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D-depleted organic matter and graphite in the Abee enstatite chondrite

Remusat, L. and Rouzaud, J.-N. and Charon, E. and Le Guillou, C. and Guan, Y. and Eiler, J. M. (2012) D-depleted organic matter and graphite in the Abee enstatite chondrite. Geochimica et Cosmochimica Acta, 96 (1). pp. 319-335. ISSN 0016-7037. doi:10.1016/j.gca.2012.07.031.

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A combination of NanoSIMS and High resolution transmission electron microscopy (HRTEM) imaging along with Raman spectroscopy was used to characterize the carbonaceous phases in HF/HCl residue of the Abee enstatite chondrite. This acid residue hosts a very D-depleted component (δD = −480‰). This residue is a mixture of graphite and highly disordered insoluble organic matter. The latter exhibits a significant mesoporosity (i.e., 200–500 nm scale), and also shows concentric and elongated stacks of polyaromatic layers. Insoluble organic matter is shown to be the most D-depleted component in Abee. We also determined, by using NanoSIMS, carbon isotopic composition of graphite and insoluble organic matter in the acid residue (δ^(13)C = −11.3 ± 2.9‰ and −28.4 ± 2.2‰, respectively). We identified graphite in metal-rich clasts and in the matrix of Abee, associated with enstatite, sulfide and metal, but we could not localize highly disordered organic matter in our section. Regardless, given the vulnerability of organic matter to thermal degradation, we suggest that it was added to Abee parent body during the latest stage of its formation, after any thermal metamorphism or partial melting of Abee parent body. A genetic link between organic matter and graphite in Abee is excluded based on our HRTEM and carbon isotopic data. The differences in carbon isotopic compositions between these phases are consistent with previous data obtained by stepwise heating experiments and indicate that graphite is not derived from a pure thermal solid-state graphitization of the organic matter. Rather, we suggest that graphite precipitated from a melt rich in C during the partial melting of the Abee parent body. Insoluble organic matter in Abee has the lowest D/H ratio among the extraterrestrial organics. Organics in most carbonaceous and ordinary chondrites are believed to have been subjected to irradiations in low temperature environments, resulting in a dramatic isotopic fractionation through exchange and reaction with gaseous molecular hydrogen during their synthesis. In contrast, Abee organic matter was likely synthesized in a neutral (i.e., not ionized) environment where thermodynamic processes at equilibrium most likely controlled its isotopic composition. This organic matter could have been accreted in a minor component of Abee like the dark inclusions without (or prior to) exposure to the radiation responsible for D enrichments in other meteoritic organics. During the last brecciation events that have affected the Abee parent body, these inclusions could have been mixed with other Abee components. The properties of this organic matter can be interpreted as an indication that thermodynamic processes acted in the synthesis of organic matter in the protosolar disk, in addition to ion/molecule and gas/grains reaction witnessed by the D-rich insoluble organic matter contained in carbonaceous chondrites.

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Guan, Y.0000-0002-7636-3735
Additional Information:© 2012 Elsevier Ltd. Received 11 October 2011; accepted in revised form 18 July 2012; available online 2 August 2012. Associate Editor: Sara S. Russell. We are grateful to M. Garcia (curator of the Caltech GPS sample collection) for having kept preciously the acid residues prepared 30 years ago. Michèle Bourot-Denise and Brigitte Zanda are thanked for providing Abee polished section and for helpful discussions. We also thank A. Rubin, S. Russell and two anonymous referees for their helpful reviews and suggestions. L.R. thanks the Programme National de Planétologie (PNP) of CNRS/INSU for financial support.
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Programme National de Planetologie (PNP)UNSPECIFIED
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Record Number:CaltechAUTHORS:20121203-111703108
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Official Citation:L. Remusat, J.-N. Rouzaud, E. Charon, C. Le Guillou, Y. Guan, J.M. Eiler, D-depleted organic matter and graphite in the Abee enstatite chondrite, Geochimica et Cosmochimica Acta, Volume 96, 1 November 2012, Pages 319-335, ISSN 0016-7037, 10.1016/j.gca.2012.07.031. (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35766
Deposited By: Ruth Sustaita
Deposited On:03 Dec 2012 19:38
Last Modified:09 Nov 2021 23:17

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