First-principles calculations of equilibrium silicon isotope fractionation among mantle minerals

Our calculation predicts significant Si isotope fractionation between mantle minerals, e.g., perovskite vs. ringwoodite, majorite vs. pyroxene, and olivine vs. its polymorphs even at high pressure and temperature conditions of deep mantle. The Si CN in silicate melt increases with increasing pressure, implying that Si isotope fractionation between silicate and metal could be a function of pressure. Our results suggest that Si isotopic fractionation factor between silicate and metal may decrease with increasing pressure; consequently, Si isotopic fractionation factor obtained from low pressure experiments may not be applicable to Si isotope fraction during core formation which occurred at high pressure. Finally, Si isotopes could also be fractionated between perovskite-rich mantle and residual melt during magma-ocean cooling in the lower mantle because of their different Si CNs. If such primordial signature is not destroyed and partially preserved through the Earth's history, significant Si isotope heterogeneity could still exist between the upper and lower mantle.