Meteoritics 29, 780-790 (1994) 0 Meteoritical Society, 1994. Printed in USA.

Alteration of calcium- and aluminium-rich inclusions in the Murray (CM2) carbonaceous chondrite

MARTINR. LEE^* ANDRICHARDC. GREENWOOD^

1Department of Physics, University of Essex, Colchester CO4 3SQ, U.K. 2Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K

*Present address: Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, U.K.

(Received 1993 December 23; accepted in revised form 1994 June 13)

Abstract-Four different types of calcium- and aluminium-rich inclusions (CAIs) have been identified in the CM2 chondrite Murray, three of which contain alteration products. Two types of altered CAIs, spinel inclusions and spinel-pyroxene inclusions, contain primary spinel (* perovskite f hibonite f diopside) and secondary Fe-rich serpentine phyllosilicates (f tochilinite * calcite). Original melilite in these CAIs is inferred to have been altered during aqueous activity in the parent body and Fe-rich serpentines, tochilinite and calcite were formed in its place.

The other type of altered CAI is represented by one inclusion, here called MCA-1. This CAI contains primary spinel, perovskite, fassaite and diopside with secondary calcite, paragonite, Mg-Al-Fe phyllosilicates and a Mg-A-Fe sulphate. Importantly, MCA-1 is similar in both primary and secondary mineralogy to a small number of altered CAIs described fiom other CM2 meteorites including Essebi, Murchison and a CM2 clast from Plainview. Features that these CAIs have in common include an unusually large size, a CV3-like primary mineralogy and the presence of secondary aluminosilicates and calcite. The Al-rich alteration products in MCA-1 are also reminiscent of secondary minerals in reffactory inclusions from CV3 meteorites, which have previously been interpreted to form by interaction of the inclusions with solar nebula gases. In common with the other types of altered CAIs in Murray, MCA-1 is inferred to have experienced its main phase of alteration in a parent body environment. The Mg-Al-Fe phyllosilicates, calcite and the Mg-Al-Fe sulphate fonned following aqueous alteration of an Al-rich precursor. eossiblv Ca dialuminate. This episode of parent body alteration may have overprinted an earlier phase of alteration in a solar nebula env&dnment &om which only paragoniie remaid.

INTRODUCTION

The CM2 carbonaceous chondrites contain objects 4 mm, which consist of refractory minerals including spinel, perovskite, hibonite and pyroxene. These Ca- and Al-rich inclusions (CAIs) are of great interest because, on the basis of thermodynamic calculations (Lord, 1965; Grossman, 1972), their constituent minerals are predicted to be the first phases formed from the early solar nebula -4.55 Ga ago. Detailed study of CAIs in CM2 meteorites has, however, shown that they contain Fe-rich phyllosilicates, calcite and Ca sulphates (Macdougall, 1979, 198 1; Armstrong et al., 1982; MacPherson et al., 1983, 1984, 1988; Lee, 1993; Greenwood et al., 1994), minerals that could not have grown from high temperature nebular gases. The CM2 matrices also host abundant phyllosilicates, calcite and occasionally Ca sulphate, and these minerals are most commonly interpreted to have formed during aqueous activity in a parent body environment (McSween an& Richardson, 1977; Kerridge and Bunch, 1979; McSween, 1979a; Tomeoka and Buseck, 1985; Lee, 1993; Zolensky et al., 1993; Greenwood et al., 1994). The physical and chemical conditions under which CAIs in CM2 meteorites were altered are uncertain. Both solar nebula and parent body environments have been suggested (Armstrong et al., 1982; MacPherson et al., 1983; Lee and Barber, 1991; Greenwood et al., 1994).

In addition to the aforementioned altered CAIs, a small number of unusual CAIs that contain Na-, Al- and C1-rich alteration products such as feldspathoids and Al-rich phyllosilicates, have been described from CM2 meteorites (MacPherson et al., 1983; El Goresy et al., 1984). The Na- Al- and C1-rich minerals are relatively common alteration products of refractory inclusions in CV3 meteorites including Allende (Hashimoto and Grossman, 1987), Kaba (Fegley and Post, 1985) and Mokoia (Cohen et al., 1983) and C03 meteorites such as Y791717 (Tomeoka et al., 1992). These CAIs are interpreted to have been altered by interaction with solar nebula gases (Grossman and Steele, 1976; Allen et al., 1978; Grossman et al., 1979; Hashimoto, 1992; Tomeoka et al., 1992).

It is the aim of this paper to describe Murray CAIs, with emphasis on their alteration products, and to discuss the environment( s) within which alteration may have taken place. Most Murray CAIs have a secondary mineral assemblage that is comparable to their enclosing meteorite matrix. However, the alteration products of one distinctive Murray CAI are more reminiscent of secondary phases in refractory inclusions from CV3 meteorites. Thus, juxtaposed in Murray are CAIs whose alteration products could, on the basis of previous studies, be interpreted as the product of both parent body and solar nebula environments.

EXPERIMENTAL This study was carried out using one demountable thin section (surface

area 55.6 mm2) and one conventional polished thin section (60.0 mm2) of the Murray meteorite (CM2 carbonaceous chondrite), both supplied by the Natural History Museum, London. Murray was seen to fall in Kentucky, U.S.A. in 1950. The CAIs were located by systematically traversing the thin sections using a Cambridge Instruments 5360 scanning electron microscope, and analysing all likely objects with a Link Systems energy-dispersive X-ray detector. Once found, those CAIs that were sufficiently large (greater than -30 pn in diameter) and intact were imaged using backscattered electrons (BSE) and quantitatively analysed by electron microprobe. Mineral analyses were performed using a Hitachi S2500 SEM (15kV accelerating voltage, 2 nA current) fitted with a Link Systems An10/55S energy-dispersive analytical system.

Following imaging and analysis, a number of CAIs were prepared for TEM investigation. Copper discs with a central hole 100-600 ,urn in diameter were glued with an epoxy resin overthe CAIs of interest, and then the disc with CAIs attached was extracted t?om the thin section. Following a brief wash in acetone, the CAIs were Ar-ion beam thinned until parts of them were eledron- transparent. A JEOL 200CX transmission electron microscope (TEM) operated at 200 kV was used for imaging, and analysis of the thinned CAIs. X-ray spectra were acquired using a high angle, Link Systems energy-dispersive X-ray detector. These spectra were quantified by the thin film technique of Cliff and Lorimer (1975), using k-factors experimentally determined from mineral standards.

