Petrogenesis of olivine-phyric shergottites Sayh al Uhaymir 005 and Elephant Moraine A79001 lithology A

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1 Pergamon doi: /s (03) Geochimica et Cosmochimica Acta, Vol. 67, No. 19, pp , 2003 Copyright 2003 Elsevier Ltd Printed in the USA. All rights reserved /03 $ Petrogenesis of olivine-phyric shergottites Sayh al Uhaymir 005 and Elephant Moraine A79001 lithology A CYRENA ANNE GOODRICH 1,2, * 1 Hawaii 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, USA 2 Max-Planck-Institut für Chemie, P.O. Box 3060, D Mainz, Germany (Received July 18, 2002; accepted in revised form February 21, 2003) Abstract Martian meteorites Sayh al Uhaymir (SaU) 005 and lithology A of EETA79001 (EET-A) belong to a newly emerging group of olivine-phyric shergottites. Previous models for the origin of such shergottites have 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 that may 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 the more magnesian olivine, and with chromian ulvöspinel rims in the more ferroan olivine and the groundmass. Crystallization histories for both rocks were determined from petrographic features (textures, crystal shapes and size distributions, phase associations, and modal abundances), mineral compositions, and melt compositions reconstructed from magmatic inclusions in olivine and chromite. The following observations indicate that the chromite and most magnesian olivine (Fo in SaU 005; Fo in EET-A) and pyroxenes (low-ca pyroxene [Wo 4 6] of mg and augite of mg 78 in SaU 005; orthopyroxene [Wo 3 5] of mg in EET-A) in these rocks are xenocrystic. (1) Olivine crystal size distribution (CSD) functions show excesses of the largest crystals (whose cores comprise the most magnesian compositions), indicating addition of phenocrysts or xenocrysts. (2) The most magnesian low-ca pyroxenes show near-vertical trends of mg vs. Al 2 O 3 and Cr 2 O 3, which suggest reaction with a magma. (3) In SaU 005, there is a gap in augite composition between mg 78 and 73. (4) Chromite cores of composite spinel grains are riddled with cracks, indicating that they experienced some physical stress before being overgrown with ulvöspinel. (5) Magmatic inclusions are absent in the most magnesian olivine, but abundant in the more ferroan, indicating slower growth rates for the former. (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 total crystallization sequence of this melt consistent with the overall crystallization history of the rock or its bulk modal mineralogy. Further, the following observations indicate that in both SaU 005 and EET-A the fraction of solid xenocrystic or xenolithic material is small (in contrast to previous models for EET-A), and most of the material in the rock formed by continuous crystallization of a single magma (possibly mixed). (1) CSD functions and correlations of crystal size with composition show that most of the olivine (Fo in SaU 005; Fo in EET-A) formed by continuous nucleation and growth. (2) Groundmass pigeonites are in equilibrium with this olivine, and show continuous compositional trends that are typical for basalts. (3) The CSD function for groundmass pigeonite in EET-A indicates continuous nucleation and growth (Lentz and McSween, 2000). (4) The melt trapped in olivine of Fo 76 to 67 in EET-A has a predicted crystallization sequence similar to that inferred for most of the rock and produces an assemblage similar to its modal mineralogy. (5) Melt trapped in late olivine (Fo 64) in SaU 005 has a composition consistent with the inferred late crystallization history of the rock. The conclusion that only a small fraction of either SaU 005 or EET-A is xenocrystic or xenolithic implies that both rocks lost fractionated liquids in the late stages of crystallization. This is supported by: (1) high pigeonite/plagioclase ratios; (2) low augite contents; and (3) olivine CSD functions, which show a drop in nucleation rate at high degrees of crystallization, consistent with loss of liquid. For EET-A, this fractionated liquid 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 commonly divided into basaltic and lherzolitic types. The basaltic * Author to whom correspondence should be addressed (cyrena@ higp.hawaii.edu) shergottites are pyroxene-plagioclase basalts, and the lherzolitic shergottites are olivine-pyroxene cumulates derived from basaltic magmas (McSween and Treiman, 1998). However, recently discovered Martian meteorites include olivinephyric 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

