|
Extreme Si-29 and Si-30 enrichments in SiC-containing
graphites from the Murchison meteorite.
Croat T. K. and Stadermann F. J. (2008)
Bull. Am. Astron. Soc. 40, AAS 212th Meeting, #03-32.
ABSTRACT
Coordinated TEM and NanoSIMS isotopic studies have revealed
two SiCs found as internal grains within graphite from the Murchison
meteorite. These composite grains (e.g. SiCs found within graphite) are
of particular interest, as they are a clear case in which both
carbonaceous presolar phases form from the same star. Further, NanoSIMS
isotopic analyses reveal large 29Si and 30Si
enrichments in both SiCs; the most extreme (#5) with δ29Si=1290±50
‰, δ30Si=1040±50 ‰ and 12C/13C=240±9
and the other (#6) with δ29Si=120±30 ‰, δ30Si=380±45
‰ and 12C/13C=123±4 (all with 1 σ
errors).
Si anomalies of the magnitude seen in SiC #5 (e.g. both δ29Si
and δ30Si > +1000 ‰) have only been seen in one other
unusual SiC out of over 20,000 measured grains. Such extreme
anomalies indicate a massive star origin (either Type II SN or
Wolf-Rayet). Although the less extreme SiC #6 does fall near the
isotopic range of SiC-Z grains (of postulated low-metallicity AGB
origin), that it is also found within graphite suggests its origin may
be the same as that of #5. Despite their isotopic similarities, the two
SiCs show differences in their degree of disorder, crystallite domain
size, and their position within the host graphite. The properties of
SiCs #5 and #6, as well as TEM and C isotopic data from other
SiC-containing graphites, suggest that these SiCs with graphites
are different from the overall presolar SiC population.
|