DESCRIPTION Spinel, Spinel-pyroxene and Spinel-pyroxene-olivine Inclusions

A total of 85 CAIs were found in the two Murray thin sections (abundances of 0.90 CAIdmmz and 0.58 CAIs/mm2), of which 75 were sufficiently large (>30 pm) and intact to be studied in detail

780

Alteration of CAIs in the Murray chondrite 78 1

by imaging and X-ray analysis. One of these (called MCA-1) is deralogically different from all of the others and will be described separately. The other 74 CAIs can be subdivided into one of three mineralogical groups (Table 1).

Spinel Inclusions-Spinel inclusions have a core of spinel (i perovskite), which is generally rimmed by a single layer of Fe- rich phyllosilicates, a few pm in thickness. These CAIs are deralogically and morphologically comparable to sp-py inclu- sions but lack their outer diopside rim.

Spinel-pyroxene Inclusions -The majority of Murray CAIs (70%) belong to the spinel-pyroxene (sp-py) group (Table 1). m e e textural sub-types can be distinguished, namely (1) compact (2) granular and (3) composite. Most numerous are compact inclu- ions, which have a sub-circular to irregular core of spinel (f perovskite f hibonite f calcite), rimmed by a 2-8-pm thick layer of Fe-rich phyllosilicate and/or a 1-4-pm thick layer of diopside (Fig. 1). Parts of the phyllosilicate and pyroxene rim layers are often absent, indicating removal by abrasion before or after incorporation into the Murray parent body (Fig. 2). The granular sub-type of sp-py inclusion has a poorly structured core of fine-grained spinel intergrown with patches of Fe-rich phyllo- silicate, which is surrounded by a continuous diopside rim. bclusions of the composite sub-type are chain-like aggregates of individual, 30-50 pm sized sp-py objects (Fig. 3). Each sp-py object has a core of spinel + perovskite (f hibonite f calcite), which is rimmed by phyllosilicate and an outer selvage of diopside (Fig. 3).

Spinel-pyroxene-olivine Inclusions -Two sub-types of spinel- pyroxene-olivine (sp-py-01) inclusions have been recognised. In the most common sub-type, one or more pm-sized aggregates of granular spinel and pyroxene crystals are mantled by homogeneous forsterite. More rare are inclusions that have a core of spinel concentrically rimmed by layers of diopside and forsterite (Fig. 4). Within one such CAI, perovskite, Fe-Ni metal, pentlandite and FeS were identified within the inner diopside rim (Fig. 4).

Rim Phyllosilicates

The Fe-rich phyllosilicate rim around spinel and sppy inclusions comprises ,urn to sub-pm-sized phyllosilicate crystals, which in TEM have a -0.7 nm (001) lattice fringe and basal layer electron diEaction spacing, characteristics typical of serpentine-

FIG. 1. A compact sp-py inclusion (MCA-2) (BSE image). Within the spinel (Sp) core lies perovskite (I%), calcite (Cc) and radiating laths of hibonite (Hi). The CAI is rimmed by Fe-rich phyllosilicate (Ph) and a selvage of diopside (Di).

FIG. 2. A compact sp-py inclusion (BSE image). Most ofthe rim sequence has been removed from the spinel (Sp) and perovskite (Pv) core. Phyllosilicate (Ph) and diopside (Di) rim layers only survive along one edge of the CAI. This CAI is a transitional form between a sp-py inclusion and spinel inclusion.

TABLE 1. THE abundances, sizes and mineral compositions of the three main types of CAIs in Murray from two thin sections.

Number (%) Size: (range) (mean) spinel inclusions 9 12.2 40-277 pm 150 pm sp-py inclusions 56 75.7 33-1380 km 122 pm sp-py-01 inclusions 9 12.2 54-353 pm 132 pm

Hibonite Perovskite Phyllosilicate Calcite n (%o) n (%) n (%I n (%)

spinel inclusions 0 0 5 55.6 7 77.8 I 11.1

sp-py-01 inclusions 0 0 5 55.6 4 44.4 0 0 sp-py inclusions 9 16.1 32 57.1 53 94.6 4 7. I

In the upper table the abundances and size ranges of the three main types of CAIs are listed. The lower table gives the abundances of different minerals within the CAIs. n denotes the number of CAIs of that type which contain the mineral and (%) denotes what percentage of the total number of CAIs of that type contains the mineral. All CAIs contain spinel.

782 Lee and Greenwood

FIG. 3. A composite sp-py inclusion (BSE image). The CAI is an aggregate of a number of sp-py objects each of which has a spinel + perovskite core (SP) and rim layers of Fe-rich phyllosilicate (Ph) and diopside. The diopside rims are too narrow to be resolved at this magnification. The CAI is enclosed within a dust mantle (DM).

group minerals. The only exception was the inner rim to one sp-py inclusion (Fig. 5), which was shown by TEM images and selected area electron diffraction (SAED) patterns, to comprise an intergrowth of -0.70 nm serpentine with -1.08 nm tochilinite and -1.78 nm coherently interstratified serpentine-twhilinite.

Dust Mantles

Approximately 50% of Murray CAIs are enclosed within a phyllosilicate-dominated "dust mantle" that has a clastic appearance and is a few tens of pm in thickness. Dust mantles smooth out the external profile of the CAIs by infilling embayments but thinning over promenances (Figs. 3, 6) . The TEM and BSE images show that the dust mantles are formed of serpentine

FIG. 5 . A tochilinite-rimmed compact sp-py inclusion (BSE image). The CAI has a core of spinel (Sp) and an outer diopside rim (Di). Tochilinite and serpentine (TS) form an inner rim.

phyllosilicates and tochilinite together with tiny metal and silicate grains. These structures, termed "accretionary dust mantles" by Metzler et al. (1 992), are common in other CM2 meteorites where they surround both chondrules and CAIs (Brearley and Geiger, 1991, 1993; Metzler etal., 1992; Zolensky et al., 1993).

The MCA-1 Inclusion

The CAI MCA-1 is 670 pm long by 300 pm wide and enclosed within a dust mantle 30-70 pm in thickness (Figs. 6a, b). This inclusion appears to be made of one polymineralic band, 40-100 pm in width, which is contorted to form an oval-shaped object. The internal and external surfaces of the band are rimmed by a single 5-10-pm thick layer of diopside. Narrow parts of the band contain only granular spinel. In wider areas, spinel forms an inner rim to a core of spinel, perovskite, fassaite and calcite crystals which lie supported within a groundmass of finely crystalline Mg- Al-Fe phyllosilicates and a Na-A1 silicate.

