Geology Reference
In-Depth Information
The apparent underrepresentation of HEDs in Antarctica
might simply reflect a statistical overabundance of ordi-
nary chondrites owing to a lack of pairing among the lat-
ter. A decrease in unique ordinary chondrites by a factor
of 2-3 would increase the percentage of HEDs at levels
similar to falls and hot deserts.
At the same time, this decrease would serve to increase
the abundance of carbonaceous chondrites in Antarctica
to levels that exceed the modern falls. In this sense, the
difference between hot and cold deserts is striking. Hot
deserts have a paucity of CM chondrites relative to other
types of carbonaceous chondrites or the population
as  a  whole. Most of the Antarctic CM chondrites are
relatively pristine, with intact fusion crust, and show a
young terrestrial age of ~10,000 years, both of which
suggest that the Antarctic is a relatively gentle environ-
ment that preserves smaller samples. Further, it is likely
that the cold storage of Antarctica would preserve
hydrated carbonaceous chondrites better than would the
hot desert environment, in which both chemical (e.g.,
rain) and physical (e.g., freeze-thaw) weathering would
aggressively destroy such meteorites. Interestingly, type
1 carbonaceous chondrites, in which virtually all mafic
silicates have altered to hydrated phyllosilicates, attest
to  the relative importance of chemical and physical
preservation. CM1 chondrites were first discovered in
Antarctica and are almost exclusively known from
that  continent (Plate  14). Their survival suggests that
chemical weathering has produced minimal alteration
of  these indurated samples. In addition, there are two
CR1 Antarctic meteorites, and these may be all we have
in the  world's collections (Plate  27). In contrast, CI1
chondrites are absent, despite the collection of 20,000+
samples. Unlike CM1 chondrites, CI1 chondrites are
both heavily aqueously altered and poorly lithified. The
absence of chondrules prevents them from having cohe-
sive strength. While there are only five known CI mete-
orites in the world's collections (Alais, Ivuna, Orgueil,
Tonk, and Revelstoke), none have been found in the
Antarctic or in the world's hot deserts (with the exception
of some possible meta-CI's in the Japanese collection
[ Tonui et al ., 2003; Zolensky et al ., 2005]. Given the
extremely fragile nature of these meteorites, it could be
that even modest physical weathering in the extreme
physical conditions in Antarctica effectively destroys
these rare samples.
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