Effects of hydrogen and iron on the silicon diffusivity of wadsleyite

We determined the temperature dependence of Si diffusion rates in polycrystalline Mg₂SiO₄ wadsleyite under nominally dry conditions (20-60wt.ppmH₂O), at a pressure of 18GPa and temperatures between 1450 and 1600°C. Diffusion experiments were conducted using an isotopic tracer diffusion method. Diffusion profiles were obtained by a depth-profiling mode using a secondary ion mass spectrometer. Diffusion coefficients were calculated from the obtained diffusion profiles by numerical simulation after correcting for the convolution effect using surface roughness. Since all the obtained diffusion profiles were composed of volume and grain-boundary diffusion regimes, we determined the volume and grain-boundary diffusion coefficients simultaneously. Si diffusion rates in Mg₂SiO₄ wadsleyite with 20-60wt.ppmH₂O were about half an order of magnitude slower than those with 14-507wt.ppmH₂O reported by Shimojuku et al. [Shimojuku, A., Kubo, T., Ohtani, E., Yurimoto, H., 2004. Si self-diffusion in wadsleyite: implications for rheology of the mantle transition zone and subducting plates. Geophys. Res. Lett. 31, doi:10.1029/2004GL020002] with correcting the convolution effect in both data. The Si diffusivity in Mg₂SiO₄ wadsleyite was comparable for the volume diffusion and the grain-boundary diffusion in (Mg_0.9Fe_0.1)₂SiO₄ wadsleyite with similar water contents reported by Shimojuku et al. [Shimojuku, A., Kubo, T., Ohtani, E., Nakamura, T., Okazaki, R., Dohmen, R., Chakraborty, S., 2009. Si and O diffusion in (Mg,Fe)₂SiO₄ wadsleyite and ringwoodite and its implications for the rheology of the mantle transition zone. Earth Planet. Sci. Lett. 284, 103-112]. Analysis of point defect chemistry based on the positive dependence between Si diffusion rates and water content implies that Si diffusion in Mg₂SiO₄ wadsleyite under hydrous conditions occurs by a vacancy diffusion mechanism. Enhancement of the Si diffusivity through hydrogen incorporation possibly leads to water weakening in wadsleyite, because Si is the slowest diffusing species.