Extreme Si-29 And Si-30 Enrichments In SiC-containing Graphites From The Murchison Meteorite

Details Extreme Si-29 And Si-30 Enrichments In SiC-containing Graphites From The Murchison Meteorite Extreme Si-29 And Si-30 Enrichments In SiC-containing Graphites From The Murchison Meteorite

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...212.0332c&link_type=abstract

American Astronomical Society, AAS Meeting #212, #3.32; Bulletin of the American Astronomical Society, Vol. 40, p.190

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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 sigma 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 [1] 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 [2], suggest that these SiCs with graphites are different from the overall presolar SiC population. [1] Amari S. et al. (1999) ApJ. 517, L59. [2] Croat T.K. and Stadermann F.J. (2008) Lunar Planet. Sci. XXXIX, 1739.

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