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Origins of Magnetite Nanocrystals in Martian Meteorite ALH84001The Martian meteorite ALH84001 preserves evidence of interaction with aqueous fluids while on Mars in the form of microscopic carbonate disks. These carbonate disks are believed to have precipitated 3.9 Ga ago at beginning of the Noachian epoch on Mars during which both the oldest extant Martian surfaces were formed, and perhaps the earliest global oceans. Intimately associated within and throughout these carbonate disks are nanocrystal magnetites (Fe3O4) with unusual chemical and physical properties, whose origins have become the source of considerable debate. One group of hypotheses argues that these magnetites are the product of partial thermal decomposition of the host carbonate. Alternatively, the origins of mag- netite and carbonate may be unrelated; that is, from the perspective of the carbonate the magnetite is allochthonous. For example, the magnetites might have already been present in the aqueous fluids from which the carbonates were believed to have been deposited. We have sought to resolve between these hypotheses through the detailed characterized of the compo- sitional and structural relationships of the carbonate disks and associated magnetites with the orthopyroxene matrix in which they are embedded. Extensive use of focused ion beam milling techniques has been utilized for sample preparation. We then compared our observations with those from experimental thermal decomposition studies of sideritic carbonates under a range of plausible geological heating scenarios. We conclude that the vast majority of the nanocrystal magnetites present in the car- bonate disks could not have formed by any of the currently proposed thermal decomposition scenarios. Instead, we find there is considerable evidence in support of an alternative allochthonous origin for the magnetite unrelated to any shock or thermal processing of the carbonates.
Document ID
20090038967
Acquisition Source
Johnson Space Center
Document Type
Preprint (Draft being sent to journal)
Authors
Thomas-Keprta, Kathie L.
(Jacobs Technologies Engineering Science Contract Group Houston, TX, United States)
Clemett, Simon J.
(Jacobs Technologies Engineering Science Contract Group Houston, TX, United States)
Mckay, David S.
(NASA Johnson Space Center Houston, TX, United States)
Gibson, Everett K.
(ARES Corp. Houston, TX, United States)
Wentworth, Susan J.
(Jacobs Technologies Engineering Science Contract Group Houston, TX, United States)
Date Acquired
August 24, 2013
Publication Date
January 1, 2009
Publication Information
Publication: Geochimica et Cosmochimica Acta
Publisher: Elsevier
Volume: 73
Issue: 21
ISSN: 0016-7037
Subject Category
Geophysics
Report/Patent Number
JSC-CN-19246
Distribution Limits
Public
Copyright
Other

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