Oxygen Fugacity of the Martian Mantle From Pyroxene/Melt Partitioning of REE

This study is part of an ongoing effort to calibrate the pyroxene/melt REE oxybarometer for conditions relevant to the martian meteorites. Redox variations have been reported among the shergottites. Wadhwa used the Eu and Gd augite/melt partitioning experiments of McKay, designed for the LEW86010 angrite, to infer a range of fo2 for the shergottites. Others inferred fo2 using equilibria between Fe-Ti oxides. There is fairly good agreement between the Fe-Ti oxide determinations and the estimates from Eu anomalies in terms of which meteorites are more or less oxidized. The Eu anomaly technique and the Fe-Ti oxide technique both essentially show the same trend, with Shergotty and Zagami being the most oxidized and QUE94201 and DaG 476 being the most reduced. Thus, the variation in fo2 appears to be both real and substantive. However, although the redox trends indicated by the two techniques are similar, there is as much as two log unit offset between the results of three researchers. One explanation for this offset is that the Eu calibration used for the shergottites was actually designed for the LEW86010 angrite, a silica-undersaturated basalt whose pyroxene (diopside) compositions are rather extreme. To correct this, experiments have been conducted on the redox relationship of Eu partitioning relative to Sm and Gd for pyroxene/melt compositions more relevant to Martian meteorites. We report here preliminary results for experiments on pigeonite/melt partitioning as a function of fO2.