Chromium in olivine in all eleven samples was found to be dominated by the divalent species, with valences ranging from 2.10 卤 0.02 (1蟽) to 2.46 卤 0.04. The non-chromite-bearing ureilites have the most reduced Cr, with a weighted mean valence of 2.12 卤 0.01, i.e., Cr2+/Cr3+ = 7.33. All low-Fo chromite-bearing ureilites have more oxidized Cr, with valences ranging from 2.22 卤 0.03 to 2.46 卤 0.04. EET 96328, whose chromite grain we interpret as a late-crystallizing phase, yielded a reduced Cr valence of 2.15 卤 0.07, similar to the non-chromite-bearing samples. Based on the measured Cr valences, magmatic (1200-1300 掳C) oxygen fugacities (fO2) of the non-chromite-bearing samples were estimated to be in the range IW鈭?.9 to IW鈭?.8 (assuming basaltic melt composition), consistent with fO2 values obtained by assuming olivine-silica-iron metal (OSI) equilibrium. For the primary chromite-bearing-ureilites, the corresponding fO2 were estimated (again, assuming basaltic melt composition) to be 鈭糏W to IW+1.0, i.e., several orders of magnitude more oxidizing than the conditions estimated for the chromite-free ureilites. In terms of Fo and Cr valence properties, ureilites appear to form two groups rather than a single 鈥淐r-valence (or fO2) vs. Fo鈥?trend. The chromite-bearing ureilites show little variation in Fo (鈭?4-76) but significant variation in Cr valence, while the non-chromite-bearing ureilites show significant variation in Fo (鈭?7-95) and little variation in Cr valence. These groups are unrelated to petrologic type (i.e., olivine-pigeonite, olivine-orthopyroxene, or augite-bearing). The chromite-bearing ureilites also have lower contents of Cr in olivine than most non-chromite-bearing ureilites, consistent with predictions based on Cr olivine/melt partitioning in spinel saturated vs. non-spinel-saturated systems.
Under the assumption that at magmatic temperatures graphite-gas equilibria controlled fO2 at all depths on the ureilite parent body, we conclude: (1) that ureilite precursor materials having the Fo and Cr valence properties now observed in ureilites are unlikely to have been preserved during planetary processing; and (2) that the Fo and Cr valence properties now observed in ureilites are consistent with having been established by high-temperature carbon redox control over a range of depths on a plausible-sized ureilite parent body. The apparent limit on ureilite Fo values around 74-76 suggests that the precursor material(s) had bulk mg# 猢?that of LL chondrites.