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

100
Classification of Meteorites
classified as a CV anomalous (Koeberl et al.,
1987; Rubin, 1988b; Weisberg et al., 1996b), and
CK anomalous (Kallemeyn, 1996).
Figure 13 Combined elemental maps of the: (a) CK4
carbonaceous chondrite breccia Karoonda and (b) the
H/L3.6 ordinary chondrite Tieschitz. Karoonda consists
of coarse-grained matrix containing abundant plagioclase
(pl) and high-Ca pyroxene (cpx) grains, and rare
poorly recognized chondrules and AOAs. Outlined
region contains rare plagioclase grains and wellrecognized
chondrules and may represent CK material
of lower petrologic type. Tieschitz contains abundant
type I and type II chondrules of porhyritic (PO, POP,
PP), radial pyroxene (RP), and cryptocrystalline (CC)
textures; Al-rich chondrules and CAIs are rare. Most
chondrules are surrounded by Al-rich (bluish) finegrained
material composed of a nepheline-like phase
resulted from in situ aqeuous alteration (source
Hutchison et al., 1998).
Ningqiang is an ungrouped carbonaceous
chondrite that shares many petrologic, as well
as oxygen isotopic and TL characteristics with
CV and CK chondrites (Rubin et al., 1988b;
Kallemeyn et al., 1991; Koeberl et al.,
1987; Mayeda et al., 1988; Guimon et al., 1995;
Kallemeyn, 1996). As a result, it has been
1.05.2.4.2 Ordinary chondrites
H, L, and LL chondrites. Ordinary chondrites
(Figure 13(b)) are characterized by: (i) magnesium-normalized
refractory lithophile abundances
,0.85 £ CI chondrites (Figure 2);
(ii) oxygen isotopic compositions plotting above
the terrestrial fractionation line (Figure 4);
(iii) high abundance of chondrules with nonporphyritic
and FeO-rich (type-II) chondrules being
common and aluminum-rich chondrules being
rare; (iv) rarity of CAIs and AOAs; and (v) a large
range in degree of metamorphism (petrologic
types 3–6), with several type 3.0–3.1 ordinary
chondrites (e.g., Semarkona and Bishunpur)
showing evidence for aqueous alteration
(e.g., Hutchison et al., 1987, 1998; Alexander
et al., 1989; Sears and Weeks, 1991; Sears et al.,
1980, 1991).
Abundance ratios (CI- and Mg-normalized) of
the refractory lithophile elements and of selenium
and zinc are very similar for the three ordinary
chondrite groups; siderophile element abundances
decrease and oxidation state increases through the
sequence H–L–LL (Figures 2 and 7). Based on
evidence for differences in impact history (there is
an H-chondrite cluster of cosmic-ray exposure ages
at ,7 Ma and an L-chondrite cluster of outgassing
ages ,900 Ma; Keil, 1964; Heymann, 1967;
Crabb and Schultz, 1981) and the scarcity of
evidence for intergroup mixing in ordinary chondrite
breccias, it is generally accepted that the H, L,
and LL chondrites are formed in at least three
separate parent asteroids (Kallemeyn et al., 1989).
A number of taxonomic parameters are used to
resolve the individual ordinary chondrite groups.
Small, systematic textural differences among
the ordinary chondrite groups are observed for
metal abundances and chondrule size (Table 2).
On a plot of cobalt concentration in kamacite
versus Fa content in olivine, there is a hiatus
between H and L, but no hiatus between L and
LL chondrites (Figure 14). Olivine composition
is not a reliable indicator of group
for unequilibrated ordinary chondrites of low
petrologic type (,3.5) and siderophile element
abundances are used instead (Kallemeyn et al.,
1989; Sears et al., 1991). Several chondrites
(Bjurböle, Cynthiana, Knyahinya, Qidong, Xi
Ujimgin) fall between the main L and LL
clusters (Figure 14), possibly indicating formation
on a separate parent asteroid (Kallemeyn
et al., 1989). Based on abundance of siderophile
elements and compositions of olivine and kamacite
(Figure 14) that are intermediate between
H and L chondites, Tieschitz (Figure 13(b)) and