INTERSTELLAR GRAPHITE IN METEORITES - ISOTOPIC COMPOSITIONS AND STRUCTURAL-PROPERTIES OF SINGLE GRAPHITE GRAINS FROM MURCHISON

One hundred forty-three carbon grains, ranging in size from 2 to and 8 mu m, from two chemical and physical separates from the Murchison CM2 chondrite, were analyzed by ion microprobe mass spectrometry for their C- and N-isotopic compositions. Both separates are enriched in the exotic noble gas component Ne-E(L). Ninety grains were also analyzed for their H and O contents and 118, for Si. Thirteen grains were analyzed by micro-sampling laser Raman spectroscopy. Round grains have large C-isotopic anomalies with C-12/C-13 ratios ranging from 7 to 4500 (terrestrial ratio = 89). Nitrogen in these grains is also anomalous but shows much smaller deviations from the terrestrial composition, N-14/N-15 ratios ranging from 193 to 680 (terrestrial ratio = 272). Spherulitic aggregates and non-round compact grains have normal C-isotopic ratios but N-15 excesses (up to 35%). Raman spectra of the analyzed grains indicate varying degrees of crystalline disorder of graphite with estimated in-plane crystallite dimensions varying from 18 Angstrom (highly disordered, similar to terrestrial kerogen) to similar to 750 Angstrom (well-crystallized graphite). Element contents of H, O, and Si are correlated with one another, and H and O are probably present in the form of organic molecules. On the basis of morphology, the round grains fall into two groups: grains with smooth, shell-like surfaces (''onions'') and grains that appear to be dense aggregates of small scales (''cauliflowers''). ''Onions'' tend to have lower trace element contents, isotopically light C (C-12/C-13 > 89) and a high degree of crystalline order, whereas ''cauliflowers'' have a larger spread in trace element contents and C-isotopic ratios (they range from isotopically light to heavy) but tend to have a low degree of crystalline order. However, these differences exist only on average, and no clear distinction can be made for individual grains. A few limited conclusions can be drawn about the astrophysical origin of the carbon grains of this study. The N-15 excesses in spherulitic aggregates and non-round grains can be explained as the result of ion-molecule reactions in molecular clouds. The round grains, on the other hand, must have formed in stellar atmospheres (circumstellar grains). Grains with isotopically light C must have formed in stellar environments characterized by He-burning, either in the atmosphere of Wolf-Rayet stars during the WC phase or in the He-burning, C-12-rich zone of a massive star, ejected by a supernova explosion. Isotopically heavy C is produced by H-burning in the CNO cycle. Possible sources for grains with heavy C are carbon stars (AGE stars during the thermally pulsing phase) or novae, but the detailed distribution of C-12/C-13 ratios agree neither with the distribution observed in carbon stars nor with theoretical predictions for these two types of stellar sources.