Tracer diffusion coefficients of Ca, Sr, Ba, and Co in molten olivine tholeiite basalt have, been determined between 1260° and 1440°C at 1 atm. The diffusion coefficients range from 10−7 to 10−6 cm2 s−1, and the activation energies range from 39 to 44 kcal mol−1. The diffusion coefficients decrease with increasing ionic radius, but the observed relationship does not conform to the Stokes-Einstein relation. The results of our experiments suggest that the characteristic diffusion distances, estimated by the expression (Dt)1/2, exceed 1 km only for very long times of 109 years or more. Consequently, chemical diffusion does not contribute significantly to large-scale differentiation and mixing processes in a partially molten mantle. A layer of 1-km half width that has been depleted in incompatible trace elements (e.g., by extraction of melt) can be replenished by diffusion from the adjacent material to one-half its original concentration in 3×108 years (or 0.9 in 8×109 years). Diffusive transport is efficient in small-scale processes (<1 km). Diffusion in a partially molten asthenosphere greatly increases the rate of local equilibration, and it contributes significantly to small-scale mixing and contamination processes in magma chambers and conduits.