Enhancement of cation diffusion rates across the 410-kilometer discontinuity in Earth's mantle

Rates of cation diffusion (magnesium, iron, and nickel) have been determine d in olivine and its high-pressure polymorph, wadsleyite, at 9 to 15 gigapa scals and 1100 degrees to 1400 degrees C for compositions that are relevant to Earth's mantle. Diffusion in olivine becomes strongly dependent on comp osition at high pressure. In wadsleyite, diffusion is one to two orders of magnitude faster than in olivine, depending on temperature. Homogenization of mantle heterogeneities (chemical mixing) and mineral transformations inv olving a magnesium-iron exchange will therefore occur considerably faster i n the transition zone than at depths of Less than 410 kilometers.