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Late Reduction Textures in Almahata Sitta UreiliteThe Almahata Sitta ureilite, derived from asteroid 2008 TC3, consists of many individual fragments recovered from the Nubian dessert strewn field [1]. Like most ureilites, it contains abundant carbon and exhibits examples of disequilibrium textures that record a late reduction event accompanied by rapid cooling (tens of degC/h) from high temperatures (1150-1300 C). Variations in Fe/Mg of silicate minerals are accompanied by variations in Fe/Mn, indicating loss of Fe into metal [2]. In coarser-grained fragments of Almahata Sitta, olivine exhibits irregular high mg# rims in contact with networks of interstitial metal 5- 20 microns in typical thickness. This is a common ureilite texture thought to be driven by the reaction of graphite to a CO gas phase and the concurrent reduction of FeO in olivine to Fe metal, with excess silica going primarily into pyroxene (2MgFeSiO4 + C approaches MgSiO4 + MgSiO3 + 2Fe + CO) [3, see also 4,5,6]. Other fragments of Almahata Sitta exhibit anomalous textures such as fine grain size, high porosity, and abundant graphite. Within these fragments pyroxene locally exhibits high-mg# rims in contact with metal and a discreet silica phase, suggesting that the reduction mechanism MgFeSi2O6 + C approaches MgSiO3 + Fe + SiO2 + CO. Metals in Almahata Sitta are particularly unaltered in comparison to ureilite finds. Variations in minor and trace element composition of this metal might partly result from localized dilution as iron is supplied by reduction of silicates.
Document ID
20090033121
Acquisition Source
Johnson Space Center
Document Type
Conference Paper
Authors
Herrin, J. S.
(NASA Johnson Space Center Houston, TX, United States)
Le, L.
(NASA Johnson Space Center Houston, TX, United States)
Zolensky, M. E.
(NASA Johnson Space Center Houston, TX, United States)
Ito, M.
(NASA Johnson Space Center Houston, TX, United States)
Jenniskens, P.
(Search for Extraterrestrial Intelligence Inst. Mountain View, CA, United States)
Shaddad, M. H.
(Khartoum Univ. Sudan)
Date Acquired
August 24, 2013
Publication Date
January 1, 2009
Subject Category
Chemistry And Materials (General)
Report/Patent Number
JSC-CN-18801
JSC-CN-18799
Meeting Information
Meeting: 41st annual meeting of the Division of Planetary Sciences of the American Astronomical Society
Location: Puerto Rico, NM
Country: United States
Start Date: October 4, 2009
End Date: October 9, 2009
Distribution Limits
Public
Copyright
Other

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