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 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 ...

The End of Interstellar Chemistry as the Origin of Nitrogen in Comets and Meteorites

“…The lack of any significant 13 C enhancement in the material with the largest 15 N content rules out a nucleosynthetic origin for the nitrogen fractionation and suggests that, as with deuterium, the 15 N-rich material results from low-temperature interstellar chemistry (Kerridge, Chang, & Shipp 1987;Messenger & Walker 1997;Alexander et al 1998). Despite the common belief that significant 15 N fractionation could occur in interstellar chemistry, until recently there had been little work on this problem (Adams & Smith 1981).…”

On the Formation of Amide Polymers via Carbonyl–amino Group Linkages in Energetically Processed Ices of Astrophysical Relevance

“…Additionally, more than 80 amino acids were identified in meteorites such as Murcheson and the Lonewolf Nunataks 94102 (Kvenvolden et al 1970;Cronin & Chang 1993;Glavin et al 2006;Elsila et al 2007). A detailed isotopic analysis (D/H; N) provided compelling evidence that these molecules were formed in the ISM (Robert & Epstein 1982;Yang & Epstein 1983;Cronin & Chang 1993;Alexander et al 1998). However, with the exception of formamide (HCONH 2 ; Jones et al 2011), the underlying formation mechanisms of the COM carrying the peptide bond are far from being understood.…”

Graves Nunataks 95209: A snapshot of metal segregation and core formation

“…Lancet and Anders (1970) originally suggested that the large isotopic difference between the carbonates (d 13 C > +40&) and organics (d 13 C < À10&) could be explained by formation of the organics by a FTT process and presented experiments demonstrating a C-isotope fractionation between hydrocarbons and CO 2 of the right magnitude and direction during a FTT process. However, it is now widely accepted that the organics in chondrites have an interstellar heritage (Alexander et al, 1998) and did not form from FTT reactions of nebular CO. Moreover, FTT experiments by Yuen et al (1990) under more realistic nebular conditions than those considered earlier (Lancet and Anders, 1970) indicated much more limited isotopic fractionations, though still in the direction that oxidized C is heavier than reduced C. It is now believed that the carbonates in carbonaceous chondrites formed by precipitation during aqueous alteration on the meteoritesÕ parent asteroids.…”