116 <strong>Classification</strong> <strong>of</strong> Meteorites Table 8 Major characteristics <strong>of</strong> some ungrouped nonchondritic meteorites. Meteorite Silicate mineralogy <strong>Classification</strong> Refs. ALHA77255 Silica Anomalous iron [1] Bocaiuva Ol (Fa 8 ), Opx (Fs 8 ), Pl (An 49 ), Silicate-bearing [2] Cpx (Fs 5 Wo 42 ) iron, Off Deep Springs Silicate-bearing iron [3] Divnoe Ol (Fa 20 – 28 ), Px (Fs 20 – 28 Wo 0.5 – 2.5 ), Pl (An 45 – 32 ) Ol-rich primitive [4–6] achondrite Enon Ol (Fo 91 ), Opx (Fs 11 Wo 2 ), Cpx (Fs 4 Wo 44 ), Stony iron [7,8] Pl (An 15 Ab 79 ), Chr, Wt, Tr, Schr Guin Iron with affinities to [9] IIE, IAB, <strong>and</strong> IIICD LEW86211 Troilite–metal intergrowth with Anomalous [1,10] olivine–pyroxene inclusions LEW88763 Ol (71 vol.%; Fo 63 – 64 ), Px (7%; Fs 29 Wo 4 þ Aug (Fs 16 Wo 38 ), Primitive achondrite [11] Pl (10%; Ab 55 – 74 An 19 – 44 ), Met þ Tr þ Chr þ Ilm (6%) Mbosi Pl, Px, Chr, Tr, Schr, Sil Silicate-bearing iron [12] NWA 176 Ol (20.9 vol.%, Fa 11 ), Opx (17.5%, Fs 11 Wo 3 ), Cpx (1.3%, Silicate-bearing iron [13] Fs 6 Wo 42 ), Pl (3.9%, An 50 Ab 47 ), Mer (0.2%), Chr (0.1%) Puente del Ol (23 wt.%, Fa 4 ), Px (14%, Fs 6 Wo 1 ), Cr-Di (15%, Fs 3 Wo 47 ), Silicate-bearing iron [14] Zacate Pl (15%, An 14 Ab 82 ), Grph (27%), Tr, Chr, Daub, Met QUE93148 Ol (65 vol.%), Opx (13%), Met (22%), Aug (,0.1%) Olivine-rich achondrite [15] Sombrerete Opx (14.7%, En 68 ), albitic glass (66.7%), Pl (9%), Cl-Apt (8%), Silicate-bearing iron [16,17] Ks, Trid, Chr, Ilm, Rt Tucson Ol (66 vol.%, Fo 99 – 100 ), En (30%, 0.5–21 wt.% Al 2 O 3 ), Di (3%, 5–18 wt.% Al 2 O 3 ), Pl, Sp, Brz Silicate-bearing iron [18] Brz ¼ brezinaite; Chr ¼ chromite; Cl-Apt ¼ Cl-apatite; Cpx ¼ clinopyroxene; Daub ¼ daubreelite; Di ¼ diopside; Grph ¼ graphite; Ilm ¼ ilmenite; Ks ¼ kaersutite; Mer ¼ merrillite; Met ¼ FeNi-metal; Ol ¼ olivine; Opx ¼ orthopyroxene; Pl ¼ plagioclase; Px ¼ pyroxene; Rt ¼ rutile; Schr ¼ schreibersite; Tr ¼ troilite; Trid ¼ tridymite; Wt ¼ whitlockite. [1]—Clayton <strong>and</strong> Mayedsa (1996); [2]—Malvin et al. (1985); [3]—Schaudy et al. (1972); [4]—Petaev et al. (1994); [5]—McCoy et al. (1992a); [6]—Weigel et al. (1996); [7]—Bunch et al. (1970); [8]—Kallemeyn <strong>and</strong> Wasson (1985); [9]—Rubin et al. (1986); [10]—Prinz et al. (1991b); [11]—Swindle et al. (1998); [12]—Olsen et al. (1996a); [13]—Liu et al. (2001); [14]—Olsen et al. (1996b); [15]—Goodrich <strong>and</strong> Righter (2000); [16]—Prinz et al. (1982); [17]—Malvin et al. (1984); [18]—Nehru et al. (1982). Some irons with Ni contents ,6% consist almost entirely <strong>of</strong> kamacite <strong>and</strong> show no Widmanstätten pattern; they are called hexahedrites (H) (Figure 27(f)). 1.<strong>05</strong>.4.6 Silicate-bearing IVA Irons The IVA iron meteorite group contains four silicate-bearing members: Bishop Canyon, Gibeon, Steinbach, <strong>and</strong> São João Nepomuceno (Mittlefehldt et al., 1998). Silicates in IVA irons are nonchondritic in their mineralogy (Table 6) <strong>and</strong> consist <strong>of</strong> orthobronzite–clinobronzite– tridymite-rich inclusions in Steinbach <strong>and</strong> São João Nepomuceno <strong>and</strong> <strong>of</strong> individual SiO 2 grains in Bishop Canyon <strong>and</strong> Gibeon (e.g., Ulff-Möller et al., 1995; Scott et al., 1996; Haack et al., 1996; Marvin et al., 1997). 