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104 their textures are igneous or metamorphic. They are generally thought to be ultrametamorphosed chondrites or residues of very low degrees of partial melting. Several groups of achondrites and iron meteorites are genetically related and were possibly derived from single asteroids or planetary bodies (acapulcoites and lodranites; winonaites, silicate inclusions in IAB and IIICD irons; howardites, eucrites and diogenites; shergottites, nakhlites, Chassigny and ALH84001). 1.05.4.1 Primitive Achondrites Several groups of stony (silicate-rich) meteorites that have essentially bulk chondritic compositions but achondritic textures formed by low degrees of melting and have collectively been referred to as “primitive achondrites” (Prinz et al., 1983b). These include the acapulcoites, lodranites, and winonaites. Two groups of silicatebearing iron meteorites, IAB and IIICD, may have originated from the same parent body as the winonaites. Some of the silicate inclusions in IIE irons also resemble primitive achondrites. Classification of Meteorites 1.05.4.1.1 Acapulcoites and lodranites The acapulcoites and lodranites are fine- to medium-grained equigranular rocks composed of orthopyroxene, olivine, Cr-diopside, Naplagioclase, FeNi-metal, schreibersite, troilite, whitlockite, Cl-apatite, chromite, and graphite (Figures 16(a) and (b)). Although this mineral assemblage is rather similar to that of ordinary chondrites, and rare relict chondrules have been reported in several acapulcoites (e.g., Schultz et al., 1982; McCoy et al., 1996), their mineral compositions, mineral abundances, and grain size differ from ordinary chondrites (Table 6). There is no clear-cut distinction between acapulcoites and lodranites. Typically, acapulcoites (Figure 16(a)) are fine grained (150–230 mm), with approximately chondritic abundances of olivine, pyroxene, plagioclase, metal, and troilite, whereas lodranites (Figure 16(b)) are coarse grained (540–700 mm) and depleted in troilite and plagioclase (Nagahara, 1992; McCoy et al., 1996; Mittlefehldt et al., 1998). The bulk oxygen isotopic compositions of acapulcoites and lodranites are similar and form a cluster with significant dispersion in D 17 O(Figure 17). Figure 16 Photomicrographs of primitive achondrites in transmitted light with partially crossed nicols. (a) Acapulcoite ALH81315 and (b) lodranite LEW88280 have similar mineralogy (mostly orthopyroxene, olivine, Cr-diopside, Na-plagioclase, FeNi-metal, and troilite), but different grain size. (c) Silicate inclusion in IAB iron Caddo County is a basaltic clast composed of plagioclase (pl, gray), clinopyroxene (cpx, whitish) and olivine (ol, pink). (d) Silicate inclusion in IAB iron Campo del Cielo has chondritic mineralogy ((a, b)—photograph courtesy of D. Mittlefehldt; (c, d)—photograph courtesy of G. Benedix).
Table 6 Synopsis of petrologic characteristics of the asteroidal, nonchondrite groups, and models of their origin. Group Mineralogy Texture Origin Acapulcoites–lodranites Olivine (Fo 87 – 97 ), orthopyroxene (En 86 – 92 ), clinopyroxene (En 50 – 54 Wo 43 – 46 ), metal, ^troilite, ^Na-plagioclase (An 12 – 31 ) IAB–IIICD silicate inclusions, winonaites Olivine (Fa 1–8 ), orthopyroxene (Fs 1–9 ), clinopyroxene (Fs 2–4 Wo 44 – 45 ), Naplagioclase (An 11 – 22 Ab 76 – 85 ), troilite, metal, daubreilite, schreibersite, graphite IIE silicate inclusions Olivine (Fo 79 – 86 ), orthopyroxene (Fs 14 – 17 ), clinopyroxene (Fs 6–18 Wo 36 – 45 ), Naplagioclase (An 0–14 Ab 9–93 ), troilite, metal Nonmagmatic irons IAB, IIE, IIICD metal phase Angrites Al–Ti-diopside (TiO 2 , 1–5%, Al 2 O 3 , 4–12%; Fs 12 – 50 ), anorthite (An 86 – 100 Ab 0–13 ), Ca-olivine (Fo 32 – 89 ), kirschsteinite Aubrites Fine- to coarse-grained equigranular Fine-grained equigranular Chondritic to gabbroic Metal, troilite, schreibersite, ^graphite Coarse-grained, generally exsolved Impact melting (?) Enstatite, pigeonite, diopside, forsterite, Si-bearing kamacite, Na-plagioclase (An 2–24 Ab 75 – 94 ), Fe, Mn, Mg, Cr, Ti, Na, K-sulfides Brachinites Olivine (Fo 65 – 70 ), augite (En 44 – 48 Wo 39 – 47 ), ^orthopyroxene (En 69 – 72 ), Fe-sulfide, ^Na-plagioclase (An 22 – 33 Ab 68 – 76 ) Ureilites Olivine (Fo 74 – 95 ), orthopyroxene (En 75 – 87 ), pigeonite (En 68 – 87 Wo 2–16 ), augite (En 52 – 60 Wo 32 – 39 ) HED (howardites, eucrites, diogenites) Orthopyroxene (En 14 – 79 ), olivine (Fo 56 – 73 ), chromite, pigeonite, Ca-plagioclase (An 73 – 96 Ab 5–23 ), silica Mesosiderites Orthopyroxene (En 41 – 77 ), olivine (Fo 63 – 92 ), chromite, pigeonite, Ca-plagioclase (An 91 – 93 Ab 7–9 ), silica, phosphates Silicates and oxides in Olivine (Fo 80 – 90 ), chromite, ^low-Ca pallasites pyroxene (En 83 – 91 ) Pallasite and mesosiderite metal phase Fine- to coarse-grained igneous Breccias of igneous lithologies Medium- to coarse-grained equigranular Coarse-grained granular Breccias (howardites), fine- to coarse-grained basalts (eucrites), coarse-grained orthopyroxenites (diogenites) Breccias of igneous lithologies, impact melts Coarse-grained, rounded to angular Ultrametamorphism, low-degree partial melting ^ melt migration Ultrametamorphism, low-degree partial melting, impact mixing Metamorphism, low-degree partial melting, melt migration, shock Crystallized melts, cumulates Melted and crystallized residues from partial melting Metamorphosed chondrite, partial melting residue Partial melting residue Cumulates, crystallized melts, impact mixing Impact melted crustal igneous lithologies Cumulates from core–mantle boundary Metal, troilite, schreibersite, ^graphite Coarse-grained, generally exsolved Fractional crystallization of core Magmatic irons Metal, troilite, schreibersite, ^graphite Coarse-grained, generally exsolved Fractional crystallization of core After Mittlefehldt et al. (1998).
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: abundances and chondrule sizes by c
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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