2017 年 46 巻 1 号 p. 1-14
In this article, I review four topics of my past researches that are intended to bring new insights about structure and dynamics of the Earth's mantle. Firstly, the phase boundary of dissociation of ringwoodite to bridgmanite plus periclase, so called post-spinel transition, in Mg2SiO4 was determined by means of in-situ X-ray diffraction. This study showed that the Clapeyron slope of this reaction was − 0.4 MPa/K, which is much smaller than previously considered. This shallow slope suggests that seismically observed depression of the 600-km discontinuity under subduction zones cannot be explained by a temperature effect only, and that the 660-km discontinuity will not be a barrier of mantle convection. Secondly, the thermal expansion coefficients of four major mantle minerals were determined by using a new multi-anvil press with an oscillation system, which enables us to obtain high-quality diffraction patterns at high temperatures. Together with the pressures of the olivine-wadsleyite transition in (Mg,Fe)2SiO4, an adiabatic geotherm in the whole mantle was estimated. This profile shows that adiabatic gradients in the upper and lower mantle, respectively are 0.4-0.6 and 0.3-0.5 K/km. Thirdly, the hopping and proton conductions of olivine, wadsleyite and ringwoodite were measured under conditions of their stability fields. The magnitudes of hopping and proton conductions, respectively, are larger and smaller than previously considered, which suggests that the mantle transition zone is essentially dry. Fourthly, water-content dependence of Si lattice and grain-boundary self-diffusion in forsterite was measured to show that their water-contents are around 0.3, which is significantly smaller than those obtained by deformation experiment. In addition, water-content exponent of lattice self-diffusion coefficient of oxygen is zero within uncertainty. These water-content exponents suggest that effects of water on mantle rheology could be limited.
