Matrix mineralogy of the Lancé CO3 carbonaceous chondrite: A transmission electron microscope study

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Geochrmica d Cosmochrmico Acta Vol. 54. pp. 1155-l 163 Copyright 0 1990 Pergamon Press pk. Printed in U.S.A.

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Matrix mineralogy of the LancC CO3 carbonaceous chondrite: A transmission electron microscope study LINDSAY P. KELLER and PETER R. BUSECK Departments of Geology and Chemistry, Arizona State University, Tempe, AZ 85287- 1404, USA

(Received June 8, 1989; accepted

in revised form .Jamrary 26, 1990)

Abstract-The LancC CO3 carbonaceous chondrite (CC) is less altered than the CI and CM chondrites and so provides a view of the mineralogy and textures resulting from the earliest stages of aqueous alteration of CCs. Matrix olivine in LancC has been partly altered to fine-grained, Fe-bearing serpentine and poorly crystalline Fe3+ oxide, a process that required both hydration and oxidation. Serpentine occurs as discrete packets separated from the olivine surfaces by the Fe’+ oxide. The Fe released during the dissolution of olivine was partly incorporated into the serpentine: the remainder was oxidized to form Fe3+ oxide. Matrix metal was also altered to produce Fe oxides, leaving the residual metal enriched in Ni. Olivine grains in Lanct matrix contain channels along their [ 1001 and [OOl] directions. The formation and convergence of such channels resulted in a grain-size reduction of the olivine. The alteration was pervasive but incomplete, suggesting a limited availability of fluid. A brief study of two other CO chondrites, Kainsaz and Warrenton, shows that these meteorites do not contain phyllosilicates in their matrices, although both contain Fe” oxide between olivine grains. Prior to its alteration, LancC probably resembled Kainsaz, an unaltered CO3 chondrite. The alteration assemblage in Land is only slightly different from that in Mokoia and essentially the same as that in C3 xenoliths from Murchison. Alteration products in LancC show greater similarities to CI than to CM chondrites. INTRODUCTION

carbonaceous chondrites and the processes responsible for its formation. We studied three CO chondrites, Kainsaz, Lanci, and Warrenton, all of which are falls. Of these, only Lance contains significant amounts of hydrous phases: thus, it is the main emphasis of this paper. Despite previous studies (KERRIDGE, 1964; MICHEL-LEVY, 1969; KURAT, 1973, 1975). the identities of the hydrous phases in LancC are uncertain. More importantly, the textural relationships between hydrous phases and other matrix minerals are unknown. Our observations from a TEM study of the matrix mineralogy of the Lanct CO chondrite show that Fe-rich olivines and metal have been altered to Fe-bearing serpentine and ferrihydrite. Preliminary results were reported by KELLER and BUSECK (1988).

BECAUSE OF THEIR COMPOSITIONAL similarities to the Sun, carbonaceous chondrites are widely considered to be among the most primitive materials available for the study of the early solar system. Many carbonaceous chondrites, however, show evidence of having experienced fluid interactions, probably in parent bodies. While some carbonaceous chondrite groups (e.g., the CI and CM chondrites) retain primitive chemical compositions, they have been altered so extensively that their prealteration mineralogy and textures are difficult or impossible to decipher. In contrast, most CO and CV chondrites are altered only slightly and thus serve as important indicators of the earliest stages of aqueous alteration in carbonaceous chondrites as well as their prealteration appearance. The large surface area of carbonaceous chondrite matrix makes it particularly vulnerable to alteration by heating or by interaction with fluids. Until the application of the transmission electron microscope (TEM) to meteorite studies, mineralogical investigations of carbonaceous chondrite matrix were hampered by its fine-grained nature. The use of the TEM for the study of CI and CM carbonaceous chondrites has led to new insights regarding their history (e.g., TOMEOKA and BUSECK, 1983, 1985, 1988; MACKINNON and BUSECK, 1979; BARBER, 198 1, 1985; KERRIDGE, 1964). Although much has been learned about CI and CM chondrites, relatively little is known about the character of matrix in CO and CV carbonaceous chondrites. Ornans-type (CO) chondrites are not abundant (13 falls and finds are listed by SEARS and DODD, 1988), and they remain one of the least studied of the carbonaceous chondrite groups. We have undertaken a study of CO chondrites in order to understand more about the matrix mineralogy of

