with a mean of Fa₁₇ (CaO 0.2%–0.4%); pyroxene compositions are in the range Wo1–10Fs12–18.

MESOSIDERITES

FIGURE 4-18

LEW86210 (9.2 g).—This is a reddish-brown weathered pebble with a high content of metal. Thin section examination reveals a granular aggregate of about 50% nickel-iron and 50% silicates. Pyroxene is the predominant silicate, with lesser amounts of plagioclase, as clastic grains up to 1.5 mm in maximum dimension in a matrix of metal and comminuted silicates. Minor weathering is indicated by small areas of brown limonite. Microprobe analyses show pyroxene in the composition range Wo2–8Fs24–61; most grains are Fs24–31, with a mean of Wo₄Fs₂₉. The plagioclase composition is An89–92. One grain of olivine, Fa₄₅, was analysed.

QUE86900 (1532.3 g).—This meteorite, from the Queen

FIGURE 4-18.—Photomicrographs of mesosiderites: a, LEW86210; b, QUE86900. Both photographs show clasts of orthopyroxene and plagioclase enclosed in metal matrices. The metal in QUE86900 is heavily weathered. Width of field in each photo is 2.3 mm.

Alexandra Range, has isolated blebs of dull black fusion crust on about 10% of the surface, the remainder being weathered red-brown. Shallow regmaglypts are present. Large green platy pyroxene crystals are abundant, the largest being 1.5 cm long. Cleaving the stone revealed an interior with numerous platy and dark rounded inclusions in a highly weathered, red-brown, metal-rich matrix. A thin section shows plagioclase and pyroxene clasts, up to 1.5 mm across, in an opaque matrix of nickel-iron and troilite; the nickel-iron is extensively weathered to limonite. Most of the pyroxene is hypersthene, but some pigeonite is present; the composition ranges are Wo1–11Fs21–64En31–78, with a mean of Wo₃Fs₃₃. Most plagioclase compositions are in the range An90–96, but a few more sodic grains were analysed.

Mittlefehldt (1988) has measured the REE distribution in a gabbroic clast from this meteorite.

Literature Cited

Berkeley J.L.

1988.    LEW85313 Howardite: Clues to the Diogenite-Cumulate Eucrite Connection? In Lunar and Planetary Science XIX, pages 63–64. Houston: The Lunar and Planetary Institute.

Clayton, R.N., and T.K. Mayeda

1988.    Ureilites Are Not Igneous Differentiates. In Lunar and Planetary Science XIX, pages 197–198. Houston: The Lunar and Planetary Institute.

Crozaz, G., and L. Lundberg

1988.    Rare Earth Elements (REE) in the Unique Achondrite LEW86010. In Lunar and Planetary Science XIX, pages 231–232. Houston: The Lunar and Planetary Institute.

Delaney, J.S.

1987.    The Lewis Cliff 85305 Eucrite, and Shock Effects in Eucrites. Meteoritics, 22:365–366.

Gibson, E.K.

1987.    Carbon and Sulfur Abundances in Antarctic Meteorites. Meteoritics, 22:384–385.

Grady, M.M., and C.T. Pillinger

1989.    Nitrogen and Carbon in ALH85085—Links with Bencubbin? In Lunar and Planetary Science XX, pages 351–352. Houston: The Lunar and Planetary Institute.

Grossman, J.N., A.E. Rubin, and G.J. MacPherson

1988.    ALH85085: A Unique Volatile-poor Carbonaceous Chondrite with Possible Implications for Nebular Fractionation Processes. Earth and Planetary Science Letters, 91:33–54.

Kallemeyn, G.W., A.E. Rubin, and J.T. Wasson

1991.    The Compositional Classification of Chondrites, V: The Karoonda (CK) Group of Carbonaceous Chondrites. Geochimica et Cosmochimica Acta, 55:881–892.

Kozul, J., and R.H. Hewins

1988.    LEW85300, 02, 03 Polymict Eucrite Consortium. In Lunar and Planetary Science XIX, pages 645–648. Houston: The Lunar and Planetary Institute.

Lugmair, G.W., S.J.G. Galer, and R. Loss

1989.    Rb-Sr and other Isotopic Studies of the Angrite LEW86010. In Lunar and Planetary Science XX, pages 604–605. Houston: The Lunar and Planetary Institute.

McKay, G., D. Lindstrom, L. Le, and S.-R. Yang

1988.    Experimental Studies of Synthetic LEW86010 Analogs: Petrogenesis of a Unique Achondrite. In Lunar and Planetary Science XIX, pages 760–761. Houston: The Lunar and Planetary Institute.

McKay, G., D. Lindstrom, S.-R. Yang, and J. Wagstaff

1988.    Petrology of Unique Achondrite Lewis Cliff 86010. In Lunar and Planetary Science XIX, pages 762–763. Houston: The Lunar and Planetary Institute.

McSween, H.Y.

1987.    Aqueous Alteration in Carbonaceous Chondrites: Mass Balance Constraints on Matrix Mineralogy. Geochimica et Cosmochimica Acta, 51:2469–2477.

Mittlefehldt, D.W.

1988.    Petrogenesis of the Mesosiderite Regolith. In Lunar and Planetary Science XIX, pages 788–789. Houston: The Lunar and Planetary Institute.

Mittlefehldt, D.W., and M.M. Lindstrom

1988.    Geochemistry of Diverse Lithologies in Antarctic Eucrites. In Lunar and Planetary Science XIX, pages 790–791. Houston: The Lunar and Planetary Institute.

Ott, U., H.P. Lohr, and F. Begemann

1987.    Noble Gases in ALH84025: Like Brachina, Unlike Chassigny. Meteoritics, 22:476–477.

Prinz, M., M.K. Weisberg, and C.E. Nehru

1988.    LEW86010, a Second Angrite: Relationship to CAI’s and Opaque Matrix. In Lunar and Planetary Science XIX, pages 949–950. Houston: The Lunar and Planetary Institute.

Prinz, M., M.K. Weisberg, C.E. Nehru, and J.S. Delaney

1986.    A Second Brachina-like Meteorite. In Lunar and Planetary Science XVII, pages 679–680. Houston: The Lunar and Planetary Institute.

Rubin, A.E.

1990.    Kamacite and Olivine in Ordinary Chondrites: Intergroup and Intragroup Relationships. Geochimica et Cosmochimica Acta, 54:1217–1232.

Rubin, A.E., and G.W. Kallemeyn

1989.    Carlisle Lakes and Allan Hills 85151 : Members of a New Chondrite Grouplet. Geochimica et Cosmochimica Acta, 53:3035–3044.

1990.    Lewis Cliff 85332: A Unique Carbonaceous Chondrite. Meteoritics, 25:215–225.

Ryder, G., and A.V. Murali

1987.    Mineralogy and Chemistry of Antarctic Aubrites. Meteoritics, 22:495–496.

Schwarz, C., G. Ryder, and A.V. Murali

1986.    Chemistry of Mineral Separates from Three Antarctic Aubrites from Allan Hills. Meteoritics, 21:507.

Scott, E.R.D.

1988.    A New Kind of Primitive Chondrite, Allan Hills 85085. Earth and Planetary Science Letters, 91:1–18.

Steele, I.M., and J.V. Smith

1976.    Mineralogy of the Ibitira Eucrite and Comparison with Other Eucrites and Lunar Samples. Earth and Planetary Science Letters, 33:67–78.

Takeda, H., H. Mori, and H. Ogata

1988.    Mineralogy of Magnesian and Calcic Groups and Formation Conditions of Ureilites. In Lunar and Planetary Science XIX, pages 1173–1174. Houston: The Lunar and Planetary Institute.

Van Schmus, W.R., and J.A. Wood

1967.    A Chemical-Petrologic Classification for the Chondritic Meteorites. Geochimica et Cosmochimica Acta, 31:747–765.

Warren, P.H., and G.W. Kallemeyn

1987.    A Trio of Meteoritic Dunites, and New Data for Shergotty. In Lunar and Planetary Science XVIII, pages 1056–1057. Houston: The Lunar and Planetary Institute.

Wasson, J.T., and G.W. Kallemeyn

1990.    Allan Hills 85085: A Subchondritic Meteorite of Mixed Nebular and Regolithic Heritage. Earth and Planetary Science Letters, 101: 148–161.

Weisberg, M.K., M. Prinz, and C.E. Nehru

1988.    Petrology of ALH85085: A Chondrite with Unique Properties. Earth and Planetary Science Letters, 91:19–32.

Weisberg, M.K., M. Prinz, C.E. Nehru, R.N. Clayton, and T.K. Mayeda

1989.    ALH85151 and Carlisle Lakes 001: Members of a New Chondrite Group. In Lunar and Planetary Science XX, pages 1191–1192. Houston: The Lunar and Planetary Institute.

Wlotzka, F., and E. Jarosewich

1977.    Mineralogical and Chemical Compositions of Silicate Inclusions in the El Taco, Campo del Cielo, Iron Meteorite. In B. Mason, editor, Mineral Science Investigations, 1974–1975. Smithsonian Contributions to the Earth Sciences, 19:104–125.

5. Descriptions of Iron Meteorites

Roy S. Clarke, Jr.

Roy S. Clarke, Jr., Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560.

This section provides brief descriptions of four octahedrites, a group IAB silicate-rich specimen, and three anomalous meteorites. The descriptions are based on material prepared previously for the Antarctic Meteorite Newsletter and on recently published data. The specimens considered are listed with their weights and classifications in Table 5-1.

Octahedrites

ALH84165 (94.7 g).—This is an aerodynamically shaped specimen (4.3 × 3.3 × 1.5 cm) from the Allan Hills (Figure 5-1). Distinct anterior and posterior surfaces are present, both covered with thin coatings of reddish brown secondary iron oxides. The anterior surface is smoother than the posterior and has a uniform radius of curvature over most of its area. The posterior radius of curvature is uniform and larger than that of the anterior surface. The anterior radius does, however, decrease markedly approaching the join of the two surfaces. Anterior surface ablation-melt has accumulated on the posterior side of the join, resulting in an approximately 1 mm flange around the specimen (Figure 5-1, bottom). Delicate streamers of fusion crust remain on the edges of the anterior surface recording the flow of melt away from the stagnation point at the center the surface. The axis of oriented flight was perpendicular to the surface as shown in Figure 5-1, top, at the stagnation point.

A median slice perpendicular to both the plane of the flange and the long axis of the specimen was taken to the left of the small depression in Figure 5-1, bottom, providing an area of approximately 3 cm² for metallographic examination. The anterior edge of the slice is essentially free of fusion crust, but a heat-altered zone of kamacite transformed to α₂ penetrates 1.5 to 2 mm into the body of the specimen. The posterior edge has a continuous fusion crust accumulation 0.2 to 0.3 mm thick, and the kamacite along this edge has also been penetrated to a depth of 1.5 to 2 mm by α₂ structure. At the flange intersections, fusion crust accumulation is as thick as 1.5 mm.

Taenite lamellae, taenite-plessite areas, and cellular plessite areas are present in a well developed Widmanstätten pattern. The few opportunities for band width measurements suggest a value of about 1 mm, and the bulk Ni value is 7.76 wt% (Jarosewich, 1990). Grain boundary schreibersite is present, as is schreibersite associated with taenite. Kamacite is mottled and contains a pronounced ε-structure and occasional remnant Neumann bands. These observations suggest a chemical Group IIIAB classification, as has been established by Wasson et al. (1989).

GRO85201 (1401 g).—This specimen (13 × 8 × 3.5 cm) from Grosvenor Mountains was even more dramatically shaped by aerodynamic forces than ALH84165. It is roughly the shape and size of an extended and slightly curved adult human hand with closed fingers and thumb (Figure 5-2). The convex surface was the anterior surface during stable oriented flight (Figure 5-2, top). A distinct stagnation point may be seen in the center of this surface, from which streamers of melt flowed to a circumferential lip that separates the anterior and posterior surfaces (Figure 5-2, bottom). The anterior surface is dark and has a reddish cast due to terrestrial oxidation. There are distinct patches of black fusion crust and areas of bare, shiny metal

FIGURE 5-1.—ALH84165 is an aerodynamically shaped individual covered with a thin coating of secondary iron oxides. (Top) Looking down on the anterior surface. The plane of the photograph is perpendicular to the orientation of the individual during atmospheric passage. The long axis is 4.3 cm. (Bottom) Specimen rotated 180° around the long axis and photographed from an oblique angle to show the accumulation of fusion crust forming the flange separating anterior and posterior surfaces.

