Effect of pressure on Fe–Mg, Ni and Mn diffusion in (FexMg1−x)2SiO4 olivine

The pressure dependence of Fe–Mg interdiffusion coefficients, as well as of diffusion coefficients of the trace elements Ni and Mn in olivine, have been determined from diffusion anneals carried out using oriented single crystals in multianvil presses. This is the first determination of activation volumes of diffusion of trace elements (Ni, Mn) in olivine. Results from an isothermal polybaric (1673 K, 6–12 GPa) and an isobaric polythermal (1623–1823 K, 12 GPa) set of experiments were combined to constrain an internally consistent set of Arrhenius parameters for the diffusion processes. It is found that within the uncertainty of the data, (a) diffusion rates of Fe–Mg, Mn and Ni are very similar to each other at all conditions, consistent with more detailed observations from 1 atmosphere experiments (Petry et al., 2004), (b) apparent activation volumes using different fitting protocols lie between 4 and 7 cm3 mol−1, but a value of 5.3 ± 1.0 cm3 mol−1 (cf. Misener, 1974; Farber et al., 2000), describes the pressure dependence of diffusivity of these elements in olivine over its entire stability range (in pressure) adequately along the Ni–NiO fO2 buffer, (c) the activation volume at constant fO2 is higher, ∼7 cm3 mol−1 and (d) at depths corresponding to the lowermost stability of olivine (∼12 GPa), activation energies of diffusion are ∼100 kJ mol−1 higher than at near surface conditions, which means that closure temperatures are higher by several hundred degrees. geothermal gradients within the Earth, diffusion rates increase with depth in the lithosphere but decrease with depth, in spite of the increase in temperature, in the convective asthenospheric part of the mantle. Consequently, there is a maxima in diffusion rates at the lithosphere–asthenosphere boundary and for the olivine part of the mantle, this is likely to be the region where most geochemical mixing and eradication of heterogeneities occur.