Irregular to euhedral crystals of spinel, 0.3-15.0 pm, are the most abundant constituents of the band interior. The TEM images show that some spinel crystals poikilitically enclose small inclu- sions rich in Si and Ca, possibly diopside, fassaite or gehlenitic melilite. Perovskite forms rounded crystals, 1.3-13.0 pm, some of which are closely associated with fassaite (Fig. 7), but most lie supported within the phyllosilicate groundmass. Fassaite occurs mainly within the interior of the band, whereas most rim pyroxene is diopside. Fassaite crystals are anhedral and range in size from sub-pm to -50 pm. The two most prominent fassaites have their outer surfaces decorated by small perovskite crystals (Fig. 7). One anhedral calcite crystal, 44 pm in diameter was found in the band interior. The BSE and TEM images reveal numerous inclusions within the calcite that include perovskite and Al-rich phyllosilicate. The interface between calcite and phyllosilicate is serrated (Fig. 8), indicating that calcite crystallized after phyllosilicates.

Most of the groundmass within which spinel, perovskite and fassaite crystals lie is formed of Mg- Al- and Fe-bearing phyllo- silicates. The TEM images show that coarse, lath-shaped crystals, 52000 nm long (normal to c*) by 5200 nm wide (parallel to c*), are intergrown with much smaller, randomly-oriented phyllo-

FIG. 4. A sp-py-01 inclusion (BSE image). The CAI has a spinel core (Sp), which is rimmed by diopside (Di) and forsterite (Fo). The dioPside Contains inclusions of pentlandite (Pe), perovskite (Pv), Fe-sulphide (FeS) and Fe-Ni metal (FeNi).

silicates 5200 nm long by 550 nm wide (Fig. 9a). The small phyllosilicate crystals have a -0.7 m lattice fringe spacing, consis- tent with serpentine or a clay mineral such as kaolinite. Most of

Alteration of CAIs in the Murray chondrite 783

1

mantle

. 1 5 0 m ,

FIG. 6. The CAI MCA-1. (a) BSE image of MCA-1. (b) Corresponding sketch showing the distribution of minerals in MCA-1. The diopside outer rim of the hand is shaded black.

the larger laths also have a -0.7 nm lattice fringe spacing, although a few contain coherently interstratified packets, 3-10 nm wide with a -1.4 nm fringe spacing (Fig. 9b). The DO01 SAED patterns of some -0.7 nm laths display a distinct streaking of k # 3n reflections parallel to c*, indicative of stacking faults in their lattice (Fig. 10). The coarse laths gave good SAED patterns (Fig. lo), but d-spacings obtained from them, as well as the more finely crystalline phyllosilicates (Table 2), are a poor match for common -0.7 nm phyllosilicates. This in part reflects uncertainties about the precise value of the camera factor for calibrating diffraction patterns and the extensive polymorphism and chemical substitution typical of phyllosilicates.

Within the phyllosilicate groundmass of MCA-1, a Mg-Al-S-Fe Phase was identified by TEM that does not diffract electrons strongly in any orientation, indicating that it is non-crystalline. a s could either be because it is a glass, or because it has become amOrphous as a result of beam damage. Most glasses previously described from CAIs are silica-rich (Reid et al. 1974; Ireland et al. 1991), and so the Mg-A-S-Fe phase is unlikely to be a glass. n u s , this phase is inferred to be a beam-sensitive mineral. One Possibility is that it is a hydrous sulphate because Lee (1993) identified gypsum (hydrous Ca sulphate) within Cold Bokkeveld ( c m ) CAIs, which also beam damaged rapidly. The only other mineral identified within the band is an uncommon Na-A1 silicate, Which forms irregularly shaped crystals -5 pm. Identification of

FIG. 7. A BSE image of fasSaite (Fa) crystals in the core of MCA-1. Perovskte (h) decorates the outer surface of the fassaites but only rarely occurs within them. The fassaite crystals lie in a groundmass of Mg-Al-Fe phyllosilicates (MAF), perovskite (h) and spinel (Sp). Part of the diopside outer rim (Di) of the CAI is also visible.

this mineral by TEM has again been hindered by beam damage which renders it amorphous.

The pyroxene layer that rims all band surfaces is composed of tightly interlocking diopside crystals 1.4-2.2 pm. None of the crystals examined by TEM showed signs of alteration or contained any inclusions.

MINERAL CHEMISTRY AND MINERALOGY Spinel and Perovskite

Spinel is a major phase in all Murray CAIs studied but shows very little chemical variation within or between CAI types. Most spinels analysed were near-pure MgAl204 (Table 3). Perovskite is also compositionally invariant and is near-pure CaTiO3 (Table 3). Hibonite

Hibonite was found only in sppy inclusions, and few chemical differences could be detected between individual crystals (Table 3). Murray hibonite is compositionally comparable to hibonites in CAIs from Murchison (MacPherson et af., 1983) and Cold Bokkeveld (Greenwood et af., 1994). Diopside and Fassaite

variable with Pyroxene in sp-py and sp-py-ol inclusions is compositionally

concentrations ranging from 3.01 to 21.75 wt%

FIG. 8. Bright-field TEM image of the serrated interface between calcite (Ca) and Mg-Al-Fe phyllosilicates (MAF) in the core of MCA-1.

784 Lee and Greenwood

FIG. 9 The Mg-Al-Fe phyllosilicates from MCA-1. (a) Bright-field TEM image. The phyllosilicate aystals are of variable size and include relatively large laths lying in a groundmass of much fmer crystals. The darkest aystals are diffracting electrons most strongly. (b) TEM lattice image of a coarse Mg- Al-Fe phyllosilicate lath. The majority of the lattice fringes have a 4 . 7 nm spacing, but between them are coherently interstratified packets with fringes spaced -1.4 nm apart.

and Ti02 concentrations reaching 9.26 wt% (Table 3). Fassaite in CAIs can contain appreciable Ti3+ in addition to Ti4+ (Dowty and Clark, 1973; Simon et al., 1991). Analyses of Murray fassaites typically have low cation totals (Table 3, analysis 8), consistent with the presence of significant Ti3+. If stoichiometry is assumed, recalculation of analysis 8 (Table 3) indicates a Ti3+/(Ti3+ + Ti4+) ratio of 0.5. Allende Type B CAIs have Ti3+/(Ti3+ + Ti4+) ratios of between 0.07 and 1.0 (Stolper, 1982).

The pyroxene, which rims MCA-1, is diopside, whereas in the band interior it is a fassaite with unusually high Ti02 concentrations, up to -11.0 wt0/0. MacPherson et al. (1983) have described a CAI in Murchison (called SH4) whose fassaites had -14.2 wt% TiOz. The Murchison CAI SH4 has other similarities with MCA-1, which will be discussed later.

