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Supernova graphite in the NanoSIMS: carbon,
oxygen and titanium isotopic compositions of a spherule and
its TiC sub-components
Stadermann F. J., Croat T. K., Bernatowicz T. J., Amari
S., Messenger S., Walker R. M., and Zinner E. (2004)
Geochim. Cosmochim. Acta 69, 177-188.
doi:10.1016/j.gca.2004.06.017
ABSTRACT
Presolar graphite spherules from the Murchison
low-density separate KE3 contain a large number of internal
TiC crystals, which range in size from 15 to 500 nm. We have
studied one such graphite grain in great detail by
successive analyses with SEM, ims3f SIMS, TEM and NanoSIMS.
Isotopic measurements of the bulk particle in the ims3f
indicate a supernova origin for this graphite spherule. The
NanoSIMS measurements of C, N, O and Ti isotopes were
performed directly on TEM ultramicrotome sections of the
spherule, allowing correlated studies of the isotopic and
mineralogical properties of the graphite grain and its
internal crystals. We found isotopic gradients in
12C/13C and
16O/18O from the core of the graphite
spherule to its perimeter, with the most anomalous
compositions being present in the center. These gradients
may be the result of different degrees of isotopic exchange
with isotopically normal material, either in the laboratory
or during the particle's history. No similar isotopic
gradients were found in the 16O/17O
and 14N/15N ratios, which were normal
within analytical uncertainty throughout the graphite
spherule. Due to an unusually high relative O yield,
internal TiC crystals were easily located during NanoSIMS
imaging measurements. It was thus possible to determine
isotopic compositions of several internal TiC grains
independent of the surrounding graphite matrix. These TiC
crystals are significantly more anomalous in their O
isotopes than the graphite, with
16O/18O ratios ranging from 14 to 250
(compared to a terrestrial value of 499). Even the most
centrally located TiC grains show significant variations in
their O isotopic compositions from crystal to crystal.
Measurement of the Ti isotopes in three TiC grains found no
variations among them and no large differences between the
compositions of the different crystals and the bulk graphite
spherule. However, the same three TiC crystals vary by a
factor of 3 in their 16O/18O ratios.
It is not clear in what form the O is associated with the
TiC grains and whether it is cogenetic or the result of
surface reactions on the TiC grains before they accreted
onto the growing graphite spherule. The latter case would
imply that individual TiC grains formed in an environment
with uniform Ti isotopic composition in the supernova
ejecta, but were exposed to a gas with variable
18O/16O ratios before being
incorporated into the graphite spherule. The presence of
short-lived 44Ti in one of the TiC subgrains
confirms the identification of this graphite spherule as a
supernova condensate.
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