Seismic tomography indicates that the descent of some slabs is impeded in the region from 660 to 1000 km depth, referred to as the mid-mantle. It is also indicated that the ascent of plumes might be accelerated in the mid-mantle. A recent evaluation of viscosity variation suggested that the viscosity may increase or have a maximum in the mid-mantle. This viscosity increase or maximum may impede the slab subduction and accelerate the plume upwelling. However, there is no first-order phase transition in the mid-mantle, and therefore, the possible viscosity variation should be caused by the rheological change of the dominant lower-mantle constituent mineral, bridgmanite. The absence of seismic anisotropy in the significant part of the lower mantle suggests the dominance of diffusion creep. Element diffusivities and grain size are two essential factors of diffusion creep, and defect chemistry controls element diffusivity. Therefore, the defect chemistry, diffusivity, and grain-growth rate of bridgmanite under mid-mantle conditions are to be investigated in this project.
Although the multianvil press is the most reliable tool to investigate mineral properties under high P-T conditions, the pressure and temperature ranges of the conventional multianvil technology are insufficient for the present purpose. However, we recently developed multianvil technology that can generate mid-mantle conditions. Therefore, we determine the defect chemistry, element diffusivity, and grain growth rate of bridgmanite under mid-mantle conditions using our advanced multianvil technology to understand the origin of the viscosity increase that may cause the slab stagnation and plume acceleration in the mid-mantle.
Until recently, the majority of geophysicists and mineralogists considered that it should be at the 660-km discontinuity where physical properties significantly change because of the first-order phase transition of the dissociation of ringwoodite to bridgmanite plus ferropericlase. As mentioned above, however, the geophysical observations have declined this expectation and shown that the change occurs in the mid-mantle. Therefore, Clarifying the origin of the change in rheology in the mid-mantle is currently an essential task for geophysics and mineralogy.
The current project also has particular importance in high-pressure science. Before our technological development, the pressure limit of multianvil presses with carbide anvils has been kept up to 26 GPa, which has remained unchanged for a few decades. Although much higher pressures (~120 GPa) can be generated using sintered diamond anvils, the use of sintered diamond anvils is impractical due to the required extremely high experimental skills and extremely high prices. Our technology can generate pressures to 40 GPa easily and 50 GPa with some special efforts. Thus we have extended the pressure range by 1.5 to 2 times wider. We also attempt to expand the temperature range of multianvil experiments. Although the practical temperature range of multianvil experiments is 300 to 2000 K, a recent innovation allows us to generate a temperature of 4000 K using a boron-doped diamond heater. We attempt to make this technique more practical for many multianvil workers.
We further aim to expand the pressure range at extremely high temperatures by combining a boron-doped diamond heater and sintered diamond anvils. Our final target is the simultaneous generation of 80 GPa and 2500 K. There are many applications of these technologies not only in Earth science but also in material science. Further development of multianvil technologies through the current project will bring tremendous benefit to both scientific communities.
