Isotopic, optical, and trace element properties of large single SiC grains from the Murchison meteorite

Journal Article · Tue Mar 31 23:00:00 EST 1992 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(92)90237-D· OSTI ID:5263148

Virag, A; Wopenka, B; Zinner, E ^[1]; Amari, Sachiko ^[2]; Anders, E; Lewis, R S ^[3]

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1. Washington Univ., St. Louis, MO (United States)
2. Univ. of Chicago, IL (United States) Washington Univ., St. Louis, MO (United States)
3. Univ. of Chicago, IL (United States)

Forty-one large SiC grains from the Murchison CM2 chondrite, ranging up to 15 {times} 26 {mu}m, were analyzed by ion probe mass spectrometry for the isotopic compositions of C, N, Mg, and Si, and the concentrations of Al, Ti, V, Fe, Zr, and Ba. Most grains were also examined by Raman spectroscopy. The majority have large isotopic anomalies, with {sup 13}C/{sup 12}C and {sup 14}N/{sup 15}N up to 30X and 9X solar, and {sup 29,30}Si enriched by up to 102{per thousand}. Only two grains, characterized by extremely heavy carbon give evidence for fossil {sup 26}Mg, with ({sup 26}Al/{sup 27}Al){sub 0} ratios of 2.1 {times} 10{sup {minus}3} and 3.9 {times} 10{sup {minus}3}. On the basis of C and Si isotopic composition, twenty-nine of the grains fall into three compact clusters, presumably from three discrete sources. Two of these clusters are anomalous and comprise only grains of cubic structure. The third, isotopically, normal cluster contains only anhedral, noncubic grains; and although contamination cannot be categorically excluded, an origin in a reducing environment in the early solar system is a viable possibility. A few conclusions can be reached about the astrophysical origin of the coarse-grained SiC. The C and N isotopic compositions of the anomalous grains are not very diagnostic, being consistent with H-burning in the CNO cycle. The very existence of SiC requires C-rich stars, of C/O > 1. The Si-isotopic compositions qualitatively show the signature of neutron capture in He-burning shells of highly evolved stars, narrowing the choice to asymptotic giant branch (AGB) or Wolf-Rayet stars. AGB stars are the more likely candidates, as only they can provide high mass loss rates and hence the high gas densities required for growth of large SiC grains.

• OSTI ID: 5263148
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 56:4; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ALKALINE EARTH ISOTOPES
ALKALINE EARTH METALS
ALUMINIUM
BARIUM
CARBON COMPOUNDS
CARBON ISOTOPES
CHEMICAL ANALYSIS
CHEMICAL COMPOSITION
CHEMISTRY
CHONDRITES
ELEMENTS
GEOCHEMISTRY
IRON
ISOTOPE RATIO
ISOTOPES
LASER SPECTROSCOPY
MAGNESIUM ISOTOPES
MASS SPECTROSCOPY
METALS
METEORITES
NITROGEN ISOTOPES
PETROCHEMISTRY
PETROGENESIS
QUANTITATIVE CHEMICAL ANALYSIS
QUANTITY RATIO
RAMAN SPECTROSCOPY
SILICON COMPOUNDS
SILICON ISOTOPES
SPECTROSCOPY
STONE METEORITES
TITANIUM
TRACE AMOUNTS
TRANSITION ELEMENTS
VANADIUM
ZIRCONIUM