SMITHSONIAN CONTRIBUTIONS TO THE
EARTH SCIENCES
NUMBER 1
George SwitZer Partially Melted
and William G. Melson Tr ? -r? i ?
Kyanite rxlogite
from the
Roberts Victor Mine,
South Africa
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ABSTRACT
Three specimens have been studied of the rare kyanite eclogite nodules in kimber-
lite from the Roberts Victor mine, South Africa. All are essentially the same
with the primary assemblage: kyanite, omphacite, garnet, diamond (in one sample),
chrome diopside, and rutile. There is also present a fine-grained secondary assemblage
that appears in two forms: (1) primary omphacite altered to a mixture of plagioclase,
clinopyroxene, and possibly glass; and (2) thin layers along omphacite, kyanite, and
garnet grain boundaries. These layers have a clear-cut igneous texture and consist of
plagioclase microlites with glass or devitrified glass, or plagioclase microlites and
subhedral augite, with or without glass. Hornblende, spinel, and calcite are accessories,
and analcite fills vesicles. Corundum and mullite occur at the margins of kyanite
grains.
The glass in the secondary assemblage has a composition roughly equivalent to
what one might expect if it was derived by incongruent melting of omphacite, followed
by partial crystallization. Omphacite at one atmosphere pressure begins to melt at
about 1030? C and melting is complete at about 1260? C. At 30 kilobars (O'Hara and
Yoder, 1967) melting begins at about 1570? C and is complete at 1600? C. Thus,
sudden pressure release of an eclogite at high temperature could cause partial'melting
of omphacite.
These kyanite eclogites clearly contained an interstitial melt that has been rapidly
cooled. Evidence points to this melt having been generated mainly by partial melting
of primary omphacite rather than by introduction of an externally derived melt. The
partial melting may have occurred in response to one of the following three processes
or some combination of them :
1. Increase in temperature at constant pressure.
2. Introduction of water into the eclogite at constant temperature and constant
total pressure.
3. Release of pressure at constant temperature.
The third process seems to offer the most reasonable explanation for the partial
melting.
Introduction
Of the several rock types found as nodules in kim-
berlite in South Africa, one of the most interesting and
the least abundant is the kyanite eclogite from the
Roberts Victor mine.
The first description of this rock seems to have
been in a letter from Professor Beck of Freiburg,
Germany, to E. H. V. Melville, who read the letter
at the 16 July 1906 meeting of the Geological Society
of South Africa (Melville, 1907). Further descriptions
have been published by Johnson (1907) and Wil-
liams (1932). In these studies one of the most in-
George Switzer and William G. Melson, Department of Min-
eral Sciences, Smithsonian Institution, Washington, D.C.
20560.
triguing features of the rock, namely evidence of
partial melting, was overlooked.
The purpose of this paper is to describe the evi-
dence of partial melting and to discuss possible causes.
The primary mineral assemblage of these kyanite eclo-
gites is also of special interest and will be dealt with
in another paper.
Three specimens of kyanite eclogite were available
for study, USNM 87375, 110752, and 111060. The lat-
ter two were recently donated by the Geological De-
partment of De Beers Consolidated Mines, Ltd.,
through Mr. D. Du Toit.
Evidence of partial melting is so pronounced in the
Roberts Victor mine kyanite ecolgite that it was in
these it was first recognized. Once having been rec-
1
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
ognized, other eclogites (without kyanite) examined
from the Roberts Victor mine revealed that some
of these too have interstitial glass, possibly derived
by partial melting. A study of the secondary assemblage
of normal (non-kyanite bearing) eclogite from the
Roberts Victor mine is underway and will be reported
on separately.
Mineralogy
The Roberts Victor mine kyanite eclogite is made
up of distinctly different primary and secondary min-
eral assemblages. The primary assemblage is a course
aggregate of garnet, omphacite (largely altered), and
kyanite, with diamond (in one specimen), chrome di-
opside, and rutile as accessories. Microprobe analyses
made possible the elucidation of a complex, fine-grained
secondary assemblage that appears to be largely the
result of partial melting of primary omphacite. Table
1 lists the primary and secondary mineral assemblages.
OMPHACITE.?This primary mineral played a key
role in the generation of the secondary assemblage.
Data on relicts of unaltered omphacite (Figure 1) are
given in Table 2.
