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American Mineralogist; November 2007; v. 92; no. 11-12; p. 2002-2005; DOI: 10.2138/am.2007.2724
© 2007 Mineralogical Society of America
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Letter

Valence state partitioning of Cr between pyroxene-melt: Effects of pyroxene and melt composition and direct determination of Cr valence states by XANES. Application to Martian basalt QUE 94201 composition

J.M. Karner1,*, J.J. Papike1, S.R. Sutton2,3, C.K. Shearer1, G. McKay4, L. Le5 and P. Burger1

1 Astromaterials Institute, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
2 Department of Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, U.S.A.
3 Consortium for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.A.
4 Mail Code ST, NASA JSC, Houston, Texas, 77058, U.S.A.
5 ESC Group, JE23, Houston, Texas, 77058, U.S.A.

Correspondence: * E-mail: jkarner{at}unm.edu

DCr augite/melt is approximately double that of DCr pigeonite/melt in synthetic Martian basaltic samples equilibrated at the same fO2. This increase is not related to changing fO2 and the valence of Cr, but rather to the increased availability of elements for coupled substitution with the Cr3+ ion, namely Na and Al. The availability of Al and Na to partition into pyroxene is due to delayed nucleation of plagioclase for the QUE 94201 Martian basalt composition. Direct valence state determination by XANES shows that Cr3+ is the dominant valence state in pyroxene at IW-1, IW, and IW+1. Trivalent Cr is apparently much more compatible in the pyroxene structure than divalent Cr, and thus an increasing DCr for both augite/melt and pigeonite/melt with increasing fO2 is a function of the increased activity of Cr3+ in the crystallizing melt.

Key Words: Chromium • partitioning • multivalent elements • pyroxene • XANES • QUE 94201






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