Mg-Fe interdiffusion rates in wadsleyite and the diffusivity jump at the 410-km discontinuity

Mg-Fe interdiffusion rates have been measured in wadsleyite aggregates at 16.0-17.0 GPa and 1230-1530 °C by the diffusion couple method. Oxygen fugacity was controlled using the NNO buffer, and water contents of wadsleyite were measured by infrared spectroscopy. Measured asymmetric diffusion profiles, analyzed using the Boltzmann-Matano equation, indicate that the diffusion rate increases with increasing iron concentration and decreasing grain size. In the case of wadsleyite containing 50-90 weight ppm H₂O, the Mg-Fe interdiffusion coefficients at compositions of Mg/(Mg + Fe)=0.95 in the coarse-grained region (about 60 μm) and 0.90 in the fine-grained region (about 6 μm) were determined to be a D_Xmg=0.95 (m² s^-1) = 1.24 × 10^-9 exp[-172 (kJ mol^-1)/RT] and D_Xmg=0.90 (m² s^-1) = 1.77 × 10^-9 exp[-143 (kJ mol^-1)/RT], respectively. Grain-boundary diffusion rates were estimated to be about 4 orders of magnitude faster than the volume diffusion rate. Grain-boundary diffusion dominates when the grain size is less than a few tens of microns. Results for the nominally dry diffusion couple in the present study are roughly consistent with previous studies, taking into account differences in pressure and grain size, although water contents of samples were not clear in previous studies. We observed that the diffusivity is enhanced by about 1 order of magnitude in wadsleyite containing 300-2100 wt. ppm H₂O at 1230 °C, which is almost identical to the enhancement associated with a 300 °C increase in temperature. It is still not conclusive that a jump in diffusivity exists between olivine and wadsleyite because water contents of olivine in previous diffusion studies and effects of water on the olivine diffusivity are uncertain.