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

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96 Classification of Meteorites Figure 10 Combined elemental maps of the: (a) CR carbonaceous chondrite PCA91082, (b) CH carbonaceous chondrite PAT91546, (c) CB a carbonaceous chondrite Gujba, and (d) CB b carbonaceous chondrite Hammadah al Hamra 237. (a) The CR chondrite PCA91082 contains large porphyritic, metal-rich (mt), magnesian (type I) chondrules, heavily hydrated fine-grained matrix, and rare anorthite-rich chondrules (ARC) and CAIs. Many chondrules are surrounded by coarse-grained, igneous rims. (b) The CH chondrite PAT91546 contains abundant small chondrules and chondrule fragments, FeNi-metal grains, and CAIs. Interchondrule matrix material is virtually absent; heavily hydrated matrix lumps (mx) are present instead. Most chondrules are FeNi-metal-free and have cryptocrystalline (CC) and barred olivine textures; chondrules of porphyritic textures (PO) are rare. (c) The CB a chondrite Gujba consists of large chondrule fragments with cryptocrystalline and very fine grained textures, and FeNi-metal grains. (d) The CB b chondrite Hammadah al Hamra 237 contains abundant FeNi-metal, chondrules, and rare CAIs. Chondrules have either cryptocrystalline (reddish colors) or skeletal olivine (bluish colors) textures. Matrix material is absent. with a positive nickel versus cobalt trend and a solar Ni/Co ratio; (ii) small chondrule and CAI sizes (average diameter ,20 mm; ,90 mm inthe CH chondrite Acfer 182); (iii) dominance of cryptocrystalline (CC) chondrules; (iv) high (,0.5 wt.%) Cr 2 O 3 contents of chondrule olivines; (v) lack of matrix and presence of heavily hydrated lithic clasts mineralogically similar to those in CB chondrites instead; (vi) nearly solar bulk refractory lithophile element abundances (Figure 2); (vii) large depletions in bulk volatile and moderately volatile element abundances (Figure 2); (viii) whole-rock oxygen isotopic compositions plotting along the CRmixing line (Figure 4); (ix) large positive anomalies in d 15 N(Figure 6); and (x) presence of a unique component of subsolar rare gases (Grossman et al., 1988; Scott, 1988; Weisberg et al., 1988; Wasson and Kallemeyn, 1990; Bischoff et al., 1993b). When the ALH85085 meteorite was first described, it sparked much controversy because
Classification of Chondritic Meteorites 97 Figure 11 Backscattered electron images (a, b) and elemental maps in Ni Ka X-rays (c, d) of the: (a) CR chondrite PCA91082, (b) CH chondrite PCA91467, (c) CB a chondrite Gujba, and (d) CB b chondrite Hammadah al Hamra 237. (a) In the CR chondrite PCA91467, FeNi-metal occurs as nodules in chondrules, igneous rims around chondrules and in the matrix. (b) In the CH chondrite PAT91546, FeNi-metal occurs largely as irregularly shaped grains outside chondrules. (c) In the CB a chondrite Gujba, most FeNi-metal grains are texturally similar to metal-sulfide aggregates in the CB b chondrite Hammadah al Hamra 237. Although there are compositional variations between metal grains, each grain is compositionally uniform. (d) In Hammadah al Hamra 237, FeNi-metal occurs exclusively outside chondrules; ,20% of all metal grains are compositionally zoned, with cores enriched in Ni relative to edges. Some of the zoned metal grains contain silicate inclusions. Metal-sulfide aggregates are compositionally uniform. of these unusual characteritics. Wasson and Kallemeyn (1990) suggested that it is a modified “subchondritic” meteorite formed as a result of a highly energetic asteroidal collision. More recently, it has been argued that the refractory inclusions, chondrules, and zoned metal grains in these meteorites are in fact pristine nebular products (Meibom et al., 1999; Krot et al., 2000c; McKeegan et al., 2003). CB (Bencubbin-like) chondrites. The CB chondrites (Figures 10(c), (d), and 11(c), (d)) are a group of very metal rich (60–70 vol.%) meteorites including Bencubbin, Gujba, Weatherford, Hammadah al Hamra (HH) 237, and Queen Alexandra Range (QUE) 94411 which is paired with QUE94627 (Weisberg et al., 2001). Characteristics of the CB chondrites include: (i) very high metal/silicate ratios (Table 2); (ii) extreme depletion in moderately volatile lithophile elements (Figure 2); (iii) extreme enrichment in d 15 N(Figure 6); (iv) bulk oxygen isotopic compositions plotting near CR chondrites (Figure 4); (v) bulk refractory lithophile element abundances of (1.1–1.5) £ CI (Figure 2(a)); (vi) FeNi-metal with a positive nickel versus cobalt trend and a solar Ni/Co ratio; (vii) magnesium-rich (Fa #4 ,Fs #4 ), FeNi-metalfree chondrules with only nonporphyritic (cryptocrystalline and skeletal-olivine) textures;
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 13: (1992), Rubin et al. (1985), Rubin
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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