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

116
Classification of Meteorites
Table 8
Major characteristics of some ungrouped nonchondritic meteorites.
Meteorite Silicate mineralogy Classification 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, and 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 and 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 and 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 and 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 of kamacite and show no
Widmanstätten pattern; they are called hexahedrites
(H) (Figure 27(f)).
1.05.4.6 Silicate-bearing IVA Irons
The IVA iron meteorite group contains four
silicate-bearing members: Bishop Canyon,
Gibeon, Steinbach, and São João Nepomuceno
(Mittlefehldt et al., 1998). Silicates in IVA irons
are nonchondritic in their mineralogy (Table 6)
and consist of orthobronzite–clinobronzite–
tridymite-rich inclusions in Steinbach and São
João Nepomuceno and of individual SiO 2 grains
in Bishop Canyon and Gibeon (e.g., Ulff-Möller
et al., 1995; Scott et al., 1996; Haack et al., 1996;
Marvin et al., 1997).
1.05.5 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, and Chassigny). However, this designation
is no longer comprehensive, and the simple
term “martian meteorites” is recommended instead
(Treiman et al., 2000). The martian meteorites
are volcanic or subvolcanic and plutonic rocks.
Their diverse, highly fractionated compositions,
and young crystallization ages (1.3 Ga and
possibly ,180 Ma), suggest that they are derived
from a large, planet-sized body (e.g. Wood and
Ashwal, 1981; Ashwal et al., 1982; Jones, 1986).
Their unique oxygen isotopic compositions
(Figure 28) and FeO/MnO ratios (Figure 29)
indicate that this body is not Earth or Moon.
Similarities between the isotopic compositions
of nitrogen and noble gases of the martian
atmosphere (as determined by the Viking Landers)
and those trapped in impact-produced
glasses in some shergottites suggest that it is
Mars (e.g., McSween and Treiman, 1998;
Bogard et al., 2000; Nyquist et al., 2001;
McSween, 2002 and references therein; see
Chapter 1.22).
1.05.5.1 Shergottites
The shergottites are the most abundant (18 out
of 26) and the most diverse of the martian
meteorite subgroups. They are commonly divided
into two types: basaltic and lherzolitic.