The origin of chondritic macromolecular organic matter: A carbon and nitrogen isotope study

Abstract

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 ∼δ 13 C = 30%, the N isotopes range from δ 15 N ' ‐40 to 260%. 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 δ 15 N = ‐20 and ‐40% and a narrow combustion interval, the other having a broader release profile and a composition of δ 15 N ∼ 260%. 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 15 N‐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.