PREVIOUS

WORK

The CO chondrites are largely unaltered, and all are classified as petrologic type 3 (MCSWEEN, 1977). Within the CO group, there exists a sequence related to the varied intensity of thermal metamorphism as expressed in the compositions of the ferromagnesian silicates and metal (MCSWEEN, 1977). Compositional and mineralogical data suggest that these chondrites experienced maximum post-accretional temperatures of 350 to 450°C. In addition to thermal metamorphism, several CO chondrites contain evidence of fluid alteration. In a reconnaissance study of carbonaceous chondrite matrix, KERRIDGE (1964) described fibrous phyllosilicates in Lank and Ornans that he identified as clinochrysotile on the basis of electron diffraction data (no chemical analyses were reported). MICHELLEVY (1969) described clay-like hydrous silicates as alteration products of matrix olivine in LancC. VAN SCHMUS (1969) referred to “fibrous microcrystalline material” associated with 1155

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and pyroxene in chondrules of Felix. Aqueous transport of Fe and Mg was suggested by KERRIIXZ. (1972), who observed veins of Fe enrichment in olivinc from chondrules in Warrenton. KURAT (1973. 1975) and KUR~I- and KRACHER (1980) observed serpentinized objects in Lan& matrix and suggested that these indicate metsomatic alteration. Al-poor serpentine was identified in the matrix Allan Hills A77307 (IKEDA,1983). Ikeda observed that glassy material in chondrules had altered to Al-rich chlorite or berthierine plus smectite and suggested that the alteration of chondrules and matrix occurred prior to accretion. KEC‘K and SEARS (1987) observed traces of red cathodoluminescence in LancC that they attributed to extraterrestrial aqueous alteration. Clearly. there is much evidence for the occurrence of products of aqueous alteration. although no concensus exists regarding their mineralogical character or details of their formation.

olivine

of

MATERIALS

AND

METHODS

A total of six thin sections (two thin sections each) were made from chips of LancC (Me 135 1). Kainsaz (Me 2654), and Warrenton (#376a). The thin sections were analyzed using a JEOL JXA 8600 electron microprobe equipped for scanning electron microscopy. Specimens for TEM analysis were extracted from thin sections. mounted on Cu grid supports, ion thinned, and C-coated. For Lanck. three TEM specimens of 3-mm diameter were extracted From each thin section and ion milled. A total of six TEM specimens wcrt‘ prepared from Kainsaz and Warrenton. TEM observations were recorded from the thin edges of the holes milled in the sample. Highresolution images were obtained using a JEOL 200CX TEM operated at 200 kV and a JEOL 4000EX TEM operated at 400 kV. EnergSdispersive, X-ray spectroscopy (EDS) analyses and selected-area electron diffraction (SAED) patterns were obtained using a Philips 4007 TEM operated at 120 kV. EDS spectra were accumulated for 150 to

250 seconds with detector deadtimes of 1Sto 30%. 1hc EUS analysts while semi-quantitative, allow the determination of approximate mineral stoichiometries that aid in confirming minerai identifications based upon SAED patterns. The accuracy of routine analyses is es timated to be - 10%’ relative.

0BSERVATIONS

FIG. I. Low-magnification TEM image of Lance! matrix. The arrows point to areas of phyllosilicates and poorly crystalline material between grains of olivine (01).