1.<strong>05</strong>.5 MARTIAN (SNC) METEORITES Twenty-six meteorites are believed to be martian rocks. This group was formerly referred to by the acronym SNC (for Shergottites, Nakhlites, <strong>and</strong> Chassigny). However, this designation is no longer comprehensive, <strong>and</strong> the simple term “martian meteorites” is recommended instead (Treiman et al., 2000). The martian meteorites are volcanic or subvolcanic <strong>and</strong> plutonic rocks. Their diverse, highly fractionated compositions, <strong>and</strong> young crystallization ages (1.3 Ga <strong>and</strong> possibly ,180 Ma), suggest that they are derived from a large, planet-sized body (e.g. Wood <strong>and</strong> Ashwal, 1981; Ashwal et al., 1982; Jones, 1986). Their unique oxygen isotopic compositions (Figure 28) <strong>and</strong> FeO/MnO ratios (Figure 29) indicate that this body is not <strong>Earth</strong> or Moon. Similarities between the isotopic compositions <strong>of</strong> nitrogen <strong>and</strong> noble gases <strong>of</strong> the martian atmosphere (as determined by the Viking L<strong>and</strong>ers) <strong>and</strong> those trapped in impact-produced glasses in some shergottites suggest that it is Mars (e.g., McSween <strong>and</strong> Treiman, 1998; Bogard et al., 2000; Nyquist et al., 2001; McSween, 2002 <strong>and</strong> references therein; see Chapter 1.22). 1.<strong>05</strong>.5.1 Shergottites The shergottites are the most abundant (18 out <strong>of</strong> 26) <strong>and</strong> the most diverse <strong>of</strong> the martian meteorite subgroups. They are commonly divided into two types: basaltic <strong>and</strong> lherzolitic.
Martian (SNC) Meteorites 117 Figure 27 Polished <strong>and</strong> etched slabs <strong>of</strong> iron meteorites. (a) The fine octahedrite IIICD iron Carlton. (b) The medium octahedrite IIIAB iron Casas Gr<strong>and</strong>es. (c) The coarse octahedrite IIE silicate-bearing iron Weekeroo Station. Silicate inclusions in (c) consist <strong>of</strong> plagioclase, orthopyroxene, <strong>and</strong> clinopyroxene. (d) The coarse octahedrite IAB silicatebearing iron Campo del Cielo. (e) The coarsest octahedrite IIAB iron Santa Luzia. (f) The hexahedrite IIAB iron Bennett County (photograph courtesy <strong>of</strong> the Smithsonian Institution). The basaltic shergottites consist predominantly <strong>of</strong> clinopyroxene (pigeonite <strong>and</strong> augite) <strong>and</strong> plagioclase (now shock-produced glass or maskelynite), <strong>and</strong> have basaltic or diabasic textures. The absence <strong>of</strong> olivine in these rocks <strong>and</strong> their low Mg#s (23–52) indicate that they crystallized from fractionated magmas (Stolper <strong>and</strong> McSween, 1979). Many <strong>of</strong> them (including
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