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

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108 pyroxene, calcium-rich olivine, and anorthitic plagioclase (Table 6 and Figure 18); spinel, troilite, kirschsteinite, whitlockite, titanomagnetite, and FeNi-metal are accessory (e.g., Prinz et al., 1977; McKay et al., 1988, 1990; Yanai, 1994; Mittlefehldt et al., 1998). Although oxygen isotopic compositions of angrites are indistinguishable from those of the HED meteorites, brachinites, and mesosiderites (Figure 17), the unusual mineralogies and compositions of the angrites suggest that they are not related to any of these other meteorite groups. Angrites are the most highly alkali-depleted basalts in the solar system and have low abundances of the moderately volatile element gallium. However, they are not notably depleted in the highly volatile elements—bromium, selenium, zinc, indium, and cadmium—compared to lunar basalts and basaltic eucrites. All angrites have superchondritic Ca/Al ratios, which contrasts with the chondritic Ca/Al ratios of eucritic basalts (Mittlefehldt et al., 1998). 1.05.4.2.2 Aubrites The aubrites (or enstatite achondrites) are highly reduced enstatite pyroxenites. Except Shallowater, which has an igneous texture (Keil et al., 1989), all are fragmental (Figure 19) or, less commonly, regolith breccias, as indicated by their high contents of solar-wind-implanted noble gases (e.g., Keil, 1989). They consist mostly (,75–95 vol.%) of nearly FeO-free enstatite, but also contain lesser amounts of albitic plagioclase and nearly FeO-free diopside and forsterite (e.g., Watters and Prinz, 1979). Clasts of both igneous and impact-melt origin are common, and the precursors to the breccias were mostly coarse-grained (probably plutonic) orthopyroxenites (Okada et al., 1988). Other phases include accessory silicon-bearing FeNi-metal Classification of Meteorites (e.g., Casanova et al., 1993) and a host of unusual sulfides in which elements that are normally lithophile are chalcophile, such as titanium in heideite (FeTi 2 S 4 )(Keil and Brett, 1974) and calcium in oldhamite (CaS) (Wheelock et al., 1994). Similarities in mineralogy and oxygen isotopic compositions (Figures 4 and 17) suggest that aubrites are related to enstatite chondrites, although they are not thought to have formed on the same parent bodies. Arguments have been presented that the brecciated aubrites, the igneous Shallowater aubrite, and the H- and L-group enstatite chondrites represent samples from four different asteroidal parent bodies (Keil, 1989). 1.05.4.2.3 Brachinites The brachinites are medium- to coarse-grained (0.1–2.7 mm) equigranular dunitic wehrlites, consisting dominantly of olivine (74–98%); they also contain augite (4–15%), plagioclase (0–10%), orthopyroxene (traces), chromite (0.5–2%), Fe-sulfides (3–7%), minor phosphates, and FeNi-metal (Table 6 and Figure 20). Lithophile element abundances in Brachina are close to chondritic and are unfractionated, whereas other brachinites show depletions in aluminum, calcium, rubidium, potassium, and sodium (Mittlefehldt et al., 1998). Siderophile element abundances in brachinites are nearchondritic (,(0.1–1) £ CI). Trapped noble gases show planetary-type patterns. Oxygen isotopic compositions are indistinguishable from those of HED meteorites (Figure 17), but brachinites are clearly not from the HED parent body. The question of whether brachinites are primitive, partial melt residues, or igneous cumulates (Warren and Kallemeyn, 1989; Nehru et al., 1996) remains open. Petrologic and geochemical differences between the different brachinites suggest that they may not have all formed by the same process (Mittlefehldt et al., 1998; Mittlefehldt and Berkley, 2002). Figure 19 Hand-specimen photograph of the Bustee aubrite showing the brecciated texture of the rock (9 cm in horizontal dimension) (photograph courtesy of the Smithsonian Institution). 1.05.4.2.4 Ureilites The ureilites (Figure 21) are carbon-bearing ultramafic rocks composed of olivine, pyroxene, and ,10% dark interstitial material consisting of varying amounts of carbon, metal, sulfides, and minor fine-grained silicates (Mittlefehldt et al., 1998). The characteristic features of ureilites that distinguish them from other achondrite groups include: (i) high CaO contents in olivine and pigeonite; (ii) high Cr 2 O 3 contents in olivine; (iii) high (up to ,5 wt.%) carbon contents; (iv) presence of reduced rims on olivine (and sometimes pyroxene) where in contact with graphite; and (v) large range in oxygen isotopic compositions (Figure 17) which plot along
