Oxygen fugacity of martian basalts from electron microprobe oxygen and TEM-EELS analyses of Fe-Ti oxides

The stoichiometry of titanomagnetite spinel in the martian basaltic meteori tes is assessed using quantitative analysis of oxygen measured by electron microprobe and electron energy loss spectroscopy in the transmission electr on microscope. The spinels are stoichiometric within the errors of the tech niques, enabling the calculation of oxygen fugacity with confidence. The ox ygen fugacity is calculated using the Ghiorso-Sack and Ca-QUIIF models, whi ch also yield estimates of temperature. The oxygen fugacity of the martian basalts increases from 3 log units below the QFM buffer for QUE 94201 to QF M - 1.8 for EETA 79001 (both lithologies), to QFM - 1.0 for Shergotty, Zaga mi, and Los Angeles. Dar al Gani 476 spinels contain significant MgAl2O4 an d FeCr2O4 components, complicating the use of Fe-Ti oxide models. The oxyge n fugacity of Dar al Gani 476 is estimated to be 1.5 log units below QFM, o n the basis of the Ghiorso-Sack model. The absolute error on the oxygen fug acity estimates is +/-0.5 log units; however, a consistent electron micropr obe analytical routine was applied to all of the basalts, and the relative uncertainty is closer to 0.2 log units. Oxyexsolution has occurred in QUE 9 4201, but reconstruction of pre-exsolution titanomagnetite compositions per mits the calculation of oxygen fugacity. Subsolidus reactions involving oxi des and adjacent Fe-rich silicates are documented and the use of the Ca-QUI IF model for calculation of oxygen fugacity from these phases is discussed.