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

More documents

Recommendations

Info

102 Classification of Meteorites Figure 15 Combined elemental maps (a, c, d) and Mg Ka X-ray map (b) of the (a, b) EH3 enstatite chondrite ALH81189, (c) the Kakangari-like chondrite LEW87232, and (d) the R3-5 chondrite breccia PRE95404. The EH3 chondrite ALH81189 (a, b) is composed of enstatite-rich chondrules and metal-sulfide nodules; forsterite-bearing (bright grains in (b)) chondrules are rare; CAIs are very rare; matrix material is very minor (difficult to recognize). (c) The Kakangari-like meteorite LEW87232 consists of crystalline matrix largely composed of magnesian pyroxenes and olivine, magnesian pyroxene-rich chondrules, and rare CAIs. (d) The R3-5 chondrite breccia PRE95404 contains two lithologically different protions. The lower part of the section consists of type-3 material; it contains abundant chondrules, large sulfides nodules (sf), and very rare CAIs surrounded by a fine-grained matrix material. The rest of the section represents type-4 material; it contains poorly defined chondrules and relatively coarse-grained matrix. Ferromagnesian silicates in chondrules and matrix have similar ferroan compositions; magnesium-rich chondrules and chondrule fragments are rare. and carbonaceous classes, although these meteorites have properties related to each (Srinivasan and Anders, 1977; Davis et al., 1977; Brearley, 1989; Weisberg et al., 1996a). The K chondrites have: (i) high matrix abundance (70–77 vol.%) like carbonaceous chondrites (Table 2); (ii) metal abundances (6–9 vol.%) similar to H chondrites; (iii) average olivine (Fa 2 ) and enstatite (Fs 4 ) compositions indicating an oxidation state intermediate between H and enstatite chondrites; (iv) mineralogically unique matrix composed largely of enstatite (Fs 3 ); (v) refractory lithophile element abundances (,1 £ CI) similar to those of the ordinary chondrites (Figure 2); (vi) volatile lithophile element abundances similar to ordinary chondrites; (vii) chalcophile element abundances between those of H and enstatite chondrites (Figure 2); (viii) whole-rock oxygen isotopic compositions that are below the terrestrial fractionation line (Figure 4), near the CR, CB, and CH chondrites (CR-mixing line); and (ix) abundance and isotopic compositions of nitrogen and carbon within the range of CV chondrites (Figures 5 and 6). Lea County 002 was originally classified as a Kakangari-like chondrite, although Weisberg et al. (1996a) reported differences in matrix
abundances and chondrule sizes by comparison to Kakangari/Lewis Cliff 87232 (33 vol.% versus 70–77 vol.% and 1.0–1.25 mm versus 0.25–0.5 mm, respectively). CAIs in Kakangari and Lea County 002 appear to be different as well (Prinz et al., 1991a). Mineralogical observations (Krot, unpublished) and oxygen isotopic compositions (Figure 4) suggest instead that Lea County 002 is a CR chondrite. For more on IDPs, see Chapter 1.26. 1.05.2.4.5 R (Rumuruti-like) chondrites The R chondrites (Figures 16(d)) are characterized by: (i) a high abundance of matrix (,50 vol.%); (ii) high oxidation states, reflected in the occurrence of abundant NiO-bearing olivine with Fa 37 – 40 (in equilibrated R chondrites) and nearly complete absence of FeNi-metal; (iii) refractory lithophile and moderately volatile element abundances similar to those in ordinary chondrites (,0.95 £ CI); (iv) absence of significant depletions in manganese and sodium and enrichment in volatile elements such as gallium, selenium, sulfur, and zinc, which distinguish R chondrites from ordinary chondrites (Figure 2); (v) whole-rock D 17 O values higher than for any other chondrite group (Figure 4); and (vi) extreme rarity of CAIs (Weisberg et al., 1991; Bischoff et al., 1994; Rubin and Kallemeyn, 1994; Schulze et al., 1994; Kallemeyn et al., 1996; Russell, 1998). Most R chondrites are metamorphosed (petrologic type .3.6) and brecciated (Figure 15(d)); they have typical light/dark structure and solarwind implanted rare gases and are best described as R3–6 regolith breccias (Bischoff, 2000). 1.05.3 CLASSIFICATION OF IDPS Based on the bulk chemistry, IDPs are divided into two groups: (i) micrometer-sized chondritic particles and (ii) micrometer-sized nonchondritic particles. A particle is defined as chondritic when magnesium, aluminum, silicon, sulfur, calcium, titanium, chromium, manganese, iron, and nickel occur in relative proportions similar (within a factor of 2) to their solar element abundances, as represented by the CI carbonaceous chondrite composition (Brownlee et al., 1976). Chondritic IDPs differ significantly in form and texture from the components of known carbonaceous chondrite groups and are highly enriched in carbon relative to the most carbon-rich CI carbonaceous chondrites (Rietmeijer, 1992; Thomas et al., 1996; Rietmeijer, 1998, 2002). The chondritic IDPs occur as aggregate particles (make up ,90% of chondritic IDPs) and nonaggregate particles. A subdivision of chondritic IDPs includes: (i) particles that form Classification of Nonchondritic Meteorites 103 highly porous (chondritic porous (CP) IDPs) and somewhat porous aggregates (chondritic filled (CF) IDPs); (ii) smooth particles (chondritic smooth (CS) IDPs); and (iii) compact particles with a rough surface (chondritic rough (CR) IDPs) (MacKinnon and Rietmeijer, 1987; Rietmeijer and Warren, 1994; Rietmeijer, 1998, 2002). Three groups of chondritic IDPs have been distinguished using infrared spectroscopy: (i) olivine-rich particles þ carbon phases; (ii) pyroxene-rich particles þ carbon phases; and (iii) layered lattice silicate-rich particles þ carbon phases (Sandford and Walker, 1985; MacKinnon and Rietmeijer, 1987). The layered lattice silicaterich particles are further divided into serpentinerich and smectite-rich particles (Rietmeijer, 1998). In an attempt to reconcile various chondritic IDP classifications, Rietmeijer (1994) proposed to classify IDPs on the basis of properties of identifiable textural entities in these particles and their porosity as aggregate IDPs and collapsed aggregate IDPs. In highly porous aggregates the constituents are loosely bound (CP IDPs). Most aggregates are more compact with little pore space (CF IDPs). The collapsed aggregate particles typically have a smooth surface (CS IDPs). The nonchondritic IDPs consist mostly of magnesium-rich olivines and pyroxenes, and FeNi-sulfides (Rietmeijer, 1998). Chondritic materials are often attached to the surface of nonchondritic IDPs, which suggests that the groups have a common origin. For more on IDPs, see Chapter 1.26. 1.05.4 CLASSIFICATION OF NONCHONDRITIC METEORITES The nonchondritic meteorites contain virtually none of the components found in chondrites and were derived from chondritic materials by planetary melting, and fractionation caused their bulk compositions to deviate to various degrees from chondritic materials. The degrees of melting that these rocks experienced are highly variable and, thus, these meteorites can be divided into two major categories: primitive and differentiated (Figure 1). There is no clear-cut boundary between these categories, however. The differentiated meteorites were derived from parent bodies that experienced large-scale partial melting, isotopic homogenization (ureilites are the only exception), and subsequent differentiation. Based on abundance of FeNi-metal, these meteorites are commonly divided into three types: achondrites (metal-poor), stony irons, and irons; each of the types contains several meteorite groups and ungrouped members. The primitive nonchondritic meteorites have approximately chondritic bulk compositions, but
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: Figure 14 Fayalite (Fa) content (mo
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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?