05 Classification of.. - Department of Earth and Planetary Sciences

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94 Classification of Meteorites Figure 8 Combined elemental maps (Mg—red, Ca— green, Al—blue) of: (a) the CM carbonaceous chondrite Murchison and (b) ungrouped CM-like carbonaceous chondrite Tagish Lake. Murchison contains abundant magnesian chondrules (red) with porphyritic textures, CAIs (bluish), and heavily hydrated matrix (purple) containing relatively rare grains of carbonate (green). Most chondrules and CAIs are surrounded by finegrained accretionary rims. Tagish Lake contains higher proportion of heavily hydrated matrix and carbonates, and fewer chondrules and CAIs. Many chondrules are surrounded by fine-grained accretionary rims. AR ¼ accretionary rim; CAI ¼ Ca–Al-rich inclusion; chd ¼ chondrule; crb ¼ carbonates; mx ¼ matrix. The nature of this reheating (its peak temperature and duration) remains poorly understood; two mechanisms discussed in the literature include impact shock heating and thermal metamorphism (e.g., Ikeda and Prinz, 1993). CO (Ornans-like) and ungrouped CO/CM-like chondrites. Characteristics of the CO group (Figure 9(a)) include: (i) abundant (35–45 vol.%) relatively small (,150 mm), rounded chondrules Figure 9 Combined elemental maps of: (a) the CO3.1 carbonaceous chondrite Kainsaz and (b) ungrouped CO/CM-like carbonaceous chondrite Acfer 094. Kainsaz contains abundant small chondrules, CAIs, and AOAs. Acfer 094 is texturally and mineralogically similar to CO chondrites, but contains higher abundance of matrix. AOA ¼ amoeboid olivine aggregate; BO ¼ barred olivine chondrule; PO(P) I, II ¼ type I (II) porphyritic olivine (pyroxene) chondrule. (Rubin, 1989); (ii) a high proportion of metal-rich magnesian chondrules; ferroan (type II) chondrules are also common; (iii) abundant CAIs and AOAs (,10 vol.%); (iv) a relatively low abundance of matrix (30–45 vol.%); (v) a metamorphic sequence (3.0–3.7), in many ways analogous to that of type-3 ordinary chondrites (McSween, 1977a; Scott and Jones, 1990); and (vi) secondary alteration minerals in chondrules and CAIs that are similar to those in CV chondrites (e.g., nepheline, sodalite, ferrous olivine, hedenbergite, andradite, and ilmenite, with phyllosilicates being virtually absent; e.g., Keller and Buseck (1990a), Tomeoka et al.
(1992), Rubin et al. (1985), Rubin (1998), and Russell et al. (1998a)). Acfer 094 (Figure 9(b)) is an ungrouped, type-3 carbonaceous chondrite breccia with mineralogical, petrological, nitrogen isotopic, and oxygen isotopic affinities to the CM and CO groups (Newton et al., 1995; Greshake, 1997). Its bulk chemical composition is similar to that of CM chondrites, and oxygen isotopic composition (Figure 4) and matrix modal abundance are similar to those of CO chondrites. In contrast to CM chondrites, Acfer 094 shows no evidence for aqueous alteration and thermal metamorphism and has different carbon isotope compositions. It has higher abundances of presolar SiC and diamonds than any other chondrite (Tagish Lake is the only exception). Adelaide is an ungrouped, type-3 carbonaceous chondrite with affinities to the CM–CO clan, but appears to have escaped the thermal metamorphism and alteration commonly observed in the CM and CO carbonaceous chondrites (Fitzgerald and Jones, 1977; Davy et al., 1978; Kallemeyn and Wasson, 1982; Hutcheon and Steele, 1982; Kerridge, 1985; Brearley,1991;HussandHutcheon,1992;Krotetal., 2001b,c). MAC88107 and MAC87300 comprise a grouplet of types 2 and 3 carbonaceous chondrites with bulk chemical compositions intermediate between CO and CM chondrites, and oxygen isotopic compositions similar to the CK chondrites (Clayton and Mayeda, 1999). In contrast to the CK chondrites, they show no evidence for thermal metamorphism (Krot et al., 2000d). Contrary to the CM chondrites, which have no detectable natural or induced thermoluminescence (TL), MAC87300 and MAC88107 have both (Sears et al., 1991). They experienced very minor aqueous alteration that appears to have occurred at higher temperatures (,120–220 8C) than that inferred for CM chondrites (,25 8C; Clayton and Mayeda, 1999), and resulted in formation of saponite, serpentine, magnetite, nickel-bearing sulfides, fayalite, and hedenbergite (Krot et al., 2000d). CR (Renazzo-like) and ungrouped CR-like chondrites. Characteristics of the CR group (Figures 10(a) and 11(a), and Table 2) include: (i) abundant large FeNi-metal-rich, porphyritic, magnesian (type-I) chondrules, typically surrounded by multilayered coarse-grained, igneous rims that in many cases contain a silica phase; (ii) high (,0.5 wt.