Effects of hydrogen and iron on the silicon diffusivity of wadsleyite

Details Effects of hydrogen and iron on the silicon diffusivity of wadsleyite Effects of hydrogen and iron on the silicon diffusivity of wadsleyite

Nov 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010pepi..183..175s&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 183, Issue 1-2, p. 175-182.

Physics

Scientific paper

We determined the temperature dependence of Si diffusion rates in polycrystalline Mg 2 SiO 4 wadsleyite under nominally dry conditions (20-60 wt. ppm H 2 O), at a pressure of 18 GPa 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 Mg 2 SiO 4 wadsleyite with 20-60 wt. ppm H 2 O were about half an order of magnitude slower than those with 14-507 wt. ppm H 2 O 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 Mg 2 SiO 4 wadsleyite was comparable for the volume diffusion and the grain-boundary diffusion in (Mg 0.9 Fe 0.1 ) 2 SiO 4 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) 2 SiO 4 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 Mg 2 SiO 4 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.

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