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

Abstract

Experiments on a Martian basalt composition show that DV augite/melt is greater than DV pigeonite/melt in samples equilibrated under the same fO2 conditions. This increase is due to the increased availability of elements for coupled substitution with the V3+ or V4+ ions, namely Al and Na. For this bulk composition, both Al and Na are higher in concentration in augite compared with pigeonite; therefore more V can enter augite than pigeonite. Direct valence state determination by XANES shows that the V3+ and V4+ are the main V species in the melt at fO2 conditions of IW-1 to IW+3.5, whereas pyroxene grains at IW-1, IW, and IW+1 contain mostly V3+. This confirms the idea that V3+ is more compatible in pyroxene than V4+. The XANES data also indicates that a small percentage of V2+ may exist in melt and pyroxene at IW-1. The similar valence of V in glass and pyroxene at IW-1 suggests that V2+ and V3+ may have similar compatibilities in pyroxene.