PII S0016-7037(99)00015-0
New lithologies in the Zagami meteorite: Evidence for fractional crystallization of a single
magma unit on Mars
T. J. MCCOY,1,2,3,* M. WADHWA,4,5 and K. KEIL1,6
1Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA
2Code SN4, NASA/Johnson Space Center, Houston, Texas 77058, USA
3Department of Mineral Sciences, MRC 119, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA
4McDonnell Center for the Space Sciences and Department of Earth and Planetary Sciences, Washington University,
St. Louis, Missouri 63130, USA
5Department of Geology, The Field Museum, Roosevelt Rd. at Lake Shore Dr., Chicago, Illinois 60605, USA
6Hawaii Center for Volcanology, University of Hawaii, Honolulu, HI 96822
(Received November 5, 1997; accepted in revised form January 8, 1999)
Abstract?Zagami consists of a series of increasingly evolved magmatic lithologies. The bulk of the rock is
a basaltic lithology dominated by pigeonite (Fs28.7?54.3), augite (Fs19.5?35.0) and maskelynite (Ab42?53).
Approximately 20 vol.% of Zagami is a basaltic lithology containing FeO-enriched pyroxene (pigeonite,
Fs27.0?80.8) and mm- to cm-sized late-stage melt pockets. The melt pockets are highly enriched in olivine-
bearing intergrowths, mesostases, phosphates (both whitlockite and water-bearing apatite), Fe,Ti-oxides and
sulfides. The systematic increases in abundances of late-stage phases, Fs and incompatible element (e.g., Y
and the REEs) contents of pigeonite, Ab contents of maskelynite, and FeO concentrations of whitlockite all
point to a fractional crystallization sequence.
The crystallization order in Zagami and the formation of these various lithologies was controlled by the
abundances of iron, phosphorus, and calcium. During fractional crystallization, iron and phosphorus enrich-
ment occurred, ultimately forcing the crystallization of calcium phosphates and olivine-bearing intergrowths.
The limited amount of calcium in the melt and its partitioning between phosphates and silicates controlled the
crystallization of phosphates, plagioclase, pigeonite, and augite. The presence of these FeO-enriched, water-
poor late-stage lithologies has important implications. Discrepancies between experimental and petrologic
studies to infer the history of basaltic shergottites may be partially explained by the use of starting
compositions which are too FeO-poor in the experimental studies. The water-poor nature of the late-stage melt
pockets suggests crystallization from a very dry magma, although whether this magma was always dry or
experienced significant near-surface degassing remains an open question. Finally, the presence of fractional
crystallization products within Zagami suggests that this may be a relatively common process on
Mars. Copyright ? 1999 Elsevier Science Ltd
1. INTRODUCTION
Martian (SNC) meteorites provide our only opportunity to
study the products of Martian magmatic processes in our lab-
oratories. These meteorites provide a mechanism to unravel
both the early differentiation history of Mars and the physical
and chemical processes acting during more recent (1.3 Ga to
present) events. Differences between these meteorites detail the
range of igneous processes occurring on Mars. Elephant Mo-
raine A79001 was the first Martian meteorite documented to
contain large-scale heterogeneity (McSween and Jarosewich,
1983), providing important clues to the processes occurring
within individual magma units. In this work, we demonstrate
that Zagami also is heterogeneous on a large scale, and that
study of the diverse lithologies can provide insights into the
magmatic processes occurring during its crystallization.
The Zagami meteorite fell as a single stone of ;18 kg in
Nigeria in 1962. It is a fine- to medium-grained rock of basaltic
composition and one of the basaltic shergottites, a group which
includes Shergotty, EET A79001, QUE 94201 and Dar al Gani
476. Stolper and McSween (1979) provided a comprehensive
petrologic and experimental study of the small amount of
material (;250 g) from Zagami available at the time. In 1988,
the main mass of this meteorite became available for study,
stimulating work by Treiman and Sutton (1992) and McCoy et
al. (1992). Even these studies demonstrated only minor differ-
ences (e.g., grain size, oxide abundances) between different
pieces of Zagami. In 1992, Vistisen et al. (1992) documented
the existence of a cm-sized olivine-rich sample of Zagami,
suggesting that a more thorough study of all available material
might provide additional clues to the genesis of this important
Martian rock (McCoy et al., 1993; Wadhwa et al., 1993).
We report here the results of an extensive petrologic and
geochemical study of Zagami. The rock exhibits a range of
lithologies that become increasingly enriched in iron oxide and
incompatible elements (e.g., P, Ti, Zr, and REEs). These lithol-
ogies range from the basaltic lithology studied by Stolper and
McSween (1979) to late-stage melt pockets that are highly
enriched in phosphates, mesostases, sulfides, and oxides. We
suggest these lithologies formed through extensive fractional
crystallization of the magma unit of which Zagami is a sample.
The type of differentiation observed in this meteorite may have
occurred commonly in magma bodies on Mars.
*Author to whom correspondence should be addressed (USA
mccoy.tim@nmnh.si.edu).
Pergamon
Geochimica et Cosmochimica Acta, Vol. 63, No. 7/8, pp. 1249?1262, 1999
Copyright ? 1999 Elsevier Science Ltd
Printed in the USA. All rights reserved
0016-7037/99 $20.00 1 .00
1249
2. SAMPLES AND TECHNIQUES
We studied multiple samples of Zagami, examined each under the
binocular microscope, photographed each piece and prepared thin
sections of appropriate materials. Robert Haag provided access to ;5.2
kg of Zagami with a sawn surface area of ;910 cm2. Polished thin
sections UH 217 and 218 were prepared from this material. Access to
a 442 gram sample (USNM 6545) and polished thin section (USNM
6545-1) was provided by the Smithsonian Institution. Lise Vistisen and
colleagues provided material from a large, late-stage melt pocket and
we prepared two polished thin sections (UH 233, 234) from this
material. UH sections are deposited in the collection of the Hawaii
Institute of Geophysics and Planetology.
All thin sections were examined in reflected and transmitted light.
