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

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110 Classification of Meteorites Figure 21 Photomicrographs in transmitted light with crossed polars (a, b, d) and reflected light (c) of ureilites Kenna (a), LEW88774 (b), ALH83014 (c), and Hughes 009 (d). (a) Kenna contains coarse-grained olivine and pigeonite in equilibrated (note abundant triple junctions), foliated texture. Abundant black material along olivine grain boundaries and in crosscutting veins consists of fine-grained graphite and reduction rims/veins characteristic of ureilites.(b) LEW88774 has large oikocryst of low-Ca pyroxene (now orthopyroxene with augite exsolution lamellae, oriented diagonally upper right to lower left) enclosing irregularly shaped, embayed and corroded grains of augite (blue) which are in optical continuity with one another (as can be seen in the similar orientation of shock-produced twins) and appear to be remnants of once single crystals of primary augite which reacted with the liquid from which the oikocryst crystallized. (c) In ALH83014, ureilite with very low shock level, graphite grains occur along grain boundaries and within olivine grains as large, euhedral crystals rather than the fine-grained masses seen in most ureilites. (d) The olivine–augite–orthopyroxene ureilite Hughes 009 shows highly equilibrated texture typical of olivine–pigeonite ureilites. Olivine contains magmatic inclusions (center of crystal, bottom). (1979, 1997) in his layered crust model of the HED parent body. Howardites. These are lithified regolith from the surface of the HED parent body, as indicated by high contents of solar-wind-implanted noble gases in their fine-grained, clastic, impactproduced matrices (e.g., Suess et al., 1964) (Figure 22(a)). They are polymict breccias and consist mostly of eucritic and diogenitic material with .10 vol.% of orthopyroxene as well as some olivine. In addition, they contain abundant impact-melt clasts and breccia fragments, and occasionally carbonaceous chondrite xenoliths (e.g., Metzler et al., 1995; Pun et al., 1998). Eucrites. These are pyroxene–plagioclase basalts and are subdivided into three major subclasses: the noncumulate eucrites (basaltic eucrites), the cumulate eucrites, and the polymict eucrites. Polymict eucrites (Figure 22(b)) are polymict breccias consisting mostly of eucritic material, but they also contain ,10 vol.% of a diogenitic component in the form of orthopyroxene. Noncumulate eucrites originally formed as quickly cooled surface lava flows (unequilibrated noncumulate eucrites), but most were subsequently metamorphosed (metamorphosed noncumulate eucrites). Unequilibrated noncumulate eucrites, also referred to as the unmetamorphosed or least-metamorphosed noncumulate eucrites or Pasamonte-type eucrites, are surface lava flows (Figure 22(c)) that cooled quickly (Walker et al., 1978). As a result of their fast cooling, their pyroxenes (pigeonite of Mg# ,70–20) are zoned, and exsolution lamellae are only visible by TEM. These rocks have experienced only minor metamorphism (e.g., Takeda and Graham, 1991). Metamorphosed (equilibrated) noncumulate eucrites are also collectively referred to as the ordinary eucrites. They include the Juvinas type (main group) and the Stannern and Nuevo Laredo types. They are unbrecciated or monomictbrecciated, metamorphosed basalts (Figure 22(d)) and contain homogeneous low-calcium pigeonite (Mg# ,42–30) with fine exsolution lamellae of high-calcium pyroxene. The high abundance
Classification of Nonchondritic Meteorites 111 Figure 22 Thin section photomicrographs in transmitted light with crossed polars of typical HED meteorite types. (a) Howardite Kapoeta consisting of mineral fragments of highly variable grain sizes. (b) Polymict eucrite Y74450 consisting of basaltic rock (upper right) and mineral fragments. (c) Unequilibrated noncumulate eucrite Pasamonte showing the typical basaltic texture of plagioclase (light) and pyroxene (colored). (d) Metamorphosed (equilibrated) noncumulate eucrite Ibitira showing a recrystallized texture, with plagioclase (white) and pyroxene (colored). The round, dark areas in the center, bottom, and the left of image are vesicles. (e) Cumulate eucrite Serra de Magé. The rock consists of large crystals of plagioclase (lighter material, with straight twin lamellae) and mostly orthopyroxene with complex augite exsolution lamellae (darker material, with irregular, sometimes worm-like exsolution lamellae). (f) Diogenite Johnstown illustrating the highly brecciated nature of the rock that consists essentially entirely of orthopyroxene (after Keil, 2002). of metamorphosed noncumulate eucrites suggests that the HED parent body experienced global metamorphism and that most of the original unequilibrated noncumulate eucrites have been extensively metamorphosed and transformed into the metamorphosed noncumulate eucrites (e.g., Yamaguchi et al., 1996, 1997). That many noncumulate eucrites had been extensively thermally metamorphosed was recognized early, and prompted Takeda and Graham (1991) to establish a classification of increasing metamorphism from type 1 to type 6. It should be noted that this scale is mainly based on the compositions and textures of the pyroxenes of these rocks and, thus, is different from the well-known metamorphic (petrologic type) scale of ordinary chondrites established by Van Schmus and Wood (1967). Cumulate eucrites include the Binda type and Moore County type. They are coarse-grained gabbros, and many are unbrecciated (Figure 22(e)). They contain orthopyroxene inverted from low-calcium clinopyroxene (Mg# ,67–58) and orthopyroxene inverted from pigeonite (Mg# ,57–45). Northwest Africa 011 (NWA 011) is a basaltic eucrite (Yamaguchi et al., 2002) composed of coarse-grained (,1 mm in size) anhedral clinopyroxene (pigeonite and relict augite) and a fine-grained to stubby (50–100 mm in size) plagioclase (Or 0.3 – 0.5 Ab 11.0 – 22.1 ). The oxygen isotopic composition of NWA 001 is similar to CR chondrites (Yamaguchi et al. (2000); Figure 17). Diogenites. These are coarse-grained, cumulate orthopyroxenites that are usually highly
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: Figure 20 Photomicrographs of brach
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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