Martian Meteorites

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doi:10.1016/S0016-7037(03)00171-6

Petrogenesis of olivine-phyric shergottites Sayh al Uhaymir 005 and Elephant Moraine

A79001 lithology A

CYRENAANNEGOODRICH1,2,*1Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa,

1680 East-West Road, Honolulu, HI 96822, USA2Max-Planck-Institut fur Chemie, P.O. Box 3060, D-55020 Mainz, Germany

(Received July 18, 2002; accepted in revised form February 21, 2003)

AbstractMartian meteorites Sayh al Uhaymir (SaU) 005 and lithology A of EETA79001 (EET-A) belongto a newly emerging group of olivine-phyric shergottites. Previous models for the origin of such shergottiteshave focused on mixing between basaltic shergottite-like magmas and lherzolitic shergottite-like material.Results of this work, however, suggest that SaU 005 and EET-A formed from olivine-saturated magmas thatmay have been parental to basaltic shergottites.

SaU 005 and EET-A have porphyritic textures of large (up to 3 mm) olivine crystals (25% in SaU 005;13% in EET-A) in finer-grained groundmasses consisting principally of pigeonite ( 50% in SaU 005;

60% in EET-A), plagioclase (maskelynite) and 7% augite. Low-Ti chromite occurs as inclusions in themore magnesian olivine, and with chromian ulvospinel rims in the more ferroan olivine and the groundmass.Crystallization histories for both rocks were determined from petrographic features (textures, crystal shapesand size distributions, phase associations, and modal abundances), mineral compositions, and melt compo-sitions reconstructed from magmatic inclusions in olivine and chromite. The following observations indicatethat the chromite and most magnesian olivine (Fo 7470 in SaU 005; Fo 8177 in EET-A) and pyroxenes(low-Ca pyroxene [Wo 46] ofmg 7774 and augite ofmg 78 in SaU 005; orthopyroxene [Wo 35] ofmg8480 in EET-A) in these rocks are xenocrystic. (1) Olivine crystal size distribution (CSD) functions showexcesses of the largest crystals (whose cores comprise the most magnesian compositions), indicating additionof phenocrysts or xenocrysts. (2) The most magnesian low-Ca pyroxenes show near-vertical trends ofmg vs.Al2O3and Cr2O3, which suggest reaction with a magma. (3) In SaU 005, there is a gap in augite compositionbetweenmg 78 and 73. (4) Chromite cores of composite spinel grains are riddled with cracks, indicating thatthey experienced some physical stress before being overgrown with ulvospinel. (5) Magmatic inclusions areabsent in the most magnesian olivine, but abundant in the more ferroan, indicating slower growth rates for theformer. (6) The predicted early crystallization sequence of the melt trapped in chromite (the earliest phase) in

each rock produces its most magnesian olivine-pyroxene assemblage. However, in neither case is the totalcrystallization sequence of this melt consistent with the overall crystallization history of the rock or its bulkmodal mineralogy.

Further, the following observations indicate that in both SaU 005 and EET-A the fraction of solidxenocrystic or xenolithic material is small (in contrast to previous models for EET-A), and most of thematerial in the rock formed by continuous crystallization of a single magma (possibly mixed). (1) CSDfunctions and correlations of crystal size with composition show that most of the olivine (Fo 6962 in SaU005; Fo 7653 in EET-A) formed by continuous nucleation and growth. (2) Groundmass pigeonites are inequilibrium with this olivine, and show continuous compositional trends that are typical for basalts. (3) TheCSD function for groundmass pigeonite in EET-A indicates continuous nucleation and growth (Lentz andMcSween, 2000). (4) The melt trapped in olivine of Fo 76 to 67 in EET-A has a predicted crystallizationsequence similar to that inferred for most of the rock and produces an assemblage similar to its modalmineralogy. (5) Melt trapped in late olivine (Fo 64) in SaU 005 has a composition consistent with theinferred late crystallization history of the rock.

The conclusion that only a small fraction of either SaU 005 or EET-A is xenocrystic or xenolithic impliesthat both rocks lost fractionated liquids in the late stages of crystallization. This is supported by: (1) highpigeonite/plagioclase ratios; (2) low augite contents; and (3) olivine CSD functions, which show a drop innucleation rate at high degrees of crystallization, consistent with loss of liquid. For EET-A, this fractionatedliquid may be represented by EET-B. Copyright 2003 Elsevier Ltd

1. INTRODUCTION

Eighteen of the 26 meteorites believed to be Martian rocks

are classified as shergottites (Table 1). Shergottites are com-

monly divided into basaltic and lherzolitic types. The basaltic

shergottites are pyroxene-plagioclase basalts, and the lher-

zolitic shergottites are olivine-pyroxene cumulates derived

from basaltic magmas (McSween and Treiman, 1998). How-

ever, recently discovered Martian meteorites include olivine-

phyric basalts (Table 1)that appear to represent a third type of

shergottite(Goodrich, 2002).The relationship of these rocks to

the basaltic and lherzolitic shergottites is not clear. Are they

products of mixing between basaltic shergottite-like magmas* Author to whom correspondence should be addressed ([email protected]).

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Geochimica et Cosmochimica Acta, Vol. 67, No. 19, pp. 37353771, 2003Copyright 2003 Elsevier Ltd

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and lherzolitic material? Do they represent magmas that could

have been parental to both basaltic and lherzolitic shergottites?

Or are they products of entirely different magma types and/or

processes? This paper examines the petrogenesis of olivine-

phyric shergottites Sayh al Uhaymir (SaU) 005 and lithology A

of Elephant Moraine A79001 (EET-A).

1.1. Basaltic and Lherzolitic Shergottites

The basaltic shergottites Shergotty, Zagami, QUE 94201,

Los Angeles, NWA 480, NWA 856 and Dhofar 378 (Table 1),

consist predominantly of clinopyroxene (pigeonite and augite)

and plagioclase (now shock-produced glass or maskelynite),

and have basaltic or diabasic textures. Shergotty and Zagami

contain cumulus pyroxene(Stolper and McSween, 1979; Mc-

Coy et al., 1992),whereas QUE 94201 and Los Angeles (which

have higher plagioclase/pyroxene ratios) may represent magma

compositions(Kring et al., 1996; McSween et al., 1996; Rubin

et al., 2000; Mikouchi et al., 2001a; McKay et al., 2002).

Nevertheless, the absence of olivine in all basaltic shergottites,

and their low bulkmg (100 molar Mg/[Mg Fe]) values

of23 to 52(Table 1), indicate that they represent fraction-

ated, rather than primary, magmas (Stolper and McSween,

1979).

The lherzolitic shergottites ALHA77005, LEW 88516, Y

793605, YA 1075 and GRV 99027 (Table 1), consist predom-

inantly of coarse-grained olivine and poikilitic pigeonite(Mc-

Sween et al., 1979a, 1979b; Harvey et al., 1993; Ikeda, 1994a,

1997; Treiman et al., 1994a). In contrast to the basaltic sher-

gottites, they have much lower plagioclase contents and higher

bulkmg values (70), and they contain chromite. Their min-

eralogy is consistent with early accumulation from magmas

having the crystallization sequence of inferred primary basaltic

shergottitic magmas(McSween et al., 1979a, 1979b).

1.2. Olivine-Phyric Shergottites

In the context of the division of shergottites into basaltic and

lherzolitic types, EETA79001 was always an anomaly.

EETA79001 consists of two lithologies separated by an obvi-

ous contact(Steele and Smith, 1982; McSween and Jarosewich,

1983). Lithology B (EET-B) is a clinopyroxene-plagioclase

rock resembling the basaltic shergottites. Lithology A (EET-

A), however, is distinct from either the basaltic or the lher-

zolitic shergottites, and in the light of recent discoveries can be

considered to be the first known olivine-phyric shergottite. It

contains megacrysts of olivine, orthopyroxene, and chromite in

a finer-grained groundmass of pigeonite and plagioclase. Pet-

Table 1. Shergottitesa

ol (%)b Fo opx (%) pig (%) aug (%) plag (%) Oxidesc mgd Ref.

Basaltic shergottites

Shergotty 36.3 33.5 23.3 ti, il 46 1, 2Zagami 36.5 36.5 21.7 ti, il 52 1, 2EET-Be 39.5 20.0 29.1 ti, il 43 1, 2

QUE 94201 33.7 10.1 46.0 ti, il 43 3, 4Los Angeles 3844 4345 ti, il 23 5, 6NWA 480 41 31 25 ti, il 34 7NWA 856 45 23 23 ti, il 49 8Dhofar 378 49 47 ti, il 9

Olivine-phyric shergottites

EET-Ae 1013 8152 3.47.2 5563 3.26.5 1618 chr, ti, il 61 2, 10, 11DaG 476f 1424 7962 1.53.5 5053 tr2.9 1217 chr, ti, il 68 12, 13, 14SaU 005g 2129 7462 1 48 7 15 chr, ti, il 68 15, 16Dhofar 019 712 7325 5763 2627 chr, ti, il 58 17NWA 1068h 21 7242 52 22 chr, ti, il 59 18NWA 1195 8160 19

Lherzolitic shergottites

ALHA77005 60.2 7369 9.5 3.7 9.5 chr-ti, il 71 2, 20LEW 88516 45.9 7064 25.3 12.0 7.0 chr-ti, il 70 20, 21Y 793605 40.4 7564 51 7.4 chr-ti, il 70 22, 23YA 1075 7568 24GRV 99027 73 25

a Abbreviations: ol olivine; Fo forsterite; opx orthopyroxene; pig pigeonite; aug augite; plag plagioclase (maskelynite); ref. reference; EET Elephant Moraine; QUE Queen Alexandra Range; NWA North West Africa; DaG Dar al Gani; SaU Sayh al Uhaymir,ALH Allan Hills; LEW Lewis Cliffs; Y Yamato.

b Modal abundances.c chr chromite; ti titanomagnetite (chromian ulvospinel); il ilmenite; chr-ti chromite-titanomagnetite solution.d 100 molar Mg/(Mg Fe) in bulk composition.e EETA79001 lithology A and lithology B.f And possibly paired meteorites DaG 489/734/670/876. Range of Fo contents varies among specimens.g And possibly paired meteorites SaU 008/051/094/060/090. Range of Fo contents varies among specimens.h Possibly paired with NWA 1110 (26).

References: (1) McSween (1985) and references therein. (2) Banin et al. (1992) and references therein. (3) McSween et al. (1996). (4) Dreibus etal. (1996). (5) Rubin et al. (2000). (6) Mikouchi (2000). (7) Barrat et al. (2002a). (8) Jambon et al. (2002). (9) Ikeda et al. (2002). (10) Steele andSmith (1982). (11) McSween and Jarosewich (1983). (12) Zipfel et al. (2000). (13) Folco et al. (2000). (14) Mikouchi et al. (2001b). (15) Zipfel (2000)

and this work. (16) Dreibus et al. (2000). (17) Taylor et al. (2002). (18) Barrat et al. (2002b). (19) Irving et al. (2002). (20) Treiman et al. (1994a).

