High pressure melting of Fe-Si alloys with applications to the lunar core composition and dynamo processes

Earth’s magnetosphere is generated by convective dynamo action within its liquid metallic outer core. This same core-driven dynamo process has been inferred for other terrestrial planetary bodies which either presently possess a magnetosphere, or may have in the past. These bodies include Ganymede, Mercury, the Moon, and Asteroid 4 Vesta. However, understanding these core processes requires that the core composition be known. By experimentally determining the solid-liquid phase transitions of core-relevant alloys, the likely compositions of these terrestrial cores may be constrained.

            Experiments were conducted on 8 Fe-Si alloys in the range of Fe-5 wt% Si to Fe-33 wt% Si (FeSi) using a 1000-ton cubic anvil press, at pressures of 3-5 GPa and temperatures into the liquid state. A central 5-hole BN cylinder held 5 different Fe-Si sample compositions simultaneously with a thermocouple located at the base of the BN cylinder, and was surrounded by a graphite furnace within a pyrophyllite cubic pressure cell. Following quenching of each experiment, the samples were analysed by electron microprobe for composition and texture. From these analyses, the solidus and liquidus boundaries were mapped across the aforementioned compositional range at of 3, 4, and 5 GPa.

            It was determined that the melting boundary for 3-5 GPa was roughly 50-150 K higher than that of 1 atm, with a eutectic composition of Fe-20 wt% Si. Across the 3-5 GPa range, there was an increase in the melting boundary of roughly 50-75 K. Using pressure and temperature estimates from previous core modelling studies, a range of approximately 10-15 wt% Si was suggested for the core of the Moon.