Figure 20 Photomicrographs of brachinites—(a) Brachina and (b) ALH84025—in transmitted light with partially crossed nicols. Brachina has an equigranular texture of olivine, augite, and plagioclase, with minor chromite, FeNi-metal, and sulfides. ALH84025 has an equigranular texture of olivine and augite with fine twin lamellae and interstitial troilite (black). Note coarse grain size (compared to Brachina) and absence of plagioclase (photograph courtesy of M. Wadhwa). the line with slope ,1 nearly identical to the carbonaceous chondrite anhydrous mineral line (Goodrich, 1992; Clayton and Mayeda, 1996). Based on the mineralogy and petrology, ureilites are divided into three groups (lithological types): (i) olivine–pigeonite ureilites, (ii) olivine–orthopyroxene–(augite) ureilites, and (iii) polymict ureilites. The olivine–pigeonite ureilites (Figure 21(a)), which constitute .90% of all ureilites, are coarse grained (average grain size ,1 mm), highly equilibrated assemblages of ,80 vol.% of olivine ranging in composition from Fo 76 to Fo 95 (olivine compositions within each ureilite are homogeneous), and uninverted pigeonite. Many have pronounced fabric, defined by both a lineation and a foliation (Figure 21(a)). Graphite occurs mainly along grains boundaries (Figure 21(c)). Most of the olivine–orthopyroxene–(augite) ureilites have poikilitic textures, consisting of large (up to ,5 mm) oikocrysts of low-calcium pyroxene (either orthopyroxene or inverted Classification of Nonchondritic Meteorites 109 pigeonite) enclosing rounded chadocrysts of olivine and augite (Figure 21(b)); all these minerals may contain magmatic inclusions (Figure 21(d)), which are absent in olivine– pigeonite ureilites (Goodrich et al., 2001). META78008, one of the olivine–orthopyroxene–(augite) ureilites, contains some pigeonite (Berkley and Goodrich, 2001). The polymict ureilites are fragmental breccias and regolith breccias containing solar-windimplanted gases. They consist almost entirely of mineral and lithic clasts of olivine–pigeonite and olivine–orthopyroxene–(augite) ureilite materials. In addition, there are plagioclase clasts, feldspathic melts clasts, Ca–Al–Ti-rich clasts that resemble angrites, heavily hydrated clasts of CI-like materials, and chondritic fragments (Prinz et al., 1986, 1987, 1988; Ikeda and Prinz, 2000; Ikeda et al., 2000; Kita et al., 2000). The olivine–pigeonite ureilites are now accepted to be residues from at least 15% melting, the minimum required to eliminate plagioclase from a generally chondritic parent body (Warren and Kallemeyn, 1992; Scott et al., 1993; Goodrich et al., 2002), whereas the olivine–augite–orthopyroxene ureilites appear to be cumulates (Goodrich et al., 2001). The small percentage of feldspathic material in polymict ureilites may sample the missing basaltic crust of the ureilite parent body. 1.05.4.2.5 HED meteorites: howardites, eucrites, diogenites The HED meteorites have traditionally been classified into one group, because there is strong evidence that they originated on the same asteroidal parent body (Drake, 2001). This evidence includes their identical oxygen isotopic compositions (Figure 17), similarities in Fe/Mn ratios of pyroxenes (e.g., Papike, 1998), the occurrence of polymict breccias consisting of materials of eucritic and diogenitic parentage (e.g., the howardites; Fredriksson and Keil (1963)), and the existence of rocks intermediate between diogenites and cumulate eucrites (Takeda and Mori, 1985). Similarities in the mineralogical compositions of the HEDs and the surface mineralogy of asteroid 4 Vesta, as determined by ground-based visible and near-infrared spectroscopy, suggest that the HEDs are impact ejecta off 4 Vesta (McCord et al., 1970). This proposal has been strengthened by the discovery of the so-called “Vestoids” of the Vesta family, which are small (,10 km diameter) asteroids that are dynamically linked to Vesta (e.g., Binzel and Xu, 1993). The HEDs are discussed in order of increasing depth in their parent body, as suggested by Takeda
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: compositions to the winonaites and
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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