Phase analyses were conducted with Cameca Camebax and Cameca
SX-50 electron microprobes. For analyses of pyroxene and olivine, a
1-mm beam and 15-nA current were used, whereas analyses of maske-
lynite, mesostases, and phosphates utilized a 10-nA current and 5- to
10-mm beam size to reduce volatile loss. Well-known standards were
used. Concentrations of REEs and other selected trace and minor
elements were measured in situ in various minerals present in sections
of the late-stage melt pockets (UH 233) and the dark, mottled lithology
(UH 217, 218) with the Washington University modified CAMECA
IMS-3f ion microprobe. Experimental techniques for ion microprobe
analyses are described in detail by Zinner and Crozaz (1986) and
Lundberg et al. (1988).
3. RESULTS
Zagami consists of a series of increasingly evolved mag-
matic lithologies. The bulk of the rock (termed normal Zagami,
;80 vol.%) is the basaltic lithology studied by Stolper and
McSween (1979), McCoy et al. (1992), Treiman and Sutton
(1992) and Wadhwa et al. (1994). The remaining ;20 vol.% is
a basaltic lithology that is enriched in FeO, has a heterogeneous
distribution of pyroxene and maskelynite on a mm-scale, and
was termed the dark, mottled lithology by McCoy et al. (1995).
It contains mm- to cm-sized late-stage melt pockets that are
highly enriched in olivine, SiO2, phosphates, Fe, Ti-oxides, and
sulfides (Vistisen et al., 1992; McCoy et al., 1993). These
pockets make up ;10 vol.% of the dark, mottled lithology, or
;2 vol.% of the rock. The largest of these pockets was termed
the DN lithology by Vistisen et al. (1992). We do not use the
DN nomenclature in this article, but do present data from this
material because it is the largest, most evolved late-stage melt
pocket from the Zagami material studied by us. Shock-melt
veins and pockets are also present in Zagami and have previ-
ously been discussed by McCoy et al. (1992) and Marti et al.
(1995).
3.1. Textures
The complete range of lithologies known to exist in Zagami
have been observed in two samples from the Haag collection (a
1.85 kg part slice and a 2.78 kg end piece) and in the 442 g
sample from the Smithsonian (USNM 6545) and, therefore,
spatial associations can be observed (Figs. 1a?c). The bulk of
the samples are composed of normal Zagami and are light-
colored and fine grained. The FeO-rich lithology is darker in
color and mottled in appearance. These two lithologies are
separated by a reasonably sharp, although not necessarily
straight, contact (Figs. 1a,c). Finally, within the dark, mottled
lithology occur the late-stage melt pockets that range in size
from a few hundred microns to more than a cm in diameter.
The dark, mottled lithology (Fig. 2a) is generally similar to
normal Zagami in mineralogy and texture and consists mostly
of a homogeneous mixture of maskelynite and pyroxene. Am-
phiboles, although observed in only four melt inclusions in
three separate pyroxene grains in this lithology, occur in the
Mg-rich cores of pyroxenes, as is also the case in normal
Zagami (McCoy et al., 1992). Grain sizes (0.24?0.31 mm) of
Fig. 1. Hand sample photographs of Zagami. (a) Cut face of a 2.78
kg end piece with dark, mottled lithology at the top and normal Zagami
at the bottom. Shock-melt veins cross-cut the specimen. Width 5 18
cm. (b) Close-up view of the dark, mottled lithology. Field of view is
7 cm. (c) Sample USNM 6545 (;5 cm across) with the dark, mottled
lithology (upper left) separated from normal Zagami (lower right) by a
planar contact. The dark, mottled lithology contains late-stage melt
pockets, impact-melt pockets, and elongate maskelynite.
1250 T. J. McCoy, M. Wadhwa, and K. Keil
the dark, mottled lithology are comparable to those of normal
Zagami (0.17?0.36 mm; McCoy et al., 1992). However, unlike
normal Zagami, minerals in the dark, mottled lithology do not
exhibit a preferred alignment (using the method of Darot,
1973). It also differs from normal Zagami in containing mm-
sized areas substantially enriched in pyroxene or plagioclase. In
all sections of the dark, mottled lithology, pyroxene clumps up
to ;4 mm in diameter are found (Fig. 2a). The centers of these
clumps typically contain FeO-rich pyroxene, are depleted in
maskelynite and are enriched in phosphates, Fe, Ti-oxides,
pyrrhotite, and mesostasis (Fig. 3). We also observed string-
ers of large (up to 3 mm) maskelynite grains that originate at
the boundary between the dark, mottled lithology and nor-
mal Zagami and are oriented perpendicular to this boundary
(Fig. 2b).
The smallest of the late-stage melt pockets in the dark,
mottled lithology are similar mineralogically to those in normal
Zagami (Stolper and McSween, 1979) and contain mesostases,
phosphates, Fe-Ti oxides, and pyrrhotite. The largest of these
melt pockets, typified by the Vistisen et al. (1992) sample,
occur within the pyroxene clumps and contain pyroxene,
maskelynite, Fe,Ti oxides, pyrrhotite, whitlockite, and rare
apatite as mm-sized phenocrysts; abundant mesostases and
fayalite-bearing intergrowths are also present. The largest such
late-stage melt pocket contains only 13.6% pyroxenes (mostly
pigeonite; augite is absent as a phenocryst phase but is present
within the fayalite-bearing intergrowths), far less than the 70?
80% typical of the rest of Zagami (Fig. 4a,b; Table 1). Phases
that are normally found in minor abundances (, 3%) in normal
Zagami are prominent in this pocket, including Fe,Ti-oxides
(5.9%), pyrrhotite (1.6%), and phosphates (11.4%). Apart from
these phenocryst phases, this largest melt pocket is composed
of a diverse set of intergrowths. Adjacent to the pigeonite
phenocrysts occur a vermicular intergrowth of augite, fayalite
and a silica-rich material (probably SiO2 1 maskelynite 1 K-
feldspar) (Fig. 5a). Adjacent to large phosphate grains, fayalite
occurs with wormy inclusions of SiO2 1 maskelynite 1 K-
feldspar 6 whitlockite 6 Fe,Ti oxides 6 sulfides (Fig. 5b).
Fayalite comprises 25?50 vol.% of these two intergrowths.
