Thermodynamic Properties of Liquid Fe-Mg Alloys Under Outer-Core Conditions Using First-Principles Molecular Dynamics

Magnesium (Mg) partitioning behavior between solid and liquid iron (Fe) shows that Mg slightly favors liquid Fe under conditions at the Earth’s core. This means that the Earth’s outer core may contain more Mg than previously thought. However, geophysical properties such as density (ρ) and sound velocity (VP) of liquid Fe-Mg alloys under outer-core conditions have yet to be explored. Considering that the liquid outer core includes approximately 10 wt.% light elements, here we established an equation of state (EoS) of liquid Fe-Mg alloys with Mg less than 10 wt.% to study the thermodynamic properties under outer-core conditions using first-principles molecular dynamics. The results show that adding Mg to liquid Fe will clearly reduce the ρ, isothermal bulk modulus (KT), and adiabatic bulk modulus (KS). Meanwhile, it will increase the VP. In order to access the Mg content in the outer core, the ρ and VP of liquid Fe-Mg alloys along the geotherm are compared with the preliminary reference Earth model. Assuming Mg is the only light element, the maximum content of Mg required is approximately 2.9–6.1 wt.% due to the temperature uncertainty at the inner core boundary (ICB). Further, considering the geochemical constraints (the partition coefficient between liquid and solid Fe, molten Fe-alloy, and silicate melts), the content of Mg is further constrained to below 0.5 wt.%.