2 3736 C. A. Goodrich Table 1. Shergottites a ol (%) b Fo opx (%) pig (%) aug (%) plag (%) Oxides c mg d Ref. Basaltic shergottites Shergotty ti, il 46 1, 2 Zagami ti, il 52 1, 2 EET-B e ti, il 43 1, 2 QUE ti, il 43 3, 4 Los Angeles ti, il 23 5, 6 NWA ti, il 34 7 NWA ti, il 49 8 Dhofar ti, il 9 Olivine-phyric shergottites EET-A e chr, ti, il 61 2, 10, 11 DaG 476 f tr chr, ti, il 68 12, 13, 14 SaU 005 g chr, ti, il 68 15, 16 Dhofar chr, ti, il NWA 1068 h chr, ti, il NWA Lherzolitic shergottites ALHA chr-ti, il 71 2, 20 LEW chr-ti, il 70 20, 21 Y chr-ti, il 70 22, 23 YA GRV 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 ulvöspinel); 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 et al. (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 and Smith (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). 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 olivinephyric shergottites Sayh al Uhaymir (SaU) 005 and lithology A of Elephant Moraine A79001 (EET-A) 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 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 bulk mg ( 100 molar Mg/[Mg Fe]) values of 23 to 52 (Table 1), indicate that they represent fractionated, rather than primary, magmas (Stolper and McSween, 1979). The lherzolitic shergottites ALHA77005, LEW 88516, Y , YA 1075 and GRV (Table 1), consist predominantly 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 shergottites, they have much lower plagioclase contents and higher bulk mg values ( 70), and they contain chromite. Their mineralogy is consistent with early accumulation from magmas having the crystallization sequence of inferred primary basaltic shergottitic magmas (McSween et al., 1979a, 1979b) 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 obvious 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 lherzolitic 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-

3 Petrogenesis of SaU 005 and EETA79001 lithology A 3737 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 magma similar 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 suggested 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 orthopyroxene, and chromite grains in a finer-grained groundmass of pigeonite and maskelynite (Zipfel et al., 2000), and was thus the first 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) have suggested 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; Pätsch et al., 2000). However, detailed mineralogical differences between the two meteorites 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 distinguish 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 mixtures 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 features (textures, crystal shapes and size distributions, phase associations and modal abundances) and mineral (olivine, pyroxenes and spinels) compositions to reconstruct its crystallization 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 crystallization 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 relationships between these olivine-phyric shergottites and the basaltic 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-scattered electron images, X-ray maps, and quantitative analyses were obtained using the JEOL JXA 8900RL electron microprobe at Johannes Gutenberg Universität in Mainz, and the JEOL JSM-LV5900 scanning electron microscope and Cameca SX-50 electron microprobe at the University of Hawaii. Operating conditions for quantitative analyses were 15-keV accelerating potential and 12 to 30 na beam current for analyses of silicate phases, and 20-keV accelerating potential and 20 to 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 were applied to the analyses. For analyses of glasses, a defocussed beam ( 2- m diameter) was used wherever the size of the area permitted. To test whether loss of alkali elements occurred in these analyses, some glasses were analyzed using 10-keV accelerating potential, 5-nA beam current, and a 5- m beam diameter. These tests did not show significantly higher values for Na 2 OorK 2 O compared with 15-keV analyses. Analyses of mixed phases (designated broad-beam analyses) were performed with a defocussed beam ( 2 5- m diameter). 3. GENERAL PETROGRAPHY AND MINERAL COMPOSITIONS 3.1. SaU 005 SaU 005 has a porphyritic texture of large olivine crystals (commonly in clusters) in a finer-grained groundmass consisting principally of low-ca pyroxene and maskelynite (Figs. 1a and 1c). Chromite occurs as rare inclusions in magnesian olivine, and more abundantly with chromian ulvöspinel rims as inclusions in more ferroan olivine and discrete grains in the groundmass. Occurrences and compositions of spinels are described 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 (maximum length and average width) were measured for all olivine crystals ( 50 m) in an area (excluding shock-melted veins and pockets) of 325 mm 2 (276 crystals, yielding a density of 85 crystals/cm 2 ). Individual crystals in clusters were distinguished optically, and then measurements were made on combined elemental X-ray maps (e.g., Fig. 1a). Seventy-four percent 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

4 3738 C. A. Goodrich 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 augite and is coarser-grained than the groundmass of lithology A. Olivine abundance is 25% in SaU 005 and 12% in lithology A of EETA79001 (EET-A). Larger areas of olivine in SaU 005 are clusters of two to five crystals. Note that olivines in EET-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. 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 2mm in length all have aspect ratios of 2 to 3.5. Compositions were obtained for 113 olivine crystals, comprising all those ( 50 m in size and not obviously shock melted) in an area of 100 mm 2. 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 predominantly 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 75 77) having Wo contents consistent with orthopyroxene composition (Wo 4 6) occur as rounded, 50 to 200 m sized inclusions in the highly magnesian (Fo 73 74) cores of the largest olivine crystals (Fig. 3a; Table 3). They have 0.5 to 1.0% Al 2 O 3, 0.4 to 0.5% Cr 2 O 3 and 0.05 to 0.1% TiO 2 (Fig. 4a). Magnesian augite (mg 78, Wo 34 35), with 1.8 to 2% Al 2 O 3 (Table 3), also occurs in several of the inclusions. Low-Ti chromite grains are commonly 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 ( 100 m), irregularlyshaped grains, and was determined by automated point-counting of combined elemental X-ray maps to comprise at most 7%

