Matrix and fine-grained rims in the unequilibrated CO3 chondrite, ALHA77307: Origins and evidence for diverse, primitive nebular dust components

Matrix and fine-grained rims in the unequilibrated CO3 chondrite, ALHA77307: Origins and evidence for diverse, primitive nebular dust components

Geochimica PI Cosmwhimico Copyright 0 1993 Pergamon Acta Vol. 57, pp. 1521-1550 Pm.? Ltd. Printed in U.S.A. 0016-7037/93/$6.00 + .oo Matrix and f...

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Geochimica PI Cosmwhimico Copyright 0 1993 Pergamon

Acta Vol. 57, pp. 1521-1550 Pm.? Ltd. Printed in U.S.A.

0016-7037/93/$6.00

+

.oo

Matrix and fine-grained rims in the unequilibrated CO3 chondrite, ALHA77307: Origins and evidence for diverse, primitive nebular dust components ADRIAN J. BREARLEY Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87 13I- 1126, UsA (Received June 9, 1992; accepted in revisedform September 10, 1992) Abstract-The mineralogical and chemical ch~~e~stics of dark matrix and fine-grained rims in the unequilibrated CO3 chondrite ALHA have been investigated in detail by scanning electron microscopy, electron microprobe analysis, and transmission electron microscopy. Matrix and fine-gmined rims on chondrules and other objects are compositionally homogeneous on the scale of 10 Km, and there are no compositional differences between matrix and rims. Mineralogically, both the matrix and rims are extremely diverse and consist of a highly unequilibrated assemblage of Si- and Fe-rich amorphous material, divine, pyroxene, Fe,&%metal, magnetite, ~ndandite, pyrrhotite, anhydrite, and mixed layer phyllosilicate phases. Several distinct components can be recognized within the matrix and rims based on their textural and compositional characteristics, which appear to represent basic fine-grained units of nebular dust. The microstructures of these different components show that they have experienced significantly different thermal histories. Unlike the ordinary chondrites, an origin for any significant component of the matrix or rims in ALHA from chondrules is improbable, based on compositional and microstructural evidence. The matrix and rim components in ALHA formed by di~uilib~um conden~tion processes as fine-grained amo~hous dust that is represented by the abundant amo~hous component in the matrix. Such condensation could have occurred under a variety of conditions, at different times and locations within the solar nebula, or possibly earlier in a circumsteilar environment. Subsequent thermal processing of this primitive condensate material, in a variety of environments in the nebula, resulted in partial or complete recrystallization of the fine-grained dust. The intimate association of fine-grained components with disparate compositions and thermal histories shows that mixing of finegrained dust within the nebula must have been extremely thorough. INTRODUCI’ION

orites (e.g., TOMEOKA and BUSECK, 1988), have provided important new insights into the nature of aqueous alteration processes in an asteroidal environment. Similarly, the matrices of CV3 meteorites have also been characterized in some detail ( KORNACKI and WOOD, 1984; COHEN et al., 1983; PECK, 1984). In contrast, the CO3 carbonaceous chondrites have been one of the least studied groups of carbonaceous chondrites, not only in terms of their matrix mineralogy, but general mineralogy and petrography as well. MCSWEEN ( 1977) suggested that there may be a metamorphic sequence among the CO3s, based on a study of six CO3 chondrites. Since that time a number of additional CO3 chondrites have become available as a result of finds in Antarctica. Studies of these additional chondrites have now demonstrated unequivocally that such a metamorphic sequence does exist, which is broadly comparable with that observed in the ordinary chondrites (Scan and JONES, 1990 ) . The main difference is that the CO3 chondrites only exhibit a limited range of metamorphic e&&s which varies between petrologic types 3.0 to 3.7 (based on the petrologic criteria of SCOTT and JONES, 1990). The recent TEM study of KELLERand BUSECK f 1990) has provided new data on the matrix mineralogy of the CO3 chondrites Land and Warrenton and has revealed that Land underwent mild aqueous alteration, probably on a parent body. However, these studies have concentrated largely on the nature of the alteration and have not specifically addressed the origin of the precursor, anhydrous matrix material itself. In any case, both La& (type 3.4) and Warrenton

THE CARBONACEOUSCHONDRITES contain a wealth of information reflecting processes which occurred in the very earliest stages of the fo~ation of our solar system. Not only do they contain minerals which formed as a result of diverse processes such as melting, evaporation, and condensation, but they have also been shown to contain ultrafine-grained diamond and silicon carbide which have an interstellar origin (e.g., LEWIS et al., 1987; BERNATOWICZet al., 1987). These inte~tell~ materials appear to reside in the dark, fine-g&r& matrix, which is an abundant component in all the carbonaceous chondrite groups (CI, CM, CV, and CO). Whilst the origin of the interstellar gmins is relatively clear, the origin of the matrix materials which host them is still the subject of considerable conjecture. For example, it is not yet clear what propo~ion of matrix could be of interstellar origin or whether the bulk of matrix represents interstellar material that has been processed in a variety of ways within the solar nebula or subsequently on a parent body. In addition, the relationship between chondrules and matrix in carbonaceous chondrites (and indeed, ordinary chondrites) is still unclear. The nature and origin of matrix materials in carbonaceous chondrites have been studied extensively over the last twenty years. In particular, the mineralogical characteristics of matrix in the CM2 carbonaceous chondrites have received a considerable amount ofattention (e.g., BARBER, 198 1; TOMEOKA and BUSECK, 1985). These, and similar studies of CI mete1521

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(type 3.6 ) have suffered parent body metamorphism (SCOTT and JONES, 1990), which may have considerably modified their matrix mineralogy. The fine-grained constituents of matrix are likely to be especially susceptible to even mild metamorphism and probably undergo rapid recrystallization and modification, which will obliterate primary features of the matrix mineralogy. In order to understand the nature and origin of matrix in unequilibrated CO3 chondrites more fully I have carried out a detailed study of matrix in the least equilibrated CO3 chondrite, ALHA (type 3.0; Sco-r-r and JONES, 1990). Such unequihbrated chondrites probably represent the least processed meteoritic samples of early solar system materials available for study. As such they are of special significance as they may preserve, at least in part, a record of processes which occurred during the formation and evolution of the solar nebula.

PREVIOUS WORK There has been some discussion in the literature about the exact classification of ALHA77307. SCOT et al. ( 198 1) classified this meteorite as a CO3 chondrite based on its petrographic properties, but the compositional data of KALLEMEYN and WASZBN( 1982) revealed that it has some unusual features. For example, the Cd, Ge, Se, and Zn abundances are all higher than typical CO chondrites, a fact that led KALLEMEYN and WASSON( 1982) to suggest that ALHA may have closer affinities to CM chondrites. On the basis of these data KALLEMEYNand WASSON( 1982) suggested that ALHA may belong to a new grouplet intermediate between CM and CO chondrites. Part of the problem of the classification of ALHA may be related to the fact that only five CO chondrites had been analyzed at the time, so that the sample size is not exactly representative, as noted by RUBINet al. ( 1985) and RECK and SEARS( 1987). An alternative, and perhaps more viable explanation for this difference in volatile elements, is that ALHA appears to have a higher matrix abundance than other CO3 chondrites (40-50 ~01%;Scorr, 1984; cf. -30 ~018, MCSWEEN, 1977). If the volatiles are concentrated in the matrix component as seems likely, it would be expected that the volatile abundances in ALHA should indeed be higher than other CO3 chondrites. This would also account for the fact that Mg is lower by approximately 15% than typical CO values, since matrix is Fe-rich and chondrules in CO3 chondrites are dominantly Mg-rich types ( IKEDA, 1983; Scorr and JONES, 1990). The general consensus based on a number of studies (SCOTT et al., 1981; RUBIN et al., 1985; RECK and SEARS, 1987; Scorr and JONES, 1990) is that there are a significant number of similarities between ALHA and other CO3 chondrites and that the classification of ALHA as a CO3 chondrite is appropriate. Whilst chondrules in ALHA and isolated grains have been studied in some detail (SCOTTand JONES,1990, JONES,1992), matrix has received little attention. IKEDA ( 1983) described evidence for aqueous alteration effects in the matrix and chondrules of ALHA and suggested that alteration occurred prior to accretion of the meteorite. In addition METZLERet al. ( 1988) reported some preliminary analytical data for fine-grained rims on chondrules in ALHA77307. However, no detailed mineralogical analysis of matrix has been undertaken to date.

MATERIALS AND EXPERIMENTAL

METHODS

Samples of ALHA were provided by the Antarctic Meteorite Working Group. Demountable doubly polished petrographic thin sections were prepared from a chip of the meteorite, mounted using an acetone soluble epoxy, Loctite 4 14. At each stage of the sample preparation the samples were impregnated with epoxy to prevent disintegration during polishing. The samples were initially studied using light optical microscopy. followed by scanning electron mi-

croscopy (SEM), electron probe microanalysis, and finally transmission electron microscopy (TEM). This procedure allows complete characterization of the same region of the sample to be carried out at all scales. SEM (backscattered electron imaging) and electron microprobe analyses were carried out on a JEOL JXA733 Superprobe operating at 15 kV. Broad beam analyses of fine-grained matrix and rim materials were obtained using a 10 pm beam, and point analysis of individual mineral grains in the chondrules and matrix were carried out using a 1 pm beam diameter. All the analyses were performed using a beam current of 20 nA and Bence-Albee corrections were applied to the data. After detailed petrographic studies of the samples by SEM and microprobe, slotted copper grids were glued to the thin section over areas of interest, using an acetone insoluble epoxy. After immersion in acetone for 5-8 h the samples could be easily removed from their glass mounting slides. These samples were subsequently prepared for transmission electron microscope studies by conventional ion-beam milling techniques, using a Gatan ion beam mill. Transmission electron microscopy was carried out on a JEOL 2000FX analytical transmission electron microscope operating at 200 kV. A Tracer Northern TN 5500 energy dispersive X-ray analysis system was used to obtain in situ quantitative analytical data em-

ploying the ClifiXorimer thin film approximation for data reduction. Experimental k-factors, determined from a variety of mineral standards, were used throughout. PETROGRAPHY As noted earlier, ALHA contains a very high abundance of opaque, fine-grained matrix, which occurs throughout the meteorite. Petrographic studies of matrix in ALHA77307, by optical microscopy, are difficult due to the fine-grained character of its constituent components. However, backscattered electron imaging has revealed a wealth of textures and features which show that matrix has two distinct modes of occurrence in ALHA77307. The first and most striking occurrence is as abundant rims around chondrules, ameboid olivine inclusions and other objects. The second occurrence is a continuous matrix, which is present interstitial to chondrules and mineral fragments, etc., and occurs throughout the meteorite. The majority of chondrules have extremely well-defined rims (Fig. la-c) which frequently exhibit sets of radial cracks that emanate from the chondrule-rim interface. On any given chondrule the rim thickness may vary considerably, but generally lies in the range IO-100 pm, with IO-30 pm being most common. Most rims are continuous, but on some chondrules they may be locally absent as shown in, for example, Fig. lc. The interface between fine-grained rims and matrix is often extremely well defined by a narrow layer (~5 pm in thickness) of magnetite and sulfides (e.g., Fig. lc). This type of layer is often discontinuous, but in some cases may be present over - 70-80s of the rim. Characteristically, most rimmed chondrules show well-defined outlines and do not appear to be brecciated. However, some chondrules were clearly fragmented before formation of the rims and have angular, irregular outlines. For example, Fig. lc shows a type II FeO-rich porphyritic olivine chondrule, which has clearly been fragmented, as indicated by the presence of a crystal which has been broken (as indicated by the discontinuous Mg-Fe zoning pattern). Although many rims in ALHA are texturally very distinct, some rims are less well defined, such as the example shown in Fig. Id. In this case, a rim appears to be present over part of the chondrule, which con-

