HR: 14:10h
AN: V33C-03    [Abstracts]
TI: Second critical endpoint in the basalt-H₂O system
AU: * Mibe, K
V33C-03 AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kanzaki, M
V33C-03 AF: Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori, 682-0193 Japan
AU: Kawamoto, T
V33C-03 AF: Institute for Geothermal Science, Graduate School of Science, Kyoto University, Beppu, Oita, 874-0903 Japan
AU: Matsukage, K N
V33C-03 AF: Department of Environmental Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki, 310-0056 Japan
AU: Fei, Y
V33C-03 AF: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd, Washington, DC 20015-1305 United States
AU: Ono, S
V33C-03 AF: Institute for Research on Earth Evolution (IFREE), Japan Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, Kanagawa, 237-0061 Japan
AB: The water released from subducting slab plays an important role in mass transport within subduction zones. Therefore, understanding the phase relations in the basalt-H₂O system is fundamental for clarifying the subduction zone magmatgism. In order to determine the second critical endpoint in the basalt (MORB)-H₂O system, experiments were conducted using X-ray radiography technique together with Kawai-type double-stage multi-anvil high pressure apparatus (SPEED-1500) installed at SPring-8, Japan. Direct X-ray beam, which passes through the anvil gaps of SPEED-1500 and sample under high pressure, is observed with an X-ray camera. The sample container should not react with hydrous samples, but should be x-ray transparent. We, therefore, developed a new sample container, which is composed of a metal tube and a pair of single crystal diamond lids put on both ends of metal tube. The sample in the metal container can directly be observed through the diamond lids with X-ray radiography. The experimental conditions are at pressures from 1.8 to 4.0 GPa and at temperatures up to about 1400 deg. C. Pressure is applied first, and then temperature is increased. In the experiments at pressures below the second critical endpoint, the experimental P-T path encounters the stability field of aqueous fluid + silicate melt. In this case, one phase (either aqueous fluid or silicate melt) forms spheres in the other phase because of their differences in the interfacial tension. Therefore, round shape is expected to be observed in the radiographic images. On the other hand, in experiments beyond the second critical endpoint, round shape should not be observed because supercritical fluid is the only phase existing at high temperature. In the experiments up to 3 GPa, two phases (fluid and melt) were observed. Above 3 GPa, however, we could not distinguish two phases in the radiographic images, indicating that aqueous fluid and silicate melt can coexist up to 3 GPa and there is no difference between these two phases above 3 GPa. It could be concluded that the second critical endpoint in the system basalt (MORB)-H₂O occurs at around 3 GPa. Our experimental results suggest that the aqueous fluid and silicate becomes indistinguishable at the depths deeper than 90 km in subducting hydrous slab. The fluid released from the slab in the for-arc region could be one phase (aqueous fluid) or two phases (aqueous fluid + hydrous silicate melt) depending on the slab temperature, whereas the released fluid beneath the volcanic front and in the back-arc side should be the only one supercritical fluid phase.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005