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& Planetary Science 40, Nr 9/10, 1413–1431 (2005) Abstract available online at http://meteoritics.org

Fine-grained dust rims in the carbonaceous : Evidence for alteration

Ansgar GRESHAKE1*, Alexander N. KROT2, George J. FLYNN3, and Klaus KEIL2

1Institut f¸r Mineralogie, Museum f¸r Naturkunde, Humboldt-Universit‰t zu Berlin, Invalidenstrasse 43, 10115 Berlin, Germany 2Hawai’i Institute of Geophysics and Planetology, SOEST, University of Hawai’i at Manoa, Honolulu, Hawai’i 96822, USA 3Department of Physics, SUNY Plattsburgh, Plattsburgh, New York 12901, USA *Corresponding author. E-mail: [email protected] (Received 04 November 2004; revision accepted 29 March 2005)

Abstract–The Tagish Lake consists of heavily aqueously altered , CAIs, and larger mineral fragments in a fine-grained, phyllosilicate-dominated matrix. The vast majority of the coarse-grained components in this are surrounded by continuous, 1.5 to >200 μm wide, fine-grained, accretionary rims, which are well known from belonging to petrological types 2 and 3 and whose origin and modification is still a matter of debate. Texturally, the fine-grained rims in Tagish Lake are very similar throughout the entire meteorite and independent of the nature of the enclosed object. They typically display sharp boundaries to the core object and more gradational contacts to the meteorite matrix. Compared to the matrix, the rims are much more fine- grained and characterized by a significantly lower porosity. The rims consist of an unequilibrated assemblage of phyllosilicates, Fe,Ni sulfides, , low-Ca pyroxenes, and forsteritic , and are, except for a much lower abundance of carbonates, very similar to the Tagish Lake matrix. Electron microprobe and synchrotron X-ray microprobe analyses show that matrix and rims are also very similar in composition and that the rims differ significantly from matrix and bulk meteorite only by being depleted in Ca. X-ray elemental mapping and mineralogical observations indicate that Ca was lost during aqueous alteration from the enclosed objects and preferentially crystallized as carbonates in the porous matrix. The analyses also show that Ca is strongly fractionated from Al in the rims, whereas there is no fractionation of the Ti/Al-ratios. Our data suggest that the fine-grained rims in Tagish Lake initially formed by accretion in the solar nebula and were subsequently modified by in situ alteration on the parent body. This pervasive alteration removed any potential evidence for pre- accretionary alteration but did not change the overall texture of the Tagish Lake meteorite.

INTRODUCTION intermediate between the two classes. Based on their mineralogical investigations, Zolensky et al. (2002) argued The Tagish Lake meteorite, which fell on January 18, that Tagish Lake is distinct from all known C1- and C2-type 2000 in the Yukon Territory, Canada, is a type 2 carbonaceous carbonaceous and represents an entirely new type chondrite with both similarities and differences to CI and CM of C2 meteorite. As the reflectance spectrum of Tagish Lake chondrite groups (Brown et al. 2000). While its high is similar to spectra of D-type , it was suggested that proportion of matrix and high amount of volatile elements this meteorite may represent the first material available on suggest a close relationship to CI chondrites, the presence and Earth from such very primitive and thermally unprocessed mineral chemistry of chondrules and refractory inclusions, asteroids (Hiroi et al. 2001), although some ∼10 μm IDPs the sulfide , and the bulk chemistry are more like exhibit reflection spectra similar to P- and D-type asteroids as CM and different from CI carbonaceous chondrites. As well (Bradley et al. 1996). summarized by Simon and Grossman (2003), several One of the striking similarities of Tagish Lake with CM previous studies of Tagish Lake’s bulk chemistry, oxygen carbonaceous chondrites is the presence of fine-grained, isotopic composition, bulk organic , and other organic matrix-like rims around coarse-grained components, such as compounds found that the meteorite is different from both Ca, Al-rich inclusions (CAIs), chondrules, and mineral CM and CI chondrites, yet in some properties it is fragments. Fine-grained rims around different kinds of

