To investigate the critical phenomena between aqueous fluid and hydrous silicate magma in the Earth's interior, a new method for making direct observations of immiscible fluids was developed using X-ray radiography technique together with the Kawai-type double stage multianvil system driven by a DIA-type cubic press (SPEED-1500) installed at BL04B1, SPring-8, Japan. Basalt-H2O and Sr-plagioclase-H2O systems were investigated. A new sample container comprising of a metal (Pt or AuPd) tube with a pair of lids made of single crystal diamonds was used under pressures between 2 and 5 GPa and temperatures up to 1500°C. The sample inside the container can be directly observed through the diamond lids.
At around 800-900°C and pressures up to about 3 GPa in both basalt-H2O and Srplagioclase-H2O systems, we observed light-gray spherical bubbles moving upward in the darkgray matrix. The light-gray spheres that absorb less X-rays represent an aqueous fluid, whereas the dark-gray matrix represents a silicate melt. At least up to 1200°C, immiscible two phases (i.e., both aqueous fluid and silicate melt) were observed up to about 3 GPa in the basalt-H2O system. At pressures above about 4 GPa, no bubbles appeared when heating to about 1200°C, suggesting the possibility that the second critical end-point in basalt-H2O system could be around 3 to 4 GPa. Our new technique can be applied to the direct observations of various coexisting 2-fluids under deep mantle conditions that could not be achieved by previous experimental methods.