FIGURE 5-2.—GRO85201 is an unusually well preserved oriented individual. (Top) Looking down on the anterior surface. The plane of the photograph is perpendicular to the orientation of the individual during stable atmospheric flight. The stagnation point is in the center of the photograph. It is from this point that delicate streamers of melt flowed radially to the edge of the surface. The long axis is 13 cm. (Bottom) Specimen rotated about 60° around the long axis in (Top), showing the posterior surface and the accumulation of melt at the edge of the anterior surface.

TABLE 5-1.—Antarctic iron meteorites, 1984–1986.

    Sample number | Weight (g) | Classification
ALH84165 | 94.7 | IIIAB, Om
GRO85201 | 1401 | IIIAB, Om
EET84300 | 72.7 | IAB, Off
LEW86220 | 25.0 | Silicate-rich IAB
LEW86540 | 21.0 | IIICD, Off
ALH84233 | 13.6 | Ungr, Anom
LEW85369 | 6.3 | Ungr, Anom
LEW86211 | 163 | Ungr, Sulfide-rich

associated with the metal streamers. The posterior surface is darker, also has a slight reddish cast, and has a uniform matte appearance. The specimen appears to have had a long terrestrial residence, but its delicate ablation-produced markings are remarkably well preserved.

A slice, perpendicular to both the surface illustrated in Figure 5-2, top, and the long axis of the specimen, was removed approximately one-third of the way from the small end, yielding an 11 cm² surface for examination. A regular Widmanstätten pattern with kamacite band widths of 0.8 to 1.5 mm is present. Plessite areas and centers of taenite bands have a martensitic structure. Grain boundary schreibersite and occasional large rhabdites are present. Weathering has penetrated into the interior along grain boundaries. The anterior edge has a rim of terrestrial oxide generally less than 0.1 mm thick and containing a few areas of remnant fusion crust. The posterior edge is uniformly covered with layered and slightly weathered fusion crust 1.0 to 1.5 mm thick. Interior to both edges is an α₂ structure typical of heat-altered zones. This α₂ structure blends into a much coarser martensite structure that is present throughout the kamacite in the section. These observations, combined with a bulk Ni value of 8.59 wt% (Jarosewich, 1990), suggest that the specimen is cosmically heat-altered and a member of group IIIAB as has been established by Wasson (1990).

EET84300 (72.7 g).—This irregularly shaped specimen (3.5 × 3 × 2 cm) from Elephant Moraine is severely weathered and is covered with a reddish brown coating of secondary oxides containing small amounts of adhering soil material. After the internal structure of the meteorite had been examined, it was realized that surface silicates on one part of the surface are meteoritic and not terrestrial silicates.

An off-center section through the specimen provided an area of 4 cm² for metallographic examination. About two-thirds of this area is uninterrupted metal, while the remaining one-third contains silicate clusters and individual silicate crystals dispersed in metal. These silicate associations comprise about 5% of the total area. The metal is polycrystalline, containing three cm-size areas of finest octahedrite structure with band widths around 0.1 mm. These three large areas are separated by two comparatively thin bands of kamacite containing median grain boundary schreibersite. The silicate-rich areas are more complex and contain smaller areas of finest octahedrite structure. The slice is rimmed with an essentially continuous band of secondary oxides about 0.2 mm thick. A few small metal areas near the rim contain α₂ structure, its deepest penetration being less than 0.5 mm.

Dominant features within the metal areas are a Widmanstätten pattern of taenite, cellular plessite, and kamacite lamellae. Schreibersite is found at junctions of kamacite lamellae, at centers of kamacite lamellae, and within plessite areas. A number of graphite inclusions are present, ranging in size from a few hundredths of a mm to 0.5 mm. Tiny inclusions of daubreelite were tentatively identified.

Angular silicates occur as clusters of grains and as individual grains within metal. They range from transparent to translucent, and from water-clear to slightly colored. Some of the coloring is probably due to weathering. A 0.5 mm transparent and slightly green crystal proved on electron microprobe examination to be diopside. Other silicates identified were plagioclase (An₁₂), olivine (Fa₁), and pyroxene (Fs₆). The cluster silicates are coarsely crystalline and associated with polycrystalline troilite, small amounts of metal, and schreibersite and/or cohenite.

This specimen is a silicate- and graphite-rich polycrystalline finest octahedrite with a bulk Ni value of 9.71 wt% (Jarosewich, 1990). It belongs to chemical group IAB (Wasson et al., 1989) and is distinct from IAB meteorite EET83333 on the basis of both structure and chemical composition (Clarke, 1989; Wasson et al., 1989; Jarosewich, 1990).

LEW86220 (25.0 g).—This weathered, elongated pebble (4 × 1 × 1.5 cm), was described as an iron meteorite with silicate inclusions by Mason and Martinez (1988). Their macroscopic description noted that the interior “appears to be coarse-grained silicate minerals, some green, some yellow—all stained by oxidation.” The microscopic description noted that most silicate grains are in the 0.3–1.2 mm range, and that they are olivine and pyroxene with minor plagioclase and limonite veining. “Microprobe analyses give the following compositions: olivine, Fa₇; pyroxene, Wo₂Fs₉; plagioclase, An₁₅; one grain of diopside, Wo₄₃Fs₄, was analyzed.”

Additional information given here is based on low power microscopic examination of specimen fragments resulting from section preparation, and on the microscopic examination of a second polished thin section (LEW86220,2; 0.8 cm²) and a polished section LEW86220,3; 0.9 cm²).

The exteriors of the fragments are covered with a polished terrestrial weathering crust of patchy reddish brown to black material. Positive evidence for melt crust was not found, but some of the black material appeares to represent the surface expression of heat-altered silicates. Two small surface areas in the mm-size range have a jade-green hue, while a third such surface area was at an edge, revealing both exterior and interior surfaces. Its interior expression was that of cleavage surfaces of coarse, transparent green crystals, undoubtedly the diopside mentioned above. Approximately 10 such areas were seen on broken surfaces, one of which was 2.5 mm long. A crude estimate based on two sawn surfaces and the two polished section surfaces indicate that 30%v40% of the surface area consists of irregularly distributed metal.

Polished section and polished thin section examination revealed no melt crust, but there is a well developed heat-altered zone. Several areas along silicate edges have 20–50 μm rims of dendritic magnetite precipitated during the ablative, high temperature vitrification of the silicates, indicating that very little material has been lost from this surface. Sulfide veining and kamacite transformed to α₂ penetrate approximately one mm into the specimen. Weathering veins penetrate into the interior of the specimen, particularly along the edges of metal areas; and areas of metal transformed to oxide are prominent near exterior surfaces.

The metal is predominantly kamacite, with occasional well developed Neumann bands. Several taenite-plessite areas associated with schreibersite are present within kamacite. On the basis of optical examination alone, the association observed is kamacite in contact with a very thin border of “clear taenite 1” (˜1 μm), followed by a region of equal thickness of “cloudy zone,” followed by a comparatively broad area of “clear taenite 2,” and finally transformation into a coarse martensite which occupies most of the area of the structure. This thin border with kamacite is probably tetrataenite, but the optical anisotropy needed for confirmation can not be seen on such a thin rim. Troilite is irregularly distributed, much less abundant than metal, and mainly occurs as single crystals. Several small chromites were also recognized.

The information available suggests that this specimen passed through at least part of the atmosphere as an individual and that it is a silicate-rich area from a group IAB iron meteorite.

LEW86540 (21.0 g).—This aerodynamically shaped, tektite-like specimen is a slightly out-of-round (2.2 × 2.4 cm) disk (1.1 cm thick), with a small segment (2–3 mm wide, 2 mm deep) missing from its edge. Anterior and posterior surfaces intersect at a sharp edge which undulates only slightly from a plane. Maximum thickness is approximately at the center of the disk. The two surfaces are smooth, and they have essentially uniform radii of curvature, the radius of the anterior surface being 2–3 cm and that of the posterior surface 6–8 cm.

Surfaces are predominantly reddish-brown due to weathering, have small black patches or remnant fusion crust, and have several patches of iridescent coloring near the edge. Fine streamers of fusion crust (˜0.1 mm) radiate from the stagnation point at the center of the anterior surface to the edge, with a very small accumulation of fusion crust at the edge. Indications of a very fine Widmanstätten pattern stand in relief in several areas of the posterior surface and can be seen under low magnification. Wasson (1990) has classified the meteorite as a finest octahedrite of chemical group IIICD. He reports bulk values of 18.7 wt% Ni and 0.59 wt% Co.

A slice perpendicular to the plane defined by the edge of the specimen, and slightly to one side of the center of the disk, provided a surface area of 2 cm² for metallographic examination. The edge of this section corresponding to the anterior surface has a 10 to 100 μm thick rim of terrestrial weathering products. Remnant fusion crust is present within it and is partially invaded by terrestrial oxides. On the posterior edge, similar fusion crust/terrestrial oxide associations extend along about two-thirds of its length. A layered fusion crust accumulation 120 μm thick has collected at an intersection of anterior and posterior surfaces. Throughout the entire section, kamacite has been heat-altered to α₂. Widths of kamacite lamellae range from 15 to 40 μm, their lengths being 10 to 100 times their widths. Schreibersites within 800 μm of the anterior surface have been heat-altered and survive in the form of quenched eutectic melts. Within 800 μm of the posterior surface, both eutectic melts and schreibersites melted at their edges are observed. Schreibersites are small (in the 50 to 100 μm range), numerous, and found within kamacite lamellae or at lamellae junctions. A small number of electron microprobe analyses indicate that schreibersite Ni contents range from 46 to 50 wt%. Taenite-bordered martensitic plessite occupies 60% or more of the surface area, and its Ni content is 18 to 19 wt%, in good agreement with Wasson’s (1990) bulk Ni value.

Ungrouped Meteorites

ALH84233 (13.6 g).—This reddish-brown and rounded specimen (2 × 1.5 × 1 cm) has a protrusion on one side giving it a shape suggestive of a teardrop, suggesting atmospheric ablation was an important shape-forming process. Several very small areas of remnant fusion crust were tentatively identified, but fusion crust has mainly been obliterated by weathering. One side of the specimen has been more severely weathered, suggestion it was the underside while exposed on the surface.

A median slice through the long axis of the specimen provided an area of approximately 1.5 cm² for metallographic examination. Much of the edge of the specimen has been penetrated by 0.1 mm of terrestrial weathering, and in a few areas both remnant fusion crust and weathering products were observed. The metal appears to have been single crystal kamacite with an occasional subgrain boundary that has been transformed throughout by atmospheric heating to a coarse martensite or α₂ structure. Its bulk Ni value as determined by electron microprobe is somewhat greater than 6 wt%. No significant troilite or schreibersite was observed in the metal.

At the base of the elongation where it joins the main body of the specimen, there is a 1 mm² area of weathered silicate material. Its external edge has been melted by atmospheric ablation and contains small bodies of troilite and kamacite. On the basis of a small number of microprobe analyses, the silicates seem to be uniform in composition. Olivine (Fa₂₀),

pyroxene (Fs₁₇), and plagioclase (Ab₈₃An₁₂Or₄) are present. Prinz et al. (1991) have recently given more detailed information on the silicates.

The data given here does not place this meteorite into one of the recognized structural of chemical classification catagories. On the basis of trace element data on the metal, Wasson (1990) has found this to be an ungrouped meteorite.

LEW85369 (6.3 g).—This specimen from Lewis Cliff is small (1.5 × 1.5 × 0.8 cm), irregularly shaped, pitted, weathered, and covered with a reddish-brown coating of secondary oxides.