1413 © The , 2005. Printed in USA. 1414 A. Greshake et al. objects are found in many meteorites belonging to the 1998; Gounelle and Zolensky 2001). A significant influence unequilibrated petrological types 2 and 3 in various chondrite of terrestrial alteration on the distribution of especially groups (see e.g., Metzler and Bischoff 1996 and references calcium in the Tagish Lake samples studied here can thus be therein). The formation of such rims has been controversial excluded. since their first detailed description by Kurat (1970) in the H3 Two demountable polished thin sections of Tagish Lake chondrite Tieschitz. The different formation mechanisms now were studied in transmitted and reflected light by optical proposed include: 1) condensation of fine-grained material microscopy, backscattered electrons (BSE) by scanning from an impact-produced hot gas (Kurat 1970); 2) accretion electron microscopy (SEM), electron microprobe analysis of dust onto the surfaces of coarse-grained core objects in the (EPMA), transmission electron microscopy (TEM), and solar nebula (e.g., Allen et al. 1980; Bunch and Chang 1980; synchrotron X-ray fluorescence (SXRF) analysis. BSE King and King 1981; Scott et al. 1984; Rubin 1984; images were obtained using JEOL JSM-6300, -5900LV, and MacPherson et al. 1985; Rubin and Wasson 1987; Brearley Zeiss DSM-962 scanning electron microscopes. Quantitative and Geiger 1991; Tomeoka et al. 1991; Nakamura et al. 1991; mineral analyses were performed with a JEOL JXA-8800L Metzler et al. 1992; Brearley 1993; Metzler and Bischoff electron microprobe operated at 15 kV, a probe current of 1996; Hua et al. 1996, 2002); 3) formation by “regolith 15 nA, and a beam size of 1–2 μm. Major and minor element gardening” during multiple stages of aqueous alteration and bulk compositions of the rims and the matrix were determined brecciation of the enclosed objects on the parent body (Sears by electron microprobe analysis applying a defocused 25 and et al. 1993; Tomeoka and Tanimura 2000); 4) alteration of the 50 μm electron beam, respectively. For analyses of enclosed objects (e.g., Richardson 1981; Sears et al. 1991, phyllosilicates in rims and matrix, a defocused beam of 5– 1993); 5) shock melting caused by impacts into the regolith 10 μm was used to avoid beam damage. Suitable mineral (Bunch et al. 1991); and 6) devitrification of quenched standards including anorthoclase, basaltic glass, chromite, melts (Hutchison and Bevan 1983). Browning chromium augite, diopside, ilmenite, microcline, and et al. (2000) studied rim textures surrounding individual , all certified by the United States National silicate grains in CM chondrites and concluded that these Museum as reference samples for electron microprobe rims formed by in situ alteration on the parent body. analysis (Jarosewich et al. 1980), were applied to calculate the However, it should be pointed out that such rims are mineral compositions. The X-ray elemental maps were texturally quite different from the fine-grained dust mantles acquired on the same electron microprobe and on a Cameca found in Tagish Lake and CM chondrites and thus require a SX-50 electron microprobe at 15 kV accelerating voltage, 50– distinct formation mechanism. 100 nA beam current, and a beam size of approximately 1– In order to address the origin of fine-grained, matrix-like 2 μm. rims and their relation to the meteorite matrix material, we After detailed petrographic studies, slotted Cu grids were performed a detailed study of the petrography and chemical glued on areas of interest of the thin section, removed and ion- compositions of the fine-grained rims in Tagish Lake. thinned using a GATAN duo ion beam mill. During thinning, the samples were cooled by liquid nitrogen to avoid ion beam SAMPLES AND ANALYTICAL METHODS damage. TEM studies were carried out with a Philips CM20 analytical TEM operated at 200 kV and equipped with a The Tagish Lake samples provided for this study were Tracor Northern energy dispersive X-ray detector. collected in spring 2000 and, although not disaggregated, may For trace element analyses, the samples were removed have been in contact with liquid lake water and snow melt for from the glass slide and mounted on Kapton film with a small several months (Zolensky et al. 2002). To quantify potential drop of silicon oil. Trace element concentrations were changes of the meteorite’s bulk chemistry due to terrestrial measured in fragments from four rims, five matrix areas, and alteration, Friedrich et al. (2003) and Dreibus et al. (2004) two chondrules using the X-ray microprobe at the National analyzed pristine and altered samples of the Tagish Lake Synchrotron Light Source (Brookhaven National meteorite. They found that the altered samples had suffered Laboratory). Depending on the size of the individual losses of the volatile halogens Cl, Br, and I as well as of the fragment, up to four analyses were carried out on each sample moderately volatile elements Na, K, and P, most likely due to using a synchrotron X-ray beam of about 15 × 15 μm in size. dissolution of halogen salts in the ice water (Friedrich et al. Average compositions were calculated for matrix, chondrules, 2003; Dreibus et al. 2004). In contrast, the studies showed that and each rim analyzed. The analytical errors of the SXRF Ca, S, and Se abundances are almost identical in both the analyses are on the order of ±10% for each value. pristine and the altered samples, indicating that carbonates, sulfides, and sulfates remained unaffected during residence in RESULTS the lake water and snow melt (Friedrich et al. 2003; Dreibus et al. 2004). This observation is also supported by the total Petrography and Mineralogy of Rimmed Objects absence of carbonate and sulfate veins and fillings, which would be indicative of leaching of Ca and S (e.g., Barrat et al. Texturally, Tagish Lake is a porous composed of Fine-grained dust rims in the Tagish Lake carbonaceous chondrite 1415

Fig. 2. a) A backscattered electron image showing the typical texture of the carbonate-poor lithology of Tagish Lake. Heavily altered chondrules are set into a fine-grained phyllosilicate-dominated matrix. All chondrules are surrounded by fine-grained rims with distinctly lower porosity than the matrix. b) BSE image of a large porphyritic -pyroxene chondrule (type IAB) with a broad, fine-grained rim. Mesostasis and Ca pyroxene have been replaced by fine- and coarse-grained phyllosilicates, and only forsteritic olivine remained unaltered. chd = chondrule; FGR = fine-grained rim; ol = olivine; phy = phyllosilicates.

lithology (Figs. 1 and 2). According to the olivine and pyroxene textures, the is S1 (unshocked) (Scott et al. 1992; Zolensky et al. 2002). Fig. 1. a) Combined X-ray elemental map for Mg (red), Ca (green) In the sections studied, four different kinds of coarse- α α and Al K (blue). b) Ca K X-ray elemental map of the ungrouped grained anhydrous components were found that are rimmed carbonaceous chondrite Tagish Lake. The meteorite contains olivine- rich (red grains) chondrules replaced to varying degrees by by fine-grained material: 1) chondrules; 2) CAIs; 3) mineral phyllosilicates (bluish) and surrounded by fine-grained rims. fragments; and 4) -rich aggregates (Fig. 3). Chondrules and fine-grained rims are generally depleted in Ca Chondrules occur in the size range of a few μm up to relative to matrix, which contains abundant carbonates (green dots). 1 mm and are dominantly FeO-poor porphyritic olivine (type Fine-grained rims around two chondrules are outlined in (a). CAI = IA; Fig. 3a) and olivine-pyroxene chondrules (type IAB; Ca,Al-rich inclusion; chd = chondrule; chd psd = chondrule pseudomorph; crb = carbonates; FGR = fine-grained rim. Chondrules Fig. 2b). In such chondrules, the constituent olivines are are marked by asterisks in (b). nearly pure forsterites (Fa0.3–1.8) that contain considerable amounts of MnO (up to 0.43 wt%), CaO (up to 0.28 wt%), carbonate-poor and carbonate-rich lithologies distinguished and Cr2O3 (up to 0.53 wt%; Table 1). Frequently, small Fe,Ni mainly by different abundances of carbonates, magnetites, metal inclusions are present in the olivines. Pyroxene in type and sulfides (Zolensky et al. 2002). All sections investigated IAB chondrules is orthopyroxene (Fs0.7–1.6Wo0.5–1) with up to in this study belong to the more abundant carbonate-poor 0.78 wt% Al2O3 and 0.49 wt% Cr2O3 (Table 1). 1416 A. Greshake et al.

Fig. 3. BSE images of various rimmed objects in Tagish Lake. a) Magnesian porphyritic olivine chondrule (type IA). b) Ferrous porphyritic olivine chondrule (type IIA) with compositionally zoned olivine grains. Parts of the formerly glassy mesostasis are still Al- and P-rich. c) Magnesian barred olivine chondrule. The mesostasis has been completely replaced by phyllosilicates. d) Magnesian olivine chondrule with dendritic texture. e) Porphyritic olivine chondrule with Fe,Ni-sulfide layer at the chondrule-rim interface. f) Two individually rimmed type IA chondrules attached to each other. FGR = fine-grained rim; mes = mesostasis; ol = olivine; phy = phyllosilicates; sf = sulfide. Fine-grained dust rims in the Tagish Lake carbonaceous chondrite 1417

Fig. 3. Continued. BSE images of various rimmed objects in Tagish Lake. g) Heavily altered CAI composed of carbonates, phyllosilicates, and a few remaining spinel grains. h) Isolated magnesian olivine grain mantled by a fine-grained rim. The olivine contains one altered and one unaltered Fe,Ni-metal inclusion. i) Cluster of numerous small magnetite grains surrounded by a fine-grained rim. j) Completely altered chondrule surrounded by a fine-grained rim. alt met = altered Fe,Ni metal; crb = carbonate; FGR = fine-grained rim; ol = olivine; phy = phyllosilicates; sp = spinel.