In petrographic thin sections, Lance matrix is line grained and orange-red in color. Low-magnification TEM images show that matrix is mainly comprised of small grains of olivine mixed with variable amounts of phyllosilicate and other poorly crystalline material (Fig. I). Minor matrix phases include Ca-rich and Ca-poor clinopyroxene, metal, and oxides. Olivine in LancC matrix is Fe-rich, fine-grained (co.1 to 10 Km in diameter) and fragmented, with the smaller fragments altered to phyllosilicates and F$’ oxide. (We use the term “Fe3’ oxide” to describe the poorly crystalline Fe-rich phase(s) that are similar to ferrihydrite. We have not directly determined the oxidation state of Fe in these materials: we assume that it is +3 based on the orange-red color of matrix. These materials are discussed in greater detail in the Discussion section.) EDS analyses of olivine fall within the range of 40 to 50 mol% fayalite. Typically. the olivine analyses contain <1 wt% each of CaO and A1203. Zoning in matrix olivines was not observed. Most grains are rounded, but some are euhedral. There is no apparent correlation between crystal morphology and composition. Many euhedral olivine grains contain channels that parallel their ( 100) and (00 1) directions. The channels are up to 50 nm long and occur in a semiperiodic fashion (Fig. 2); they are approximately 20 nm apart and are filled with a chaotic mixture of fine-grained phyllosilicate and Fe3+ oxide. Some olivine grains contain planar features parallel to (100) that give rise to streaking in SAED

Mineralogy of the matrix of Lance patterns. These are similar to the planar zones observed in Mokoia 03 matrix olivine by TOMEOU and BWECK ( 1986b, 1990). One olivine grain in LancC matrix, when viewed down [OO13. displays semi-periodic indentations arrayed along the crystal edges parallel to both the [ 100] and [0 101 directions. These features appear as pits in TEM images, but they may instead be grooves in the olivine surface (parallel to [OOI]) seen edge on. The pits are only 1 to 5 nm across and - 1 nm deep (Fig. 3). SAED patterns of the olivine containing pits show streaking along [ 1001. The phyllosilicates have a fibrous appearance and occur with Fe3+ oxide and poorly crystalline material between olivine grains (Fig. 1) and in veins in larger olivine grains (Fig. 4). (The term “fibrous” is used to describe the two-dimensional appearance of crystals that may, in fact, be tabular or platy in three dimensions.) Some phyllosilicates show an orientation relationship to the olivine such that their (00 1) layers are roughly parallel ta (100) of olivine. Phyllosilicates also occur draped around olivine grains. More commonly, they occur as random packets separated from the olivine surface either by a layer of Fe3’ oxide or by other poorly crystalline material (Fig. 5). The phyllosilicate packets are up to 10 nm wide, from IO to 50 nm long, and show (001) basal fringe spacings of 0.7 nm (Fig. 6). The phyllosilicates in veins are coarser grained and are up to 300 nm long. The larger phyllosilicate grains have a 0.7-nm fringe spacing, but contain more defects and terminated layers (apparent edge dislocations) than do the fine grains (compare Fig. 6 to Fig. 7). Fe?+ oxide is ubiquitous in LancC matrix. The material is poorly crystalline, with SAED patterns containing only difuse diffraction rings. Two SAED patterns are common, one with spacings of w-0.25, -0.22, -0.20, and -0.14 nm, and an-

FIG. 3. High-resolution TEM image of pits or grooves (periodic white spots at the crystal margins) in euhedral matrix olivine. The arrows point towards individual pits. Note the serpentine packets in matrix.

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FIG. 4. Log-mag~i~cation TEM image of a vein of alteration material cutting through a large grain of matrix olivine (al). The sides of the vein are lined by Fe’+ oxide. while the middle of the vein is filled with fibrous serpentine crystals.

and -0.16 nm. The Fe’+ oxide is finely dispersed throughout the intergranular areas, except where coatings have formed at olivine-matrix interfaces (Figs. 5 and 8). These coatings were only observed on crystal faces of euhedral olivine grains and along veins. EDS analyses ofaltered areas were obtained using analytical electron microscopy (AEM). The phyllosili~a~es are too fine

other with spacings of -0.27

FIG. 6. High-resolution TEM image of matrix phyllosilicates showing the 0.7 nm basal fringe spacing.

