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The thermodynamic properties of iron silicate liquids at high pressures and temperatures are poorly constrained, even though they are important for understanding the thermal and chemical evolution of a magma ocean. Here we report the results of the P-V-T equation of state, thermodynamic properties, and spin transition of iron in FeSiO3 liquid at 2500-6000 K and pressure conditions spanning the entire mantle using first-principles molecular dynamics simulations. Our calculations predict that FeSiO3 liquid undergoes a linear high-to-low spin transition over a broad pressure interval (>296 GPa), and the spin state of iron in FeSiO3 liquid is mainly the high-spin state near the conditions of the core-mantle boundary. Our results of FeSiO3 liquid adiabats show that iron content has little effect on the adiabatic temperature profile of liquid (Mg,Fe)SiO3 in a magma ocean.

Plain Language Summary A magma ocean is likely to have existed in the early history of Earth due to terrestrial planet accretion and the Moon-forming giant impact. The thermodynamic properties of silicate liquids have significant effects on the evolution of the magma ocean which are closely related to compositions and structures of the modern mantle. However, thermodynamic properties of iron silicate liquid are poorly constrained, and the concentration of iron silicate liquid was likely higher than that today in an early magma ocean. In this study, we report the thermodynamic properties and spin transition of iron in FeSiO3 liquid using first-principles molecular dynamics simulations. Our calculations show that the FeSiO3 liquid undergoes a linear transition from a high-spin to low-spin state over a broad pressure interval. Surprisingly, the addition of iron has little effect on the temperature structure of a magma ocean. Our results provide new insight into the evolution of the magma ocean.