(21) Dreibus et al. (1992). (22) Ikeda (1997). (23) Warren and Kallemeyn (1997). (24) Yanai (2002). (25) Lin et al. (2002). (26) Goodrich et al. (2003).

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rogenetic studies of EET-A have largely focussed on modelling

it as a mixture of basaltic and lherzolitic shergottite types.

Textural and compositional characteristics of the megacrysts

suggested disequilibrium with the groundmass, which led to the

idea that they are xenolithic remnants of assimilated ultramafic

material. McSween and Jarosewich (1983) calculated that the

groundmass of EET-A could be produced by mixing 10%

olivine, 26% orthopyroxene and 0.5% chromite with a magmasimilar to EET-B. However,Wadhwa et al. (1994)showed that

the energy required to assimilate this material was more than

could plausibly be provided by latent heat of crystallization. An

alternative model discussed (McSween and Jarosewich, 1983;

McSween, 1985; Wadhwa et al., 1994) was magma mixing,

with the megacrysts originating as phenocrysts in one of the

magmas. Mittlefehldt et al. (1999), however, showed from

trace element data that the lherzolitic endmember in any mixing

model for EET-A should have contained little melt, and sug-

gested that the energy constraints of assimilation could be

satisfied by impact melting. Although no one model has been

generally accepted for the origin of EET-A, the idea that its

megacrysts are in some sense xenolithic has, and some authors(e.g.,Treiman, 1995)have even referred to them as lithology X.

DaG 476 (discovered in Libya in 1998/1999) is a porphyritic

olivine basalt consisting of large olivine crystals, lesser or-

thopyroxene, and chromite grains in a finer-grained ground-

mass of pigeonite and maskelynite(Zipfel et al., 2000),and was

thus thefirst example of a lithology like EET-A to be found as

a whole meteorite. Its general similarity to EET-A led to the

idea that its megacrysts might be xenolithic, and to discussion

of mixing models for its petrogenesis(Folco et al., 2000; Zipfel

et al., 2000; Mikouchi et al., 2001b; Wadhwa et al., 2001).

However, textural and chemical characteristics of DaG 476

make mixing models for its origin less compelling than in the

case of EET-A, and some authors (Zipfel et al., 2000) havesuggested that DaG 476 represents a previously unrecognized

type of shergottitic magma.

SaU 005 (found in Oman 1 yr after the discovery of DaG

476) is similar to DaG 476 in texture, mineralogy and mineral

compositions, bulk chemical composition, and exposure age

(Zipfel, 2000; Dreibus et al., 2000; Patsch et al., 2000). How-

ever, detailed mineralogical differences between the two me-

teorites and the large distance between the two sites at which

they were found, indicate that they are not paired (Zipfel,

2000). SaU 094 (Gnos et al., 2002) has been possibly paired

with SaU 005(Grossman and Zipfel, 2001).

Three more shergottites (Dhofar 019, NWA 1068 and NWA

1195) with mineralogical and textural similarities to EET-A,

DaG 476, and SaU 005 have recently been discovered. It is

clear that these six rocks share petrographic features that dis-

tinguish them from either of the two established shergottite

types (Table 1), and I have proposed(Goodrich, 2002)that they

be designated by the term olivine-phyric shergottite. Their

relative abundance suggests that they may not simply be mix-

tures of basaltic shergottite-like and lherzolitic shergottite-like

materials. They may instead represent distinct martian magma

types (e.g., Zipfel et al., 2000; Taylor et al., 2002), possibly

more primitive than any previously recognized (Irving et al.,

2002).In light of these new developments, it seems worthwhile

to reconsider the petrogenesis of EET-A, through a comparison

with other olivine-phyric shergottites.

This paper addresses the petrogenesis of SaU 005 and

EET-A. For each of these rocks, I examine petrographic fea-

tures (textures, crystal shapes and size distributions, phase

associations and modal abundances) and mineral (olivine, py-

roxenes and spinels) compositions to reconstruct its crystalli-

zation history. I then use magmatic inclusions in olivine and

chromite to determine the compositions of magmas that were

present at various stages of its history, and compare the crys-tallization sequences predicted for these magmas to its modal

mineralogy, mineral compostions, and inferred crystallization

sequence to test various petrogenetic models. In addition, I use

the compositions of these magmas to examine possible rela-

tionships between these olivine-phyric shergottites and the ba-

saltic and lherzolitic shergottites.

2. SAMPLES AND ANALYTICAL METHODS

Three thin sections (#s 1, 3 and 4) and one thick section of SaU 005,three thin sections (. . .,76; . . .,68; and . . .,94) of EETA79001, and one

thin section (. . .,29) of ALHA77005 were studied. Back-scatteredelectron images, X-ray maps, and quantitative analyses were obtained

using the JEOL JXA 8900RL electron microprobe at Johannes Guten-berg Universitat in Mainz, and the JEOL JSM-LV5900 scanning elec-tron microscope and Cameca SX-50 electron microprobe at the Uni-

versity of Hawaii. Operating conditions for quantitative analyses were15-keV accelerating potential and 12 to 30 nA beam current foranalyses of silicate phases, and 20-keV accelerating potential and 20to 30 nA beam current for analyses of chromite and Fe-Ti oxides.Natural and synthetic oxides and silicates were used as standards.Counting times ranged from 10 to 40 s. PAP -z corrections wereapplied to the analyses. For analyses of glasses, a defocussed beam(2-m diameter) was used wherever the size of the area permitted. Totest whether loss of alkali elements occurred in these analyses, someglasses were analyzed using 10-keV accelerating potential, 5-nA beamcurrent, and a 5-m beam diameter. These tests did not show signifi-cantly higher values for Na2O or K2O compared with 15-keV analyses.Analyses of mixed phases (designated broad-beam analyses) were

performed with a defocussed beam (25-m diameter).

3. GENERAL PETROGRAPHY AND MINERALCOMPOSITIONS

3.1. SaU 005

SaU 005 has a porphyritic texture of large olivine crystals

(commonly in clusters) in a finer-grained groundmass consist-

ing principally of low-Ca pyroxene and maskelynite (Figs. 1a

and 1c). Chromite occurs as rare inclusions in magnesian

olivine, and more abundantly with chromian ulvospinel rims as

inclusions in more ferroan olivine and discrete grains in the

groundmass. Occurrences and compositions of spinels are de-

scribed in detail in section 4.1.

The abundance of olivine was determined by automated

point-counting of X-ray maps to be 21 to 29% (by area), similar

to that (25%) reported by Zipfel (2000). Grain shapes are

commonly subhedral to euhedral (Figs. 1a and 1c). Sizes (max-

imum length and average width) were measured for all olivine

crystals (50 m) in an area (excluding shock-melted veins

and pockets) of 325 mm2 (276 crystals, yielding a density of

85 crystals/cm2). Individual crystals in clusters were distin-

guished optically, and then measurements were made on com-

bined elemental X-ray maps (e.g., Fig. 1a). Seventy-four per-

cent of all crystals have lengths 0.5 mm, with a maximum in

the distribution at 0.25 mm (Fig. 2a). For the remaining 25%,

abundance decreases regularly with increasing length up to 2

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mm. Only a few larger crystals (up to 3 mm) were observed.

A crystal size distribution (CSD) plot of ln(n) vs. length, where

n is the slope of the cumulative crystals per volume vs. length

function (Marsh, 1988), is linear with negative slope (1.74)

for crystals 0.1 to 1.5 mm in length, shows a dropoff for smaller

crystals, and is horizontal for larger crystals (Fig. 2c). Most

crystals have aspect ratios of 1 to 2; however, crystals 2 mm

in length all have aspect ratios of 2 to 3.5.

Compositions were obtained for 113 olivine crystals, com-

prising all those (50 m in size and not obviously shock

melted) in an area of 100 mm2. For every crystal, maximum

and minimum forsterite (Fo) contents were determined through

a combination of line profiles and point analyses, using Mg and

Fe X-ray maps as a guide in selecting locations for analysis. For

zoned crystals, maximum Fo contents are always located near

centers and minimum Fo contents are always located at edges.

In most cases, zonation is concentrically regular or subregular.

Forsterite contents range from 74 to 62 (Fig. 2e; Table 2).

Crystals 0.5 mm in length (74% of all crystals) are predom-

inantly Fo 65 to 63 in composition, and only slightly zoned. All

crystals 1 mm in length have minimum (edge) Fo contents

65. Most have maximum Fo contents extending only to Fo

71. Only the few crystals 2 mm in length have more

magnesian (up to Fo 74) core compositions.

Magnesian low-Ca pyroxenes (mg 7577) having Wo con-

tents consistent with orthopyroxene composition (Wo 4 6)

occur as rounded, 50 to 200 m sized inclusions in the highly

magnesian (Fo 7374) cores of the largest olivine crystals (Fig.

3a; Table 3). They have 0.5 to 1.0% Al2O3, 0.4 to 0.5%

Cr2O3 and 0.05 to 0.1% TiO2 (Fig. 4a). Magnesian augite (mg

78, Wo 34 35), with 1.8 to 2% Al2O3(Table 3), also occurs in

several of the inclusions. Low-Ti chromite grains are com-

monly associated with these pyroxenes (Fig. 3a).

The groundmass contains 48% low-Ca pyroxene and 15%

maskelynite, and has an average grain size of 130 m

(Zipfel, 2000).Augite occurs as small (100m), irregularly-

shaped grains, and was determined by automated point-count-

ing of combined elemental X-ray maps to comprise at most 7%

Fig. 1. SaU 005 (a, c) and EETA79001 (b, d). Combined elemental X-ray maps (Red Ca K, Green Fe K, Blue

Al K) in (a, b). Olivine is light to medium green, pigeonite is dark green to brown, maskelynite is blue. Augite,phosphates, and (in SaU 005 only) veins of terrestrial Ca-carbonate are red to orange. Image of EETA79001 (b) shows

contact between lithology A (top) and lithology B (bottom). Lithology B has a higher abundance of maskelynite and augiteand is coarser-grained than the groundmass of lithology A. Olivine abundance is 25% in SaU 005 and 12% in lithologyA of EETA79001 (EET-A). Larger areas of olivine in SaU 005 are clusters of two to five crystals. Note that olivines inEET-A are more strongly zoned than those in SaU 005. Collages of back-scattered electron images (BEI) in (c, d). Small,bright grains are chromite and Fe-Ti oxides.