In all chondrules of these two types, the high-Ca grained rims include FeO-poor barred olivine chondrules pyroxenes and the entire mesostasis have been aqueously (Fig. 3c), FeO-poor olivine chondrules with dendritic textures altered to dominantly Mg-rich phyllosilicates, which are (Fig. 3d), and FeO-rich porphyritic olivine chondrules (type much more coarse-grained than the rim material (Figs. 2b and IIA; Fig. 3b). Again, the mesostasis of such chondrules has 3; Tables 2–3); also, no augite overgrowths on pyroxene been replaced by phyllosilicates, whereas even the FeO-rich phenocrysts were found. While several Fe,Ni metal inclusions olivines appear almost unaffected by alteration. In some IIA in the forsteritic olivine have been replaced by alteration type chondrules, relict olivines that display chemical zoning products (Table 4), their host olivines and also the constituent from Fa0.8 in the core to Fa35 at the rim are present (Fig. 3b; orthopyroxenes generally show only few signs of alteration. Table 1). In one of these chondrules, an exceptional case was Occasionally, entire chondrules are almost completely altered found: parts of the formerly glassy Al- and P-rich mesostasis to hydrous phases, leaving only rare relict forsterites (“chd retained their primary compositional signature (Fig. 3b). psd” in Figs. 1 and 3j). Independent of size, mineralogy, and chemical Two IA chondrules were found to be surrounded by a composition, several chondrules display ellipsoidal shapes broad (up to 35 μm wide) Fe,Ni-sulfide band located between most likely caused by deformation during mild shock the silicate portion of the chondrule and the fine-grained rim metamorphism (Figs. 2a, 3b, and 3d). Such chondrule (Fig. 3e). flattening was observed for samples of the Murchison CM2 Less abundant chondrule types with pronounced fine- chondrite experimentally shocked to only 4 GPa (Tomeoka 1418 A. Greshake et al. ments. types and in mineral NiO S Total O 2 0.02 0.31 0.16 79.2 0.02 0.33 0.47 86.2 0.05 99.6 0.8 0.05 100.9 1.6 0.5 0.05 100.0 0.8 1.0 0.05 99.2 0.7 0.05 100.2 0.6 0.05 98.8 0.8 0.03 100.8 0.2 0.05 100.8 43.2 0.05 100.2 0.4 0.05 100.2 0.3 0.05 100.0 0.4 0.05 100.0* < < < < < < < < < < < < < < NiO Total Fa Fs Wo O OK 2 2 ne in various chondrule 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.13 100.6 1.7 0.02 0.02 0.02 0.02 0.02 0.020.02 0.07 99.4 46.3 0.020.02 0.06 0.06 99.9 100.7 23.8 37.8 < < < < < < < < < < < < < < < < Na olivine; px = low-Ca pyroxene. OK 2 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 < < < < < < < < < < < < < < < Na < chondrule types and of unaltered mineral frag acing olivine and pyroxe 0.03 21.9 0.34 0.18 0.04 0.56 0.19 73.0 0.03 25.3 0.32 0.27 0.16 0.23 0.07 84.0 < < 0.03 39.0 0.30 0.03 56.2 0.79 0.03 56.7 0.14 < < < mineral; min. frag. = mineral fragment; ol FeO MnO MgO CaO FeO MnO MgO CaO 3 3 O O 2 2 Cr Cr analyses of primary phases in various analyses of alteration products repl 3 3 O 2 O 0.01 0.36 0.81 0.29 56.4 0.23 0.01 0.50 1.2 0.12 55.4 0.20 0.01 0.20 32.7 0.37 30.2 0.46 2 eral fragment; ol = olivine; px pyroxene. < < Al < Al small grain size; chd. = chondrule; min 2 2 0.04 0.02 0.34 21.6 0.29 38.9 0.15 0.04 0.04 0.46 0.49 1.2 0.04 0.78 0.33 0.50 0.03 39.0 0.57 0.04 0.02 0.33 0.64 0.11 55.8 0.22 0.04 0.03 0.40 0.61 0.24 56.6 0.26 0.04 0.03 0.47 0.80 0.43 55.3 0.24 0.04 0.08 0.53 1.7 0.19 55.9 0.06 0.04 0.05 0.14 0.25 0.10 57.4 0.28 0.04 0.05 0.53 36.5 0.55 27 0.28 0.04 0.07 0.09 0.42 0.10 57.1 0.13 0.04 0.13 0.17 0.34 0.04 0.14 0.17 0.41 0.04 0.04 0.02 0.18 37.8 0.51 24.6 0.46 < < < < < < < < < < < < < < < TiO TiO 2 SiO 2 SiO -"- ol 38.4 IIA ol 41.4 -"- px 59.3 -"- px 58.6 -"- ol 41.9 -"- ol 41.9 IAB ol 41.4 -"- ol 41.9 -"- ol 42.5 Min. frag. ol 35.7 -"- ol 42.1 Min. frag. ol 42.4 I, BO ol 42.2 0.24 IA ol 42.3 Chd. type Min. Data in wt%; *normalized to 100% due Chd. type Data in wt%; chd. = chondrule; min. frag. min -"--"- ol ol 36.6 35.7 -"-Min. frag., px-"- 28.9 0.22 39.2 3.5 38.3 0.17 0.20 2.4 4.5 2.9 0.50 11.8 0.59 6.2 0.15 7.8 18.9 0.11 31.2 0.38 0.07 0.18 0.06 0.06 1.2 0.03 0.50 0.88 0.12 70.7 80.6 -"--"- IABType -"--"- 40.5 39.1I, BO 38.0Min. frag., ol 0.25 0.11 40.2 38.6 0.13 37.3 44.6 4.3 3.5 0.11 0.29 4.6 0.33 0.13 4.0 2.9 2.7 3.6 4.1 6.8 4.3 0.43 18.1 3.9 16.2 3.1 0.63 19.6 8.1 0.15 22.0 0.17 19.0 8.0 0.31 22.1 20.9 0.09 0.28 21.5 0.13 0.31 25.0 0.34 18.4 0.44 21.3 0.36 1.8 0.33 0.65 0.47 0.50 0.13 0.08 0.35 0.14 0.49 0.11 0.44 0.55 0.15 0.38 0.12 0.35 0.40 0.09 0.54 0.21 91.0 84.4 0.99 90.2 0.50 89.3 89.8 IA 39.9 0.16 3.9 3.3 17.2 0.22 20.1 0.44 0.18 Table 1. Representative electron microprobe Table 2. Representative electron microprobe Table fragments. Fine-grained dust rims in the Tagish Lake carbonaceous chondrite 1419

