Mg-Fe and Mg-Ni interdiffusion coefficients have been measured in single-crystal olivine (α-(Mg, Ni, Fe)2SiO4), polycrystalline β phase (β-(Mg, Ni, Fe)2SiO4), and silicate spinel (&ggr;-(Mg, Ni, Fe)2SiO4) at 1473 K between 1 and 14 GPa under controlled thermodynamic conditions. In olivine, DMg-Niα ranges from 10-17 m2 s-1 at 1 GPa to 4¿10-18 m2 s-1 at 9 GPa, and DMg-Feα ranges from 3¿10-15 m2 s-1 at 1 GPa to 10-15 m2 s-1 at 4 GPa. At 9 GPa the cation diffusion rates in &ggr; spinel are ~3 orders of magnitude higher than those in olivine; between 9 and 14 GPa, DMg-Ni&ggr; decreases from 8¿10-15 to 9¿10-16 m2 s-1, and DMg-Fe&ggr; decreases from 2¿10-13 to 5¿10-14 m2 s-1. Between 10 and 14 GPa the diffusion coefficients for both Fe and Ni are similar in magnitude and pressure dependence between β phase and &ggr; spinel. The activation volumes for Mg-Ni diffusion are 3.2 and 6.7 cm3 mol-1 in the α and &ggr;(β) phases, respectively, and for Mg-Fe diffusion are 5.4 and 6.1 cm3 mol-1 in the α and &ggr;(β) phases, respectively. Our results demonstrate that extreme differences in transport properties are expected to occur across the 400 km discontinuity. A model for electrical conductivity in the upper mantle involving two mechanisms for electrical conduction, with one mechanism being associated with a hydrogen or defect-related conduction process and the other involving a straightforward application of the Nernst-Einstein equation to our cationic diffusion data, reproduces geomagnetic sounding results for the upper mantle and shallow transition zone. This modeling yields an order of magnitude increase in electrical conductivity across the 400 km discontinuity, with little or no change in conductivity occurring near 520 km depth. Our modeling suggests that divalent cation diffusion is the dominant mechanism of charge transport between 400 and 670 km depth and that the high-pressure olivine polymorphs dominate the electrical behavior of the deep upper mantle. Comparison between field-based deep-mantle conductivity profiles derived from magnetotelluric studies and extrapolation of our model suggests that a discontinuity in electrical transport properties exists at the base of the transition zone as well. Similarly, given the dramatic differences in chemical diffusion between different polymorphs of olivine, it is possible that other transport property-mediated processes, such as viscous flow, could dramatically change across the 400 and 670 km discontinuities; such changes are less likely to occur at 520 km depth. ¿ 2000 American Geophysical Union