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of the rock (this number includes whitlockite and Ca-carbon-

ates, which were not distinguished from augite). Minor phases

in the groundmass are whitlockite, pyrrhotite, pentlandite, chro-

mite, chromian ulvospinel (or titanomagnetite) and ilmenite.

Maskelynite compositions as reported by Zipfel (2000) are

An51-65Or0.3-0.9and were not investigated in this work.

Low-Ca pyroxenes in the groundmass are pigeonite, with

Wo 6 and mg 75 to 67 (Table 3). They have abundant,

shock-produced twin lamellae typical of pigeonite. No optically

or compositionally distinct cores of orthopyroxene were ob-

served, consistent with the report ofZipfel (2000). Composi-

tional trends (Fig. 4a)show generally increasing Al2O3 (start-

Fig. 2. Olivine in SaU 005 and EET-A. (a, b) Histograms of number of crystals versus maximum length. SaU 005: 276

crystals in 325 mm2

(85 crystals/cm2

). EET-A: 46 crystals in 195 mm2

(24 crystals/cm2

). (c, d) Crystal size distribution(CSD) plots, based on data in (a, b), of ln( n) vs. length, where n is dNV*/dL and NV* is cumulative number of crystals pervolume (NV NA

1.5 ). Lines are regressions through the ranges of lengths shown. (e) Maximum (central) and minimum

(edge) forsterite (Fo) contents for all crystals in an area of 100 mm2 in SaU 005. Crystals 0.5 mm in length (74% of allcrystals) are mostly Fo 65 to 63 in composition, and only slightly zoned. All crystals 1 mm in length have minimum Focontents 65. Most have maximum Fo contents Fo 71. Only the few crystals 2 mm in length have more magnesiancore compositions, up to Fo 74.



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ing from values comparable to those found in the

orthopyroxene) and Wo as mg decreases from 75 to 71, and

decreasing Al2O3 and Wo as mg decreases from 71 to 68.

Cr2O3 contents decrease and TiO2 contents increase (again,

starting from values comparable to those found in the orthopy-

roxene) over the entire compositional range, but show increasesin slope from mg 71 to 68. In addition, both Wo and Al2O3(and possibly also Cr2O3) show notable spikes (nearly vertical

trends) at the most magnesian compositions (mg 74 75),

which are similar to those reported for the most magnesian

pigeonite in DaG 476 (Zipfel et al., 2000). Augites in the

groundmass (Wo 30 34) have compositions ofmg 73 to

69, with Al2O3 contents of 1.8 to 2.5% (Table 3).

3.2. EET-A

EET-A has a porphyritic texture (Figs. 1b and 1d)with 15

vol.% megacrysts in a finer-grained groundmass (Steele and

Smith, 1982; McSween and Jarosewish, 1983).The megacrystsconsist of olivine (10 13%) and low-Ca pyroxene (2 4%)

that is referred to in the literature as orthopyroxene. Olivine and

low-Ca pyroxene megacrysts are generally isolated from one

another, but some composite grains have been observed (Mc-

Sween and Jarosewich, 1983).Low-Ti chromite grains occur as

inclusions in the megacrysts and with chromian ulvospinel rims

in the groundmass, and are commonly considered part of the

megacryst assemblage. Spinels are described in section 4.2.

The groundmass consists of pigeonite (55 63%), augite (3

6%) and maskelynite (16 18%), with minor ulvospinel, ilmen-

ite, phosphate, pyrrhotite and mesostasis (Steele and Smith,

1982; McSween and Jarosewich, 1983).The CSD function for

groundmass pyroxenes is linear with negative slope in the

range 0.1 to 0.4 mm(Lentz and McSween, 2000).

Steele and Smith (1982) and McSween and Jarosewich

(1983) described olivine grains in EET-A as having highly

irregular external forms. However, observations made in this

study show that many grains are subhedral (Figs. 1b and 1d),

and many of those with highly irregular shapes appear to have

been sheared and/or disrupted by veins of late shock melt.

Olivine sizes (maximum length) were measured for all crystals

in an area of195 mm2 (46 crystals, yielding a density of 24

crystals/cm2). The distribution peaks at 1 mm, with 57% of

crystals having lengths between 0.9 and 1.4 mm (Fig. 2b).

Sizes extend up to 2.7 mm(McSween and Jarosewich [1983]

report megacryst sizes as large as 5 mm, but it is not clear if

these are single crystals). The crystal size distribution (CSD)

function (Fig. 2d) is linear with negative slope (1.35) for

crystals 0.9 to 1.9 mm in length, shows an extreme dropoff

for smaller crystals, and is horizontal for larger crystals. Oli-

vine compositions range from Fo 81 to 53 (Steele and Smith,

1982; McSween and Jarosweich, 1983). In this study, it was

observed that the most magnesian compositions (Fo 76) are

rare and occur only in the core regions of the largest ( 1.5 mm)crystals. Most crystals are zoned from Fo 76 to 63 (similar to

crystals described bySteele and Smith, 1982). McSween and

Jarosewich (1983)andSteele and Smith (1982)emphasized the

irregularity of zonation contours, but many crystals appear

concentrically zoned. Smaller crystals tend to consist entirely

of more FeO-rich compositions.

Low-Ca pyroxene megacrysts consist of irregularly-shaped,

magnesian cores with more ferroan coronas (Figs. 3b and 3c).

Steele and Smith (1982) and McSween and Jarosewich (1983)

noted that although it is difficult to determine the structural

state of EET-A low-Ca pyroxenes from optical properties, due

to shock effects, the more Mg-rich compositions appear to be

orthorhombic. In this work it was found that the magnesiancores of low-Ca pyroxene megacrysts are commonly free of

twin lamellae, while their coronas, as well as all groundmass

low-Ca pyroxenes, show shock-produced twinning typical of

pigeonite (Fig. 3b). This distinction is correlated with differ-

ences in minor element trends. Although the cores have a

limited range of mg (80 82), they show large variations in

Al2O3 (0.4 2.1 wt.%) and Cr2O3 (0.51.1%) contents,

even within single grains, resulting in nearly vertical mg-Al2O3and mg-Cr2O3 trends (Fig. 4b). Al2O3 and Cr2O3 contents are

positively correlated, and the highest concentrations of these

elements occur near the outer edges of the grains. Wo contents

of cores are mostly 2 to 3 (consistent with orthopyroxene

compositions), though the data hint at a nearly verticalmg-Wotrend as well. Small inclusions (50 mm) of low-Ca pyroxene

observed in olivine of Fo 73 are similar in composition (mg81,

Wo 2), and also show significant variation in Al2O3and Cr2O3(Fig. 4b). In contrast, coronas around the cores and all ground-

mass low-Ca pyroxenes have less magnesian (mg 78) com-

positions and show much shallower compositional trends (Fig.

4b). Al2O3and Wo increase as mg decreases to 60, and then

decrease as mg decreases to 60. Cr2O3 decreases continu-

ously as mg decreases, but shows a slight spike at mg 60.

TiO2 contents are very low (0.05%) in the magnesian cores,

and increase continuously with decreasing mg in coronas and

groundmass low-Ca pyroxene (Fig. 4b). The most ferroan

low-Ca pyroxene analyzed in this study was mg 57, but

McSween and Jarosewich (1983) report compositions extend-

ing to mg 50. Augites in the groundmass range in compo-

sition from mg 65 to 50. The general compositional trends

observed here for low-Ca pyroxenes are consistent with those

reported by McSween and Jarosewich (1983). However, the

distinct vertical trends shown by the megacryst cores (Fig. 4b)

have not previously been described.

Section . . .,68 has an exceptionally large (5 mm) low-Ca

pyroxene megacryst, called X-14(Berkley et al., 1999; Berkley

and Treiman, 2000), with several isolated, irregularly-shaped

patches (or cores) that are more magnesian (mg 84 86) than

cores of the low-Ca pyroxene megacrysts found in most thin

sections. Data from Treiman and Berkley (private communica-

Table 2. Olivine in SaU 005.a

1b 2 3 4

SiO2 38.2 37.3 36.7 36.2Cr2O3 0.1 0.06 0.05 bd1

c

FeO 23.7 26.2 30.4 32.5MgO 37.6 35.1 31.7 29.8

MnO 0.50 0.53 0.58 0.62CaO 0.19 0.39 0.26 0.26Total 100.3 99.6 99.7 99.4

Fo 73.9 70.5 65.3 62.0

a Selected analyses showing full range of forsterite (Fo) contents.b Center of large crystal in Fig. 3a.c Below detection limit.

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Fig. 3. Orthopyroxenes in SaU 005 and EET-A. (a) Orthopyroxene (mg7775) in SaU 005 occurs only as rounded, 50to 200 m-sized inclusions (dark grey) in the most magnesian (Fo 74 71) olivine, in some cases associated with magnesian(mg 78) augite (not distinguishable at the contrast level shown). White grains are low-Ti chromite. This olivine crystal isone of the largest (seeFig. 2), and is zoned from Fo 74 in the center to Fo 65 at the edges (wingson either side are separatecrystals). Collage of BEI. (b) Megacryst consisting of magnesian (mg82 80) orthopyroxene core surrounded by pigeonitein EET-A. Crossed-polarized transmitted light. Core is free of twin lamellae, while rim shows polysynthetic twinningcharacteristic of pigeonite. (c) BEI of same grain as in (b). White dots correspond to analysis points labelled opx coresinFigure 4b.



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tion) and obtained in this work show that these cores (Wo 3 4)

have near-vertical trends in Al2O3 and Cr2O3, similar to those

of cores in the common megacrysts but offset to higher mg

(Fig. 4b).

McSween and Jarosewich (1983) designated low-Ca py-

roxenes with Wo 3 to 5 (corresponding to mg 73) as

orthopyroxene and those with Wo 5 (mg 73) as pigeonite.

In this paper, I will refer only to low-Ca pyroxene identified by

the near-vertical minor element trends shown inFigure 4b(and

commonly also by the absence of twin lamellae) as orthopy-

roxene (regardless of its present structural state). Other low-Ca

pyroxene (identified by shallower minor element trends and the

presence of abundant twin lamellae) will be referred to as

pigeonite (despite the fact that the most magnesian members

have low Wo contents consistent with orthopyroxene compo-

sitions).

4. SPINELS IN SaU 005 AND EET-A

4.1. Spinels in SaU 005

The main occurrences of spinels in SaU 005 are summarized

inFigure 5. Low-Ti chromite occurs as individual grains (type

1) included in olivine of Fo 74 to 70 composition, and in

composite grains as cores rimmed by chromian ulvospinel

(type 2) included in olivine of Fo 69 to 62 and in the

groundmass. Chromian ulvospinel also occurs as individual

grains included in olivine of Fo 69 to 62 and in the groundmass,

and as daughter crystals in melt inclusions that occur in Fo 69

to 62 olivine. In addition, a magnetite-rich spinel occurs in

micron to submicron-sized intergrowths with pyroxene in oli-

vine of all compositions.

