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

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events (see Chapter 1.07–1.09). Although most
chondrites experienced thermal processing on their
parent asteroids, such as aqueous alteration,
thermal, and shock metamorphism, they did not
experience melting and igneous differentiation,
and thus largely preserve records of physical and
chemical processes in the solar nebula. Deciphering
these records is a primary goal of chondrite
studies.
Nonchondritic meteorites lack chondritic textures
and are formed by partial or complete
melting and planetary differentiation of chondritic
precursor asteroids or larger planetary bodies
(Mars, Moon), and hence provide unique opportunities
to study these processes on extraterrestrial
bodies (see Chapter 1.11, 1.12, 1.21, and 1.22).
Several groups of nonchondritic meteorites
experienced only low degrees of melting, and
have largely retained their chondritic bulk compositions.
To emphasize the relatively unprocessed
nature of these nonchondrites and their
intermediate status relative to chondrites and
highly differentiated meteorites, they are referred
to as primitive achondrites and discussed as a
separate category of nonchondritic meteorites.
1.05.2 CLASSIFICATION OF CHONDRITIC
METEORITES
1.05.2.1 Taxonomy
Based on bulk chemistry (Figures 2 and 3),
bulk oxygen isotopic compositions (Figure 4),
mineralogy, petrology (Figures 8–13 and 15), and
proportions of various chondritic components
(Table 2), 14 chondrite groups have been
recognized. Several other chondrites are mineralogically
and/or chemically unique and defy
classification into the existing chondrite groups;
these are commonly called ungrouped chondrites
(Table 1 and Figure 1).
Conventionally, a chondrite “group” is defined
as having a minimum of five unpaired chondrites
of similar mineralogy, petrography, bulk isotopic
properties, and bulk chemical compositions in
major, nonvolatile elements. According to this
definition, K chondrites could be considered as a
grouplet: there are only two known K chondrites
(see K (Kakangari-like) chondrites).
Thirteen out of the 14 chondrite groups
comprise three major classes: carbonaceous (C),
ordinary (O), and enstatite (E), each of which
contains distinct groups.
The term carbonaceous is somewhat of a
misnomer, because only the CI, CM, and CR
chondrites are significantly enriched in carbon
relative to noncarbonaceous chondrites (Figure 5).
The CI chondrites lack chondrules. Carbonaceous
and noncarbonaceous chondrites can be resolved
Classification of Meteorites
(with some exceptions) by several characteristics:
(i) mean refractory lithophile/Si abundance
ratios relative to CI chondrites ($1.00 in carbonaceous
chondrites, #0.95 in noncarbonaceous
chondrites); (ii) oxygen isotopic composition
(D 17 O #22‰ for carbonaceous chondrites,
except CI; D 17 O$21‰ for noncarbonaceous
chondrites); (iii) refractory inclusion abundances
($0.1 vol.% in carbonaceous chondrites, except
CI; #0.1 vol.% in noncarbonaceous chondrites);
and (iv) matrix/chondrule modal abundance ratio
($0.9 in carbonaceous chondrites, except CH and
CB; #0.9 in noncarbonaceous chondrites, except
K chondrites) (Kallemeyn et al., 1996).
There are eight well-resolved groups of carbonaceous
chondrites: CI, CM, CR, CH, CB, CV,
CK, and CO. The letters designating the groups
refer to a typical chondrite (commonly fall)
in the group: CI (Ivuna-like), CM (Migheilike),
CO (Ornans-like), CR (Renazzo-like),
CH (ALH85085-like), CB (Bencubbin-like), CV
(Vigarano-like), and CK (Karoonda-like). The
exception to this rule is the CH group, in which the
“H” refers to high metal abundance and high iron
concentration (Bischoff et al., 1993b).
Ordinary chondrites are divided into three
groups: H, L, and LL. The letters designating the
groups refer to the bulk iron contents: H chondrites
have high total iron contents, L chondrites have
low total iron contents, and LL chondrites have
low metallic iron relative to total iron, as well
as low total iron contents.
Enstatite chondrites comprise two groups with
different contents of metallic iron: EH and EL.
Additionally, there is an ungrouped E chondrite,
LEW87223 (Grossman et al., 1993).
R (Rumuruti-like) and K (Kakangari-like)
chondrite groups are different from other chondrites
and have been suggested to represent
additional chondrite classes.
The term clan is used as a higher order of
classification than group. It was originally defined
to encompass chondrites that have chemical,
mineralogical, and isotopic similarities and were
believed to have formed at about the same time in
the same small region of the solar nebula, in a
narrow range of heliocentric distances (Kallemeyn
and Wasson, 1981; Kallemeyn et al., 1996).
Weisberg et al. (1995a) used the term for
chondrites that have chemical, mineralogical,
and isotopic similarities suggesting a petrogenetic
kinship, but have petrologic and/or bulk chemical
characteristics that challenge a group relationship.
Four carbonaceous chondrite clans have been
recognized: (i) the CR clan, which includes the
CR, CH, CB, and a unique chondrite LEW85332;
(ii) the CM–CO clan; (iii) the CV–CK clan;
and (iv) the CI clan (Kallemeyn et al., 1996).
The noncarbonaceous chondrites constitute three
clans: OþR, E, and K.