A Pyroxenite Mantle on Mercury? Experimental Insights from Enstatite Chondrite Melting at Pressures up to 5 GPa

Abstract

Enstatite chondrites (EC) are potential source material for the accretion of Mercury due to their reduced nature and enrichment in volatile elements. Understanding their melting properties is therefore important to better assess a scenario where Mercury formed from these chondrites. Here, we present experimental data on the partial melting of a modified EH4 Indarch EC, which was adjusted to have 18\% more metallic Si than SiO$_2$ in mass, yielding an oxygen fugacity of 3.7 below the iron--wüstite redox buffer and 12 wt\% Si in the metal. Experiments were performed from 0.5 to 5 GPa. Results indicate that the stability field of enstatite expands relative to olivine. This expansion is likely due to the presence of Ca--S and Mg--S complexes in the silicate melt, which enhance SiO$_2$ activity and promote enstatite crystallization. Additionally, sulfides show enrichment in Mg and Ca, up to 22 and 13 wt\% respectively, the main remaining cations being Fe, Cr, and Mn. These high Mg and Ca contents are observed at low temperatures and high silica content in the silicate melt, respectively. High-pressure melts (2 to 5 GPa, 160--400 km depth in Mercury) are Mg-rich, similar to those in Mercury's high-magnesium region (HMR), while low-pressure melts (0.5 to 1 GPa, 40--80 km depth) are Si-rich, comparable to the northern volcanic plains (NVP). Results suggest that a large fraction of Mercury's surface aligns compositionally with these melts, implying that Mercury's mantle could predominantly have a pyroxenitic composition. However, regions with differing compositions, such as aluminum-rich areas like the Caloris basin, suggest local variability in mantle geochemistry. Overall, our results show that if Mercury formed from materials similar to EC, batch melting of its primitive pyroxenite mantle would yield magmas with compositions resembling those of most rocks observed on the surface.