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.