98 (viii) high (,0.5 wt.%) Cr 2 O 3 contents in chondrule olivines; (ix) rarity <strong>and</strong> uniform 16 O- depletion <strong>of</strong> CAIs; <strong>and</strong> (x) virtual absence <strong>of</strong> fine-grained matrix material <strong>and</strong> presence <strong>of</strong> rare heavily hydrated lithic clasts instead (Lovering, 1962; Mason <strong>and</strong> Nelen, 1968; Kallemeyn et al., 1978; Newsom <strong>and</strong> Drake, 1979; Weisberg et al., 1990, 2001; Rubin et al., 2001; Krot et al., 2001a, 2002; Greshake et al., 2002). Based on petrologic <strong>and</strong> chemical differences among the CB chondrites, they have been divided into two subgroups: CB a , which includes Bencubbin, Weatherford, <strong>and</strong> Gujba; <strong>and</strong> CB b , which includes HH 237 <strong>and</strong> QUE94411 (Weisberg et al., 2001). Differences between the subgroups include FeNi-metal abundances, CAI abundances, chondrule sizes, compositional range <strong>of</strong> FeNi-metal, <strong>and</strong> nitrogen isotopic compositions (Figures 6, 10(c), (d), <strong>and</strong> 11(c), (d)). The CB a contains ,60 vol.% metal, centimeter-sized chondrules, <strong>and</strong> exceptionally rare CAIs (the only CAI was found in Gujba; Weisberg et al., 2002); FeNimetal ranges from 5 wt.% Ni to 8 wt.% Ni, <strong>and</strong> d 15 N is up to ,1,000‰. The CB b contains .70 vol.% metal, millimeter-sized chondrules; FeNi-metal ranges from 4 wt.% Ni to 15 wt.% Ni; refractory inclusions are common; <strong>and</strong> d 15 Nis up to ,200‰. CV (Vigarano-like) <strong>and</strong> ungrouped CV-like chondrites. The CV chondrites (Figure 12) are characterized by: (i) millimeter-sized chondrules with mostly porhyritic textures, most <strong>of</strong> which are magnesium rich <strong>and</strong> ,50% <strong>of</strong> which are surrounded by coarse-grained igneous rims; (ii) high matrix/chondrule ratios (0.5–1.2); (iii) unique presence <strong>of</strong> abundant salitehedenbergite ^ <strong>and</strong>radite nodules in the matrix (Krot et al., 1998); (iv) high abundance <strong>of</strong> millimeter-to-centimeter-sized CAIs <strong>and</strong> AOAs; <strong>and</strong> (v) common occurrence <strong>of</strong> igneous melilitespinel-pyroxene ^ anorthite (type-B) CAIs (MacPherson et al., 1988). The CV chondrites are a diverse group <strong>of</strong> meteorites that was originally divided into oxidized (CV Ox ) <strong>and</strong> reduced (CV R ) subgroups, based principally on modal metal/magnetite ratios <strong>and</strong> nickel content <strong>of</strong> metal <strong>and</strong> sulfides (McSween, 1977b). Weisberg et al. (1997a) subdivided the oxidized CV chondrites into the Allende-like (CV OxA ) <strong>and</strong> Bali-like (CV OxB ) subgroups (Figure 13). Matrix/chondrule ratios increase in the order CV R (0.5–0.6), CV OxA (0.6–0.7), <strong>and</strong> CV3 OxB (0.7–1.2); metal/ magnetite ratios decrease in the same order (McSween, 1977b; Simon et al., 1995). Oxygen isotopic compositions <strong>of</strong> the CV chondrites show a wide range along a slope ,1 line on the three isotope diagram, with the CV OxB chondrites being slightly depleted in 16 O relative to the CV R <strong>and</strong> <strong>Classification</strong> <strong>of</strong> Meteorites CV OxA chondrites (Weisberg et al., 1997a; Figure 1). There are significant mineralogical differences between the CV subgroups. These differences are largely secondary <strong>and</strong> resulted from late-stage alteration that affected, to various degrees, most <strong>of</strong> the CV chondrites (Krot et al., 1995, 1998). For example, primary minerals in chondrules in CV