Carbon isotopic compositions of Antarctic carbonaceous chondrites with relevance to the alteration and existence of organic matter

Hiroshi Naraoka, Akira Shimoyama, Osamu Matsubaya, Kaoru Harada
1997 Geochemical Journal  
Bulk carbon isotopic compositions of 26 Antarctic carbonaceous chondrites (33 specimens) are re ported and discussed with relevance to the processes involved in their alteration and the existence of solvent-extractable organic matter. The 613C values (relative to PDB) vary from -16 .6 to +0.9%0. The average value (-6.2%0) is higher than that for non-Antarctic carbonaceous chondrites by -6%0 . The difference may be explained by contamination of terrestrial organic carbon on non-Antarctic
more » ... es and/or different populations of parent bodies. Most Antarctic carbonaceous chondrites have an apparent trend ("a major sequence") that the S13C value becomes higher with increasing carbon content . The trend is likely to be explained by a mixing model of two components; isotopically heavy and labile component (solvent-ex tractable organic matter and carbonate), and isotopically light and inert acid-insoluble component (kerogen like matter). In addition, an unique group (altered specimens) is characterized by relatively high content and isotopically light carbon. For seven CM chondrites, S13C values of residues after the hot H20 and HCl/HF treatment are also reported. Despite considerable isotopic variations of bulk carbon , the 613C values of acid-insoluble resi dues may be classified into two groups; isotopically light (-15 to -13%0) and heavy (-9 to -7%0) ones . Most kerogen-like matters belong to the light group, which are isotopically similar to non-Antarctic kero gen-like matter. The altered specimens have isotopically light bulk carbon (-15%0), which is similar to those of their kerogen-like matter; they probably lost heavy components such as solvent-extractable organic matters (e.g., amino acid and carboxylic acid) and carbonates during aqueous alteration and thermal metamorphism on parent bodies. This is in accord with mineralogical and oxygen isotope studies . The presence of solvent-extractable organic matter is one of the important factors controlling bulk carbon isotopes in carbonaceous chondrites.
doi:10.2343/geochemj.31.155 fatcat:4cw7hfsdmfd35fj4skfg7e4gua