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

88
Classification of Meteorites
there does not appear to be a resolvable difference
between many groups within the chondrite
classes. For example, bulk oxygen isotopic
compositions of EH and EL enstatite chondrites
completely overlap, as do those of the type-3 H, L,
and LL ordinary chondrites. The CO, CV, and CK
carbonaceous chondrites also have similar bulk
oxygen isotopic compositions.
1.05.2.2.3 Bulk nitrogen and carbon
abundances and isotopic
compositions
Nitrogen abundances and isotopic compositions
(Figure 6) can differentiate most of the
existing chondrite groups (Kung and Clayton,
1978; Kerridge, 1985). Carbon abundance and
isotopic compositions (Figure 5) can differentiate
only CI and Mighei-like (CM) carbonaceous
chondrite groups.
Figure 3 Al/Mn versus (Zn/Mn) £ 100 atom ratios
in chondritic meteorites (sources Kallemeyn and
Wasson, 1981, 1982, 1985; Kallemeyn et al., 1978,
1989, 1991, 1994, 1996; Wasson and Kallemeyn, 1988;
Kallemeyn, unpublished).
carbonaceous chondrite group (CB) is the only
exception). Carbonaceous chondrites are enriched
in refractory lithophile elements relative to CI,
and depleted in volatile elements. Siderophile and
chalcophile elements show volatility-controlled
abundance patterns.
Elemental abundance patterns for ordinary,
Rumuruti-like (R), and Kakangari-like (K) chondrites
are fairly flat and enriched relative to CI
for lithophile and refractory lithophile elements.
Enstatite chondrites have the lowest abundance of
refractory lithophile elements.
In addition to abundance patterns, other bulk
compositional criteria for resolving the chondrite
groups involve compositional diagrams of
elements of different volatility, e.g., Sb/Ni versus
Ir/Ni, Sc/Mg versus Ir/Ni, and Al/Mn versus
Zn/Mn (Figure 3).
1.05.2.2.2 Bulk oxygen isotopic compositions
Most chondrite classes (ordinary, carbonaceous,
and enstatite chondrites) plot in unique
positions on a three-isotope oxygen plot (Figure 4).
Carbonaceous and K chondrites fall below the
terrestrial fractionation line (CI chondrites are the
only exception), whereas ordinary, enstatite, and
R chondrites each form well-defined clusters on or
above the terrestrial fractionation line. However,
1.05.2.2.4 Oxidation state
The chondrite groups record a wide range
of oxidation states that were probably established
through a combination of nebular and
asteroidal processes (Rubin et al., 1988a;
Krot et al., 2000a). The oxidation state is reflected
by the distribution of iron among its three
common oxidation states: 0 (FeNi-metal and
Fe-sulfides), þ2 (silicates), and þ3 (oxides). The
oxidation states of chondrite groups can be
summarized in a Urey–Craig diagram, where
iron present in metal and sulfide phases is plotted
versus iron present in silicate and oxide phases
(Figure 7). Oxidation state generally increases in
the order enstatite–ordinary–carbonaceousþR
chondrite classes, with K chondrites being intermediate
between ordinary and enstatite chondrites.
Within the ordinary chondrites, the oxidation state
increases from H to L to LL, as is reflected in
an increase in the Fe/(Fe þ Mg) ratios of their
olivine and pyroxene. For carbonaceous chondrites,
oxidation state increases in the order
CB–CH–CR–CO–CV–CK–CM–CI.
We note, however, that this classification
parameter does not generally reflect the oxidation
state of individual chondritic components,
e.g., CAIs in all chondrite groups formed under
highly reducing conditions (see Chapter 1.08), and
magnesian (type-I) and ferrous (type-II) chondrules
(both of which occur in the same meteorites)
require formation under different redox
conditions (see Chapter 1.07). In addition, the
role of nebular and asteroidal processes in
establishing of the oxidation states of chondrites
remains controversial (e.g., Krot et al., 2000a).