Olivine

Olivine only occurs in sp-py-ol inclusions and is a near-pure forsterite (Table 3). Such a composition is typical of olivines previously described from Murchison and Cold Bokkeveld CAIs (MacPherson et al., 1983; Greenwood et al., 1994).

Spinel and SP-PY Inclusion Rim Phyllosilicates

Phyllosilicates that rim spinel and sp-py inclusions are Fe-rich (Table 4), although FeMg ratios are variable, ranging from 4.8- 8.6. The major element compositions and structural formulae of these phyllosilicates, together with their -0.7 nm basal layer spacings determined by TEM, are consistent with Fe-rich

FIG. 10. The SAED pattern of a coarse Mg-Al-Fe phyllosilicate lath from the core of MCA-1. The basal row has spots with a -0.7 nm (001) spacing, but reflections in the two k # 3n rows are streaked, which is indicative of sigruficant stacking disorder in the phyllosilicate lattice.

serpentines. These phyllosilicates may also have significant concentrations of A1 (Table 4), which are greater than those of matrix phyllosilicates (Table 4, analysis 5). The tochilinite-rich rim around one sp-py inclusion (Fig. 5) contained up to 37.0 wt% SO3 and 4.9 wt% NiO. The S and Ni concentrations are highest in the parts of the rim with brightest BSE contrast (Fig. 5). Calcite, Sodium-Aluminum Silicate and Phyllosilicates in MCA-1

The one crystal of calcite analysed in MCA-1 is near-pure CaC03, a composition that is consistent with the majority of CM2

TABLE 2. THE d-spacings of Mg-Al-Fe phyllosilicate crystals from the core of MCA-1 determined by electron diffraction (in nm).

A B C

0.71'

0.46* 0.36

0.24 0.23*

0.18

0.15 0.15 0.14

1.43' 0.71' 0.71'

0.47 0.36 0.36 0.26 0.27-0.24- 0.26 0.24 0.24

0.23 0.20 0.20 0.18

0.17 0.16

The d-spacings have been calculated from two separate zone axes of coarse laths (A and B) and a ring pattern from a large area of finely crystalline phyllosilicate (C). The diffraction pattern corresponding to zone axis A is shown in Fig. 10. Owing to uncertainties about the precise value of the camera factor, and to aid comparison, the d-spacing data has been normalised to a (001) reflection spacing of 0.71 nm. = present in some patterns, * = streaked row of reflections, i- = accurate to B .01 nm,

= broad area of diffuse reflections.

Alteration of CAIs in the Murray chondrite 785

TABLE 3. REPRESENTATIVE electron microprobe analyses of spinel, hibonite, pyroxene and olivine.

Spinel Spinel Hib. Hib. Pyrox. Pyrox. Pyrox. Pyrox. Olivine Olivine 1 2 3 4 5 6 7 8 9 10

MgO 28.69 28.67 2.77 2.67 17.86 15.78 14.41 8.47 56.70 56.25

SiO, ------ _ _ _ _ _ _ ------ 0.37 52.29 50.22 46.83 35.62 42.56 42.67 CaO 0.13 ------ 8.07 8.04 24.35 24.50 24.36 24.28 0.11 0.18 TiO, 0.38 ------ 5.01 4.27 1.35 1.73 ------ 9.26 0.22 ------

0.29 0.36 MnO ______ __-__- _ _ _ _ _ _

FeO ______ 0.47 0.68 0.30 0.58 0.73 0.51 0.56 0.43 0.64

Al,O, 70.71 70.83 82.93 83.60 3.01 6.87 13.11 21.75 ------ _ _ _ _ _ _

V,03 0.37 0.39 0.24 ------ _---_- _ _ _ _ _ _ ______ 0.88 ______ __---_ Cr203 ------ 0.24 ______ ______ -_---- 0.22 0.31 ------ __-_-_

_____- ------ _ _ _ _ _ _ _____- ----__ __--__

Total 100.28 100.60 99.70 99.25 99.44 100.05 99.53 100.82 100.02 100.39 ~~ ~~

Number of cations per formula unit

Mg 1.01 1.01 0.47 0.45 0.97 0.85 0.78 0.46 1.98 1.97 A1 1.97 1.97 11.07 11.16 0.13 0.29 0.56 0.94 ----- ____- Si _____ ___-- _ _ _ _ _ 0.04 1.90 1.82 1.70 1.30 1.00 1.00 Ca 0.00 ----- 0.98 0.98 0.95 0.95 0.95 0.95 0.00 0.00 Ti 0.01 ----- 0.43 0.36 0.04 0.05 ----- 0.26 0.00 ----- V 0.01 0.01 0.02 -----

0.01 0.01 ----- ----- 0.01 Cr 0.01 Mn

Fe _____ 0.01 0.06 0.03 0.02 0.02 0.02 0.02 0.01 0.01 Total 3.00 3.00 13.03 13.02 4.01 3.99 4.02 3.96 2.99 3.00 0 4.00 4.00 19.00 19.00 6.00 6.00 6.00 6.00 4.00 4.00

Analyses 1 to 8 = sp-py inclusions, analyses 9 & 10 = sp-py-01 inclusions. All analyses are expressed as wt%, (------) denotes not detected. Hib. = hibonite, Pyrox. = pyroxene.

_____ 0.03 ----- _____ ------ _ _ _ _ _ _ _ _ _ _ _____ 0.00 ----- _ _ _ _ _

_____ ----- _____ _____ _____ ------ _____ _-_-_ _____

matrix calcite (Johnson and Prim, 1993). The Na-A1 silicate from MCA-1 is chemically most similar to paragonite, Na2A4[Si&12020] (OH)4, a muscovite-like mica in which K has been partly or completely substituted by Na (Deer et al., 1962) (Table 4). The C1 and K in these analyses are probably derived from paragonite, but at least some of the Fe is most likely a contaminant from surrounding Mg-Al-Fe phyllosilicates. Rapid beam damage of paragonite in the TEM was also noted by Ahn et a/. (1985), who ascribed it to volatilization or Na difision.

The majority of phyllosilicates in MCA-1 are Mg- Al- and Fe- rich (Table 4). Additionally, many analyses have traces of Na, S and C1. Such compositions are closest to a chlorite such as ferroan chochlore. However, TEM images and SAED patterns show that most of the phyllosilicates in MCA-1 have a -0.7 nm basal layer spacing (Table 2). Chlorite, which has a -1.4 nm basal layer spacing does occur in MCA-1 (Fig. 9b), but is comparatively rare. The chemically most similar -0.7 nm phase to the MCA-1 Phyllosilicate is berthierine, although berthierines have a higher FeMg ratio.