OxB chondrites are replaced by phyllosilicates, magnetite, nickel-rich sulfides, fayalite (Fa 95 – 100 ), <strong>and</strong> hedenbergite (Hua <strong>and</strong> Buseck, 1995; Krot et al., 1998). Chondrules in CV OxA chondrites are replaced by magnetite, nickel-rich sulfides, fayalitic olivine (Fa 30 – 60 ), nepheline, <strong>and</strong> sodalite. Chondrules in CV R chondrites are virtually unaltered (Kimura <strong>and</strong> Ikeda, 1998; Krot et al., 1995, 1998). Although all <strong>of</strong> the CV chondrites have been classified as type 3, matrices <strong>of</strong> CV OxB chondrites contain abundant phyllosilicates; phyllosilicates are rare in both the CV R <strong>and</strong> CV OxA chondrites (Tomeoka <strong>and</strong> Buseck, 1982, 1990; Keller <strong>and</strong> Buseck, 1990b; Keller <strong>and</strong> McKay, 1993; Keller et al., 1994; Zolensky et al., 1993). Coolidge <strong>and</strong> Loongana 001 comprise a grouplet <strong>of</strong> carbonaceous chondrites with chemical, oxygen isotopic <strong>and</strong> petrographic characteristics similar to those <strong>of</strong> the CV chondrites: high matrix modal abundance (,20–30 vol.%), 1–2 vol.% refractory inclusions, <strong>and</strong> high refractory lithophile abundance ratios (,1.35 £ CI). However, both meteorites are <strong>of</strong> higher petrologic type, 3.8–4, <strong>and</strong> have smaller chondrules <strong>and</strong> lower volatile element abundances than CV chondrites (Kallemeyn <strong>and</strong> Wasson, 1982; Scott <strong>and</strong> Taylor, 1985; Noguchi, 1994; Kallemeyn <strong>and</strong> Rubin, 1995). CK (Karoonda-like) <strong>and</strong> CK/CV-like chondrites. Most <strong>of</strong> the CK chondrites (Figure 13(a)) are <strong>of</strong> high petrologic type (4–6) <strong>and</strong> are characterized by: (i) high abundance <strong>of</strong> matrix; (ii) large porphyritic chondrules (700–1,000 mm), with nonporphyritic chondrules <strong>and</strong> igneous rims around chondrules being virtually absent; (iii) high degree <strong>of</strong> oxidation indicated by high Fa contents <strong>of</strong> olivine (Fa 29 – 33 ), nearly complete absence <strong>of</strong> FeNi-metal (Figure 7), high nickel content in sulfides, <strong>and</strong> abundant magnetite which commonly contains ilmenite <strong>and</strong> spinel exsolution lamellae; (iv) rarity <strong>of</strong> CAIs <strong>and</strong> AOAs; (v) large compositional variations in plagioclase (An 45 – 78 ); (vi) bulk oxygen isotopic compositions plotting within the CO <strong>and</strong> CV fields (Figure 4); (vii) bulk refractory lithophile element abundances (,1.2 £ CI) between those <strong>of</strong> CV <strong>and</strong> CO chondrites (Figure 2); <strong>and</strong> (viii) depletion in moderately volatile elements relative to CV <strong>and</strong> CO chondrites (Figure 2) (Kallemeyn et al., 1991).
Figure 12 Combined elemental maps <strong>of</strong> the CV carbonaceous chondrites: (a) Allende (CV OxA ), (b) Kaba (CV OxB ), <strong>and</strong> (c) Leoville (CV R ). The CV chondrites contain large CAIs, AOAs, <strong>and</strong> chondrules, <strong>and</strong> fine-grained matrix. Most chondrules have porphyritic textures <strong>and</strong> magnesium-rich compositions; plagioclase-rich chondrules (PRCs) are relatively common. The CV matrices contain abundant secondary Ca-, Fe-rich pyroxenes (green spots). The Kaba matrix is hydrated; matrices in Leoville <strong>and</strong> Allende are anhydrous. Image <strong>of</strong> the Allende meteorite is not representative; large CAIs are relatively rare.
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