A median section through the specimen provided an area of 0.8 cm² for metallographic examination. The surface is comprised of about 8 roughly equidimensional metal grains in the 1 to 3 mm² size range. Two of these grains have a martensite structure that contains martensite subgrains bordered by thin, irregular bands of kamacite. The large kamacite grains appear to be single crystal kamacite free of inclusions. Individual kamacite grains react differently to nital etchant, some becoming very dark on only brief exposure. Preliminary electron microprobe analysis indicates that the kamacite contains slightly more than one weight percent Si. Terrestrial weathering has penetrated into the center of the sample along kamacite/kamacite grain boundaries and along cleavages. The external surface of the slice is bordered for the most part by about a 0.1 mm rim of terrestrial weathering products. Small areas of remnant fusion crust remain within and under the weathering products. Interior to this and around most of the exterior surface is a heat-altered zone, up to 1.5 mm thick at one point. The two thickest areas of heat-altered zone are at opposite ends of the section along its long axis, suggesting that this may have been an oriented individual during atmospheric passage.

The data given here are insufficient to place this meteorite into one of the recognized classification catagories. On the basis of trace element data, Wasson (1990) has found it to be an ungrouped meteorite with compositional similarities to the Horse Creek iron meteorite and to metal nodules from the Mount Egerton stony iron meteorite.

LEW86211 (163.1 g), 86498 (134.2 g).—These two specimens of 4.5 × 4 × 4 cm and 5 × 3 × 3 cm respectively are distinct from all other meteorites in appearance and physical properties and are obviously paired.

This macroscopic description is based on a 72.6 g piece of LEW86211, 4.1 × 2.9 × 2.8 cm, broken from the larger mass during processing. Two-thirds of its surface area is original exterior and the remainder is interior surface. The exterior is a weathered fusion crust, predominantly brown with suggestions of red and yellow, containing occasional patches of retained black fusion crust. There are areas of iridescent coloring, and while smooth to the touch, the surface has an unusual roughness and angularity of appearance when viewed with low magnification. The areas of interior surface have a hackly appearance and a predominately yellow color that varies from pale yellow to bronze yellow. Under magnification (x20) the yellow material is seen to be present as globules, as fracture surfaces and crystal faces, and as fine filaments. A few small areas appear to be vugs coated with a drusy material. Whether these vug fillings are indigenous or a weathering phenomenon is not clear, but they may well prove to be indigenous. A few small areas of silicate are present as is an occasional chromite.

Metallographic examination was performed on a 6 cm² polished and etched slice removed from the piece described above. A thin heat altered zone is present and can be recognized along edges of both the triolite and metal areas, as well as along edges of the occasional silicate-rich area. The matrix is coarsely crystalline troilite that contains globular metal, with metal and troilite present in roughly equal amounts (40% to 60% troilite depending on the particular subarea selected). This structure suggests an eutectic intergrowth. Troilite crystals are in the mm-size range and are cracked on a one-tenth mm scale. A typical metal globule is 0.3 × 0.6 mm, although particles larger or much smaller are also present. A martensitic pattern develops on the metal with etching, with centers of these regions containing 8–9 wt% Ni and their edges as high as 20 wt% Ni. Schreibersite was not identified, but preliminary electron microprobe analyses indicated an unusually high phosphorus content within the martensite (0.5 to 0.6 wt%). This is suggestive of an unusually high cooling rate.

The occasional silicate-rich areas are fine-grained, some of these areas are vesicular, and weathering veins have penetrated into some of them. Prinz et al. (1991) have described these associations as highly reduced and consisting of olivine, orthopyroxene, and clinopyroxene with no feldspathic components. Oxygen isotope data establishes it as distinct from other meteorites, and the authors suggest that it is most similar to the low-Ni extreme of the HE meteorites. Wasson (1990) has classified LEW86211 as an ungrouped meteorite compositionally similar to, but in significant ways different from, low-Ni HE meteorites.

Discussion

This group of eight meteorites illustrates the unusual distribution of Antarctic iron meteorite types as compared to types recovered elsewhere. An unusually high proportion of ungrouped or anomalous meteorites is observed (Clarke, 1986; Wasson, 1990). Group IIIAB irons are the most abundant among non-Antarctic meteorites, but ALH84165 and GRO85201 are the only two IIIABs so far recovered by ANSMET expeditions. Both the great distance separating their recovery sites and observed chemical differences (Wasson et al., 1989; Wasson, 1990) suggest that they represent separate falls.

Literature Cited

Clarke, R.S., Jr.

1986.    Antarctic Iron Meteorites: An Unexpectedly High Proportion of Falls of Unusual Interest. [Abstract] In J.O. Annexstad, L. Schultz, and H. Wänke, editors, Workshop on Antarctic Meteorites. LPI Technical Report, 86–01, pages 28–29. Houston: The Lunar and Planetary Institute.

1989.    Descriptions of Some Antarctic Iron Meteorites. In U.B. Marvin and G.J. MacPherson, editors, Field and Laboratory Investigations of Meteorites from Victoria Land and the Thiel Mountain Range, Antarctica, 1982–1983 and 1983–1984. Smithsonian Contributions to the Earth Sciences, 28:61–63.

Jarosewich, E.

1990.    Chemical Analyses of Meteorites: A Compilation of Stony and Iron Meteorites. Meteoritics, 25:323–337.

Mason, B., and R. Martinez

1988.    LEW86220. Antarctic Meteorite Newsletter, 11(1):20.

Prinz, M., N. Chatterjee, M.K. Weisberg, R.N. Clayton, and T.K. Mayeda

1991.    Silicate Inclusions in Antarctic Irons. [Abstract.] In Lunar and Planetary Science XXII, pages 1101–1102. Houston: The Lunar and Planetary Science Institute.

Wasson, J.T.

1990.    Ungrouped Iron Meteorites in Antarctica: Origin of Anomalously High Abundances. Science, 249:900–902.

Wasson, J.T., X. Ouyang, J. Wang, and E. Jerde

1989.    Chemical Classification of Iron Meteorites: XI. Multi-Element Studies of 38 New Irons and the High Abundance of Ungrouped Irons from Antarctica. Geochimica et Cosmochimica Acta, 53: 735–744.

6. Terrestrial Ages of Victoria Land Meteorites Derived from Cosmic-Ray-Produced Radionuclides

John Evans, John Wacker, and James Reeves

John C. Evans, John F. Wacker, and James H. Reeves, Battelle, Pacific Northwest Laboratories, P.O. Box 999, Richland, Washington 99352.

Since 1979, Battelle, Pacific Northwest Laboratories (Battelle-Northwest) has continued to assay Antarctic meteorites for the cosmic-ray-produced radionuclide, ²⁶Al(t1/2 ˜ 720,000 years). This nuclide is assayed rapidly and nondestructively by multiparameter gamma-ray spectroscopy. These measurements have been intended to provide a basis for estimation of the terrestrial residence time of meteorites that have been collected on the Antarctic ice sheet. While the ²⁶Al method alone cannot provide a completely reliable terrestrial age for individual samples, ²⁶Al data can be used to evaluate trends for large groups of samples and provide useful guidance for selecting samples for more detailed study by other methods. Other investigators have performed analyses for several other cosmic-ray-produced radionuclides on some of these same samples.

In addition to ²⁶Al, the isotopes studied and tabulated in this article include ⁵³Mn (t1/2 = 3.7 million years), ¹⁰Be (t1/2 = 1.5 million years), ³⁶Cl (t1/2 = 300,000 years), and ¹⁴C (t1/2 = 5730 years). Very sophisticated and labor intensive techniques that involve complicated physical and chemical separations are used for those analyses. Manganese-53 is measured by neutron activation while ¹⁴C, ¹⁰Be, and ³⁶Cl are measured by accelerator mass spectrometry.

Previous issues of Smithsonian Contributions to the Earth Sciences have reviewed the status of cosmogenic radionuclide research on Antarctic meteorites (Evans and Rancitelli, 1980; Evans, Reeves, and Rancitelli, 1982; Nishiizumi, 1984). This article updates that information but expands the data base considerably with the addition of previously unreported information on 214 meteorites. The Battelle-Northwest program has measured ²⁶Al in 422 Victoria Land meteorites (Evans, Rancitelli, and Reeves, 1979; Evans and Reeves, 1987), and these data are tabulated in this article, together with published data on four other radionuclides. The tabulation includes 22 ¹⁰Be, 79 ⁵³Mn, 54 ³⁶Cl, and 15 ¹⁴C measurements. Only information on samples studied in the Battelle-Northwest program is considered here; the tabulation is thus not completely comprehensive. Multiple measurements on the same meteorite have also been excluded in the interest of simplicity. Our information is representative, although the total data base on cosmogenic radionuclides available in the published literature is somewhat larger than that presented. A more complete review of all available data for Antarctic as well as non-Antarctic meteorite finds and falls has recently been compiled and published by Nishiizumi (1987).

Several factors influence the interpretation of cosmogenic radionuclide data when calculating a terrestrial age. Ideally we may assume that cosmic ray interactions in the meteorite preatmospheric main mass produce long-lived radionuclides at a known and constant rate. If the cosmic ray exposure time is long, compared to the half-life of the isotope of interest, the meteorite will fall to earth with a constant saturated activity that will then decay exponentially. The amount of decay provides a measure of the residence time of the meteorite on earth. Saturation activities for ²⁶Al were calculated by Evans and Reeves (1987) to be 59 and 55 dpm/kg for L and H chondrites, respectively. The expected saturation activity of ureilites is given by Wilkening et al. (1973) as 58 ± 4 dpm/kg. The calculations were based on the production rate data of Fuse and Anders (1969). Similarly, the corresponding saturation activities for ⁵³Mn, ¹⁰Be, ³⁶Cl, and ¹⁴C are 414, 20, 22.8, and 60 dpm/kg respectively, according to the evaluation of Reedy, Arnold, and Lal (1983).

Several factors may modify the activity of cosmogenic radionuclides found at the time of fall. For example, shielding may reduce the production rate for either abnormally large or unusually small preatmospheric bodies. That effect is more significant for low-energy products such as ²⁶Al, ⁵³Mn, or ¹⁴C and less important for high-energy products such as ³⁶Cl. Another factor affecting production is that meteorites may occasionally experience unusual irradiation histories (e.g., solar cosmic ray effects that produce elevated activity). Low-energy products such as ²⁶Al and ⁵³Mn can be affected by a solar component particularly in very small objects. Finally, and most importantly, the cosmic ray exposure period in space following breakup from the parent body may not be long enough to reach radioactive saturation for the isotope of interest. That effect is quite significant for ⁵³Mn or ¹⁰Be and is of occasional significance for ²⁶Al. The half-life of ³⁶Cl is sufficiently short that the isotope is almost always found at the saturation value. Chlorine-36 is thus clearly the isotope of choice for the time scale of interest in Antarctica.

In this work, we consider terrestrial ages derived from ³⁶Cl to be the most reliable. Other cosmic ray produced radioisotopes with appropriate half-lives for terrestrial age determination include ⁸¹Kr (t1/2 = 210,000 years), ⁵⁹Ni (t1/2 = 76,000 years), and ⁴¹Ca (t1/2 = 100,000). A few ⁸¹Kr based ages have been included in this work although the data were not included in the table in the interest of conserving space. For more details see Freundel et al. (1986). ⁸¹Kr data are relatively scarce at this time as the nuclide is generally present in low abundances in meteorites and is difficult to measure. Interpretation of ⁸¹Kr data is complicated since the production rate of the isotope is sensitive to trace element chemistry. ⁵⁹Ni and ⁴¹Ca data are also rather scarce. Interpretation of data for those isotopes is complicated by neutron capture effects which are typically very size dependent. Terrestrial ages can also be derived with some degree of assurance by combining ²⁶Al and ⁵³Mn data. The longer lived ⁵³Mn is used to estimate exposure age, and the shorter lived ²⁶Al is used to estimate terrestrial age. A convergent solution to a common pair of terrestrial and exposure ages can be obtained by numerical solution to the pair of simultaneous radioactive saturation and decay equations after only a few iterations.

