Compositional diversity and stratification of the Martian crust: Inferences from crystallization experiments on the picrobasalt Humphrey from Gusev Crater, Mars

Phase equilibrium experiments on a liquid composition of the Gusev Crater rock Humphrey (with added F and Cl) with 0.07 wt% and 1.67 wt% water have revealed the range in compositional diversity of lavas that could arise from fractionation at the base of a thick martian crust. Humphrey composition melts with 0.07 wt% water produce ne-normative alkalic residual melts at 9.3 kbar, silica progressively decreases with increasing degree of fractionation. This decrease in silica is accompanied by Fe-, Ti-, and P-enrichment. With 1.67 wt% bulk water content, Humphrey-like melts instead crystallize to produce residual melts that show progressive silica-enrichment, Al-enrichment, and Fe-depletion, progressing from mildly alkalic hy-normative basalt to trachyandesite. Coupled with the production of such residual liquids by fractionation is the formation of ultramafic to mafic cumulus lithologies that would remain at the base of the Martian crust. This process would lead to significant compositional stratigraphy of the crust that contains elements of both trends, as early hydrous primary magmas gave way to drier more depleted primary magmas upon dehydration of the magma source regions by repeated episodes of partial melting. The possibility of significant contribution of late silica-poor, Fe-enriched basalt, residual to fractionation at depth, to the present martian surface, makes it difficult to reliably use surface lithologies to constrain the compositional characteristics of the mantle magmatic source regions and the primary melts produced from them.