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In situ survey of graphite in unequilibrated
chondrites: Morphologies, C, N, O, and H isotopic
ratios
Mostefaoui S., Zinner E., Hoppe P., Stadermann F.
J., and El Goresy A. (2003) Meteorit. Planet. Sci.
40, 721-743.
ABSTRACT
We performed in situ morphological and isotopic studies
of graphite in the primitive chondrites Khohar (L3),
Mezö-Madaras (L3), Inman (L3), Grady (H3), Acfer 182
(CH3), Acfer 207 (CH3), Acfer 214 (CH3), and St. Marks (E4).
Various graphite morphologies were identified, including
book, veins, fibrous, fine-grained, spherulitic, granular,
and cliftonite. SIMS measurements of H-, C-, N-, and
O-isotopic compositions of the graphites revealed large
variations in the isotopic ratios of these four elements.
The delta 15N and delta 13C values
show significant variations among the different graphite
types without displaying any strict correlation between the
isotopic composition and morphology. In the Khohar vein
graphites, large 15N excesses are found, with
delta 15Nmax ~+955‰, confirming
previous results. Excesses in 15N are also
detected in fine-grained graphites in chondrites of the CH
clan, Acfer 182, Acfer 207, and Acfer 214, with delta
15N ranging up to +440‰. The 15N
excesses are attributed to an interstellar origin. They are
likely produced by ion-molecule reactions at low
temperatures in the interstellar molecular cloud (IMC) from
which the solar system formed. Significant variations in the
carbon isotopic ratios are detected between graphite from
different chondrite groups, with a tendency for a systematic
increase in delta 13C from ordinary to enstatite
to carbonaceous chondrites. These variations are interpreted
as being due to small- and large-scale carbon isotopic
variations in the solar nebula.
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