The cBΩ model, which suggests the defect Gibbs energy is proportional to the isothermal bulk modulus and the mean volume per atom, is first introduced to predict self-diffusion coefficients of oxygen in various silicate and oxide minerals in terms of available elastic data. We develop a new approach to determine constant c in the cBΩ model on the basis of the observed compensation effect between the activation energies and pre-exponential factors, which is critical to the diffusivity prediction. Under anhydrous conditions, the validity of this model is tested by the experimentally determined oxygen self-diffusion coefficients. Our results show that the absolute oxygen diffusion rates derived from the cBΩ model are in agreement with experimental data in a variety of rock-forming minerals including olivine, MgSiO3 perovskite, spinel, and zircon.

[1] Si and Mg self-diffusion coefficients were measured simultaneously in single crystals of MgSiO 3 perovskite under lower mantle conditions. There is little difference in Si volume diffusivity measured directly using single crystals (this study) and those retrieved from experiments with polycrystals (earlier studies). This agreement between studies establishes the reliability of Si diffusion coefficients measured in perovskite. Within the uncertainties of our measurements, no anisotropy in the diffusion of either Si or Mg could be resolved. À11 m 2 /s for Mg, DH = 308 kJ/mol for Si, and 305 kJ/mol for Mg. Mg diffusivity in MgSiO 3 perovskite is distinctly lower than those measured in olivine, wadsleyite, and ringwoodite. We find that Mg has very similar diffusivity to Si in perovskite. As a consequence, the rheological properties of the lower mantle may be controlled by the coupled motion of Si and Mg. A point defect-based model is discussed that may account for the diffusion behavior of Si and Mg in MgSiO 3 perovskite. Our data indicate that, within realistic ranges of temperature, grain size, and state of stress, both diffusion creep as well as dislocation creep may be observed in the lower mantle.

No abstract