grained to yield precise compositional data for a single packet: however, EDS spectra of alteration products indicate major Fe, Si, and Mg. This observation is consistent with the results of MICHEL-LEVY ( 1969). who obtained similar compositions from EDS spectra of LancC matrix. Our EDS analyses of alteration products and matrix olivine are plotted in Fig. 0. We define alteration products as the material that occurs between mineral grains in matrix. These areas include abundant phyllosilicates and oxides, but areas containing recognizable grains of ferromagnesian silicates, metals. and other anhydrous or refractory minerals were avoided. The minor Al in

some EDS analyses was combined the data.

with SI tb~-~‘asc In planing

DISCUSSION

Based upon our imaging and chemical data. we conclude that the LancC alteration products are mostly serpentine and f;e” oxide mixed in various proportions. The EDS analyses of alteration material are scattered along and slightly above

Si+AI

FIG. 7. Coarse-grained sepentine with numerous defects and terminated layers. The defects are best observed by looking at the image at a shallow angle along the arrows.

FIG. 9. A plot of TEM-EDS analyses. in atomic percent, ofalteration material (solid boxes) between olivine grains in Lance matrix. Also shown is the range in composition of matrix olivine (open boxes connected by a line) and the estimated sepentine composition (asterisk). Fa = fayalite. Fo = forsterite. Fs = ferrosilitc. En = enstatite. and 1-1 -- lizardite.

Mineralogy of the matrix of Lanct a line between the Fe corner of the diagram and the average matrix olivine composition (Fig. 9). As noted above, the serpentine packets are too small to analyze by AEM; however, because the alteration material is a mixture of Fe3’ oxide and serpentine, we can estimate a serpentine composition by drawing a line from the Fe corner of the diagram through Fass (average matrix olivine) until the line intersects the serpentine composition line. Such an extrapolation leads to an approximate composition of (Mg2.zFeo.,)Si,0S(OH), . The serpentine probably formed by aqueous alteration of matrix olivine according to:

= fMgz.zFeo.8)Si20S(OH)j + FeO, with subsequent or simultaneous oxidation to convert the Fe*+ to Fe3” oxide, which is immobile. The fact that most serpentine is separated from the olivine surfaces suggests that much of it formed from material released from the olivine grains during alteration. The scatter of the EDS data in Fig. 9 suggests a heterogeneous distribution of alteration products. The olivine/serpentine textures resemble those in terrestrial serpentinites (CRESSEY,1979). Alternatively, oriented intergrowths of serpentine and olivine may have been disrupted during regolith “gardening.” From a consideration of Fig. 9> it is apparent that the alteration products are richer in Fe than matrix olivine. This result suggests that either Fe was added in the altered areas or Mg and Si were removed. A similar trend is observed in terrestrial altered olivines by SMITH et al. (1987). They suggested that alteration products became more Fe-rich as the degree of weathering increased because of Mg and Si loss by leaching. While this process may have occurred in Lance, we believe its effect was overshadowed by the release of Fe to matrix as a result of the alteration of other Fe-rich matrix phases in addition to olivine. A probable source of additional Fe is the oxidation of matrix metal, leaving the residual metal enriched in Ni. Most of the Fe released from metal was oxidized to form Fe3’ oxide. This mechanism could explain the paucity of metal in matrix as well as the composition of the metal grains that remain: Ni-rich taenite and minor chromite occur in Lance matrix, while kamacite is absent. Nirich taenite is in direct contact with Fe3+ oxide, but not with magnetite. The composition of taenite in matrix is richer in Ni than the average value of taenite for Lance reported by MCSWEEN( 1977). However, McSween could not analyze the submicron metal particles in Lance matrix using an electron microprobe. The lack of kamacite and the presence of Nirich taenite and minor chromite in matrix suggests that Lance is more oxidized than most CO chondrites. Ornans also contains only Ni-rich taenite (MCSWEEN, 1977).