Mesostases are composed of maskelynite 1 SiO2 6 K-felds-
par. The K-rich mesostases tend to be finer-grained than the
K-poor variety. Late-crystallizing mesostases, Fe,Ti-oxides and
pyrrhotite can all occur as mm-sized particles.
3.2. Major Element Mineral Compositions
Pigeonites in normal Zagami range in composition from
Fs28.7Wo15.4 to Fs54.3Wo9.7, whereas augites range from
Fs19.5Wo34.4 to Fs35.0Wo32.7 (McCoy et al., 1992; McCoy,
unpublished data). Pigeonites in the dark, mottled lithology and
its late stage melt pockets exhibit a very broad range of com-
positions (Fs27.0?80.8), as do the augites (Fs19.3?56.2); note that
these compositions range to much higher FeO concentrations
than those in normal Zagami (Fig. 6; Table 2). The most
FeO-enriched grains always occur in the centers of the late-
stage melt pockets, where Mg-rich pyroxenes are absent (Fig.
3). The largest late-stage melt pocket contains pigeonites with
compositions from Fs56.0Wo23.6 to Fs70.7Wo11.3, substantially
enriched in FeO relative to normal Zagami. Augite is not a
phenocryst phase in this melt pocket, but does occur within
fayalite-bearing intergrowths (Fs46.6?53.6) and as isolated
grains in the mesostases (Fs53.2?56.2Wo40.3?42.0; Table 2,
Fig. 6).
Maskelynite compositions in normal Zagami range from
Fig. 2. (a) Polished thin section (UH 217) of the dark, mottled
lithology. Prominent features include a large shock-melt pocket (sm)
and a pyroxene clump (pc). (b) PTS USNM 6545-1, with the dark,
mottled lithology at the top and normal Zagami at the bottom, separated
by a planar contact. Prominent in the dark, mottled lithology are the
elongate maskelynite stringers (ms). Sections are 2.6 cm in diameter.
1251Fractional crystallization on Mars
Ab42?53Or1?4 (Stolper and McSween, 1979). Maskelynite in
the dark, mottled lithology and its late-stage melt pockets
(Ab50.2?58.8; Fig. 7, Table 3) overlaps with this range and
extends it to more sodic compositions. Maskelynite pheno-
crysts in the largest late-stage melt pocket lie towards the sodic
end of this range (Ab56.0?58.8).
Zagami contains a variety of mesostasis materials in late-
stage melt pockets, which appear to be mixtures of plagioclase
1 SiO2 6 K-feldspar or shock-produced glasses of these three
phases (Stolper and McSween, 1979). Potassium-poor mesos-
tasis occurs as pockets up to a millimeter in size and in the
fayalite-bearing intergrowths. Individual analyses of the K-
poor mesostasis (Table 4) vary significantly in their normative
quartz abundances and exhibit a range in feldspar composition
(calculated from the abundances of Na2O, CaO and K2O and
normalized to 100%). The K-poor mesostasis appears to be a
mixture of SiO2, which comprises 0?91.4 normative % of
individual analyses, and maskelynite. The maskelynite compo-
nent ranges in composition from Ab58.3Or4.0 to Ab72.6Or2.7
(Table 4), consistently more albitic than maskelynite phenoc-
rysts in the dark, mottled lithology (Fig. 7). Potassium-rich
mesostasis occurring both as large pockets and in the fayalite
intergrowths have comparable ranges of compositions, scatter-
ing in an array from orthoclase-rich (up to Ab22.6Or72.7) toward
the composition of the K-poor mesostasis, reaching
Ab76.0Or18.0 (Fig. 7). The majority of analyses fall in the range
An3?16Or57?73, with normative quartz ranging from 30?42%
(Table 4, Fig. 7). These compositions suggest that the K-rich
mesostases is a mixture of SiO2, maskelynite and K-feldspar.
Phosphates are abundant in late-stage melt pockets of all
sizes. In the largest of these, phosphates comprise 11.4 vol.%.
Although both whitlockite and apatite occur in the melt pock-
ets, the former is by far the more abundant. Whitlockites vary
in FeO from 2.65?5.15 wt.% FeO (Table 5), ranging to signif-
icantly higher concentrations than found in normal Zagami
(2.59?3.47 wt.% FeO; McCoy et al., 1992; McCoy, unpub-
lished data). Not surprisingly, the largest late-stage melt pocket
has the most FeO-enriched whitlockites (4.96?5.15 wt.% FeO).
Apatite occurs as large phenocrysts and they contain small
amounts of FeO, MnO and MgO (Table 5). Stoichiometries
calculated on the basis of 12 oxygens consistently reveal a
deficiency in F and Cl relative to the ideal value of 1 (Table 5).
This deficiency would correspond to the presence of ;0.50
wt.% water. Watson et al. (1994) determined a water content of
;0.3?0.4 wt.% for one of these apatite grains by ion micro-
probe analysis, confirming the presence of structural pre-ter-
restrial water.
Fayalite has previously been observed in normal Zagami,
with a range of compositions from Fa90?93 (Stolper and Mc-
Sween, 1979). This mineral occurs in fine-grained intergrowths
in the late-stage melt pockets, particularly in the largest of these
pockets. In one late-stage melt pocket, a fayalite grain 150 mm
in diameter was observed. Fayalite compositions in different
textural settings are all FeO-rich and comparable in composi-
tion (Table 6), with a range from Fa90.0?96.7. Fayalite occurring
with FeO-rich phases (e.g., augite, ilmenite) have lower FeO
concentrations than those occurring with phosphates.
Stolper and McSween (1979) reported that Fe,Ti oxides in
normal Zagami included ilmenite (Ilm95Hm5) and titanomag-
netite (Mt37Usp63), suggesting crystallization at an (fO2 close
Fig. 3. BSE photomosaic of the pyroxene clump from Fig. 2a. Pyroxenes in the center of the clump are highly enriched
in FeO compared to those on the edges. Opaques (o), phosphates (p), and mesostases (m) are abundant in the center of the
clump. Field of view is ;7.5 mm.