Table 3. Representative electron microprobe analyses of alteration products replacing mesostasis in various chondrule types. Chd. type SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2OK2O NiO S Total IA 42.6 0.11 3.5 0.88 9.6 0.13 26.0 0.26 0.31 0.04 0.60 0.10 84.1 -"- 43.5 0.29 3.1 0.63 11.4 0.06 26.8 0.32 0.22 0.11 1.1 0.17 87.7 -"- 43.0 0.22 3.7 0.96 8.7 0.10 30.7 0.18 0.31 0.03 0.57 0.06 88.5 IAB 41.9 0.13 3.1 0.65 10.3 0.08 26.5 0.28 0.40 0.14 0.82 0.08 84.4 -"- 42.8 0.20 5.0 1.1 12.6 0.13 25.9 0.19 0.60 0.22 0.81 0.14 89.7 -"- 41.7 0.15 5.3 2.3 13.7 0.06 26.0 0.13 0.51 0.06 0.13 0.10 90.1 IIA 41.3 <0.04 3.5 0.61 10.3 0.16 25.1 0.11 0.43 0.06 0.62 0.12 82.4 -"- 45.4 <0.04 4.6 0.76 11.4 0.04 26.8 0.13 0.61 0.05 0.55 0.10 90.5 I, BO 33.7 0.20 14.0 0.75 12.6 0.06 25.0 0.29 0.29 0.09 0.73 0.21 87.9 Data in wt%; chd. = chondrule.

Table 4. Representative electron microprobe analyses of typical metal alteration products in IAB chondrules and forsteritic olivine fragments. Chd. type SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2OK2O NiO S Total IAB 37.8 <0.04 4.8 2.1 18.2 0.15 23.6 0.37 0.53 0.06 2.3 0.51 90.5 Min. frag., ol 28.2 <0.04 2.6 1.5 17.9 0.22 19.0 0.40 0.31 0.03 2.9 0.98 74.1 -"- 37.9 <0.04 6.9 1.2 16.7 <0.03 22.1 0.14 0.94 0.11 0.29 0.44 86.8 Data in wt%; chd. = chondrule; min. frag. = mineral fragment; ol = olivine. et al. 1999), a pressure corresponding to the shock class S1 clusters are often found to be pseudomorphs after hexagonal assigned to the Tagish Lake chondrite (Stˆffler et al. 1991; pyrrhotite grains (Fig. 4a; Zolensky et al. 2002). Several of Scott et al. 1992; Zolensky et al. 2002). the 45 μm to 1 mm-sized clusters composed of ∼50 to several Ca, Al-rich inclusions and other refractory objects in hundred magnetites of different morphologies are surrounded Tagish Lake are also strongly affected by alteration, as are the by fine-grained rims (Fig. 3i). chondrules (Fig. 3g). Typically, the primary phases are extensively replaced by coarse-grained, FeO-poor Mineralogy and Petrography of Fine-Grained Rims phyllosilicates and with up to 5.2 wt% FeO and 3.9 wt% MnO. While other authors reported -rich Most chondrules, mineral fragments and refractory inclusions and small grains of perovskite (e.g., Zolensky et al. objects in Tagish Lake are surrounded by continuous, 2002; Simon and Grossman 2003), in this study, only spinel phyllosilicate-dominated, fine-grained rims, which can < (0.14 TiO2, 69.0 Al2O3, 0.18 Cr2O3, 1.4 FeO, 0.03 MnO, clearly be distinguished from the enclosed objects and the < < 29.1 MgO, 0.02 CaO, 0.06 Na2O, 0.02 K2O, 0.13 NiO; data meteorite matrix (Figs. 1–3). in wt%; normalized to 100% and corrected assuming that all Texturally, the fine-grained rims exhibit very similar SiO2 comes from the surrounding material) was identified as characteristics throughout the entire meteorite that are an unaltered refractory phase. Also, olivine-rich objects independent from the nature of the enclosed object (Fig. 5). largely consisting of refractory forsterite and lacking any The thicknesses of the rims vary between 1.5 and >200 μm (altered) mesostasis were only rarely found in the thin and are positively correlated with the diameters of the sections studied. enclosed objects, i.e., large objects are surrounded by broad Mineral fragments rimmed by fine-grained material rims and small constituents by thin layers (Fig. 6). Most rims include forsteritic olivine, FeO-rich and sometimes contain radial cracks extending from the object-rim to the compositionally zoned olivine, and FeO-poor low-Ca rim-matrix boundary and typically do not continue into the pyroxene (Fig. 3h). Similar to chondrules, all high-Ca matrix or the core objects (Figs. 2 and 3). The rims generally pyroxenes are almost entirely altered to Mg-rich exhibit sharp boundaries with the enclosed objects and show a phyllosilicates, whereas olivine and orthopyroxene show no clearly recognizable but more diffuse transition to the matrix or only minor degrees of alteration along fractures and around of the meteorite (Figs. 2, 3, and 5a). The rims are much finer- Fe,Ni-metal inclusions (Fig. 3h, Tables 2 and 4). grained and less porous than the chondrule, CAI, and mineral Magnetite occurs as large single spherules, clusters of fragment alteration products and the meteorite matrix (Figs. 3 tiny euhedral or also rounded crystals, stacked platelets- and 5b). The rim-chondrule boundaries are occasionally plaquettes, and as framboids (Fig. 4). All these morphologies decorated by opaque layers of either Fe,Ni sulfides or have been observed in various types of carbonaceous magnetites, which clearly belong to the enclosed object chondrites (e.g., Hua and Buseck 1998). The magnetite (Fig. 3e). One type IA chondrule was found in which the 1420 A. Greshake et al.