As noted above, Fe3+ oxide in Lance produces two different diffraction patterns. The first is consistent with “well-crystallized” or “six-line” ferrihydrite (5Fe203 +9H20) (TOWE and BRADLEY, 1967; CHUKHROV, 1976; CARLSON and SCHWERTMANN,198 1; EGGLETONand FITZPATRICK, 1988). The diffraction pattern of well-crystallized ferrihydrite con-

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tains a diagnostic line at approximately 0.197 nm. The interpretation of the second diffraction pattern is more ambiguous; it resembles that of several hydrous ferric oxides, including poorly crystalline ferrihydrite (CARLSON and SCHWERTMANN,1981; EGGLETONand FITZPATRICK, 1988; TOMEOKA and BUSECK, 1988) and ferroxyhyte (G-FeOOH; CHUKHROV, 1976): We cannot rule out the possibility that some of the Fe3’ oxide is a terrestial weathering product, even though Lance is a fall. Some olivine grains are coated with a thin layer devoid of serpentine (Fig. 5). These layers appear as bands of dark contrast (darker than the adjacent olivine and surrounding material) along faces of euhedral olivines and along the sides of veins. We speculate that these layers represent Fe3+ oxide coatings that formed from the Fez+ that was released during olivine alteration. The coatings vary in thickness, from 1- to 2-nm on euhedral olivine grains and up to lOO-nm thick along the sides of veins. Similar surface coatings have been proposed from X-ray photoelectron spectroscopy (XPS) studies of fayalite dissolution (SCHOTT and BERNER, 1983: SIEVER and WOODFORD, 1979). In Lanct, these coatings mainly occur on euhedral olivine grains. The presence of these coatings suggests that local differences in redox potential existed during alteration. The mo~hology of the Fe3* oxide in Lance is distinctly different from that reported from other chondrites. This difference may be related to the source of the Fe. In the Orgueil (Cl) chondrite, TOM~OKA and BUSECK(1988) showed that much of the ferrihydrite occurs as large granular masses of small particles that are probably pseudomorphs after framboidal magnetite. They also observed one-gained fe~hydrite intergrown with smectite. BREARLEY(1988) described Fe”’ oxide in the matrix of the Kakangari chondrite as featherlike aggregates of <5-nm particles. He also observed Fe3+ oxide occurring as thin rims on troilite grains. In Lance, most ferrihydrite does not occur in aggregates, but is finely dispersed throughout matrix and probably formed from Fe that was released from olivine and metal during alteration. The ferrihydrite contains appreciable Si and Mg, consistent with its adsorptive properties (CARLSON and SCHWERTMANN,198 1; SCHULZet al., 1987). We have not observed the correlation between Fe, Ni, and S described by TOMEOKAand BLJSECK (1988), who suggested that Ni and S were adsorbed on ferrihydrite particles. Olivine

SEM, electron microprobe, and X-ray powder diffraction studies have shown that fayalitic olivine is the dominant mineral in Lance matrix (MI~HEL-LEAFY,1969; KURAT, 1973, 1975). Both anhedral and euhedral olivine grains occur in Lance matrix. Similar olivine morphologies occur in the matrices of other C3 chondrites (PECK, 1983, 1984). Evidence from TEM images suggests that large olivine grains in Lance were fragmented by the formation and convergence of channels along cleavage planes. resulting in mixtures of angular grains and rounded grains (Fig. 2). This process probably operated in concert with the fragmentation of olivine during regolith gardening. One of the main results of alteration was a net grain-size reduction of matrix olivine.

