Experimental constraints on the origin of Martian meteorites and the composition of the Martian mantle

Abstract

The major element compositions of partial melts of a chondritic (Homestead L5) model Mars mantle have been determined at 4.7–5.0 GPa in a multi-anvil device over a temperature interval that ranged from near solidus to near liquidus conditions. The purpose of these experiments is to determine if the major element abundances of Martian basalts (shergottite meteorites) or their parent magmas can be derived from a model Martian mantle composition at high pressure. Partial melting of our model composition at 4.7–5.0 GPa produces liquids with super-chondritic CaO/Al 2O 3 similar to those of proposed Martian basalt parent magmas. The concentrations of CaO and Al 2O 3 in the high-pressure experimental liquids are, however, lower than in Martian basalt parent magmas. We conclude that matches for both ratios and concentrations involving CaO and Al 2O 3 between current model Mars compositions and proposed Martian basalt parent magmas would require at least two stages of magmatic differentiation. For example, partial melting at 5 GPa (425 km depth in Mars) produces a magma having super-chondritic CaO/Al 2O 3, but subsequent, lower pressure differentiation of olivine (±low-Ca pyroxene) is needed to increase the CaO and Al 2O 3 concentrations to those of calculated Martian basalt parent magmas. This two-stage polybaric differentiation would be consistent with either a magma ocean or mantle plume-melting scenario. On the other hand, these multi-stage differentiation scenarios cannot reconcile the significant mismatch between the FeO content of our experimental liquids and Martian basalt parent magmas. A remedy for this apparent inconsistency might require a bulk Martian mantle or shergottite parent source region with a composition and Mg# closer to that of H-chondrites, but still much less magnesian than are terrestrial upper-mantle basalt source regions.