PRESOLAR SIC IN CHONDRITES - HOW VARIABLE AND HOW MANY SOURCES Journal Article uri icon

DCO ID 11121/7107-5655-3473-3737-CC

in language

  • eng

year of publication

  • 1993

abstract

  • The carbon and silicon isotopic compositions of 246 isotopically anomalous SiC grains found in relatively low concentration acid residues prepared from nine chondrites are reported. However, two of these residues, from Leoville (CV3) and Qingzhen (EH3), only produced one anomalous SiC grain each. The other meteorites studied included Murchison and six UOCs. The results for Murchison are broadly similar to those found by previous Murchison SiC studies. However, no two Murchison studies are identical, implying that despite the number of grains analyzed to date, we still do not have a representative sample of Murchison SiC. Statistical comparisons between the SiC in Murchison and other meteorites are also hampered by the small sample sizes relative to the range of isotopic compositions observed. Nevertheless, of the six UOCs studied only the results obtained from Inman are clearly different from Murchison. Several previous studies have concluded that most of the SiC formed in AGB stars, although the silicon isotopes remain problematic. However, few, if any, C-rich AGB stars have C-12/C-13 ratios less than 20. Those SiC grains with the isotopically heaviest carbon (C-12/C-13 < 20) may have come from s-process poor J- and R-type carbon stars.
  • The carbon and silicon isotopic compositions of 246 isotopically anomalous SiC grains found in relatively low concentration acid residues prepared from nine chondrites are reported. However, two of these residues, from Leoville (CV3) and Qingzhen (EH3), only produced one anomalous SiC grain each. The other meteorites studied included Murchison and six UOCs. The results for Murchison are broadly similar to those found by previous Murchison SiC studies. However, no two Murchison studies are identical, implying that despite the number of grains analyzed to date, we still do not have a representative sample of Murchison SiC. Statistical comparisons between the SiC in Murchison and other meteorites are also hampered by the small sample sizes relative to the range of isotopic compositions observed. Nevertheless, of the six UOCs studied only the results obtained from Inman are clearly different from Murchison. Several previous studies have concluded that most of the SiC formed in AGB stars, although the silicon isotopes remain problematic. However, few, if any, C-rich AGB stars have C-12/C-13 ratios less than 20. Those SiC grains with the isotopically heaviest carbon (C-12/C-13 < 20) may have come from s-process poor J- and R-type carbon stars. Four small groups of grains, which appear to require a minimum of four distinct SiC sources, have been identified in a compilation of 308 individual SiC analyses but the overall lack of groupings in the compilation suggest that either there were a large number of stellar sources or that there were a few sources whose isotopic compositions varied considerably. Simple calculations suggest that one could expect 10 to 100 AGB stars to have contributed SiC to a protosolar system embedded in a mature molecular cloud. Even a few 10s of sources are enough to explain the apparent differences seen in the various studies of Murchison SiC and, with the exception of Inman, the differences observed from meteorite to meteorite. This number of sources would also appear to favor galactic chemical evolution as the explanation of the slope 1.3 array formed by SiC in silicon three isotope plots rather than explanations which appeal to a single star or rare high mass AGB stars.

volume

  • 57