Diffusion in MgO at high pressures: Constraints on deformation mechanisms and chemical transport at the core‐mantle boundary

High‐pressure experiments on diffusion in MgO were performed to model the rheological and chemical transport properties of the lower mantle. Lattice and grain boundary diffusion coefficients for Mg, O and Al were determined at 2273 K and pressures up to 25 GPa. The results for pure MgO are in excellent agreement with first‐principles calculations. In samples doped with Al 2 O 3 , cation vacancies were found to attach to Al impurities with a binding energy of ∼0.7 eV. Our results predict that a transition from diffusion creep to dislocation creep will occur in the deep lower mantle if the shear stress exceeds ∼1–10 MPa, for a grain size of ∼0.1–1 mm. Diffusion through periclase is fast enough to allow substantial chemical exchange across the core‐mantle boundary since core formation, with length scales of ∼1–10 km for lattice diffusion and 100 km for grain boundary diffusion.