Oxygen isotope fractionation in mantle minerals

The increment method is adopted to calculate oxygen isotope fractionation factors for mantle minerals, particularly for the polymorphic phases of MgSiO 3 and Mg 2SiO 4. The results predict the following sequence of 18 O enrichment: pyroxene (Mg, Fe, Ca) 2Si 2O 6>olivine (Mg,Fe) 2SiO 4>spinel (Mg,Fe) 2SiO 4>ilmenite (Mg,Fe,Ca)SiO 3>perovskite (Mg,Fe,Ca)SiO 3. The calculated fractionations for the calcite perovskite (CaTiO 3) system are in excellent agreement with the 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 18 O relative to the perovskite structured silicates in the lower mantle but depleted in 18 O relative to olivines and pyroxenes in the upper mantle. The oxygen isotope layering of the mantle might 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 can be described by the following sequence of 18 O enrichment: upper crust>lower crust>upper mantle>transition zone>lower mantle>

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 of18O-enrichment:pyroxene (Mg, Fe, Ca)2Si2O6>olivine (Mg, Fe)2SiO4 > spinel (Mg, Fe)2SiO4> ilmenite (Mg, Fe, Ca) SiO3>perovskite (Mg, Fe, Ca) SiO3. The calculated fractionations for the calcite-perovskite (CaTiO3) System are in excellent agreement with the 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 in18O relative to the perovskite-structured silicates in the lower mantle but depleted in18O relative to olivines and pyroxenes in the upper mantle. The oxygen isotope layering of the mantle might 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 can be described by the following sequence of18O-enrichment:upper crust>lower crust>upper mantle>transition zone>lower mantle>core.