Al,Si-interdiffusion in bridgmanite and the viscosity of the lower mantle

• Applicant: Dr. Nobuyoshi Miyajima, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry ; Geophysics ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 404345772

The viscosity of the Earth’s mantle is a key parameter in all model calculations on mantle convection and on the thermal state of the Earth. Yet, the viscosity of the lower mantle, which is the largest reservoir of our planet, is only poorly constrained. The viscosity of solids is related to the deformation creep rate. Creep deformation at high temperature is due to transport of matter by the motion of lattice defects. Therefore, viscosities can be estimated from diffusion data using the Nabarro-Herring and Weertman models. In this proposal, I show some preliminary data, which suggest that silicon diffusivities in majorite and silicate perovskite (bridgmanite) are different and that the diffusivity of silicon in perovskite is strongly dependent on the Al content in the system. In the proposed project, Si diffusivity in silicate perovskite, the dominant phase of Earth´s lower mantle, shall be further investigated: (i) Al,Si-interdiffusion in magnesium silicate perovskite shall be systematically studied using multi-anvil press. (ii) The dependence of Si diffusivity on chemical composition and Al substitution mechanism shall be studied using different diffusion couples (e.g., with vacancies). (iii) From the experimental results, the viscosity in the lower mantle shall be estimated, assuming that plastic deformation rate is controlled by the slowest diffusing species in the constituent minerals. The results of this study will provide important constraints on the mode of convection and chemical mixing in Earth´s mantle. The relative importance of layered versus whole-mantle convection and the likelihood of the survival of primordial, undepleted reservoirs in the deep mantle can only be quantitatively addressed with the viscosity data that are expected to result from this study.

• DFG Programme: Research Grants