5 Petrogenesis of SaU 005 and EETA79001 lithology A 3739 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 mm 2 (85 crystals/cm 2 ). EET-A: 46 crystals in 195 mm 2 (24 crystals/cm 2 ). (c, d) Crystal size distribution (CSD) plots, based on data in (a, b), of ln(n) vs. length, where n is dn V */dl and N V * is cumulative number of crystals per volume (N V N A 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 mm 2 in SaU 005. Crystals 0.5 mm in length (74% of all crystals) are mostly Fo 65 to 63 in composition, and only slightly zoned. All crystals 1 mm in length have minimum Fo contents 65. Most have maximum Fo contents Fo 71. Only the few crystals 2 mm in length have more magnesian core compositions, up to Fo 74. of the rock (this number includes whitlockite and Ca-carbonates, which were not distinguished from augite). Minor phases in the groundmass are whitlockite, pyrrhotite, pentlandite, chromite, chromian ulvöspinel (or titanomagnetite) and ilmenite. Maskelynite compositions as reported by Zipfel (2000) are An Or and 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 observed, consistent with the report of Zipfel (2000). Compositional trends (Fig. 4a) show generally increasing Al 2 O 3 (start-

6 3740 C. A. Goodrich ing from values comparable to those found in the orthopyroxene) and Wo as mg decreases from 75 to 71, and decreasing Al 2 O 3 and Wo as mg decreases from 71 to 68. Cr 2 O 3 contents decrease and TiO 2 contents increase (again, starting from values comparable to those found in the orthopyroxene) over the entire compositional range, but show increases in slope from mg 71 to 68. In addition, both Wo and Al 2 O 3 (and possibly also Cr 2 O 3 ) 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 of mg 73 to 69, with Al 2 O 3 contents of 1.8 to 2.5% (Table 3) EET-A Table 2. Olivine in SaU 005. a 1 b SiO Cr 2 O bd1 c FeO MgO MnO CaO Total Fo a Selected analyses showing full range of forsterite (Fo) contents. b Center of large crystal in Fig. 3a. c Below detection limit. 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 megacrysts consist 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 ulvöspinel 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 ulvöspinel, ilmenite, 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 of 195 mm 2 (46 crystals, yielding a density of 24 crystals/cm 2 ). 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. Olivine 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 by Steele and Smith, 1982). McSween and Jarosewich (1983) and Steele 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 magnesian cores 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 differences in minor element trends. Although the cores have a limited range of mg ( 80 82), they show large variations in Al 2 O 3 ( wt.%) and Cr 2 O 3 ( %) contents, even within single grains, resulting in nearly vertical mg-al 2 O 3 and mg-cr 2 O 3 trends (Fig. 4b). Al 2 O 3 and Cr 2 O 3 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 vertical mg-wo trend as well. Small inclusions ( 50 mm) of low-ca pyroxene observed in olivine of Fo 73 are similar in composition (mg 81, Wo 2), and also show significant variation in Al 2 O 3 and Cr 2 O 3 (Fig. 4b). In contrast, coronas around the cores and all groundmass low-ca pyroxenes have less magnesian (mg 78) compositions and show much shallower compositional trends (Fig. 4b). Al 2 O 3 and Wo increase as mg decreases to 60, and then decrease as mg decreases to 60. Cr 2 O 3 decreases continuously as mg decreases, but shows a slight spike at mg 60. TiO 2 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 extending to mg 50. Augites in the groundmass range in composition 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-

7 Petrogenesis of SaU 005 and EETA79001 lithology A 3741 Fig. 3. Orthopyroxenes in SaU 005 and EET-A. (a) Orthopyroxene (mg 77 75) in SaU 005 occurs only as rounded, 50 to 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 is one of the largest (see Fig. 2), and is zoned from Fo 74 in the center to Fo 65 at the edges ( wings on either side are separate crystals). Collage of BEI. (b) Megacryst consisting of magnesian (mg 82 80) orthopyroxene core surrounded by pigeonite in EET-A. Crossed-polarized transmitted light. Core is free of twin lamellae, while rim shows polysynthetic twinning characteristic of pigeonite. (c) BEI of same grain as in (b). White dots correspond to analysis points labelled opx cores in Figure 4b.

