Silicon and magnesium diffusion in a single crystal of MgSiO₃ perovskite

Si and Mg self-diffusion coefficients were measured simultaneously in single crystals of MgSiO₃ 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. Diffusion of Si and Mg in perovskite are described by an Arrhenius equation, D=D₀ exp (-H/RT) at 25GPa, with D ₀=5.10×10^-11m²/s for Si and 4.99×10^-11m²/s for Mg, H=308kJ/mol for Si, and 305kJ/mol for Mg. Mg diffusivity in MgSiO₃ 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 MgSiO₃ 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.