Isotope fractionation induced by phase transformation: First-principles investigation for Mg2SiO4

With increasing pressure, olivine transforms to wadsleyite, ringwoodite, and finally dissociates into bridgmanite and ferropericlase, producing dramatic structure changes along these phase transformations. Here we investigated the equilibrium isotope fractionation of Mg, Si, and O among these phases using first-principles calculations. Both Mg and Si have measurable isotope fractionations among these phases, even at mantleʹs pressure–temperature conditions. Wadsleyite and ringwoodite are depleted in heavy Si but enriched in heavy Mg relative to olivine. Bridgmanite is depleted in heavy Si and Mg among all phases. Increasing pressure can slightly reduce the size of Si isotope fractionation but significantly depress Mg isotope fractionation among these phases. Mg and Si isotope fractionation among Mg2SiO4 polymorphs may provide a promising way to “probe” the depth and temperature of origin of mantle xenoliths.