Matrix and rims in CO3 chondrite ALHA

tains large ( lo-20 pm) metal grains, and is clearly layered. However, over a substantial part of the chondrule surface, no obvious rim is present, although a large region of matrix is attached to the chondrule. The abundance of well-defined rims and the absence of chondrule fragments with partial rims indicates that whilst ALHA has probably suffered some brecciation in a parent body environment, this brecciation was certainly not extensive. METZLER and BISCHOFF ( 1989) have described a CM2 chondrite, Yamato 791198, which has pristine, unbrecciated, texture consisting essentially of rimmed chondrules. Fragments of rock occur in ALHA which may be broadly comparable to this type of texture. An example is shown in Fig. 1e, in which several chondrules (some with fine-grained rims) occur in a cluster, embedded in a continuous fine-grained matrix. The presence of fragments of this type supports the idea that brecciation of ALHA was limited. Regions of interchondrule matrix are abundant throughout the meteorite and can be considered as a groundmass to chondrules, silicate fragments, etc. Continuous regions of matrix can extend for several hundred microns .or can be small fragments of matrix just a few tens ofmicrons in extent, interstitial to chondrules. The larger regions of matrix typically contain small chondrules as well as isolated olivine, pyroxene and metal grains, etc. (Fig. 1f). Low magnification images of rim and matrix materials in ALHA indicate that they are texturally quite homogeneous. High magnification BSE imaging of both chondrule rims and matrix clearly reveal the extremely fine-grained nature of the rim materials. The only clearly resolvable phases are grains of magnetite and olivine which have a grain size of 10 pm or less. The majority of magnetite grains lie in the I- 10 pm range and are disseminated throughout an ultrafinegrained silicate matrix (Fig. 2a-d). Angular grains of forsteritic olivine occur in both matrix and rims but, as can be seen from the BSE images, are not very common. It is noteworthy that Fe-rich olivine fragments have not been observed in the rims of FeO-rich chondrules, but forsteritic olivines are present. Chondrule-rim interlaces are extremely sharp and there is no evidence of chemical or textural modification of either the chondrule or the rim at the interface. The vast majority of rims show no evidence of mineralogical or compositional layering, such as occurs in rims in the CM2 meteorites. However, some rims (Figs. 2a, Id) occur which appear to be more Mg-rich in composition (as indicated by variations in backscattered electron contrast) in an irregular zone adjacent to the chondrule interface, giving some suggestion of layering. This observation is supported by the fact that the inner part of the rim also has a lower abundance of magnetite and sulfides. It appears that compositional layering is present to a limited extent within some rims in ALHA77307. Texturally, chondrule rim material and interchondrule matrix appear to be very similar for the most part (e.g., Fig. 2d). However, some regions of matrix occur which have higher abundances of very fine-grained opaques, mostly magnetite, although some sulfides are also present. In addition, the abundance of fragmental material, e.g., olivines, appears to be typically higher than rim material, although

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the abundance varies somewhat from region to region and is often indistinguishable from rims. MATRIX AND RIM COMPOSITIONS The compositions of rim and matrix material in ALHA have been studied extensively by electron microprobe using both focused ( 1 pm) and broad beam ( 10 pm) techniques. Typically, 5-20 analyses were obtained on individual rims from different chondrules and from a number of different regions of matrix. On the 10 pm scale, matrix and rims show a remarkable compositional homogeneity. Individual analyses obtained from both chondrule rims and matrix using a 10 pm beam are plotted on a ternary Si-FeMg element wt% diagram in Fig. 3a and b, and representative average analyses for matrix and a number of different chondrule rims are reported in Table 1. The rim and matrix datasets show a very similar degree of compositional variation which defines a linear trend from the Fe apex towards a point on the Si-Mg join with Mg/(Mg + Si) = 0.40. Some analyses in both subsets of data lie off this line, but this is relatively rare. Figure 3c shows the average bulk compositions for seventeen fine-grained chondrule rims in ALHA77307. The data are from a variety of chondrule types. Whilst there is clearly some compositional spread in these data, they are relatively tightly constrained (the variation in Mg/( Mg + Fe) is < 10%) and define the same linear compositional trend as individual analyses. The only other published data for matrix in ALHA are from SCOTT and JONES ( 1990) who reported the average of twenty-four 50-pm beam analyses. Although the data from this study are broadly comparable with the data of SCOTT and JONES ( 1990) there are some notable differences, which are worthy of comment. The average total reported by SCOTT and JONES ( 1990) is 80.92 wt%. This is significantly lower than the totals in this study which typically lie in the range 89-9 1 wt%. As a consequence of the difference in totals the SiOz, FeO, and MgO concentrations of Sco-rr and JONES ( 1990) tend to be lower than the results produced in this study, whilst concentrations for all other elements are comparable. The totals reported by SCOTT and JONES( 1990) are typical of matrices of chondrites which are hydrous, but the matrix of ALHA is essentially anhydrous. The discrepancy may arise from the fact that the matrix of ALHA is highly fractured (e.g., Fig. la-f) on a scale of 30-40 pm, such that analyses carried out using a 50 pm beam would cover a number of fractures. This would significantly affect the totals in comparison with a 10 pm beam analysis. The average element /Si ratios from this study are within 10% of the values obtained by SCOTT and JONES (1990) with the exception of Mn (14%), Mg (20%), Ca ( 30% ) , Na ( 50% ) , and S ( 25% ) . The large discrepancies for S and Na may be due to the fact that these elements are volatile and were lost to a greater extent using a 10 pm beam, since their apparent concentrations are lower in this study. Magnesium is much higher in the SCOTT and JONES ( 1990) analysis, which could reasonably be attributable to the fact that more large grains of forsteritic olivine were included in the 50-Mm beam analysis. The analyses were also carried out on a different electron microprobe ( ARL EMX )

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FE. 1. Backscattered electron images illustrating the petrographic characteristics of dark, tine-grained chondrule rims and interchondrule matrix in ALHA77307. (a) Fine-grained rim around a type 1. MgO-rich porphyritic olivine chondrule. The rim is almost completely continuous and has a maximum width of -30 pm. (b) Continuous, finegrained rim around an irregularly-shaped type 1, MgO-rich porphyritic olivine chondrule. The rim varies significantly in thickness from just a few microns up to 50 pm. The boxed region at left center is shown in mom detail in Fig. 2a. (c) Fine-grained rim surrounding a type 11,FeO-rich, porphyritic olivine chondrule. The lower center and right of the chondrule clearly represent a fracture surface which cuts across crystals and Fe-Mg zoning profiles. The interface between the zoned olivinc (lower right) and the rim is shown in more detail in Fig. 2b. (d) Small, MgGrich, porphyritic olivine chondrule with a large region of fine-grained matrix attached to it. over 100 Hm in thickness and 200 pm wide.

Mat&and

rimsin CO3 chondriWALHA77307

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(e) Cluster of three MgO-rich porphyritic olivine and pyroxene chondrules set in a fine-grained matrix. No clear rims appear to be present. The chondrules appear to be embedded in a continuous matrix. (f) Region of continuous, finegrained interchondrule matrix, several hundred microns in size which contains small chondrules, silicate and metal fragments, etc.

using different standards, which might also account for some of the discrepancies. Figures 4a-d illustrate some of the major elemental correlations observed in the data for rims and matrix in ALHA77307. It is apparent from the ternary Si-Fe-Mg (element wt%) diagrams (Fig. 3a-c) that these elements are correlated. The relationship between Si and Fe is shown in Fig. 4a and confirms the strong negative correlation between the two elements. Magnesium shows a similar negative correlation with both Si and Fe. Minor element correlations are not as clearly defined as those involving the major elements, but some do exist. For example, Na and K (Fig. 4b) show a strong positive correlation which is extremely well defined, whilst S and Ni (Fig. 4c) show a rather weak positive correlation, although there is considerable scatter in the data. Su~~singly, there are no clear correlations among the refractory elements, Ca, Ti, and Al. For example, data for Ca and Al (Fig. 4d) show considerable scatter in the data, although the bulk of the analyses tend to cluster at low Ca concentrations, but show no apparent trend. The high Ca values may be attributable to the local occurrence of secondary anhydrite in the matrix, which were unavoidably included in the analysis spot. If this is the case then analyses with high Ca probably do not represent primary concentrations. Despite the general lack of clear correlations among the minor elements, these elements are extremely useful for

comparing the compositions of fine-grained matrix and rims, when plotted on normalized element ratio diagrams. Data for three separate rims and average matrix in ALHA are shown in Fig. 5, normalized to CI chond~te values and Si. Both matrix and rims in ALHA are significantly fractionated in a number of elements relative to CI abundances. Whilst there are clearly some variations in the normalized element ratios for individual elements, average analyses for rims on different chondrules appear to be remarkably similar. In addition, the shapes of the patterns for individual rims are also similar in terms of their elemental enhancements and depletions. In all cases Ni, K, and Al are all significantly enriched in the matrix relative to CI chondrite values, whilst Fe exhibits a slight enrichment. All the other elements are depleted relative to CI values, with Ca, S, and Na exhibiting the strongest depletions. Titanium, m~esium, chromium, and manganese appear to behave rather similarly and show comparatively small, but consistent depletions. The remarkable similarity of all the rims and matrix, in terms of their normalized element ratio patterns, suggests that they represent material from a similar reservoir. Calcium and aluminum are clearly fractionated relative to one another in matrix and rims with C?abeing significantly depleted relative to Al. The Ca/Al ratios of rims and matrix lie in the range 0.067 to 0.41 I with a mean of 0.227. This value is substantially below the solar value ( 1.069)) but lies within the range of matrix values found by MCSWEENand

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FIG. 2. Higher magnification backscattered electron images illustrating the tine-grained characteristics of rim and interchondrule matrix in ALHA77307. (a) Boxed region from chondrule rim in Fig. lb showing the extremely sharp nature of the interface between the chondrule (CH) and its surrounding rim. The bright. micron-sized crystals in the rim are magnetite and dark fragments (arrowed) are forsteritic olivine. The backscattered electron contrast from the extremely fine-grained rim material is variable suggesting compositional variations on the micron scale. Also note the presence of irregularly shaped MgO-rich regions close to the chondrule edge (upper center) suggesting that some compositional layering may be present in this rim. Magnetite also appears to be less abundant in the inner part of the rim. (b) Fine-grained rim adjacent to the zoned olivine crystal shown in Figure lc. The edge of the rim is clearly defined by the presence of clusters of fine-grained magnetite crystals. No evidence of compositional layering is present.

MatfiXand rims in CO3 chondritc ALHA RICHARDSON (1977)for the CO3 chondrites (0.159-0.99). The matrix Ca/Al ratios of the five CO3 chondrites analyzed by MCSWEEN and RICHARDSON( 1977) have a bimodal distribution, with three (Isna, Omans, and Warrenton) having Ca/Al ratios between 0.159 and 0.259 (all falls). ALHA values are entirely consistent with these values, so there is no mason to attribute the matrix depletion in Ca to weathering effects. Although Ca could be decoupled from Al as a result of aqueous alteration, as appears to have happened in CI chondrites ( BREARLEYand PRINZ, 1992)) the absence of evidence for significant, pervasive, aqueous activity in Lance and Warrenton ( KELLERand BUSECK, 1990 ) , argues against such an explanation for the CO3 chondrites. There is certainly evidence for aqueous alteration locally in ALHA77307, involving mobilization of Ca, but the limited scale on which such alteration has operated precludes the possibility of producing the observed Ca/Al fractionation. It seems likely that the depletion in Ca in matrix was established in the nebula and is not the result of secondary aqueous alteration processes. The bulk Ca/Al ratio of ALHA (0.939) is noteworthy ( KALLEMEYNand WASSON, 1982) because it is significantly lower than the mean value for CO3 meteorites (1.104) (WASSON and RALLEMEYN, 1988). The observed depletion of Ca in ALHA was attributed by RALLEMEYN and WASSON ( 1982) to weathering effects in Antarctica. An alternative explanation is that the bulk composition reflects the higher proportion of matrix material in ALHA in comparison with other CO3 meteorites (as noted earlier for Mg and volatile contents in ALHA77307). MINERALOGY OF MATRIX AND RIMS After detailed electron microprobe and backscattered electron studies, a number of regions of matrix and chond~le rims were selected for study by t~nsmission electron microscopy. Although the compositional data reported above show that the matrix is relatively homogeneous on a 10 brn scale, the TEM studies have revealed that mineralogically, the matrix and rims of ALHA are extremely heterogeneous and complex, and have a highly unequilibrated mineral assemblage, consisting of a number of distinct components. These individual components can frequently be identified on both compositional and textural grounds. The TEM studies also show that mineralogically there are no differences between matrix and rims, indicating that the two have a common origin. The following phases, in roughly decreasing order of abundance, constitute the bulk of both rims and matrix: amorphous silicate component, olivine, pyroxene, magnetite, kamacite, pentlandite, pyrrhotite, anhydrite, and disordered mixed layer phyllosilicate phases. The general microstructures and detailed occurrence and compositions of different mineral phases within rims and matrix are discussed in detail below.