From chemical and mineralogical data, the Mg-Al-Fe Phyllosilicate is interpreted to be a mixture of -0.7 nm Al-rich Phyllosilicates with -1.4 nm chlorite. The -0.7 nm phyllosilicate is predominantly berthierine, possibly interstratified or intergrown With other -0.7 nm phyllosilicates such as Fe-rich serpentine and a Mg-rich phyllosilicate like Al-lizardite or amesite. The S in some analyses suggests minor interstratifications of tochilinite, although none were imaged.

Magnesium-Aluminum-Iron Sulphate in MCA-1 This phase has a highly unusual composition (Table 4), and no

minerals could be found to match it. The only mineral with a similar composition is magnesian aluminocopiate, (Mg,Al)(Fe,Al)4(SO& (OH)2.20H20. As previously stated, a hydrous composition for this phase would agree with the observation that it beam damages rapidly.

DISCUSSION

Alteration of Spinel and SP-PY Inclusions Iron-rich phyllosilicate r ims have been previously described

from CAIs in Murchison (Macdougall, 1981; MacPherson et af., 1983, 1984; Ireland, 1988) and Cold Bokkeveld (Greenwood et al., 1994). These phyllosilicates were interpreted to have formed as a result of alteration in the solar nebula by MacPherson et al. (1983), whereas Greenwood et al. (1994) favoured a parent body environment.

Solar Nebula Alteration-The interpretation by MacPherson et al. (1983) that Murchison CAI rim phyllosilicates were formed in the solar nebula is partly based on previous petrographic observations. They noted that Macdougall(l981) had found a CAI whose rim phyllosilicates had been partly broken off, placing spinel and hibonite in direct contact with the matrix. A petrographically similar CAI has been found during this study (Fig. 2). Owing to the absence of nearby fragments of the broken rim described by Macdougall (1981), MacPherson et al. (1983) suggested that the

786 Lee and Greenwood

CAI rim was not altered to phyllosilicates in the area of the parent body that was sampled by Murchison. However most CM2 meteorites, including Murchison and Murray, are regolith breccias, which have experienced extensive mechanical reworking (regolith gardening) that was active during aqueous alteration of their matrices (Tomeoka and Buseck, 1985). In addition, Armstrong,et al. (1982) have described fragments of the rim of a CAI from Murchison (the Blue Angel), which were scattered throughout the matrix adjacent to the CAI. Thus, we think it is equally as plausible that the rim phyllosilicates described by Macdougall (1981) could have become detached from their host CAI and intermixed with the matrix after they had been formed by parent body aqueous alteration of an anhydrous precursor.

Parent Body Aqueous Alteration-Fe-rich serpentine and tochilinite, which are major constituents of Murray phyllosilicate

TABLE 4. Representative electron microprobe and ATEM analyses of phyllosilicates, paragonite and the unknown sulphate.7

Phyllosilicates Parag. Unk. 1 2 3 4 5 6 7

6.04 2.7* 0.67 0.6* 1.03 --- --_ 10.61 7.7 19.15 15.7 19.1 0.69 35.2 8.36 7.0 27.79 25.4 3.7 39.63 26.6 30.46 24.7 30.04 24.5 31.1 47.59 3.7 1.10 0.5 2.03 1.4 5.7 0.67 15.9

0.19 --- ____ 1.45 m a 0.43 ----

0.71 0.4 0.66 1.0 0.8 0.28 0.8

____ ____ ____ -- 0.9 ---- ____ 48.0859.1 19.11 31.5 34.8 3.22 14.5

3.9 ---- 0.6

-___ ____ _-__ _-__ I_- ____

___-- ____ ---_ 0.5 ____ ____

__-_ __- ____ Total 100.00 100.0 100.00 100.0 100.0 100.00 100.0

Na Mg Al Si S c1 K Ca Ti Cr Fe Ni Total 0

0.13 1.63 1.02 3.14 0.08 ---- __-_ 0.08 ____ ---_ 4.15

10.23 14.00

Number of cations per formula unit

0.17 2.50 2.86 2.63 0.13

0.02 ____ 0.06 ____ ____ 1.40

9.77 14.00

____

____ 2.21 2.81 2.31 0.10 __-- ____

0.10 0.04

2.48

10.05 14.00

--__ __-

____ 2.76 0.42 3.02 0.42 ____ ---

0.08

0.07 2.82 0.30 9.89

14.00

-_-_

1.45 0.13 5.78 5.88 0.06 0.25 0.07 0.04 ____ --__

0.33

13.99 22.00

-___

0.13* 1.33 0.80 0.09 0.30

n.a ____ 0.02 ____ ____ 0.31 0.01 2.98 4.00

Analysis 1 = mean of 10 analyses of sp-py inclusion rim phyllosilicates (electron microprobe). Analysis 2 = mean of 6 analyses of the phyllosilicate rim to MCA-1 (ATEM). Analysis 3 = mean of 2 analyses of MCA-1 internal phyllosilicate (electron microprobe). Analysis 4 = mean of 20 analyses of MCA-I intmal phyllosilicates (ATEM). Analysis 5 = mean of 10 analyses of matrix phyllosilicates surrounding MCA-1 (ATEM). Analysis 6 (Parag.) = MCA-1 paragonite (electron microprobe). Analysis 7 (Unk.) = unknown sulphate &om MCA-1 (ATEM). (---) denotes below detection limits, na denotes not analysed. * ATEM Na data is unreliable. t All data expressed as wt% oxide. Electron microprobe results normalized to 100 *'Yo to facilitate comparison with ATEM data.

rims, also comprise a significant proportion of Murray matrix (12 wt% tochilinite, 35 wt% cronstedtite, 65 wt% serpentine (McSween, 1987)) and dust mantles (9 wt% tochilinite, 26 wt% cronstedtite, 65 wt% serpentine (Metzler et af., 1992)). The chemical composition of CAI rim phyllosilicates however, especially their F a g ratios (4.8-8.6), is considerably greater than that of average Murray matrix (2.97, McSween and Richardson, 1977) and dust mantles (2.49, Metzler et af., 1992). These mean values hide the fact that the mineralogical and chemical composition of the matrix and dust mantles is highly variable. Micron-sized areas of concentrated tochilinite and cronstedtite are very common in the matrices of CM2 meteorites (Tomeoka and Buseck, 1985; Metzler et al., 1992). In addition, the outer parts of dust mantles in Murchison are dominated by cronstedtite and tochilinite (Brearley and Geiger, 1993). Thus, areas of Fe-rich serpentine and tochilinite can occur in the meteorite matrix and dust mantles as well as CAI rims.

