Development of high pressure-temperature experimental techniques and studies of phase relations and crystal chemistry of mantle minerals

Abstract

The Kawai-type multi-anvil press (KMA) is one of the powerful high-pressure appraratus to investigate the Earth’s interior. Because KMA can produce a stable pressure-temperature field in a relatively large sample volume, it provides reliable results. Here I introduce recent progress on studies of deep earth science, especially showing my research achievements. I have developed KMA techniques for ultra-high pressure generation and in-situ X-ray diffraction, which have been applied for studies on phase relations and crystal chemistry of mantle-constituent minerals up to mid-mantle conditions. I have studied crystal chemistry of bridgmanite up to 52 GPa in pyrolite and basaltic systems, and found that oxygen vacancy components in bridgmanite disappear around 40 GPa corresponding to 1000 km depth. This phenomenon can explain stagnation of subducting slabs around 1000 km depth. In-situ X-ray diffraction techniques to precisely and accurately determine phase stability have been developed and applied to determine bridgmanite-forimng reaction boundaries: Mg₂SiO₄ post-spinel, MgSiO₃ akimotoite-bridgmanite, and Mg₃Al₂Si₃O₁₂ post-garnet transitions. These studies provide insights into the structures and dynamics between 660 km and 1000 km depths. In addition, I also introduce my recent studies on deep water cycle in the mantle: high-pressure phase relations and water partioning of mantle minerals under hydrous conditions. Water partitioning between nominally anhydrous minerals (NAMs) of olivine and its high-pressure polymorphs and hydrous phases has been studied. The results indicate that these NAMs are nearly dry when coexisting with hydrous minerals, resulting in factors of deep-forcus earthquakes and slab stagnation in a wet subducting plate. Phase relations of aluminous silica minerals under hydrous conditions have been also studied. I found that CaCl₂-type aluminous silica is stable even at top-lower mantle conditions and can accommodate more than 1 wt% water in the crystal structure, suggesting deep water cycle by silica minerals.