8 3742 C. A. Goodrich Table 3. Pyroxenes in SaU 005 and EET-A. SaU 005 EET-A opx a aug b pig c pig d pig e pig f pig g aug h opx i opx J opx k pig l pig m pig n SiO TiO Al 2 O Cr 2 O FeO MgO MnO CaO Na 2 O Total Wo mg 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. tion) and obtained in this work show that these cores (Wo 3 4) have near-vertical trends in Al 2 O 3 and Cr 2 O 3, similar to those of cores in the common megacrysts but offset to higher mg (Fig. 4b). McSween and Jarosewich (1983) designated low-ca pyroxenes 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 in Figure 4b (and commonly also by the absence of twin lamellae) as orthopyroxene (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 compositions). 4. SPINELS IN SaU 005 AND EET-A 4.1. Spinels in SaU 005 The main occurrences of spinels in SaU 005 are summarized in Figure 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 ulvöspinel (type 2) included in olivine of Fo 69 to 62 and in the groundmass. Chromian ulvöspinel 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 olivine of all compositions 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% ulvöspinel (100 molar 2Ti/[2Ti Cr Al]) component, and show a small variation in Cr# (molar Cr/[Cr Al]) from 0.77 to Their magnetite (100 molar Fe 3 /[Fe 3 Cr Al 2Ti]) components, calculated from electron microprobe analyses following the method of Carmichael (1967), are 2 to 3% (Fig. 7a). They show a normal zonation trend, with fe# (molar Fe 2 /[Fe 2 Mg]) increasing and Cr# dereasing from center to edge (Fig. 8a). The observed variation in fe# ( ), however, is small, which indicates that subsolidus Fe/Mg reequilibration has occurred. Olivine-spinel Fe/Mg equilibration temperatures determined from the calibration of Fabries (1979) are 820 C for the edges of the grains and 900 C for their centers Type 2 composite Chromite-Ulvöspinel 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 ulvöspinel rims are distinguished from one another texturally (Figs. 9a and 9b). Cores are pervaded by short thin cracks ( 1 20 m long, submicron in width), which commonly end abruptly at the core-rim boundary, 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

9 Petrogenesis of SaU 005 and EETA79001 lithology A 3743 Fig. 4. Plots of mg vs. Al 2 O 3,Cr 2 O 3, Wo and TiO 2 for low-ca pyroxenes 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 and from Berkley and Treiman, private communication) show nearly vertical trends in Al 2 O 3,Cr 2 O 3 and possibly also Wo, which are distinct from the shallower trends shown by pigeonite (coronas on cores and in groundmass). In SaU 005, similar spikes in Al 2 O 3,Cr 2 O 3 and Wo are shown by the most magnesian groundmass pigeonite. back-scattered electron images (Figs. 9a and 9b) and, except in rare cases, is difficult to see in reflected light. Cores are anhedral 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 ulvöspinel ( %), and a small range of Cr-Al variation similar to that of type 1 chromites (Cr# decreasing from center to core/rim boundary). Their magnetite contents 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 ulvöspinel rims follow a trend of ulvöspinel variation ( 10 42%) at nearly constant Al content (Fig. 6a; Fig. 10a, profile 1; Table 4, analyses 5 and 6). Magnetite contents ( 2 9%) and fe#s ( ) are generally higher than those of cores. They are zoned (from core/rim boundary to edge of grain) with Cr# decreasing ( ) as magnetite and fe# increase (Figs. 7b and 8b). There is a distinct gap in ulvöspinel component (from %) between cores and rims (Fig. 6a), and a discontinuous change in zonation trend. Pattern 2 (Fig. 6b; Fig. 10a, profile 2; Table 4, analyses 7 9) 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 ulvöspinel (up to 7%) and magnetite components. There remains a small gap in ulvöspinel 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) ulvöspinel and magnetite contents, with decreasing Cr# and fe# (Figs. 6c, 7c, and 8c). It is similar to that of pattern 2 cores, but at slightly higher Al contents and extending to higher ulvöspinel 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 ulvöspinel 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 increases (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 ulvöspinel or ilmenite. Rims have a faintly-revealed fine lamellar structure, which is probably a result of late oxidation-exsolution of ilmenite Pyroxene-Spinel intergrowths Micron to submicron-sized pyroxene-spinel inclusions (Fig. 9e), similar to those described by Zipfel et al. (2000) in DaG 476 and by Ikeda (2001) in DaG 735 (paired with DaG 476),