1252 T. J. McCoy, M. Wadhwa, and K. Keil
Fig. 4. Photomicrographs of a section of the largest late-stage melt pocket in both transmitted, plane polarized (a), and
reflected light (b). Width 5 4 mm. (a) Clear maskelynite (m), pigeonite (p), and skeletal whitlockite (w) are the major
phenocryst phases. (b) Other phases include Fa-bearing intergrowths (f), the opaques Fe, Ti-oxides, and pyrrhotite (o), and
mesostases (me) (both K-rich and K-poor).
1253Fractional crystallization on Mars
to the quartz-fayalite-magnetite buffer. These same phases are
found in all lithologies of Zagami, and our analyses (Table 7)
show only very minor compositional differences between
lithologies, with titanomagnetite ranging in composition from
Usp63?68 and titanomagnetite from Hm4?5. The inference that
all the lithologies in Zagami crystallized near the QFM buffer
is consistent with the presence of SiO2, fayalite and titanomag-
netite as major constituents of the late-stage melt pockets. It is
interesting to note that while the late-stage melt pockets are
similar in degree of fractionation to QUE 94201, the latter
formed at much more reducing conditions (McSween et al.,
1996). Three pyrrhotite grains in the largest late-stage melt
pocket are identical in composition to those of normal Zagami
(Fe0.94S; Stolper and McSween, 1979).
3.3. Trace Element Mineral Compositions
REE patterns of the various minerals in the dark, mottled
lithology (Table 8, Fig. 8a) are similar to those in normal
Zagami (Wadhwa et al., 1994). Low- and high-Ca pyroxenes
have the characteristic LREE-depleted patterns (typically
somewhat steeper for pigeonite than for augite), with small Eu
anomalies (Eu/Eu* ;0.6?0.8, where Eu* is the value interpo-
lated between chondrite-normalized values of Sm and Gd).
However, the range of absolute abundances of incompatible
trace elements in pyroxenes in the dark, mottled lithology is
significantly different from that in normal Zagami. Although
there is some overlap, pyroxene compositions in the dark,
mottled lithology extend to much higher REE and other incom-
patible element concentrations than those in normal Zagami.
This is evident from comparing the compositional range of
pigeonites in the dark mottled lithology with that in normal
Zagami (Fig. 9).
Maskelynite in the dark, mottled lithology has a LREE-
enriched pattern with a strong positive Eu anomaly (Eu/Eu*
; 60?80). Apatite has a relatively flat REE pattern, with La
; 15?20 3 CI. Whitlockite has the highest REE abundances of
all the analyzed minerals and is the main carrier of REEs (as is
the case for all shergottites); La abundances are 480?650 3 CI
in whitlockite in the dark, mottled lithology as compared to
400?480 3 CI in whitlockite in normal Zagami. As in the case
of whitlockite in normal Zagami (Wadhwa et al., 1994), the
REE pattern of this mineral (with the exception of the small Eu
anomaly) in the dark, mottled lithology is parallel to that of the
Zagami bulk rock.
We also report the trace element compositions of minerals in
the largest of the melt pockets (Table 8, Fig. 8b). The range of
incompatible trace element abundances in pigeonite of this melt
pocket has significant overlap with that in normal Zagami, but
extends to somewhat higher concentrations than even the dark,
mottled lithology (Fig. 9).
Maskelynite in this largest late-stage melt pocket has a
LREE-enriched pattern, with a strong positive Eu anomaly
(Eu/Eu* ; 60?90). Apatite in this pocket has relatively higher
REE abundances (La ; 20?30 3 CI) than that typically found
in the dark, mottled lithology. Moreover, the REE pattern of
this apatite appears to be more fractionated (i.e., HREE de-
pleted and with a more pronounced negative Eu anomaly) than
that of apatite in the dark, mottled lithology. REE abundances
in whitlockite of this melt pocket (La ; 500?540 3 CI) fall
within the range of REE concentrations in whitlockite in the
dark, mottled lithology.
4. CRYSTALLIZATION HISTORY OF ZAGAMI
The presence of related magmatic lithologies occurring
within a single Martian meteorite can help to elucidate the
process of differentiation within magma units on Mars. The
data presented here and in previous studies (Stolper and Mc-
Sween, 1979; McCoy et al., 1992; Treiman and Sutton, 1992)
suggest that the sequence of normal Zagami to the dark, mot-
tled lithology to the late-stage melt pockets is, by and large, a
sequence of progressively increasing fractional crystallization,
resulting in the formation of increasingly evolved residual
melts (Table 9). The abundance of late-stage phases, most
notably fayalite, increases systematically from normal Zagami
to the dark, mottled lithology to the late-stage melt pockets, as
does the Fs and incompatible element (e.g., Y and the REEs)
contents of pigeonite, the Ab contents of maskelynite, and the
FeO concentrations of whitlockite. It is noted, however, that the
crystallization sequence within the late-stage melt pockets may
not necessarily be identical owing to their isolation from each
other and local compositional variations within their immediate
environments of crystallization.
Table 1. Modal analyses (vol.%) and average grain sizes of the three magmatic lithologies of Zagami, as obtained by study of polished thin sections.
Phase
Normal Zagami Dark, mottled lithology Late-stage melts
UNM 991 UNM 994
USNM
6545-1
USNM
6545-1 UH 217 UH 28
UH
233, 234
Pyroxene 77.1 80.4 73.6 71.3 76.9 80.3 13.6
Maskelynite 17.6 10.3 21.4 21.6 13.6 9.3 19.0
Mesostasis 1.8 3.7 1.7 3.3 2.9 4.1 8.1
Fe, Ti-Oxides 1.5 2.6 1.7 1.3 2.6 2.6 5.9
Pyrrhotite 0.6 0.6 0.4 0.7 0.6 0.6 1.6
Phosphates 0.5 1.3 1.1 1.4 1.9 2.2 11.4
Shock melt 0.1 0.9 ? 0.2 1.1 0.5 ?
Fa Intergrowth ? ? ? Tr ? Tr 39.9
No. of points 2000 2000 1054 1415 1667 1942 2010
Grain size (mm) 0.24 0.36 0.17 0.24 0.30 0.31 ?
Alignment Yes No No No No No ?