ALTERED OMPHACITE.?The omphacite in these
rocks is altered almost completely to a peculiar yet
characteristic gray to greenish-gray, nearly opaque
product. The grain size of the alteration product
ranges from submicroscopic to a birefringent aggregate
in which at least two phases can be seen but not posi-
tively identified by optical properties or microprobe
analysis (Figures 1, 2). Some areas that exhibit a
blotchy appearance in ordinary light, under crossed
nicols show curved, stellate plagioclase crystals, giving
a texture very similar to that frequently seen in
quenched silicate melts (Figure 2).
Peaks on an x-ray diffraction pattern can be ac-
counted for by plagioclase and cl.inopyroxene but the
composition of these phases cannot be determined by
this method. The presence of glass as a third phase is
suspected but has not been proven.
Electron microprobe analyses of the alteration
product are given in Table 2. Areas selected for
analysis were those that in thin section were extremely
fine grained, and the electron beam was widened to
20^. Comparison with the analyses of fresh omphacite
shows that the reaction was essentially isochemical.
SECONDARY IGNEOUS ASSEMBLAGE.?Most of the
minerals in the secondary assemblage occur as thin
layers along omphacite, kyanite, and garnet grain
boundaries (Figure 3) and, in part, replace the pri-
TABLE 1.?Mineralogy of Roberts Victor mine kyanite
eclogite
Primary assemblage Secondary assemblage
Garnet (grossular - alman-
dine-pyrope)
Omphacite
Diamond (noted only in
111060)
Chrome diopside (noted only
in 110752)
Rutile (noted only in 87375)
Altered omphacite (plagio-
clase +clinopyroxene+
glass?)
Plagioclase (An29)
Augite (only in 87375 and
111060)
Fassaite (only in 110752)
Spinel
Hornblende
Glass
Analcite
Calcite
Corundum
Mullite
TABLE 2.?Omphacite and altered omphacite from Roberts
Victor mine kyanite eclogite
Fresh omphacite Altered omphacite
111060 110752 110752? 87375 *
SiOj
TiO2
A12O3
Fe2O8
FeO
MgO
CaO
Na2O
K2O
a
yZ
Ac
2V( + )Ac
Jd
Ts
Hd
Di
52.1
0.3
17.5
0.3
1.2
7.2
11.9
6.8
0.3
1.664] Na
1.6721? 0.002
1.688J
36?
70?
2
46
14
3
35
53.3
0.3
17.9
0.4
1.4
6.4
12.1
6.4
0.2
3
42
14
4
36
56.0
0.3
18.6
1.9
6.6
11.3
5.0
0.3
56.4
0.4
20.2
2.3
4.2
9.2
5.2
0.0
1 Analyses by electron microprobe.
2 Based on other analyses of similar omphacites Fe assigned
on basis that 0.8 of the total Fe is Fe2+. Only specimen 111060
contains enough fresh omphacite for determination of optical
properties.
3 Average of five analyses. Range: SiO2 55.4?56.6; TiO2
0.3-0.3; A12O3 18.1-19.0; Fe 1.5-1.5; MgO 6.2-7.0; CaO
10.8-12.3; Na2O 4.5-5.5; K2O 0.3-0.3. All Fe taken as FeO.
4 Average of four analyses: Range: SiO2 55.8-56.8; TiO2
0.3-0.4; A12O3 20.1-20.5; Fe 1.6-1.9; MgO 3.8-4.6; CaO
8.6-9.7; Na2O 4.7-5.5. All Fe taken as FeO.
NUMBER 1
FIGURE 1.?Relicts of omphacite (white) in altered omphacite (gray). USNM 111060.
mary omphacite. Under high magnification the layers
have a clear-cut igneous texture (Figure 4) showing
plagioclase microlites with glass or devitrified glass,
or plagioclase microlites and subhedral augite, with
or without glass. Hornblende, spinel, and calcite are
accessories, and analcite fills vesicles. Where the
secondary igneous assemblage cuts garnet it is fre-
quently composed entirely of augite, plagioclase, and
spinel, without glass, and has ophitic texture.
Data on the minerals of the secondary igneous as-
semblage follow.
Plagioclase: This mineral has typical microlite
habit and twinning, little or no zoning, and
composition An29.
Augite, fassaite, and hornblende: Augite, a major
mineral of the secondary assemblage, in thin section
is subhedral and colorless. The augite in specimen
87375 is Al-rich and Ti-poor as compared with
normal augite. In specimen 110752 the "augite" con-
tains 18.3 percent A12O3, placing it much closer to
fassaite than augite in composition, although the CaO
content is a little low for this mineral. (Tabulation
of fassaite analyses in Deer, Howie and Zussman,
1963, shows CaO?*25 percent). To our knowledge
such a fassaite-like pyroxene has not been reported
in a similar paragenesis.
