Aqueous alteration of the Bali CV3 chondrite: Evidence from mineralogy, mineral chemistry, and oxygen isotopic compositions

Geochimica et Cosmochimica Acta, Vol. 58, No. 24, pp. 5589-5598, 1994 Copyright 0 1994 Elsevier Science Ltd Printed in theUSA. All rights reserved 0016-7037/94 $6.00 + .oO

Pergamon

0016-7037(94)00350-5

Aqueous alteration of the Bali CV3 chondrite: Evidence from mineralogy, mineral chemistry, and oxygen isotopic compositions LINDSAY P. KELLER,’ KATHIE L. THOMAS,’ ROBERT N. CLAYTON,~ TOSHIKO K. MAYEDA,~

JOHN M. DEHART,~ and DAVID S. MCKAY 4 ‘MVA, Inc., 5500 Oakbrook Parkway, Suite 200, Norcross, GA 30093, USA ‘C23, Lockheed Engineering and Sciences Co., 2400 NASA Rd. I, Houston, TX 77058, USA ‘Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA 4SN, NASA Johnson Space Center, Houston, TX 77058. USA (Receiwd

November

9, 1993: accepred in revistdfirrm

September

29, 1994)

Abstract-A petrographic, geochemical, and oxygen isotopic study of the Bali CV3 carbonaceous chondrite revealed that the meteorite has undergone extensive deformation and aqueous alteration on its parent body. Deformation textures are common and include flattened chondrules, a well-developed foliation, and the presence of distinctive ( 100) planar defects in olivine. The occurrence of alteration products associated with the planar defects indicates that the deformation features formed prior to the episode of aqueous alteration. The secondary minerals produced during the alteration event include well-crystallized Mg-rich saponite, framboidal magnetite, and Ca-phosphates. The alteration products are not homogeneously distributed throughout the meteorite, but occur in regions adjacent to relatively unaltered material, such as veins ofaltered material following the foliation. The alteration assemblage formed under oxidizing conditions at relatively low temperatures (< 100°C). Altered regions in Bali have higher Na, Ca, and P contents than unaltered regions which suggests that the fluid phase carried significant dissolved solids. Oxygen isotopic compositions for unaltered regions in Bali fall within the field for other CV3 wholerocks, however, the oxygen isotopic compositions of the heavily altered material lie in the region for the CM and CR chondrites. The heavy-isotope enrichment of the altered regions in Bali suggest alteration conditions similar to those for the petrographic type-2 carbonaceous chondrites. INTRODUCTION THE CV3 CARBONACEOUSchondrites

were originally thought to have escaped the effects of aqueous alteration that has extensively modified the mineralogy of the CI and CM chondrites. However, recent transmission electron microscope (TEM ) studies have shown that hydrated minerals are locally well-developed in some CV3 falls (KELLER and BUSECK, 1990a,b;

TOMEOKA

and

BUSECK, 1990;

KELLER and

a combination of analytical techniques including scanning electron microscopy (SEM), TEM, and cathodoluminescence (CL) spectroscopy. Oxygen isotopic compositions were obtained for whole-rock samples of both altered and unaltered regions. Preliminary results were reported by KELLER and THOMAS (1991).

METHODS

AND MATERIALS

We studied three different samples of the Bali CV3 chondrite. The first two samples. from the Naturhistorisches Museum in Vienna, were a small < 1 cm fragment with intact fusion crust, and a fragment (- 1 g) from the interior of the main mass. Both of the Vienna samples contain distinct chondrules and inclusions set in a dark, fine-grained matrix, with a pronounced foliation defined by flattened chondrules and inclusions, however, the sample with the intact fusion crust has been extensively altered (see Results), whereas the interior fragment is relatively unaltered. In addition, we obtained a polished probe mount of Bali from the Smithsonian Institution (USNM 48391). This section appears to be largely unaltered with‘the exception of a vein of altered material that follows the foliation. Petrographic thin sections were prepared from the Vienna samples, observed in the SEM, and analyzed by electron microprobe. For our TEM studies, regions of interest were extracted from the thin sections, attached to Cu grid supports, and ion-thinned using 5 kV Ar ions. A total of ten ion-thinned samples from two thin sections were examined in the TEM. The TEM observations were made using a JEOL 2OOOFX TEM equipped with a LINK energy-dispersive X-ray (EDX) spectrometer. Phyllosilicate analyses were obtained using a JEOL IOOCX TEM equipped with a PGT EDX spectrometer. Quantitative EDX analyses of silicates have relative errors of -5% for major elements based on counting statistics and errors associated with the determination of experimental k-factors. Electron microprobe analyses of Bali matrix were obtained using an electron beam that was defocussed to 10 pm in diameter. Mineral standards were analyzed using the same conditions. Relative errors

