In situ survey of graphite in unequilibrated chondrites: Morphologies, C, N, O, and H isotopic ratios

Abstract

We performed in situ morphological and isotopic studies of graphite in the primitive chondrites Khohar (L3), Mezˆ-Madaras (L3), Inman (L3), Grady (H3), Acfer 182 (CH3), Acfer 207 (CH3), Acfer 214 (CH3), and St. Marks (EH5). Various graphite morphologies were identified, including book, veins, fibrous, fine-grained, spherulitic, and granular graphite, and cliftonite. SIMS measurements of H, C, N, and O isotopic compositions of the graphites revealed large variations in the isotopic ratios of these four elements. The δ 15 N and δ 13 C values show significant variations among the different graphite types without displaying any strict correlation between the isotopic composition and morphology. In the Khohar vein graphites, large 15 N excesses are found, with δ 15 Nmax ∼+955‰, confirming previous results. Excesses in 15 N are also detected in fine-grained graphites in chondrites of the CH clan, Acfer 182, Acfer 207, and Acfer 214, with δ 15 N ranging up to +440‰. The 15 N excesses are attributed to ion-molecule reactions at low temperatures in the interstellar molecular cloud (IMC) from which the solar system formed, though the largest excesses seem to be incompatible with the results of some recent calculation. Significant variations in the carbon isotopic ratios are detected between graphite from different chondrite groups, with a tendency for a systematic increase in δ 13 C from ordinary to enstatite to carbonaceous chondrites. These variations are interpreted as being due to small- and large-scale carbon isotopic variations in the solar nebula.