Effect of pressure on Fe–Mg interdiffusion in (FexMg1−x)O, ferropericlase

Fe–Mg interdiffusion in ferropericlase has been studied at pressures between 8 and 23 GPa and temperatures between 1653 and 2073 K, using a multianvil apparatus. The compositions of the single crystal diffusion couples were XFeO=0.07 for one crystal and XFeO=0.37 for the other, although in one case pure MgO was used instead of XFeO=0.07. Fe–Mg interdiffusion at oxygen fugacities buffered by Ni–NiO, including 1 bar data at 1673 and 1873 K of Mackwell et al. (in preparation), can be described for 0.07<XFeO<0.37 by:DFe–Mg=D0 expAXFeORT exp−EA+VAPRTwhere the pre-exponential factor D0=9.8(±0.7)×10−6 m2 s−1, the constant A=132±13 kJ mol−1, the activation energy EA=255±16 kJ mol−1, the activation volume VA=3.3±0.1 cm3 mol−1, XFeO is mole fraction of FeO in (FexMg1−x)O, T is absolute temperature, and P is pressure. This result can be used to constrain Fe–Mg interdiffusion coefficients of ferropericlase in the Earth’s lower mantle. At the top of the lower mantle, at 23 GPa and 2000 K, diffusivities will be 0.5×10−13 m2 s−1 and 1.1×10−13 m2 s−1 for XFeO=0.1 and 0.2, respectively. Extrapolating the results to higher pressures suggests that diffusive interaction at the core–mantle boundary will extend at most ∼15 km into the lower mantle.