?, not present; Tr, ,0.1 %.
1254 T. J. McCoy, M. Wadhwa, and K. Keil
Fig. 5. Reflected light photomicrographs (fields of view 5 130 mm) of late-stage fayalite-bearing intergrowths. (a)
Pigeonite phenocrysts (p) bounded by a vermicular intergrowth of augite (a), fayalite (f), and K-rich mesostasis (k). (b)
Whitlockite (w) phenocryst bounded by a coarser-grained mixture of fayalite (f), K-rich mesostasis (k), smaller whitlockites
(w), and opaque phases (pyrrhotite and Fe,Ti-oxides) (o).
1255Fractional crystallization on Mars
Considerable debate remains about the crystallization history
of Zagami. McCoy et al. (1992) suggested a two-stage mag-
matic history, where homogeneous Mg-rich pyroxenes crystal-
lizing in a deep-seated (1?2 kbar, 7.5?15 km depth on Mars),
slowly-cooling magma chamber, with subsequent emplacement
of an ;10-m thick flow, which cooled at ,0.5?C/h. Slow
cooling was supported by the work of Brearley (1991). In
contrast, Treiman and Sutton (1992) inferred a single-stage
magmatic history with relatively rapid cooling (2?20?C/h).
Since that time, experiments by McCoy and Lofgren (1996)
have confirmed the need for slow cooling during crystallization
to produce textures similar to those of Zagami. However,
measurements of lower than expected water concentrations in
the kaersutitic amphibole occurring within melt inclusions in
the cores of Mg-rich pyroxenes in Zagami (Watson et al., 1994)
and recent experimental work by Popp et al. (1995a,b) suggest
that these low-OH kaersutites may be stable at lower pressures,
and, therefore, at shallower depth than previously assumed.
Whether or not Zagami experienced a period of crystallization
at depth, it clearly experienced a period of relatively slow
cooling and crystallization in a moderately-thick magma body
at or near the surface of Mars. Our new results suggest that
significant fractional crystallization occurred during this period.
The crystallization order in Zagami can be inferred from the
Fig. 6. Pyroxene quadrilateral showing compositions of pyroxenes from normal Zagami (McCoy et al., 1992), the dark,
mottled lithology, and the largest late-stage melt pocket. Each lithology contains increasingly FeO-enriched pigeonite,
reflecting formation of these lithologies via fractional crystallization.
Table 2. Representative analyses of pigeonite and augite illustrating the range of variability.
Pigeonite Augite
Dark, mottled lithology Late-stage melts Dark, mottled lithology Late-stage melts
SiO2 53.9 51.2 50.6 49.1 47.9 46.6 48.1 47.9 48.0 53.1 51.7 49.7 48.6 48.5 46.5
Al2O3 0.68 1.25 0.79 0.59 0.63 0.61 0.84 0.69 0.39 0.94 1.34 0.92 0.97 0.69 n.d.
FeO 17.4 24.3 30.2 34.5 38.1 44.1 33.1 37.1 40.8 12.3 17.3 25.5 27.6 31.3 30.8
MgO 22.0 15.37 13.1 9.06 5.21 3.12 6.73 5.64 5.79 17.0 14.2 8.72 5.23 4.1 1.74
CaO 6.02 7.14 5.31 5.96 6.94 3.85 10.9 8.38 5.11 16.3 14.5 13.8 17.1 15.4 18.8
Total 100.11 99.23 100.06 99.17 98.69 98.28 99.67 99.71 100.09 99.57 99.04 98.66 99.50 99.99 98.77
Fs 27.0 40.7 50.1 59.2 67.7 80.8 56.0 64.1 70.7 19.3 28.3 43.4 46.9 53.6 53.2
Wo 12.0 15.3 11.3 13.1 15.8 9.0 23.6 19.0 11.3 33.0 30.4 31.0 37.2 33.8 41.5
n.d., not determined.
1256 T. J. McCoy, M. Wadhwa, and K. Keil
mineralogies of the various lithologies and their modal abun-
dances (Fig. 10). Petrographic (McCoy et al., 1992) and exper-
imental (Stolper and McSween, 1979; McCoy and Lofgren,
1996) studies suggest that pigeonite was the liquidus phase.
McCoy et al. (1992) suggested that augite co-crystallization
commenced shortly afterwards, but ceased at ;15% crystalli-
zation of the bulk melt, at which time FeO-enrichment is
observed in pigeonite rimming the homogeneous Mg-rich
cores. McCoy et al. (1992) argued that this marked the point of
eruption of the phenocryst-bearing lava flow, with augite crys-
tallization commencing again, followed by plagioclase. For
much of the crystallization history of Zagami, augite, pigeonite,
and plagioclase were co-crystallizing. Physical segregation of
early-formed, relatively magnesian pyroxene likely formed
normal Zagami, possibly through processes such as crystal
settling, melt migration, and flow differentiation. A break in
augite compositions between the dark, mottled lithology and its
late-stage melt pockets likely reflects cessation of augite crys-
tallization and the onset of phosphate crystallization. This ap-
pears to have occurred after about 95 vol.% of the rock had
crystallized (Fig. 10). During crystallization, P concentrated in
the melt until it reached saturation and calcium phosphates
began to crystallize. The melt was too poor in calcium to
support crystallization of phosphates, plagioclase, pigeonite,
and augite and, therefore, augite crystallization ceased. Plagio-
clase crystallization may well have been stabilized by Al in the
melt. At ;98% crystallization, the melt became too FeO-rich to
crystallize pigeonite and, instead, a fayalite-SiO2 mixture
formed. At this stage, much of the melt had crystallized and the
melt pockets were isolated from each other. As noted previ-
ously, their crystallization histories were controlled by local
compositional variabilities in the late-stage melts. In some
pockets, sufficient P was present to allow for the crystallization
of phosphates, producing the phosphate-bearing fayalite inter-
growths. In other late-stage melts, the breakdown of pigeonite
and the absence of sufficient P allowed augite to crystallize
once again, producing the augite-bearing fayalite intergrowths.