10 3744 C. A. Goodrich Fig. 5. Occurrences of spinels and melt inclusions in SaU 005 and EET-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 of low-ti chromite cores with chromian ulvöspinel rims, as well as individual grains of chromian ulvöspinel, occur as inclusions in the more ferroan olivine (Fo for SaU 005 and Fo for EET-A) and in the groundmass in both rocks. Melt inclusions occur in low-ti chromites in all settings in both rocks. Melt inclusions occur in olivine only of Fo 69 to 62 in SaU 005; these inclusions commonly contain daughter crystals of chromian ulvöspinel, but do not contain chromite. Melt inclusions occur in olivine only of Fo 76 to 60 in EET-A; these inclusions commonly contain trapped crystals of low-ti chromite, without ulvöspinel. The most magnesian olivine cores in both rocks are devoid of melt inclusions. In both rocks, olivine contains tiny exsolutions of pyroxene plus a magnetite-rich spinel (not illustrated). occur in olivine of all compositions. They tend to be elongate in shape, showing parallel alignment along crystallographic directions 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 pigeonite of Wo 11 to 16 and the spinel to be Ti-poor and magnetite-rich 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 as inclusions in a magnesian range of olivine compositions (in this case Fo 81 60), while type 2 composite grains of chromite cores with chromian ulvöspinel 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 crystals in melt inclusions that occur in Fo 76 to 60 olivine. In addition, tiny pyroxene-spinel intergrowths were observed in olivine Type 1 chromites Chromites included in olivine are significantly more abundant than in SaU 005. They are 15 to 40 m in size, with Fig. 6. Compositions of spinels in SaU 005 and EET-A in the system chromite (Cr) spinel (Al) ulvöspinel (2Ti). (a) SaU 005. Type 1 chromites (inclusions in olivine of Fo 74 70) in black. Type 2 composite grains of chromite cores with chromian ulvöspinel rims (which occur as inclusions in olivine of Fo 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 around melt inclusions in type 2 chromite cores in red. (d) EET-A. Type 1 chromites (inclusions in olivine of Fo 81 60) in black. Type 2 composite spinels (which occur as inclusions in olivine of Fo and in the groundmass) in green. In contrast to SaU 005, type 2 spinels in EET-A show zonation pattern 1 only. Spinels in ALHA77005 shown in magenta for comparison. euhedral to subhedral shapes. They have low-ti compositions similar to type 1 chromites in SaU 005, with nearly constant ulvöspinel content of 2% (Fig. 6d) and magnetite contents of 3% (Fig. 7d). Likewise, they show limited normal zonation, but with higher Cr#s ( ) and fe#s ( ), and a slightly larger range of fe# variation (Fig. 8d). Olivinespinel equilibration temperatures are 950 to 1000 C (Fabries, 1979) Type 2 composite Chromite-Ulvöspinel 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

11 Petrogenesis of SaU 005 and EETA79001 lithology A 3745 Table 4. Spinels in SaU 005 and EET-A SiO TiO Al 2 O Cr 2 O FeO MgO MnO CaO ZnO V 2 O Total Cations on the basis of 4 oxygen atoms Si Al Cr Fe Ti V Mg Fe Zn Mn Ca Ulvö Sp Chr Mag fe# Cr# (1 13) 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 of profile 1 in Fig. 10a. (7 9) 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 profile 4 in Fig. 10a. (14 17) Type 2. Points 1, 12, 13 and 18 of profile 5 in Fig. 10b. 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 5 7). Cores are similar to type 1 chromites, with nearly constant ulvöspinel of 2 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. 7d and 8d). 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 ulvöspinel rims (Table 4, analyses 16 and 17) follow a trend of ulvöspinel variation ( 18 67%) at nearly constant Al content (Fig. 6d; Fig. 10b, profiles 5 7). Magnetite contents ( 4 9%) and fe#s ( ) are higher than those of rims in SaU 005 (Figs. 7d and 8d), and increase from core/rim boundaries to edges of grains with decreasing Cr# (Fig. 10b, profiles 5 7). The gap in ulvöspinel 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 5 7). 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 discussed above for SaU 005) may be exsolved ulvöspinel or ilmenite. Rims commonly have a fine lamellar structure (Fig. 9d), which is probably a result of late oxidation-exsolution of ilmenite 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 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 ( m in size) occurring in association with pyroxene and maskelynite (spinel grains included in oikocrystic olivine were specifically avoided) to provide the best analogy to type 2 spinels in SaU 005 and

12 3746 C. A. Goodrich 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 ulvöspinel rims), zonation patterns 1 and 2. Cores of different grains shown 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 same symbols in blue. 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-toedge 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 ulvöspinel 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 ulvöspinel content such as that seen in SaU 005 and EET-A spinels. These compositional data are in agreement with those reported previously by McSween et al. (1979b) and Ikeda (1994a) for chromites occurring as inclusions in late crystallizing phases (ferroan pyroxenes and maskelynite) in ALHA MELT INCLUSIONS IN OLIVINE AND CHROMITE IN SaU 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 mineralogy and mineral compositions, to be patches of groundmass. 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 H 2 O) were excluded from further study.