THOMAS, 199 1; GRAHAM and LEE,

1992; KELLER and 1993). In this study, we have investigated the extent of preterrestrial aqueous alteration in samples of the Bali CV3 chondrite. Our initial work has shown that alteration products are not homogeneously distributed in Bali; regions of heavily altered material occur adjacent to much less altered areas. This distribution of alteration products presents a unique opportunity to study in detail the sequence of mineralogical and chemical changes that accompany aqueous alteration in this carbonaceous chondrite and to document reaction mechanisms and the conditions of alteration. Bali is an observed fall ( 1907, in the Central African Republic) and belongs to the oxidized subgroup of the CV3 chondrites ( MCSWEEN, 1977 ). A preliminary petrographic and mineralogical study of Bali was reported by HOINKES and KURAT ( 1975 ), who noted that many of the chondrules and inclusions were partly altered to hydrous silicates and that metal blebs had been replaced by fine-grained oxidesulfide intergrowths. The basic petrographic properties of Bali relative to other members of CV group are described in MCSWEEN ( 1977). We studied the mineralogy, petrography, and chemistry of altered and unaltered regions of Bali using MCKAY,

5589

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L. P. Keller et al

of < 1% were obtained for major elements based upon counting statistics. We collected one hundred and fifty matrix analkseq from random areas in two thin sections excluding monomineralic the same size as the incident beam.

grains of

The cathodoluminescence (CL) properties of the Bali samples were determined using a Nuclide Luminoscope attached to a binocular micro5copc. .A 35 mm camera attached 10 one of the oculars was IIWI lo obtain color photographs of the Cl. respon\c. (‘I photomosaics ucrc prepared for two of the thin sections. Oxygen Isotope compositions were determined by the methods of (‘I ,A\ ION and MAYEI)A ( 1963. 19X3) on two samples from Vienna (17662 ). In each case. both thedark phyllosilicate-rich altered material and the adjacent whole-rock wcrc analyzed. The 12662 samples were analyzed in the 1080’5 and the precise mineralogy of the analyzed aamplcs is not hnown. Oxygen isotopic compositions were also ohtaincd from two additional samples of Bali that A’erc selected atielpetrographic studies using light and electron microscop>. Isotopic compositions are reported in the conventional A-nolation relative to the SMOU’ standard.

We have found that regions within Bali have undergone significant aqueous alteration, but the alteration products are not homogeneously distributed. The analyzed fragments contain extensively altered areas, while other fragments arc relatively unaltered. Therefore, in the following sections we tirst discuss the characteristics of the least-altered regions, and then consider the heavily altered regions.

Least-Altered

--Ileast-altered 15

10

5-

Matrix Olivines (mol % Fa) FIG. 2. Histogram of matrix olivine compositions determined by ‘TEM-EDX analysis in (a) least-altered regions and in (h) heavily altered areas.

Regions

In polished thin sections. Bali shows typical CV features including a high chondrule to matrix ratio and an abundance of Ca- and Al-rich inclusions. Chondrules and inclusions in Bali are deformed and display flattened cross sections with aspect ratios approaching 2: I. Bali exhibits a well-developed foliation, although the deformation effects are not as severe as those exhibited by Leoville. Chondrules and chondrule mesostases appear fresh and unaltered. The opaque assemblages ( mostly sulfide-magnetite aggregates) in chondrules and in matrix show little evidence for rust staining. Matrix in the least-altered regions is relatively pristine and consists

