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

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86 events (see Chapter 1.07–1.09). Although most chondrites experienced thermal processing on their parent asteroids, such as aqueous alteration, thermal, and shock metamorphism, they did not experience melting and igneous differentiation, and thus largely preserve records of physical and chemical processes in the solar nebula. Deciphering these records is a primary goal of chondrite studies. Nonchondritic meteorites lack chondritic textures and are formed by partial or complete melting and planetary differentiation of chondritic precursor asteroids or larger planetary bodies (Mars, Moon), and hence provide unique opportunities to study these processes on extraterrestrial bodies (see Chapter 1.11, 1.12, 1.21, and 1.22). Several groups of nonchondritic meteorites experienced only low degrees of melting, and have largely retained their chondritic bulk compositions. To emphasize the relatively unprocessed nature of these nonchondrites and their intermediate status relative to chondrites and highly differentiated meteorites, they are referred to as primitive achondrites and discussed as a separate category of nonchondritic meteorites. 1.05.2 CLASSIFICATION OF CHONDRITIC METEORITES 1.05.2.1 Taxonomy Based on bulk chemistry (Figures 2 and 3), bulk oxygen isotopic compositions (Figure 4), mineralogy, petrology (Figures 8–13 and 15), and proportions of various chondritic components (Table 2), 14 chondrite groups have been recognized. Several other chondrites are mineralogically and/or chemically unique and defy classification into the existing chondrite groups; these are commonly called ungrouped chondrites (Table 1 and Figure 1). Conventionally, a chondrite “group” is defined as having a minimum of five unpaired chondrites of similar mineralogy, petrography, bulk isotopic properties, and bulk chemical compositions in major, nonvolatile elements. According to this definition, K chondrites could be considered as a grouplet: there are only two known K chondrites (see K (Kakangari-like) chondrites). Thirteen out of the 14 chondrite groups comprise three major classes: carbonaceous (C), ordinary (O), and enstatite (E), each of which contains distinct groups. The term carbonaceous is somewhat of a misnomer, because only the CI, CM, and CR chondrites are significantly enriched in carbon relative to noncarbonaceous chondrites (Figure 5). The CI chondrites lack chondrules. Carbonaceous and noncarbonaceous chondrites can be resolved Classification of Meteorites (with some exceptions) by several characteristics: (i) mean refractory lithophile/Si abundance ratios relative to CI chondrites ($1.00 in carbonaceous chondrites, #0.95 in noncarbonaceous chondrites); (ii) oxygen isotopic composition (D 17 O #22‰ for carbonaceous chondrites, except CI; D 17 O$21‰ for noncarbonaceous chondrites); (iii) refractory inclusion abundances ($0.1 vol.% in carbonaceous chondrites, except CI; #0.1 vol.% in noncarbonaceous chondrites); and (iv) matrix/chondrule modal abundance ratio ($0.9 in carbonaceous chondrites, except CH and CB; #0.9 in noncarbonaceous chondrites, except K chondrites) (Kallemeyn et al., 1996). There are eight well-resolved groups of carbonaceous chondrites: CI, CM, CR, CH, CB, CV, CK, and CO. The letters designating the groups refer to a typical chondrite (commonly fall) in the group: CI (Ivuna-like), CM (Migheilike), CO (Ornans-like), CR (Renazzo-like), CH (ALH85085-like), CB (Bencubbin-like), CV (Vigarano-like), and CK (Karoonda-like). The exception to this rule is the CH group, in which the “H” refers to high metal abundance and high iron concentration (Bischoff et al., 1993b). Ordinary chondrites are divided into three groups: H, L, and LL. The letters designating the groups refer to the bulk iron contents: H chondrites have high total iron contents, L chondrites have low total iron contents, and LL chondrites have low metallic iron relative to total iron, as well as low total iron contents. Enstatite chondrites comprise two groups with different contents of metallic iron: EH and EL. Additionally, there is an ungrouped E chondrite, LEW87223 (Grossman et al., 1993). R (Rumuruti-like) and K (Kakangari-like) chondrite groups are different from other chondrites and have been suggested to represent additional chondrite classes. The term clan is used as a higher order of classification than group. It was originally defined to encompass chondrites that have chemical, mineralogical, and isotopic similarities and were believed to have formed at about the same time in the same small region of the solar nebula, in a narrow range of heliocentric distances (Kallemeyn and Wasson, 1981; Kallemeyn et al., 1996). Weisberg et al. (1995a) used the term for chondrites that have chemical, mineralogical, and isotopic similarities suggesting a petrogenetic kinship, but have petrologic and/or bulk chemical characteristics that challenge a group relationship. Four carbonaceous chondrite clans have been recognized: (i) the CR clan, which includes the CR, CH, CB, and a unique chondrite LEW85332; (ii) the CM–CO clan; (iii) the CV–CK clan; and (iv) the CI clan (Kallemeyn et al., 1996). The noncarbonaceous chondrites constitute three clans: OþR, E, and K.
Classification of Chondritic Meteorites 87 Figure 2 Magnesium- and CI-normalized bulk lithophile (a, c) and siderophile (b, d) element abundances of the carbonaceous (a, b) and noncarbonaceous (c, d) chondrite groups (sources Kallemeyn and Wasson, 1981, 1982, 1985; Kallemeyn et al., 1978, 1989, 1991, 1994, 1996; Kallemeyn, unpublished). 1.05.2.2 Primary Classification Parameters 1.05.2.2.1 Bulk chemical compositions The bulk compositions of chondrites closely match the compositions of the solar photosphere, with the exception of a few highly volatile elements (hydrogen, carbon, nitrogen, helium) and lithium (see Chapter 1.03). The chondrite groups also show different levels of depletions in moderately volatile elements (e.g., manganese, sodium, potassium, gallium, antimony, selenium, and zinc). The elemental abundances of bulk Ivuna-like carbonaceous chondrites (CI) are viewed as a measure of average solar-system abundances and are used as a reference composition for many solar-system materials. Bulk compositions of other chondrite groups are similar to, but distinct from, CI chondrites, most nonvolatile elements varying within a factor of 3 relative to CI abundances (Figure 2). Each chondrite group has a unique and well-defined elemental abundance pattern (the Bencubbin-like
Page 1 and 2: 1.05 Classification of Meteorites A
Page 3: Introduction 85 Table 1 Meteorite g
Page 7 and 8: 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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