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

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Table 3 Summary of criteria for petrologic types. Criterion Petrologic type 1 2 3 4 5 6 Homogeneity of —————— .5% mean deviations —————— ,5% ——————— Homogeneous ——————— olivine and low-Ca pyroxene compositions Structural state of low-Ca pyroxene —————— Predominantly monoclinic —————— .20% monoclinic ,20% monoclinic Orthorhombic Feldspar —————— Minor primary grains only —————— Secondary, ,2 mm grains Secondary, 2–50 mm grains Secondary, .50 mm grains Chondrule glass Altered, mostly absent a Clear, isotropic, variable abundance ———————————— Devitrified, absent ————————————— Maximum Ni in metal ,20 wt.%; taenite ———————————— .20 wt.% Kamacite and taenite in exsolution relationship ————————————— minor or absent Mean Ni in sulfides .0.5 wt.% —————————————————————— ,0.5 wt.% —————————————————————— Matrix All fine-grained, opaque Mostly fine, opaque Clastic, minor opaque —————————— Transparent, recrystallized ——————————— coarsening from 4 to 6 Chondrule–matrix intergration No chondrules ———— Chondrules very sharply defined ———— Chondrules well defined Chondrules readily delineated Chondrules poorly defined Carbon (wt.%) 3–5 0.8–2.6 ——————————————————————— ,1.5 ——————————————————————— Water (wt.%) 18–22 2–16 0.3–3 ——————————————— ,1.5 ——————————————— After Van Schmus and Wood (1967), with modifications from Sears and Dodd (1988) and Brearley and Jones (1998). a Chondrule glass is rare in CM2 chondrites, but is preserved in many CR2 chondrites.
Classification of Chondritic Meteorites 93 Table 4 Classification scheme for shock metamorphism in chondrites. Shock stage Description Effect resulting from equilibration peak shock pressure Shock pressure Olivine Plagioclase Orthopyroxene (GPa) a S1 Unshocked Sharp optical extinction, ,4–5 irregular fractures S2 Very Undulutory extinction, Undulutory extinction, Undulutory extinction, 5–10 weakly shocked irregular fractures irregular fractures irregular and some planar fractures S3 Weakly shocked Planar fractures, undulutory extinction, irregular fractures Undulutory extinction Clinoenstatite lamellae on (100), undulutory extinction, planar and irregular fractures 15–20 S4 Moderately Weak mosaicism, Undulutory extinction, 30–35 shocked planar fractures partially isotropic, planar deformation features S5 Strongly Strong mosaicism, Maskelynite 45–55 shocked planar fractures, planar deformation fractures S6 Very Solid-state Shock melted Majorite, melting 75–90 strongly recrystallization (normal glass) shocked and staining, ringwoodite, melting Shock Whole-rock melting (impact melt rocks and melt breccias) melted Sources: Stöffler et al. (1991) and Rubin et al. (1997). Shock effects in ordinary chondrites are characterized by effects in olivine and plagioclase; shock levels in carbonaceous chondrites are characterized by effects mostly in olivine (Scott et al., 1992); shock levels in enstatite chondrites are characterized by effects mostly in orthopyroxene. The prime shock criteria for each shock stage are in italics. a Shock pressures for ordinary chondrites only. Table 5 Classification of chondritic breccias. Breccia type Lithic fragments Regolith breccias Fragmental breccias Impact melt breccias Granulitic breccias Primitive breccias Description Xenolithic—clasts of different chemical class to host Cognate—impact melt fragments Fragmental debris on surface of body (contain solar-wind gases, solar flare tracks, agglutinates, etc.), usually have light–dark structure Fragmental debris with no regolith properties Unmelted debris in igneous matrices Metamorphosed fragmental breccias Type-3 ordinary chondrite breccias After Keil (1982). Tagish Lake (Figure 8(b)) is an ungrouped carbonaceous chondrite with CI and CM affinities that has higher abundances of carbon (5.8 wt.%) and presolar nanodiamonds than any other chondrite (Brown et al., 2000; Clayton and Mayeda, 2001; Grady et al., 2002; Mittlefehldt, 2002; Zolensky et al., 2002). The ungrouped meteorites Belgica-7904, Yamato-86720, Yamato-82162, and Dhofar-225 have petrographic, mineralogical, and chemical affinities to the CM and CI carbonaceous chondrites (e.g., Tomeoka et al., 1989; Tomeoka, 1990; Paul and Lipschutz, 1990; Bischoff and Metzler, 1991; Ikeda, 1992; Hiroi et al., 1993a, 1996; Clayton and Mayeda, 1999; Ivanova et al., 2002), but have distinct oxygen isotopic compositions (Figure 4). In contrast to CI and CM chondrites, these meteorites experienced aqueous alteration followed by reheating and dehydration (e.g., Akai, 1988; Yanai et al., 1995). The reheating resulted in partial loss of carbon, nitrogen, and water, structural transformation of serpentine to olivine, and precipitation of abundant fine-grained troilite (possibly by decomposition of tochilinite).
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 9: 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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