Melting of Model Martian Mantle at High-pressure: Implications for the Composition of the Martian Basalt Source Region

High-pressure melting experiments on the Homestead L5 ordinary chondrite were performed at 5 GPa in a multi-anvil device over a temperature interval that ranged from near solidus to near liquidus conditions. Near solidus silicate liquids of Homestead have higher CaO/Al2O3 and lower Mg# compared to solidus liquids of terrestrial peridotite (e.g.: KLB-1) at this pressure. The silicate portion of Homestead is similar to proposed model Martian mantle compositions and therefore these experiments may place constraints on the nature of melts derived from the Martian interior at high pressure. For example, the super-chondritic CaO/Al2O3 of several proposed shergottite parent magmas fall within the range of the experimental Homestead melts, however the CaO and Al2O3 contents of the shergottite parent magmas are higher than Homestead melts. Thus a model Martian mantle composition can produce shergottite CaO/Al2O3 by partial melting at a depth of ~425 km, however a second stage of shallow level magmatism with olivine (± low-Ca pyroxene) fractionation is required to match CaO and Al2O3 values. FeO content of Homestead partial melts at 5 GPa are much higher than are those of shergottite parent magmas, implying that the shergottite source mantle could be lower in FeO than the Dreibus and Wanke [1] Martian mantle composition (DW hereafter). Background: The main goal of this study is to use high-pressure, high-temperature melting experiments to determine if it is possible to derive the major element abundances of Martian basalts (shergottites) or their parent magmas from a model Martian mantle composition. Earlier melting experiments on FeO-rich bulk compositions [2-6] similar to the composition in our current study have cast some doubt on the feasibility of generating shergottite parent magmas by near-eutectic melting in the incipient garnet stability field. These studies showed that shergottite-like magmas, with super-chondritic CaO/Al2O3, cannot be produced by partial melting in the pressure range 1.5-3.0 GPa (~150-300 km depth in Mars). However, it is well established from numerous phase equilibrium studies at P>3 GPa on chondritic compositions and on Earth mantle peridotites [7-9] that increasing pressure up to ~15 GPa continuously expands the stability field of garnet at the expense of all other crystalline phases, and decreases garnet solubility in silicate melt. Hence, it can be expected that partial melts at P>3 GPa will be characterized by increasing CaO/Al2O3, approaching values observed in shergottites. Therefore we were motivated to determine if the partial melting of a model Martian mantle composition at higher pressures could give rise to shergottites or their parent magmas.