In this article, the following recent advances in the deep mantle rheology are reviewed. (1) High-precision global gravity data put new constraints on the present deformation of the deep mantle and, in turn, mantle viscosity. (2) Anisotropic structure has been revealed in the mantle transition zone and the lower mantle. (3) Pressure condition of strain-rate controlled deformation experiments has been expanded to 15-25 GPa at 1700-2100 K. New insight into dynamics of the Earth's interior should be brought by interpreting the new geophysical observations with rheological data that will be obtained by the advanced deformation experiments.
Diffusion experiments of garnet, wadsleyite, and ringwoodite, which are the major constituents of the mantle transition zone, have been conducted using the Kawai-type multi-anvil high-pressure apparatus. Si diffusion rates appear to be the slowest in the major element in these minerals, and they control the rates of plastic deformation. Based on the Si diffusion coefficients, rheological properties of the mantle transition zone are discussed.
Measurements of thermal conduction for materials in the Earth's upper mantle and the transition zone under high pressure have been developed by using a Kawai-type apparatus, with a one-dimensional plane pulse-heating method. This method is a predominant one for study in mantle materials under pressure because it requires comparatively small amount of sample and is applicable to materials with anisotropy in thermal conduction. Thermal conductivities and thermal diffusivities of olivine, garnet, pyroxenes, serpentine and talc were measured under high-pressure using this method.
Thermal conductivity of lower mantle minerals is essential for controlling the rate of core heat loss and long-term thermal evolution of the Earth, but it has been poorly constrained at the high pressures of the Earth's lowermost mantle. We have newly developed an apparatus for measuring the thermal diffusivity using a pulsed light heating thermoreflectance technique under high pressure in a diamond anvil cell. The new method enabled us to determine the lattice thermal diffusivity of both MgSiO3 perovskite and post-perovskite, the main constituent of the Earth's lower mantle, at room temperature and at high pressures up to 144 GPa greater than the core-mantle boundary pressure. Lattice thermal conductivity of perovskite-dominant lowermost mantle assemblage obtained in this study is about 11 W·m−1·K−1, while post-perovskite bearing rocks exhibit ～60% higher conductivity. Such perovskite value is comparable to the conventionally assumed lowermost mantle conductivity of 10 W·m−1·K−1.
An experimental method to study the seismic attenuation factor (Q−1) and anelastic properties of materials at high pressure and high temperature has been established by using the multianvil high-pressure deformation device (D-DIA) and a synchrotron X-ray radiography at SPring8. Time resolved images of the sample and reference material provide their strain as a function of time during cyclic loading. Attenuation is determined as the tangent of the angle of phase lag between the sample and the reference material. A newly installed short period sinusoidal cyclic loading oil pressure system enable us to determine minimal strain of the sample in a wide frequency range from 2 to 0.01 hertz on olivine aggregates at 1 GPa and up to 1673 K. The detectable minimum strain is around 5×10−5. Several test experiments exhibited resolvable Q−1 (10−2) above 1273 K. The results are generally consistent with previously reported data.
Untransformed metastable minerals exist under the low-temperature conditions of the subducting plates. Here I report the experimental studies on the transformation kinetics of pyroxene and garnet at high-pressure and high-temperature conditions. Slow kinetics of the pyroxene-garnet transformation due to the low atomic diffusion rate indicates that large low-density metastable regions would exist in the slab, greatly contributing to the slab stagnation around the mantle transition region.
Physical properties at ambient condition are only a small fraction of the real figure of materials, and their hidden properties are revealed by pressurization. I and collaborators have studied on physical properties of elements under high pressure using first-principles calculations based on the density functional theory. In this article, I briefly introduce our previous and current works on pressure-induced structural and superconducting transitions in hydrogen, calcium, yttrium, iron, copper, gold, carbon, phosphorus, oxygen, iodine, and argon.
The high-pressure behavior of SiO2 glass has attracted considerable attention because of its importance not only in condensed-matter physics and materials science but also in geophysics. We have conducted X-ray absorption/diffraction measurements and optical-microscope observation to understand the behavior of SiO2 glass under hydrostatic and uniaxial compression. SiO2 glass undergoes pressure-induced structural transformations first in the intermediate-range order (the network structure consisting of SiO4 tetrahedra) and then in the short-range order (the coordination number). Under uniaxial compression, the atomic arrangement in intermediate-range order deforms largely and a large differential strain remains after decompression. This article attempts to review these interesting behaviors, mainly based on our recent researches.