Table 6-1 summarizes a large body of cosmogenic radionuclide data measured on Antarctic meteorites allocated from the U.S. collection. With only five exceptions, all terrestrial ages given in the last column of Table 6-1 were calculated or recalculated specifically for this tabulation. The terrestrial age estimate for the lunar meteorite, ALHA81005, was taken from the work of Nishiizumi, Reedy, and Arnold (1988). Terrestrial age estimates for ALHA77005 (shergottite), ALHA78132 (eucrite), ALHA79017 (eucrite), and EETA79005 (eucrite) were taken from the age calculations of Freundel et al. (1986) based on their ⁸¹Kr measurements. ³⁶Cl data were used whenever available to produce firm estimates of terrestrial age; those data are shown as numbers with a one standard deviation error. The ³⁶Cl method is capable of measuring terrestrial ages greater than approximately 70,000 to 90,000 years. In a few cases, ¹⁴C data were used for an age calculation, although most meteorites have been found to be older than the greatest age measurable with ¹⁴C (30,000 to 50,000 years). Ages shown in the table as approximate are based on a combination of ²⁶Al and ⁵³Mn data. In most cases where comparison is possible, the two methods agree reasonably well; however, the ³⁶Cl method is clearly more accurate and capable of resolving shorter ages. The remaining terrestrial age estimates in Table 6-1 are based only on ²⁶Al and should be used with great caution. Those ages are intended as conservative limits based on worst-case arithmetic propagation of 1 sigma errors in both the measured and the expected value. That procedure will, in most cases, significantly overestimate the terrestrial age. A minimum upper limit of 200,000 years was assumed for terrestrial ages based on ²⁶Al alone. That number is somewhat arbitrarily based on the authors’ best judgement as to the minimum amount of decay resolvable by ²⁶Al data alone.

The samples in Table 6-1 have a wide range of terrestrial ages, with a few specimens having sufficiently short terrestrial ages that they still contain detectable ¹⁴C, while a number of samples measured for ³⁶Cl have ages of nearly 1 million years; the oldest sample measured to date is ALHA78153 (970,000 ± 100,000 years). The 970,000 year terrestrial age for ALHA78153 shown in Table 6-1 is derived from ³⁶Cl data; however, a nearly identical age estimate can be obtained from the ²⁶Al data as well. A number of other samples have been found recently that have similarly low ²⁶Al contents and may have terrestrial ages up to 1 million years. Currently available evidence continues to suggest that approximately 1 million years is the approximate age cutoff for the residence time in the Allan Hills region. There are not enough measurements available now to draw firm conclusions about the other regions sampled; however, Nishiizumi (1984) has suggested that the Allan Hills meteorites have a wider range of terrestrial ages and are generally older than other Victoria Land and Yamato meteorites. The distribution of ages and the age cutoff represent important constraints for models of ice movement, meteorite accumulation, and destruction of meteorites by weathering in the Victoria Land region.

Figure 6-1 shows the distribution of ²⁶Al contents for the Antarctic meteorites collected in the Victoria Land region compared with the equivalent treatment for a selection of falls and finds collected worldwide outside Antarctica. For this purpose we have assumed that the terrestrial ages of chondrites outside the Antarctic are, in general, short compared with the half-life of ²⁶Al (i.e., less than 100,000 years). The worldwide data include 135 samples taken from a variety of sources (Evans, Reeves, Rancitelli, and Bogard, 1982; Cressy, 1976; Cameron and Top, 1974; Heyman and Anders, 1967). Approximately one-third of the data originated in our laboratory. The frequency distribution of worldwide data (solid bars) has been renormalized on the figure to give the same integrated

FIGURE 6-1.—Distribution of ²⁶Al contents of ordinary chondrite finds from the Victoria Land region of Antarctica (crosshatched bars) as compared with the distribution worldwide for both falls and finds (solid bars). The frequency distribution of worldwide data has been adjusted to have the same integrated area as for Antarctic specimens. All data are normalized to an L chondrite composition using the saturation values computed from the production rates of Fuse and Anders (1969), and nominal chondritic compositions. No correction was made for exposure age. Worldwide data were taken from a variety of sources (Evans, Reeves, Rancitelli, and Bogard, 1982; Cressy, 1976; Cameron and Top, 1974; Heyman and Anders, 1967).

area as the Antarctic data (crosshatched bars). Only ordinary chondrites (H, L, and LL) are included. The data for H chondrites were multiplied by a factor of 1.07 to correct for the expected differences in production rates caused by compositional differences (Fuse and Anders, 1969). The Antarctic data show a definite shift to lower values. The shift is evident in the excess of samples with ²⁶Al contents between 35 and 50 dpm/kg as well as a corresponding depletion above 65 dpm/kg. Interpretation of the excess of samples with low values is somewhat complicated by the inclusion of a number of paired specimens which may be associated with showers; however, the low values are associated with a wide range of meteorite types and cannot be attributed strictly to shower effects. It thus appears from a strictly statistical basis that the average age of the collection is a significant fraction of the half-life of ²⁶Al. This observation is consistent with other observations based on more direct methods of terrestrial age determination (i.e., ³⁶Cl or ⁸¹Kr dating) as discussed above.

Funding for this work was provided by the National Science Foundation, Division of Polar Programs under grant DPP-8745437 and the National Aeronautics and Space Administration under contract NASA No. 86-229. The authors are indebted to Dr. Kunihiko Nishiizumi for his assistance in providing data and references for the assembly of Table 6-1.

TABLE 6-1.—Activity and terrestrial age estimates for Victoria Land meteorites¹.

Literature Cited

Andrews, H.R., R.M. Brown, G.C. Ball, N. Burn, Y. Imahori, J.C.D. Milton, W.J. Workman, and E.L. Fireman

1982.    ¹⁴C Content of Antarctic Meteorites Measured with the Chalk River MP Tandem Accelerator. [Abstract.] In Abstracts, Fifth International Conference on Geochronology, Cosmochronology, and Isotope Geology, Nikko, Japan, pages 8–9.

Cameron, I.R., and Z. Top

1974.    Measurement of ²⁶Al in Stone Meteorites and Its Use in the Determination of Orbital Elements. Geochimica et Cosmochimica Acta, 38: 899–909.

Cressy, P.J.

1976.    Cosmogenic Radionuclides in Stone Meteorites. NASA TN D-8241.

Evans, J.C., and L.A. Rancitelli

1980.    Terrestrial Ages. In U.B. Marvin and B. Mason, editors, Catalog of Antarctic Meteorites, 1977–1978. Smithsonian Contributions to the Earth Sciences, 23:45–46.

Evans, J.C., L.A. Rancitelli, and J.H. Reeves

1979.    ²⁶Al Content of Antarctic Meteorites: Implications for Terrestrial Ages and Bombardment History. In Proceedings of the Tenth Lunar and Planetary Science Conference, pages 1061–1072. New York: Pergamon Press.

Evans, J.C., and J.H. Reeves

1987.    ²⁶Al Survey of Antarctic Meteorites. Earth and Planetary Science Utters, 82:223–230.

Evans, J.C., J.H. Reeves, and L.A. Rancitelli

1982.    Aluminum-26: Survey of Victoria Land Meteorites. In U.B. Marvin and B. Mason, editors, Catalog of Meteorites from Victoria Land, Antarctica, 1978–1980. Smithsonian Contributions to the Earth Sciences, 24:70–74.

Evans, J.C., J.H. Reeves, L.A. Rancitelli, and D.D. Bogard

1982.    Cosmogenic Nuclides in Recently Fallen Meteorites: Evidence for Galactic Cosmic-Ray Variations During the Period 1967–1978. Journal of Geophysical Research, 87:5577–5591.

Fireman, E.L.

1979.    ¹⁴C and ³⁹Ar Abundances in Allan Hills Meteorites. In Proceedings of the Tenth Lunar and Planetary Science Conference, pages 1053–1060. New York: Pergamon Press.

1980.    Carbon-14 and Argon-39 in ALHA Meteorites. In Proceedings of the Eleventh Lunar and Planetary Science Conference, pages 1215–1221. New York: Pergamon Press.

1983.    Carbon-14 Ages of Antarctic Meteorites. In Lunar and Planetary Science XIV, pages 195–196. Houston: The Lunar and Planetary Institute.

Fireman, E.L., and T. Norris

1981.    Carbon-14 Ages of Allan Hills Meteorites and Ice. In Proceedings of the Twelfth Lunar and Planetary Science Conference, pages 1019–1025. New York: Pergamon Press.

Freundel, M., L. Schultz, and R.C. Reedy

1986.    Terrestrial ⁸¹Kr-Kr Ages of Antarctic Meteorites. Geochimica et Cosmochimica Acta, 50:2663–2673.

Fuse, K., and E. Anders

1969.    Aluminum-26 in Meteorites VI, Achondrites. Geochimica et Cosmochimica Acta, 33:1783–1980.

Goswami, J.N., and K. Nishiizumi

1983.    Cosmogenic Records in Antarctic Meteorites. Earth and Planetary Science Letters, 64:1–8.

Heyman, D., and E. Anders

1967.    Meteorites With Short Cosmic-ray Exposure Ages, As Determined From Their Al²⁶ Content. Geochimica et Cosmochimica Acta, 31:1973–1981.

Imamura, M., K. Nishiizumi, and M. Honda

1979.    Cosmogenic ⁵³Mn in Antarctic Meteorites and their Exposure History. Memoirs of the National Institute of Polar Research (Japan), special issue, 15:227–242.

Moniot, R.K., T.H. Kruse, W. Savin, C. Tuniz, T. Milazzo, G.S. Hall, D. Pal, and G.F. Herzog

1982.    Beryllium-10 Contents of Stony Meteorites and Neon-21 Production Rate. In Lunar and Planetary Science XIII, pages 536–537. Houston: The Lunar and Planetary Institute.

Nishiizumi, K.

1984.    Cosmic-Ray Produced Nuclides in Victoria Land Meteorites. In U.B. Marvin and B. Mason, editors, Field and Labratory Investigations of Meteorites from Victoria Land, Antarctica. Smithsonian Contributions to the Earth Sciences, 26:105–109.

1987.    ⁵³Mn, ²⁶Al,¹⁰Be, and ³⁶Cl in Meteorites: Data Compilation. Nuclear Tracks Radiation Measurement 13:209–273.

Nishiizumi, K., J.R. Arnold, D. Elmore, R.D. Ferraro, H.E., Gove, R.C. Finkel, R.P. Beukens, K.H. Chung, and L.R. Killius

1979.    Measurements of ³⁶Cl in Antarctic Meteorites and Antarctic Ice using a Van de Graaff Accelerator. Earth and Planetary Science Utters, 45:285–292.

Nishiizumi, K., J.R. Arnold, D. Elmore, X. Ma, D. Newman, and H.E. Gove

1983.    ³⁶Cl and ⁵³Mn in Antarctic Meteorites and ¹⁰Be-³⁶Cl Dating of Antarctic Ice. Earth and Planetary Science Utters, 62:407–417.

Nishiizumi, K., J.R. Arnold, M. Imamura, T. Inoue, and M. Honda

1982.    Cosmogenic Radionuclides in Antarctic Meteorites. In Seventh Symposium on Antarctic Meteorites, pages 52–54. Tokyo, Japan: National Institute of Polar Research.

Nishiizumi, K., J.R. Arnold, J. Klein, and R. Middleton

1982.    ¹⁰Be and Other Radionuclides in Antarctic Meteorites and Associated Ice. Meteoritics, 17: 260–261.

Nishiizumi, K., D. Elmore, P.W. Kubik, G. Bonani, M. Suter, W. Wolfli, and J.R. Arnold

1986.    Age of Antarctic Meteorites and Ice II. [Abstract.] In Lunar and Planetary Science XVII, pages 724–725. Houston: The Lunar and Planetary Institute.

Nishiizumi, K., J. Klein, R. Middleton, D. Elmore, P.W. Kubik, and J.R. Arnold

1986.    Exposure History of Shergottites. Geochimica et Cosmochimica Acta, 50:1017–1021.

Nishiizumi, K., M.T. Murell, J.R. Arnold, D. Elmore, R.D. Ferraro, H.E. Gove, and R.C. Finkel

1981.    Cosmic-Ray Produced ³⁶Cl and ⁵³Mn in Allan Hills-77 Meteorites. Earth and Planetary Science Utters, 52:31–38.

Nishiizumi, K., R.C. Reedy, and J.R. Arnold

1988.    Exposure History of Four Lunar Meteorites. [Abstract.] Meteoritics 23:294–295.

Reedy, R.C, J.R. Arnold, and D. Lal

1983.    Cosmic Ray Record in Solar System Matter. Science, 219:127–135.

Tuniz, C., D.K. Pal, R.K. Moniot, W. Savin, T.H. Kruse, G.F. Herzog, and J.C. Evans

1983.    Recent Cosmic Ray Exposure History of ALHA 81005. Geophysical Research Utters, 10:804–806.

Vogt, S., U. Herpers, R. Sarafin, P. Signer, R. Wieler, M. Suter, and W. Wolfli

1986.    Cosmic Ray Records in Antarctic Meteorites. [Abstract.] In Proceedings of the International Workshop on Antarctic Meteorites, pages 55–57. Houston: The Lunar and Planetary Institute.