In specimen 110752 the fassaite is more coarsely
crystalline than the augite in the other two specimens.
It occurs either as monomineralic rims around garnet,
intergrown with plagioclase, or ophitically enclosing
plagioclase. In thin section it is colorless.
Hornblende, a minor constituent in all three speci-
mens, is subhedral to euhedral, brown and pleochroic
brown to light brown in thin section.
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
FIGURE 2.?Quench texture in altered omphacite. Crystals are plagioclase. Grossed nicols.
USNM 87375.
Microprobe analyses of these three minerals are
given in Table 3.
Spinel: Minute, dark green octahedrons of spinel
are common in the secondary igneous assemblage, par-
ticularly where the igneous assemblage is in contact
with garnet. Its composition by microprobe analysis is
approximately Sp40 He60.
Glass: Local areas of residual glass are a prominent
feature of the secondary igneous assemblage. The glass
fills interstices in plagioclase-augite, is pale brown in
color, and isotropic. Some areas are completely clear,
others contain from a few to abundant microlites, and
some have the typical appearance of devitrified glass.
TABLE 3.?Augite, fassaite, and hornblende from Roberts
Victor mine kyanite eclogites 1
Augite Fassaite
in 87375 in 110752
Hornblende
in 87375
Hornblende
in 110752
SiO2
TiO2
A12O3
Fe
MgO
GaO
Na2O
K2O
51.3
0.6
7.4
8.9
11.8
18.6
0.6
0.0
41.7
0.7
18.3
7. 1
7.5
19.9
1.4
0.0
38.6
2.2
17.4
8.2
13.3
10.4
4.0
0.6
37.4
0.6
19.3
7.4
11.3
12. 1
3.0
1.5
1 Analyses by electron microprobe.
NUMBER I
The best examples of fresh glass are in specimen 87375.
A microprobe analysis is given in Table 4.
TABLE 4.?Glass in kyanite eclogite from the Roberts
Victor mine l
SiO2
TiO2
A12O3
MgO
CaO
Na2O
K2O
Fe
Weight percent
47.7
0.6
23.1
1.6
1.0
9.7
1.9
4.8
Norm2
or 11.1
ab 39.8
an 4.7
ne 22.7
ol 8.6
mg 4.9
il 1.2
c 3.4
1 Specimen 87375.
2 In calculation of norm, iron was assigned one-half to Fe2+
and one-half to Fe3+ (3.1 FeO and 3.4 Fe2O3). With this
assignment of iron the sum is 92.1. The difference is probably
water.
Analcite: White, massive analcite fills vesicles in sec-
ondary igneous texture layers. Identification was by
microprobe analysis and x-ray diffraction.
Calcite: Calcite is a rare accessory in the secondary
igneous assemblage.
Corundum: Corundum crystals occur commonly at
the margins of kyanite grains. Identification was by
microprobe analysis and optical properties.
Mullite: A frequently seen phase in thin sections is
a fibrous mineral in kyanite that microprobe analysis
demonstrated to be compositionally identical to mullite
(Table 5).
TABLE
SiO2
A12O3
Total
5.?Mullite in
Mullite in
110752
25.9
73.6
99.5
Roberts Victor
Mullite
theoretical
28.2
71.8
100.0
mine kyanite eclogite
Kyanite
theoretical
36.8
63.2
100.0
In thin section the mullite has the following optical
properties: parallel extinction, length slow, index of
refraction less than that of kyanite.
Incongruent Melting of Omphacite
The garnet and kyanite in these eclogites are not al-
tered, except locally in small volumes along their bound-
aries. The igneous mineral assemblage and the com-
position of the interstitial glass suggests that they were
involved in only a minor way in the generation of the
melt. On the other hand, the pervasive alteration of
omphacite and the features of the igneous assemblage
indicate that omphacite played a major role in the
formation of the melt and its melting behavior thus
is of special interest here.
The system nepheline-diopside-silica has been in-
vestigated by Schairer and Yoder (1960) at one at-
mosphere, and the results can be used to qualitatively
interpret partial melting of natural omphacite at low
pressure. Mixtures along the diopside-jadeite join begin
to melt at about 1120? C when these mixtures contain
more than 15 percent diopside. The first liquids
(equivalent to residual liquids) are highly undersatu-
rated. The norm of such a liquid would contain mainly
albite and nepheline, a feature that characterizes the
composition of the glass in one of the kyanite eclogites
(Table 6).
