Transport Properties of Fe2SiO4 Melt at High Pressure From Classical Molecular Dynamics: Implications for the Lifetime of the Magma Ocean

The transport properties of silicate melts played important roles in various geodynamic processes during the evolution of the Earth. The dependence of transport properties on the temperature and pressure of Fe-bearing melts has not been systematically explored, although Fe-bearing silicate melts are thought to have represented important portions of the early Earth and Earth's current mantle. Based on large-scale molecular dynamics calculations, the pressure and temperature dependences of the self-diffusivity and viscosity of fayalite (Fe2SiO4) from 1600 to 4000K and up to 50GPa have been investigated. The results indicate that the self-diffusion coefficient increases and viscosity decreases with increasing temperature; the calculated temperature dependence of transport coefficients can be fit with an Arrhenian relation below 5GPa, while viscosity exhibits a crossover from Arrhenius to non-Arrhenius behavior at pressure higher than 20GPa. With increasing pressure, the mobility of Fe2SiO4 melt is inhibited along isotherms, and Fe2SiO4 melt is more viscous than Mg2SiO4 melt at high pressures. Combining these results with those from previous studies of magma ocean isentropes, the associated viscosity profiles of Fe2SiO4 melt are discussed. Our results indicate that iron-rich silicate melts would have decelerated the cooling rate in the lower mantle and had a great influence on the physical properties of the lower mantle portion of the magma ocean.