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114 Classification of Meteorites Figure 25 Polished slabs of the main-group pallasites: (a) Brenham and (b) Ahumada. Brenham (30 cm in horizontal dimension) contains well-rounded olivines dispersed in metal. Ahumada (15 cm in horizontal dimension) contains angular and subangular olivine grains in metal. Photograph courtesy of the Smithsonian Institution. close in composition to IIF irons and has higher nickel and iridium contents than those of the main group pallasites (Scott, 1977). Kracher et al.(1980), therefore, suggested that Eagle Station grouplet pallasites and IIF irons formed in a close proximity in the solar nebula region but not on the same parent asteroid. Eagle Station pallasites have oxygen isotopic compositions quite similar to those of CV chondrites (Figures 4 and 17), suggesting another linkage (Clayton and Mayeda, 1996). 1.05.4.4.3 Pyroxene–pallasite grouplet The pyroxene–pallasite grouplet consists of Vermillion and Yamato-8451 (Boesenberg et al., 2000). These meteorites contain ,14–63 vol.% olivine, 30–43 vol.% metal, 0.7–3 vol.% pyroxene, 0–1 vol.% troilite, and minor whitlockite. The occurrence of millimeter-sized pyroxenes distinguishes these pallasites from main-group and Eage Station grouplet pallasites (Hiroi et al., 1993b; Boesenberg et al., 2000; Yanai and Kojima, 1995). The pyroxene pallasites have metal of different composition from other pallasites, possibly indicating formation on a different parent asteroid (e.g., Mittlefehldt et al., 1998). Metal of Y-8451 has same nickel and iridium content as the Eagle Station grouplet, but the gold content is like that of the main group pallasites (Wasson et al., 1998). Metal in Vermillion has nickel, iridium, and gold contents similar to main group pallasites, but has different contents of moderately volatile siderophile elements (e.g., gallium and germanium) than any other group of pallasites (e.g., Mittlefehldt et al., 1998). Oxygen isotopes of pyroxene pallasites are also distinct from those of main-group pallasites (Figure 17). 1.05.4.5 Irons Iron meteorites are commonly classified on the basis of chemical and structural properties. 1.05.4.5.1 Chemical groups On plots of log (E) versus log (Ni), where E denotes some well-determined trace element,
,85% of iron meteorites fall into one of 13 chemical groups: IAB, IC, IIAB, IIC, IID, IIE, IIF, IIIAB, IIICD, IIIE, IIIF, IVA, and IVB. With two exceptions, these groups fall within restricted areas on a log (Ge) versus log (Ni) plot (Figure 26(a)), whereas on log (Ir) versus log (Ni) plot the same samples form narrow, elongated fields spanning a considerable range in iridium contents (Figure 26(b)). It is customary to refer to these clusters as groups as if they contain at least five members. Individual samples that fall outside defined chemical groups are called ungrouped or anomalous iron meteorites, and some of these form grouplets of two to four individuals. The taxonomic significance of gallium and germanium derives from the fact that they are the most volatile siderophile elements (Wasson and Wai, 1976; Wai and Wasson, 1979) and tend to be strongly fractionated between different iron groups. Early determinations of these elements (Lovering et al., 1957) resolved only four groups, designated by Roman numerals I–IV in order of decreasing content of gallium and Classification of Nonchondritic Meteorites 115 germanium. Letters were added later to distinguish additional groups. Not all of these groups proved to be independent, leading to the current (confusing) nomenclature, in which some groups have a combination of two letters after their Roman numeral. Chemical evidence indicates that magmatic iron meteorite groups formed by fractional crystallization, most likely in the cores of differentiated asteroids. Of the 13 major iron meteorite groups, only the IAB and IIICD groups have broad ranges in nickel and some trace elements. These ranges are difficult to explain by simple fractional crystallization or a metallic core. In addition, many members of these groups contain silicate inclusions, which are roughly chondritic in mineralogy and chemical compositions, but the textures are achondritic, exhibiting recrystallized textures. Silicate inclusions in IAB irons are linked through oxygen isotopic compositions and mineral compositions to winonaites and may be related to IIICD irons (Figure 17). About 15% of all known iron meteorites do not belong to one of the above defined 13 chemical groups (Wasson and Kallemeyn, 1990). If each ungrouped individual or grouplet represents a separate parent body, then iron meteorites sample between 30 and 50 asteroids (Scott, 1979; Wasson, 1990). Mineralogy of several ungrouped iron meteorites with silicate inclusions and known oxygen isotopic compositions are listed in Table 8, which also contains information about several ungrouped nonchondritic meteorites. Figure 26 Plots of: (a) Ni versus Ge and (b) Ni versus Ir showing the fields for the 13 iron meteorite groups (source Scott and Wasson, 1975). 1.05.4.5.2 Structural groups The metallic phase of many iron meteorites shows a texture called the Widmanstätten pattern, an oriented intergrowth of body-centered cubic a-FeNi (kamacite), and high-nickel regions composed of several phases (a tetragonal FeNi phase (tetrataenite), a body-centered cubic FeNi phase (a 2 -FeNi), and awaruite (FeNi 3 )). Kamacite forms lamellae with a nickel content of ,6%. The width of the kamacite lamellae is a function of the cooling rate and, hence, is approximately constant in a given iron meteorite, but varies widely between different meteorites. Because kamacite lamellae are oriented along octahedralplanes, irons which show this structure are called octahedrites (Figures 27(a)–(e)). According to the width of the kamacite lamellae, octahedrites are further subdivided into coarsest (Ogg, bandwidths .3.3 mm), coarse (Og, 1.3–3.3 mm), medium (Om, 0.5–1.3 mm), fine (Of, 0.2–0.5 mm), and finest (Off, ,0.2 mm). Plessitic octahedrites (Opl) are transitional to ataxites (D).
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 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: 1.05.4.4 Pallasites Pallasites are
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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