The CM2 matrix and dust mantle phyllosilicates are most commonly interpreted to be the products of parent body aqueous alteration (McSween and Richardson, 1977; McSween, 1979a, b, Bunch and Chang, 1980; Barber, 1981, 1985; Tomeoka and Buseck, 1985; Brearley and Geiger, 1991, 1993; Zolensky et al., 1993; Greenwood et al., 1994), although an origin in the solar nebula has been suggested by Metzler et al. (1 992). Chemical and mineralogical similarities between CAI rim phyllosilicates and the matrix and dust mantle phyllosilicates outlined above could suggest that the rim phyllosilicates were also formed by parent body aqueous alteration. In the following section, we outline a model of how this alteration could have taken place.

Parent Body Alteration Model-Greenwood et al. (1 994) argued that the precursor to Fe-rich phyllosilicate rims around Cold Bokkeveld spinel and sp-py inclusions was melilite. An inner rim layer of gehlenitic melilite is present in several Murchison CAIs (MacPherson et al., 1983; Simon et af., 1993, 1994) and is a common feature of lightly-altered CAIs from ALH85085 and Acfer 182, two unclassified chondritic meteorites (Grossman et al., 1988; Kimura et al., 1993; Bischoff et al., 1993). From the morphological and mineralogical similarity between CAIs in Murchison, ALH85085 and Acfer 182 and those in Murray, we would suggest that the latter could have also originally possessed an inner melilite rim.

If spinel and sp-py inclusion rim phyllosilicates had formed by aqueous alteration of a gehlenitic melilite precursor, they should be Al-rich because A1 is relatively immobile during low-temperature aqueous alteration. The rim phyllosilicates do contain more A1 than Murray matrix phyllosilicates (Table 4, analyses I , 2 and 5), but Al concentrations are still significantly less than would be expected. In strongly alkaline solutions however, alumina is highly soluble (Mason, 1966) and Zolensky (1984) has suggested that such conditions (a pH of 10-12) prevailed during early aqueous alteration of CM2 parent bodies and concurrent formation of tochilinite. Thus, the discrepancy between observed and expected A1 concentrations can be reconciled if the spinel and sp-py inclusions were altered during early phases of aqueous activity.

Tomeoka and Buseck (1985) interpreted CM2 matrix tochilinite and serpentine to have formed by multi-stage parent body aqueous alteration. In their model, following early alteration of kamacite, pore fluids were initially Fe-rich and precipitated tochilinite and Fe-rich serpentines. With time, the more resistant matrix silicates such as olivine and pyroxene began to alter,

Alteration of CAIs in the Murray chondrite 787

TABLE 5. COMPARISON between the major element composition of Mg-Al-Fe phyllosilicates in MCA-I and other previously recorded Al-rich phyllosilicates.

1 2 3 4 5 6*

MgO 19.15 23.82 20.62 32.11 21.17 ----- AI,O, 27.79 24.15 6.35 22.78 23.23 _____ SiO, 30.04 29.02 37.46 32.61 30.37 -___- FeO 19.11 22.24 34.86 1 1.43 20.24 --___ Total 96.09 99.23 99.29 98.93 95.01 -----

Mg/Si 0.82 1.06 0.71 1.27 0.90 1.04 AllSi 1.05 0.95 0.19 0.80 0.87 0.99 Fe/Si 1.06 I .28 1.55 0.58 1.11 1.11

1 = Mean of two electron microprobe analyses of MCA-I Mg-Al-Fe phyllosilicates, 2 = spinach from an Essebi chondrule (analysis number 3 in Richardson, 1981), 3 = an analysis of berthierine from the matrix of a Murchison xenolith (MXI) (Olsen et al. 1988). 4 = clinochlore from the matrix of Nogoya (Zolensky et al.. 1993), 5 = mean of two analyses of high Al, intermediate mg phyllosilicates from Belgica 7904 chondrule rims (Ikeda and Prinz, 1993), 6 = Al-rich phyllosilicates from CM clasts in Plainview (Nozette and Wilkening, 1982). Analyses 2 , 3 and 4 normalised to 100 wt% prior to tabulation of major element concenrations. *Only element ratios were given by Nozette and Wilkening (1982).

liberating Mg and Si into the matrix, which helped transform tochilinite into Fe- and Mg-bearing serpentines. It is during this time that the majority of the matrix and dust mantle phyllosilicates were formed. Thus, if spinel and sp-py inclusion rims were originally dominated by gehlenitic melilite and were altered relatively early by high pH fluids, Fe-rich serpentine and/or tochilinite would have been most likely to have formed in their place.

Calcite has been previously described by Armstrong et al. (1982) and MacPherson et al. (1983) from Murchison CAIs and Greenwood et al. (1 994) in Cold Bokkeveld CAIs. MacPherson et al. (1983) concluded that calcite formed by interaction with solar nebula gases, whereas Armstrong et al. (1 982) and Greenwood et al. (1994) both favoured a parent body environment. Calcite in Murray CAIs is also interpreted to be a product of parent body aqueous alteration and is inferred to have formed at a similar time as the rim phyllosilicates. Calcite is abundant in the matrix of Murray (Kerridge and Bunch, 1979; Grady et al., 1988) where it is frequently intergrown with serpentine and tochilinite (Bunch and chang, 1980). Calcite and phyllosilicates are interpreted to have formed during the same phase of aqueous alteration in the matrix of Murray and other CM2 meteorites (Bunch and Chang, 1980; Johnson and Prinz, 1993).

Alteration of MCA-1

hclusion MCA- 1 is morphologically and mineralogically different from all other CAIs investigated during this study. Most noticeable is its unusually large sue and the presence of Ti-rich fasaite, mica, Al-rich phyllosilicates and a Mg-Al-Fe sulphate. Features that MCA-1 has in common with other Murray CAIs include the abundance of spinel and perovskite, a diopside outer

and a dust mantle. Spinel, perovskite and fassaite are inferred to have formed first, producing the band that was rimmed by diopside before the whole CAI was enclosed within an anhydrous dust mantle.

Calcite, paragonite, Mg-Al-Fe phyllosilicates and the Mg-Al-Fe sUIPhate in MCA-1 are all interpreted to be alteration products. The Mg-Al-Fe phyllosilicates are mineralogically and chemically different from those phyllosilicates that rim other Murray CAIs and

also distinct from CM2 matrix and dust mantle phyllosilicates.

Calcite however is a characteristic component of CM2 meteorites, occurring in both altered CAIs and the matrix. The two environments in which MCA-1 could have been altered are (1) the solar nebula and (2) a parent body; each will be discussed in turn.