The final crystallization products are the mesostases and
Fig. 7. Compositions of maskelynite, K-rich and K-poor mesostases from the dark, mottled lithology and its late-stage
melt pockets. Maskelynite is slightly more albitic than in normal Zagami. Mesostases consist of feldspar 1 SiO2 6
K-feldspar or shock-produced glasses of these compositions. The K-poor mesostasis is consistently more albitic than
maskelynite.
Table 3. Representative analyses of maskelynite from the dark,
mottled lithology and its late-stage melt pockets, illustrating the range
of compositional variability.
Dark, mottled Late-stage melt
SiO2 56.8 58.0 59.0 58.6 59.4
Al2O3 26.4 25.9 25.1 25.3 24.9
FeO 0.78 0.72 1.04 0.63 0.79
CaO 9.98 9.25 7.95 8.42 7.90
Na2O 5.81 6.27 6.62 6.44 6.95
K2O 0.37 0.42 0.65 0.60 0.80
Total 100.18 100.57 100.39 99.99 100.74
Ab 50.2 53.8 57.9 56.0 58.8
Or 2.1 2.3 3.7 3.5 4.4
1257Fractional crystallization on Mars
opaque phases (e.g., pyrrhotite, Fe-Ti-oxides). Therefore, the
late crystallization history of Zagami is largely controlled by
the abundances of iron, calcium, and phosphorus.
The general picture, outlined above, of the crystallization
history of Zagami is supported by the trace element microdis-
tributions in minerals in the different lithologies (i.e., normal
Zagami, the dark, mottled lithology and the late-stage melt
pockets in the latter). The data presented here clearly indicate
that these lithologies are related by progressive fractional crys-
tallization. Most noteworthy is the similarity of the REE pat-
terns of the same mineral in each of these lithologies (Fig. 8).
It has been previously demonstrated that the REE patterns of
melts in equilibrium with early- as well as late-formed minerals
of the basaltic shergottites are parallel to the REE pattern of
their respective bulk rocks, suggesting closed system fractional
crystallization following some degree of crystal accumulation
(Wadhwa et al., 1994; Lundberg et al., 1988). Therefore, the
noted similarity of REE patterns of minerals between normal
Zagami and the more highly evolved lithologies (data for which
are presented here; Fig. 8) then implies that these lithologies are
related by fractionation of the same parent magma. In addition,
the fact that the incompatible trace element composition of
pigeonites in the dark, mottled lithology and in the largest of
the late-stage melt pockets overlaps that of normal Zagami, and
extends to higher concentrations than in the latter (Fig. 9)
supports progressive fractional crystallization to produce the
dark, mottled lithology and the late-stage melt pockets, follow-
ing the formation and segregation of minerals comprising nor-
mal Zagami from the parent magma. Finally, the higher abun-
dances of REEs in phosphates of the dark, mottled lithology
compared to normal Zagami also point towards a more evolved
composition for the latter. However, one may note that al-
though the REE concentrations in whitlockite of the largest
late-stage melt pocket are, on average, higher than that in
normal Zagami, they fall within the range of REE abundances
found in whitlockites of the dark, mottled lithology. We sug-
gest that this may be due to the local variability in the compo-
sition (particularly, the phosphorous content, which would de-
termine when the whitlockite began crystallization and,
therefore, the REE content this mineral would incorporate) of
the melt from which this melt pocket formed, relative to the
average composition that formed the dark, mottled lithology.
The idea that the bulk compositions of Shergotty and Zagami
are not melt compositions, but record some degree of crystal
Table 4. Compositions of K-poor and K-rich mesostases from late-stage melt pockets in the dark, mottled lithology illustrating the range of
compositions.
K-poor mesostases K-Rich mesostases
SiO2 58.7 59.7 78.3 78.8 97.6 98.3 74.5 77.9 77.4 76.9
Al2O3 24.7 24.7 13.4 12.9 2.37 1.34 12.5 11.8 11.6 12.0
FeO 0.74 0.59 0.37 0.45 0.11 0.09 1.45 1.28 0.97 0.74
CaO 7.70 7.63 3.52 4.00 0.51 0.09 1.92 0.84 0.68 0.63
Na2O 7.23 6.74 4.53 4.03 0.84 0.59 2.34 1.83 1.80 1.67
K2O 0.62 0.84 0.30 0.29 0.04 0.22 7.04 7.28 7.49 8.16
Total 99.69 100.20 100.42 100.47 101.47 100.63 99.75 100.93 99.90 100.10
Qz 0 1.4 43.5 46.5 91.4 93.8 30.5 36.5 36.5 34.5
Ab 60.8 58.5 68.0 62.7 72.6 76.0 29.1 25.9 25.2 22.6
An 35.9 36.7 29.0 34.4 24.7 6.0 13.3 6.6 5.3 4.6
Or 3.4 4.8 3.0 2.9 2.9 18.0 57.6 67.5 69.5 72.7
See text for explanation of end member compositions.
Table 5. Representative analyses of whitlockite and apatite from late-stage melt pockets in the dark, mottled
lithology illustrating the range of compositional variability.
Whitlockite Apatite
SiO2 0.10 0.07 0.10 0.11 0.50 0.44 0.69
FeO 3.09 3.89 4.96 5.15 0.78 0.94 1.12
MnO 0.11 0.18 0.22 0.22 0.14 0.15 0.10
MgO 1.97 1.65 0.85 0.59 0.07 0.06 n.d.
CaO 47.1 47.1 46.7 46.8 53.4 54.2 53.1
Na2O 1.14 1.20 1.22 1.17 b.d. b.d. n.d.
P2O5 44.7 44.7 45.3 44.8 40.7 40.1 41.2
F b.d b.d b.d. b.d. 1.12 1.25 1.27
Cl b.d. b.d. b.d. b.d. 2.92 3.04 2.36
Total 98.21 98.79 99.35 98.84 99.63 100.18 99.84
O?F,Cl ? ? ? ? 1.12 1.22 1.20
Total ? ? ? ? 98.51 98.96 98.64
n.d., not determined; b.d., below detection.
1258 T. J. McCoy, M. Wadhwa, and K. Keil
accumulation was suggested by Stolper and McSween (1979).