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The semi-periodic spacing ofchannels and pits is intriguing. These zones of preferential alteration may be related to compositional heterogeneities, to lattice misfit between regions in the parent olivine, or to periodic defects. If the periodicity results from chemical variations. then perhaps the olivine grains contain fine-scale (on the order of IO- to 30-nm) intergrowths of alternating Mg- and Fe-rich compositions. Analytical limitations at this time preclude testing this hypothesis. We see evidence that periodically spaced planar defects in olivine may have provided sites for the initiation ofchannels. There are narrow (< I nm). planar features (Fig. IO) that parallel (IOO), the dominant slip system in olivine. Similar features also occur in some CV3 chondrites. We have observed unaltered planar defects in olivine from amoeboid aggregates in Allende (Fig. I 1). Planar features with similar orientations also occur in Mokoia (TOMEOKA and BUSEC‘K, 1986b. 1990), although the planar zones in Mokoia are -2 nm wide and contain oriented mixtures of smectite and Fe-oxide. The channels in Lance olivines have a terrestrial counterpart. SMITH et al. ( 1987) observed etch channels with a semiperiodic spacing of -20 nm in terrestrial Fe-rich olivines. They suggested that the periodicity may arise from compositional differences or from modification of surface layers after the initiation of dissolution.

The members of the CO group are similar to one another in their bulk compositions and textures (MCSWEEN, 1977). We have briefly studied two other CO chondrites. Kainsaz and Warrenton, in order to compare their matrix mineralogy to that of Lance.

FIG. IO. High-resolution TEM image of planar defects along ( 100) in LancC matrix olivine. The arrows point towards three such defects.

FIG. I I High-resolution TEM image of planar drtects along in olicine from an .Allende amoeboid olivine aarcpatc

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Kainsaz is reported to be one of the least altered members of the CO group (AHRENS et al., 1973: Mc’SWEE~. 1977). and our TEM observations lead to a similar conclusion. Kainsaz is essentially unaltered; however, minor oxidation has produced poorly crystalline Fe oxide that in places imparts a red color to matrix; the color suggests Fe”, Kainsaz matrix consists of compositionally heterogeneous. tine-graincd olivine and pyroxenes. Olivine compositions fall between Fa?,, and Faso, while pyroxenes tend to be either enstatite or hedenbergite. In addition to these major phases. fine-grained kamacite (Feg4Ni6), taenite (Fe55Ni45), and a single grain of’ well-crystallized graphite was observed. The graphite surrounds a kamacite grain in matrix. Unlike Lance, the metal in Kainsaz matrix has the same composition as that in chondrules, suggesting that Kainsaz matrix has not undergone the same degree of oxidation as Lance. Kainsaz is probably a good model for the prealteration appearance of La&. As with Kainsaz and Lance, Warrenton matrix consists mainly of Fe-rich ohvine. EDS analyses of matrix olivines show them to be remarkably constant in their composition, with all analyses falling between Fai5 and Faso. Ni-rich taenite occurs in Warrenton matrix. but kamacite 1s absent. The minerals in matrix have undergone some oxidation, as shown by the presence of poorly crystalline Fe”’ oxide. However. no other alteration products were observed.

Hydrous alteration products have been reported from other type-3 carbonaceous and ordinary chondrites (e.g., OLSEN et al., 1988; TOMEOKA and BUS~XK, 1986a,b; HUTC‘FJISONet al.. 1987; KELLER and BUSECF;. 1989). In all of these cases, hydration of matrix phases has been accompanied by simultaneous or subsequent oxidation, resulting in a mixture