Fig. 4. BSE images of different magnetite morphologies in Tagish Lake. a) Magnetite cluster pseudomorph after hexagonal pyrrhotite. b) A close-up of the same cluster showing the diverse morphologies of magnetites, i.e., framboids, spherules, and stacked platelets. c) A close-up of two magnetite framboids consisting of tiny rounded grains. d) A close-up of stacked magnetite platelets and tiny individual euhedral to rounded magnetite crystals. alteration products (phyllosilicates) of the primary chondrule plaquettes, and framboids) to those observed in the Tagish phases seem to crosscut the rim into the meteorite matrix Lake matrix (Fig. 5d), supporting this conclusion. (Fig. 5c). Some rims show a pronounced internal layering Carbonates are less abundant in the rims compared to the with two layers of different compositions (Figs. 5a and 5b). In matrix. Grain sizes of the different components vary from <1 these cases, the layer appearing bright in BSE images is more to 25 μm, and, except for the much lower abundance of fine-grained and enriched in magnetite and sulfides. Usually, carbonates, the rims are mineralogically very similar to the the FeO-rich layer forms the outer and the FeO-poor layer the matrix of Tagish Lake. inner part of the dust rim (Fig. 5a). However, an opposite case TEM investigations of several rims reveal the presence was also observed (Fig. 5b). of both fine-grained phyllosilicates with entangled ribbon- Mineralogically, the rims consist of unequilibrated like structures and more coarse-grained phyllosilicate flakes assemblages of dominant phyllosilicates, Fe,Ni sulfides of (Fig. 7a). These differently shaped aggregates are found as mostly intermediate pyrrhotite- composition mostly distinct regions adjacent to each other on a sub-μm (Zolensky and Di Valentin 1998; Zolensky et al. 2002), as scale (Fig. 7a). While the ribbon-like aggregates consist of well as pentlandite, Fe,Ni metal, magnetites, low-Ca intimately intergrown (1.0–1.2 nm lattice fringes) pyroxenes, and forsteritic olivines with sometimes high and serpentine (0.7 nm lattice fringes), the coarse-grained MnO contents (Figs. 5e and 5f). Magnetites in the rims clumps and flakes are clearly dominated by curved saponite exhibit very similar morphologies (rounded crystals, crystals with only minor amounts of serpentine (Figs. 7a and Fine-grained dust rims in the Tagish Lake carbonaceous chondrite 1421

Fig. 5. BSE images illustrating the characteristic features of fine-grained rims in Tagish Lake. a) Layered rim with FeO-poor portion at the chondrule-rim interface. Note the sharp rim-chondrule and the more gradual rim-matrix boundaries. b) Layered rim with FeO-rich portion at the chondrule-rim interface. c) Chondrule phyllosilicates crosscutting the fine-grained rim into the meteorite matrix. d) Magnetites of various morphologies (rounded crystals, framboids, and platelets) in a fine-grained rim. e) Large magnetite, sulfide, and Mg-rich olivine in a compact fine-grained rim matrix. f) Coarse phyllosilicates and tiny sulfides in a fine-grained rim. chd = chondrule; chd phy = chondrule phyllosilicates; cph = coarse phyllosilicates; FGR = fine-grained rim; mt = magnetite; ol = olivine; sf = sulfide. 1422 A. Greshake et al.

compositions resulting from abundant Mg-rich phyllosilicates to more Fe-rich compositions caused by heterogeneous distributions of small magnetites, sulfides and more rarely Fe- rich phyllosilicates. This heterogeneous distribution of the opaque phases most likely also accounts for the slightly different molar Fe/(Mg + Fe) ratios that average at 0.35 for the matrix and at 0.41 for the rims. Compared to fine-grained rims in CM chondrites (Metzler et al. 1992; Zolensky et al. 1993), the rims in Tagish Lake show a broad compositional overlap with CM rims both extending to Mg-rich compositions (Zolensky et al. 1993). By normalizing the compositions of matrix and rims to the bulk composition of the Tagish Lake meteorite, it is confirmed that matrix and rims are very similar in composition and differ from the bulk meteorite by being slightly depleted in Na and slightly enriched in S (Fig. 9). Most notably, the rims are strongly depleted in Ca and slightly enriched in Ti compared to bulk Tagish Lake. The only bulk compositional differences between matrix and rims are a significant depletion of Ca and a small enrichment of Ti in the Fig. 6. Correlation of rim thicknesses and diameters of the enclosed rim material relative to the matrix (Fig. 9). This remarkable objects. depletion of Ca in the rims and the core objects compared to the matrix is illustrated by X-ray elemental mapping of a 7d). The anhydrous rim components, i.e., tiny, mostly single rimmed chondrule and of a larger area of Tagish Lake rounded magnetites, lath shaped to euhedral Fe,Ni sulfides, (Figs. 1 and 10). and often randomly corroded silicates in the size range of The microprobe analyses also show that Ca is strongly several tens of nm to about 1 μm, are interspersed within the fractionated from Al in the rims, whereas in the matrix the Ca/ phyllosilicate dominated groundmass of the rims (Figs. 7a–c). Al ratios scatter over a wide range (Fig. 11). The average Ca/ These observations indicate that the fine-grained rims are also Al ratios of the rims of 0.26 and the matrix of 0.71 are, on the TEM scale very similar to the matrix of the carbonate- however, both clearly below the ratio of 1.0 for the Tagish poor lithology of Tagish Lake (Keller and Flynn 2001; Lake bulk composition. In contrast to calcium, the Mikouchi et al. 2001; Zolensky et al. 2002). geochemically less mobile Ti is significantly less fractionated in rims and matrix (Fig. 12). The only small variability in the Chemical Composition of Matrix and Fine-Grained Rims Ti/Al ratios averages at 0.07 for the rims and 0.04 for the matrix compared to 0.05 for the bulk meteorite. The major and minor element compositions of Tagish The results of synchrotron X-ray microprobe analyses Lake matrix and fine-grained rims, as determined by indicate that, within analytical errors, there is only slightly defocused beam electron microprobe analysis, are given in variation for moderately volatile element concentrations from Table 5 and illustrated in Fig. 9; the compositions of the rim rim to rim (Fig. 13). Average matrix and rims have almost and matrix phyllosilicates were also determined by electron identical contents of Ni, Zn, Ga, and Se. Only Cu, and Ge may microprobe analysis and are shown in Fig. 8. For analyses, be slightly enriched in the rims. Relative to the mean Tagish only areas without carbonate grains and pits were selected. Lake composition, rims and matrix are, on average, enriched The microprobe analyses yield low average totals of in Ni, Cu, Ge, Se, and Zn. These results are in good agreement 63.1 wt% for the matrix and of 76.2 wt% for the rims, with data obtained from fine-grained rims of the CO3.0 indicating the presence of water-bearing phyllosilicates and chondrite ALH A77307 (Brearley et al. 1995). No correlation an amount of porosity that is significantly higher in the matrix between element concentrations and the degree of volatility than in the rims (Table 5). Additionally, the low average totals has been observed. The averaged concentrations of may be caused by a higher percentage of saponite relative to moderately volatile elements in the two chondrules analyzed serpentine in the matrix. The analyses also suggest that the differ significantly from those in matrix and rims. While Ni, fine-grained rims are chemically homogeneous at the 25 μm Cu, Se, and Zn are depleted, Ga and Ge are enriched scale. As illustrated in a ternary Mg-Fe-Al + Si plot (Fig. 8), compared with the rims, the matrix, and the Tagish Lake the phyllosilicates occurring in rims and matrix exhibit a mean (Fig. 13). In the chondrules, decreasing concentrations striking compositional similarity. Both matrix and rim are found for Ga, Ge, Se, and Zn in the order of their phyllosilicates show a comparable spread from Mg-rich decreasing condensation temperatures. Fine-grained dust rims in the Tagish Lake carbonaceous chondrite 1423

