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Correlated isotopic and microstructural studies of
turbostratic presolar graphites from the Murchison meteorite.
Croat T. K., Stadermann F. J., and Bernatowicz T.
J. (2008)
Meteorit. Planet. Sci. 43(9), 1497-1516.
ABSTRACT
We present data from TEM and NanoSIMS investigations of
Murchison (CM2) KFC1 presolar graphites. TEM examinations of graphite
ultramicrotome sections reveal varying degrees of graphite disorder,
leading to distinctions between well-graphitized onions, more
turbostratic platy graphites, and the most disordered cauliflower
graphites. Aside from their larger size, platy graphites are roughly
similar in isotopic composition and in internal grain properties to the
well-graphitized onions. Most carbide-containing platy graphites
exhibit large s-process element enrichments (~200x solar Mo/Ti ratios),
suggesting origins predominantly in AGB carbon stars. The C isotopic
distribution of platy graphites is similar to onions, with
representatives in both 12C-depleted (5 < 12C/
13C
< 40) and 12C-enriched groups (100 < 12C/13C
< 350) and a pronounced gap in the 40 < 12C/13C
< 75 region that contains 75% of mainstream SiCs. The large 12C
enrichments combined with the extreme s-process element enrichments
suggest formation in an environment inhomogeneously enriched in the
nucleosynthetic products of thermal pulses in AGB stars. In contrast,
numerous scaly cauliflower graphites show 18O enrichments
and lack s-process-enriched carbides, suggesting a SN origin, as was
the case for many Murchison KE3 SN graphites. The more turbostratic
graphites (platy and scaly) are on average larger than onions, likely
resulting from formation in a gas with higher C number density. Oxygen
content increases progressively with increasing degree of graphite
disorder, which can stabilize these grains against further
graphitization and may be a reflection of higher O/C ratios in their
formation environments.
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