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.

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Non-equilibrium textures in metamorphic rocks contain a great deal of information on reaction kinetics and mass transfer in metamorphic processes. The reaction zone between chemically incompatible minerals (or rocks) is a good example of such non-equilibrium textures, and hence it is worth studying it in detail. Korzhinskii's chemical potential diagram is a useful method to understand the zonal sequence in the reaction zone. It can explain the variability in the zonal sequence by specifying the corresponding diffusion path on the chemical potential diagram. However, the method cannot tell us why the specific diffusion path is realized in nature. Fisher-Joesten's steady diffusion model can be considered as a quantification of Korzhinskii's chemical potential diagram, which introduces flux-ratio equations corresponding to specification of the diffusion path on the chemical potential diagram. The steady diffusion model successfully describes the stability fields of zonal sequences in the L-ratios (phenomenological coefficient ratios) diagram. Application of the steady diffusion model to an olivine-plagiocalse corona in a norite from the Ryoke Metamorphic Rock is presented. Extension of the steady diffusion model to open system is also discussed for the case of a metabasite-marble reaction zone in the Hirao Limestone and for myrmekites from the Okueyama granite. Experimental studies of the formation of reaction zones in the system dolomite-quartz-H₂O reveal temporal change in zonal sequence and mineral species of the reaction zones, showing a transition from an initial transient state to a steady state. Finally, I discuss periodic structures in rocks representing a pattern formation in a mineral-fluid system, taking an orbicular marble from the Hirao Limestone as an example. Although our understanding on such a ‘nonlinear structure’ is insufficient, I believe that there are some unknown physico-chemical processes behind such a structure.

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Magma has been deeply related to the evolution and dynamics of the Earth from the magma ocean in the early Earth to present magmatism. Especially, a hot topic is a pressure-induced change in the property/structure of magma and its correlation to the macroscopic property and microscopic structure, with the goal of understanding the deep magma. This article introduces our studies about high-pressure measurements of magma properties with focusing on three topics. First one is the gravitational stability of hydrous magma at the base of the upper mantle based on density measurements at high pressure. Our results indicated a rapid reduction of the partial molar volume of H₂O in magma with pressure, which causes enough densification of hydrous magma to be stagnant. Second, we proposed the ponded magma model at the lithosphere-asthenosphere boundary due to a decrease in the segregation rate. This model can illustrate the geophysically observed anomaly around the boundary. For density measurement of magmas under desired conditions, we succeeded in applying an X-ray absorption technique, which could provide a better understanding of magma behavior. Last topic is lunar magmatism. Compression behavior of lunar basaltic magma with high TiO₂ content indicated the density crossover between the magma and lunar mantle at a certain depth. Based on this result, subducted magma could create lateral chemical heterogeneities in the lunar mantle and, moreover, it could form the low-velocity/low-viscosity layer at the base of lunar mantle.

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Twinned crystals often grow larger and/or in different morphologies than coexisting single crystals. Such characteristics of twinned crystals are generally explained as a result of ‘reentrant corner effect’. However, observations of natural quartz crystals twinned in accordance with Japan law and computational simulations on the twin boundary structures revealed more complex growth mechanism, in which formation of dense Brazil twin lamellae at the {1122} composition plane of Japanese twin boundary causes the preferential growth of Japanese twin. The intersections of Brazil and Japanese twin boundaries were found to be structurally coherent when they have certain composition planes. This structural diversity of the {1122} Japanese twin boundaries causes development of fine Brazil twin lamellae and triggers faster growth.

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Carbonate apatite (CO₃Ap) that contains 6-9 wt% carbonate in apatitic structure is the composition of human bone. Unfortunately, CO₃Ap cannot be sintered due to the presence of CO₃. CO₃Ap block was found to be fabricated based on a compositional transformation though dissolution-precipitation reaction using a precursor such as calcite block. CO₃Ap block thus fabricated up-regulate the differentiation of osteoblastic cells and resorbed by the osteoclasts. As results of these response to cells, CO₃Ap block demonstrate much better osteoconductivity than hydroxyapatite [HAp: Ca₁₀(PO₄)₆(OH)₂)] which is currently used artificial bone substitute. Also, CO₃Ap block was found to be replace to new bone similar to autograft even though HAp would not replace to bone. Clinical trial of CO₃Ap is now in progress, and CO₃Ap will be available for its clinical use next year.

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