Abstract-The N and C abundances and isotopic compositions of acid-insoluble carbonaceous material in thirteen primitive chondrites (five unequilibrated ordinary chondrites, three CM chondrites, three enstatite chondrites, a CI chondrite and a CR chondrite) have been measured by stepped combustion. While the range of C isotopic compositions observed is only 413C = 30%0, the N isotopes range from 6I5N = 4 0 to 260%0.After correction for metamorphism, presolar nanodiamonds appear to have made up a fairly constant 3 -4 wt% of the insoluble C in all the chondrites studied. The apparently similar initial presolar nanodiamond to organic C ratios, and the correlations of elemental and isotopic compositions with metamorphic indicators in the ordinary and enstatite chondrites, suggest that the chondrites all accreted similar organic material. This original material probably most closely resembles that now found in Renazzo and Semarkona. These two meteorites have almost M-shaped N isotope release profiles that can be explained most simply by the superposition of two components, one with a composition between 615N = -20 and 40%0 and a narrow combustion interval, the other having a broader release profile and a composition of d15N = 260%0. Although isotopically more subdued, the CI and the three CM chondrites all appear to show vestiges of this M-shaped profile. How and where the components in the acid-insoluble organics formed remains poorly constrained. The small variation in nanodiamond to organic C ratio between the chondrite groups limits the local synthesis of organic matter in the various chondrite formation regions to at most 30%. The most IsN-rich material probably formed in the interstellar medium, and the fraction of organic N in Renazzo in this material ranges from 40 to 70%. The isotopically light component may have formed in the solar system, but the limited range in nanodiamond to total organic C ratios in the chondrite groups is consistent with most of the organic material being presolar. INTRODUCTIONThe majority of C in primitive chondritic meteorites is present as organic material. The synthesis of this material is poorly understood, and none of the formation models proposed thus far have been able to explain all of the observed chemical and isotopic features. The organic material in CI and CM chondrites, the most extensively studied of the chondrite groups, can be broadly divided into a soluble fraction and a much more abundant (70-95%) insoluble fraction (Becker and Epstein, 1982). The soluble fraction is a mixture that includes amino acids as well as aliphatic and aromatic hydrocarbons.The insoluble fraction is a complex macromolecular material, rich in polycyclic aromatic hydrocarbons (PAH), which has often been described as kerogen-like.For many years, the most popular hypothesis for the origin of organic material in CI and CM carbonaceous chondrites was via Fischer-Tropsch-type (FTT) synthesis in the solar nebula (e.g., Hayatsu and Anders, 1981). It was envisaged that, as the nebula cooled to 30WOO K ...

Abstract-The Saharan meteorite Acfer 094 is a unique type of carbonaceous chondrite. Mineralogical and petrological considerations and 0 isotopes are unable to distinguish unambiguously between a C03 VS. CM2 classification. The other important light elements, C and N, have systematics that do not match any previously recognised meteorite group. Particularly important in this respect is the very low CIN ratio and ol3C of the macromolecular C. Acfer 094 has more diamond and SiC, especially X type grains, than any other specimen studied, suggesting minimal thermal or aqueous processing to decrease its very primitive status.

Abstract-Diamonds isolated from primitive chondrites of the carbonaceous, ordinary and enstatite groups have been analysed by high-resolution stepped combustion, followed by measurement of their C and N isotopes using a newly adapted technique that allows quantitative measurements of C/N ratios. The~13C of the diamond is shown to vary between meteorite groups from -32 to -38%0, and the measured C/N ratios suggest that the N concentration of diamond ranges over a factor of 7 from 1800 ppm (Tieschitz) to 13,000 ppm (Adrar 003). The b 15N ofN released from diamond is constrained to -348 ± 7%0. The complexity of the C release pattern and C/N ratio during combustion implies the presence of more than one component, which suggests that either more than one type of diamond is present in the samples, or unidentified additional phases are located in the acid-resistant residue. The components are present in varying proportions between meteorite groups. The data are compatible with a model of a mix of different diamond populations (some probably presolar and some possibly solar) existing in the early solar nebula, where each population originally contributed a roughly equal amount to chondrites of every class. Subsequent metamorphism has resulted in overall variations in~13C and C/N ratios in diamond isolated from meteorites of differing petrologic grade without significantly altering the N isotopic composition. Possible ways for this to be achieved are explored.

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