13 Petrogenesis of SaU 005 and EETA79001 lithology A 3747 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 ulvöspinel rims), zonation patterns 1 and 2. Cores of different grains shown 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 same symbols in blue. Melt inclusions in olivine occur only in a limited range of host 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) crystals, 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 81 77) 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 concentrated in zones outlining the core near the core-rim boundary. General properties of the inclusions are summarized in Table 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 ). 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 12 m, and those in olivine in EET-A average 30 m. All inclusions consist principally of pyroxene and glass. In some cases the glass is homogeneous; in others it contains blebs and/or dendrites of a nearly pure silica phase. Inclusions

14 3748 C. A. Goodrich Fig. 9. BEI of spinels in SaU 005, EET-A and ALHA (a, b) SaU 005. Type 2 composite grains of low-ti chromite cores with chromian ulvöspinel 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 2 composite grains of low-ti chromite cores with chromian ulvöspinel rims, which occur as inclusions in olivine of Fo and in groundmass. Melt inclusion (round, black) in core in (d). Numbered lines correspond to compositional profiles shown in Figure 10b. (e) SaU 005. Micron to submicron-sized exsolutions of magnetite-rich spinel and pyroxene in olivine. (f) ALHA Numbered line corresponds to compositional profile shown in Figure 10b. 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 ). However, the presence of the silica phase in these areas can be inferred from large heterogeneitites in SiO 2 content, and in a few inclusions (e.g., Figs. 13b and 14a) 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 (ulvöspinel 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 daughter mineral (that is, if integrated into the trapped melt compo-

15 Petrogenesis of SaU 005 and EETA79001 lithology A 3749 Fig. 10. Representative center-to-edge compositional profiles for spinels in SaU 005, EET-A, and ALHA (a) SaU 005, type 2 composite spinels (chromite cores with ulvöspinel rims). Profiles 1 and 4 show zonation pattern 1. Profiles 2 and 3 show zonation patterns 2 and 3 respectively. Profile 4 begins at a melt inclusion, others do not. (b) type 2 composite spinels (chromite cores with ulvöspinel rims) in EET-A, and spinel in ALHA Positions of some profiles marked on BEI in Figure 9. Ulvöspinel 100 molar 2Ti/(2Ti Cr Al); magnetite 100 molar Fe 3 /(Fe 3 2Ti Cr Al); Cr# molar Cr/(Cr Al); fe# molar Fe 2 /(Fe 2 Mg).

16 3750 C. A. Goodrich 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.% pyroxene, which occurs as thick rims and/or massive/blocky crystals (Figs ) and only rarely as skeletal/dendritic crystals (e.g., Figs. 13c and 15a) Compositions Inclusions in Olivine in SaU 005 Fig. 11. Comparison between zonation pattern 3 and reaction rims around melt inclusions for cores of type 2 composite spinels in SaU 005. The two patterns are similar in showing enrichment of Al and Ti, and depletion of Cr. In addition, both show an increase in fe#. They differ in that reaction rims around melt inclusions show a depletion in magnetite component, while pattern 3 shows an enrichment. Both patterns appear to result from reaction of low-ti chromite cores with evolved liquid. Ulvöspinel 100 molar 2Ti/(2Ti Cr Al Fe 3 ); magnetite 100 molar Fe 3 /(2Ti Cr Al Fe 3 ); chromite 100 molar Cr/(2Ti Cr Al Fe 3 ); spinel 100 molar Al/(2Ti Cr Al Fe 3 ). sition they would result in a melt with an unrealistically high Cr 2 O 3 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. Pyroxenes in inclusions in olivine in SaU 005 have high Wo (44 56), Al 2 O 3 ( 7 15%) and TiO 2 ( %), and low Na 2 O( %) and Cr 2 O 3 ( %) contents (Fig. 16). They contain significant amounts of phosphorus (up to 2% P 2 O 5 ), 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 range of 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 difference 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 SiO 2 that 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% SiO 2, 17.5% Al 2 O 3, and 8% CaO, and has an extremely low Cr 2 O 3 ( 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 Na 2 O 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 Na 2 O 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). Broadbeam 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 relationship, 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% SiO 2,5%Al 2 O 3 ). Jagoutz (1989)

