CARBON ISOTOPIC COMPOSITIONS OF ANTARCTIC CARBONACEOUS CHONDRITES WITH RELEVANCE TO THE ALTERATION AND EXISTENCE OF ORGANIC-MATTER

Bulk carbon isotopic compositions of 26 Antarctic carbonaceous chondri tes (33 specimens) are reported and discussed with relevance to the pr ocesses involved in their alteration and the existence of solvent-extr actable organic matter. The delta(13)C (relative to PDB) vary from -16 .6 to +0.9 parts per thousand. The average value (-6.2 parts per thous and) is higher than that for non-Antarctic carbonaceous chondrites by similar to 6 parts per thousand. The difference may be explained by co ntamination of terrestrial organic carbon on non-Antarctic chondrites and/or different populations of parent bodies. Most Antarctic carbonac eous chondrites have an apparent trend (''a major sequence'') that the delta(13)C value becomes higher with increasing carbon content. The t rend is likely to be explained by a mixing model of two components; is otopically heavy and labile component (solvent-extractable organic mat ter and carbonate), and isotopically light and inert acid-insoluble co mponent (kerogen-like matter). In addition, an unique group (altered s pecimens) is characterized by relatively high content and isotopically light carbon. For seven CM chondrites, delta(13)C values of residues after the hot H2O and HC1/HF treatment are also reported. Despite cons iderable isotopic variations of bulk carbon, the delta(13)C values of acid-insoluble residues may be classified into two groups; isotopicall y light (-15 to -13 parts per thousand) and heavy (-9 to -7 parts per thousand) ones. Most kerogen-like matters belong to the light group, w hich are isotopically similar to non-Antarctic kerogen-like matter. Th e altered specimens have isotopically light bulk carbon (similar to-15 parts per thousand), which is similar to those of their kerogen-like matter; they probably lost heavy components such as solvent-extractabl e organic matters (e.g., amino acid and carboxylic acid) and carbonate s 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.