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Mineralogy of the matrix of Lance of phyllosihcates and Fe-rich minerals. The main differences between alteration assemblages are in the species of phyllosilicate (either serpentine or smectite) and Fe-rich phases (magnetite, maghemite, or ferrihydrite). These differences are probably related to small variations in temperature and oxygen fugacity during alteration, the amount of fluids available, and the time available for alteration. Clues regarding which of these factors is most important can be found in a comparison of the alteration assemblage in Lance to other type3 chondrites. OLSEN et al. (1988) described several C3 xenohths from the Murchison CM chondrite. One of these xenoliths (MX 1) shares features with both CV and CO chondrites. Of particular interest is the observation that MXl has undergone both oxidation and hydration to produce Fe-enriched phases and a phyilosi~icate that they suggested was similar to berthierine. The formation of phyllosilicate was attributed to hydrothermal alteration of Fe-rich olivine (Faj6 to Fad9) at low temperatures (< 100°C) on the xenolith parent body. A distinct Fe oxide phase was not reported in their study. The alteration products in MX I matrix bear a striking resemblance to those in Lance matrix; each contains an Fe-bearing phyllosilicate with a 0.7-nm basal spacing and evidence for the oxidation of Fe. Presumably, each has undergone a similar alteration process. The matrix mineralogy of Lance is also similar to that of the Mokoia CV3 chondrite. Both meteorites have Fe-rich olivine as the major matrix constituent. Additionally, both chondrites exhibit textures suggesting that olivine was partly altered to phyllosihcate. In Mokoia, Fe-rich olivines have experienced partial iddingsitization (TOMEOKAand BUSECK, 1986a, 1986b, 1990) a topotactic replacement of ohvine by a mixture of smectite (saponite) and Fe oxide. The formation of saponite in Mokoia is thought by TOMEOKA and BUSECK (1986a) to be similar to the low-temperature deuteric alteration of olivine described by EGGLETON(1984). The presence of serpentine instead of saponite in Lance suggests that its alteration occurred at higher temperatures than the alteration of Mokoia. Phyllosihcates in the Kaba CV3 chondrite resemble those in Mokoia (KELLER and BUSECK, 1989). HUTCHISONet al. (1987) described the occurrence of hydrous silicates in the Semarkona LL3 chondrite and observed that the ferromagnesian silicates in matrix had been altered to Fe”-bearing smectite and maghemite. Oxidation of matrix metal was proposed to account for the N&rich taenite and minor magnetite in the matrix. The alteration assemblage in Semarkona is more similar to Mokoia than to Lance. Bishunpur, another LL3 chondrite, also contains phyllosilicates in matrix, although they are incompletely characterized.

The taenite and kamacite in CO chondrites record minimum equilibration temperatures of 400 to 450°C (WOOD, 1967), while thermoluminescence data suggest maximum metamorphic temperatures of -500°C (KECK and SEARS, 1987). These results place an upper limit to the temperature at which the alteration in Lance could have occurred (assuming it was an equi~ib~um process). Se~entinization of forsterite could have occurred at temperatures as low as 274 K

at estimated nebular water fugacities (HASHIMOTO and GROSSMAN,1987). Solid solution with Fe reduces the thermal stability of serpentine (MOODY, 1976). Thus, the formation of Fe-bearing serpentine from Fe-rich olivines could have occurred at even lower temperatures than suggested by HASHIMOTOand GROSSMAN(1987). Isotopic evidence suggests that the alteration of CI and CM chondrites required high fluidrock ratios (CLAYTON and MAYEDA, 1984). In contrast, Lance matrix was only partly altered to serpentine and Fe3+ oxide, suggesting a limited availability of fluid. This hypothesis is supported by experimental work on hydration rates in dunites by WEGNER and ERNST ( 1983) who showed that the serpentinization of dunites is a geologically rapid process that is controlled by the diffusion of H20 atong grain boundaries. Calculations by OLSENet al. ( 1988) show that at the temperatures proposed for hydration reactions (CLAYTON and MAYEDA, 1984; HASHIMOTOand GROSSMAN,1987) significant quantities of phyllosilicates could have formed over short time periods. The timing of the alteration is more difficult to constrain than temperatures, because clear cross-cutting relationships are not observed. The interaction of Lance matrix with fluid could have occurred during the mild heating event that left a thermal metamorphic imprint on the CO chondrites. This metamorphic event would have provided enough heat to form serpentine rather than lower temperature phyllosilicates (i.e., smectites) during aqueous alteration. The alteration probably occurred on the Lance parent body. PRINN and FECLEY ( 1990) showed that the formation of serpentine in a nebular setting would be kinetically inhibited at the low temperatures that we propose existed during the alteration of Lance. A common component of CV3, C03, and LL3 matrix is Fe-rich olivine. We believe that if the aqueous alteration of matrix occurs at moderate to low tempemtures, the phyllosilicates that result are serpentines, some of which contain minor Fe. At lower temperatures or with prolonged alteration, the alteration products of olivine are smectites. As these general observations rely on a rather small database, other type3 chondrites containing phyllosihcates in their matrices should be studied using TEM. The alteration assemblage in Lance shows similarities to CI and CM chondrites in that Fe-bearing serpentine is a major matrix constituent: however, Lance lacks the exotic PCP phases of CM chondrites (TOMEOKA and BUSECK, 1985; MACKINNON and BUSECK, 1979; BARBER, 1981) and the smectites of CI chondrites (TOMEOKA and BUSEC?K,1988). Because of the similar occurrence of Fe oxide with phyllosilicates, Lance shows a greater mineralogical resemblance to Cl than to CM chondrites. If Lance had undergone more extensive alteration (hydration and oxidation) at lower temperatures to form smectite in addition to serpentine, a CIlike material might have formed. This hypothesis is consistent with that for the prea~teration mineralogy of the Orgueil Cl chondrite proposed by TOMEOKAand B~JSF.CK( 1988). CONCLUSIONS Our TEM investigation of Lance matrix has revealed that fine-graincd matrix minerals (mostly olivine and metal) have been partly altered to a mixture of Fe3+ oxide and Fe-bearing