4.1.1. Type 1 chromites

Chromites included in olivine, which are rare, are subhedral

to anhedral grains 25 to 35 m in size (e.g., Fig. 3a). They

have Cr-rich, low-Ti compositions (Fig. 6a;Table 4,analyses 1

and 2) with 1.7 to 2.0% ulvospinel (100 molar 2Ti/[2Ti Cr Al]) component, and show a small variation in Cr# (molar

Cr/[Cr Al]) from 0.77 to 0.81. Their magnetite (100

molar Fe3/[Fe3 Cr Al 2Ti]) components, calculated

from electron microprobe analyses following the method of

Carmichael (1967), are 2 t o 3 % (Fig. 7a). They show a

normal zonation trend, with fe# (molar Fe2/[Fe2 Mg])

increasing and Cr# dereasing from center to edge (Fig. 8a). The

observed variation in fe# (0.77 0.78), however, is small,

which indicates that subsolidus Fe/Mg reequilibration has oc-

curred. Olivine-spinel Fe/Mg equilibration temperatures deter-

mined from the calibration of Fabries (1979) are 820C for

the edges of the grains and 900C for their centers.

4.1.2. Type 2 composite Chromite-Ulvospinel grains

Eight composite spinel grains included in Fo 69 to 62 olivine

and eighteen occurring in the groundmass were examined in

detail. They have identical properties in the two occurrences.

Chromite cores and chromian ulvospinel rims are distinguished

from one another texturally (Figs. 9a and 9b). Cores are per-

vaded by short thin cracks (120 m long, submicron in

width), which commonly end abruptly at the core-rim bound-

ary, while rims are almost completely crack-free (though in

some cases systems of larger cracks extend through both core

and rim). This textural distinction is revealed most clearly in

Table 3. Pyroxenes in SaU 005 and EET-A.

SaU 005 EET-A

opxa augb pigc pigd pige pigf pigg augh opxi opxJ opxk pigl pigm pign

SiO2 54.3 50.9 53.8 53.7 52.3 52.5 52.5 51.3 54.9 54.3 55.2 54.7 50.8 51.3TiO2 0.07 0.18 0.10 0.11 0.18 0.28 0.52 0.58 0.04 0.08 0.05 0.05 0.42 0.51

Al2O3 0.55 2.0 0.73 0.88 1.39 1.22 0.83 1.8 0.42 2.1 0.50 0.46 1.2 0.73Cr2O3 0.43 0.91 0.47 0.43 0.54 0.56 0.34 0.83 0.52 1.0 0.64 0.54 0.49 0.28FeO 14.1 8.9 15.3 15.7 15.0 16.7 18.4 12.6 13.1 12.5 12.0 13.9 21.3 23.7

MgO 27.2 17.7 25.5 25.7 24.3 21.9 22.0 16.8 30.1 27.1 30.4 27.6 17.9 18.2MnO 0.48 0.33 0.55 0.53 0.55 0.58 0.61 0.45 0.46 0.43 0.42 0.51 0.68 0.72CaO 2.0 17.1 3.7 3.0 5.7 5.8 4.5 14.1 0.93 2.8 1.0 2.0 6.5 4.6

Na2O 0.05 0.29 0.05 0.07 0.08 0.09 0.12 0.24 0.07 0.11 0.03 0.03 0.11 0.07Total 99.2 98.3 100.2 100.1 100.0 99.6 99.8 98.7 100.5 100.4 100.2 99.8 99.4 100.1Wo 3.9 35.2 7.3 6.0 11.1 11.8 9.1 29.7 1.8 5.5 1.9 3.8 13.5 9.5

mg 77.5 78.0 74.8 74.5 74.3 70.0 68.1 70.4 80.4 79.4 81.9 78.0 60.0 57.8

a High-mg, low-Wo inclusion in Fo 74 olivine (Fig. 3a).b High-mg augite inclusion in Fo 74 olivine (Fig. 3a).c High-mg, low-Al groundmass pigeonite.d High-mg, low-Wo groundmass pigeonite.e High-mg, high-Al and high-Wo groundmass pigeonite.f Low-mg, highest-Wo, high-Al groundmass pigeonite.g Lowest-mg groundmass pigeonite.h Groundmass augite.i Lowest-Al opx, from core in Figs. 3b and 3c.J Highest-Al opx, from core in Figs. 3b and 3c.k Highest-mg opx, from core in Figs. 3b and 3c.l Low-Wo, high-mg pigeonite from corona in Figs. 3b and 3c.m Highest-Wo and -Al groundmass pigeonite.n Lowest-mg groundmass pigeonite.

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back-scattered electron images (Figs. 9a and 9b)and, except in

rare cases, is difficult to see in reflected light. Cores are anhe-

dral to subhedral, sometimes with corroded and/or embayed

shapes. Overall grain shapes are anhedral to euhedral and are

not necessarily controlled by core shapes. Rims are not always

present on all sides of the cores; i.e., cores extend to the edges

of the grains in some places (e.g., Fig. 9a lower left and right

corners) but not in others. Core sizes range from 10 to 300

m, and overall grain sizes range from 25 to 370 m.

Cores have low-Ti compositions similar to those of type 1

chromites. Three distinct patterns of core-rim zonation were

observed. Individual grains commonly show different of these

patterns (including all 3) in different profiles. In pattern 1 (Fig.

6a;Fig. 10a, profile 1; Table 4, analyses 3 and 4) cores have

nearly constant ulvospinel (2.4 0.4%), and a small range of

Cr-Al variation similar to that of type 1 chromites (Cr# de-

creasing from center to core/rim boundary). Their magnetitecontents are 1 to 3% (Fig. 7b). Fe#s are uniform within most

cores, and vary from 0.74 to 0.81 among cores (Fig. 8b). One

exceptional core shows normal fe#-Cr# zonation similar to that

of type 1 chromites.

Chromian ulvospinel rims follow a trend of ulvospinel vari-

ation (10 42%) at nearly constant Al content (Fig. 6a; Fig.

10a,profile 1; Table 4, analyses 5 and 6). Magnetite contents

(29%) and fe#s (0.77 0.85) are generally higher than

those of cores. They are zoned (from core/rim boundary to edge

of grain) with Cr# decreasing (0.81 0.75) as magnetite and

fe# increase (Figs. 7b and 8b). There is a distinct gap in

ulvospinel component (from 3.510%) between cores and

rims (Fig. 6a), and a discontinuous change in zonation trend.Pattern 2 (Fig. 6b;Fig. 10a,profile 2;Table 4,analyses 79)

differs from pattern 1 only in that cores deviate from a trend of

strict Cr-Al variation, showing slight enrichment (from center

to core/rim boundary) in ulvospinel (up to 7%) and magnetite

components. There remains a small gap in ulvospinel content

between cores and rims. Pattern 3 (Fig. 6c;Fig. 10a,profile 3;

Table 4, analyses 10 12) occurs where rims are absent and

cores extend to the edge of the grain. For grains included in Fo

69 to 62 olivine, this occurs only where the grains protrude

from their olivine hosts into the groundmass. This pattern is

characterized by smoothly increasing (from center to edge)

ulvospinel and magnetite contents, with decreasing Cr# and fe#

(Figs. 6c,7c,and8c). It is similar to that of pattern 2 cores, butat slightly higher Al contents and extending to higher ul-

vospinel contents (13%) that bridge the gap seen in patterns

1 and 2.

Superimposed on these general compositional patterns are

several other effects. Near melt inclusions, cores show strong

deviations from the patterns described above. They are depleted

in chromite and magnetite and enriched in spinel and ul-

vospinel components, and have lower fe#s (Fig. 10a,profile 4;

Table 4,analysis 13). They show a zonation pattern (Figs. 6c,

7c, 8c) that is similar to the pattern 3 trend but with the

significant exception that magnetite decreases instead of in-

creases (Fig. 11). This pattern was also observed around some

large cracks. In addition, cores show an unusual pattern of

back-scattered electron contrast, which in some grains (Fig. 9a)

has a fine lamellar structure. Quantitative analyses failed to

resolve chemical variations that might be responsible for this

pattern, but X-ray imaging suggests variations in Ti content,

possibly indicating exsolution of ulvospinel or ilmenite. Rims

have a faintly-revealedfine lamellar structure, which is prob-

ably a result of late oxidation-exsolution of ilmenite.

4.1.3. Pyroxene-Spinel intergrowths

Micron to submicron-sized pyroxene-spinel inclusions (Fig.

9e), similar to those described byZipfel et al. (2000) in DaG

476 and by Ikeda (2001)in DaG 735 (paired with DaG 476),

Fig. 4. Plots of mg vs. Al2O3, Cr2O3, Wo and TiO2 for low-Capyroxenes in (a) SaU 005 and (b) EET-A. In EET-A both the common

orthopyroxene cores and the unusual X-14 (data from this work andfrom Berkley and Treiman, private communication) show nearly ver-

tical trends in Al2O3, Cr2O3 and possibly also Wo, which are distinct

from the shallower trends shown by pigeonite (coronas on cores and ingroundmass). In SaU 005, similar spikes in Al2O3, Cr2O3 and Wo are

shown by the most magnesian groundmass pigeonite.

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occur in olivine of all compositions. They tend to be elongate

in shape, showing parallel alignment along crystallographicdirections in their hosts. In some cases the pyroxene and

chromite occur in a symplectic intergrowth. They appear to be

more abundant in the small, ferroan olivines than in the more

magnesian olivine, and as reported by Ikeda (2001) for DaG

735, also slightly larger. Analyses were obtained from only a

few of the larger inclusions, showing the pyroxene to be pi-

geonite of Wo 11 to 16 and the spinel to be Ti-poor and

magnetite-rich.

4.2. Spinels in EET-A

The main occurrences of spinels in EET-A are analogous to

those in SaU 005 (Fig. 5). Type 1 low-Ti chromites occur asinclusions in a magnesian range of olivine compositions (in this

case Fo 81 60), while type 2 composite grains of chromite

cores with chromian ulvospinel rims occur as inclusions in the

more ferroan range of olivine compositions (Fo 59 53) and in

the groundmass. Low-Ti chromite also occurs as trapped crys-

tals in melt inclusions that occur in Fo 76 to 60 olivine. In

addition, tiny pyroxene-spinel intergrowths were observed in

olivine.

4.2.1. Type 1 chromites

Chromites included in olivine are significantly more abun-

dant than in SaU 005. They are 15 to 40 m in size, with

euhedral to subhedral shapes. They have low-Ti compositions

similar to type 1 chromites in SaU 005, with nearly constant

ulvospinel content of2% (Fig. 6d) and magnetite contents of

3% (Fig. 7d). Likewise, they show limited normal zonation,

but with higher Cr#s (0.87 0.84) and fe#s (0.78 0.82),

and a slightly larger range of fe# variation (Fig. 8d). Olivine-

spinel equilibration temperatures are 950 to 1000C(Fabries,

1979).

