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

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88 Classification of Meteorites there does not appear to be a resolvable difference between many groups within the chondrite classes. For example, bulk oxygen isotopic compositions of EH and EL enstatite chondrites completely overlap, as do those of the type-3 H, L, and LL ordinary chondrites. The CO, CV, and CK carbonaceous chondrites also have similar bulk oxygen isotopic compositions. 1.05.2.2.3 Bulk nitrogen and carbon abundances and isotopic compositions Nitrogen abundances and isotopic compositions (Figure 6) can differentiate most of the existing chondrite groups (Kung and Clayton, 1978; Kerridge, 1985). Carbon abundance and isotopic compositions (Figure 5) can differentiate only CI and Mighei-like (CM) carbonaceous chondrite groups. Figure 3 Al/Mn versus (Zn/Mn) £ 100 atom ratios in chondritic meteorites (sources Kallemeyn and Wasson, 1981, 1982, 1985; Kallemeyn et al., 1978, 1989, 1991, 1994, 1996; Wasson and Kallemeyn, 1988; Kallemeyn, unpublished). carbonaceous chondrite group (CB) is the only exception). Carbonaceous chondrites are enriched in refractory lithophile elements relative to CI, and depleted in volatile elements. Siderophile and chalcophile elements show volatility-controlled abundance patterns. Elemental abundance patterns for ordinary, Rumuruti-like (R), and Kakangari-like (K) chondrites are fairly flat and enriched relative to CI for lithophile and refractory lithophile elements. Enstatite chondrites have the lowest abundance of refractory lithophile elements. In addition to abundance patterns, other bulk compositional criteria for resolving the chondrite groups involve compositional diagrams of elements of different volatility, e.g., Sb/Ni versus Ir/Ni, Sc/Mg versus Ir/Ni, and Al/Mn versus Zn/Mn (Figure 3). 1.05.2.2.2 Bulk oxygen isotopic compositions Most chondrite classes (ordinary, carbonaceous, and enstatite chondrites) plot in unique positions on a three-isotope oxygen plot (Figure 4). Carbonaceous and K chondrites fall below the terrestrial fractionation line (CI chondrites are the only exception), whereas ordinary, enstatite, and R chondrites each form well-defined clusters on or above the terrestrial fractionation line. However, 1.05.2.2.4 Oxidation state The chondrite groups record a wide range of oxidation states that were probably established through a combination of nebular and asteroidal processes (Rubin et al., 1988a; Krot et al., 2000a). The oxidation state is reflected by the distribution of iron among its three common oxidation states: 0 (FeNi-metal and Fe-sulfides), þ2 (silicates), and þ3 (oxides). The oxidation states of chondrite groups can be summarized in a Urey–Craig diagram, where iron present in metal and sulfide phases is plotted versus iron present in silicate and oxide phases (Figure 7). Oxidation state generally increases in the order enstatite–ordinary–carbonaceousþR chondrite classes, with K chondrites being intermediate between ordinary and enstatite chondrites. Within the ordinary chondrites, the oxidation state increases from H to L to LL, as is reflected in an increase in the Fe/(Fe þ Mg) ratios of their olivine and pyroxene. For carbonaceous chondrites, oxidation state increases in the order CB–CH–CR–CO–CV–CK–CM–CI. We note, however, that this classification parameter does not generally reflect the oxidation state of individual chondritic components, e.g., CAIs in all chondrite groups formed under highly reducing conditions (see Chapter 1.08), and magnesian (type-I) and ferrous (type-II) chondrules (both of which occur in the same meteorites) require formation under different redox conditions (see Chapter 1.07). In addition, the role of nebular and asteroidal processes in establishing of the oxidation states of chondrites remains controversial (e.g., Krot et al., 2000a).
Classification of Chondritic Meteorites 89 Figure 4 Bulk oxygen isotope compositions of chondrite groups and ungrouped chondrites (source Clayton and Mayeda, 1999). 1.05.2.3 Secondary Classification Parameters 1.05.2.3.1 Petrologic type Van Schmus and Wood (1967) introduced a classification scheme that provides a guide to the degree of thermal and aqueous alteration experienced by a chondrite. According to this widely used scheme, chondrites are divided into petrologic (or petrographic) types 1–6. The sequence type 3 (commonly called unequilibrated) to type 6 (commonly called equilibrated) represents an increasing degree of chemical equilibrium and textural recrystallization, presumably due to thermal metamorphism. Type 1 represents higher degree of aqueous alteration compared to type 2, based largely on the abundance of hydrous silicates. Type 3 chondrites are widely considered the least modified by secondary processes (see also Chapter 1.07). We note, however, that this scheme cannot be applied to several CI and CM chondrites (e.g., Belgica- 7904) that appear to have experienced aqueous alteration followed by thermal metamorphism and dehydration. Several ordinary and enstatite chondrites that experienced extensive recrystallization and possibly melting have been designated
Page 1 and 2: 1.05 Classification of Meteorites A
Page 3 and 4: Introduction 85 Table 1 Meteorite g
Page 5: Classification of Chondritic Meteor
Page 9 and 10: Classification of Chondritic Meteor
Page 13 and 14: (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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