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General Microstructures The TEM observations indicate that the matrix is essentially anhydrous, although some phyllosilicate phases have been found locally. Initial observations of rim and matrix material clearly show that they differ considerably in texture from any of the other CO3 chondrites which have been studied previously by TEM techniques, e.g.,. Land, Kainsaz, and Warrenton ( RELLER and BUSECK, 1990; A. I. Brearley, unpubl. data). These meteorites have a matrix which is dominated by fine-grained, but well-c~~l~z~ olivine. In comparison, the matrix of ALHA is dominated by an amorphous or microcrystalline component which occurs throughout rim and matrix material, and is often closely associated with olivine. Figure 6a shows a low-magnification image of a typical region of the matrix with an inset dilliaction pattern showing diffuse diffraction rims. Set within the amorphous component are varying proportions of crystalline minerals with a uniformly small grain size ( ~0.2 pm). Some larger olivine, pyroxene, and magnetite grains, which can reach 4 pm in size, also occur but are rare. The amorphous component is clearly not the result of electron-beam i~adiation of beam-sensitive silicate minerals such as clays. Beam damage of silicate materials is often characterized by the formation of voids and, because the material is undergoing structural changes involving a change in volume, the thin foil will flex and consequently the sample will drift. None of these features has been observed in the amorphous component in ALHA77307, which appears identical whether viewed under extremely low-dose conditions or at high-image brightness for extended time periods. Many chondrite matrices contain two distinct components: a elastic, relatively coarse-grained, often angular component and a very fine-grained, nonelastic component (e.g., ASHWORTH, 1977 ) . It is generally accepted that the c&tic component is derived from fwmentation of larger grains, possibly from chondrules. In the case of the larger grains in the matrix of ALHA77307, this distinction is not always clear, because in the majority of cases the grains are clearly not angular, and based on compositional and crystallographic criteria (see discussion below) cannot have been derived from chondrules. Overall, the elastic component in the rims and matrix of ALHA has an extremely low abundance, based on TEM and BSE observations. Amorphous Component As noted above, the dominant matrix component in ALHA is amorphous material which acts as a groundmass in which fine-grained crystalline phases occur (Fig. 6b,c). This amorphous component appears to exist as distinct units within the matrix, which can be distinguished by their relative abundance of crystalline phases. This can vary from around 20 ~01% crystalline material (Fig. 6d) up to 50 ~01%

(c) Further example of a fine-grained rim which shows no evidence of compositional Iayering. Magnetite crystals are distibut~ relatively hom~en~u~y ~rou~out the rim and localfy angular fragments of fo~te~tic olivine are present (arrowed). (d) Region of fine-grained inte~hond~le matrix, showing considerable com~tional and mineralogical heterogeneity on a fine scale. Micron-sized forsteritic olivines (arrowed) and magnetite are relatively abundant.

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Si @I

Fe

Si (c)

J. Si FIG. 3. Com~sit~onal

Fe

7-.-T-r?-% Mg

data for rims and inter~hondruIe matrix in ALHA determined by electron microprobe using a IOym beam. (a) Individual microprobe analyses From seventeen chondruie rims plotted in an Si-Fe-Mg (element wt%) ternary diagram. The data define a clear trend from the Fe apex towards the Si-Mg join and define a line which would intercept this axis at approximately Mg/ (Si + Mg) = 40 if extended. Some analyses show a strong enrichment in Fe which is probably attributable to the presence of coarser-grained magnetite in the anaiysis spot. (b) Similar plot of analyses for interchondrule matrix, which define the same trend as that for chondrule rims and shows approximately the same range of compositions. (c) Average compositions for seventeen individual fine-grained chondrule rims in ALHA77307. The data show a rather restricted range of compositional variation,

(Fig. 6e) and the grain size of the crystalline phases also varies considerably (compare Fig. 6b and c with Fig. 6e). It is sometimes difficult to clearly define the boundaries between individual regions, as the change in abundance of crystalline phases may often be gradational, although in some cases can be sharply defined. Individual distinct regions or units may be of the order of I to 5 pm in size. Electron diffraction and high-resolution TEM work on the amorphous material show that although in many places it is truly amorphous, as indicated by diffuse diffraction rings in electron diffraction patterns (Fig. 7a), in same cases local ordering has occurred. In these regions, wavy and distorted lattice fringes are present with lattice spacings of 0.7, 1.O, and 1.4 nm (Fig. 7b-d). The lattice fringes only extend over very short distances (2-3 nm) and may represent protophyiiosiiicates which are forming due to recrystallization of the amorphous component. Aithough these phases cannot be identified unequivocally, their basal spacings suggest that serpentine and smectite group minerals are present. The I .4-nm phase exhibits contmst in high-resolution images which is consistent with chlorite. The abundance of these protophyiiosiiicates varies considerably from extremely minor development (< 1 ~01%) up to extremely high proportions of crystaiiites (>SO ~01%). The formation of these phases may have taken place as a result of Antarctic weathering or possibly due to aqueous alteration on a parent body. Wherever this incipient r~~s~iii~tion~ alteration occurred, it clearly only affected localized regions of the amorphous material. Compositions of the amorphous component in the matrix were determined by analytical electron microscopy. These data reveal that this material has an extremely wide range of compositions, both within the same rim and from rim to rim. These variations can occur on a very localized scale indicating considerable heterogeneity on the fine scale. The majority of analyses are SiO&h (>50 wt%), but SQ can vary from 18-7 1 wt%. Fe0 and MgO are the other major ~om~nents. Almost all the analyses contain significant concentrations ( I-8 wt% f of A&03, NiO, S and, somewhat surprisingly, P205, which can also reach concentrations as high as 8 wt% in exceptional cases. Representative analyses covering the range of compositions exhibited by this amorphous component are reported in Table 2. The data are also presented in a ternary Si-Fe-Mg etement weight percent diagram in Fig. 8, along with the fields for the bulk composition of rims and matrix, as well as the range of oiivine compositions observed in the matrix. The plot shows that the bulk compositions of fine-grained matrix and rims lie within the field defined by the amorphous component, olivine and the Fe apex (Fe,Ni metal and magnetite). The possible origins and significance of the amorphous phase are discussed in detail later. Olivine

After the amorphous component, oiivine is undoubtedly the most abundant phase within the rims and matrix of ALHA77307. its absolute mode is difficuh to establish, because of the considerable local variability in its abundance, but it is probably approximately 30-40 ~01%. A number of distinct types of olivine can be identified in

Matrix and rims in CO3 chondrite

TABLE in ALH

SO2 TiO2 A’203 Cr203 Fe0 MnO M8O CaO Na,O W’ NiO p2°5 so3 Total

Analyses

1. Electron microprobe All301.

1529

ALHA

analyses of selected rims and regions of interchondrule

matrix

29.26

29.61

30.50

30.46

29.68

30.00

31.43

29.76

29.12

30.17

0.06 4.31 0.31

0.04 4.48 0.31

0.06 4.31 0.37

0.06 4.53 0.32

0.04 4.20 0.34

0.05 3.69 0.32

0.05 4.30 0.34

0.06 4.06 0.34

0.08 4.09 0.43

0.06 3.60 0.38

30.79 0.05

35.17 0.17

3S.38 0.12

31.88 0.21

33.88 0.14

34.36 0.10

35.60 0.11

30.00 0.11

33.42 0.19

34.76 0.28

30.46 0.22

31.61 0.22

13.23 0.90 0.23 0.20 3.07 0.17 3.56

13.47 0.49 0.25 0.18 3.50 0.14 3.31

15.25 0.70 0.14

14.10 0.69 0.28

14.00 0.68 0.27

14.30 0.67 0.2 1

15.09 0.72 0.26

13.38 0.85 0.31

0.12 3.08 0.22 2.46

0.09 2.95 0.21 2.15

0.09 3.51 0.24 3.21

0.08 4.60 0.25 2.53

0.11 3.20

15.59 1.19 0.33 0.10

15.85 0.97

0.13 4.51 0.21 2.20

14.82 0.68 0.18 0.09 3.15 0.40 2.43

4.26

2.64 0.35 2.91

0.23 0.08 3.19 0.23 2.99

90.70

91.28

90.47

91.14

89.67

90.12

88.46

91.36

90.87

88.00

90.53

3.97 0.35

1-8. Individual rims on chondrules.

Analyses 9- 11.Matrix analyses. Numbers in brackets represent number of analyses used for average.

the rims and matrix based on their morphological and chemical characteristics. The most abundant type is olivine which is intimately associated with the amorphous component already described. For example, the bulk of the crystalline material shown in Fig. 6a and b is olivine. These olivines are fine grained (~300 nm) and are usually anhedral or subrounded, although occasional elongate or platy morphologies have been observed (Fig. 9a). The coarsest grained olivines typically occur where the volume of amorphous material is lowest. The olivines exhibit an unusual patchy or mottled diffraction contrast, indicating that they are either poorly crystalline or highly strained (Fig. 9a-c) and they sometimes contain planar defects (Fig. 9a,b). Clusters of anhe&al grains are also common (Fig. SC). The boundaries between the amorphous material and the olivines also appear to be poorly defined or diffuse. Analyses of this type of olivine indicate that they are stoichiometric and show that they have a broad range of compositions between FaT2 and Fala (Fig. 1Ob). The second distinct type of olivine, within rims and matrix of ALHA77307, is forsteritic in composition (Fo~~+,~; Fig. 10a) and exhibits a subhedral morphology. Olivines of this type are well crystallized and occur embedded within the regions of amorphous material and fine-grained olivines described above (Fig. 9d,e). They are typically much coarser grained than the former type of olivine and have a size range between 200 nm to >4 pm, with grains greater than 1 Frn being most common. The third type of olivine is extremely fine grained ( < 100 nm) and has very Fe-rich compositions ( F%a_ss). These crystals are euhedral and occur as distinct, small clusters ~500 nm in size, which comprise distinct textural units within the matrix. Finally, distinct olivines which have compositions like those of the so-called LIME (Low-Iron, Manganese-Enriched)

olivines reported by KLO~K et al. (1989) in interplanetary dust particles, carbonaceous, and unequilibrated ordinary chondrites also occur in the matrix of ALHA77307. These forsteritic olivines (FOG ; Fig. 10~) have MnO contents between 1.5 and 2.5 wt%. They occur as both isolated grains - 1 pm in size, such as those shown in Fig. 9d and e and as aggregates ( - 1 pm across) consisting of anhedral crystals ( - 100 nm in size; Fig. 9f), which coexist with MnO-rich ( -2 wt%) enstatites. The morphology of individual crystals within these clusters suggests that they have not been extensively annealed: grain boundaries are curved and 120” triple junctions are not evident. Pyroxenes Pyroxene is much less abundant than olivine in the matrix and rims of ALHA77307, but also occurs in a number of different morphological types. The majority are low-Ca pyroxenes with compositions in the range Eng9-Ena, with more Mg-rich compositions being the most common. Rare augites also occur. At least three different morphological types of pyroxene have been found, some having distinct compositions. All appear to be present in approximately the same abundance. The most distinctive type occur as angular, fmgmental grains (Figs. 6c, 1 la) which can probably be considered to be elastic grains, comparable with those observed in ordinary chondrites (ASHWORTH, 1977; BREARLEY et al., 1989; ALEXANDER et al., 1989) and the unique chondrite, Kakangari ( BREARLEY, 1989a). These grains typically have grain sizes of 1- 1.5 pm and are invariably Mg-rich, low-Ca pyroxenes. Unlike the elastic pyroxene grains in ordinary chondrites or Kakangari, which are typically twinned, monoclinic, low-Ca pyroxene with minor intergrown orthopyroxene, the ALHA elastic pyroxenes appear to be