Fl
largely of fine-grained ( < 10 pm) olivine, high- and low-Ca pyroxene, Fe-Ni sulfides (mostly pyrrhotite). rare Ca-phosphates, and feldspars. Rare flakes of fine-grained (< IO nm) phyllosilicates ( I .O nm basal spacings) occur between olivine and pyroxene grains, but the vast majority of pore spaces are devoid of phyllosilicates and are filled instead with very finegrained ( 4 100 nm) olivine and pyroxene fragments (Fig. I ). EDX analyses of matrix olivine grains (including some probable chondrule fragments) range from Falo to Faho. The submicrometer matrix olivines, which are the most susceptible to metamorphic reequilibration, show a narrower range of compositions (analyses by TEM/EDX) from Fa3X_s4with a strong maximum at FaStI (Fig. 2a). TEM images of matrix olivines show that they typically contain a high density of planar features ( -S X lO’“/cm’) that parallel the olivine a-b planes (Fig. 3 ). These features produce strong streaking along a* in selected-area electron diffraction (SAED) patterns. High resolution TEM images of the planar features (Fig. 4) show lattice offsets along ( IOO), which indicates that these are stacking faults and not lamellae of a second phase. Considerable strain is associated with the planar defects, as evidenced by the contrast in the TEM images (Figs. 3-5 ). Some of the stacking faults occur en echelon with partial dislocations separated by relatively strain-free regions ( Fig. 5 ) The planar features in Bali olivines are identical to those in Grosnaja (KELLER and MCKAY, 1993) and they have the same orientation as the planar features observed in Mokoia olivines ( T~MEOKA and BUSECK, 1990). An important difference between the planar features in Bali and Mokoia is that the planar features in Mokoia consist of lamellae of secondary phases. typically magnetite. It is not clear whether the planar features in Mokoia were initially stacking faults

Aqueous alteration of a carbonaceous chondrite

FIG. 3. TEM image of a typical matrix olivine grain showing a high density of ( 100)planar features. Inset is an electron diffraction pattern showing streaking along a*. that were subsequently altered, or whether they represent a simple exsolution during alteration. Oxygen isotopic compositions (Table 1) from two leastaltered whole-rock samples of Bali fall within the field for CV chondrites and along the I60 mixing line defined by Al-

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lende CAls (Fig. 6). &I80 and 6”O (in permil relative to SMOW) are - 1.21 and -5.53, respectively, for a fragment of the least-altered material that was characterized by TEM methods (point Rl ). An additional whole rock me~urement made in 1982 gave &‘*O of 4.69 and b”O of -0.04 (point R2). These two measurements indicate either extreme heterogeneity in the distribution of the oxygen isotopes (e.g., a sampling artifact), or that the fragment analyzed in 1982 may actually have been a mixture of unaltered and altered material. Support for the former interpretation comes from the presence of visible small white inclusions in Rl, which are probably %-rich. Support for the latter inte~retation comes from the observation that the composition of R2 is more ‘“O-depleted than any other CV3 whole-rock. The data points closest to R2 come from Mokoia, Grosnaja, and Leoville, all of which contain regions of hydrous alteration. Large differences in oxygen isotopic compositions in small wholerock samples of CV meteorites have been observed previously (CLAYTON, 1993), and reflect the enormous isotopic heterogeneity of their constituent materials. The cathodoluminescence properties of the least-altered material in Bali are consistent with a low petrologic type either 3.0 or 3.1. Several type A I and A2 chondrules occur in our thin sections and their mesostases display bright yellow CL indicating that very little if any metamorphic heating occurred on the Bali parent body (see SEARS et al., 1992, for a discussion ofthe chondrule classification scheme used here).

FIG. 4. High resolution TEM image of planar defects in Bali olivine. There is a slight offset associated with the defects that can be seen by rotating the image by 90” and viewing at a low angle.

s.592

I.. P. Keller et al

FIG. 5. High resolution TEM image of stacking faults in matrix olivine. .Thc dark area\ along the defects result\ from strain caused by the lattice mismatch across the stackinp faults.