4.2.2. Type 2 composite Chromite-Ulvospinel grains

As in SaU 005, composite spinels that occur as inclusions in

ferroan olivine and those that occur in the groundmass have

virtually identical properties. They consist of cores and rims

that are distinguished from one another texturally (Figs. 9c and

9d). Cores are pervaded by cracks that end abruptly at the

core/rim boundary, while rims are largely crack-free (as in SaU

005, systems of larger cracks can extend through both core and

Fig. 5. Occurrences of spinels and melt inclusions in SaU 005 andEET-A. Low-Ti chromite occurs as inclusions in olivine of Fo 74 to 70

in SaU 005 and Fo 81 to 60 in EET-A. Composite grains consisting oflow-Ti chromite cores with chromian ulvospinel rims, as well asindividual grains of chromian ulvospinel, occur as inclusions in the

more ferroan olivine (Fo 69 62 for SaU 005 and Fo 59 53 for EET-A)and in the groundmass in both rocks. Melt inclusions occur in low-Tichromites in all settings in both rocks. Melt inclusions occur in olivine

only of Fo 69 to 62 in SaU 005; these inclusions commonly containdaughter crystals of chromian ulvospinel, but do not contain chromite.

Melt inclusions occur in olivine only of Fo 76 to 60 in EET-A; theseinclusions commonly contain trapped crystals of low-Ti chromite,without ulvospinel. The most magnesian olivine cores in both rocks are

devoid of melt inclusions. In both rocks, olivine contains tiny exsolu-tions of pyroxene plus a magnetite-rich spinel (not illustrated).

Fig. 6. Compositions of spinels in SaU 005 and EET-A in the system

chromite (Cr) spinel (Al) ulvospinel (2Ti). (a) SaU 005. Type 1chromites (inclusions in olivine of Fo 74 70) in black. Type 2 com-posite grains of chromite cores with chromian ulvospinel rims (whichoccur as inclusions in olivine of Fo 69 62 and in groundmass),zonation pattern 1, in green. (b) SaU 005. Type 2, zonation pattern 2.

(c) SaU 005. Type 2, zonation pattern 3, in black. Reaction rims aroundmelt inclusions in type 2 chromite cores in red. (d) EET-A. Type 1chromites (inclusions in olivine of Fo 81 60) in black. Type 2 com-posite spinels (which occur as inclusions in olivine of Fo 59 52 and inthe groundmass) in green. In contrast to SaU 005, type 2 spinels in

EET-A show zonation pattern 1 only. Spinels in ALHA77005 shown inmagenta for comparison.

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rim). Cracks in the cores are more extensively developed into

branching systems, and more easily seen in reflected light than

in SaU 005. Both cores and overall grain shapes range from

anhedral to euhedral. Core sizes range from 80 to 400 m

and overall grain sizes range from 100 to 500 m.

Core-to-rim compositional zonation is also similar to that of

SaU 005, except that only pattern 1 is observed (Fig. 6d;Fig.

10b,profiles 57). Cores are similar to type 1 chromites, with

nearly constant ulvospinel of2 to 3% (Fig. 6d), magnetite of

3% (Fig. 7d; Table 4, analyses 14 and 15), and a limited

range in Cr# variation. However, Cr#s of type 2 cores are

distinctly lower than those of type 1 chromites, whereas in SaU

005 they are similar(Figs. 7dand8d). Also in contrast to SaU

005, most cores show significant variation in fe# (Fig. 8d) from

0.77 to 0.88, and are normally zoned (e.g., Fig. 10b, profile

5).

Chromian ulvospinel rims (Table 4, analyses 16 and 17)

follow a trend of ulvospinel variation (18 67%) at nearly

constant Al content (Fig. 6d;Fig. 10b,profiles 57). Magnetite

contents (4 9%) and fe#s (0.88 0.92) are higher than

those of rims in SaU 005 (Figs. 7dand 8d), and increase from

core/rim boundaries to edges of grains with decreasing Cr#

(Fig. 10b,profiles 57). The gap in ulvospinel content between

cores and rims is larger than for type 2 chromites in SaU 005

(Fig. 6); in addition, there appears to be a significant gap in

magnetite content (Fig. 7d; Fig. 10b, profiles 57).

Although cores do extend to edges of grains in places (e.g.,

Fig. 9c, bottom), no deviations in their composition such as

zonation pattern 3 in SaU 005 were observed. Furthermore,

reaction rims were not observed around melt inclusions. Some

cores contain fine lamellae of a Ti-rich phase, which (as dis-

cussed above for SaU 005) may be exsolved ulvospinel or

ilmenite. Rims commonly have a fine lamellar structure (Fig.

9d), which is probably a result of late oxidation-exsolution of

ilmenite.

4.2.3. Pyroxene-Spinel intergrowths

Micron to submicron-sized intergrowths of pyroxene and

spinel, similar to those in SaU 005, are abundant in olivine of

all compositions in EET-A. No analyses of either the spinel or

the pyroxene were obtained.

4.3. Spinels in ALHA77005

To compare spinels in SaU 005 and EET-A with spinels

known to be a cumulus phase in another shergottite, spinels in

lherzolitic shergottite ALHA77005 were examined. Those

studied were larger grains (50 200 m in size) occurring in

association with pyroxene and maskelynite (spinel grains in-

cluded in oikocrystic olivine were specifically avoided) to

provide the best analogy to type 2 spinels in SaU 005 and

Table 4. Spinels in SaU 005 and EET-A.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

SiO2 0.25 0.31 0.19 0.18 0.17 0.18 0.15 0.19 0.17 0.49 0.18 0.17 0.18 0.24 0.25 0.17 0.11TiO2 0.63 0.71 0.95 1.21 3.91 13.31 1.57 2.34 6.49 0.96 1.41 4.49 1.50 0.66 0.92 6.31 19.23

Al2O3 8.68 10.81 8.21 8.97 7.95 7.03 10.00 11.02 9.54 8.29 9.08 11.20 14.25 8.14 7.85 6.16 4.00Cr2O3 56.4 53.0 57.3 56.1 52.5 33.3 53.4 50.9 44.8 57.1 55.4 45.6 50.4 57.4 56.1 45.6 22.7

FeO 28.3 29.1 27.6 28.0 30.2 38.8 29.5 30.4 34.1 27.5 28.2 31.1 28.1 28.5 30.7 37.7 49.5MgO 4.49 4.22 4.92 5.03 5.06 5.48 4.48 4.72 4.80 4.83 5.12 5.69 5.33 4.63 3.00 2.92 3.00MnO 0.33 0.40 0.37 0.41 0.37 0.51 0.41 0.42 0.47 0.41 0.36 0.40 0.38 0.40 0.40 0.39 0.55

CaO 0.03 0.02 0.01 0.05 0.05 0.18 0.03 0.01 0.03 0.03 0.08 0.20 0.06 0.00 0.00 0.01 0.03ZnO 0.11 0.13 0.05 0.05 0.12 0.03 0.08 0.10 0.07 0.04 0.08 0.06 0.05 0.03 0.07 0.08 0.04V2O3 0.71 0.78 0.79 0.85 0.85 1.96 0.94 1.09 1.30 0.79 0.78 1.11 0.98 0.56 0.66 1.14 2.46

Tot al 9 9.9 99.5 100. 4 100.9 101.2 100.8 100.6 101.2 101.8 100.4 100.7 100.0 101.2 100.6 100.0 100.5 101. 6

Cations on the basis of 4 oxygen atoms

Si 0.009 0.011 0. 006 0.006 0.006 0.006 0.005 0.006 0.006 0.017 0.006 0.006 0.006 0.008 0.009 0.006 0. 004

Al 0.353 0.438 0. 333 0.360 0.320 0.284 0.402 0.438 0.379 0.335 0.365 0.446 0.556 0.330 0.324 0.255 0. 165Cr 1.540 1.441 1. 556 1.511 1.416 0.901 1.442 1.356 1.196 1.550 1.493 1.219 1.320 1.561 1.555 1.266 0. 629

Fe3 0.045 0.052 0. 034 0.038 0.034 0.070 0.045 0.051 0.055 0.027 0.043 0.071 0.018 0.052 0.045 0.107 0. 119Ti 0.016 0.018 0. 024 0.031 0.100 0.343 0.040 0.059 0.165 0.025 0.036 0.114 0.037 0.017 0.024 0.167 0. 507V 0.020 0.022 0. 022 0.023 0.023 0.054 0.026 0.030 0.035 0.022 0.021 0.030 0.026 0.015 0.019 0.032 0. 069

Mg 0.231 0.216 0. 252 0.256 0.257 0.280 0.228 0.237 0.241 0.247 0.260 0.286 0.263 0.237 0.157 0.153 0. 157Fe2 0.771 0.786 0. 760 0.760 0.827 1.041 0.797 0.807 0.907 0.763 0.761 0.807 0.760 0.767 0.854 1.000 1. 331Zn 0.003 0.003 0. 001 0.001 0.003 0.001 0.002 0.003 0.002 0.001 0.002 0.001 0.001 0.001 0.002 0.002 0. 001

Mn 0.010 0.012 0. 011 0.012 0.011 0.015 0.012 0.012 0.013 0.012 0.010 0.011 0.011 0.012 0.012 0.012 0. 016Ca 0.001 0.001 0. 000 0.002 0.002 0.007 0.001 0.001 0.001 0.001 0.003 0.007 0.002 0.000 0.000 0.000 0. 001

Ulvo 1.7 1.9 2.5 3.1 10.2 35.3 4.1 6.0 16.8 2.5 3.7 11.6 3.8 1.7 2.4 17.0 52.6Sp 17.9 22.3 16.9 18.3 16.2 14.6 20.4 22.3 19.4 17.1 18.5 22.7 28.2 16.7 16.4 13.0 8.6Chr 78.1 73.2 78.9 76.7 71.9 46.5 73.2 69.1 61.0 79.0 75.7 62.0 67.1 79.0 78.8 64.5 32.7

Mag 2.3 2.6 1.7 1.9 1.7 3.6 2.3 2.6 2.8 1.4 2.2 3.6 0.9 2.6 2.3 5.5 6.2fe# 0.769 0.784 0. 751 0.748 0.763 0.788 0.778 0.773 0.790 0.755 0.745 0.738 0.743 0.764 0.845 0.867 0. 895

Cr# 0.813 0.767 0. 824 0.807 0.816 0.761 0.782 0.756 0.759 0.822 0.804 0.732 0.704 0.826 0.828 0.832 0. 792

(113) SaU 005. (14 17) EET-A. (1) Type 1, center of grain. (2) Type 1, edge of grain. (3 6) Type 2, pattern 1. Points 1, 9, 10 and 14 ofprofile 1 in Fig. 10a. (79) Type 2, pattern 2. Points 1, 6 and 8 of profile 2 in Fig. 10a. (10 12) Type 2, pattern 3. Points 1, 5, 7 of profile 3 in Fig.10a. (13) Type 2, reaction rim around melt inclusion in core. Point 0 of pro file 4 in Fig. 10a. (14 17) Type 2. Points 1, 12, 13 and 18 of pro file 5in Fig. 10b.

