Prediction of high-temperature oxygen isotope fractionation factors between mantle minerals

The increment method is adopted to calculate oxygen isotope fractionation factors for mantle minerals, particularly for the polymorphic phases of MgSiO3 and Mg2SiO4. The results predict the following sequence nf O-18-enrichment: pyroxene (Mg,Fe,Ca)(2)Si2O6>olivine (Mg,Fe)(2)SiO4>spinel (Mg,Fe)(2)SiO4>ilmenite (Mg,Fe,Ca)SiO3>perovskite (Me,Fe,Ca)SiO3. The calculated fractionations for the calcite-perovskite (CaTiO3) system are in excellent agreement with experimental calibrations. If there would be complete isotopic equilibration in the mantle, the spinel-structured silicates in the transition zone are predicted to be enriched in O-18 relative to the perovskite-structured silicates in the lower mantle but depleted in O-18 relative to olivines and pyroxenes in the upper mantle. The oxygen isotope layering of the mantle would essentially result from differences in the chemical composition and crystal structure of mineral phases at different mantle depths. Assuming isotopic equilibrium on a whole earth scale, the chemical structure of the Earth's interior call be described by the following sequence of O-18-enrichment: uppr crust>lower crust>upper mantle>transition zone>lower mantle>core.