Wilkening, L.L., G.F. Herman, and E. Anders

1973.    Al-26 in Meteorites VII: Ureilites, Their Unique Radiation History. Geochimica et Cosmochimica Acta, 37:1803–1810.

7. Natural Thermoluminescence Levels and the Recovery Location of Antarctic Meteorites

Fouad A. Hasan, Roberta Score, Benjamin M. Myers,
Hazel Sears, William A. Cassidy, and Derek W.G. Sears

Fouad A. Hasan, Benjamin M. Myers, Hazel Sears, and Derek W.G. Sears, Cosmochemistry Group, Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701. Roberta Score, Planetary Materials Laboratory, Lockheed, NASA Johnson Space Center, Houston, Texas 77058. William A. Cassidy, Department of Geology and Planetary Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260.

In this article, the levels of natural thermoluminescence in 379 Antarctic meteorites are reported; 86 samples were from the 1985 collection at the Allan Hills site, 88 were collected in the Lewis Cliff region during the 1985–1986 season and 165 were collected at the Lewis Cliff sites during the 1986–1987 field season. An additional four samples came from the Allan Hills site in 1986, and 36 came from six other ice fields. The distributions of natural thermoluminescence data for the 1985 Allan Hills and 1986 Lewis Cliff collections are similar, with peaks in the histograms at 32–63 krad with values ranging between <1 krad and 327 ± 3 krad. On the basis of a study of 23 meteorites of known ²⁶Al content, the natural TL data for these two collections seem consistent with the majority of the meteorites having terrestrial ages of 150,000 ± 100,000 years, while a smaller fraction fall in the 400,000 ± 200,000 year range. On the other hand, samples from the 1985 Lewis Cliff collections show a greater proportion of samples with natural thermoluminescence in the 5–20 krad range, consistent with the majority having ages in the order of 400,000 ± 200,000 years. The difference within the samples collected at Lewis Cliff is related to collection site, the meteorites collected at Meteorite Moraine having a peak at 50–63 krad, with relatively little spread in natural TL, while the meteorites collected on the Lewis Cliff Ice Tongue include a larger proportion with natural TL in the 5–20 krad range. There seems to be a trend in natural TL with location on the Lewis Cliff Ice Tongue, with natural TL levels tending to be higher in the northern part of the Tongue compared with the southern part, but it is not clear if this is an effect of concentration mechanism or a few major pairings. 14% of the 379 samples have low natural TL (<5 krad), which can be attributed to recent heating (close solar passage, shock, or atmospheric heating). Three meteorites have extremely high natural TL strongly suggestive of high radiation doses or low temperatures in space (these are ALH85033, LEW85448, and LEW86286), while a further 18 have natural TL values in excess of 100 krad and possibly experienced high extraterrestrial radiation doses or low temperatures.

Introduction

The mineralogical and petrographic surveys made on Antarctic meteorites as part of their initial description identify meteorites which are members of rare or previously unknown classes. However, they do not generally identify members of well-populated classes which have experienced unusual histories, such as particularly long or short terrestrial ages, unusual orbits, or other events resulting in anomalous thermal or radiation histories. To some degree, ²⁶Al measurements provide such information. Natural thermoluminescence (TL) measurements provide data which is complementary to the information available using cosmogenic isotopes and other techniques, and can help in the identification of meteorites with interesting and unusual histories. They can also provide an indication of relative terrestrial ages, and TL measurements can be useful in identifying fragments of a single meteorite (i.e., pairing). The natural thermoluminescence of meteorites was recently reviewed by Sears and Hasan (1986) and Sears (1988).

Thermoluminescence is a measure of the number of excited state electrons in a suitable crystalline lattice. In the chondritic meteorites the lattice is usually feldspar, but it can, on occasion, be various calcic minerals, forsterite, enstatite, quartz, or other trace constituents. The means of excitation is the passage of ionizing radiations, and natural thermoluminescence measurements have successfully been applied to radiation dosimetry and pottery dating for many years; in both cases the TL level is used to determine the absorbed dose. In meteorites, because of the larger dose rates and longer time scales involved, the number of excited electrons is determined by an equilibrium between the rate of excitation and de-excitation, the de-excitation being a thermally controlled process. Simple theoretical treatments yield the following relationship between natural TL level (k), dose rate (R) and temperature (T, in °K):

where the other parameters are factors describing the TL process: s, the Arrhenius factor; k is Boltzmann’s constant; E is the difference in energy of the excited state electrons and the conduction band; a is the rate constant for excitation of electrons; km is the maximum TL that can be induced (e.g., Sears, 1988). Any event in the history of a meteorite which involves a change in radiation dose and temperature will affect natural TL levels to some extent.

The calculated black-body temperature for a meteorite at 1 a.u. is 240 K (assuming plausible values for size and albedo) and dose rates are in the order of 1 rad/year. On earth, temperatures and dose rates are probably around 293 and 1 mrad/year, respectively. A relationship between natural TL and terrestrial age is therefore to be expected as the equilibrium TL level falls from its extraterrestrial value to its terrestrial value (Sears and Mills, 1974; Melcher, 1981a; McKeever, 1982). Thus, taking s = 2 × 10¹³/sec, E = 1.43 eV (McKeever, 1982), this simple treatment suggests that at temperatures observed in the central U.S. (say 15° C) natural TL levels decay by a factor of 20 in about 5 × 10⁴ years, while at Antarctic temperatures (say −5° C) the TL fades by a factor of about 20 in 3 × 10⁶ years. This relationship between natural TL and terrestrial age is borne out, to some degree quantitatively, by studies in which natural TL levels are compared with terrestrial ages determined by cosmogenic isotopes (Sears and Durrani, 1980; Hasan et al., 1987, see below). It is the case, however, that a small proportion of meteorites have low levels of natural TL which cannot be attributed to large terrestrial ages, and these must have experienced a recent heating event sufficient to lower their natural TL.

At a perihelion of 0.722 a.u. (the smallest perihelion observed for bright fireballs falling into the Prairie Network camera system), the black body temperature is 236° K. The Lost City meteorite had an orbit with a perihelion of 0.967 a.u., at which distance the black body temperature is 275° K. Substitution of these temperature values into the above equation shows that a factor of 100 difference in natural TL level could result from these differences in perihelion (McKeever and Sears, 1980; Melcher, 1981b).

Of course, there are other means by which the meteorite could have been heated, such as shock-heating. The temperatures involved do not have to be high, but it is necessary for the heating to have occurred sufficiently recently that the equilibrium has not had time to re-establish itself. This time period is unclear, but theoretical treatments seem to indicate time periods in the order of 10⁵ years. Another major heating event is passage through the atmosphere, but the effects are limited to the outermost centimeter or less (Vaz, 1971; Sears, 1975; Melcher, 1979). In addition, a factor of two variation in natural TL levels due to gradients in the cosmogenic dose rate throughout meteorites has been reported (Vaz, 1971; Sears, 1975; Lalou et al., 1970).

Many of these factors appear to have been involved in determining the natural thermoluminescence levels in 23 Antarctic meteorites of known ²⁶Al activity discussed by Hasan et al. (1987). In Figure 7-1 the earlier data have been replotted using our current methods of natural TL data reduction (see below) and normalizing the ²⁶Al activity to Si, the major target for ²⁶Al production. (These changes result in a slightly improved correlation coefficient, 0.69 compared to 0.62 in the paper by Hasan et al. (1987). While 17 meteorites lie on or near a regression line, six plot well below the line and have presumably suffered recent heating. Two of the six are shock-blackened L6 chondrites (RKPA79001 and RKPA80202, which may be paired), and the heat associated with the shock event may have lowered their natural TL. Two others, which also are paired, have rather high ²⁶Al activities; this situation is analogous to that for Malakal which is known to have experienced an unusual thermal and radiation history (Cressy and Rancitelli, 1974; Melcher, 1981a,b). These matters were discussed by Hasan et al. (1987). Based on our interpretations of Figure 7-1, we may assume that natural TL values of significantly less than ˜5 krad imply recent heating, while values greater than this seem to be reflecting differences in history analogous to those responsible for the ²⁶Al range, predominantly differences in terrestrial age (and, on occasion, low cosmic ray exposure age). We can get some feel, albeit very approximate, of the terrestrial ages involved from a compilation of terrestrial ages by Nishiizumi (1984); five of the meteorites in the natural TL-²⁶Al “cluster,” between 30–80 krad and 250–350 dpm/kg Si, have a mean terrestrial age of 150,000 ± 100,000 years, while six meteorites in the “cluster” corresponding to 10–30 krad and 150–250 dpm/kg Si have a mean terrestrial age 400,000 ± 200,000 years. These individuals and their terrestrial ages are listed in table 2 of Hasan et al. (1987). ALHA78008 has an unusual radiation history. Its low ²⁶Al activity (and presumably its low natural thermoluminescence relative to other meteorites involved in the regression line) reflects low cosmic ray exposure, rather than simply a particularly large terrestrial age (Nishiizumi et al., 1979).

In the present paper, we report natural TL data for 379 Antarctic meteorite fragments, we present histograms for most of them (separated into various find sites), and we discuss some possible interpretations of these data.

Experimental Procedures

Our data are listed in Table 7-1. Most of the data were gathered by Fouad Hasan, while data for 27 samples were

FIGURE 7-1.—Plot of natural thermoluminescence against ²⁶Al activity for 23 Antarctic meteorites. (The samples are from collections made between 1976 and 1981 at Allan Hills, Meteorite Hills, and Reckling Peak and may be identified from table 2 in Hasan et al., 1987.) The data are from Hasan et al. (1987), and references therein, but the TL data have been converted to doses using the method of Hasan et al. (1989) and the Al-26 activities have been calculated relative to Si assuming 17.2% Si for H chondrites and 18.9% Si for L chondrites. The solid line is a regression line through 17 meteorites and has the equation Log(Natural TL) = 0.00250 ²⁶Al + 0.8610, and correlation coefficient 0.69. The broken lines connect samples for which there is evidence for pairing (Scott, 1984).

TABLE 7-1.—Natural thermoluminescence data for Antarctic meteorites. These data supersede previous TL data (October 1987, April 1988, and February 1989 data sets). The quoted uncertainties are the standard deviations shown by replicate measurement of a single aliquot.

