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Kinetic model of carbonate dissolution in Martian meteorite ALH84001

Kopp, Robert E. and Humayun, Munir (2003) Kinetic model of carbonate dissolution in Martian meteorite ALH84001. Geochimica et Cosmochimica Acta, 67 (17). pp. 3247-3256. ISSN 0016-7037. http://resolver.caltech.edu/CaltechAUTHORS:KOPgca03

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Abstract

The magnetites and sulfides located in the rims of carbonate globules in the Martian meteorite ALH84001 have been claimed as evidence of past life on Mars. Here, we consider the possibility that the rims were formed by dissolution and reprecipitation of the primary carbonate by the action of water. To estimate the rate of these solution-precipitation reactions, a kinetic model of magnesite-siderite carbonate dissolution was applied and used to examine the physicochemical conditions under which these rims might have formed. The results indicate that the formation of the rims could have taken place in < 50 yr of exposure to small amounts of aqueous fluids at ambient temperatures. Plausible conditions pertaining to reactions under a hypothetical ancient Martian atmosphere (1 bar CO2), the modern Martian atmosphere (8 mbar CO2), and the present terrestrial atmosphere (0.35 mbar CO2) were explored to constrain the site of the process. The results indicated that such reactions likely occurred under the latter two conditions. The possibility of Antarctic weathering must be entertained, which, if correct, would imply that the plausibly biogenic minerals (single-domain magnetite of characteristic morphology and sulfide) reported from the rims may be the products of terrestrial microbial activity. This model is discussed in terms of the available isotope data and found to be compatible with the formation of ALH84001 rims. Particularly, anticorrelated variations of radiocarbon with δ13C indicate that carbonate in ALH84001 was affected by solution-precipitation reactions immediately after its initial fall (ca. 13,000 yr ago) and then again during its recent exposure prior to collection.


Item Type:Article
Additional Information:Author postprint. Published version -- Copyright © 2003 Elsevier B.V. Received 16 May 2002; accepted 8 August 2002. Available online 27 August 2003. This paper is dedicated to Robert N. Clayton on the occasion of his retirement from the faculty of The University of Chicago. This study was made possible by the loan of ALH84001 samples by the NASA Johnson Space Center (JSC) and the Meteorite Working Group. We are particularly thankful to Marilyn Lindstrom for her efforts as curator at the JSC. Discussions with Conel Alexander, David Archer, Nabil Z. Boctor, Robert N. Clayton, Joseph Kirschvink, Stephen Mojzsis, Ken Nealson, Andrew Steele, and Benjamin Weiss illuminated various aspects of this work. We thank Oleg S. Pokrovsky and two anonymous reviewers for comments that greatly improved the manuscript and Greg Herzog for editorial handling. Partial support through NASA NAG 5-9800 (M. Humayun) is acknowledged. A Special Issue Dedicated to Robert Clayton. Geochimica et Cosmochimica Acta, Volume 67, Issue 17, Pages 3097-3298 (1 September 2003).
Subject Keywords:Martian meteorites, ALH84001
Record Number:CaltechAUTHORS:KOPgca03
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:KOPgca03
Alternative URL:http://dx.doi.org/10.1016/S0016-7037(02)01114-6
Official Citation:R.E. Kopp and M. Humayun (2003). Kinetic model of carbonate dissolution in Martian meteorite ALH84001. Geochim. Cosmochim. Acta 67, 3247-3256. doi:10.1016/S0016-7037(02)01114-6
Usage Policy:You are granted permission to make copies of the article for your own personal use, including for your own classroom teaching use. Copyright (C) 2003 Elsevier Ltd.
ID Code:7190
Collection:CaltechAUTHORS
Deposited By: Robert E. Kopp
Deposited On:16 Jan 2007
Last Modified:26 Dec 2012 09:29

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