Based on the partitioning of Cr and V between pigeonite cores and bulk composition, we estimate that martian basalt QUE 94201 crystallized at an fO2 between IW+0.2 and IW+0.9. These estimates are based on calibration curves for DCr, DV, and DCr/DV (pyroxene/melt) derived from experimental charges that were synthesized at fO2 conditions of IW-1, IW, and IW+1. We believe our fO2 estimate is robust because (1) the fO2 is measured in the earliest crystallizing pyroxenes; (2) the calibration curves are based on the same bulk composition as the natural sample; and (3) that bulk composition represents a melt from the martian mantle, so an accurate DCr and DV are measured. Presently, the two best candidates for martian melts, Y 980459 and QUE 94201, indicate an fO2 of IW to IW+1 for the upper martian mantle.

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