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

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(viii) high (,0.5 wt.%) Cr 2 O 3 contents in chondrule
olivines; (ix) rarity and uniform 16 O-
depletion of CAIs; and (x) virtual absence of
fine-grained matrix material and presence of rare
heavily hydrated lithic clasts instead (Lovering,
1962; Mason and Nelen, 1968; Kallemeyn et al.,
1978; Newsom and Drake, 1979; Weisberg et al.,
1990, 2001; Rubin et al., 2001; Krot et al., 2001a,
2002; Greshake et al., 2002).
Based on petrologic and chemical differences
among the CB chondrites, they have been divided
into two subgroups: CB a , which includes Bencubbin,
Weatherford, and Gujba; and CB b , which
includes HH 237 and QUE94411 (Weisberg et al.,
2001). Differences between the subgroups include
FeNi-metal abundances, CAI abundances, chondrule
sizes, compositional range of FeNi-metal,
and nitrogen isotopic compositions (Figures 6,
10(c), (d), and 11(c), (d)). The CB a contains
,60 vol.% metal, centimeter-sized chondrules,
and exceptionally rare CAIs (the only CAI was
found in Gujba; Weisberg et al., 2002); FeNimetal
ranges from 5 wt.% Ni to 8 wt.% Ni, and
d 15 N is up to ,1,000‰. The CB b contains
.70 vol.% metal, millimeter-sized chondrules;
FeNi-metal ranges from 4 wt.% Ni to 15 wt.%
Ni; refractory inclusions are common; and d 15 Nis
up to ,200‰.
CV (Vigarano-like) and ungrouped CV-like
chondrites. The CV chondrites (Figure 12) are
characterized by: (i) millimeter-sized chondrules
with mostly porhyritic textures, most of which
are magnesium rich and ,50% of which are
surrounded by coarse-grained igneous rims;
(ii) high matrix/chondrule ratios (0.5–1.2);
(iii) unique presence of abundant salitehedenbergite
^ andradite nodules in the matrix
(Krot et al., 1998); (iv) high abundance of
millimeter-to-centimeter-sized CAIs and AOAs;
and (v) common occurrence of igneous melilitespinel-pyroxene
^ anorthite (type-B) CAIs
(MacPherson et al., 1988).
The CV chondrites are a diverse group of
meteorites that was originally divided into oxidized
(CV Ox ) and reduced (CV R ) subgroups,
based principally on modal metal/magnetite ratios
and nickel content of metal and sulfides
(McSween, 1977b). Weisberg et al. (1997a)
subdivided the oxidized CV chondrites into the
Allende-like (CV OxA ) and Bali-like (CV OxB )
subgroups (Figure 13). Matrix/chondrule ratios
increase in the order CV R (0.5–0.6),
CV OxA (0.6–0.7), and CV3 OxB (0.7–1.2); metal/
magnetite ratios decrease in the same order
(McSween, 1977b; Simon et al., 1995). Oxygen
isotopic compositions of the CV chondrites show
a wide range along a slope ,1 line on the three
isotope diagram, with the CV OxB chondrites being
slightly depleted in 16 O relative to the CV R and
Classification of Meteorites
CV OxA chondrites (Weisberg et al., 1997a;
Figure 1).
There are significant mineralogical differences
between the CV subgroups. These differences are
largely secondary and resulted from late-stage
alteration that affected, to various degrees, most of
the CV chondrites (Krot et al., 1995, 1998). For
example, primary minerals in chondrules in
CV OxB chondrites are replaced by phyllosilicates,
magnetite, nickel-rich sulfides, fayalite (Fa 95 – 100 ),
and hedenbergite (Hua and Buseck, 1995; Krot
et al., 1998). Chondrules in CV OxA chondrites are
replaced by magnetite, nickel-rich sulfides, fayalitic
olivine (Fa 30 – 60 ), nepheline, and sodalite.
Chondrules in CV R chondrites are virtually
unaltered (Kimura and Ikeda, 1998; Krot et al.,
1995, 1998).
Although all of the CV chondrites have been
classified as type 3, matrices of CV OxB
chondrites contain abundant phyllosilicates;
phyllosilicates are rare in both the CV R and
CV OxA chondrites (Tomeoka and Buseck, 1982,
1990; Keller and Buseck, 1990b; Keller and
McKay, 1993; Keller et al., 1994; Zolensky
et al., 1993).
Coolidge and Loongana 001 comprise a grouplet
of carbonaceous chondrites with chemical,
oxygen isotopic and petrographic characteristics
similar to those of the CV chondrites: high matrix
modal abundance (,20–30 vol.%), 1–2 vol.%
refractory inclusions, and high refractory lithophile
abundance ratios (,1.35 £ CI). However,
both meteorites are of higher petrologic type,
3.8–4, and have smaller chondrules and lower
volatile element abundances than CV chondrites
(Kallemeyn and Wasson, 1982; Scott and Taylor,
1985; Noguchi, 1994; Kallemeyn and Rubin,
1995).
CK (Karoonda-like) and CK/CV-like chondrites.
Most of the CK chondrites (Figure 13(a))
are of high petrologic type (4–6) and are
characterized by: (i) high abundance of matrix;
(ii) large porphyritic chondrules (700–1,000 mm),
with nonporphyritic chondrules and igneous rims
around chondrules being virtually absent; (iii) high
degree of oxidation indicated by high Fa contents
of olivine (Fa 29 – 33 ), nearly complete absence of
FeNi-metal (Figure 7), high nickel content in
sulfides, and abundant magnetite which commonly
contains ilmenite and spinel exsolution
lamellae; (iv) rarity of CAIs and AOAs;
(v) large compositional variations in plagioclase
(An 45 – 78 ); (vi) bulk oxygen isotopic compositions
plotting within the CO and CV fields
(Figure 4); (vii) bulk refractory lithophile element
abundances (,1.2 £ CI) between those of CV and
CO chondrites (Figure 2); and (viii) depletion in
moderately volatile elements relative to CV and
CO chondrites (Figure 2) (Kallemeyn et al.,
1991).