Oxygen Isotope Fractionation in MgSiO3 and Mg2SiO4 Polymorphs: Implications for the Chemical Structure of the Mantle and Isotopic Nonequilibuium in Mantle Assemblages

摘要

The modified increment method has been applied to the calculation of oxygen isotope fraetionation factors for the polymorphic phases of MgSiO3 and Mg2Si04. The results suggest the following sequence of l8O- enricliment in the different structures of the mantle silicates: pyroxene Mg2Si20r, > olivine Mg2Si04 > spinel-structured Mg2Si0j > ilmenite-structured MgSiO, > perovskite-structured MgSiO,. Assuming isotopic equilibrium between the various phases of the mantle, the chemical structure of the mantle can be described by the following sequence of "O-enrichment: upper mantle > transition zone > lower mantle, Such an oxygen isotope layering results from differences in the chemical composition and crystal structure of mineral phases at different mantle depths.