Heavily Altered Regions

Matrix

The heavily altered regions in Bali are readily identified in the optical microscope by an abundance of opaques in matrix (mostly magnetite) and by the lack of interference colors (under crossed polars) within chondrules). The majority of chondrules and inclusions in the heavily altered regions are largely replaced by phyllosilicates and are pale tan in transmitted light. In the Smithsonian probe mount (USNM 48391). a vein of heavily altered material (including chondrule pseudomorphs) -2-3 mm wide follows the foliation and is bounded by more pristine (i.e.. least-altered) material. Chondrules and inclusions in heavily altered regions are surrounded by distinctive rims (up to 200 grn wide) that are nearly devoid of magnetite and consist of tine-grained olivine and pyroxenes intergrown with abundant phyllosilicates (Fig. 7a, compare to Fig. 7b). The compositions of rim olivines are tightly clustered about Fax, ( Fa35 to Fa4,)). TEM observations of these rims reveal that they are texturally indistinguishable from matrix except for their paucity of magnetite.

shaped

crystals

Sample RI R2 Al A2

least-altered least-altered heavilv altered heavil; altered

6180

6”O

-1.21 4.69 8.70 7.02

-5.53 -0.04 3.20 2.49

with

( 100) partings

(Fig.

8, compare

to Fig.

I ). The compositions

of matrix olivines are distinctly different from the least-altered regions; EDX analyses of matrix ol-

12

-4

TABLE 1. Oxygen isotopic compositlons of least-altered and heavily altered samples of the Bali CV3 chondrite.

in the heavily altered regions tend to be than in the least-altered areas and occur as lath-

olivines

finer-grained

0

4

$0

8

12

16

20

24

(% rel. SMOW)

FIG. 6. Oxygen isotopic compositions of least-altered (RI ) and heavily altered (A I ) portions of Bali in relation to other carbonaceous chondrite whole-rock compositions (data from CLAYTON and MAYLDA. 1984. 1989). Reference lines are 7‘F = terrestrial fractionation line. and CA1 = refractor> inclusion mixing line. Points A2 and R2 are from a dark inclusion and adjacent host material from another sample of Bali. Altered samples Al and A2 show heavy-isotope enrichment. similar to that seen in CM and CR matrix, due to aqueous alteration at low temperatures C4 100°C).

Aqueous alteration of a carbonaceous chondrite

(4

(b) FIG. 7. (a) Backscattered SEM image of inclusions and chondrule pseudomorphs in heavily altered Bali showing the opaque-depleted rims surrounding the objects; (b) Backscattered SEM image of a region of least-altered Bali. The small bright grains in matrix are mostly Fe-Ni sulfides.

ivines show a bimodal distribution with most analyses clustering about Faas , and a second lesser cluster at FaaO (Fig. 2b). As in the least-altered samples, the olivine grains have a high density of planar defects along ( 100). In the heavily altered samples, alteration products are associated with some of the defects and large olivine grains have apparently broken into many smaller grains by fracture along the defects. Figure 9 shows an olivine grain in Bali matrix that has been partly replaced by phyllosilicate such that the ( 100) planes of olivine parallel the (00 I ) planes of the phyllosihcate. In one SAED pattern, weak reflections from a spine1 phase (probably magnetite) occur in olivine; however, magnetite lamellae are not common. Low-Ca pyroxene in matrix is also subject to alteration, with (00 1) of the phyllosihcates approximately perpendicular to opx ( 100) (Fig. 10). Matrix in the heavily altered areas contains abundant packets of phyllosilicates showing 1.O nm basal spacings in HRTEM images (Figs. 910) and these phyllosilicates have compositions approaching endmember saponite ( Fig. 11). The saponites are well crystallized and have molar Mg/( Mg + Fe) ratios ranging from

5593

FIG. 8. Low magnification TEM image of heavily-altered Bali matrix showing numerous lath-shaped olivine grains with ( 100) parting.