TABLE 6.?Norms of primary omphacite and secondary
glass in Roberts Victor mine kyanite eclogite
Omphacite
in 111060
Omphacite
in 110752
Glass in
87375
or
ab
an
ne
di
ol
il
mg
c
ab + ne + di
1.7
24.6
16.4
17.9
33.0
5.2
0.6
0.5
75.5
1. 1
30.4
19.7
12.8
31.3
3.6
0.6
0.7
74.5
11. 1
39.8
4.7
22.7
4.2
1.2
4.9
3.4
62.5
Melting experiments* by the authors on omphacite
from a Roberts Victor mine eclogite show that melt-
ing begins at about 1030? C and is complete at about
1260? C. This lowering of the liquidus and solidus tem-
peratures compared to the synthetic system (Schairer
and Yoder, 1960) is probably due to the FeO content
of the natural omphacite, the only important con-
1 Charges placed in 60 Pd 40 Ag crucibles, or Pt crucibles
above 1200? C, and crucibles sealed in evacuated silica
glass tubes; vertical quench furnace, Pt 10 Rd thermocouple
calibrated against Au and synthetic diopside standards; furn-
ace temperature readings ?2? C; liquidus and solidus tem-
peratures ?10? C.
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
FIGURE 3.?Layers of the secondary igneous assemblage along grain boundaries of altered
omphacite (black) and garnet (white). USNM 87375.
stituent not included in the synthetic system. Ompha-
cite also melts incongruently at high pressure, but
detailed phase diagrams have not been worked out.
O'Hara and Yoder (1967, fig. 8) have shown that at
30 kilobars omphacite begins to melt at about 1570?
G and is completely melted at 1600? G. The composi-
tion of first liquids from the melting of omphacite at
high pressure has not been determined, but it is
reasonable to assume that they will contain considerable
normative nepheline and albite.
Cause of Partial Melting
These kyanite eclogites clearly contained interstitial
magma that has been rapidly cooled. Two hypotheses
may be offered to account for the presence of this
magma:
1. Introduction of an externally derived melt from
the kimberlitic magma.
2. Partial melting of the primary assemblage.
We believe that the latter hypothesis is much more
likely, based on the following evidence: (1) the per-
vasive occurrence of the secondary igneous assemblage
in very thin layers along all grain boundaries, com-
bined with no more nor less of the assemblage toward
the margin of the nodules; and (2) the composition
of secondary assemblage can be accounted for by par-
tial incongruent melting of the primary omphacite.
The partial melting may have occurred in response
NUMBER 1
FIGURE 4.?Closeup view of a secondary igneous assemblage layer showing plagioclase micro-
lites with interstitial glass. The bordering material is altered omphacite. USNM 87375.
to one of the following three processes, or to some com-
bination of them:
1. Increase in temperature at constant pressure.
2. Introduction of water into the eclogite at con-
stant temperature and constant total pressure.
3. Release of pressure at constant temperature.
The merits of each of these hypotheses are discussed
in the following sections.
INCREASE IN TEMPERATURE AT CONSTANT PRES-
SURE.?This mechanism might be invoked, for ex-
ample, by considering immersion of an eclogite nodule
in kimberlitic magma. Based on the present state of
knowledge of the melting of omphacite at high and
low pressure as reviewed earlier, one cannot cate-
gorically choose between partial melting at high or low
pressure to explain the decomposition of the ompha-
cite in these kyanite eclogites, if indeed partial melting
did occur at constant pressure. Actually, the nature
of the nodules argues against this interpretation,
which assumes slow heating (and slow cooling) under
constant pressure. If the specimens had undergone
such a history, it seems likely that the partial melting
along grain boundaries would have caused disaggre-
gation of the nodule. Such disaggregation obviously
did not occur.
INTRODUCTION OF WATER AT CONSTANT TEMPERA-
TURE AND CONSTANT TOTAL PRESSURE.?The presence
of hornblende and analcite in the secondary igneous
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
FIGURE 5.?Mullite (M) and corundum (C) on the margin of a kyanite grain (K). The lines
in kyanite are twin lamellae. USNM 87375.
assemblage shows conclusively that the interstitial
magma did contain dissolved water. Indeed, the
vesicles observed in specimen 87375 show that, at some
stage in the crystallization of this magma, the water
content became high enough for the water vapor
pressure of the magma (combined with the vapor
pressure of other dissolved volatiles) to equal the
total pressure.
