The Effect of Water on Fe-Mg Interdiffusion Rates in Ringwoodite and Implications for the Electrical Conductivity in the Mantle Transition Zone

We determined the kinetics of Fe-Mg interdiffusion in ringwoodite aggregates as a function of water content (up to ~6,000 wt. ppm H₂O) at 20 GPa and 1,373–1,673 K by the diffusion couple method. The dependence of Fe-Mg interdiffusivity (D_Fe-Mg) on Fe concentration was determined using the Boltzmann-Matano method. The experimentally reported D_Fe-Mg in ringwoodite within 0 ≤ X_Fe ≤ 0.1 could be fitted by the relation (Formula presented.), where E* = (1 − X_Fe)E_Mg + X_FeE_Fe (E_Mg = 140 ± 5 kJ/mol, E_Fe = 4 ± 2 kJ/mol), D₀ = 5:59^+2:90_−1:91× 10⁻¹⁰ m²/s, n = −0.21 ± 0.10, r = 0.25 ± 0.03, and α = −24 ± 4. The water content exponent r of 0.25 suggests a nonnegligible role of water in enhancing Fe-Mg interdiffusion in ringwoodite. The length scale over which the chemical heterogeneities are homogenized by Fe-Mg interdiffusion in the mantle transition zone is estimated to be only a few hundred meters even assuming the whole Earth age. Comparison between the conductivities predicted from Fe-Mg interdiffusion and those obtained from magnetotelluric surveys suggests that around 0.1 wt.% water can account for the high conductivity anomalies (~10^−0.6–10⁻¹ S/m) observed in the lower part of the mantle transition zone.