Fig. 7. Transmission electron microscope images of fine-grained rims in Tagish Lake. a) Bright field image showing the typical microtexture of the rims. Coarse- and fine-grained phyllosilicates are present in distinct, adjacent regions and serve as groundmass to small anhydrous mineral phases including magnetite, olivine and Fe,Ni sulfides; the latter are present as lath-shaped and euhedral crystals. b) Bright field image of a randomly corroded olivine embedded into coarse phyllosilicates. c) Bright field image of a euhedral sulfide and a rounded magnetite crystal. d) High-resolution TEM image of saponite displaying 1.2 nm interlayer spacings. cph = coarse phyllosilicates; fph = fine phyllosilicates; mt = magnetite; ol = olivine; sap = saponite; sf = sulfide. 1424 A. Greshake et al.

Table 5. Defocused beam electron microprobe analyses of matrix and rims in Tagish Lake. Matrix Rims* n = 79 n = 80 1i 1o 2i 2o 3i 3o 4i 4o

SiO2 22.4 27.7 32.6 29.6 31.7 29.5 31.2 26.8 31.5 33.3 TiO2 0.07 0.13 0.26 0.21 0.18 0.16 0.20 0.26 0.20 0.19 Al2O3 1.9 2.4 3.2 2.9 2.5 2.6 2.9 2.7 2.6 2.6 Cr2O3 0.32 0.5 0.59 0.51 0.58 0.51 0.61 0.52 0.41 0.52 FeO 16.3 21.0 21.3 25.6 19.4 24.7 20.0 28.5 21.0 17.0 MnO 0.2 0.17 0.11 0.13 0.14 0.12 0.05 0.10 0.05 0.10 MgO 15.2 16.0 15.9 14.5 15.5 14.6 14.9 15.1 17.0 16.6 CaO 0.97 0.45 0.54 0.57 0.41 0.46 0.36 0.48 0.21 0.25 Na2O 0.22 0.34 0.29 0.29 0.23 0.24 0.25 0.24 0.32 0.36 < K2O 0.06 0.02 0.05 0.04 0.04 0.04 0.03 0.03 0.04 0.04 NiO 1.5 1.9 2.1 1.9 2.6 1.9 2.2 2.0 5.1 2.7 S 3.9 5.5 1.9 2.3 1.9 2.6 2.0 3.7 2.9 1.6 Total 63.0 76.1 78.8 78.6 75.2 77.4 74.7 80.4 81.3 75.3 Data in wt%; *non-layered rims; i = inner rim; o = outer rim. 1–3: Fe/Mg(inner) < Fe/Mg(outer); 4: Fe/Mg(inner) > Fe/Mg(outer); n = number of analyses.

DISCUSSION

Several of the observed mineralogical and chemical rim characteristics allow one to constrain the origin of the rims around anhydrous components in Tagish Lake. In the discussion, we will focus on the three main mechanisms proposed for the formation of fine-grained rims: regolith gardening, in situ alteration of the enclosed objects, and accretion onto the enclosed objects.

Formation of the Fine-Grained Rims

Regolith gardening for the origin of the fine-grained rims has been proposed by a number of investigators (e.g., Sears et al. 1993; Tomeoka and Tanimura 2000). However, the fine- grained rims in Tagish Lake completely surround the coarse- grained objects (CAIs, chondrules, and mineral fragments) and show no evidence for fragmentation. These observations strongly argue against formation of the rims by regolith Fig. 8. Composition of phyllosilicate-dominated regions in fine- gardening. Such highly disruptive processes should have grained rims (filled circles) and matrix (open squares) from Tagish produced abundant fragments of partly rimmed objects, Lake, as determined by defocused beam electron microprobe analyses. The composition of fine-grained rims from different CM completely rimmed lithic clasts composed of chondrules, chondrites (grey triangles; data from Metzler et al. 1992) are plotted CAIs, and/or mineral fragments, and also pervasive cracks for comparison. not restricted to specific constituents (e.g., Metzler et al. 1992; Metzler and Bischoff 1996; Hua et al. 2002). Compared (carbonate-rich and poor) together with the restriction of observations in many even strongly brecciated and/or cracks to only the fine-grained rims may further indicate that metamorphosed CM chondrites, the fine-grained rims in Tagish Lake is not brecciated on the chondrule but only on Tagish Lake often contain radial cracks which do not extend larger scale. into the meteorite matrix (e.g., Metzler et al. 1992, Brearley In situ alteration of the enclosed objects (e.g., Sears et al. 1993; Hua et al. 2002). However, because none of the cracked 1991) is also unlikely for the origin of the fine-grained rims, rims is fragmented, it seems highly unlikely that the cracks based on convincing textural and compositional evidence: 1) formed by collision of the rimmed objects prior or during the the fine-grained rims exhibit very similar mineralogical aggregation of Tagish Lake. Instead, we suggest that these characteristics independent of the nature of the enclosed cracks formed during in situ aqueous alteration due to volume object; 2) no replacement of any chondrule or CAI mineral by change (e.g., oxidation of metal to magnetite), as previously fine-grained rim material has been observed; 3) alteration reported from the ungrouped carbonaceous chondrite MAC products of mesostasis and high-Ca pyroxene in the rimmed 88107 (Krot et al. 2000). The presence of different lithologies chondrules and of refractory phases in CAIs are Fine-grained dust rims in the Tagish Lake carbonaceous chondrite 1425

