EVIDENCE FOR A REDUCED, FE-DEPLETED MARTIAN MANTLE SOURCE REGION OF SHERGOTTITES

Estimates of the oxygen fugacity of the source regions of martian sher gottites are obtained from coexisting Fe-Ti oxides and from Fe3+/Fe2ratios in pyroxenes. These estimates are one to four log(10) units low er than previously reported values, and indicate that the shergottite source region has an oxidation state about two or three orders of magn itude below the quartz-fayalite-magnetite (QFM) oxygen reference buffe r. An approximation of the major element composition of the shergottit e source region is obtained utilizing the MELTS petrologic modeling so ftware. A bulk composition of the shergottite source region is derived , which is consistent with the generation of sher-gottites upon 3% par tial melting at 10 kbar. This composition is decidedly less enriched i n Na2O and FeOTot than previous estimates. When compared to the bulk c omposition of the terrestrial mantle, the source region of the shergot tites is lower in CaO, Al2O3, Na2O and TiO2, and higher in MgO.