It is important to determine the stability of water-bearing minerals at high pressure and high temperature in order to accurately understand the water transport processes in the Earth’s deep mantle. In this article, previous experimental studies and our recent works on the stability of water-bearing minerals under high pressures and high temperatures, were reviewed. We clarified the high-temperature phase relations of hydrous aluminosilicates and the stability of hydrous SiO₂ stishovite by experimental techniques using multi-anvil apparatus. Based on these results, deep water transportation into the lower mantle were discussed.

Recent studies revealed that iron-oxides exhibit various stoichiometry under high pressure and temperature conditions corresponding to the Earth’s mantle. The stability and the physical properties of the high-pressure iron oxides have important implications for the structure and evolution of the Earth’s deep mantle. The iron-oxides are likely stable above approximately 10 GPa with considerably high solubility of the divalent and trivalent cations such as Mg²⁺ and Al³⁺. Some iron-oxides can be stable under relatively low oxygen fugacity, in which the diamond is stable. The phases show characteristic physical properties, such as electrical conductivity, due to the mixed valence states of iron. We summarize the recent findings on high-pressure iron oxides and their implications for the Earth and planetary sciences.

Nitrogen is abundant on the Earth’s surface as it is an essential element of life and occupies 78 vol.% of the Earth’s atmosphere. However, the relative abundance of nitrogen normalized by carbonaceous chondrite in the bulk silicate Earth is depleted compared to other volatile elements. This is the so-called “missing” nitrogen which has been an unraveled mystery in geochemistry and the Earth’s interior is one of the candidates for solving it. In this paper, the behavior of nitrogen in the deep Earth is introduced, mainly about nitrogen differentiation by the core formation, formation of nitrogen reservoir through solidification of the magma ocean, and nitrogen transportation via subducting slabs.

Extensive melting of early Earth’s mantle is considered to occur due to accretionary impacts, resulting in a deep magma ocean. Oxygen fugacity of the mantle is thought to have increased over accretion and core formation. However, the effect of oxygen fugacity on the solidus of mantle materials has not been constrained. Here we discuss the effect of oxygen fugacity based on results on the solidus temperature of mantle pyrolite at a high oxygen fugacity. We find that the solidus of pyrolite under oxidizing conditions is significantly lower than those under more reducing conditions. The strong influence requires reassessment of models of early Earth thermal evolution and core formation.

Methane hydrate, a representative gas hydrate, is considered one of the major constituent materials of icy bodies such as Uranus, Neptune, and Saturn’s moon Titan. However, its physicochemical properties under a wide range of temperature and pressure conditions that correspond to those inside icy bodies have not been fully elucidated. In this article, we focus on the latest findings from our research and introduce the phase transition mechanism, structural evolution, and stability of methane hydrate observed under various temperature and pressure conditions.

This article provides an overview of the author’s 35-year exploration of the physical properties of materials under low-temperature and high-pressure conditions. It outlines the development of various hydrostatic pressure apparatuses during this period, describing their unique features and the representative research outcomes they enabled. High-pressure studies continue to be a vital methodology for uncovering novel physical phenomena, and their role in materials research is anticipated to expand further. By summarizing key advancements and technical innovations, the article aims to offer useful perspectives and contribute to the broader understanding and development of high-pressure research.