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

available online at http://meteoritics.org 721 © The Meteoritical Society, 2005. Printed in USA. In situ survey of graphite in unequilibrated chondrites: Morphologies, C, N, O, and H isotopic ratios Smail MOSTEFAOUI1*, Ernst ZINNER2, Peter HOPPE1, Frank J. STADERMANN2, and Ahmed EL GORESY1 1Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany 2Laboratory for Space Sciences and Physics Department, Washington University, St. Louis, Missouri 63130, USA *Corresponding author. E-mail: [email protected] (Received 13 October 2003; revision accepted 20 March 2005) 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 δ15N and δ13C 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 15N excesses are found, with δNmax ∼+955‰, confirming previous results. Excesses in 15N are also detected in fine-grained graphites in chondrites of the CH clan, Acfer 182, Acfer 207, and Acfer 214, with δ15N ranging up to +440‰. The 15N 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 δ13C from ordinary to enstatite to carbonaceous chondrites. These variations are interpreted as being due to smalland large-scale carbon isotopic variations in the solar nebula.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 δ15N and δ13C 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 15N excesses are found, with δNmax ∼+955‰, confirming previous results. Excesses in 15N are also detected in fine-grained graphites in chondrites of the CH clan, Acfer 182, Acfer 207, and Acfer 214, with δ15N ranging up to +440‰. The 15N 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 δ13C from ordinary to enstatite to carbonaceous chondrites. These variations are interpreted as being due to smalland large-scale carbon isotopic variations in the solar nebula.