0.8-I .O. Saponite grain size shows a positive correlation with apparent pore size ranging from a few nanometers up to several 100 micrometers. Calcium phosphates are common in heavily altered regions as micrometer-sized aggregates of anhedral grains. SAED patterns indicate that the Ca-phosphates are predominantly whitlockite and apatite. The apatite is probably hydroxyapatite, as thin window EDX analyses of the apatites showed no detectable Cl or F. Rare fine-grained (< 10 pm) pentlandites are the dominant sulfide phase in the heavily-altered regions. A striking feature in backscattered SEM images of the heavily altered regions is the extensive development of magnetite aggregates (up to 50 pm in dia.) in matrix that show preferred orientation along the foliation in matrix (Fig. 7). The majority of the magnetite aggregates are irregularly shaped and consist of numerous, tightly packed, 0. I- to 0.3pm single crystals (Fig. 12) that strongly resemble the framboidal magnetites found in the CI chondrites ( KERRIDGE et al.. 1979) and those in dark inclusions in CR chondrites

FIG. 9. High-resolution TEM image of an oriented intergrowth of saponite in olivine in Bali matrix. The saponite shows I-nm basal spacings.

5594

1.. P. Keller et al

FIG. IO. HRTEM

image of saponite surrounding

parallel and subparallel to the pyro~nc

a pyroxene grain in

Bali matrix.

The phyllosilicate

layers arc

( 100)

( WEISBERG et al.. I993 ). The plaquette-morphology of magnetite known from Cl chondrites was not observed in Bali. EDX analyses of the magnetites indicate that they are nearly pure Fe oxide with
to the (,-axis). (Fig. 13).

Geochemistry Regions

and are intimately

of Least-Altered

intergrown

with saponite

and Heavily Altered

The mineralogical differences between heavily altered and least-altered regions of Bali suggest that chemical differences might also exist, To investigate the chemical changes that have occurred during aqueous alteration. we obtained defocussed electron microprobe analyses of fine-grained matrix in both lightly- and heavily altered areas. We collected eighty

Si + Al

A

monm

saponite

PAgUN Oct. of

Y

gMonits/

OCt.

FK;, I I, A plot TEM-EDX analyses showing the range of saponitc and mica compositions in Bali. Triangular diagram plots tetrahcdral cations (Si and Al) at the top. and the octahedral cations on the bottom (Mg + Fe. trioctahedral clays) and (Al. dioctahedral clays). Matrix phyllosilicates arc nearly endmember saponites. whereas the micas observed in the CAI are highly aluminous.

5595

Aqueous alteration of a carbonaceous chondrite 2. The average compositionof

TABLE

lightlyand heavilyalteredBali

matrix from defocused electronmicroprobe analyses. lightly-

heavily

range

0.39 3.16 22.6 29.4 1.17 2.42 0.09 1.67 0.11 0.44 0.23 35.0 2.45

N&O

AhO,

MO

Si02 PZOS :0

CaO TiOi cr201 MllO Fe0 Ni

FIG. 12. Backscattered SEM image of framboidal magnetite aggregate in heavily altered Bali matrix.

analyses from heavily altered regions, and seventy analyses of least-altered matrix from random areas in two thin sections excluding monomineralic grains of the same size as the beam. Table 2 contains the average of the analyses for both regions along with the range of values that were observed. Figures 14- 17 show the distribution of analyses for selected elements which exhibit significant differences between least- and heavily altered regions. Taken at face value, the distributions suggest that extensive diffusion and element redistribution occurred during alteration, especially with regard to Na, S, Ca, P, Mg, and Fe concentrations. The average Na, Ca, and P in heavily altered regions is higher than in the lightly altered regions and is consistent with the higher modal abundances of Nasaponite and Ca-phosphates in these regions (Figs. 14, 15). The difference between Mg and Fe abundances in the heavily and least-altered regions are partly artifacts of the analysis procedures (Fig. 16). The difference in Fe results from the

0.03 -2.96 1.03 - 6.59 3.09 -32.02 3.69 -39.73 0.02 -11.65 0.13 - 23.60 0.01 - 0.99 0.11 - 13.72 0.01 -0.62 0.13 -1.66 0.03 -0.37 13.77 -73.69 0.39 - 16.94

0.54 3.50 22.9 35.5 0.47 0.66 0.13 3.51 0.07 0.41 0.23 31.2 0.69

0.04 -5.54 0.65- 12.42 5.37 -35.70 16.46 - 49.56 0 03 _ 3.16 0.06 - 1.64 0.01 - 1.34 0.12 -25.46 0.01 -0.17 0.01 -2.27 0.03-0.61 5.17 -65.40 0.05 - 1.42

