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Repost: C1 Carbonaceous Chondrites
Ken Irving schrieb:
> I am looking for information on the series of arguments and evidence
> that C1 Carbonacous Chondrites were inherent of the original matter
> in the solar system.
Hello Ken,
Here are excerpts of the copies of three mails I sent to our List some
time ago. I hope this is helpful.
Best wishes,
Bernd
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Here are some references that may help us better understand the term
"p r i m i t i v e":
a) SEARS D.W. (1978) The Nature and Origin of Meteorites:
The CB* chondrites, and particularly the C1 chondrites, are usually
considered to be the most primitive - in the sense of least altered -
samples of the solar system (pp. 64-65).
The first feature to note is that the chondrites closely resemble the
Sun ... For C1 chondrites - the least altered class of meteorites - the
agreement could hardly be better (p. 88).
*C chondrites other than CV (Van Schmus and Hayes, 1974)
b) KERRIDGE J.F. and BUNCH T.E (1979) Aqueous Activity On Asteroids:
Evidence from Carbonaceous Meteorites (in Asteroids, ed. T. Gehrels,
Univ. of Arizona Press):
Bulk contents of calcium and magnesium in CI chondrites provide a
reasonable match with solar abundances (Anders, 1971; Holweger, 1977)
... indicating that CI parental material was neither depleted nor
enriched in these elements (p. 748).
c) NORTON O.R. (1998) Rock From Space II, p. 191:
Carbonaceous chondrites are rare, primitive, and organic ... They show
practically no thermal metamorphism, and compared to other meteorites,
their composition is closest to that of the Sun. For these reasons,
carbonaceous chondrites are consisered the most primitive meteorites.
--------------- snip ---------------
And here is some more on "primitiveness":
d) ANDERS E. et al. (1993) Interstellar grains in primitive meteorites:
Diamond, silicon carbide and graphite (Meteoritics 28, 490-514):
Excerpt from the abstract, p. 490: Primitive meteorites contain a few
parts per million (ppm) of pristine interstellar grains that provide
information on nuclear and chemical processes in stars.
e) NEWTON J. et al. (1995) Açfer 094, a uniquely primitive carbonaceous
chondrite from the Sahara (Meteoritics 30, 47-56):
Abstract - The Saharan meteorite Açfer 094 is a unique type of
carbonaceous chondrite. Mineralogical and petrological considerations
and O isotopes are unable to distinguish unambiguously between CO3 vs.
CM2 classification. The other important light elements, C and N, have
systematics that do not match any previously recognised meteorite group.
Particularly important in this respect is the very low C/N ratio and
d13C of the macromolecular C. Açfer 094 has more diamond and SiC,
expecially X type grains, than any other specimen studied, suggesting
minimal thermal or aqueous processing to decrease its very primitive
status.
Excerpt from the introduction, p. 47: Initial studies involving other
stable isotopes indicate that, whatever its exact classification, Açfer
094 is primitive in that it contains isotopically anomalous C and N
(Newton et al., 1992 a, b). Now that these investigations have been
extended, it is possible to conclude that the meteorite is very
primitive indeed and may not be directly related to existing types of
carbonaceous chondrites.
--------------- snip ---------------
Another very interesting paper:
i) BREARLEY A.J. (1997) Phyllosilicates in the matrix of the unique
carbonaceous chondrite Lewis Cliff 85332 and possible implications for
the aqueous alteration of Cl chondrites (Meteoritics 32-3, 1997, 377):
Excerpt from the abstract:
The compositional and mineralogical data suggest that matrix of LEW
85332 may represent a very early stage in the type of aqueous alteration
experienced by the CI chondrites, although it is improbable that LEW
85332 was a precursor to the CI chondrites, because of its high
abundance of chondrules.
Excerpt from the introduction:
The carbonaceous chondrites represent a diverse group of meteorites
whose properties have provided a wealth of information on the early
history of our solar system. Continuing studies of these meteorites have
shown that although the bulk of them can be readily classified into four
major groups, the CI, CM, CO and CV chondrites, there are still a large
number of enigmatic chondrites that have properties setting them apart
from these groups ...
Unique carbonaceous chondrites provide an important source of new
information with regard to the diversity of materials that formed in the
early solar system. Such information is extremely important because it
furthers our understanding of the processes that formed these meteorites
and underscores the fact that the constraints on nebular and parent body
processes as recorded in existing chondrite groups must constantly be
revised. The discovery of ALH 85085 is an excellent example of this type
of process (Scott, 1988; Weisberg et al., 1988; Grossman et al., 1988).
Best wishes,
Bernd
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