More recently, several authors (e.g., Harvey et al., 1996; Mc-
Sween et al., 1996; Mikouchi et al., 1996) have suggested that
QUE 94201 may represent a melt composition unaffected by
crystal accumulation. With these new lithologies in Zagami, we
have, for the first time, sampled both crystal cumulates and
residual melts in a single rock, allowing us to better constrain
the bulk composition of this magma. A perplexing feature of
shergottites is that although the homogeneous, Mg-rich cores in
Shergotty and Zagami comprise only 10?20% of the rocks
(McCoy et al., 1992), crystallization experiments on bulk com-
positions of these rocks produce 35?45% pyroxene having the
composition of the Mg-rich cores (Stolper and McSween, 1979;
McCoy and Lofgren, 1996; Hale et al., 1998). Rather than
completely reflecting crystal accumulation, as suggested by
Stolper and McSween (1979), this may reflect the fact that the
actual bulk composition from which Zagami crystallized differs
substantially from the normal Zagami lithology used by Stolper
and McSween (1979) and McCoy and Lofgren (1996). Using
data from these experiments, we calculate a KD for FeO in the
first crystallizing pigeonite of ;0.76. This would suggest that
the natural magnesian pyroxenes (;Fs30) might form as the
first crystallizing phase from a magma with ;24.7 wt.% FeO.
By comparison, the bulk composition of normal Zagami con-
tains only 18.2 wt.% FeO (McCoy et al., 1992). Additionally,
even the intercumulus melt (the bulk composition without the
Mg-rich pyroxene cores) contained only 18.5 wt.% FeO (Mc-
Coy et al., 1992). However, a bulk composition based on
mineral abundances in the various lithologies (Table 1), min-
eral compositions (McCoy et al., 1992; McCoy, unpublished
data; this work) and estimates of the abundances of the various
lithologies would contain ;19.1 wt.% FeO. This value should
be considered a minimum FeO concentration for the true bulk
composition from which Zagami crystallized, because the FeO-
rich lithologies may be substantially more abundant in the
magma body than observed in the main mass of this meteorite.
This suggests that crystallization experiments using an FeO-
enriched composition would produce fewer magnesian py-
roxenes and might explain some of the discrepancies between
experimental and petrologic studies.
An interesting feature of these late-stage magmatic litholo-
gies (i.e., the dark, mottled lithology and the melt pockets) is
the near-absence of structurally-bound water. If the Zagami
parent magma had contained any water, one might expect the
late-stage crystallization products such as apatite to be enriched
Table 7. Representative analyses of ilmenite and titanomagnetite in
the three lithologies of Zagami illustrating the range of compositional
variability.
Normal
Zagami
Dark,
mottled lithology
Late-stage
melt pockets
(1) (2) (1) (2) (1) (2)
TiO2 49.7 22.1 50.2 24.4 50.3 24.3
Al2O3 0.09 2.76 0.08 1.89 0.07 2.22
Cr2O3 0.15 2.16 0.21 1.56 b.d. b.d.
MgO 0.82 0.44 0.92 0.74 0.19 0.14
FeO 47.3 70.8 47.1 69.4 47.8 71.5
MnO 0.71 0.56 0.64 0.64 0.75 0.64
Total 98.77 98.82 99.15 98.63 99.11 98.8
(1) Ilmenite, (2) Titanomagnetite; b.d. 5 below detection limits.
Table 6. Compositions of fayalite in late-stage melt pockets in the
dark, mottled lithology illustrating the range of compositional variabil-
ity.
(1) (2) (3) (4) (5)
SiO2 30.2 28.9 29.3 29.4 29.5
FeO 64.6 67.3 68.4 68.2 66.8
MnO 1.84 1.86 1.91 1.87 1.76
MgO 4.01 1.52 1.22 1.30 2.31
CaO 0.37 0.42 0.33 0.44 0.24
Total 101.02 100.00 101.16 101.21 100.61
Fa 90.0 96.1 96.5 96.7 90.9
(1),(2) In augite-bearing fayalite intergrowth, (3) In Phosphate-
bearing fayalite intergrowth, (4) In whitlockite, (5) In Fe, Ti-oxide n.d.,
not determined; b.d., below detection.
Table 8. Representative rare earth element abundances (in ppm) in minerals in the dark, mottled lithology and the late-stage melt pockets. Numbers
in parentheses are 1s errors (from counting statistics only) in the last significant digits.
Dark, mottled lithology Late-stage melt pockets
Pigeonite Augite Maskelynite Apatite Whitlockite Pigeonite Augite Maskelynite Fayalite Apatite Whitlockite
La 0.015 (2) 0.052 (7) 0.109 (4) 3.7 (2) 130 (2) 0.014 (3) 0.081 (8) 0.050 (4) b.d. 5.4 (4) 132 (2)
Ce 0.049 (4) 0.21 (2) 0.211 (8) 11.2 (4) 314 (4) 0.056 (8) 0.38 (4) 0.081 (7) b.d. 16.9 (9) 314 (4)
Pr 0.012 (2) 0.036 (6) 0.024 (2) 1.4 (1) 40 (1) 0.014 (3) 0.083 (7) 0.008 (1) b.d. 2.5 (2) 40 (1)
Nd 0.066 (7) 0.32 (3) 0.074 (4) 8.1 (3) 202 (4) 0.09 (1) 0.69 (3) 0.026 (4) b.d. 12.8 (5) 208 (4)
Sm 0.047 (7) 0.23 (3) 0.016 (3) 2.9 (2) 75 (3) 0.076 (11) 0.36 (3) 0.014 (4) b.d. 4.3 (4) 72 (3)
Eu 0.015 (2) 0.079 (8) 0.50 (2) 1.0 (1) 23 (1) 0.027 (3) 0.11 (1) 0.40 (3) b.d. 1.2 (1) 18 (1)
Gd 0.11 (1) 0.42 (5) 0.023 (3) 4.1 (3) 105 (4) 0.15 (2) 0.89 (7) 0.013 (4) b.d. 6.2 (7) 91 (4)
Tb 0.024 (3) 0.093 (14) 0.004 (1) 0.71 (6) 19 (1) 0.046 (8) 0.20 (2) 0.002 (1) 0.007 (3) 1.1 (1) 18 (1)
Dy 0.27 (1) 0.78 (5) 0.019 (2) 5.4 (2) 139 (3) 0.45 (3) 1.74 (6) b.d. 0.073 (10) 7.3 (4) 128 (3)
Ho 0.060 (6) 0.14 (2) b.d. 1.0 (1) 28 (1) 0.09 (1) 0.43 (3) b.d. 0.038 (6) 1.5 (1) 28 (1)
Er 0.23 (1) 0.44 (3) b.d. 2.9 (1) 83 (2) 0.43 (2) 1.37 (5) b.d. 0.13 (1) 3.0 (2) 67 (2)
Tm 0.034 (5) 0.073 (12) b.d. 0.39 (3) 11 (1) 0.071 (1) 0.18 (1) b.d. 0.045 (6) 0.40 (4) 10 (1)
Yb 0.26 (1) 0.51 (4) b.d. 2.4 (1) 75 (3) 0.59 (5) 1.47 (6) b.d. 0.52 (3) 2.2 (2) 50 (2)
Y 1.62 (7) 3.99 (18) 0.058 (4) 33.4 (6) 961 (5) 3.32 (11) 8.58 (16) 0.030 (3) 0.76 (4) 31.2 (9) 568 (4)
b.d. 5 below detection limit of the ion microprobe.
