We will clarify reasons for viscosity increase in the mid-mantle!

A seismic tomography study [Fukao and Obayashi, 2012] depicted slab images in the circum-Pacific regions, which showed that, although slabs under and near Japan and under the middle part of Southern America stagnate above the 660-km discontinuity, the majority of other slabs stagnate between 660 and 1000 km depths. Another tomographic study [French and Romanowicz, 2015] depicted plume images in the whole Earth, which showed that, although vertically low-velocity columns are seen below hotspots in the deep mantle, they become tomographically invisible or unclear above a depth of 1000 km. These studies imply that mantle viscosity vastly changes above a depth of 1000 km. Evaluation of 1-dimensional viscosity distribution by Rudolf et al. [2015] suggested mantle viscosity increases from 660 to 1000 km depth.   

Such viscosity increase inferred from geophysical studies should be explained based on mineral physics. Bridgmanite is the dominant mineral in the lower mantle, and therefore, it is the first candidate responsible for the viscosity increase. However, bridgmanite and other minerals do not show any first-order phase transition in the mid-mantle. Therefore it is expected that the mechanical properties of bridgmanite change under conditions of the mid-mantle. Since the strain rate in the lower mantle should be meager, the diffusion creep should be the primary creep mechanism. Therefore, we hypothesize that the element diffusivity and grain size of bridgmanite should change in the mid-mantle because these two properties control the diffusion creep of solids. Furthermore, defect chemistry controls element diffusivity in crystalline solids. Hence, the defect chemistry of bridgmanite under mid-mantle conditions should be understood.

For these reasons, we investigate chemistry, element diffusivity and grain growth rate of bridgmanite under mid-mantle conditions.

To investigate the defect chemistry, high-quality samples equilibrated under well-controlled chemical environments are indispensable. For this reason, the use of large volume presses is mandatory, but pressure conditions that  large-volume presses can generate are limited, usually to 26 GPa. However, we have developed experimental technology to practically generate pressures higher than 27 GPa by large-volume presses [Ishii et al., 2016; 2017]. We investigate the chemistry and transport properties of bridgmanite under mid-mantle conditions using this technology.