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

Abstract

Silicon isotope fractionation factors for mantle silicate minerals, including olivine, wadsleyite, ringwoodite, pyroxenes, garnet (pyrope), majorite, and Mg-perovskite, are calculated using density functional theory. Our results show that equilibrium fractionations of Si isotopes are negligible among pyroxenes, olivine, and pyrope, but are significant between olivine and its polymorphs (wadsleyite and ringwoodite). There is also significant Si isotope fractionations between mantle minerals with different Si coordination numbers (CN), such as Mg-perovskite (CN=6) and olivine polymorphs (CN=4). When in equilibrium with each other, 30Si/28Si decreases in the order of olivine>pyroxenes>wadsleyite>majorite>ringwoodite>Mg-perovskite.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.