Fe-Mg interdiffusion in (Mg,Fe)SiO3 perovskite

(Mg,Fe)SiO₃ perovskite is the dominant mineral in the lower mantle. Understanding the chemical and physical properties of this phase is therefore essential for understanding processes such as metal-silicate separation during core formation and mantle convection. An important parameter in describing the kinetics of such processes is the diffusion coefficient as function of P, T and fO₂. Although there is a relatively large database for diffusion coefficients in metals, oxides and silicates at 1 bar, high pressure results are still scarce due to experimental difficulties. Developments in multianvil techniques during the last 20 years allow the synthesis of high pressure phases such as (Mg,Fe)SiO₃ perovskite. Only in the last few years have diffusion experiments became possible as the result of improved pressure-temperature control, the development of large volume presses and new analytical techniques. However, there has been only one study of diffusion in perovskite (Si self diffusion, Yamazaki et al., 2000, PEPI, 119, 299-309). In this study of Fe-Mg interdiffusion, polycrystalline samples of (Mg,Fe)SiO₃ perovskite were synthesized and then used for diffusion experiments at conditions of 23-25 GPa, 1750^° C to 2100^° C with run durations of 4 to 24 hours. In a first set of experiments we used MgO single-crystal capsules together with added Fe foil which buffer the fO₂ at relatively reducing conditions. Fe-Mg diffusion profiles are extremely short even after 24 h and cannot be measured by the electron microprobe. We therefore used EDX-TEM with which concentration profiles down to approximately 10 nm in length can be measured with relatively good counting statistics. Diffusion profiles measured on our samples are 100-600 nm long. In one sample we clearly observe that diffusion can be influenced by subgrain boundaries. Such regions of the sample have to be carefully avoided when analyzing diffusion profiles. At 1750^oC and 24 GPa a Fe-Mg interdiffusion coefficient of 3.66 x 10^-20 m²/sec is derived from our measurements. Fe-Mg interdiffusion in perovskite is therefore several orders of magnitude slower than in other phases at comparable temperatures. Currently, experiments are being performed on samples contained in Re-capsules to produce a more oxidizing environment. These type of experiments show that it is possible to investigate transport properties of lower mantle minerals. Future progress in experimental equipment will allow higher pressures to be achieved with large sample volumes and long run durations.