OXYGEN-ISOTOPE FRACTIONATION IN MANTLE MINERALS

The increment method is adopted to calculate oxygen isotope fractionat ion factors for mantle minerals, particularly for the polymorphic phas es of MgSiO3 and Mg2SiO4. The results predict the following sequence o f O-18-enrichment: pyroxene (Mg, Fe, Ca)(2)Si2O6 > olivine (Mg, Fe)(2) SiO4 > spinel (Mg, Fe)(2)SiO4 > ilmenite (Mg, Fe, Ca) SiO3 > perovskit e (Mg, Fe, Ca) SiO3. The calculated fractionations for the calcite-per ovskite (CaTiO3) system are in excellent agreement with the experiment al 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-structure d silicates in the lower mantle but depleted in O-18 relative to olivi nes and pyroxenes in the upper mantle. The oxygen isotope layering of the mantle might result from differences in the chemical composition a nd crystal structure of mineral phases at different mantle depths. Ass uming isotopic equilibrium on a whole earth scale, the chemical struct ure of the Earth's interior can be described by the following sequence of O-18-enrichment: upper crust > lower crust > upper mantle > transi tion zone > lower mantle > core.