1259Fractional crystallization on Mars
in their hydrous component. However, apatite, which com-
prises ,0.5 wt.% of the largest, late-stage melt pocket and
contains ;0.5 wt.% water, accounts for less than 25 ppm water
in this highly evolved lithology. Because one of the most
evolved lithologies in Zagami is so dry, the melt from which it
crystallized must also have been extremely dry. This does not
solve the more interesting problem of whether the original
magma was dry, as one might infer from the work of Watson
et al. (1994) and Popp et al. (1995a, b), or if it experienced
significant devolatilization during its emplacement history prior
to crystallization, as has been suggested by McSween and
Harvey (1993). However, the inference of an extremely dry
magma at the time of crystallization suggests that if degassing
did occur, it must have been an extremely efficient process.
Schaber et al. (1978) demonstrated that 10-m thick flows are
extremely common in the Tharsis region of Mars and compa-
rably thick near-surface intrusives must also be abundant. Since
Zagami may have crystallized from a flow of similar thickness,
it may be that rocks with multiple lithologies like those in
Zagami are fairly common on Mars. While evidence for large-
scale differentiation of basalts has been observed (e.g., Mustard
et al., 1997), future missions might expect to find compositional
differentiation within most magma units on Mars.
5. CONCLUSIONS
Zagami experienced a prolonged history of fractional crys-
tallization, producing a variety of lithologies which are increas-
ingly enriched in FeO and incompatible elements (e.g., P, Ti,
Zr, REEs).
The bulk composition of the magma unit from which Zagami
formed is likely to be significantly more FeO-rich than previ-
ously assumed. This may partially explain discrepancies in the
magmatic history derived from petrologic and experimental
studies.
The magma from which Zagami crystallized was remarkably
dry, with the most evolved lithology containing ;25 ppm of
Fig. 8. Representative REE abundances (normalized to CI values of
Palme et al., 1981) in minerals of (a) dark, mottled lithology and (b) the
largest of the late-stage melt pockets. HREE abundances in maskelynite
and LREE abundances in fayalite are not shown because they are below
the detection limit of the ion microprobe. Error bars are 1s from
counting statistics only; in cases where error bars cannot be seen, they
are smaller than the data points.
Fig. 9. Zr vs. Y abundances in pigeonites of normal Zagami (crosses;
data from Wadhwa et al., 1994), dark, mottled lithology (filled squares)
and the largest of the late-stage melt pockets (open squares). Note that
the x- and y-axes are plotted on logarithmic scales.
Table 9. Parameters indicating that the sequence of normal Zagami
to the dark, mottled lithology to the late-stage melt pockets is a
sequence of increasing fractional crystallization, resulting in the for-
mation of increasingly evolved residual melts.
Normal
Zagami
Dark, mottled
lithology
Late-stage
melts
Fa intergrowth (wt.%) 0.0 Trace 39.9
Fs in pigeonite 28.7?54.3 27.0?80.8 56.0?70.7
Y in pigeonite (ppm) 1.1?4.2 1.6?7.5 2.0?8.8
Ab in maskelynite 42?53 50.2?58.8 56.0?58.8
FeO in whitlockite (wt.%) 2.6?3.5 3.1?5.2 5.0?5.2
1260 T. J. McCoy, M. Wadhwa, and K. Keil
structurally-bound water. It is unclear whether the magma was
dry at the time of its formation or experienced extremely
efficient outgassing prior to crystallization.
Differentiation of magma units at or near the surface of Mars
may be a fairly common process and products of this differen-
tiation may be expected while sampling magma units during
future Mars missions.
Acknowledgments?Samples and thin sections were generously made
available by L. Vistisen and D. Petersen (Niels Bohr Institute, Copen-
hagen), M. B. Madsen (H. C. ?rsted Institute, Copenhagen), R. Haag
(Tucson, AZ), G. J. MacPherson (Smithsonian Institution) and A. J.
Brearley (University of New Mexico). We particularly thank the first
three colleagues for bringing these highly-evolved lithologies to our
attention and spurring our renewed interest in Zagami. Helpful discus-
sions and assistance by G. J. Taylor and L. Leshin are appreciated.
Expert technical assistance was provided by T. Servilla, T. Hulsebosch,
D. McGee, T. Tatnall, P. Bernhard, V. Yang, and S. Wentworth.
Substantive reviews by H. Y. McSween, Jr. and L. A. Leshin substan-
tially improved and clarified the paper. Financial support was provided
by a National Research Council Postdoctoral Fellowship at NASA
Johnson Space Center (T. J. M.), NASA grants NAGW-3281 and NAG
5-4212 (K. Keil, PI) and NAG-9-55 (G. Crozaz, PI). This is Hawaii
Institute of Geophysics and Planetology publication no. 1028 and
School of Ocean and Earth Science and Technology publication no.
4732.
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