Specimen number | Natural TL (krad at 250° C) | Specimen number | Natural TL (krad at 250° C) | Specimen number | Natural TL (krad at 250° C)
ALH85016 | 40 ± 3 | ALH85112 | 48.0 ± 0.5 | GR085211 | 39 ± 1
ALH85017 | 3.6 ± 0.6 | ALH85114 | 8.92 ± 0.07 | GR085212 | 74 ± 1
ALH85018 | 33.9 ± 0.6 | ALH85115 | 10 ± 1 | GR085213 | 105 ± 1
ALH85020 | 39 ± 2 | ALH85118 | 38.0 ± 0.3 | GR085214 | 74 ± 2
ALH85023 | 55 ± 1 | ALH85119 | -   - | GR085215 | 0.80 ± 0.04
ALH85026 | 36 ± 2 | ALH85120 | 10.1 ± 0.1 | GR085216 | 12.5 ± 0.1
ALH85027 | 67 ± 4 | ALH85122 | 1.9 ± 0.4 | GR085218 | 5.6 ± 0.2
ALH85028 | 18 ± 1 | ALH85123 | 74 ± 2 |  |
ALH85029 | 28.2 ± 0.9 | ALH85124 | 6.9 ± 0.3 | LEW85301 | 0.41 ± 0.03
ALH85030 | 29 ± 1 | ALH85125 | 10.1 ± 0.1 | LEW85303 | 31 ± 2
ALH85031 | 0.9 ± 0.4 | ALH85127 | 1.5 ± 0.2 | LEW85305 | 0.036 ± 0.005
ALH85033 | 258 ± 3 | ALH85128 | 4.3 ± 0.1 | LEW85306 | -   -
ALH85034 | 40 ± 3 | ALH85129 | 36 ± 2 | LEW85313 | 4.4 ± 0.04
ALH85035 | 6.0 ± 0.1 | ALH85131 | 32.2 ± 0.5 | LEW85314 | 35.8 ± 0.5
ALH85037 | 6.2 ± 0.4 | ALH85132 | 13 ± 1 | LEW85315 | 22.5 ± 0.6
ALH85038 | 27.2 ± 0.3 | ALH85133 | 57 ± 4 | LEW85316 | 52 ± 2
ALH85039 | 26.6 ± 10.3 | ALH85135 | 43.6 ± 0.5 | LEW85317 | 35.8 ± 0.2
ALH85040 | 40.5 ± 0.2 | ALH85136 | 20 ± 1 | LEW85318 | 12.2 ± 0.1
ALH85041 | 17.8±10.6 | ALH85137 | 2.0 ± 0.3 | LEW85319 | 7.3 ± 0.1
ALH85042 | 48 ± 1 | ALH85141 | 21.5 ± 0.4 | LEW85321 | 41.8 ± 0.8
ALH85043 | 107 ± 11 | ALH85142 | 26.2 ± 0.2 | LEW85322 | 41 ± 1
ALH85044 | 20.2 ± 0.3 | ALH85143 | 5.7 ± 0.1 | LEW85323 | 6.6 ± 0.1
ALH85045 | 63 ± 2 | ALH85144 | 96 ± 2 | LEW85324 | 32 ± 3
ALH85048 | 15.1 ± 0.4 | ALH85146 | 59 ± 2 | LEW85325 | 20 ± 1
ALH85052 | 2.1 ± 0.5 | ALH85151 | 155 ± 2 | LEW85327 | 0.37 ± 0.05
ALH85054 | 7.7 ± 0.6 | ALH85152 | 88 ± 2 | LEW85329 | 26 ± 1
ALH85056 | 2.5 ± 0.1 | ALH85155 | 104 ± 17 | LEW85330 | 12.4 ± 0.3
ALH85059 | 50 ± 4 | ALH85156 | 8.8 ± 0.2 | LEW85331 | 47 ± 2
ALH85062 | 59 ± 6 | ALH86600 | 25.4 ± 0.5 | LEW85333 | 52 ± 2
ALH85063 | 9.0 ± 0.2 | ALH86601 | 4 ± 0.4 | LEW85334 | 26.2 ± 0.5
ALH85065 | 35.4 ± 0.3 | ALH86602 | 10.0 ± 0.2 | LEW85335 | 6.9 ± 0.1
ALH85066 | 77 ± 2 | ALH86603 | 80 ± 2 | LEW85336 | 82 ± 4
ALH85070 | 104 ± 2 |  |  | LEW85337 | 10.2 ± 0.4
ALH85071 | 5.3 ± 0.1 | BOW85800 | 43 ± 3 | LEW85338 | 25.6 ± 0.6
ALH85073 | 74.2 ± 0.8 |  |  | LEW85340 | 27.1 ± 0.8
ALH85075 | 45.9 ± 0.7 | DOM85501 | 4.0 ± 0.5 | LEW85341 | 14 ± 1
ALH85076 | 16.9 ± 0.4 | LOM85502 | 34 ± 2 | LEW85343 | 40 ± 1
ALH85077 | 8.3 ± 0.2 | DOM85503 | 36.3 ± 0.8 | LEW85345 | 43 ± 3
ALH85079 | 92 ± 1 | LOM85504 | 52 ± 1 | LEW85346 | 50 ± 5
ALH85080 | 57 ± 5 | DOM85505 | 14.6 ± 0.3 | LEW85347 | 52 ± 5
ALH85082 | 14.7 ± 0.4 | LOM85506 | 56 ± 1 | LEW85348 | 19.5± 0.6
ALH85083 | 52 ± 5 | DOM85508 | 20.6 ± 0.7 | LEW85350 | 10.0± 0.2
ALH85084 | 45 ± 2 | DOM85509 | 54.1 ± 0.7 | LEW85351 | 6.4 ± 0.2
ALH85086 | 54 ± 4 | DOM85510 | 63 ± 2 | LEW85352 | 6.6 ± 0.3
ALH85087 | 34 ± 2 |  |  | LEW85353 | 0.5 ± 0.2
ALH85090 | 80 ± 4 | EET86800 | 2.7 ± 0.1 | LEW85354 | 1.9 ± 0.4
ALH85091 | 24 ± 2 | EET86801 | 7.7 ± 0.1 | LEW85356 | 14 ± 1
ALH85094 | 96 ± 2 | EET86802 | 29 ± 1 | LEW85357 | 6.4 ± 0.2
ALH85097 | 93 ± 2 | GEO85700 | 9.9 ± 0.2 | LEW85359 | 24 ± 1
ALH85098 | 5.6 ± 0.2 | GEO85701 | 72 ± 2 | LEW85360 | 0.16 ± 0.01
ALH85100 | 57 ± 2 |  |  | LEW85362 | 8.0 ± 0.3
ALH85102 | 1.7 ± 0.2 | GRO85203 | 110 ± 4 | LEW85368 | 0.32 ± 0.02
ALH85103 | 58 ± 1 | GRO85204 | 43 ± 2 | LEW85371 | 24 ± 3
ALH85104 | 0.56 ± 0.09 | GRO85205 | 25.8 ± 0.3 | LEW85373 | 20 ± 3
ALH85105 | 60 ± 1 | GRO85207 | 77.3 ± 0.4 | LEW85379 | 14.4 ± 0.3
ALH85107 | 19.1 ± 0.6 | GRO85208 | 71.8 ± 0.2 | LEW85380 | 6.9 ± 0.1
ALH85108 | 2 ± 1 | GRO85209 | 18 ± 6 | LEW85381 | 7.8 ± 0.3
ALH85110 | 148 ± 2 | GRO85210 | 23.2 ± 0.3 | LEW85383 | 34 ± 2
LEW85384 | 48 ± 0.5 | LEW86035 | 96.0 ± 0.2 | LEW86199 | 27.6 ± 0.2
LEW85385 | 8.2 ± 0.1 | LEW86037 | 79 ± 1 | LEW86203 | 16 ± 3
LEW85386 | 0.090 ± 0.004 | LEW86039 | 5 ± 2 | LEW86204 | 9.7 ± 0.1
LEW85398 | 14 ± 1 | LEW86040 | 139 ± 5 | LEW86206 | 96 ± 1
LEW85402 | 31.0 ± 0.9 | LEW86041 | 2.34 ± 0.08 | LEW86207 | 8 ± 1
LEW85403 | 18.2 ± 0.4 | LEW86043 | 24.8 ± 0.3 | LEW86211 | 6 ± 1
LEW85404 | 35 ± 1 | LEW86044 | 21.9 ± 0.9 | LEW86213 | 13 ± 2
LEW85405 | 5.9 ± 0.2 | LEW86047 | 32.2 ± 0.8 | LEW86215 | 9.0 ± 0.9
LEW85406 | 22 ± 0.4 | LEW86050 | 136 ± 14 | LEW86225 | 42.5 ± 0.7
LEW85413 | 7.9 ± 0.2 | LEW86053 | 52 ± 5 | LEW86226 | 64 ± 2
LEW85418 | 10.1 ± 0.2 | LEW86055 | 37.1 ± 0.5 | LEW86228 | 55.8 ± 0.9
LEW85420 | 96 ± 4 | LEW86056 | 11 ± 0.6 | LEW86232 | 62 ± 4
LEW85423 | 31.8 ± 0.2 | LEW86057 | 77 ± 6 | LEW86238 | 59 ± 9
LEW85426 | 6.5 ± 0.1 | LEW86060 | 1.4 ± 0.07 | LEW86241 | 20 ± 1
LEW85427 | 77 ± 4 | LEW86070 | 58 ± 1 | LEW86249 | 17 ± 1
LEW85428 | 13.6 ± 0.4 | LEW86072 | 60.5 ± 0.6 | LEW86250 | 45 ± 2
LEW85429 | 54 ± 1 | LEW86073 | 47 ± 4 | LEW86251 | 55.8 ± 0.7
LEW85433 | 93 ± 1 | LEW86074 | 47 ± 1 | LEW86252 | 155 ± 19
LEW85441 | 0.60 ± 0.01 | LEW86076 | 42.7 ± 0.9 | LEW86255 | 60.7 ± 0.6
LEW85443 | 46 ± 1 | LEW86077 | 101 ± 6 | LEW86258 | 4 ± 2
LEW85445 | 25.4 ± 0.5 | 1JEW86078 | 107 ± 4 | LEW86266 | 24.2 ± 0.6
LEW85446 | 48 ± 1 | LEW86079 | 66.7 ± 0.4 | LEW86268 | 16 ± 1
LEW85448 | 206 ± 5 | LEW86081 | 93 ± 1 | LEW86273 | 31 ± 1
LEW85449 | 6.2 ± 0.4 | LEW86083 | 36 ± 2 | LEW86282 | 89 ± 0.2
LEW85450 | 3.2 ± 0.6 | LEW86084 | 29 ± 1 | LEW86286 | 327 ± 3
LEW85451 | 0.88 ± 0.02 | LEW86085 | 19 ± 1 | LEW86295 | 5.7 ± 0.3
LEW85454 | 1.76 ± 0.01 | LEW86086 | 93 ± 2 | LEW86302 | 1.7 ± 0.1
LEW85455 | 3.4 ± 0.3 | LEW86088 | 54.9 ± 0.4 | LEW86305 | 33 ± 1
LEW85456 | 62.5 ± 0.6 | LEW86089 | 4.7 ± 0.3 | LEW86311 | 53 ± 1
LEW85457 | 1.63 ± 0.03 | LEW86090 | 6.9 ± 0.1 | LEW86312 | 19.5 ± 0.1
LEW85458 | 33 ± 0.06 | LEW86091 | 12.7 ± 0.1 | LEW86314 | 74 ± 2
LEW85459 | 1.7 ± 0.1 | LEW86096 | 76 ± 1 | LEW86317 | 68 ± 2
LEW85460 | 11.24 ± 0.5 | LEW86098 | 2.9 ± 0.1 | LEW86327 | 19.6 ± 0.1
LEW85461 | 77 ± 2 | LEW86099 | 9.7 ± 0.4 | LEW86337 | 53 ± 2
LEW85463 | 1.5 ± 0.1 | LEW86101 | 6 ± 1 | LEW86340 | 96 ± 8
LEW85464 | 12.5 ± 0.3 | LEW86104 | 27.2 ± 0.2 | LEW86344 | 19.4 ± 0.7
LEW85465 | 5.3 ± 0.2 | LEW86107 | 40 ± 2 | LEW86349 | 84 ± 2
LEW85472 | 24.2 ± 0.7 | LEW86110 | 53.2 ± 0.5 | LEW86350 | 3 ± 1
LEW86001 | 28 ± 4 | LEW86111 | 59 ± 28 | LEW86352 | 20.4 ± 0.1
LEW86002 | 13 ± 3 | LEW86115 | 52.5 ± 0.7 | LEW86354 | 25.0 ± 0.3
LEW86011 | 157 ± 1 | LEW86118 | 3.6 ± 0.65 | LEW86360 | 57 ± 1
LEW86012 | 50.8 ± 0.9 | LEW86119 | 1.7 ± 0.08 | LEW86364 | 29.7 ± 0.3
LEW86013 | 93 ± 2 | LEW86120 | 22.9 ± 0.9 | LEW86366 | 44 ± 1
LEW86014 | 93 ± 2 | LEW86123 | 21 ± 3 | LEW86367 | 8 ± 2
LEW86015 | 122 ± 6 | LEW86134 | -   - | LEW86368 | 96 ± 4
LEW86016 | 8.2 ± 0.3 | LEW86135 | 88 ± 4 | LEW86371 | 28.4 ± 0.8
LEW86017 | 17 ± 1 | LEW86138 | 196 ± 22 | LEW86376 | 9.9 ± 0.6
LEW86018 | -   - | LEW86152 | 107 ± 2 | LEW86380 | 45 ± 9
LEW86019 | 114.8 ± 0.7 | LEW86160 | 10.2 ± 0.2 | LEW86382 | 3.76 ± 0.07
LEW86020 | 37.0 ± 0.2 | LEW86161 | 69 ± 2 | LEW86385 | 50 ± 1
LEW86021 | 36 ± 2 | LEW86163 | 10 ± 3 | LEW86388 | 67 ± 2
LEW86022 | -   - | LEW86164 | 13.1 ± 0.5 | LEW86393 | 21.7 ± 0.4
LEW86023 | 32 ± 3 | LEW86165 | 50.6 ± 0.7 | LEW86395 | 22.5 ± 0.1
LEW86024 | 21.9 ± 0.1 | LEW86166 | 2.3 ± 0.2 | LEW86396 | 2.6 ± 0.2
LEW86025 | 0.9 ± 0.1 | LEW86168 | 96 ± 2 | LEW86397 | 25.9 ± 0.3
LEW86026 | 64 ± 2 | LEW86174 | 93 ± 4 | LEW86407 | 16.5 ± 0.1
LEW86028 | 38 ± 2 | LEW86181 | 16 ± 1 | LEW86418 | 0.85 ± 0.07
LEW86030 | 75. ± 0.6 | LEW86183 | 22.3 ± 0.6 | LEW86438 | 88 ± 2
LEW86031 | 127 ± 2 | LEW86186 | 17.9 ± 0.3 | LEW86442 | 28.7 ± 0.6
LEW86033 | 62 ± 2 | LEW86195 | 16.0 ± 0.5 | LEW86451 | 52 ± 2
LEW86463 | 30 ± 4 | LEW86490 | 58.5 ± 0.2 | LEW86544 | 18.9±0.3
LEW86465 | 39.6 ± 0.7 | LEW86499 | 13.5 ± 0.5 | LEW86546 | 57 ± 2
LEW86466 | 47 ± 1 | LEW86500 | 38 ± 1 | LEW86549 | 52 ± 4
LEW86470 | 21.3 ± 0.4 | LEW86503 | 28.4 ± 0.3 | QUE86900 | 16 ± 7
LEW86471 | 5 ± 0.6 | LEW86514 | 63.0 ± 0.8 | RKP86700 | 9 ± 1
LEW86472 | 42.7 ± 0.3 | LEW86515 | 54 ± 1 | RKP86701 | 37.8±0.8
LEW86473 | 87.1 ± 0.2 | LEW86522 | 0.9 ± 0.2 | RKP86702 | 12 ± 1
LEW86479 | 80 ± 10 | LEW86525 | 7.3 ± 0.1 | RKP86703 | 10.2 ± 0.4
LEW86485 | 28 ± 0.9 | LEW86528 | 27 ± 0.3 | RKP86704 | 38 ± 2
LEW86489 | 30.0 ± 0.9 | LEW86534 | 14.4 ± 0.4 | RKP86705 | 13.7 ± 0.4

