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

More documents

Recommendations

Info

118 Figure 28 Bulk oxygen isotopic compositions of SNC meteorites, lunar meteorites, and HED meteorites (source Clayton and Mayeda, 1996). Classification of Meteorites of finer-grained FeO-rich olivine, pigeonite, augite, maskelynite, and other late phases (McSween et al., 1979a,b; Treiman et al., 1994; Ikeda, 1997). McSween et al. (1979a,b) noted that the dominant mineralogy of these rocks is consistent with early crystallization from magmas having the crystallization sequence inferred for basaltic shergottites such as Shergotty. This led to their classification as shergottites. Some authors, however, feel that they should be referred to simply as martian lherzolites (e.g., Treiman et al., 1994). EET79001 is a unique shergottite consisting of two lithologies (designated A and B) separated by an obvious contact (Figure 30(a)). Lithology B is a clinopyroxene–plagioclase rock resembling the basaltic shergottites. Lithology A, however, is distinct from either the basaltic or lherzolitic shergottites. It has a porphyritic texture consisting of megacrysts of olivine, orthopyroxene, and chromite, in a finergrained pigeonite–plagioclase groundmass. Fournew shergottites (e.g., Dar al Gani 476 and Sayh al Uhaymir 005 (Figure30(b))) consist entirely of olivine–porphyritic lithologies resembling EET79001 lithology A. These meteorites have been referred to by various terms, including “basaltic shergottite” (on the basis of plagioclase content), “transitional shergottite,” or “mixed shergottite.” We recommend the term “olivinephyric shergottite,” which emphasizes their differences from basaltic shergottites such as Shergotty or QUE94201, and does not have genetic implications. Figure 29 Weight percentages of FeO and MnO in solar-system basalts and martian meteorites. Filled circles are SNC basalts, open circles are other martian meteorites, and filled squares are Mars’ surface materials. Martian meteorites have essentially the same FeO/MnO ratio as do Mars surface materials and eucrite basalts. Only Chassigny has a comparable FeO/MnO ratio to terrestrial basalts (source Treiman et al., 2000). the type shergottite, Shergotty) contain cumulus pyroxene, and have strongly foliated textures suggesting crystal accumulation in near-surface dikes or lava flows (Stolper and McSween, 1979; McCoy et al., 1992b). However, some (e.g., QUE94201 and Los Angeles) have higher plagioclase contents and may represent magma compositions (McSween et al., 1996; McKay et al., 2002; Rubin et al., 2000). The lherzolitic shergottites are magnesian (Mg# , 70) olivine–clinopyroxene–chromite cumulates, dominated by coarse-grained poikilitic pigeonite enclosing rounded olivine (and chromite) crystals, with interstitial areas 1.05.5.2 Nakhlites (Clinopyroxenites/Wehrlites) Nakhlites are clinopyroxenites or wehrlites, dominated by augite with lesser olivine. Their coarse-grained textures (Figure 30(c)) and the presence of exsolution lamellae in augite (which require slow cooling) suggest that they are cumulates. They also contain interstitial material consisting of a microcrystalline groundmass of radiating crystalline plagioclase, pigeonite, ferroaugite, titanomagnetite, pyrite, troilite, chlorapatite, and rare silica-rich glass (e.g., Berkley et al., 1980; Treiman, 1986, 1990, 1993). 1.05.5.3 Chassignite (Dunite) Chassigny is the only known martian dunite. It is an olivine–chromite cumulate consisting of 90% olivine (Fa ,32 ), 5% pyroxene, 2% feldspar (An ,20 ) that has been transformed into maskelynite, and 3% accessory minerals including chromite (Figure 30(d)). Melt inclusions in olivine contain hydrous amphibole,
Martian (SNC) Meteorites 119 Figure 30 Combined elemental maps (a–c) in Al (red), Fe (green) and Ca Ka (blue) X-rays and thin section photomicrographs in transmitted light with crossed polars (d–f) of martian meteorites: (a) Elephant Moraine (EET) 79001, (b) Sayh al Uhaymir (SaU) 005, (c) Yamato (Y) 000593, (d) Chassigny, and (e, f) ALH84001. Image of EET79001 shergottite shows a contact between lithology A (right) and lithology B (left). Lithology A is an olivinephyric basalt containing megacrysts of olivine in a finer grained groundmass consisting principally of pigeonite (green-blues) and maskelynite (magenta). Lithology B is a pyroxene–plagioclase (maskelynite) basalt. It lacks olivine, is coarser-grained than the groundmass of lithology A, and has a higher augite content (some of the blues). SaU 005 is an olivine-phyric basalt mineralogically similar to lithology A of EET79001. Olivine megacrysts is a finer-grained groundmass consisting principally of pigeonite (green-blues) and maskelynite (magenta). Y000593 is a clinopyroxenite containing augite (blue), olivine (green), and interstitial areas (red) consisting principally of radiating sprays of plagioclase and alkali feldspars. Chassigny is an olivine-chromite cumulate. Olivine grains contain hydrous amphibole-bearing melt inclusions. ALH84001 is a coarse-grained orthopyroxenite containing abundant carbonates.
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 and 32: 1.05.4.4 Pallasites Pallasites are
Page 33 and 34: ,85% of iron meteorites fall into o
Page 35: Martian (SNC) Meteorites 117 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

Create successful ePaper yourself

Delete template?

Save as template?