17 Petrogenesis of SaU 005 and EETA79001 lithology A 3751 Table 5. General properties of melt inclusions in SaU 005 and EET-A. SaU 005 in olivine (Fo 69 62) a Type I Type II SaU 005 in low-ti chromite (Fo 74 70) b EET-A in olivine (Fo 76 60) a EET-A in low-ti chromite (Fo 81 60) b Number Size (avg.) (50) m (60) m 3 70 (12) m (30) m 5 30 (12) m Daughter phases High-Ca pyx. Type 1 glass. High-Ca pyx. Type 2 glass. Low to high-ca pyx. Glass. Low & high-ca pyx. Glass. Intermediate-Ca pyx. Glass. Silica phase. Minor phases Fe-sulfide. Fe-sulfide. Phosphate. Ti-Fe oxide. Silica phase. Fe-sulfide. Silica phase. Fe-sulfide. Low-Ti chromite. Silica phase. 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.) ( 40) ( 40) ( 70) ( 70) ( 70) a Range of olivine compositions in which melt inclusions occur. b Range of compositions of olivine containing low-ti chromites (without ulvöspinel-rich rims). and Harvey et al. (1993) found evidence that a similar phase in melt inclusions in shergottites ALHA77005 and LEW is cristobalite. Variation in the composition of type 2 glass probably 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 the glass itself 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 crystallites and having high CaO contents, which suggests that incipient crystallization of further pyroxene in these areas depleted the remaining glass in Ca. Based on these observations it is concluded that, to firstorder, 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 separated out a silica-rich phase, and sometimes crystallized a small amount of additional pyroxene 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 inclusions in olivine. Pyroxenes have a broader range of lower Wo contents ( 8 43, avg. 34), significantly lower Al 2 O 3 ( %) and TiO 2 ( %), higher Cr 2 O 3 ( %), and relatively uniform lower Fe/Mg ratios ( ). They contain similar amounts of phosphorus, which again appears to be substituting for Si. Most glasses are heterogeneous and analyses show compositional 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 SiO 2 contents greater than or equal to that (68%) of type 1 glass in inclusions in olivine in SaU 005 and extending to nearly pure analyses of the silica-rich phase (95% SiO 2 ), suggesting that this phase is more abundant than in inclusions in olivine and that the bulk glass has higher SiO 2. This is supported by averages of glass analyses for inclusions from which several analyses were obtained, which show 74 to 75% SiO 2 (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% SiO 2, (Table 2, column 7), and may therefore be a good representative of the bulk glass composition for all inclusions. In addition to having higher SiO 2 than type 1 glass in inclusions in olivine, it has lower Al 2 O 3 ( 16.5%), CaO (2.4%) and Na 2 O (2.6%), higher Cr 2 O 3 ( %), and much lower Fe/Mg (1.4). Broad-beam analyses of the inclusions are consistent with mixes of the observed pyroxenes 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% P 2 O 5 ) that appears to be substituting for Si (see Goodrich, 1984) 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 compositions, only two (both of which are in Fo 60) were large enough to yield usable data. Compositions of the majority of pyroxenes in these inclusions are similar to those of pyroxenes in inclusions in chromite in SaU 005 in Al 2 O 3, TiO 2,Na 2 O and P 2 O 5 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 3 13) and high-ca (Wo 40 48) compositions (one analysis with intermediate Wo may be a result of overlap), and have low Cr 2 O 3 contents similar to pyroxenes in inclusions in olivine in SaU. Their Fe/Mg ratios are relatively homogeneous ( ). Glasses in most inclusions are also similar to glasses in

18 3752 C. A. Goodrich Fig. 12. BEI of inclusions in olivine (Fo 69 62) in SaU 005. (a-b) Type I inclusions consisting of pyroxene (thin rims and skeletal crystals) and homogeneous glass. Bright spherule in (b) is iron sulfide. (c-d) Type II inclusions consisting of pyroxene (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. inclusions in chromite in SaU 005 (Figs. 17 and 18), and appear to be mixes of a silica-rich phase and silica-depleted glass. Their SiO 2 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 SiO 2 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% SiO 2, 15% Al 2 O 3 and 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 (SiO 2 68%) large enough to resolve by EMPA (Fig. 14d). Both show a low abundance of the silica phase, indicating bulk glass compositions with lower SiO 2 than other inclusions 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 silicadepleted 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 broadbeam analyses, which are consistent with mixes of the observed pyroxenes and glasses (Fig. 18) 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 observations described above (ignoring minor phases such as phosphates) and examined for consistency between their predicted early crystallization sequences as determined by MAGPOX (Longhi, 1991) and the observed mineralogy (principally pyroxene types) of the inclusions Inclusions in Olivine in SaU 005 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 Na 2 O content for type 1 glass and the average pyroxene from Type II inclusions (Table 6). Glass and pyroxene volume proportions

