Evidence for a reduced, Fe-depleted martian mantle source region of shergottites

Abstract

Estimates of the oxygen fugacity of the source regions of martian shergottites are obtained from coexisting Fe-Ti oxides and from Fe³⁺/Fe²⁺ ratios in pyroxenes. These estimates are one to four log₁₀ units lower than previously reported values, and indicate that the shergottite source region has an oxidation state about two or three orders of magnitude below the quartz-fayalite-magnetite (QFM) oxygen reference buffer. An approximation of the major element composition of the shergottite source region is obtained utilizing the MELTS petrologic modeling software. A bulk composition of the shergottite source region is derived, which is consistent with the generation of sher-gottites upon 3% partial melting at 10 kbar. This composition is decidedly less enriched in Na₂O and FeO_Tot than previous estimates. When compared to the bulk composition of the terrestrial mantle, the source region of the shergottites is lower in CaO, Al₂O₃, Na₂O and TiO₂, and higher in MgO.