Hydrous Alteration in the Solar Nebula-Aluminosilicate alteration products are rare in CM2 CAIs but are common constituents of altered Al-rich inclusions and CAIs in CV3 meteorites such as Allende (Grossman and Steele, 1976; Allen et al., 1978; Grossman et al., 1979; MacPherson et al., 1981; Hashimoto and Grossman, 1987; Keller and Buseck, 1991) and C03 meteorites, especially Y791717 (Tomeoka et al., 1992). These minerals, which are rich in-Na and Ca, include anorthite, grossular, feldspathoids, calcic micas and a variety of Mg-A1 phyllosilicates. Hashimoto and Grossman (1 987) have identified a progressive alteration sequence in the Allende amoeboid olivine aggregates. One of the first minerals to be replaced was melilite which altered to grossular plus anorthite, the anorthite in turn being replaced by feldspathoids. With more alteration, fassaite broke down to ilmenite plus phyllosilicates. Even after melilite and fassaite had been altered, primary diopside, spinel and some perovskite remained (Hashimoto and Grossman, 1987), a very similar mineral assemblage to that in MCA-1. The Al-rich inclusions within Allende amoeboid olivine aggregates are interpreted to have imported Na20, Si02, C1, K20, CrzO3, FeO and H20 from a solar nebula gas to form the secondary minerals (Hashimoto and Grossman, 1987). Likewise, Tomeoka et al. (1992) infer solar nebula gas-derived Na, Si, C1 and Fe to have entered Y791717 CAIs during alteration.

Keller and Buseck (1991) have described a Type A Allende CAI within which there is evidence for two distinct episodes of solar nebula alteration. During the earlier event, grossular and anorthite were formed by interaction of the CAI with a volatile- and Fe-rich vapour. Later, Keller and Buseck (1991) infer this grossular and anorthite to have been partially replaced by the calcic micas clintonite and margarite respectively, during hydration in a low temperature solar nebula gas.

By analogy with the aforementioned studies on CV3 and C03 refractory inclusions, it is possible that primary gehlenitic melilite and some fassaite in MCA-1 was altered by interaction with solar nebula gases to calcite, paragonite, Mg-Al-Fe phyllosilicates and

788 Lee and Greenwood

Mg-Al-Fe sulphate. The formation of earlier, intermediate alteration products such as grossular and anorthite, again in the solar nebula, cannot be discounted, although there is no evidence for their former presence in MCA-1.

Parent Body Aqueous Alteration-One or more minerals in MCA-1 may have been altered following exposure to aqueous solutions in a parent body environment. The majority of Al-rich alteration products previously documented from CV3 refractory inclusions are interpreted to have formed in a nebula environment, but there are exceptions. Tomeoka and Buseck (1982) described Na- and C1-bearing mica and montmorillonite from an Allende fine-grained CAI, which they inferred to be the product of aqueous alteration. Tomeoka and Buseck (1990) have also described a h i g h 4 phyllosilicate (HAP) from the matrix of Mokoia (CV3), which was also found in Mokoia CATS by Cohen et al. (1983). This H A P is dominantly Na-phlogopite with minor serpentine and was again interpreted by Tomeoka and Buseck (1990) to have formed during parent body aqueous alteration.

Minerals that are structurally comparable to the Mg-Al-Fe phyllosilicates in MCA-1 have been described by Brearley and Prinz (1993) from CI-like clasts within the Nilpena polymict urelite. These phyllosilicates comprise 0.7 nm serpentine overgrown by 1.4 nm chlorite and are interpreted to have formed by parent body aqueous alteration (Brearley and Prinz, 1993). Terrestrial -1.4 nm chlorite intergrown with -0.7 nm befierine has been described by Ahn and Peacor (1985) from sedimentary rocks and Banfield et al. (1989) within retrograde metamorphic rocks. Ahn and Peacor (1985) suggest that berthierine is a metastable precursor to chlorite. By analogy, the chlorite packets within 4 . 7 nm phyllosilicate laths from MCA-1 may have formed by partial recrystallization of a berthierine precursor.

Phyllosilicates, which are chemically similar to those in MCA- 1, have been described from a number of different carbonaceous meteorites. Richardson (1 981) documented Mg-Al-Fe phyllo- silicates ("spinach") in Essebi (CM2) chondrules (Table 5), which he interpreted to have formed by aqueous alteration of mesostasis glass. An Al-rich "spinach," predominantly berthierine, has also been described from the matrix of Murchison xenoliths by Olsen et al. (1988) (Table 5). These phyllosilicates are interpreted to have formed by aqueous alteration but within a parent body different from that which the Murchison meteorite was derived. The Al-rich phyllosilicates have also been found within aqueously altered chondrules in CR2 meteorites by Weisberg and Prinz (1991). These phyllosilicates have 10-20 wt% MgO, 10-15 wt% A1203 and 20-30 wt% FeO. Zolensky et af. (1993) have described clinochlore (a chlorite) from the matrix of Nogoya, ALH84029, ALH83 100 and Cochabamba (all CM2), which is compositionally similar to the Mg-Al-Fe phyllosilicates in MCA-1 (Table 5). In common with its accompanying matrix phyllosilicates, this clinochlore is inferred to have formed by parent body alteration (Zolensky et al., 1993). Clinochlore, with 13-16 wt% A1203, has also been described from ALH84034, an Antarctic CM2 meteorite by Llorca and Brearley (1992). This clinochlore formed together with Mg-serpentine by aqueous alteration of an aluminous chondrule mesostasis. In contrast, a Mg-Al-Fe phyllosilicate that occurs in the rims of Belgica 7904 chondrules, again compositionally similar to MCA-1 phyllosilicates (Table 5) , has been interpreted to have formed by the alteration of anorthite and enstatite in a solar nebula environment (Ikeda and Prinz, 1993).

A number of unusual, altered CAIs have been described by Nozette and Wilkening (1982), MacPherson et al. (1983) and El Goresy et al. (1984), whose secondary mineral assemblage is reminiscent of that in MCA-1. Nozette and Wilkening (1982) described two CV3-like CAIs from a CM2 clast in Plainview (H5) which had been altered to a mixture of Al-rich phyllosilicates (probably chlorite), Mg- and Fe-rich phyllosilicates, calcite and Fe- Ti oxides. The major element ratios of the Al-rich phyllosilicate are closely comparable to those of Mg-Al-Fe phyllosilicate in MCA-1 (Table 5). Alteration is inferred to have taken place by the action of water in a CM2 parent body prior to brecciation and incorporation of the clast into the Plainview parent body (Nozette and Wilkening, 1982).