19 Petrogenesis of SaU 005 and EETA79001 lithology A 3753 Fig. 13. BEI of inclusions in chromite in SaU 005. All inclusions consist of pyroxene and glass. Pyroxene occurs predominantly as massive or blocky crystals, and only rarely (c) as skeletal/dendritic crystals. The glass is heterogeneous in SiO 2 content 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 in Figure 9a. Pit in inclusion in (c) is SIMS analysis spot. were weighted by reasonable estimates of density ( pyx 3.3; glass 2.3) to give weight proportions. The resulting composition is given in Table 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), consistent with the presence of high-ca pyroxene as the only daughter phase in the inclusions Inclusions in Chromite in SaU 005 The pbc of inclusions in chromite in SaU 005 can be calculated as 70 vol. % average pyroxene 30 vol. % bulk glass. Two possible compositions for the bulk glass were used, yielding two possible pbcs. Bulk glass 1 (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 in Figure 13a. Bulk glass 1 has slightly higher SiO 2 and lower Al 2 O 3 than bulk glass 2; 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 pointcounting, and in fact this composition can be satisfactorily modelled as a mixture of 52 vol.% average pyroxene 48 vol.% average glass. Pbc1 and Pbc2 are saturated only with pigeonite and evolve to pigeonite-augite cosaturation, consistent with the presence of only intermediate- and high-ca pyroxenes in the inclusions. In contrast, pbc3 is orthopyroxenesaturated. 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 significantly from the pbc of inclusions in olivine, in having lower Al 2 O 3, CaO, Na 2 O and TiO 2 contents, and higher Si/Al ratios (Table 6) Inclusions in Olivine in EET-A Because the inclusions in olivine in EET-A occur in a broad range of host compositions (Fo 76 60), it seems likely that

20 3754 C. A. Goodrich Fig. 14. BEI of inclusions in olivine (Fo 76 60) in EET-A. Inclusions consist of pyroxene and glass. The glass is heterogeneous in SiO 2 content and appears to be a mixture of a silica-rich phase (contrast in [a] has been enhanced to show this) 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 inclusions have notably less pyroxene and more aluminous compositions than those in more magnesian olivine. 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 inclusions 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 significantly more aluminous pyroxenes and bulk glasses than those in Fo 76 to 67 (consistent with their representing a more evolved melt). Therefore, because the interest here is primarily in the earliest magma, which would have been preserved most closely in the more magnesian olivine, these two inclusions have been eliminated 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 cosaturation, 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 Al 2 O 3 and high Si/Al compared to inclusions in olivine in SaU 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 Al 2 O 3 (and lower Si/Al) than pbc1, and also (as was the case for inclusions in chromite in SaU 005) appears from Figure 19c to have a smaller pyroxene (mafic) component. However, in this case, increasing its pyroxene content (by addition of pyroxene from inclusions in chromite) cannot result in a composition 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 Al 2 O 3 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 pigeonite-saturated composition (these limits are shown as an ellipse in Fig. 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 Al 2 O 3 and high Si/Al compared to the pbc of inclusions in olivine in SaU 005 (Tables 6 and 7; Fig. 19).

21 Petrogenesis of SaU 005 and EETA79001 lithology A 3755 Fig. 15. BEI of inclusions in chromite in EET-A. Inclusions consist of pyroxene and glass. The glass is heterogeneous in SiO 2 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 ulvöspinel rim can be seen in (b) Primary Trapped Liquid (PTL) Compositions 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 volumes of wall olivine in excess of the equilibrium amount. In all but the most rapidly quenched cases, this olivine quickly reequilibrates Fe and Mg with its host (or even with the larger body of magma surrounding the host crystal) and concommitant reequilibration of the residual melt in the inclusion severely 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 impervious 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 Cr 2 O 3 in spinelsaturated basaltic melts (Roeder and Reynolds, 1991). Hence, for quenched inclusions the pbc essentially preserves the composition 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 H 2 O 3 Fe 2 O 3 H 2 ) and exchange of this Fe for Cr and/or Al in the spinel (Fe 2 O 3 7 Cr 2 O 3,Al 2 O 3 ), hence depleting the inclusion in FeO and enriching it in Cr 2 O 3 and/or Al 2 O 3 (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-