A. J. Brearley

1530

35.0

~

a

30.0 G

D 25.0 D #

d

V

10.0

cn 5.0 1 0.0

0.b

I

5.0

I

10.0

I

15.0

I

20.0

1

25.0

FE (ELEMENT

I

30.0

I

35.0

I

I

40.0

45.0

5

w-r 513)

0.5

b A I9

0.4

S 0.3 5 : w

0.2

y

0.1

0.0 _ ._ 0.0

O.‘l

0.2

NU

0.3

(ELEMENT

0.i

0.5

C

WT W)

FIG. 4. Elemental variation diagrams illustrating the principal elemental correlations for matrix and chondrule rims in ALHA77307, based on electron microprobe data ( 10 pm beam). (a) Si vs. Fe (element wt’%). (b) K vs. Na (element WV%).(c)S vs. Ni (element wt%). (d) Al vs. Ca (element wt%).

dominated by the orthorhombic polymorph, with only very minor intergrown clinopyroxene. Electron diffraction patterns from these grains have strong ( 100) maxima at 1.8 nm with essentially no streaking which would be indicative of stacking disorder (Fig. 1 la). Such grains indicate either a rather slow cooling history from high temperatures or extended annealing of clinopyroxene at moderate temperature. Orthopyroxene also occurs as aggregates of crystals with grain sizes of around

0.2-0.3 Frn (Fig. 1lb). These grains are Mg-rich, but show no enrichment in MnO, and consequently appear to be distinct from the LIME (Low-Iron Manganese-Enriched) pyroxenes discussed in the following text. Some stacking disorder is present in these grains, due to the presence of minor intergrown clinopyroxene. Only one grain of low-Ca pyroxene was found which had clear 0.9 nm ( 100) spacings in electron diffraction patterns and strong streaking parallel to [ 1001 more

0

P

0.

0

P

. ’

VI

p

in I

I

-

b I

0

P I

P w I

0

.P

CA (ELEMENTwr 93)

Q

I

VI

.u

J

-I

0

.+

.

s (ELEMENT w W) ' p - - p N *u .u f cn bin 0 ho (II0

n

f m

.m 0

1532

A. J. Brearley

( -200 nm). These crystals have an anhedral morphology and are typically Mg-rich, low-Ca pyroxene, although one grain of augite was also found. Magnetite

Backscattered electron imaging and microprobe analyses of matrix and rim materials in ALHA indicate that magnetite is the coarsest-grained phase present (
Fe,Ni metal is a relatively common phase in the matrix and rims of ALHA77307. It appears to occur exclusively as extremely fine-grained spherical to subspherical crystals embedded within the amorphous/microcrystalline material (Fig. 6c, 13a,b). All the grains observed are kamacite with Ni contents of around 4-5 element wt% and grain sizes of between 25 and 200 nm. In rare cases, where the matrix has been affected by aqueous alteration, the metal grains may be rimmed by an extremely narrow (~5 nm) film of an unidentified iron oxide phase (Fig. 13a). However, the majority of crystals show no evidence of any type of replacement and have sharp interfaces with the coexisting amorphous material. Sulfides

Sulfides are comparatively abundant within the matrix of ALHA and, like kamacite, appear to occur exclusively as fine-grained crystals within regions of the amorphous/microcrystalline material (e.g., Fig. 6c,d). All the grains observed are less than 0.2 I.crn in size and are either Ni-bearing pyrrhotite or pentlandite. No troilite has been observed, although it occurs elsewhere in the meteorite as relatively large grains (40-300 pm). Some pyrrhotite grains contain around 7-8 atom% Ni, which suggests that very fine-grained pentlandite may be present in some sort of intergrowth. It has, however,

I

0 CHONDRULERIM 0 CHONDRULERIM A CHONDRULERIM OMATRM I

I I

I I

II

Irl I

1

-

Al Ca Ti Mg Cr Mn Na K Fe Ni S FIG. 5. Normalized element ratio diagram showing averaged data for interchondrule matrix and three separate fine-grained chondrule rims. The data are normalized to Si and CI chondrite values (from ANDERS and GREVESSE, 1989). The shapes of the patterns are remarkably similar for all the rims and matrix indicating that they essentially represent the same material.

not been possible to confirm that this is the case. Pentlandite is much less common than pyrrhotite and is found as isolated subhedral to euhedral crystals with an unusual mottled diffraction contrast (Fig. 14). Electron diffraction patterns from these grains show asterism of diffraction maxima, indicating that the crystals have a very low degree of crystallinity or are highly strained. Anhydrite

Anhydrite occurs locally within the matrix of ALHA and is found exclusively in veins which cross-cut regions of fine-grained matrix (Fig. 15a). This vein-filling anhydrite occurs as relatively coarse-grained crystals with a fibrous or elongate morphology with an elongation direction parallel to (00 1; see Fig. 1Sa). The crystals always occur with their elongation direction normal to the walls of the veins. Individual crystals are 1-2 pm in length and 0.1-0.2 wrn in width. Electron diffraction patterns from regions of anhydrite show strong asterism in the diffraction maxima, indicating that the crystals have subparallel orientations (inset in Fig. 15a). In addition, individual crystals show a mottled diffraction contrast (similar to that observed in pentlandite), perhaps due to strain, poor crystallinity or the presence of subgrains (Fig. 15b). The anhydrite is also found to be relatively beam sensitive and undergoes an electron beam-induced transformation to an amorphous phase in a matter of minutes. Analytical electron microscope analyses of individual crystals typically give concentrations of Ca and S which are consistent with stoichiometric anhydrite. Occasionally, however, analyses may show enhanced Ca concentrations and low S indicating that calcite may be present intergrown on a fine scale with the anhydrite. It has not, however, been possible to confirm that this is the case by electron diffraction techniques.

Matrixand rims in CO3 chondrite ALHA Disordered Mixed Layer P~yl~i~t~ Isolated aggregates or single crystals of a highly disordered, mixed layer phyllosilicate phase also occur sporadically in the matrix of ALHA77307. This phase occurs as grains embedded within the amorphous component of the matrix, ranging in size from 0.3 pm up to over 1 pm. These grains may be the type of phyllosili~te observed by &DA ( 1983) in the matrix of ALHA77307. Figure 16 shows an example of an aggregate of grains, which are typically oriented with their basal planes normal to one another. Some of the grains show marked kinking or curvature of their basal lattice hinges. Electron difliaction and hi-re~iution TEM imaging of the phyllosilicate phase shows that it is highly disordered in character and consists of randomly interlayered phases with 0.7, 0.85,0.96, 1.08, and 1.2 nm basal spacings. The inset in Fig. 16 shows a region with interlayering of 0.85 and 1.2 nm phases. The 0.7, 1.08, and 1.2 nm phases which appear to be the most abundant in these intergrowths may represent serpentine, tochilinite, and smectite, respectively. The composition of this phase is extremely Fe-rich and closely resembles Forich serpentine compositions found in CM meteorites. A more detailed investigation of these phyllosilicate phases will be presented elsewhere. DISCUSSION The origin of matrix materials in carbonaceous and ordinary chondrites has been, and continues to be, an area of considerable controversy; and there is certainly no consensus as to how these complex materials form (see, for example, SCOTT et al., 1988, for a recent review). It is probably true to state that there are almost as many proposed theories for the origin of matrix materials as there are studies. This contrasts strongly with chondrules, for which a general consensus exists, i.e., chondrules formed by melting of preexisting dust aggregates by high-temperature processes within the solar nebula. A pa~icularly important aspect of any model for the origin of chondrules is that they formed by the same mechanism in the different chondrite groups. In contrast, based on current models for the origin of matrix, matrix in each different chondrite group formed by a different mechanism (excluding the aqueously altered CI and CM groups). This is perhaps surprising because, although matrix materials show a considerable diversity from one chondrite group to another, they share many similarities which indicate that important links exist between the components of matrix (e.g., the presence of FeO-rich olivine) in the different chondrite groups with largely anhydrous matrices (CO, CV, ordinary chond&es). Such links suggest that it may be more probable that common mechanisms, be they chemical or physical, are responsible for formation of matrix materials. In this discussion, I examine the possible origins of matrix and rim materials in ALHA based on the detailed petrographic observations described in the previous sections. These data allow assessments of possible origins for matrix in the context of current models. The petrographic and mineralogical observations presented above can be summarized into a number of important points. First, the matrix in ALHA is a highly unequilibrated

1533

assemblage of material, both amo~ho~ and crystalline, which has clearly undergone minimal parent body metamorphism. Second, a number of distinct mineralogical components can be identified within the matrix which clearly must have formed under different conditions. Third, amorphous material is a very major component and in this respect ALHA matrix appears to be similar to the least equilibrated ordinary chondrites. Fourth, despite the marked mineralogical diversity of the matrix on a very fine scale ( submicron ) , on the scale of an electron microprobe analysis ( lo-pm diameter beam) the matrix and rims are compositionally homogeneous. Finally, the mineralogy and chemistry of in~vidu~ rims and finegrained matrix are essentially compositionally identical. These data strongly suggest that rim and matrix materials in this meteorite represent the same reservoir of material and consequently have identical origins. To simplify the following discussion I wilI use the term matrix to denote both interchondrule matrix and rims, unless a distinction between the different occurrences is appropriate. POSSIBLE ORIGINS OF MATRIX AND THE MINERALOGICAL COMPONENTS OF MATRIX Current models for matrix in chondritic meteorites can be broadly divided into two views: ( 1) matrix is the result of secondary processing of other components such as chondrules and cannot be considered to be primitive (e.g., ASHWORTH, 1977; HOUSLEYand CIRLIN, 1983; ALEXANDERet al., 1989) and (2) matrix represents primitive materials which formed by processes in the early solar nebula or may consist of a combination of nebular and presolar materials (e.g., NAGAHARA, 1984, KORNACKI and WOOD, 1984; COHENet al., 1983; BREARLEYet al., 1989; BREARLEY, 1989a). This material may have been subsequently processed to some degree within the solar nebula. There is clearly a major dichotomy between any of the nebula models and models which invoke an origin for matrix by derivation from chondrules as exemplified by, for example, the work of ALEXANDER et al. ( 1989). Based on studies of matrix in unequilibrated ordinary chondrites ALEXANDERet al. ( 1989 ) have argued that matrix consists of a component derived by fragmentation of chondrules (the elastic component comprised principally of olivines and pyroxenes) and a very fine-grained component (the nonelastic component) consisting of Fe-rich olivine and amorphous material. ALEXANDERet al. ( 1989) have argued that the nonelastic matrix formed by solid state recrystallization and reaction of chondrule mesostasis with Fe,Ni metal. In contrast, nebular models generally argue for formation of matrix materials in ordinary chondrites and carbonaceous chondrites by either low-tem~rature equiiib~um condensation (e.g., NAGAHARA, 1984) or disequilibrium condensation processes (e.g., KORNACKI and WOOD, 1984; COHEN et al., 1983; BREARLEY et al., 1989; BREARLEY,1989a). These two endmember models provide a framework for examining the possible origin of matrix and rims in ALHA77307. Origin of Clastic Matrix Component In the case of the angular, relatively coarse-granted component of matrix in the ordinary chondrites, an origin by