evidence listed includes: 1) the presence of unaltered chondrule mesostasis in direct contact with rim phyllosilicates, indicating that the rim phases must have been altered before accreting onto the chondrule; 2) the presence of a rimmed chondrule fragment where olivine displays both fresh and serpentinized surfaces but no alteration at the rim- chondrule interface, indicating that the olivine was altered before the rims accreted; 3) the presence of heterogeneous disequilibrium mineral assemblages in the rims, disproving homogeneous in situ parent body alteration; and 4) the presence of altered chondrules surrounded by rims containing numerous anhydrous minerals. All these observations can only be sufficiently explained by pre-accretionary alteration of the rim and/or chondrule minerals, but not by in situ parent body alteration, which should have led to a more Fig. 9. Average concentrations of major and minor elements from the homogeneous degree of alteration of all involved phases Tagish Lake matrix (filled triangles) and fine-grained-rims (open (Bischoff 1998 and references therein). It is further suggested squares) as determined by defocused beam electron microprobe analyses. Concentrations are normalized to the Tagish Lake bulk that such pre-accretionary alteration occurred on small, composition (Brown et al. 2000). uncompacted precursor planetesimals that were later disrupted, which produced large amounts of fine-grained mineralogically and texturally different from the rim material; chondritic material altered to different degrees (Metzler et al. 4) the positive correlation of rim thickness with the diameter 1992; Bischoff 1998; Wilson et al. 1998). of the core object contradicts rim formation by in situ In contrast to this scenario, several observations have alteration (e.g., Metzler et al. 1992; Metzler and Bischoff been made in carbonaceous chondrites which seem to 1996); 5) SXRF and EMP analyses show that the rims are contradict pre-accretionary alteration and are instead more compositionally very similar throughout the entire meteorite compatible with in situ parent body alteration. For example, and significantly different from the enclosed objects. matrix and fine-grained rims within any particular meteorite Accretion onto the enclosed rims in the solar nebula for are found to be mineralogically, but not always the formation of the fine-grained rims appears to us, in compositionally, identical to one another (Rubin 1984; Scott agreement with earlier investigators, to be the only viable et al. 1984; Brearley and Geiger 1991; Zolensky et al. 1993). mechanism for their origin. According to Metzler et al. (1991, By assuming that the composition of the material was 1992), the observed positive correlation between rim determined by the initial mineralogy, whereas the current thickness and core object diameter indicates that the rims mineralogy is determined by secondary modification formed—depending on the dust/gas ratio—within several processes, it is concluded that the observed identical thousand years in a turbulent protoplanetary disk. Although mineralogy of rims and matrix was caused by post- chemically very similar, the layering of some fine-grained accretional aqueous alteration on the parent body (Zolensky rims makes sampling from compositionally different et al. 1993). This model seems to be supported by local reservoirs in the solar nebula very likely (e.g., Metzler et al. correlations between mineralogy and composition of fine- 1991, 1992; Metzler and Bischoff 1996). grained rims, indicating that aqueous alteration occurred in situ and that the resulting secondary mineral assemblage is Alteration and Modification of Fine-Grained Rims controlled by the rim’s bulk chemistry (Brearley and Geiger 1991). In addition, Hanowski and Brearley (2000) found Fe- Fine-grained rims dominantly consist of secondary rich aureoles around numerous components in CM minerals formed by aqueous alteration of anhydrous chondrites, the origin of which is incompatible with pre- precursor phases, and there is still considerable debate about acrretionary alteration. Their mass balance calculations show whether this alteration predates accretion of the rims (e.g., that, during aqueous alteration, Fe was mobilized from metal Ikeda 1983; Metzler et al. 1992; Bischoff 1998) or took place grains and is now quantitatively present in the aureoles. after rim formation on the parent body (e.g., Zolensky and Excluding any significant Fe loss due to previous pre- McSween 1988; Krot et al. 1995, 1998a; Hanowski and accretionary alteration, thus, the aureoles strongly suggest in Brearley 2000). situ parent body alteration (Hanowski and Brearley 2000). As discussed in a detailed review by Bischoff (1998), Finally, a similar degree of alteration of rims and matrix, as there are textural, mineralogical, and chemical characteristics well as a systematic Mg enrichment of serpentines in of fine-grained rims and rimmed objects, especially in CM chondrules and rims with increasing degree of alteration, are chondrites, which strongly indicate a pre-accretionary taken as arguments for in situ parent body alteration alteration of the rim material (see also Ciesla et al. 2003). The (Hanowski and Brearley 2001). 1426 A. Greshake et al.

Fig. 10. Mg, Fe, Al, and Ca Kα X-ray elemental maps of two FeO-poor porpyhritic olivine chondrules surrounding fine-grained-rims and meteorite matrix. While all unaltered olivines are of forsteritic composition, the former chondrule mesostasis and pyroxenes, which have both been replaced by phyllosilicates, are rich in Al and Fe. The Ca distribution map clearly shows that fine-grained rims and enclosed objects are depleted in Ca compared to the meteorite matrix. crb = carbonate; FGR = fine-grained rims; ol = olivine; phy = phyllosilicates.

The Tagish Lake meteorite shows strong compositional reported from fine-grained rims in various CM chondrites and mineralogical evidence for extensive in situ parent body (e.g., Metzler et al. 1992; Brearley 1996; Hanowski and aqueous alteration that affected all lithologies, including the Brearley 2001; Hua et al. 2002). Our petrographic fine-grained rims. observations suggest that Ca was lost from the rimmed Chemically, this alteration resulted in the fractionation of objects as a result of the replacement of Ca-rich phases, i.e., the refractory lithophile Ca from Al in the rims and in their Ca-rich pyroxene, chondrule mesostasis, and melilite, by Ca- general depletion in Ca compared to the bulk meteorite free/poor phyllosilicates. Because Ca and Al are refractory (Figs. 1, 10, and 11). In contrast, the less mobile Ti remained lithophile elements of similar volatility, the observed unfractionated. A similar behavior of Ca was previously fractionation of Ca from Al and mobilization of Ca suggests Fine-grained dust rims in the Tagish Lake carbonaceous chondrite 1427

