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Episodic carbonate precipitation in the CM chondrite ALH 84049: An ion microprobe analysis of O and C isotopes

Tyra, Mark and Brearley, Adrian and Guan, Yunbin (2016) Episodic carbonate precipitation in the CM chondrite ALH 84049: An ion microprobe analysis of O and C isotopes. Geochimica et Cosmochimica Acta, 175 . pp. 195-207. ISSN 0016-7037. doi:10.1016/j.gca.2015.10.034.

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We have determined the O and C isotope compositions of dolomite grains and the C isotope compositions of calcite grains in the highly altered CM1 chondrite, ALH 84049, using Secondary Ion Mass Spectrometry (SIMS). Chemically-zoned dolomite constitutes 0.8 volume percent (vol%) of the sample and calcite 0.9 vol%. Thirteen separate dolomite grains have δ13C values that range from 37 to 60 (±2) ‰, δ^(18)O values from 25 to 32 (±3) ‰, and δ^(17)O values from 10 to 16 (±3) ‰ (VSMOW). Intragrain δ^(13)C values in dolomite vary up to 10‰. The δ^(13)C values of three calcite grains are distinct from those of dolomite and range from 10 to 13 (±2) ‰ (PDB). Calcite and dolomite appear to record different precipitation episodes. Carbon isotope values of both dolomite and calcite in this single sample encompass much of the reported range for CM chondrites; our results imply that bulk carbonate C and O isotope analyses may oversimplify the history of carbonate precipitation. Multiple generations of carbonates with variable isotope compositions exist in ALH 84049 and, perhaps, in many CM chondrites. This work shows that one should exercise caution when using a clumped isotope approach to determine the original temperature and the isotopic compositions of water for CM chondrite carbonates. Less altered CM meteorites with more-homogeneous C isotope compositions, however, may be suitable for bulk-carbonate analyses, but detailed carbonate petrologic and isotopic characterization of individual samples is advised.

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Guan, Yunbin0000-0002-7636-3735
Additional Information:© 2016 Published by Elsevier Ltd. Received 5 March 2015, Accepted 29 October 2015, Available online 27 November 2015. This work was supported by NASA Grant NNG06GG37G and NNX11AK51G to A.J. Brearley (PI), the N.M. Space Grant (M.A. Tyra), and a 2010 Meteoritical Society student travel award (M.A. Tyra). I am grateful for discussions with Scott Jasechko, Maarten de Moor, and Rena Ford. Electron microscopy and electron microprobe analyses were carried out in the Electron Microbeam Analysis Facility, Department of Earth and Planetary Sciences, University of New Mexico, a facility supported by the State of New Mexico. We are particularly grateful to Mike Spilde for his assistance with the SEM and electron microprobe. US Antarctic meteorite samples are recovered by the Antarctic Search for Meteorites (ANSMET) program which has been funded by NSF and NASA, and characterized and curated by the Department of Mineral Sciences of the Smithsonian Institution and Astromaterials Curation Office at NASA Johnson Space Center. Finally, we acknowledge the hard work of Conel Alexander, Monica Grady, Chris Herd (AE) and an anonymous reviewer whose work extensively improved the manuscript.
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New Mexico Space Grant ConsortiumUNSPECIFIED
Meteoritical SocietyUNSPECIFIED
Record Number:CaltechAUTHORS:20160218-110004369
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Official Citation:Mark Tyra, Adrian Brearley, Yunbin Guan, Episodic carbonate precipitation in the CM chondrite ALH 84049: An ion microprobe analysis of O and C isotopes, Geochimica et Cosmochimica Acta, Volume 175, 15 February 2016, Pages 195-207, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:64553
Deposited By: Tony Diaz
Deposited On:18 Feb 2016 23:24
Last Modified:10 Nov 2021 23:32

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