99.63 80

100.35 70

Total N=

range

altered

altered

fact that much of the Fe in heavily altered regions is present as magnetite framboids, which were only occasionally encountered during analyses. However, the differences still provide information regarding the alteration process. The higher average Mg (and correspondingly lower average Fe) in the heavily altered regions correlates with the higher forsterite content of the matrix olivines in this region relative to the least-altered regions. The higher Mg also results from the abundant Mg-rich saponite in matrix. Heavily altered regions are more depleted in S than the least-altered material, and also have higher Ni contents (Fig. 17). Fewer sulfides are observed in the heavily altered regions and they tend to have higher Ni/Fe ratios than those from least-altered regions. An unexpected result of the microprobe analyses was that for most of the elements analyzed, the least-altered regions were more homogeneous than the heavily altered regions. DISCUSSION Bali shows evidence for a complex series of events following accretion, including lithification, deformation, and alteration. Deformation of Bali occurred following accretion, because all components show deformation textures. However, it is not clear which process was responsible for the deformation. The foliation and chondrule flattening could be a result of static compaction from overburden pressure (e.g., CAIN et

IC

.

Xl? B d

l

l heavily

0

.

I

altere d

least altered

6

p!

i,

E

f’

0

0

1

2

3

4

Calsi (w/w ratio) mktke to Cl

FIG. 13. HRTEM image of oriented saponite-mica intergrowth a Bali CAL

in

FIG. 14. A plot of P/Si and Ca/Si for heavily altered and leastaltered regions of Bali matrix showing the correlation of Ca with P in the heavily altered regions which reflects the abundance of Caphosphates.

55Yh

L. P. Keller et al.

least-altered

(solid bars)

heavily-altered

1

0.5

0

a heavily 0

(open bar?.)

1.5

2

NilSi (whv ratlo) raletlve

NaBi (w/w ratio) relative to Cl

FIG. 15.The distribution of Na relative to Si in Bali matrix. Sodium is more abundant in heavily altered regions, shows a wider distribution than in least-altered matrix, and correlates with the abundance of saponite in the heavily altered regions.

al.. 1986), or these features could have developed as result of gardening during meteoroid impacts as suggested by SNEYD et al. ( 1988). Recent experimental data show that repeated shock events can produce the requisite chondrule flattening ( NAKAMLJRA et al., 1993). We believe that the planar defects in olivine were produced during this time period, but we recognize several ambiguities in the interpretation of the planar defects. First, few experimental studies have been done on the deformation of Fe-rich olivines, and so their deformation behavior must be extrapolated from that of magnesian

0

0.2

0.4

0.6

0.6

1

1.2

1.4

1.6

1.6

2

Fe/Sl (w/wr&lo)relativeto Cl FIG.16. Histograms for the distribution of Mg/Si and Fe/Si in Bali matrix. The differences between heavily-altered and least-altered matrix correlates with mineralogical differences (see text). Vertical dashed lines are the ratios for Cl abundances.

altered altered

to Cl

FIG. 17. The distribution of Ni and S in Bali matrix. The higher average Ni in heavily altered matrix correlates with the abundance of Ni-rich sulfides. Sulfur is depleted in heavily altered matrix because of oxidation of sulfides to produce magnetite.

olivines. Second, Fe-rich olivines in some deformed meteorites (Leoville, for example) do not show the distinctive ( 100) planar defects seen in Bali ( NAKAMURA etal., 1992). While planar defects along ( 100) are uncommon in Mg-rich olivines, Fe-rich olivines develop a parting along ( 100) which probably facilitates slip along this plane. It is also possible that these defects form over a restricted range ofstrain rates. The presence of the planar defects in both heavily altered and least-altered regions of Bali suggests that deformation preceded aqueous alteration. However, it is not clear from petrography if deformation occurred contemporaneously with aqueous alteration. Because of the associated strain, the planar defects in olivines could have served as energetically-favorable nucleation sites for the growth of saponite and magnetite, and the presence of the defects may also have assisted in grain size reduction during alteration via parting along ( 100). Mineralogical