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1 P. Keller and I’. R. Buseck

serpentine. The alteration involved both hydration and oxidation of the olivine. The olivine altered via the I’ormation of channels that parallel the cleavage planes. The convergence of the channels as they grew combined with regolith “gardening” resulted in the fragmentation of large olivine grains into numerous small grains. Much of the serpentine formed from material released from olivine during its dissolution. The oxidation of matrix metal released Fe and left the residual metal Ni-enriched. While the alteration was pervasive. the alteration products are volumetrically small in quantity. suggesting a limited availability of fluid. The alteration assemblage in Lance is similar to that in C3 xenoliths in the Murchison chondrite and to that in the Mokoia 03 chondrite. A comparison of Lance to Mokoia suggests that the alteration assemblage in Lance probably reflects a higher temperature of formation. Prior to aqueous alteration, Lance was probably similar in chemistry and mineralogy to Kainsaz. an unaltered CO chondrite. Warrenton probably formed via thermal metamorphism of a Kainsaz-like precursor. The type of alteration in Lance may be indicative of the earliest stage of aqueous alteration in carbonaceous chondritcs. .-icknoM?/edg~enf.s-This work was supported

by NASA Grant NAG 9-59. We thank Dr. E. Olsen of the Field Museum ofNatural History for providing samples of Lance and Kainsaz and Dr. C. Francis of the Harvard University Mineralogical Museum for a sample of Warrenton. We thank Dr. Jung Ho Ahn for assistance with the JEOL 4OOOEX and Jim Clark for assistance with the SEM and microprobe. We also thank Tom Sharp, Kazu Tomeoka. and Jung Ho Ahn for useful discussions. The manuscript benefited from reviews by Wolfgang KlSck and two anonymous reviewers. Electron microscopy was performed at the Facility for High-Resolution Electron Microscopy in the Center for Solid State Science at Arizona State University. established with support from the National Science Foundation (NSF) Grant No. DMR-86-11609. The microprobe was obtained with funds provided by the Earth Science division of the NSF (Grant #EAR8408 163). Editorial

bundling:

H. Palme

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BARBER D. J. (1985) Phyllosilicates and other layer-structured materials in stony meteorites. Clay Minerals 20, 4 15-454. BREARLEY A. J. (1988) Nature and origin of matrix in the unique chondrite. Kakangari: A TEM investigation. .x1X Lurzar Planet. Ser. Cor$, 130- 13 I CARLSON L. and SCHWERTMANN U. (1981) Natural

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