Introduction of small amounts of water along grain
boundaries into an anhydrous eclogite at high tem-
perature and high total pressure would either generate
a small amount of hydrous magma, or lead to the
formation of a new hydrous phase, or both. If the
temperature was sufficiently high, a melt rather than
a hydrous phase would most likely form.
Kimberlitic magmas are known to contain con-
siderable H2O and CO2. Thus water could have been
introduced into the nodule from its environs. The
low K2O and high Na2O content of the residual glass
and the pervasive occurrence of the secondary igneous
assemblage along grain boundaries indicate that, if
anything was introduced from the surrounding kimber-
litic magma, it was only water plus possibly other
volatiles. It is also possible that the small amount of
water that must be accounted for was derived inter-
nally, for recent work2 has shown that practically all
"anhydrous" silicates, including pyroxene, garnet,
and kyanite, contain some water.
1 C. Frondel, personal communication.
NUMBER 1
Water, either externally or internally derived, or
both, certainly played a role in the formation of the
interstitial magma, but at the present time there seems
to be no way to evaluate its importance.
RELEASE OF PRESSURE AT CONSTANT TEMPERA-
TURE.?As discussed earlier, at 30 kilobars omphacite
begins to melt at about 1570? C, whereas at one atmos-
phere melting begins at about 1030? C. Thus, sudden
pressure release of an eclogite at high temperature
would clearly cause partial melting of omphacite. Even
if the initial temperature was as low as 1150? C and
the pressure release large, some melting would take
place.
A rapid pressure release implies that there would
also be adiabatic cooling because of insufficient time
for the nodules to exchange heat conductively with
their environs; however, for solids the amount of
such cooling is small because of their small A V. Thus,
initially melting would occur, the amount being de-
termined by the temperature at the time of pressure
release, the magnitude of the pressure drop, and the
composition of the omphacite.
It is difficult to conceive how these eclogites could
have remained intact when each grain boundary was
"coated" with interstitial magma unless the time inter-
val between melting and quenching was short. Melting
on release of pressure would be almost immediate. The
heat required for melting would come from the eclogite
itself; that is, no conductive transfer of external heat
into the nodule would be required. Quenching then
took place almost immediately after the partial melt-
ing, by cooling in the rapidly ascending kimberlite
magma. Kimberlite magmas are generally considered
to be gas-rich and may undergo rapid cooling during
ascent because of (1) expansion of an initially present
gas phase, and (2) emission of gas from the magma
due to rapid drop in pressure. The actual importance
of this cooling, however, is difficult to evaluate.
Summary
The presence of diamond in one of the specimens
studied indicates a deep-seated origin for the kyanite
eclogite nodules. The mineralogical, textural, and spa-
tial relationships of the secondary igneous assemblage
are consistent with the view that the unusual features
observed in these nodules resulted from emplacement
into the crust in a rapidly ascending kimberlite magma,
which caused first melting due to sudden release of
pressure followed by quenching due to rapid cooling
in rising, expanding gas-rich kimberlite magma.
These nodules are, to our knowledge, the first de-
scribed examples of partial melting of very deep-seated
rocks. We do not mean to imply, however, that the
partial melting of these samples is necessarily pertinent
to the generation of magma by the partial melting of
eclogite in the upper mantle. On the contrary, the
partial melting described herein is of a very specialized
nature and may be a result of the specific but as yet
quantitatively unspecified conditions existing during
the generation, ascent, and cooling of kimberlitic
magma.
References
Deer, W. A., R. A. Howie, and J. Zusman
1963. Rock-forming Minerals. London: Longmans, Green
and Co., Ltd.
Johnson, J. P.
1907. Notes on Lherzolite and Eclogite Boulders from the
Roberts-Victor Mine. Transactions of the Geolog-
ical Society of South Africa, 10:112-114.
Melvill, E. H.
1907. Proceedings of the Geological Society of South
Africa. Transactions of the Geological Society of
South Africa, 9:68.
O'Hara, M. J., and H. S. Yoder, Jr.
1967. Formation and Fractionation of Basic Magmas at
High Pressures. Scottish Journal of Geology,
3:67-117.
Schairer, J. H., and H. S. Yoder, Jr.
1960. The Nature of Residual Liquids from Crystallization,
with Data on the System Nepheline-Diopside-Silica.
American Journal of Science, 258-A: 273-283.
Williams, A. F.
1932. The Genesis of the Diamond. London: Ernest
Benn, Ltd.
U.S. GOVERNMENT PRINTING OFFICE : 1969 O - 326-596