%) Cr 2 O 3 content in chondrule olivines; (iii) abundant heavily hydrated matrix and matrix-like lithic clasts (commonly referred to in the literature as dark inclusions); (iv) abundant FeNi-metal with a positive nickel versus cobalt trend and a solar Ni/Co ratio; (v) low abundance of CAIs and AOAs; (vi) uniform 16 O-enrichment of AOAs and most CAIs; (vii) whole-rock refractory lithophile elemental abundances close Classification of Chondritic Meteorites 95 to solar (Figure 2); (viii) whole-rock oxygen isotopic compositions that form a unique CRmixing line (Figure 4); and (ix) bulk nitrogen isotopic compositions with large positive anomalies in d 15 N(Figure 6) (McSween, 1977b; Bischoff et al., 1993a; Weisberg et al., 1993; Kallemeyn et al., 1994; Krot et al., 2000b, 2002; Aléon et al., 2002). Most of the CR chondrites are of petrologic type 2 and generally contain abundant phyllosilicates, carbonates, platelet and framboidal magnetite, and sulfides (Weisberg et al., 1993). GRO95577 contains no anhydrous silicates and is classified as CR1 (Weisberg and Prinz, 2000). The Al Rais meteorite is an anomalous CR chondrite due to a high abundance of matrix component (.70 vol.%). The Kaidun meteorite is a complex chondritic breccia that appears to consist mainly of CR material, but also contains clasts of enstatite, ordinary, and several carbonaceous chondrite groups (Ivanov, 1989; Clayton and Mayeda, 1999). Sahara 00182 (SAH00182) is an ungrouped metal-rich C3 chondrite (Grossman and Zipfel, 2001; Weisberg, 2001). It contains large (up to 2 mm in diameter), metal-rich, multilayered chondrules that are texturally similar to those in the CR chondrites. However, SAH00182 differs from the CR chondrites in that its chondrules contain troilite (FeS), and the chondrules and matrix lack the hydrous phyllosilicates that are characteristic of many CR chondrites. Its whole-rock oxygen isotopic composition plots along the CCAM line (Grossman and Zipfel, 2001). The recently described metal-rich Tafassasset meteorite has an oxygen isotopic composition similar to CR chondrites and an equigranular texture (Bourot-Denise et al., 2002). It is, however, characterized by uniformly ferrous olivine (Fa 30 ) and orthopyroxene (Fs 25 ), and the presence of albitic plagioclase (An 24 – 45 ), chromite, and merrilite; it may be the first metamorphosed CR chondrite. Lewis Cliff 85332 (LEW85332) is a unique, metal-rich type-3 carbonaceous chondrite breccia with chemical, oxygen isotopic, and petrographic characteristics similar to those of the CR chondrites (Rubin and Kallemeyn, 1990; Prinz et al., 1992; Weisberg et al., 1995a; Brearley, 1997). It has a high abundance of heavily hydrated matrix (,30 vol.%), anhydrous chondrules and CAIs, and CR-like refractory lithophile element abundance ratios. Relative to the CR chondrites, however, it has smaller chondrules and higher abundances of manganese and most volatile siderophile elements. CH (ALH85085-like) chondrites. The CH chondrites (Figures 10(b) and 11(b)) are characterized by: (i) abundant FeNi-metal (,20 vol.%)
Page 1 and 2: 1.05 Classification of Meteorites A
Page 3 and 4: Introduction 85 Table 1 Meteorite g
Page 5 and 6: Classification of Chondritic Meteor
Page 11: Classification of Chondritic Meteor
Page 17 and 18: Figure 12 Combined elemental maps o
Page 19 and 20: Figure 14 Fayalite (Fa) content (mo
Page 21 and 22: abundances and chondrule sizes by c
Page 23 and 24: Table 6 Synopsis of petrologic char
Page 25 and 26: compositions to the winonaites and
Page 27 and 28: Figure 20 Photomicrographs of brach
Page 29 and 30: Classification of Nonchondritic Met
Page 31 and 32: 1.05.4.4 Pallasites Pallasites are
Page 33 and 34: ,85% of iron meteorites fall into o
Page 35 and 36: Martian (SNC) Meteorites 117 Figure
Page 37 and 38: Martian (SNC) Meteorites 119 Figure
Page 39 and 40: References 121 Benedix G. K., McCoy
Page 41 and 42: References 123 Ivanova M. A., Taylo
Page 43 and 44: References 125 McSween H. Y. Jr. an
Page 45 and 46: References 127 a large differentiat

05 Classification of.. - Department of Earth and Planetary Sciences

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