1534

A. J. Brearley

FIG. 6. Transmission electron micrographs showing the general microstructural characteristics of fine-grained chondrule rims and matrix in ALHA77307. (a) Bright field transmission electron micrograph showing a typical region of matrix which consists of submicron crystals (many are in Bragg diffracting orientations and appear dark in the image) set in a groundmass or matrix of amorphous material (which appears lighter in the image). The inset electron diffraction pattern shows a number of diffuse diffraction rings consistent with the presence of an amorphous phase. The sharp diffraction maxima are from the randomly oriented crystalline phases. (b) Bright field transmission electron micrograph of a region of matrix showing the presence of abundant, randomly oriented crystallites set in an amorphous groundmass. The inset electron diffraction pattern shows sharp diffraction rings which can be satisfactorily indexed as olivine. (c)

Matrix and rims in CO3 chondrite ALHA

1535

Bright field image of a region of matrix showing the presence of an angular, elongate crystal of low-0 pyroxene (qYx ), olivine (OL), kamacite (Kam) and pyrrhotite ( FeS) all set within an amorphous matrix. (d) Bright field image of a region of matrix, which shows an extremely low abundance of crystalline material. The amorphous component (Am) is abundant and contains rounded to subrounded blebs of Fe,Ni metal and suifides (FeS) disseminated through it. fe) Bright field image of a region of matrix which contains a significantly higher abundance of crystalline material, in this case comparatively coarse-grained olivine crystals with highly irregular crystal shapes.

fragmentation of chondrules seems most likely (e.g., ALEXANDER et al., 1989; BREARLEY et al., 1989). A similar origin

for at least some components of the matrix in ALHA appears to be viable, based on a cursory examination of the data. For example, the anhedral to subhedral grains of olivine and pyroxene and the amorphous material (which may represent chondrule mesostasis) could be indicators of a chondrule origin. However, a detailed examination of the data shows that this is highly improbable for most of the components in ALHA matrix. Some of the well-crystallized, coarser-grained olivines with high forsterite contents (e.g., Figs. 2c,d, 9d,e) could conceivably represent chondrule fmgments, as Mg-rich chondrules are the most common type in CO3 meteorites. However, it would be expected that more Fe-rich olivines would also be represented in the size fraction, but none has been observed. in addition, the olivines have subrounded morphologies, not the angular, fragmental characteristics, which are typical of elastic olivines in ordinary chondrite matrices. The second group of relatively coamegrained forsteritic ofivines have, high Mn contents (LIME olivines) and could not have been derived from chondrules, since olivines with this distinct, unusual composition do not occur in chondrules. Eyroxene fragments derived from chondrules would also be expected to occur. However, the observed pyroxene fragments present in ALHA matrix are dom-

inantly orthopyroxene, which clearly experienced slow cooling histories. Low-Ca pyroxene in chondrules in unequilibrated meteorites (ALHA included) consists almost exclusively of twinned clinopyroxene or an intergrowth of ortho and clinopyroxene (dominated by clinopyroxene). Such intetgrowths are formed as a result of inversion of protopyroxene to me&table clinopyroxene during rapid cooling of the chondrules from high temperature (SMYTH, 1974; BREARLEY and JONES,1988). Thus, the fragments of orthopyroxene cannot be derived from chondrules. Only one grain of inte~own clinopyroxene and o~hopyroxene has been observed which could conceivably be derived from a chondrule. In summary, the bulk of the elastic component of the fine-grained matrix and rims in ALHA could not have been derived from ehondrules. The elastic components in ALHA may have a common origin with the nonelastic material, but experienced somewhat different thermal histories. This possibility is examined further in the follo~ng discussion. Origin of Nonelastic Component

of Matrix

The nonelastic component of the matrix consists f as noted in the previous text) of an amorphous component, finegrained olivine and pyroxene, Fe,Ni metal, sulfides, and

1536

A. J. Brearley

FIG. 7. High-resolution transmission electron micrographs of amorphous material in ALHA chondrule rims and matrix and associated ultrafine-gr~ned crystalline phases. (a) Bright field image of a region of amo~hous material showing the typicaf phase contrast of a truly amo~hous material. The diffuse diffraction rings in the inset electron diffraction pattern are entireiy consistent with this inte~re~tion. Locally there appears to be some evidence for ordering as indicated by the presence of extremely weak, undulose lattice fringes with a basal repeat of 1 nm. (b) High-resolution transmission electron micrograph of microcrystallites within a region of amorphous material. The morphology of the crystallites suggests that they may be protophyllosilicates of some type. Based on the observed 1 and 1.4 nm lattice fringes these may be smectite and chlorite-type minerals. (c) Electron micrograph of a microcrystallite of a phase with a I .4 nm basal spacing. The spacings of the lattice fringes are consistent with a chlorite-like phase consisting of a 0.9 run talc-like layer and a 0.5 nm brucite layer. (d) Electron micrograph of extremely poorly crystalline phases with basal spacings of 1.4and 0.7 mu, probably chlorite and serpentine, respectively.

magnetite. The origin of the amorphous component is central to any discussion of the origin of matrix in ALHA77307.

Several possible origins can be invoked for amorphous materials in the matrix of chondritic meteorites. These include (a) chondruie mesostasis ejected from chondruies during chondrule formation (e.g., ALEXANDER et al., 1989; HUTCHEON and BEVAN,1983), (b) shock melt produced by shocking of fine-grained olivine, and (c) amorphous condensate material formed by disequilibrium condensation processes. Each of these possible origins is considered below. Origin jbom chondrtde mesostasis ASHWORTH ( 1977) and ALEXANDER et al. ( 1989) have

reported an amorphous

“glue”-like

material interstitial

to

line-grained

Fe-rich

olivines

in the matrix

of several

un-

equilibrated ordinary chondrites. This amorphous material is high in Si, AI, Na, and Ca and has been interpreted as representing feldspathic mesostasis produced by the fragmentation of chondtuies. For the case of the unequiI~brat~ ordinary chondrites such an origin is therefore plausible. However, compositionally the matrices of unequilibrated ordinary chondrites differ in several important respects from matrix in ALHA in being significantly enriched in SiOz, MgO, CaO and NazO. For example, UOC matrices have CaO and NazO contents of 0.9-3.5 and 0.5-3.1 wt%, respectively (Huss et al., 1981; IKEDA et al., 1981; MATSUNAMI, 1984: SCOTT et al., 1984), whereas for ALHA these ranges are significantly lower, 0.43-0.97 and 0.08-0.34 wt%, respectively. The composition of the amorphous component

Matrix and rims in CO3 chdndiiteALHA77307 TABLE 2.

Analytical electron microscope analysis of amorphous material in the matrix of ALH A77307.

SiO2

41.61

46.04

49.88

53.96

55.49

59.32

61.48

TiO2 A’203 cr203 Fe0 MnO

5.90 0.41 29.02

7.15 0.38 22.09

5.17 0.89 25.87

4.03 0.46

7.64 0.36

5.52

0.48

22.88 0.17 II.23

16.35 0.07 6.48 0.21

18.59 0.14

lo.08

0.63 9.57 0.15

2.34 0.45 9.30 0.14

MsO CaO

0.06 6.35 I .25

5.91 0.46 21.23 0.30 7.33

a.68 0.24

9.75 0.17

NiO

9.06 0.89 2.08

a.06 6.61

6.34 -

3.68 4.63

3.95 na

2.60 2.68

p2°5 S Total

0.47

0.54

-

4.30 4.12

1.2 -

0.25

71.98

1.26 1.15 0.59 4.54 2.01 I .a5 I .50 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

na = not analyzed.

present in ALHA reflects the bulk composition of matrix and is clearly not feldspathic in character as can be seen from Table 2 and Fig. 8. In addition, the Ca/Al ratio of matrix in ALHA is extremely low in comparison with typical values for chondrules, which are usually close to solar (GROSSMAN et al., 1988). Calcium and aluminum are typically concentrated in the mesostasis of chondrules, thus precluding chondrule glass as a possible precursor of matrix. These data provide strong evidence that the amorphous component of the matrix of ALHA could not have been derived from chondrule mesostasis, consistent with the evidence presented above that the elastic component of matrix derived from chondrules is also extremely low. Based on these lines of evidence, I conclude that an origin of a minor proportion ( < 1 ~01%) of the matrix components in ALHA from chondrules is viable, but the remaining 99 vol% has no relationship to chondrules.

1537

tionship between individual chondrules and associated rimming material. Furthermore, matrix and rim compositions in ALHA are remarkably homogeneous, particularly with respect to their minor element abundances. One of the principal characteristics of shock effects is that they are very heterogeneously developed. It seems highly improbable that the compositional homogeneity and common grain size of rims on many chondrules and other objects could be produced by the type of shock mechanism proposed by BUNCH et al. ( 199 1) . A shock mechanism also requires considerable disruption of the chondrule margins and incorporation of &hese materials into the rim. I have examined in detail the contact between chondrule and rim material in a large number of chondrules and found that the margins are invariably sharp (e.g., Fig. 2a-c) and show no evidence of disruption or the presence of intermediate reaction zones at the interface. For example, I have not been able to locate any fragments of Fe-rich olivine in the rims of type II, FeO-rich porphyritic olivine chondrules. In addition, as noted earlier in the BSE and TEM observations, viable fragments of chondrule minerals included in the fine-grained rims are either extremely low, or nonexistent, in many of the rims of ALHA77307. Indeed, there are clearly a number of mineralogical components within the matrix which could not be formed by such a mechanism, such as the clusters of LIME olivine and orthorhombic low-Ca pyroxene, which do not occur in chondrules. Finally, it would be expected that if rims were formed by impact of chondrules into a regolith, which already consisted of rimmed chondrules formed by a similar mechanism, fragmentation of these objects would mur, such that broken chondrules with partial rims should be present in the

Fe

Origin by shock melting of matrix material It is well established that shock-induced transformations of material can result in the formation of amorphous materials (see ST~FFLER et al., 1988, 199 1, for recent reviews). BUNCH et al. ( 199 1) have recently presented observations which suggest that some chondrule rims in ordinary chondrites were formed as a result of impact into a low-density medium such as a regolith. They argue that some fine-grained rims may have formed by fragmentation, melting and reaction of chondrule margins during impact. It is thus possible that the amorphous component in the matrix of ALHA could have formed by shock melting produced by this type of interaction. Several lines of evidence, chemical, textural, and microstructural argue against such an origin being a viable mechanism for ALHA77307. BUNCH et al. ( 1991) suggest that fine-grained rim material formed by melting and fragmentation of chondrules. Rims produced by such a process would be expected to have distinct compositional relationships with the host chondrule. As noted earlier, electron microprobe data for a large number of chondrules and their enclosing rims, show that there is no systematic compositional rela-

Si FIG. 8. Compositions of amorphous material in matrix and rims in ALHA plotted within a ternary Si-Fe-Mg (wt% element) diagram. All the analyses were obtained by analytical electron microscopy. The data exhibit a broad range, but are all Mg poor and lie close to the Si-Fe join. Also shown is the range of bulk matrix and rim compositions as well as matrix and rim ohvine compositions. The data show that the bulk matrix compositions can be fully rep resented by a three-phase triangle defined by the amorphous phase, olivine and an Fe-rich phase, such as magnetite or Fe,Ni metal.

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A. J. Brearley

FIG. 9. Transmission electron micrographs showing the range of morphologies and microstructures observed in matrix and rim olivines in ALHA77307. (a) Bright field electron micrograph of elongate, platy olivine with poorly developed, irregular facets. Locally the olivine has a strained appearance and contains planar defects (arrowed) parallel to (010). The surrounding matrix is amorphous. (b) Bright field electron micrograph of a subhedral crystal of olivine set in an amorphous matrix. The olivine has an unusual mottled contrast and contains planar defects, indicating that it is strained and may be poorly crystalline. Inset is a high-resolution image taken with the electron beam parallel to the c-axis. (c) Bright field image of a cluster of fine-grained, anhedral olivines which exhibit strain contrast indicating that they are poorly crystalline. (d) Bright field image of a subhedral, well-crystallized grain of forsteritic olivine, set in a matrix of fine-grained olivine and amorphous material. This morphologically distinct type of olivine is largely dislocation free and is significantly coarser grained than other occurrences of olivine in the matrix and rims of ALHA77307. (e) Further example of a micron-sized, well-crystallized olivine in ALHA matrix. This grain is forsteritic, but is enriched in MnO, typical of LIME olivines reported by KLOCK et al. ( 1989). (f) Distinct cluster of LIME olivines (OL) and pyroxenes (PYX) with grain sizes CO.I5 pm. The grain boundaries between individual crystals are curved, indicating that these clusters are not at textural equilibrium.

meteorite. One of the remarkable features of ALHA is that such features are not present and many objects (although not all) have rims of some sort.