Fig. 12. Ti/Si versus Al/Si plot for matrix and fine-grained rims from Fig. 11. Ca/Si versus Al/Si plot for matrix and fine-grained rims from Tagish Lake compared to Tagish Lake and CI bulk ratios. Tagish Lake compared to Tagish Lake and CI bulk ratios. chondrules and CAIs into the meteorite matrix is a general process during aqueous alteration of carbonaceous chondrites that may serve as an indicator for the degree of alteration (e.g., Brearley 1996; Krot et al. 1998b, 2003). Accordingly, the different degrees of Ca mobilization found in Tagish Lake and Murchsion (Figs. 1 and 14) convincingly indicate that Tagish Lake is more pervasively altered than this rather weakly altered CM2 chondrite. This conclusion is supported by the extreme alteration of CAIs in Tagish Lake pointing to a more progressive alteration than in any other CAI-bearing CM2 chondrite (Zolensky et al. 2002). With the exception of Ca and Ti, Tagish Lake matrix and rims show almost identical concentrations for all elements measured. However, as aqueous alteration may have also lead to a redistribution of, especially, the chalcophile elements Zn and Se, whose hosts pyrrhotite and pentlandite are often replaced by magnetite, a possible primary compositional difference between matrix and rims may have been erased. Fig. 13. Averaged concentrations of moderately volatile elements Mineralogically, rims and matrix in Tagish Lake consist from Tagish Lake matrix (filled circles), four fine-grained rims (open of saponite, serpentine, sulfides, and magnetites, in textures symbols), and chondrules (gray diamonds), as determined by that are typical for CI1 chondrites but completely unlike those synchrotron X-ray microprobe analyses. Concentrations are normalized to the Tagish Lake bulk composition and iron (Brown of CM2 chondrites (see also Keller and Flynn 2001; Mikouchi et al. 2000). et al. 2001; Zolensky et al. 2002). Also, the molar Fe/(Fe + Mg) ratios of phyllosilicates in Tagish Lake matrix (fe = 0.3– that Ca was transported via an aqueous fluid through the fine- 0.35) and rims (fe = 0.41) are more similar to the grained rims and precipitated as Ca carbonates in the phyllosilicate compositions of the CI1 chondrite Ivuna (fe meteorite’s matrix. This preferential precipitation may have ∼0.3) rather than to those of rim and matrix phyllosilicates in resulted either from the higher porosity of the matrix or was CM2 chondrites averaging at fe ∼0.7 (e.g., Zolensky et al. controlled by a chemical gradient between enclosed object, 1993; Brearley 1997; Zolensky et al. 2002). As suggested by rim, and matrix. The fact that fractionation and redistribution several studies, there is a strong correlation between the of Ca is commonly observed in the aqueously altered CM, degree of alteration and the MgO content of phyllosilicates CR, and CV carbonaceous chondrites (e.g., Fig. 14; Krot et al. replacing primary phases, i.e., the MgO content increases 1998a, 1998b) strongly suggests that mobilization of Ca from with increasing degree of alteration (e.g., Browning et al. 1428 A. Greshake et al.

observed for phyllosilicates in Tagish Lake are more likely due to fractionation of Fe from fine-grained phyllosilicates (Fe-rich serpentine) into magnetite and not indicative for advanced alteration (e.g., Brearley 1997). In addition, recent studies of iron-nickel sulfides in CM chondrites showed that the alteration index based on the MgO content of phyllosilicates may not be valid for the most thoroughly altered CMs (Chokai et al. 2004). There is, however, also strong mineralogical evidence for progressive in situ aqueous alteration of Tagish Lake. Systematic studies of sulfides in differently altered CM chondrites have shown that the amount of intermediate sulfides is directly related to the degree of alteration, i.e., pure pyrrhotite is the dominant sulfide in weakly altered CM chondrites, whereas intermediate sulfides become increasingly abundant in strongly altered CMs (Zolensky et al. 2002; Chokai et al. 2004). In Tagish Lake, the presence of both pure pyrrhotite and intermediate sulfides indicates incomplete transformation and a degree of aqueous alteration intermediate within the CM2 range (Zolensky et al. 2002). Additionally, the very similar composition of phyllosilicates in matrix and rims, the survival of dominantly very Mg-rich anhydrous silicates in chondrules, matrix, and rims, as well as the presence of clusters of framboidal magnetites in rims and matrix, prove a similar degree of alteration in all lithologies and strongly suggest in situ alteration of the rim material. However, the presence of anhydrous relict phases in matrix and rims confirm that aqueous alteration did not proceed to completion. The rare occurrence of both entirely altered chondrule pseudomorphs (Fig. 3j) and chondrules with intact Fe-rich olivines (Fig. 3b) may either reflect mixing of differently altered material during brecciation or document that aqueous alteration was locally heterogeneous. Since the meteorite matrix, the vast majority of rimmed objects, and all rims show an almost identical degree of alteration, local heterogeneities during the alteration process seem much more likely (Fig. 1). Such different degrees of in situ chondrule alteration were previously found in dark inclusions from the Efremovka CV3 chondrite (Krot et al. 1999). Fig. 14. a) Combined X-ray elemental map for Mg (red), Ca (green) The evidence presented above strongly indicates that, and Al Kα (blue). b) Ca Kα X-ray elemental map of the CM2 carbonaceous chondrite Murchison. The meteorite contains abundant apart from small localized heterogeneities, progressive parent porphyritic olivine and porphyritic olivine-pyroxene chondrules body alteration pervasively affected all Tagish Lake replaced by phyllosilicates and surrounded by fine-grained rims. lithologies and removed any potential evidence for pre- Chondrules are much less altered and fine-grained rims are better accretionary alteration. However, despite such strong defined than those in Tagish Lake. Fine-grained rims are depleted in alteration, the overall texture of Tagish Lake remained Ca relative to matrix that contains rather coarse-grained carbonates. CAI = Ca, Al-rich inclusion; chd = chondrule; crb = carbonates; FGR unchanged. Similar features were previously reported from = fine-grained rim. the CM2 chondrites Nogoya (Metzler 1995) and ALH 81002 (Hanowski and Brearley 2000, 2001), and the ungrouped 1996; Hanowski and Brearley 1997, 2001). However, the carbonaceous chondrite MacAlpine Hills (MAC) 88107 (Krot high modal abundance of magnetite in Tagish Lake clearly et al. 2000), where the mantled objects were also altered in shows that the meteorite is much more oxidized than CM2 situ on the parent body, but the primary accretionary texture is chondrites, suggesting that the Mg-rich compositions still preserved. Fine-grained dust rims in the Tagish Lake carbonaceous chondrite 1429

CONCLUSIONS Division of Chemical Sciences, under Contract No. DE- AC02-98CH10886. This is Hawai’i Institute of Geophysics These observations on fine-grained rims in the Tagish and Planetology publication No. 1420 and School of Ocean Lake carbonaceous chondrite allow us to draw the following and Earth Science and Technology publication No. 6691. conclusions: 1. Fine-grained rims are not genetically linked to the Editorial Handling—Dr. Wolf Uwe Reimold enclosed objects, excluding their formation by alteration of the core object. REFERENCES 2. Fine-grained rims show no evidence of fragmentation, indicating that they formed prior to accretion of the Allen J. S., Nozette S., and Wilkening L. L. 1980. A study of meteorite parent body. chondrule rims and chondrule irradiation records in unequilibrated ordinary chondrites. Geochimica et 3. Formation of the rims by accretion in the solar nebula Cosmochimica Acta 44:1161–1175. most convincingly accounts for all textural, Barrat J. 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