THE ORIGIN OF CHONDRITIC MACROMOLECULAR ORGANIC-MATTER - A CARBON ANDNITROGEN ISOTOPE STUDY

The N and C abundances and isotopic compositions of acid-insoluble car bonaceous material in thirteen primitive chondrites (five unequilibrat ed ordinary chondrites, three CM chondrites, three enstatite chondrite s, a CI chondrite and a CR chondrite) have been measured by stepped co mbustion. While the range of C isotopic compositions observed is only similar to delta(13)C = 30 parts per thousand, the N isotopes range fr om delta(15)N approximate to -40 to 260 parts per thousand. After corr ection for metamorphism, presolar nanodiamonds appear to have made up a fairly constant 3-4 wt% of the insoluble C in all the chondrites stu died. The apparently similar initial presolar nanodiamond to organic C ratios, and the correlations of elemental and isotopic compositions w ith metamorphic indicators in the ordinary and enstatite chondrites, s uggest that the chondrites all accreted similar organic material. This original material probably most closely resembles that now found in R enazzo and Semarkona. These two meteorites have almost M-shaped N isot ope release profiles that can be explained most simply by the superpos ition of two components, one with a composition between delta(15)N = - 20 and -40 parts per thousand and a narrow combustion interval, the ot her having a broader release profile and a composition of delta(15)N a pproximate to 260 parts per thousand. Although isotopically more subdu ed, 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-insolu ble organics formed remains poorly constrained. The small variation in nanodiamond to organic C ratio between the chondrite groups limits th e local synthesis of organic matter in the various chondrite formation regions to at most 30%. The most N-15-rich material probably formed i n 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 nanodia mond to total organic C ratios in the chondrite groups is consistent w ith most of the organic material being presolar.