Effects of phosphorus on partial melting of the Martian Mantle and compositions of the Martian Crust

Phosphorus is estimated to be ten times more enriched in the martian mantle compared to the terrestrial mantle. Yet, its effects on primary melt composition and melting phase relations in martian systems is unknown. We performed piston cylinder experiments at a constant upper mantle pressure of 2 GPa and temperatures of 1210-1450 degrees C using a model martian primitive mantle composition with P2O5 content of 0 and 0.5 wt.%. All experiments produced an assemblage of olivine + orthopyroxene + melt +/- pigeonite +/- apatite +/- spinel. Our experimental results, in combination with a previous study at similar P -T conditions and major element bulk composition but containing 0.2 wt.% bulk P2O5, show that the addition of phosphorus dramatically increases the abundance of more polymerized residual mineral such as orthopyroxene while decreasing the proportion of less polymerized residual phases such as olivine, especially at low extent of melting (similar to 11 wt.%). Such effects lead to lower SiO2 concentrations in the near-solidus melt by up to 10 wt.% for mantle P2O5 of 0.2 and 0.5 wt.%. Increasing bulk P2O5 to 0.5 wt.% also leads to slightly elevated CaO/Al2O3 ratio and increased FeO* concentration in mantle-derived melts with the latter likely due to formation of Fe-O-P complexes in the liquid. Our study suggests that elevated phosphorus in the martian mantle has important consequences regarding the composition and mineralogy of the crust, partly made with primary melts, and of the upper mantle. Because of elevated P, variably melt-depleted upper mantle of Mars is likely to be richer in orthopyroxene compared to the terrestrial mantle and the elevated P content is partly responsible for several geochemical attributes of martian basalts compared to those on Earth. Extrapolating our experimental results to a range of pressures, we suggest a depletion of P in the mantle through time, which likely contributed to major elemental compositional differences between ancient Gusev and Gale crater basalts and more recent martian meteorites. (C) 2022 Elsevier Ltd. All rights reserved.

Automated summary

Phosphorus enrichment in the martian mantle has important consequences on the composition and mineralogy of the crust and upper mantle. Experimental results show that the addition of phosphorus increases the abundance of orthopyroxene while decreasing the proportion of olivine in the residual minerals. This leads to lower SiO2 concentrations in the near-solidus melt and elevated CaO/Al2O3 ratio and FeO* concentration in mantle-derived melts.