The microstructural data obtained by TEM are also incompatible with a shock origin for the rim materials. Although the abundant amorphous material in ALHA matrix

Matrix and rims in CO3 chondtite ALHA

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FIG. 9. (Continued)

could be interpreted as a shock melt, based on the experiments of RIETMEIJER and ALBRECHT ( 199 1 ), shock pressures significantly in excess of 14.8 GPa would be required to produce significant amounts of melting, perhaps of the order of 2530 GPa. These experiments utilized fine-grained olivine as the starting material and hence closely approximate matrix material in terms of mineralogy and grain size. At shock pressures of this magnitude, dislocation development in olivine as well as formation of planar fractures and deformation features are likely to be important processes (ST~FFLER et al., 199 1). In ALHA matrix olivines are typically dislocation free, although one large olivine with a very high dislocation density was observed. In addition, shock twinning of pyroxene and shock-induced lamellae of clinopyroxene should develop in orthopyroxene. Orthopyroxene grains contain only minor clinopyroxene lamellae, which Seems to be incompatible with high shock pressures and could simply be. the result of incomplete inversion to orthopyroxene during cooling or annealing. Finally, in the experiments of RIETMEIJER and ALBRECHT ( 1991), the effects of shock are extremely heterogeneous and in the fine-grained matrix a welldefined foliation develops due to the formation of branching melt sheets. No such textures are observed in the matrix or rims of ALHA77307. Whilst the local abundance of amorphous material varies, there are no gross heterogeneities, which would be expected if shock metamorphism was an important process in the evolution of matrix. The unusual chemistry of amorphous material in ALHA also argues strongly against a melt of shock origin. If the rims formed by partial shock melting of chondrules and a matrix component rich in olivine, it would be expected that the melt would be compositionally similar to olivine, i.e., silica undersaturated and with high Mg contents. In contrast the amorphous material has high concentrations

of SiOz and is low in MgO (e.g., Fig. 8). Additionally it has high concentrations of S, NiO, and P205, which would not be expected of a shock melt. ASHWORTH (1985) in TEM studies of shocked ordinary chondrites has shown that in shock-produced melts, S and Ni form globules of sulfide and metal whilst the silicate glass remains free of these elements. In conclusion, the textural, chemical, and mineralogical data available provide no evidence to support an origin by shock melting or shock processes in general for the finegrained chondrule rims in ALHA77307. Whilst such a mechanism may have been important in the formation of rims in some of the ordinary chondrites (BUNCH et al., 199 1). it clearly did not play any significant role in the case of ALHA77307. Formation by disequilibrium condensation processes Traditional models of condensation within the solar nebula have considered equilibrium condensation as the principal mechanism for forming solid phases from a cooling vapor (e.g., LARIMER, 1967; GROSSMANand LARIMER, 1974). Implicit in the concept of equilibrium condensation is the assumption that the solar nebula was initially completely vaporized and underwent progressive cooling. An alternative mechanism of condensing solid materials from a vapor phase. is disequilibrium condensation, where the gas is rapidly supercooled. Under such conditions, kinetic factors, such as the activation energy for nucleation, override thermodynamic considerations and, as a consequence, the phases which condense are not those predicted on thermodynamic grounds (e.g., DONN and NUTH, 1985). There is a large body of experimental evidence which demonstrates that the products of disequilibrium condensation experiments on silicate-bearing compositions produce amorphous, nonstoichiometric

A. J. Brearley

a

[EEEEJ

I-

i-

10

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30

Fa

40

50

60

70

(MOLE 8)

FIG. 10. Histograms showing the compositions (in mol% Fa) of morphologically distinct olivines and pyroxenes in the matrix of ALHA77307. (a) Subhedral to euhedral, well-crystallized forsteritic olivines. (b) Fine-gained, poorly crystalline olivines associated with amorphous material. (c) LIME olivines. (d) All matrix olivines.

materials (smokes) which are metastable (e.g., DAY and DONN, 1978; NUTH and DONN, 1983; RIETMEIJER et al., 1986; RIETMEIJER and NUTH, 1991). These amorphous smokes rapidly recrystallized, even under relatively low-temperature conditions to produce submicron crystalline materials (e.g., RIETMEIJERet al., 1986; NUTH and DONN, 1983). In metallic and simple binary oxide systems the products of condensation experiments tend to be crystalline, in contrast to the amorphous products in silicate systems (STEPHENS and KOTHARI, 1978). Disequilibrium condensation studies have been promoted by the observation that the bulk of the silicate material in the interstellar medium appears to be amorphous, based on IO and 20 pm features in infrared absorption spectra (e.g., TIELENSand ALLAMANDOLA,1987). This amorphous silicate component of the interstellar medium originates as a result of condensation of dust in circumstellar environments such as the outflow from proto-planetary nebulae, and O-rich red giants. In such environments material is transported ex-

tremely rapidly from the stellar surface ( HINKLE, 1985 ) and is supercooled far above the photosphere, where condensation occurs at temperatures perhaps 400 K lower than the calculated equilibrium condensation temperature. It is thus clear that condensation of amorphous silicate material occurs in certain astrophysical environments and material formed by this type of mechanism could be present in ALHA (and hence be of presolar origin). It is also possible that condensation processes within the nebula itself could have produced amorphous silicates. The earliest model for the evolution of the solar nebula is based on the concept proposed by CAMERON( 1962) of a totally vaporized nebula, which cooled progressively, such that equilibrium condensation of refractory elements occurred at high temperatures followed by more volatile elements as the temperature fell. More recent astrophysical models for nebula evolution argue against a very hot nebula and have moved more towards the concept of a viscous accretion disk, in which temperatures at the formation location of chondritic meteorites would not have been high enough to vaporize silicate material (WOOD and MORFILL, 1988). Such models invoke a cool nebula in which localized episodic and cyclic high-temperature evaporation/condensation and melting events occur which produced chondrules, CAls, and some matrix materials from presolar interstellar dust (CLAYTON, 1978, 1982; WOOD, 1985; HUSS, 1988). These models appear to be broadly consistent with the several lines of evidence from meteorites, which argue against a completely hot nebula. For example, in recent years the discovery of a wide variety of isotopic anomalies within meteorites (see THIEMENS, 1988, for a recent review) has shown that the nebula was heterogeneous on a wide scale and that several isotopic reservoirs were present. In addition, the discovery of intact presolar grains in meteorites shows that some interstellar materials have escaped extensive thermal processing within the nebula, lending support to the concept of a cool nebula. The possibility of forming amorphous condensates within the nebula is not excluded by either hot or cold nebula models. In the cold nebula model, the nebula underwent violent, dramatic high-temperature events which produced chondrules and either formed CAIs or processed preexisting CAIs extensively. The situation for the case of a nebula which was initially completely vaporized, and underwent progressive condensation, is in fact similar, because the violent thermal events appear to have occurred after the nebula had cooled significantly. These constraints are provided by chondrule textures which show that the high-temperature events were rapid excursions from a relatively cool nebula to high temperature, since chondrules clearly cooled rapidly ( HEWINS, 1988). Such events provide the ideal mechanism for producing the conditions under which disequilibrium condensation could have occurred. For example, during high-temperature events, preexisting dust may have been melted to form chondrules, or if temperatures were sufficiently high the dust could have been evaporated and recondensed under disequilibrium conditions to form amorphous materials. Thus, the presence of amorphous condensates should be expected as a component within the nebula prior to accretion of the chondrite parent bodies. irrespective of whether the nebula was completely vaporized or not. High-temperature processing in the nebula

Matrix and rims in CO3 chondrite ALHA

DG . 11. Transmission electron micrographs illustrating the microstructures of low-Ca pyroxenes in the matrix and rims of ALHA77307. (a) Bright field image of an angular, elastic orthopyroxene grain showing the presence of minor striations parallel to ( 100) indicative of the presence of intergrown clinopyroxene, which is most abundant in the lower right-hand side of the figure. Inset are electron diffraction patterns (top right) showing only minor streaking of hO0 diffraction maxima parallel to a* and a high-resolution image showing the I .8 nm ( 100) lattice repeat of the orthopyroxene (lower left). (b) Bright field image of a grain of Mg-rich orthopyroxene from an aggregate of pyroxene crystals. The crystal clearly shows the presence of stacking disorder on ( 100) due to minor intergrown clinopyroxene. Inset electron diffraction pattern shows minor streaking parallel to a*.

was episodic, such that early formed amorphous condensates

could either themselves undergo melting to form chondrules or be annealed, revaporized, and recondensed repeatedly as the nebula evolved. I have argued elsewhere (BREARLEY, 1989a) that there is evidence for high-temperature annealing of fine-grained matrix materials (in the matrix of the unique chondrite, Kakangari), which preceded accretion. KORNACKI and WOOD ( 1984) have argued that crystalline, fine-grained

olivines in the matrix of the Allende CV3 meteorite formed in a similar manner by annealing of amorphous presolar material. Repeated thermal events would reprocess early formed condensate material in a variety of ways, such that depending on the nature of mixing in the nebula, late-formed condensate material would be more likely to survive. There is clear evidence that interstellar materials have survived in the CO3 meteorites (in addition to the other chondrite groups ) , providing support to the idea that amorphous silicate condensates of nebular or presolar origin could also be present. KERRIDGE(1985 ) reported the presence of large deuterium (D) enrichments (6D of +SOO%o ), associated with the organic component of Ornans (type 3.3; SCOTT and JONES,1990). The presence of a deuterium-enriched component suggests that the matrix of this meteorite was never heated to above 300-350°C (ROBERT and EPSTEIN, 1982).

The most likely sources of deuterium-rich material are organic molecules formed in the interstellar medium by ion-molecule reactions ( ZINNER, 1988), because such large isotopic fractionations require extremely low temperatures (~60 K) which do not appear viable within a nebular environment. Whilst the mechanisms of the reactions which form these molecules is unclear, it has been suggested that grain surface reactions are likely to be important, such that organic material will condense onto preexisting grains. Amorphous silicate grains, as a major component of interstellar dust, would have played an important role in such reactions and could have been the carriers which brought the organic components into the solar nebula. The amorphous component in ALHA could be considered a viable candidate for such material. In conclusion, the current astrophysical, isotopic, and meteoritic constraints on the nature of conditions and processes within the solar nebula, circumstellar, and interstellar environments strongly suggest that not only is disequilibrium condensation a viable mechanism for producing amorphous silicate materials, but also that these materials could have survived in unequilibrated meteorites such as ALHA77307. However, based on the mineralogical data from this study it is not possible to distinguish between a nebular or presolar origin for the amorphous material. Presolar or nebular dust

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A. J. Brearley

b

FIG. 12. Transmission electron micrographs showing the variation in morphoiogy of magnetite in the matrix and rims of ALHA77307. (a) Bright field image of a large aggregate of magnetite crystals, which consists of a number of grains with sizes of around 0.1-0.2 pm. Grain boundaries between individual crystals are straight and 120” triple junctions occur (an example is arrowed) indicating textural equilibrium. (b) Bright field image of a single, isolated magnetite crystal, with an unusual mottled diffraction contrast, which may be due to the presence of crystallographic defects. (c) Bright held image of an isolated, almost euhedral crystal of magnetite, which shows welldeveloped facets parallel to ( I 10). The crystal also contains a number of planar defects on ( 1IO), which may be cation stacking faults, such as are found in silicate spinels. Inset is a high-resolution image ([ 1I I ] zone axis) showing the 0.29 nm ( 110) lattice fringes.

would have been processed in a variety of ways and, if current

nebula models invoking high levels of turbulence (e.g., MORFILL et al., 1985). mixed thoroughly

are correct on a very

fine scale. In the following text, I examine the petrologic characteristics of ALHA matrix in the context of such a model in more detail.