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EET-A. None of the grains examined show extensive cracks

such as those seen in type 2 chromites in SaU 005 and EET-A,

or a textural distinction between cores and rims (Fig. 9f). Nine

grains were analyzed quantitatively. All have similar center-to-

edge zonation profiles, with no sharp core/rim distinction (Fig.

6d; Fig. 10b, profile 8). Grains have centers with high-Cr,

low-Ti compositions similar to type 1 and cores of type 2

chromites in EET-A, and zone smoothly with increasing ul-

vospinel and magnetite components, decreasing Cr#, and

slightly increasing fe#, generally following the trend shown by

rims of type 2 chromites in SaU 005 and EET-A. There is no

gap in ulvospinel content such as that seen in SaU 005 and

EET-A spinels. These compositional data are in agreement with

those reported previously byMcSween et al. (1979b)andIkeda

(1994a)for chromites occurring as inclusions in late crystalliz-

ing phases (ferroan pyroxenes and maskelynite) in

ALHA77005.

5. MELT INCLUSIONS IN OLIVINE AND CHROMITE INSaU 005 AND EET-A

5.1. General

Inclusions were identified optically and then examined by

SEM and EMPA. A few objects originally thought to be melt

inclusions were subsequently recognized, on the basis of min-

eralogy and mineral compositions, to be patches of ground-

mass. A few melt inclusions in SaU 005 that show signs of

terrestrial alteration (presence of carbonate veins and/or very

low analytical totals for glasses suggesting the presence of

H2O) were excluded from further study.

Fig. 7. Magnetite content vs. Cr# for spinels in SaU 005 and EET-A. (a) SaU 005. Type 1 chromites (inclusions in

olivine). (b) SaU 005. Type 2 composite spinels (chromite cores with ulvospinel rims), zonation patterns 1 and 2. Cores ofdifferent grains shown in different black symbols. Rims of same grains shown by same symbols in blue. (c) SaU 005. Type2 composite spinels, zonation pattern 3 (black) and reaction rims around melt inclusions (red). (d) EET-A. Type 1 chromites

in green. Type 2 composite spinels shown as in (b): cores of different grains different black symbols, rims of same grains same symbols in blue.

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Melt inclusions in olivine occur only in a limited range ofhost compositions in both SaU 005 and EET-A (Fig. 5). In SaU

005 they occur in the more ferroan olivine (Fo 69 62; average

64 2), located in the outer zones of large (500 m) crys-

tals, or near the centers of smaller crystals; the more magnesian

olivine cores (Fo 74 70) of large crystals are free of melt

inclusions. In EET-A, they occur in olivine of Fo 76 to 60; the

most magnesian (Fo 8177) cores and the more ferroan (Fo

59 53) outer zones of crystals are free of melt inclusions. In

both rocks, multiple inclusions per crystal are common.

Melt inclusions occur in both type 1 chromites and the

chromite cores of type 2 composite spinel grains in both SaU

005 and EET-A. Multiple inclusions per grain are common, and

in some cases (observed in SaU 005 only) they are concentratedin zones outlining the core near the core-rim boundary.

General properties of the inclusions are summarized inTable

5.In terms of most of these properties, inclusions in olivine in

SaU 005 are distinguished from the other three groups. All

inclusions are generally rounded (Figs. 1215). Inclusions in

olivine in SaU 005 range from 10 to 130 (average 60) m

in size. Inclusions of the other three groups are smaller: those

in chromite in both SaU 005 and EET-A average 12m, and

those in olivine in EET-A average 30 m.

All inclusions consist principally of pyroxene andglass.In

some cases the glass is homogeneous; in others it contains

blebs and/or dendrites of a nearly pure silica phase. Inclusions

Fig. 8. Fe# vs. Cr# for spinels in SaU 005 and EET-A. (a) SaU 005. Type 1 chromites (inclusions in olivine). (b) SaU

005. Type 2 composite spinels (chromite cores with ulvospinel rims), zonation patterns 1 and 2. Cores of different grainsshown in different black symbols. Rims of same grains shown by same symbols in blue. (c) SaU 005. Type 2 composite

spinels, zonation pattern 3 (black) and reaction rims around melt inclusions (red). (d) EET-A. Type 1 chromites in green.Type 2 composite spinels shown as in (b): cores of different grains different black symbols, rims of same grains samesymbols in blue.



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in olivine in SaU 005 include both cases (Fig. 12), which are

distinguished as Type I (pyroxene type 1 glass) and Type II

(pyroxene type 2 glass silica phase). In most inclusions of

the other three groups, interpyroxene areas are so small that it

is not possible to determine from BEIs whether the glass is

homogeneous (Figs. 1315). However, the presence of the

silica phase in these areas can be inferred from large hetero-

geneitites in SiO2 content, and in a few inclusions (e.g., Figs.

13b and14a)silica blebs or dendrites can be distinguished in

BEI but not resolved by EMPA.

Iron sulfide is a minor phase in inclusions of all groups.

Minor phosphate and Fe-Ti oxides (ulvospinel or ilmenite)

occur in Type II inclusions in olivine in SaU 005. Inclusions in

olivine in EET-A contain grains of low-Ti chromite (Figs. 14b

and 14d)whose large size precludes crystallization as a daugh-

ter mineral (that is, if integrated into the trapped melt compo-

Fig. 9. BEI of spinels in SaU 005, EET-A and ALHA77005. (a, b) SaU 005. Type 2 composite grains of low-Ti chromitecores with chromian ulvospinel rims, which occur as inclusions in olivine of Fo 69 to 62 and in groundmass. Melt inclusions

(round, black) in cores. Numbered lines correspond to compositional profiles shown in Figure 10a.(c, d) EET-A. Type 2composite grains of low-Ti chromite cores with chromian ulvospinel rims, which occur as inclusions in olivine of Fo 59 52and in groundmass. Melt inclusion (round, black) in core in (d). Numbered lines correspond to compositional profiles showninFigure 10b.(e) SaU 005. Micron to submicron-sized exsolutions of magnetite-rich spinel and pyroxene in olivine. (f)ALHA77005. Numbered line corresponds to compositional profile shown in Figure 10b.

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Fig. 10. Representative center-to-edge compositional profiles for spinels in SaU 005, EET-A, and ALHA77005. (a) SaU005, type 2 composite spinels (chromite cores with ulvospinel rims). Profiles 1 and 4 show zonation pattern 1. Profiles 2and 3 show zonation patterns 2 and 3 respectively. Profile 4 begins at a melt inclusion, others do not. (b) type 2 compositespinels (chromite cores with ulvospinel rims) in EET-A, and spinel in ALHA77005. Positions of some pro files marked on

BEI inFigure 9.Ulvospinel 100 molar 2Ti/(2Ti Cr Al); magnetite 100 molar Fe3

/(Fe3

2Ti Cr Al); Cr# molar Cr/(Cr Al); fe# molar Fe2/(Fe2 Mg).



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sition they would result in a melt with an unrealistically high

Cr2O3 content) and so were probably trapped as solid grains

along with melt. One inclusion in chromite in SaU 005 contains

a grain of olivine, which can be identified from its composition

(see below) as a daughter mineral.

Point-counting showed that inclusions in olivine in SaU 005

contain 30 to 60 vol.% pyroxene, which occurs as thin rims and

skeletal/dendritic crystals (Fig. 12). Inclusions in the other

three groups contain 50 to 90 (on average 70) vol.% pyrox-

ene, which occurs as thick rims and/or massive/blocky crystals

(Figs. 1315)and only rarely as skeletal/dendritic crystals (e.g.,

Figs. 13cand 15a).

5.2. Compositions

5.2.1. Inclusions in Olivine in SaU 005

Pyroxenes in inclusions in olivine in SaU 005 have high Wo

(44 56), Al2O3 (715%) and TiO2 (1.13.6%), and low

Na2O (0.3 0.07%) and Cr2O3(0.15 0.1%) contents (Fig.

16). They contain significant amounts of phosphorus (up to 2%

P2O5), which from structural formula calculations appears to be

substituting for Si (as in other cases of pyroxenes crystallized in

small closed systems; e.g.,Goodrich, 1984).Their Fe/Mg ratios

average 0.6 to 0.7 (it is evident from BEIs that they are

normally zoned, but the crystals are so small that the full rangeof zonation was not recorded in the analyses). Pyroxenes in

Type I inclusions show slightly less compositional variation

than those in Type II inclusions (Fig. 16). This apparent dif-

ference may be only a result of the smaller number of analyses

for Type I inclusions (25 vs. 83). Average compositions for

Type I and Type II pyroxenes are given in Table 6. The Type

II average is based on only the 52 analyses that have excellent

cation totals. The Type I average includes analyses of slightly

lesser quality, all of which show slight excesses of SiO2that are

probably due to overlap with surrounding glass. Aside from the

effect of this, and a slight difference in Fe/Mg ratio, the two

averages show only small differences that do not appear to be

significant.Glasses in Type I inclusions (type 1 glass) in olivine in SaU

005 show a relatively homogeneous composition (Fig. 16,

Table 6), both within and among inclusions. It contains 68%

SiO2, 17.5% Al2O3, and 8% CaO, and has an extremely low

Cr2O3(0.02%) content. FeO and MgO contents are also very

low (2.0 and 0.2%, respectively), and Fe/Mg ratios range

from 2 to 24 (with large errors). The only significant variation

it shows is in Na2O content, which ranges from 1.5 to 3.2%

(inclusion averages). This variation is not an analytical artifact

(see section 2), and most likely results from variable degrees of

late volatile loss (shock-induced?). The highest observed value

(3.2%) is taken to be the best estimate of Na2O content before

this loss (Table 6).

Analyses of glass (type 2 glass) and silca-rich blebs in Type

II inclusions together form extensive compositional trends that

pass through the composition of type 1 glass (Fig. 16). Broad-

beam analyses intended to sample mixes of type 2 glass and the

silica-rich phase also fall on these trends and cover nearly the

same range of compositions. It is inferred from these trends that

type 2 glass and silica-rich blebs have a complementary rela-

tionship, which can be described by the equation: type 1 glass

70 to 80% type 2 glass 20 to 30% silica phase.

