Fe-Mg interdiffusion at high pressures in mineral phases relevant for the Earths mantle

In this study Fe-Mg interdiffusion coefficients were determined at pressures between 6 and 26 GPa and temperatures between 1653 and 2273 K for various constituent minerals of the Earth's mantle employing a multianvil apparatus. Minerals investigated include olivine, wadsleyite, ferropericlase and (Mg,Fe)SiO3 silicate perovskite. For ferropericlase and olivine, oriented single crystal diffusion couples were used whereas for wadsleyite and silicate perovskite only presynthesized polycrystalline diffusion couples could be employed. As capsule material, Au and Ni-NiO capsules were chosen for olivine and Ni-NiO capsules for wadsleyite, ferropericlase and silicate perovskite. In addition, in the case of perovskite, single crystal MgO capsules in contact with metallic Fe were employed. Diffusion profiles were measured after the diffusion experiments either by electron microprobe analysis (EPMA) or for profiles < 8 microns long by energy dispersive X-ray spectrometry on a transmission electron microscope equipped with a scanning unit (EDX-STEM). In the case of olivine and silicate perovskite, diffusion coefficients were found to be essentially constant within the compositional range investigated resulting in symmetrical diffusion profiles. In this case, the profiles were fitted with an analytical solution to the diffusion equation. On the other hand, wadsleyite and ferropericlase exhibited strongly asymmetric profiles implying a strong composition dependence of Fe-Mg interdiffusion. For elucidating the compositional dependence, profiles were simulated using the finite difference method. The activation volume of olivine along the Ni-NiO buffer and the crystallographic c-direction was constrained to be 5.6 +/- 0.5 cm^3/mol. A fO2 correction leads to an activation volume of 7.4 +/- 1.0 cm^3/mol at constant fO2. The temperature effect observed at 12 GPa in Au capsules is consistent with the 1 bar activation energy employing a pressure and fO2 correction. This observation leads to the conclusion that results obtained from Au capsule experiments are consistent with an fO2 at the Ni-NiO buffer. Therefore, results obtained for wadsleyite at 15 GPa and 1673-1773 K in Ni-NiO capsules were combined with the previous results reported in the literature resulting in an activation energy of 260 +/- 30 kJ/mol along the Ni-NiO buffer at 15 GPa. The compositional dependence of diffusion is stronger than for olivine and can be described by a factor of exp((11.8 +/- 1.5)*X(Fe2SiO4)), where X(Fe2SiO4) is the mole fraction of Fe2SiO4 in the solid solution. Ferropericlase experiments were performed over a pressure range between 8 and 23 GPa leading to an activation volume of 3.3 +/- 0.1 cm^3/mol along the Ni-NiO buffer. This value corresponds to an activation volume of 5 +/- 1 cm^3/mol at constant fO2. The activation energy was determined to be 255 +/- 16 kJ/mol along the Ni-NiO buffer and the compositional dependence of exp((132 +/- 13) kJ/mol)*X(FeO)/(R*T)), where X(FeO) is the mole fraction of FeO, was found to be consistent with models at 1 bar. Fe-Mg interdiffusion in (Mg,Fe)SiO3 perovskite is orders of magnitude slower than in the other investigated phases. At oxygen fugacity conditions of the Ni-NiO buffer, the preexponential factor is (5.1 +/- 2.0)* 10^-8 m^2/sec and the activation energy is (404 +/- 144) kJ/mol. Compared to ferropericlase with X(FeO) = 0.05-0.1 the difference in the Fe-Mg interdiffusion coefficient is on the order of a factor of 20000, with the smaller diffusion coefficient in perovskite. Because perovskite is believed to be the dominant phase in the lower mantle (approximately 80 vol%), the effective diffusion coefficient De of the lower mantle should be mainly determined by the diffusion coefficient of perovskite. An effective diffusion coefficient of De = 2.4 * D(pvsk) was estimated assuming that ferropericlase occurs as isolated grains. The very low rates of diffusion in silicate perovskite and hence in the lower mantle implies that reequilibration kinetics in the lower mantle are very slow. Detailed calculations show that even on the time scale of the age of the Earth (4.5 * 10^9 years) the reequilibration distance is only on the order of 1 m. Therefore, chemical heterogeneities in the lower mantle, resulting for example from subduction, cannot effectively be erased by lattice diffusion alone. Only in the thermal boundary layer at the core-mantle boundary larger interaction distances (approximately 10-800 m), depending on the model used, might exist. During core formation, extensive reequilibration of percolating liquid metal in the lower mantle occurs on timescales of arround 100000 years whereas large diapirs would never reach a new equilibrium state with surrounding oxides and silicates solely by diffusion.