Matrix and rims in CO3 chondrite ALHA

1543

FIG. 14. Bright field transmission electron micrograph of a single isolated crystal of pentlandite set in a matrix of microcrystalline and

amorphous material. The pentlandite shows considerable strain contrast suggesting that it is poorly crystalline. The inset electron diffraction pattern (top right) shows strong asterism of the diffraction maxima consistent with a high degree of strain.

MATRIX COMPONENTS-PHYSICAL CHEMICAL PROCESSING

FIG. 13. Transmission electron micrographs of kamacite in the matrix and rims of ALHA77307. (a) Fe,Ni metal grain which has clearly undergone some aqueous alteration and is surrounded by a narrow (3 nm) rim of an iron oxide phase. The particle is embedded in a matrix of amorphous material which contains phyllosilicates with a 0.7 nm basal spacing, characteristic of serpentine. (b) Bright field image showing an irregularly shaped Fe,Ni grain embedded in a matrix of siliceous amorphous material. There is no evidence for aqueous alteration of either the metal grain or the surrounding amorphous material.

AND

The detailed TEM studies just reported reveal that there are a number of distinct mineralogical components within the matrix of ALHA77307. I consider that the following eight distinct groups, based on their chemical and morphological characteristics, can be defined as the principal components of the matrix. These are ( 1) regions of amorphous/microcrystalline material containing varying amounts of anhedral, fine-grained olivines with rather variable compositions (e.g., Fig. 6a-c), (2) isolated, well-crystalline Mg-rich (F@5_98) olivines with subhedral morphologies (e.g., Fig. 9e), (3) finegrained clusters of very fayalitic olivine (Fa~a_6~),(4) isolated, well-crystalline LIME olivines, (5) clusters of LIME olivines and pyroxenes (e.g., Fig. 9f), (6) isolated crystals of Mg-rich orthopyroxene ( Fii 11a), ( 7 ) aggregates of magnetite crystals (e.g., Fig. 12a), and (8) isolated clusters of Fe-rich mixed layer phyllosilicates (Fig. 16). In addition, there are some notable differences between different regions of matrix which consist predominantly of amorphous material. For example, some regions appear to be rather oxidized, as indicated by the presence of magnetite, whereas Fe,Ni metal is the dominant phase in other regions. Whatever the origin of the amorphous component, very different oxidation conditions were experienced on a localized scale. Furthermore, the ratio of amorphous to crystalline material varies significantly from

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A. J. Brearley

region to region within the matrix on a very localized scale. These data suggest that the matrix in ALHA consists of basic, distinct, fine-grained units, which have different chemistries, mineralogies, and thermal histories and have been juxtaposed during formation of the matrix. In other words, these units may represent individual dust grains or aggregates, which were sampled by the chondrite during accretion. Below I present arguments that the variation in the textural characteristics of some of the individual components in ALHA matrix can be rationalized relatively simply in terms ofthe formation and subsequent annealing of amorphous condensate materials to various degrees within the solar nebula. Amorphous Component

FIG. 15. Bright field transmission electron micrograph showing the typical occurrence of anhydrite in ALHA77307. (a) Vein-filling anhydrite with a fibrous morphology. The elongation direction (parallel to (00 I )) lies normal to the edges of the vein suggesting that the crystal nucleated and grew inwards from the wall of the fracture. Individual crystals are parallel to subparallel, resulting in asterism of the diffraction maxima in the inset electron diffraction pattern (top

The amorphous component in ALHA is intimately associated with crystalline phases throughout the matrix and the proportions of crystalline phases vary from one region to another. I suggest that these different regions represent distinct clusters of grains of nebular or presolar dust which have experienced different degrees of annealing and probably formed under different conditions and have varying (albeit Fe-rich) bulk compositions. Ifthis is the case then the crystalline phases present within the amorphous component should have crystallized from, and have a composition which is closely related to it. This possibility was explored in detail by carrying out analytical electron microscope analyses of coexisting finegrained olivine and amorphous material from several regions of matrix, showing a large range in olivine compositions and proportions of crystals. Olivine was chosen because it is the most abundant silicate phase, in addition to the amorphous material. The results of this study are shown in Fig. 17, a diagram of the Mg/Fe ratios of coexisting olivine and amorphous component. Whilst there is considerable spread in the data, it is evident that there is a positive correlation (albeit weak) between the host amorphous phase and the olivine, with the Mg being fractionated into the olivine. This appears to provide support for a genetic relationship between the olivine and amorphous component. One very good piece of evidence to indicate that the olivines associated with the amorphous component formed as a result of low-temperature annealing is the fact that they appear to be poorly crystalline, with abundant defects and poorly defined outlines. Evidence to support the idea that the amorphous material represents samples of material formed under different conditions comes from the presence of oxide, sulfide, and metal assemblages in different regions of matrix. Regions of amorphous matrix occur that contain metal and sulfide which can be adjacent to areas that contain magnetite intimately intergrown with amorphous material. Evidently, formation of magnetite and metal must have occurred under markedly different oxidizing conditions, at different times or locations within the nebula, but were juxtaposed by mixing of dust.

left). (b) Bright field image illustrating the mottled contrast of anhydrite, suggesting a relatively poor degree of crystallinity. Inset (top right) shows a high-resolution image of anhydrite with the (00 1) 0.69 nm lattice fringes resolved.

Matrix and rims in CO3 chondrite ALHA

FIG. 16. Bright field image of disordered mixed layer phyllosilicate phase in the matrix of ALHA77307. The grain consists of several different subgrains which are oriented with their basal planes normal to each other (as indicated by the inset electron diffraction pattern, top right). The basal lattice fringes locally show considerable curvature. The inset high-resolution image illustrates the highly disordered nature of the phase and the intergrowth of phases with 0.85 and 1.2 nm basal repeats. Locally lattice repeats of 0.7 nm also occur which may be serpentine.

The highly unusual composition of the amorphous component in the matrix is additional evidence pointing towards an origin as a result of disequilibrium condensation processes. As discussed earlier this material contains high contents of S, NiO, and sometimes PzOz , which would not be anticipated in a glass, formed by melting processes. The only other material which has similar compositions is amorphous/microcrystalline material in CM2 chondrites which contains abundant fine-grained phyllosilicates (typically serpentine; e.g., BARBER, 1981; BREARLEY, 1989b; BREARLEYand GEIGER, 1991). It is not clear in CM2 chondrites whether the amorphous/microcrystalline component is formed as a result of aqueous alteration of, for example, olivine or was already present in the meteorite prior to alteration. Based on studies of the palagonization and aqueous alteration of basaltic glass, it seems most probable that the observed textures in CM2 meteorites can be best interpreted as the formation of a gellike material from an amorphous phase by interaction with aqueous fluids (e.g., ZHOU and FYFE, 1989). Whilst there is evidence of incipient local aqueous alteration in the amorphous component of ALHA77307, as indicated by the presence of protophyllosilicates, it is clear that the bulk of this material has not been affected. For example, most regions of

1545

the amorphous component show no evidence of protophyllosilicate development, and regions of this material contain completely fresh kamacite, which shows no evidence of alteration. These data show that the compositions of the amorphous phase are a primary feature and clearly point to an unusual formation mechanism, such as disequilibrium condensation. The experimental data are currently not available to assess in detail what compositions could be produced by evaporation and disequilibrium recondensation of material of an approximately chondritic composition. However, nonstoichiometric amorphous smokes have been produced in a number of endmember systems, such as Si-0, Mg-SiO, and Mg-FeSiO (STEPHENS and KOTHARI, 1978), but in more Fe-rich systems ( Fe-SiO) there appears to be a tendency for discrete Fe- and Si-bearing phases such as maghemite and tridymite to form in addition to an amorphous Si-rich component ( RIETMEIJER and NUTH, 199 1). The condensation expe.riments of STEPHENSand KOTHARI ( 1978) on a Ni,Fe alloy were found to produce crystalline metallic products. Such data suggest that systems with chondritic abundances of most elements, including Ni and S, could condense as an amorphous phase along with Si, Fe, Mg, and Al, although this is not yet proven. However, as the concentrations of Ni and S increase in the condensing material, condensation of discrete grains of sulfides, Fe,Ni metal and oxides (depending on the oxygen fugacity) could occur along with an amorphous SiFe-Mg-bearing component. Some regions of the amorphous material with low proportions of crystalline silicate material (Fig. 6a,b) frequently contain relatively abundant subrounded Fe,Ni metal and sulfide grains, which could represent a crystalline condensate, formed under reducing conditions. One of the characteristics of amorphous condensate smokes is that they consist of delicate chains of spherical particles. No such structures exist in the matrix of ALHA77307, but if such chains were present as a component of nebular or interstellar dust they would probably be crushed and compacted during accretion of dust into larger aggregates and

10

m

! 1 I

1 I

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I+m

D

1

I

10

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1

I

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I

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70

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W(Mg+W

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olivlne

FIG. 17. Plot of the Mg/( Mg + Fe) ratios of coexisting fine-grained olivine and adjacent amorphous material from the matrix and rims of ALHA77307.

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A. J. Brearley

ultimately into the meteorites, as noted by STEPHENS and KOTHARI ( 1978). Modification of these structures would further occur as annealing and crystallization proceeded and they would, presumably, be rapidly obliterated. Some of this annealing could have occurred within a parent body environment, although based on the very unequilibrated nature of the mineral assemblage in ALHA matrix, it must have been of limited extent. Clusters of LIME Olivines and Pyroxenes KL&ZK et al. ( 1989) have argued that LIME olivines found in IDPs and matrices of unequilibrated meteorites formed as a result of equilibrium condensation. However, for IDPs it is possible that these olivines represent the products of solid state annealing (F. J. M. Rietmeijer, pers. commun., 1992). Annealing of amorphous condensate material may well be a viable origin for LIME olivines in ALHA77307. Evidence to support such a hypothesis comes from the fact that Mn-rich pyroxenes and olivines frequently exist in clusters of very fine-grained crystals, very similar in texture to those found in the matrix of Kakangari, which I have argued were formed as a result of annealing of an amorphous precursor ( BREARLEY, 1989a). In addition, the unusual compositions of these pyroxenes and olivines are compatible with such an origin. Due to kinetic factors minor elements, such as Cr and Mn, could be incorporated into Mg-olivine and pyroxene, if the crystals grew rapidly from the amorphous material. Under such disequilibrium conditions incorporation of these elements would be favored kinetically and normal crystal chemical arguments for site substitutions are overridden. Evidence for this type of behavior has been observed in meteoritic olivines ( ASHWORTH, 1979) and terrestrial almandine garnets ( BREARLEYand CHAMPNESS, 1986). Similar arguments have been used by KORNACKI and WOOD ( 1984) to account for high minor element concentrations in matrix olivines in Allende. The association of olivine and pyroxene in aggregates is explained plausibly by a model based on the experimental data on annealing of amorphous Mg-SiO smokes ( RIETMEIJER et al., 1986). Annealing initially produces an assemblage of metastable olivine and tridymite which progressively reacts to form enstaite. The association of LIME olivines and pyroxenes in clusters could be interpreted as a stranded reaction texture in which olivine reacted with metastable silica to form pyroxene. However, the reaction could not go to completion, because olivine and tridymite were not present in stoichiometric proportions. (This would be a function of the Mg/Si ratio in the starting material, prior to annealing.) Magnetite Clusters The clusters of magnetite grains and individual magnetite crystals can be fitted into a similar model involving disequilibrium condensation. As noted above in an Fe-rich system and under oxidizing conditions, magnetite could condense as very fine, crystalline grains, which would aggregate into clusters due to their magnetic properties. Studies by RIETMEIJER and NUTH ( 1991) show that maghemite can form during disequilibrium condensation experiments in the system Fe-SiO, but clearly variations in the oxygen fugacity could produce magnetite instead. Alternatively, if clusters of Fe,Ni

