Linking the Chassigny meteorite and the Martian surface rock Backstay: Insights into igneous crustal differentiation processes on Mars

Abstract

In order to use igneous surface lithologies to constrain Martian mantle characteristics, secondary processes that lead to compositional modification of primary mantle melts must be considered. Crystal fractionation of a mantle-derived magma at the base of the crust followed by separation and ascent of residual liquids to the surface is common in continental hotspot regions on Earth. The possibility that this process also takes place on Mars was investigated by experimentally determining whether a surface rock, specifically the hawaiite Backstay analyzed by the MER Spirit could produce a known cumulate lithology with a deep origin (namely the assemblages of the Chassigny meteorite) if trapped at the base of the Martian crust. Both the major cumulus and melt inclusion mineral assemblages of the Chassigny meteorite were produced experimentally by a liquid of Backstay composition within the pressure range 9.3 to 6.8 kbar with bulk water contents between 1.5 and 2.6 wt%. Experiments at 4.3 and 2.8 kbar did not produce the requisite assemblages. This agreement suggests that just as on Earth, Martian mantle-derived melts may rise to the surface or remain trapped at the base of the crust, fractionate, and lose their residual liquids. Efficient removal of these residual liquids at depth would yield a deep low-silica cumulate layer for higher magmatic water content; at lower magmatic water content this cumulate layer would be basaltic with shergottitic affinity.