Most analyses of the silica-rich blebs probably have some

overlap with glass, as the blebs are generally small, and the

pure silica phase is probably represented only by the most

silica-rich analyses (95% SiO2, 5% Al2O3). Jagoutz (1989)

Fig. 11. Comparison between zonation pattern 3 and reaction rims

around melt inclusions for cores of type 2 composite spinels in SaU005. The two patterns are similar in showing enrichment of Al and Ti,

and depletion of Cr. In addition, both show an increase in fe#. Theydiffer in that reaction rims around melt inclusions show a depletion inmagnetite component, while pattern 3 shows an enrichment. Bothpatterns appear to result from reaction of low-Ti chromite cores with

evolved liquid. Ulvospinel 100 molar 2Ti/(2Ti Cr Al Fe3); magnetite 100 molar Fe3/(2Ti Cr Al Fe3);chromite 100 molar Cr/(2Ti Cr Al Fe3); spinel 100 molar Al/(2Ti Cr Al Fe3).

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andHarvey et al. (1993)found evidence that a similar phase in

melt inclusions in shergottites ALHA77005 and LEW 88516 is

cristobalite. Variation in the composition of type 2 glass prob-

ably results from differences in the amount of silica phase

formed (rather than overlap) because analyses of glass within

each inclusion are fairly uniform (with the exception of two

inclusions [e.g.,Fig. 12d]in which theglassitself appears to

be a very fine-grained mixture of glass silica phase).

The only element for which the above relationship does not

always hold is Ca, which is too low in glasses of some Type II

inclusions (Fig. 16). This appears to be due to the presence in

these inclusions of small patchy areas containing feathery crys-

tallites and having high CaO contents, which suggests that

incipient crystallization of further pyroxene in these areas de-

pleted the remaining glass in Ca.Based on these observations it is concluded that, to first-

order, all inclusions in olivine in SaU 005 represent a single

trapped liquid composition and crystallized approximately the

same amount of pyroxene, leaving a residual liquid represented

by type 1 glass. In Type II inclusions this liquid further sepa-

rated out a silica-rich phase, and sometimes crystallized a small

amount of additional pyroxene.

5.2.2. Inclusions in Chromite in SaU 005

Both pyroxenes and glasses in inclusions in chromite in SaU

005 (Fig. 17)are compositionally distinct from those in inclu-

sions in olivine. Pyroxenes have a broader range of lower Wo

contents (8 43, avg. 34), significantly lower Al2O3(1.76.6%) and TiO2 (0.21.9%), higher Cr2O3 (1.0 2.7%), and

relatively uniform lower Fe/Mg ratios (0.47 0.1). They

contain similar amounts of phosphorus, which again appears to

be substituting for Si.

Most glasses are heterogeneous and analyses show com-

positional trends (Fig. 17)indicating that they are mixes of true

glass and a silica-rich phase similar to that in inclusions in

olivine (in one inclusion silica blebs can be seen in BEI, but

areas of silica and glass cannot be resolved by EMPA: Fig.

13b). All have SiO2contents greater than or equal to that (68%)

of type 1 glass in inclusions in olivine in SaU 005 and extend-

ing to nearly pure analyses of the silica-rich phase (95% SiO2),

suggesting that this phase is more abundant than in inclusions

in olivine and that the bulk glass has higher SiO2. This is

supported by averages ofglass analyses for inclusions from

which several analyses were obtained, which show 74 to 75%

SiO2(Table 6,column 6), and also by a high modal abundance

of silica blebs in the one inclusion in which they can be seen.

In one exceptionally large inclusion (Fig. 13a) the glass is

homogeneous, with 74% SiO2, (Table 2, column 7), and may

therefore be a good representative of the bulk glass composi-

tion for all inclusions. In addition to having higher SiO2 than

type 1 glass in inclusions in olivine, it has lower Al2O3(16.5%), CaO (2.4%) and Na2O (2.6%), higher Cr2O3 (0.5

0.6%), and much lower Fe/Mg (1.4). Broad-beam analyses of

the inclusions are consistent with mixes of the observed py-

roxenes and glasses (Fig. 17).The one grain of olivine that occurs in an inclusion in

chromite can be identified as a daughter mineral (rather than a

grain of primary olivine trapped along with melt) because it

contains significant phosphorus (0.75% P2O5) that appears to

be substituting for Si (see Goodrich, 1984).

5.2.3. Inclusions in Olivine in EET-A

Although melt inclusions were observed in olivine of Fo 76

to 60 in EET-A, most of the data were obtained from inclusions

in Fo 76 to 67. Of the inclusions in Fo 67 to 60 host compo-

sitions, only two (both of which are in Fo 60) were large

enough to yield usable data.

Compositions of the majority of pyroxenes in these inclu-

sions are similar to those of pyroxenes in inclusions in chromite

in SaU 005 in Al2O3, TiO2, Na2O and P2O5 contents (Figs. 17

and 18), and in this regard are distinct from pyroxenes in

inclusions in olivine in SaU 005 (the only analyses which fall

in the compositional range of pyroxenes in olivine in SaU 005

are those from the two inclusions in Fo 60 olivine). However,

they show a bimodal distribution of low-Ca (Wo 313) and

high-Ca (Wo 40 48) compositions (one analysis with interme-

diate Wo may be a result of overlap), and have low Cr 2O3contents similar to pyroxenes in inclusions in olivine in SaU.

Their Fe/Mg ratios are relatively homogeneous (0.40 0.10).

Glasses in most inclusions are also similar toglasses in

Table 5. General properties of melt inclusions in SaU 005 and EET-A.

SaU 005 in olivine (Fo 6962)a

SaU 005 in low-Ti chromite

(Fo 7470)bEET-A in olivine

(Fo 7660)aEET-A in low-Ti chromite

(Fo 8160)bType I Type II

Number 8 17 23 13 12Size (avg.) 1590 (50) m 10130 (60) m 370 (12) m 2050 (30) m 530 (12) m

Daughter phases High-Ca pyx.Type 1 glass. High-Ca pyx.Type 2 glass.Silica phase.

Low to high-Ca pyx.Glass.Silica phase.

Low & high-Ca pyx.Glass.Silica phase.

Intermediate-Ca pyx.Glass.Silica phase.

Minor phases Fe-sulfide. Fe-sulfide.Phosphate.

Ti-Fe oxide.

Fe-sulfide. Fe-sulfide.Low-Ti chromite.

Fe-sulfide.

Pyx morphology Thin rims.Skeletal.

Dendritic.

Thin rims.Skeletal.

Dendritic.

Massive.Blocky.

Rarely skeletal.

Thick rims.Massive.Blocky.

Massive.Blocky.

Rarely skeletal.

Vol.% pyx (avg.) 3060 (40) 3060 (40) 5080 (70) 5080 (70) 5090 (70)

a Range of olivine compositions in which melt inclusions occur.b Range of compositions of olivine containing low-Ti chromites (without ulvospinel-rich rims).



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inclusions in chromite in SaU 005 (Figs. 17and18), and appear

to be mixes of a silica-rich phase and silica-depleted glass.

Their SiO2 contents are, again, greater than or equal to that of

type 1 glass in inclusions in olivine in SaU 005, suggesting that

the bulk glass has higher SiO2 than that glass. Unfortunately,

no inclusions appear to contain homogeneous glass. The best

estimate of the bulk glass composition, given by the average of

analyses from a typical inclusion, is 77% SiO2, 15% Al2O3and 1.5% CaO (Table 7, column 2). The two inclusions that

occur in Fo 60 olivine are exceptional in that they contain

silica-rich blebs and areas of silica-depleted glass (SiO2

68%) large enough to resolve by EMPA (Fig. 14d). Both show

a low abundance of the silica phase, indicating bulk glass

compositions with lower SiO2 than other inclusions.

5.2.4. Inclusions in Chromite in EET-A

Only a few analyses of discrete pyroxenes and glasses

were obtained for inclusions in chromite in EET-A. Both are

similar to those in inclusions in chromite in SaU 005 (Fig. 18),

though pyroxenes show only intermediate Wo contents (17

36). Glass analyses show a large range in Si/Al ratio, and

clearly reflect extremely unrepresentative sampling of silica-

depleted glass and the silica-rich phase. Unfortunately, no

single inclusion provided a plausible sample of bulk glass. The

compositions of these inclusions are also represented by broad-

beam analyses, which are consistent with mixes of the observed

pyroxenes and glasses (Fig. 18).

5.3. Present Bulk Compositions (pbcs)

The present bulk composition (pbc) of an inclusion is defined

to be the bulk silicate composition of its presently visible

portion. Pbcs were constructed from the petrographic observa-

tions described above (ignoring minor phases such as phos-

phates) and examined for consistency between their predicted

early crystallization sequences as determined by MAGPOX

(Longhi, 1991)and the observed mineralogy (principally py-

roxene types) of the inclusions.


As discussed above, it appears that Type I and Type II

inclusions in olivine in SaU 005 have the same pbc, which can

be calculated simply as 40 vol.% average type 1 glass 60

vol.% average pyroxene, using the highest observed Na2O

content for type 1 glass and the average pyroxene from Type II

inclusions (Table 6). Glass and pyroxene volume proportions

Fig. 12. BEI of inclusions in olivine (Fo 69 62) in SaU 005. (a-b) Type I inclusions consisting of pyroxene (thin rimsand skeletal crystals) and homogeneous glass. Bright spherule in (b) is iron sul fide. (c-d) Type II inclusions consisting ofpyroxene (thin rims and skeletal crystals), a silica-rich phase (black blebs and dendrites), and silica-depleted glass. In (d)the glass appears to contain fine quench needles of the silica-rich phase.

3752 C. A. Goodrich


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were weighted by reasonable estimates of density (pyx 3.3;

glass 2.3) to give weight proportions. The resulting compo-

sition is given inTable 6(column 4) and shown in the Olivine

Quartz-Plagioclase (Ol-Qtz-Plag) phase system in Figure 19a.

It is saturated only with augite (not visible in Fig. 19a), con-

sistent with the presence of high-Ca pyroxene as the only

daughter phase in the inclusions.

5.3.2. Inclusions in Chromite in SaU 005

The pbc of inclusions in chromite in SaU 005 can be calcu-

lated as 70 vol. % average pyroxene 30 vol. % bulk glass.

Two possible compositions for the bulk glass were used, yield-

ing two possible pbcs.Bulk glass1 (Table 6,column 6) is an

average of mixed analyses of glass silica phase from a

typical inclusion. Bulk glass 2 (Table 6, column 7) is the

homogeneous glass in the large inclusion shown inFigure 13a.