MacPherson et al. (1983) described a large altered CAI in Murchison (SH-4) that was composed of spinel, hibonite, fassaite, perovskite, melilite, diopside, calcite and an unidentified, hydrous Na-rich feldspathoid-like phase. The environment in which SH-4 was altered is uncertain (MacPherson et al., 1983), although the occurrence of calcite and a hydrous aluminosilicate could imply similar conditions of alteration to MCA-1. A highly unusual CAI in Essebi (CM2) was documented by El Goresy et al. (1 984), which had a melilite + spinel + hibonite + perovskite core that was enclosed within a highly refractory, multi-layer rim and a A1-Si-S- C1-Mn-Fe-bearing outer mantle. Some rim melilite had been altered to sodalite, nepheline and calcite, and the mantle contained phases such as sodalite and gypsum, which were inferred to have formed during parent body processing. Although El Goresy et al. (1984) did not specify the environment in which rim melilite may have been altered, its feldspathoid-calcite alteration assemblage is clearly reminiscent of that in the two Plainview CAIs, Murchison

MCA-1 Alteration Model-The final and most significant episode of alteration of MCA-1 is interpreted to have been by aqueous processes in a parent body environment. In common with the spinel and sp-py inclusions in Murray, there are no conclusive chemical, mineralogical or petrographic characteristics of MCA-1 alteration products to unambiguously identify the environment( s) within which they formed. However as discussed, there are a number of previous descriptions of Al-rich phyllosilicates from carbonaceous meteorites, nearly all of which are interpreted to have formed by parent body aqueous alteration. Additionally, the alteration products of MCA-1 and CV3-like CAIs in Essebi, Murchison and the Plainview CM2 clast are all similar in both mineralogy and chemistry to the aqueously-altered matrices of their host CM2 meteorites. The MCA-1 inclusion must have been exposed to aqueous processes at some time in its history because its originally anhydrous dust mantle has been altered to phyllosilicates.

There are a number of possible precursors to calcite and Mg- Al-Fe phyllosilcates in MCA-1, and they can be divided into two groups: primary (calcium dialuminate [CaA407], melilite and fassaite) and secondary (grossular, anorthite and feldspathoids). Calcium dialuminate is rare in CM2 meteorites but has been found in CAIs from Murchison (Simon et af., 1994) and Acfer 182, an unclassified chondrite (Weber and Bischoff, 1992; Bischoff et al., 1993). The CAI described by Simon et al. (1994) has a Ca dialuminate + perovskite core that is surrounded by zones of hibonite + spinel and pure spinel and is concentrically rimmed by melilite, anorthite and diopside. This CAI is mineralogically

SH-4 and MCA-1.

Alteration of CAIs in the Murray chondrite 789

mmparable to one described from Acfer 182 by Bischoff et al. (1993, their Fig. 9). Assuming a Ca dialuminate/melilite pmmsor, during alteration S, Fe, H20 and COz must have been introduced to the core of MCA-1 to form the calcite and Mg-Al-Fe PhyllOsiliCateS. These ions could have been derived directly from the enclosing meteorite matrix within which S and Fe were edensively redistributed during aqueous alteration to produce serpentine and tochilinite. In addition, the abundance of calcite in the matrix of Murray (Kerridge and Bunch, 1979; Bunch and c b g , 1980) testifies to the presence of Ca and COz.

It is also possible however that calcite and phyllosilicates were produced by parent body aqueous alteration of Al-rich phases such as grossular, anorthite and feldspathoids, which themselves had formed earlier by interaction of MCA-1 with a volatile-rich nebula gas. This two-environment model is attractive because the large C ~ S in CV3 meteorites, which bear some mineralogical similarities to MCA-1, are widely interpreted to have experienced solar nebula alteration. One relic of solar nebula alteration could be the paragonite, which is relatively rich in Na, Al and C1, characteristics that are typical of solar nebula alteration products in CV3 refractory inclusions and CAIs (Hashimoto and Grossman, 1987; Keller and Buseck, 1991).

This parent body alteration scenario raises the question of why MCA-1 phyllosilicates are Al-rich when phyllosilicates that rim other Murray CAIs and are also interpreted to have formed by parent body aqueous alteration, contain significantly less Al. These differences can be reconciled if MCA-1 did originally contain Ca dialuminate, which is much richer in A1 than gehlenitic melilite (-77% AlzO3 vs. -35% Al2O3, respectively). The MCA-1 phyllosilicates also contain less Fe, although have comparable Mg concentrations to rim phyllosilicates in other Murray CAIs. This does not preclude an origin by aqueous alteration in the Murray parent body but may suggest alteration at a slightly later time, when the pore fluids had a lower FeMg ratio (i.e., Ca dialuminate was not as susceptible to aqueous alteration as melilite rims). The occasional presence of originally Ca dialuminate-bearing CAIs in CM2 meteorites may help to explain the general correlation between large, mineralogically unusual CAIs in CM2 meteorites and aluminosilicate alteration products.

The Mg-Al-Fe sulphate could have formed within MCA-1 at the same time as calcite and phyllosilicates. However, it is more likely that the sulphate precipitated later as a volumetrically minor vein-filling phase. The source of minor Na and C1 for the secondary minerals in MCA-1 is uncertain. These elements may have been sourced from the meteorite matrix or from alteration of Na- and C1-bearing minerals within MCA-1 such as feldspathoids.

CONCLUSIONS

Unambiguous determination of the environment(s) within which CATS in CM2 meteorites were altered is problematical due to the scarcity of mineralogical and petrographic characteristics that are unique to parent body and solar nebula environments. The Phyllosilicate and calcite alteration products of Murray spinel and SPPy inclusions are mineralogically and chemically comparable to minerals that are widely believed to have formed by aqueous alteration of their host meteorite matrix. The spinel and sp-py inclusions are inferred to have originally possessed an inner melilite rim that was altered early by alkaline matrix-derived fluids to phyllosilicates and calcite.

The unusual CAI MCA-1 contains Al-rich alteration products which invite comparisons with solar nebula-altered refractory inclusions in CV3 and C03 meteorites. However, it is more likely that minerals within MCA-1 such as Ca dialuminate were altered by S-, Fe- and CO2-rich matrix-derived aqueous solutions to calcite, Mg-Al-Fe phyllosilicates and a Mg-Al-Fe sulphate. The presence of paragonite in MCA-1 suggests that this CAI may also have experienced an earlier phase of alteration in a low temperature solar nebula environment. Acknowledgments-We thank Brian Diamond and Keith Moulding for their technical assistance and David Barber and Robert Hutchison for guidance during this research. Constructive reviews by Adrian Brearley and Lindsay Keller significantly improved this manuscript. This work was h d e d by research grants fiom the Natural Environment Research Council (to M.RL) and Science and Engineering Research Council (to R.C.G).

Editorial handling: K. Keil

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