gathered by Roberta Score. Over the eighteen month period during which the data were gathered, which is also the first eighteen months of the project, our techniques and procedures were constantly improved and, therefore, the data vary in quality. The procedures are essentially those described by Hasan et al. (1987), but improvements concerned primarily the criteria for the selection of samples and subsequent storage conditions. Initially, samples were taken randomly. Later, an attempt was made to take samples at least 1 cm from any obvious fusion crust. Most recently, sampling was restricted to those meteorites in which it was possible to take samples at least 1 cm away from all existing surfaces. This restricts the measurements to samples >20g. Samples in the present data base were sent from JSC to the TL laboratory at Arkansas through the United States mail. In the future, samples will be hand-carried to avoid the danger of accidental heating in transit.

We also have developed a means of standardizing our reporting procedure, so that a single datum for each meteorite is reported. The natural TL signal must be normalized to remove the major effects of meteorite-to-meteorite differences in TL sensitivity due to sample heterogeneity, shock, and metamorphism. There are two commonly used means of doing this, and the relative advantages and disadvantages of each were discussed by McKeever and Sears (1980). Here we use a composite method described by Hasan et al. (1989). It is important to note that (1) these new data replace both measures of natural TL in earlier versions of our data (Antarctic Meteoric Newsletter, 11(1), 11(2)); and (2) the uncertainties quoted refer only to the standard deviation shown by replicate measurements of a single powder and not to total accuracy, which would require measurements on several separate chips of meteorite. We quote natural TL values observed at 250° C, since this appears to be the optimum temperature: low enough to be sensitive to the events of interest and yet high enough to avoid the most trivial heating (Melcher, 1981a,b; Hasan et al., 1987, 1989).

Results and Discussion

Histograms of natural TL values for the present samples from the Allan Hills and Lewis Cliff regions are shown in Figure 7-2. The histograms have been labeled to identify individual meteorites in the belief that in this form the data will be most helpful, especially with regard to identifying paired meteorites. Unfortunately, most of the samples are unclassified at the present time, and there is little other information relevant to pairing.

The distribution in natural thermoluminescence for the 1986 samples from Lewis Cliff resembles that for the Allan Hills 1985 samples, but the Lewis Cliff 1985 samples seem to show a distribution that favors lower values (Hasan and Sears, 1988). The distribution of natural TL data for the Allan Hills 1985 meteorites shows a peak in the 32–63 krad intervals, consistent with the cluster in Figure 7-1 showing a mean terrestrial age of 150,000 ± 100,000 years. Seventy-five per cent of the samples fall in the range 5–100 krad, while three show values exceeding 125 krad, and one (ALH85033) is particularly noteworthy at 258 ± 3 krad. The other highs are 148 ± 2 krad (ALH85110) and 155 ± 2 krad (ALH85151). Such values seem high even for observed falls, and may indicate meteorites which have experienced high radiation doses or unusually low storage temperatures in space. The distribution tails more slowly toward lower values. Thirteen samples (i.e., 15% of all the ALH85 meteorites) register below the 5 krad level we would associate with heated samples, affected by small perihelia, recent shock heating, or atmospheric heating. There are some minor peaks in the histogram (e.g., those at 5.0–6.3 krad and 1.6–2.5 krad) which might indicate that some of the individuals in these peaks are paired. Of the five meteorites in the 5.0–6.3 krad range, three (ALH85071, 85098, and 85143) are H5 chondrites and are possibly paired. The other two are an H6 chondrite (ALH85037) and an LL6 chondrite (ALH85035). The group in the 1.6–2.5 krad range are two H5 chondrites (ALH85102, 85122), which may be paired, two H6 chondrites (ALH85052, 85108), which also may be paired, and one LL6 chondrite (ALH85137).

The 1985 collection from the Lewis Cliff site does not seem to show any tendency to an overall “preferred” value. Instead, values are fairly uniformly spread between 7.3 and 63 krad, although there is an indication that the distribution is bimodal with a hiatus in the 16 to 20 krad interval. The 20–63 krad range corresponds to the “cluster” in Figure 7-1 with 150,000 ± 100,000 year terrestrial ages, while the 5–16 krad range corresponds, approximately, to the “cluster” in Figure 7-1 with 400,000 ± 200,000 year terrestrial ages. There is only one Lewis Cliff sample with natural TL greater than 100 krad, namely LEW85448 (206 ± 5 krad), whereas six samples in the Allan Hills 1985 data set had values this high. Eighteen samples, making up 17% of the Lewis Cliff 1985 collection, have natural TL less than 5 krad and have presumably suffered some form of recent heating.

The shape of the histogram for the LEW86 samples resembles that of the ALH85 samples, but with a more pronounced peak at 50–63 krad. Seventy per cent of the samples have values between 13 and 100 krad, which is very similar to the group in Figure 7-1 that has a mean terrestrial age of 150,000 ± 100,000 years. There are also minor peaks at 79–100, 25–32, 7.9–10 krad, which may not be significant. High values are relatively common in the LEW86 data, 12 meteorites (7.2%) having values over 100 krad with LEW86286 at a remarkable 327 ± 3 krad, suggestive of high radiation doses or low temperatures in space. Eighteen meteorites (11% of the LEW86 samples) have values below 5 krad, suggestive of recent heating.

The difference in the distribution of natural TL data between the 1985 and 1986 collections at Lewis Cliff is related to find location. Histograms of the natural TL in samples collected on the Lewis Cliff icefields are shown in Figure 7-3. The meteorites have been separated into those found at Meteorite Moraine and those found on the Ice Tongue (Figure 7-4, 7-5), and the latter have also been subdivided according to latitude (Figure 7-4). The northernmost tip of the Tongue has not yet been systematically searched, so the lack of meteorites in the latitude intervals numbered 8–10 in Figure 7-3 is not significant. Nevertheless, it is clear that the distribution of meteorites over the region is not random, but that meteorites tend to cluster along the western edge of the Tongue. The latitude intervals are not equal, but were chosen to separate meteorites found on the Upper Ice Tongue from those found on the Lower Ice Tongue; these regions are separated by a steep step in the ice surface.

The peak in the histogram for the Lewis Cliff 1986 samples (50–63 krad), which is absent in the distribution for the 1985 Lewis Cliff samples, is due, in large part, to the meteorites from Meteorite Moraine. On the basis of the appearance of hand-specimens, considerable pairing among the Meteorite Moraine samples was suspected, and the spread in TL data is less for this site than the others. However, only 25 of our 67 Meteorite Moraine meteorites have yet been classified (10 are H5, 9 are L6, 4 are H6, 1 is LL6, and 1 is L4), while 10 of the 13 in the 50–63 krad peak are unclassified; the other three are L4, L6, and LL6.

Most of the 13% of the samples with natural TL levels below 5 krad were found on the upper part of the tongue (latitude intervals 1–5). Furthermore, there may be a weak tendency for the histograms to be skewing to higher TL values as one proceeds from the upper part of the Tongue to the lower part (i.e., proceeds north); intervals 3, 4, and 5, for instance, have about half of the samples with TL >5 krad in the range 5–20 krad, while the intervals 6 and 7 have the bulk of their samples in the range 16–100 krad. It is possible that a few major pairings are responsible for this spatial distribution. However, it is also possible that systematic variations in terrestrial age (and therefore natural TL) with location on the ice could result from the mechanisms by which the meteorites were accumulated (e.g., Bull and Lipschutz, 1982; Annexstad, 1986; Drewry, 1986). It will be necessary to make a careful study of potential pairing before seriously examining the possibility of a spatial variation in natural TL levels at this field. It will also help to have data for samples from the remainder of the ice field. The northern tip of the Lewis Cliff Ice Tongue is the prime site for the 1988–1989 field season.

Data for 36 samples collected at other sites, and a further 4 samples collected in the Allan Hills region in 1986, are listed in Table 7-1 but are too few for meaningful discussion of possible trends. Four of the samples have natural TL values less than 5 krad, and two have values in excess of 100 krad (these are listed below). Three of the 5 samples collected at Reckling Peak in the 1986–1987 season have values in the 7.9–16 krad range, while the “preferred values” among samples collected at Grosvenor Mountains and the Dominion Range during the 1985–1986 season, based on these very limited data, are in the 50–125 krad range.

Conclusions

Data for the first eighteen months of systematic measurement of natural thermoluminescence levels in Antarctic meteorites identify several meteorites with unusual histories, allow some crude sorting of meteorites by terrestrial age, and provide data which will be useful in the recognition of paired meteorites. There is evidence in the data that the patterns of natural TL levels vary with find site, which could have implications for concentration mechanisms.

Based on a comparison with Figure 7-1, and the known terrestrial ages for the samples in the plot determined from cosmogenic isotopes, the majority of samples in Allan Hills 1985 and Lewis Cliff 1986 collections have terrestrial ages in range 150,000 ± 100,000 years, with most of the remainder in the order of 400,000 ± 200,000 years. For the Lewis Cliff 1985 collection, the majority of the meteorites have natural TL values suggestive of terrestrial ages in the range 400,000 ± 200,000 years, with a smaller fraction in the 150,000 ± 100,000 year range.

Fourteen percent of the 379 Antarctic meteorites in the present study have natural TL values suggestive of recent heating (small perihelion orbits, shock heating, or heating during atmospheric passage). These are as follows:

ALH85; 017, 031, 052, 056, 102, 104, 107, 112, 115, 119, 127, 128, 137,

FIGURE 7-2.—Histograms of natural TL values in 86 meteorites from the 1985 collection at the Allan Hills, 86 meteorites collected from the Lewis Cliff region in 1985–1986 and 162 meteorites collected in the Lewis Cliff region in 1986–1987. Two samples from the LEW85 collection (LEW85305 and 85306), and three from LEW86 (LEW86022, 86018, and 86134) collections, plot off the graph to lower values.