Bulk glass 1 has slightly higher SiO2 and lower Al2O3 than

bulk glass2; otherwise they are similar. Volume proportions

were weighted by densities, using the values given above, to

yield weight proportions. Pbc1 and Pbc2 are given in Table 6

(columns 8 and 9) and shown in the Ol-Qtz-Plag system in

Figure 19b. A third pbc estimate (pbc3) is provided by the

average of all broad-beam analyses (Table 6, column 10). It

appears from this average that broad-beam analyses sampled

less pyroxene than the average abundance indicated by point-

counting, and in fact this composition can be satisfactorily

modelled as a mixture of52 vol.% average pyroxene 48

vol.% average glass. Pbc1 and Pbc2 are saturated only with

pigeonite and evolve to pigeonite-augite cosaturation, consis-

tent with the presence of only intermediate- and high-Ca py-

roxenes in the inclusions. In contrast, pbc3 is orthopyroxene-saturated. Therefore pbc1 and pbc2 appear to be more accurate

estimates of the bulk composition of these inclusions and will

be used for the reconstruction of the primary trapped liquid

(PTL) composition (with pbc3 providing an estimate of the

uncertainty in CaO content). These compositions differ signif-

icantly from the pbc of inclusions in olivine, in having lower

Al2O3, CaO, Na2O and TiO2 contents, and higher Si/Al ratios

(Table 6).


Because the inclusions in olivine in EET-A occur in a broad

range of host compositions (Fo 76 60), it seems likely that

Fig. 13. BEI of inclusions in chromite in SaU 005. All inclusions consist of pyroxene and glass. Pyroxene occurspredominantly as massive or blocky crystals, and only rarely (c) as skeletal/dendritic crystals. The glassis heterogeneousin SiO2content and in most cases appears to be a mixture of a silica-rich phase and silica-depleted glass. In rare cases (b),the silica-rich phase can be distinguished in BEI (contrast of this image has been enhanced to show this). The inclusion in

(a) is exceptionally large, and unusual in that the glass is homogeneous. This chromite grain is the same one as shown inFigure 9a.Pit in inclusion in (c) is SIMS analysis spot.



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they sample an evolving melt and should have a range of pbcs.

However, in general the data obtained here are not adequate to

distinguish differences in bulk composition among the inclu-

sions because no single inclusion is completely sampled.

Therefore, only an average pbc can be obtained. There is,

however, direct evidence for distinct bulk compositions is that

the two inclusions that occur in Fo 60 have signi ficantly more

aluminous pyroxenes and bulk glasses than those in Fo 76 to 67

(consistent with their representing a more evolved melt). There-

fore, because the interest here is primarily in the earliestmagma, which would have been preserved most closely in the

more magnesian olivine, these two inclusions have been elim-

inated from the data used to calculate the pbc.

The pbc is calculated as 70 vol.% average pyroxene 30

vol.% bulk glass, weighted by the densities used above (Table

7). The resulting composition (Fig. 19c)is saturated only with

orthopyroxene and evolves to orthopyroxene-augite cosatura-

tion, consistent with the bimodal distribution of low- and

high-Ca pyroxene compositions in the inclusions. It is similar

to the pbc of inclusions in chromite in SaU 005 in having low

Al2O3and high Si/Al compared to inclusions in olivine in SaU

005.

5.3.4. Inclusions in Chromite in EET-A

An estimate of the pbc (pbc1) of these inclusions is made as

70 vol. % average pyroxene 30 vol. % average glass (Table

7;Fig. 19c). The average of all broad-beam analyses (Table 7;

Fig. 19c) provides another estimate (pbc2). Pbc2 has higher

Al2O3 (and lower Si/Al) than pbc1, and also (as was the case

for inclusions in chromite in SaU 005) appears fromFigure 19c

to have a smaller pyroxene (mafic) component. However, in

this case, increasing its pyroxene content (by addition of py-roxene from inclusions in chromite) cannot result in a compo-

sition like pbc1, because its CaO content is already too high

(Table 7). Therefore, the PTL will be calculated from pbc1,

taking the upper limit on Al2O3 content from pbc2, and the

lower limit on CaO content from the lowest pyroxene/glass

ratio (60 vol.% pyroxene) that would still result in a pigeo-

nite-saturated composition (these limits are shown as an ellipse

inFig. 19c). The pbc of inclusions in chromite is clearly similar

to that of inclusions in olivine in EET-A and inclusions in

chromite in SaU 005 in having low Al2O3 and high Si/Al

compared to the pbc of inclusions in olivine in SaU 005 (Tables

6and 7; Fig. 19).

Fig. 14. BEI of inclusions in olivine (Fo 76 60) in EET-A. Inclusions consist of pyroxene and glass.The glass isheterogeneous in SiO2content and appears to be a mixture of a silica-rich phase (contrast in [a] has been enhanced to showthis) and silica-depleted glass. Pyroxene occurs predominantly as thick rinds or massive/blocky crystals, and only rarely as

skeletal/dendritic crystals. Bright grains in (b) and (d) are low-Ti chromite. The inclusion in (d) is one of two exceptional

inclusions (both in Fo 60 olivine) in which blebs of the silica-rich phase are large enough to analyze. These two inclusionshave notably less pyroxene and more aluminous compositions than those in more magnesian olivine.

3754 C. A. Goodrich


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5.4. Primary Trapped Liquid (PTL) Compositions

5.4.1. General

Primary trapped liquid compositions are reconstructed from

pbcs by addition of that portion of the surrounding host mineral

that crystallized from the trapped melt onto the original walls of

the inclusions, plus corrections for any chemical exchange

reactions that occurred between inclusions and their hosts.

Crystallization of the host mineral as the sole wall phase occurs

naturally, because a primary trapped melt must be saturated

with the host mineral. The host offers a ready nucleation site,

and nucleation of other phases is commonly suppressed. Melts

trapped in olivine, for example, tend to crystallize large vol-

umes of wall olivine in excess of the equilibrium amount. In all

but the most rapidly quenched cases, this olivine quickly re-

equilibrates Fe and Mg with its host (or even with the larger

body of magma surrounding the host crystal) and concommi-

tant reequilibration of the residual melt in the inclusion se-

verely depletes it in FeO (Danyushevsky et al., 2000; Gaetani

and Watson, 2000). No other important exchange reactions

occur, as olivine does not accommodate significant quantitites

of any other cations, and hence serves as a relatively impervi-

ous container. Thus, reconstruction of the PTL for an inclusion

in olivine involves only addition of olivine and exchange of Mg

for Fe. The final Fe/Mg ratio is defined by the requirement of

equilibrium with olivine of host composition. The amount of

olivine to be added, however, can only be determined if (1) the

FeO content of the PTL is known by independent means

(Danyushevsky et al., 2000) or (2) an additional constraint,

such as co-saturation of the PTL with a second phase, is

available.

In contrast, for melt inclusions in chromite crystallization of

the host phase onto inclusion walls is negligible (Kamenetsky,

1996),being limited by the low solubility of Cr2O3 in spinel-

saturated basaltic melts(Roeder and Reynolds, 1991). Hence,

for quenched inclusions the pbc essentially preserves the com-position of the PTL. Commonly, however, sufficiently rapid

cooling does not occur, and there can be Fe/Mg exchange

between inclusions and their hosts. In addition, if water is

present in the melt (even in minute quantities) a closed-system

reaction will cause oxidation of FeO in the melt (2FeO H2O

3Fe2O3 H2) and exchange of this Fe for Cr and/or Al in the

spinel (Fe2O3 7 Cr2O3, Al2O3), hence depleting the inclusion

in FeO and enriching it in Cr2O3 and/or Al2O3 (Zlobin et al.,

1990).This reaction generally leaves no record (e.g., zonation)

in the chromite, which is effectively an infinite reservoir.

These effects must be reversed to derive PTL compositions.

The proper Fe/Mg ratio can be determined by the require-

Fig. 15. BEI of inclusions in chromite in EET-A. Inclusions consist of pyroxene and glass.Theglassis heterogeneousin SiO2 content and appears to be a mixture of a silica-rich phase and silica-depleted glass. Pyroxene occurs as thick rinds

and massive/blocky crystals, and only rarely as skeletal/dendritic crystals (a). Contact between chromite core and ulvo spinelrim can be seen in (b).



22/38

ment of equilibrium with olivine inferred to have cocrystal-

lized with the host chromites. The amount of FeO that has

been lost can be determined from the constraint of olivine

saturation and/or estimated from the Al2O3 and Cr2O3 con-

tents of the inclusions (it can be assumed that latter should

be zero, and the former may be known by independent

means).


The Fe/Mg ratio of these inclusions is low (0.73, implying

equilibrium with olivine of Fo 81) indicating that they have

reequilibrated with their hosts, and so must be corrected in the

reconstruction of the PTL. The choice offinal Fe/Mg ratio is

based on the average host olivine composition (Fo 64 2;

Fe/Mg 0.56), and the PTL is defined to have Fe/Mg 1.6

(KDol/liq 0.36). The amount of olivine to add is determined

by the requirement that the PTL be cosaturated with olivine and

at least one other phase, which is reasonable because the host

olivines are relatively Fe-rich and the groundmass contains

pyroxenes of compositions appropriate for cocrystallization

with such olivine. It can be seen from Figure 19athat the PTL

must be close to cosaturation with olivine, low-Ca pyroxene

Fig. 16. Compositions of pyroxenes, glasses, and silica-rich phase in inclusions in olivine in SaU 005. All oxides in

wt.%.

3756 C. A. Goodrich


23/38

and plagioclase, and inspection of the system orthopyroxene-

plagioclase-wollastonite (Opx-Plag-Wo) shows that it must be

close to cosaturation with augite, low-Ca pyroxene and plagio-

clase. Addition of 20 mol.% olivine results in a composition

which (after correction of Fe/Mg) is co-saturated with olivineand pigeonite (Wo 9.6, mg 69) and within a few percent of

crystallization of being saturated with augite. This PTL is given

inTable 8,and is shown in the Ol-Plag-Qtz system in Figure 20

and the Opx-Plag-Wo system in Figure 21.


The Fe/Mg ratio of these inclusions is also low (0.38, im-

plying equilibrium with olivine of Fo 88 89), indicating that

they have reequilibrated with their hosts, and so must be

corrected. In this case, the PTL is assigned an Fe/Mg ratio

(0.83) based on equilibrium with the most magnesian olivine

(Fo 76) in which the inclusions occur, to yield an approxima-

tion to the earliest magma they represent. The amount of

olivine to add is determined by the requirement that the PTL be

cosaturated with olivine and low-Ca pyroxene, which is appro-

priate because of the presence in EET-A of low-Ca pyroxene

that appears to be in equilibrium with Fo 76 olivine. Addition

of20 mol.% olivine results in a composition cosaturated with

olivine of Fo 76 and low-Ca pyroxene (Wo 4) ofmg 78.

This PTL is given in Table 8and shown in Figures 20and21.

A previous estima

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