High-pressure experiments on the stability of hydrous minerals likely to be present in the Earth's mantle provide constraints on the distribution of water in the mantle, and the form in which it is stored. However, there has been confusion about compositions and structures between hydrous minerals. In regions of elevated mantle temperature, water may be stored not in hydrous minerals but in anhydrous minerals, melts, and fluids. Such water is important to understand the dynamics and evolution of the mantle, volcanism, and metasomatism. In this article, the state and migration of water in the mantle are reviewed.

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In order to understand the magmatic process, a knowledge of the physical properties of silicate melt is required. Density and viscosity are especially fundamental properties, which control the migration of magmas and the separation of residual crystals. In this article, recent advances in the investigation of silicate melt at high pressure are reviewed.

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The earth's lower mantle is mainly composed of (Mg, Fe) SiO₃ perovskite and (Mg, Fe) O magneSiOwustite. It is essential to determine the rheological properties of MgSiO₃ perovskite and periclase for understanding the rheology of the lower mantle. High pressure and temperature experiments were carried out under lower mantle conditions to determine their rheological properties. The grain growth rates of perovskite and periclase were determined to be G^{10. 6} [m] = 1×10^{-57. 4}t [sec] exp (-320. 8 [kJ/mol] /RT) and G^{10. 8} [m] =1×10^{-62. 3}t [sec] exp (-247. 0[kJ/mol]/RT), respectively, where G is grain size at time t, R is the gas constant and T is the absolute temperature. The lattice diffuSion coefficient (D₁) and grain boundary diffuSion coefficient (D_gb) of silicon in MgSiO₃ perovskite were determined at 25GPa and 1673-2073 K to be D₁ [m²/sec] = 3. 76×10^-10exp (-338 [kJ/mol] /RT) and δD_gb [m³/sec] =1. 02×10^-16exp (-303 [kJ/mol]/RT), respectively, where δ is the width of the grain boundary. The grain size of perovskite in the lower mantle is estimated to be 1-10 mm, which suggests diffusion creep (Nabarro-Herring creep) as a dominant deformation mechanism in the greater part of the lower mantle. The present results indicate that the subducting slab is much softer than the surrounding lower mantle due to the slow grain growth rate.

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Transformations of mantle minerals have a great effect on the dynamics of the descending oceanic plate through metastable reactions and changes in the microstructures of the transformed minerals. High pressure and temperature in situ X-ray observation experiments have been performed in order to reveal the mechanisms and kinetics of the olivine-spinel transformation. In the interior of cold slabs, the depth of transformation of olivine mainly depends on the growth rate of spinel. The determination of the growth rate suggests that olivine survives metastably at a depth of∼600km in cold slabs. Water, strain energy due to the transformation, and intracrystalline transformation have significant effects on both the depth and width of the field of metastable olivine.

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In Situ X-ray diffraction technique has been successfully used to determine the phase transition pressures of mantle minerals at high pressure and high temperature. This technique has an advantage in accurate estimation of pressure using internal pressure standards such as NaCl, Au, and MgO, as well as the simultaneous identification of the phases present at high temperature and high pressure. This paper reviews these studies which attempted to determine the phase boundaries based on in situ X-ray diffraction measurements, including our recent results obtained at the new synchrotron facility, Spring-8. Some technical problems inherent to this technique are discussed on the basis of these currently available experimental data.

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Recent advances in a laser-heated diamond anvil cell technique were reported in this article. The details of the double-sided laser heating system and the sample assembly were discussed to realize stable heating for a long duration and a reduction of the temperature gradient in the diamond anvil cell. Several experimental techniques for in situ X-ray observation were reviewed also. Phase relations of synthetic and natural garnets under lower mantle conditions were investigated using the laser-heated diamond anvil cell combined with synchrotron radiation.

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Basic techniques for ultrasonic measurements are discussed with emphasis placed on accuracy of sound velocity. Recent ultrasonic studies under high pressure are briefly reviewed, and a new analytical methodology called the complete thermodynamic equation of state is introduced. The potential of the method is demonstrated by analyzing a synthetic model data set. The results suggest that the method can be used to constrain thermodynamic parameters, such as heat capacity and thermal expansivity, up to pressures of the core-mantle boundary using experimental data collected only up to 10 Gpa.

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Water has two amorphous solid states at low temperatures: the low-density and high-density amorphous ices. These two amorphous solids may be related to liquid water at high temperatures. We review experiments and simulations on liquid and amorphous solid water, and discuss the possibility of the existence of a liquid-liquid phase transition in supercooled water.

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An x-ray powder diffraction technique with an imaging plate area detector is now widely used in various experimental fields including high pressure research. We developed a computer program named PIP capable of converting two-dimensional powder image to one-dimensional pattern. An aim of this program is to prepare easily, quickly, and user friendly one-dimensional diffraction patterns available for structure analysis such as Rietveld method. In this report, we answer some questions about PIP frequently asked by users.

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This paper describes the high pressure mineral physics research at SPring-8, the new third -generation synchrotron radiation facility in Hyogo, Japan. SPring-8 has four experimental stations with several pressure apparatuses for high pressure research. These facilities are open to independent researchers, and various kinds of high pressure experiments are currently being conducted and planned to reveal the physical and chemical properties of the Earth and the planetary interiors.

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