Mg/Sl (w/w ratlo) dative to Cl

lightly

and Chemical Changes during Alteration

The chemical differences between the least-altered and heavily altered regions are somewhat problematical. Sulfur is highly depleted in the heavily altered regions and was probably removed by the fluid phase. This depletion resulted from the open system behavior of S. However, with open-system behavior, one would expect that altered regions woutd become chemically more homogeneous, which is the opposite of what is observed. The chemical systematics of Ca, P, Na, Mg, and Fe (Figs. l4- 17 ) suggest nearly closed-system behavior, with only local element exchange between chondrules. inclusions, and matrix. The wide range of olivine compositions in Bali indicates that it may be one of the least equilibrated of the CV chond&es. However, the population of matrix olivine compositions is different between the least- and the heavily altered regions in Bali. Either we are observing ( 1) fine-scale variations in mineralogy due to sampling (i.e., parent body heterogeneity on a mm-scale), (2) preferential alteration of Ferich olivines, which are known to alter faster than the more magnesian varieties in certain weathering environments ( WOCELIUS and WALTHER, 1992), or (3) Fe/Mg exchange (between matrix olivines and the fluid phase or between chondrules and matrix). If the distribution of olivine com-

5597

Aqueous alteration of a carbonaceous chondrite positions presented in Fig. 2a and b are representative of the whole meteorite, then, it suggests that all three options were possible. Fluids were channelized and moved along the foliation, altering anhydrous silicates to saponite, and oxidizing sulfides to produce magnetite and Ni-rich sulfides. While much of the Fe for the framboidal magnetite was derived from oxidation of sulfides, a significant portion must have been derived from the alteration of Fe-rich olivine in matrix. Dissolution of Fe-rich matrix olivines also supplied part of the Mg and Si which were utilized in the formation of saponite.

Nature of the Fluid Phase and Alteration Conditions

The abundance of Na-saponite and Ca-phosphate in heavily altered regions suggests that the fluid phase maintained a high pH and carried significant dissolved solids (especially Na, Ca, and P), although the concentrations were not high enough to precipitate secondary carbonates and sulfates like those observed in the CI and CM chondrites ( FREDRIKSSONand KERRIDGE,1988; JOHNSONand PRINZ, 199 I ). The fluid phase was also highly oxidizing as indicated by the presence of framboidal magnetite in altered regions. The lack of Cl and F in apatite suggests that these anions were not abundant in the fluid phase. Estimating the temperature during aqueous alteration is difficult, as the alteration mineral assemblages are not useful geothermometers because saponite and magnetite are stable over a wide temperature range. Previously, others have estimated temperatures by reference to the terrestrial occurrences of the observed alteration assemblages (e.g., KELLER and BUSECK, 1990b), a reasonable assumption that is not known to be correct. Temperature estimates have also been calculated based on the thermodynamic properties of the mineral assemblage (e.g., BOURCIERand ZOLENSKY,1992), but these calculations are complicated because equilibrium must be assumed, and the thermodynamic data are usually extrapolated from higher temperature/pressure conditions. Relative alteration temperatures can be determined by comparing the assemblages in chondrites that have been altered to similar extents. For example, Grosnaja contains the assemblage serpentine-chlorite-magnetite that coexists with equilibrated matrix olivines, an assemblage which formed at higher temperatures than the saponite-magnetite association common to the other CV chondrites (KELLER and MCKAY, 1993). The temperature during the aqueous alteration of Bali is estimated to be < 100°C based on analogy to other altered CV chondrites with similar mineral assemblages. High water:rock ratios have been suggested for the aqueous alteration event(s) that have affected the CV chondrites (e.g., ZOLENSKYet al., 1993). However, it is difficult to understand how the lightly-altered CV chondrites could have seen as much water as CR chondrites and yet not be extensively altered. If it is true that CV chondrites witnessed a high water: rock ratio, then the implication is that the fluid moved through the rock quickly and thus had insufficient time for wide-scale alteration. Otherwise the calculated water:rock ratios are in error and were in fact, much lower for the saponitebearing alteration assemblages in most CV chondrites.

