Electrical Properties of Alkaline Earth Sulfides and Implications for the Interior of Mercury

Abstract

Alkaline earth sulfides are possibly abundant in the mantle of Mercury, and knowledge of their melting and transport properties is needed to investigate the structure of the planet. We report electrical experiments at pressures in the range 2–5 GPa and at temperatures up to ∼2,400 K on proposed analogs of natural sulfides, that is, Ca_1-xMg_xS with minor impurities. Electrical conductivity increases nonuniformly with temperature with no systematic dependence on cation composition. At relatively low temperatures (near 1,100 K), the conductivities span a wide range, whereas at higher temperatures the values converge within ∼0.5–7 S/m at 1,800 K and 5 GPa. The conductivity trends are complex, and likely reflect contributions from divalent cations, alkali metal and carbon impurities, which would similarly contribute to the conductivity of Mercury's crust and mantle. Melting is identified by a sharp increase in conductivity between ∼1,850 and 2,100 K at 5 GPa. These transition temperatures are consistent with the presence of impurities. Using electrical studies on relevant silicate minerals and petrological observations, we developed electrical conductivity-depth profiles of Mercury's silicate portion. Depending on the interconnectivity of the sulfide phase, the conductivity at the base of the mantle containing 8 vol.% sulfide ranges from ∼0.2 to >8 S/m. Our results can be tested with future observations from the ESA-JAXA BepiColombo mission.