FIGURE 7-3.—Histograms of natural TL values for meteorites collected in the Lewis Cliff region of the Antarctic in the 1985–1986 and 1986–1987 field seasons. Samples from Meteorite Moraine and from the Lewis Cliff Ice Tongue have been plotted separately, the latter being subdivided into 10 regions according to latitude; the sites are located in Figure 7-4. The five samples plotting off the graphs to the left are distributed over the regions as follows: region 2, LEW85305; region 3, LEW85306; region 6, LEW86134; region 10, LEW86022; Meteorite Moraine, LEW86018.

FIGURE 7-4.—Sketch map showing the locations of the regions referred to in Figure 7-3. North is to the top of the figure, the Tongue is approximately 2.3 km wide. The shading refers to moraine and snow (circles) and regions of the highest meteorite densities (dots). Regions 8–10 were searched in the 1988–1989 field season and data are not yet available.

LEW85; 301, 305, 306, 313, 327, 353, 354, 360, 368, 386, 441, 450, 451, 454, 455, 457, 459, 463,

LEW86; 018, 022, 025, 041, 060, 089, 098, 118, 119, 134, 166, 258, 302, 350, 382, 396, 418, 522,

DOM85501, EET86800, GR085215, ALH86601.

Three meteorites have exceptionally high natural thermoluminescence values which suggest high radiation doses or low storage temperatures in space; these are ALH85033, LEW85448 and LEW86286. Meteorites with natural thermoluminescence values greater than 100 krad, and for which the possibility of exposure to high radiation doses or low temperatures should be borne in mind, are:

ALH85; 043, 070, 110, 151, 155,

LEW86; 011, 015, 019, 031, 040, 050, 077, 078, 138, 152, 252.

ACKNOWLEDGMENTS.—We are grateful to Rene Martinez, Cecilia Satterwhite, and Carol Schwarz for their help in various forms, the MWG for their cooperation, Ralph Harvey for his discussions, Steve McKeever and an anonymous reviewer for helpful reviews, Raye Stucker for help in manuscript preparation, and Ursula Marvin for her gracious editorial assistance. This work is supported by grants from the National Science Foundation and the National Aeronautics and Space Administration (DPP-8613998 and NAG 9-81/natural TL, respectively, to DWGS, and DPP-8314496, to WAC).

Literature Cited

Annexstad, J.O.

1986.    Meteorite Concentration Mechanisms in Antarctica. In J.O. Annexstad, L. Schultz, and H. Wanke, editors, Workshop on Antarctic Meteorites. LPI Technical Report, 86–01, pages 23–25. Houston: The Lunar and Planetary Institute.

Antarctic Meteorite Newsletter, 11(1), February. 1988.

Antarctic Meteorite Newsletter, 11(2), August. 1988.

Bull, C., and M.E. Lipschutz

1982.    Introduction. In C. Bull and M.E. Lipschutz, editors, Workshop on Antarctic Glaciology and Meteorites. LPI Technical Report, 82–03, pages 6–26. Houston: The Lunar and Planetary Institute.

Cressy, P.J., and L.A. Rancitelli

1974.    The Unique Cosmic-ray History of the Malakal Chondrite. Earth and Planetary Science Letters, 22:275–283.

Drewry, D.J.

1986.    Entrainment, Transport, and Concentration of Meteorites in Polar Ice Sheets. In J.O. Annexstad, L. Schultz, and H. Wanke, editors, Workshop on Antarctic Meteorites. LPI Technical Report, 86–01, pages 37–47. Houston: The Lunar and Planetary Institute.

Hasan, F.A., M. Haq, and D.W.G. Sears

1987.    The Natural Thermoluminescence of Meteorites; I: Twenty-three Antarctic Meteorites of Known ²⁶Al Content. Proceedings of the 17th Lunar and Planetary Science Conference, Journal of Geophysical Research, 92:E703–E709.

Hasan, F.A., R. Score, and D.W.G. Sears

1989.    The Natural Thermoluminescence Survey of Antarctic Meteorites; A Discussion of Methods for Reporting Natural TL Data. Lunar and Planetary Science XX, pages 383–384. Houston: The Lunar and Planetary Institute.

Hasan, F.A., and D.W.G. Sears

1988.    Thermoluminescence Evidence for a Terrestrial Age Difference between Allan Hills and Lewis Cliff Meteorites. Lunar and Planetary Science XIX, pages 457–458. Houston: The Lunar and Planetary Institute.

Lalou, C., D. Nordemann, and J. Labyrie

1970.    Etude Preliminaire de la Thermoluminescence de la Meteorite Saint Severin. Compte Rendu Hebdomadaire des Séances de l’Académie des Sciences, Serie B (Sciences Physiques), 270:2401–2404.

McKeever, S.W.S.

1982.    Dating of Meteorite Falls Using Thermoluminescence: Application to Antarctic Meteorites. Earth and Planetary Science Letters, 58:419.

McKeever, S.W.S., and D.W.G. Sears

1980.    Natural Thermoluminescence of Meteorites: A Pointer to Orbits? Modern Geology, 7:137–145.

Melcher, C.L.

1979.    Kirin Meteorite: Temperature Gradient Produced during Atmospheric Passage. Meteoritics, 14:309–316.

1981a.  Thermoluminescence of Meteorites and their Terrestrial Ages. Geochimica et Cosmochimica Acta, 45:615–626.

1981b.  Thermoluminescence of Meteorites and their Orbits. Earth and Planetary Science Letters, 52:39–54.

Nishiizumi, K.

1984.    Cosmic-ray Produced Nuclides in Victoria Land Meteorites. In U.B. Marvin and B. Mason, editors, Field and Laboratory Investigations

FIGURE 7-5.—Aerial photograph of the Lewis Cliff Ice Tongue looking northeast with the tongue in the foreground. Note the step in the tongue, which is highlighted by the snow infilling. Meteorite Moraine is on the center, extreme right of the photograph. (Photograph TMA999-044 of the United States Geological Survey).

of Meteorites from Victoria Land, Antarctica. Smithsonian Contributions to the Earth Sciences, 26:105–109.

Nishiizumi, K., J.R. Arnold, D. Elmore, R.D. Ferraro, H.E. Gove, R.C. Finkel, R.P. Beukens, K.H. Chang, and L.R. Kilius

1979.    Measurements of ³⁶Cl in Antarctic Meteorites and Antarctic Ice Using a Van de Graff Accelerator. Earth and Planetary Science Letters, 45:285–292.

Scott, E.R.D.

1984.    Pairing of Meteorites Found in Victoria Land, Antarctica. In Proceedings of the Ninth Symposium on Antarctic Meteorites. Memoirs of National Institute of Polar Research (Japan), special issue, 35:102–125.

Sears, D.W.G.

1975.    Thermoluminescence Studies and the Pre-atmospheric Shape and Mass of the Estacado Meteorite. Earth and Planetary Science Letters, 26:559–568.

1988.    Thermoluminescence of Meteorites: Shedding Light on the Cosmos. Nuclear Tracks and Radiation Measurement/International Journal of Radiation Applied Instrumentation, part D, 14:5–17.

Sears, D.W.G., and S.A. Durrani

1980.    Thermoluminescence and the Terrestrial Age of Meteorites: Some Recent Results. Earth and Planetary Science Letters, 46:159–166.

Sears, D.W.G., and F.A. Hasan

1986.    Thermoluminescence and Antarctic Meteorites. In J.O. Annexstad, L. Schultz, and H. Wanke, editors, Workshop on Antarctic Meteorites. LPI Technical Report, 86–01, pages 83–100. Houston: The Lunar and Planetary Institute.

Sears, D.W.G., and A.A. Mills

1974.    Thermoluminescence and the Terrestrial Age of Meteorites. Meteoritics, 9:47–67.

Vaz, J.E.

1971.    Asymmetric Distribution of Thermoluminescence in the Ucera Meteorite. Nature Physical Science, 230:23–24.

Appendix

Tables of ANSMET Meteorites

Terminology

Class and type: A = achondrite, unique; An = angrite; Au = aubrite; C = carbonaceous chondrite; Chon. (unique) = unique or ungrouped chondrite; Di = diogenite; E = enstatite chondrite; Eu = eucrite; Ho = howardite; H = high-iron chondrite; I = iron (I, IIA, IIB, IVA = iron groups; silicate incl. = silicate inclusions); L = low-iron chondrite; LL = low-iron low-metal chondrite; M = mesosiderite; Sh = shergottite; Ur = ureilite. Chondrite petrologic type is indicated by digit following the abbreviation.

Olivine composition in mole percent fayalite (Fa), Fe₂SiO₄.

Pyroxene (orthopyroxene or low calcium clinopyroxene) composition in mole percent ferrosilite (Fs), FeSiO₃.

Degree of weathering: A = minor; metal flecks have inconspicuous rust haloes, oxide stain along cracks is minor. B = moderate; metal flecks show large rust haloes, internal cracks show extensive oxide stain. C = severe; specimen is uniformly stained brown, no metal survives, e = evaporite deposits visible to naked eye.

Degree of fracturing: A = slight; specimen has few or no cracks and none penetrate the entire specimen. B = moderate; several cracks extend across the specimen, which can be readily broken along the fractures. C = severe; specimen has many extensive cracks and readily crumbles.

Locations: ALH = Allan Hills; BTN = Bates Nunatak; BOW = Bowden Neve; DRP = Derrick Peak; DOM = Dominion Range; EET = Elephant Moraine; GEO = Geologists Range; GRO = Grosvenor Mountains; ILD = Inland Forts; LEW = Lewis Cliff; MET = Meteorite Hills; MIL = Miller Range; MBR = Mount Baldr; OTT = Outpost Nunatak; PCA = Pecora Escarpment; PGP = Purgatory Peak; QUE = Queen Alexandra Range; RKP = Reckling Peak; TIL = Thiel Mountains; TYR = Taylor Glacier.

Source of classifications: * = S.G. McKinley and K. Keil; t = S.J.B. Reed and S.O. Agrell; $ = C.B. Moore; § = M. Rhodes and S. Haggerty.

˜ = Classified using refractive indices.

Abbreviations: n.d. = no data.

TABLE A.—Meteorites listed by source area in numerical sequence (fractions of grams in weight dropped unless total weight is less than 1 gram).

TABLE B.—Meteorites listed by class and source area in numerical sequence (fractions of grams in weight dropped unless total weight is less than 1 gram).

TABLE C—ANSMET meteorite subtotals through 1986.

Class and type | Number | Class weight (g)
CHONDRITE subtotals: |  |
C2 | 62 | 6391
C3 | 1 | 181
C30 | 6 | 2379
C3V | 7 | 801
C4 | 6 | 592
Chon. (unique) | 3 | 140
E3 | 1 | 39
E4 | 17 | 301
E5 | 1 | 20
E6 | 3 | 823
H3 | 13 | 595
H4 | 132 | 84989
H4,5 | 1 | 16000
H4,6 | 1 | 314
H5 | 932 | 213553
H5,6 | 1 | 31
H6 | 355 | 51543
H? Acapulco-like | 3 | 23
L3 | 133 | 19896
L3,4 | 1 | 31
L4 | 47 | 9497
L5 | 74 | 26108
L6 | 472 | 635221
LL3 | 10 | 4159
LL4 | 3 | 8
LL5 | 11 | 561
LL6 | 79 | 7347
LL7? | 1 | 8
TOTAL | 2376 | 1081551
ACHONDRITE subtotals: |  |
A | 4 | 113
An | 1 | 7
Au | 21 | 3888
Di | 7 | 5542
Di/M | 1 | 2
Eu | 45 | 12965
Ho | 6 | 1252
Lunar | 1 | 31
Sh | 2 | 8425
Ur | 16 | 8045
TOTAL | 104 | 40270
IRON subtotals: |  |
Irons | 37 | 328080
STONY-Iron subtotals: |  |
Mesosiderites | 10 | 12852
TOTALS | 2527 | 1462753