How Primitive are CV Chondrites? Most CV chondrites are currently classified as petrographic type 3, which indicates that they have undergone little or no metamorphic heating and have not been altered by aqueous fluids since their accretion ( MCSWEEN, 1977). However, there is evidence for slight thermal metamorphism, as well as an aqueous alteration overprint in many of the CVs. Recent thermoluminescence data suggest that most CV chondrites span the range from type 3.0-3.3 ( SYMESet al., 1993 ), and several have equilibrated matrix olivine compositions (PECK, 1983; KELLER and MCKAY 1993). The unequilibrated olivine compositions coupled with the CL response indicate a low petrographic type 3.0 or 3.1 for Bali. Six of the CV3 falls have been examined using TEM including: Allende, Vigarano, Grosnaja, Mokoia, Kaba, and Bali. All six of these chondrites contain phyllosilicates, although the mineral species and proportions vary considerably. For example, Allende contains phyllosilicates, but these consist mostly of micas which are confined to the CAIs and are believed to have formed prior to accretion (HASHIMOTO and GROSSMAN, 1987; KELLER and BUSECK,199 1) . Allende matrix is essentially anhydrous. In contrast, Mokoia, Kaba, Grosnaja, and Bali all contain abundant phyllosilicates in matrix, chondrules, and inclusions. In Kaba and within regions of Bali, the proportion of phyllosilicates approaches 50% by volume. Vigarano is only slightly altered and contains locally welldeveloped phyllosilicates in matrix, but chondrules and inclusions are relatively unaltered. Phyllosilicates in CV chondrites are predominantly Na-saponites, except in Grosnaja, where serpentine and chlorite are the major clay minerals (KELLER and MCKAY, 1993 ). What is becoming clear with more detailed analyses is that CV3 chondrites commonly show the effects of aqueous alteration, and that Allende is atypical for a CV3 chondrite in that its matrix lacks phyllosilicates. For purposes of classification, some of the CV3 chondrites should be reclassified as petrographic type < 3.0 (e.g., CV2 or perhaps CV2.5) particularly Bali and Kaba. The abundance of alteration materials indicates type 2 for the heavily altered regions of Bali, which is also consistent with the oxygen isotopic data. CONCLUSIONS The Bali CV3 chondrite has been extensively altered; however, the alteration products are not homogeneously distributed. Petrographic observations indicate that Bali was deformed prior to being altered by aqueous fluids. Deformation textures include a well-developed foliation, chondrule flattening, and distinctive (100) planar defects in olivines. Aqueous alteration has resulted in the formation of abundant secondary minerals including Mg-rich saponite, framboidal magnetite, and Ca phosphates. These secondary minerals indicate low-temperature alteration (< 100°C) under highly oxidizing conditions. Altered regions within Bali contain higher Na, Ca, and P than unaltered regions suggesting element redistribution between chondrules and matrix. Mg/Fe exchange between the fluid phase, chondrules, and matrix olivines, combined with preferential alteration of Fe-rich olivine during aqueous alteration, modified the average olivine composition from FaSoin unaltered matrix to Fax, in heavily

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altered regions. The oxygen isotopic compositions of the most altered materials from Bali grade continuously into the compositions of CM and CR whole-rocks. For example, two of the CM chondrites with compositions near point A I in Fig. 6 are Bells and Nogoya. The implication is that both the raw

materials and the reaction conditions for formation of aqueous alteration products in Bali and other CV3 chondrites were similar to those in CM and CR chondrites (CLAYTON and MAYEDA, 1984; WEISBERG et al., 1993). The occurrence of alteration products in Bali that follow the foliation of the meteorite, and the presence of deformed olivines in both altered and unaltered regions indicates that

the aqueous alteration occurred on the Bali parent body and not in a nebular setting. ~cknowlrdgments-Financial support for LPK was provided by a National Research Council Fellowship at the NASA Johnson Space Center and by MVA, Inc. Electron microscopy was performed in the Electron Beam Analysis Laboratories at the Johnson Space Center. We thank Gero Kurat at the Naturhistorisches Museum in Vienna and Glenn MacPherson at the Smithsonian Institution for the samples used in this study. We thank Allan Treiman for a review of an early version of the manuscript. Constructive reviews by A. Brearley, M. Weisberg. T. Esat, and C. Koeberl are appreciated. This work was supported by NASA RTOPs 152-17-40-23 and 199-52-l I-02 (to DSM), and NSF grant EAR 92-18857 (to RNC). Editorial

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