metal condensed in a relatively reducing environment they could undergo later oxidation to form magnetite. The morphology of some of the magnetite clusters in ALHA matrix suggests that they have undergone relatively extensive annealing at some point during their history. It is possible that this could have occurred during such thermal events as formed chondrules. Clastic Coarser Grained Component-Orthopyroxenes The elastic component in ALHA may have formed by a similar mechanism to the nonelastic material, i.e., annealing of amorphous disequilibrium condensates, but simply experienced longer periods of annealing prior to accretion of the nebular dust. For example, the well-crystallized orthoenstatite crystals in ALHA matrix could result from annealing of amorphous dust at relatively high temperature within the orthopyroxene stability field. This would require temperatures of between approximately 560 and 98O”C, temperatures which could be achievable at least locally within a nebula environment. Based on the experimental data of RIETMEIJER et al. ( 1986) discussed previously, enstatite probably formed via reaction of metastable olivine and tridymite. ANNEALING PROCESSES IN CO3 CHONDRITES The role of annealing is probably of great importance in explaining the difference in the matrix mineralogy of ALHA and other CO3 meteorites. More metamorphosed CO3 chondrites such as Kainsaz (3.1), Lanct, (3.4), and Warrenton (3.6) contain matrices which consist predominantly of crystalline, fine-grained FeO-rich olivine (KELLER and BUSECK, 1990; A. J. Brearley, unpubl. data). SCOTT and JONES ( 1990) have argued that the petrologic sequence observed in the CO3 meteorites is the result of parent body metamorphism of type 3.0 (ALHA77307) material. Based on diffusion modeling in olivine, JONES and RUBIE ( 199 1) have demonstrated that this is a valid way of producing the zoning observed in chondrule olivines in higher petrologic type CO3 meteorites. The effects of parent body metamorphism on amorphous material such as occurs in ALHA can be anticipated with reasonable confidence, i.e., it will recrystallize to produce fine-grained olivine. In the case of ALHA77307, mild annealing in a parent body environment may have occurred and, in combination with compaction processes, would have resulted in a reduction in pore space in the fine-grained rims. The amorphous condensate material probably has a rather low density, significantly lower than that of the crystalline material which may be forming from it. For silicate glasses, the closest amorphous material for which density data are available, measured densities are of the order of 4 to 10% less than their crystalline counterparts (CARMICHAEL et al., 1974). Some recrystallization during parent body metamorphism would result in a decrease in volume in the fine-grained rim and matrix material, which could readily explain the sets of radial fractures in the rims and complex cracking in interchondrule matrix. In conclusion, the mineralogical and textural characteristics of many of the components of the matrix of ALHA can be reconciled with a disequilibrium condensation origin,

Matrix and rims in CO3 chondrite ALHA followed by thermal processing to different degrees, largely prior to accretion. As such, each component probably represents individual grains or clusters of nebular dust. If this is the case then it is clear that primitive dust exhibited a remarkable mineralogical and chemical diversity on a submicron scale, which has been preserved in ALHA77307. Secondary annealing processes experienced by different dust components varied considerably, from extremely mild in the case of the amorphous component, to relatively severe for the aggregates of pyroxenes, LIME olivines and magnetites. Mixing of materials with different histories was also extremely thorough, such that on the scale of microns, aggregates of dust can be considered compositionally homogeneous. EVIDENCE FOR AQUEOUS ALTERATION (EXTRATERRESTRIAL VS. ANTARCTIC) Mineralogical evidence, such as the presence of phyllosilicates and anhydrite, indicate that ALHA has experienced some degree of aqueous alteration. For any meteorite recovered from Antarctica it is important to try to distinguish the effects of Antarctic weathering, such as those described by GOODING ( 1986), from possible extraterrestrial aqueous alteration. Three distinct types of occurrence of hydrous phases are locally present in the matrix: ( 1) grains of mixed layer phyllosilicates within regions of amorphous material (e.g., Fig. 16), (2) veins of anhydrite (Fig. IS), and (3) protophyllosilicates associated with regions of amorphous material (e.g., Fig. 7b-d). For the purposes of this paper, I will examine only the occurrences of type (2) and (3). The origin of the mixed layer phyllosilicates will be discussed elsewhere (A. J. Brearley, unpubl. data). The textural evidence presented earlier strongly indicates that the development of anhydrite was a late-stage event in the evolution of matrix in ALHA77307. It occurs in veins which crosscut regions of fine-grained matrix, consistent with such an interpretation. Clearly the possibility of this material being the result of Antarctic hydrocryogenic weathering must be considered seriously. GOODING ( 1986 ) described some of the effects of Antarctic alteration of chondrites and found that gypsum commonly occurred as an alteration product. Anhydrite has not been positively identified. Gypsum is the stable Ca-sulfate at low temperature (<5O”C) and dehydrates to form anhydrite at higher temperature ( HOLSER, 1979). However, these minerals are characterized by the fact that they frequenrly exhibit metastable behavior, such that anhydrite might be metastable at low temperatures where gyp sum is favored on thermodynamic grounds. Thus, although formation of gypsum is probably most likely under the conditions pertinent to Antarctic alteration, the possibility that anhydrite could form cannot be dismissed. Anhydrite does occur in other meteorites, but appears to be extremely rare. BARBERand HUTCHISON( 199 1) reported anhydrite in a car-

* METZLERet al. ( 1988) have called fine-grained rims on chondrules in CM2, Cv3, and CO3 meteorites (including ALHA77307) accretionary dust mantles. Whilst accretionary processes are clearly a viable mechanism for the formation of rims, I prefer to use terminology which does not infer a formational process, such as finegrained rims, to describe these features.

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bonaceous chondrite clast in Bencubbin (a find) and anhydrite has recently been found in veins in the Cold Bokkeveld CM2 chondrite (M. Lee, pets. commun., 1992). In the latter case the morphology of the anhydrite and its mode of occurrence is very similar to that observed in ALHA77307. It is perhaps noteworthy that anhydrite has never been identified in CI meteorites (e.g., RICHARDSON, 1978). The anhydrite present in Cold Bokkeveld appears likely to be of extraterrestrial origin, since this meteorite is a fall. This evidence indicates that anhydrite in ALHA could potentially be the result of extraterrestrial aqueous alteration. It is certainly evident that CO3 meteorites have experienced mild aqueous alteration (KELLER and BUSECK, 1990)) but anhydrite veins have not been observed in these other CO3 meteorites. My conclusion based on these observations is that it is currently not possible to determine unequivocally whether anhydrite formation was terrestrial or extraterrestrial, until our understanding of Antarctic weathering effects has improved. Essentially the same conclusion must also be reached for the origin of protophyllosilicates which are present locally within regions of amorphous material. The style of alteration strongly resembles that produced as a result of alteration of geologic and synthetic glasses (e.g., TAZAKI et al., 1989). Metastable amorphous material would clearly be very susceptible to alteration by aqueous fluids, whether it be in the Antarctic ice or on a meteorite parent body, even at extremely low temperatures. Perhaps the most important conclusion that can be made is the alteration was clearly not pervasive, but occurred incipiently on a very localized scale.

FORMATION OF RIMS IN ALHA The origin of fine-grained rims on chondrules has been a question of some controversy over the last several years, and as with matrix, there have been several models proposed. These include formation by devitrilication of chondrule melts ( HUTCHISON and BEVAN, 1983), shock melting during impact onto a regolith (BUNCH et al., 1991), and accretion of fine-grained dust onto chondrules in the nebula ( METZLER et al., 1988, 1992).* The first two of these models have been discussed in some detail earlier and I have argued that they are not consistent with the chemical, mineralogical, and textural data presented in this paper. The petrological and mineralogical characteristics of rims in ALHA appear to be compatible with formation by accretionary processes in the nebula, such as have been described in detail by METZLER et al. (1992) for the CM2 chondrites. There are some textural and compositional differences between rims in CM2 chondrites and ALHA77307, but there are also a number of similarities. Rims in ALHA generally show weakly developed compositional and textural layering in comparison with most CM2 chond&es. However, some CM2 chondrites, such as Yamato 74662 and 793321, contain fine-grained rims which frequently do not exhibit compositional or textural zoning, and consequently are comparable with ALHA77307. The same is also true of rim compositions. Rims on different chondrules in ALHA are compositionally homogeneous, similar to Yamato 74662 and 19332 1, but this is generally atypical

A. J. Brearley

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of chondrule rims in other CM2 meteorites which are extremely heterogeneous. In pristine CM2 meteorites, such as Yamato 79 I 198, the entire meteorite is composed of chondrules and other objects rimmed by fine-grained dust mantles. There is no interchondrule matrix. However, in other brecciated CM2 chondrites, an interchondrule matrix exists, which has been termed elastic matrix by METZLER et al. ( 1992 ), because it appears to consist of fragmental material from the breakup of other components in the meteorite. This differs somewhat from interchondrule matrix in ALHA77307, which is essentially indistinguishable from fine-grained rims, both in texture, mineralogy, and composition. It is possible that some regions of what appears to be interchondrule matrix in ALHA could be produced by oblique cuts through fine-grained rims. However, this does not seem to be a reasonable explanation for regions of matrix which may extend for several hundred microns, such as that shown in Fig. If. The differences in the textural characteristics of interchondrule matrix in ALHA and brecciated CM2 chondrites suggests that brecciation in ALHA was extremely limited, and that interchondrule matrix represents dust which was accreted into lumps in the nebular rather than onto chondrules. Following the arguments of METZLER et al. ( 1992), it seems probable that the fine-grained rims on chondrules in ALHA represent fine-grained nebula dust which was accreted onto the chondrules in a nebular environment, prior to accretion of the CO3 parent body. The fact that many chondrules have rims of variable thicknesses suggests that rims may have been abraded by collision processes in the nebula, possibly during accretion. Some of this abraded material may be represented in the interchondrule matrix.

CONCLUSIONS Microstructural and electron microprobe studies of finegrained matrix and rims in ALHA have provided a wealth of information which puts important constraints on the origin of this fine-grained material. TEM studies have revealed a remarkable diversity of distinct mineralogical components, which can be identified on the basis of their chemical and textural characteristics. These components clearly experienced markedly different formational and thermal histories, but were intimately mixed on a very fine scale prior to accretion. As a consequence the matrix of ALHA is compositionally remarkably homogeneous on the scale of 10 pm or more, but on the submicron level is highly unequilibrated. I have presented arguments which show that matrix and rim materials in ALHA could not have been derived from chondrules by any reasonable mechanism, but represent relatively unprocessed nebular or presolar dust. The primitive character of this material is indicated by the presence of an ubiquitous amorphous component, which I believe represents the product of disequilibrium condensation processes within a nebular or circumstellar environment. This study further underscores the wealth of information about nebular and asteroidal processes which can be provided by studies of highly unequilibrated meteorites.

am grate&l to Robert Hutch&on, Knut Metzler, and David Kring for their very constructive and helpful reviews of the manuscript and Rhian Jones for many helpfuJ discussions. I would

Acknowledgments-l

also like to thank the Antarctic Meteorite Working Group for providing the samples of ALHA used in this study and Tom Servilla for preparing the thin sections. The assistance of Ken Nichols and Fleur Rietmeijer with the photography is greatly appreciated. Electron microprobe analysis and transmission electron microscopy were carried out in the Electron Microbeam Analysis Facility, Department of Earth and Planetary Sciences and Institute of Meteoritics, University of New Mexico. This work was funded by the Institute of Meteoritics, UNM, NASA grant NAG 9-30 (Klaus Keil, PI) and NAG 9-497 (J. J. Papike, PI). Editorial handling: C. Koeberl

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