HR: 09:30h
AN: V11C-07 INVITED     [Abstracts]
TI: Second Critical Endpoint in Peridotite-H2O System and its Bearing on the Magmatism in Subduction Zones
AU: * Mibe, K
EM: mibe@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: * Mibe, K
EM: mibe@eri.u-tokyo.ac.jp
AF: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC 20015 United States
AU: Kanzaki, M
EM: mkanzaki@misasa.okayama-u.ac.jp
AF: Institute for Study of the Earth_fs Interior, Okayama University, Misasa, Tottori, 682-0193 Japan
AU: Kawamoto, T
EM: kawamoto@bep.vgs.kyoto-u.ac.jp
AF: Institute for Geothermal Science, Graduate School of Science, Kyoto University, Noguchibaru, Beppu, 874-0903 Japan
AU: Matsukage, K N
EM: kyoko.nishihara@yale.edu
AF: Department of Environmental Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki, 310-0056 Japan
AU: Fei, Y
EM: fei@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC 20015 United States
AU: Ono, S
EM: sono@jamstec.go.jp
AF: Institute for Frontier Research on Earth Evolution (IFREE), Japan Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, Kanagawa, 237-0061 Japan
AB: Migration of fluid/melt through mantle wedge plays an important role on the magmatism in subduction zones. In general, both the solubility of water in silicate melts and the solubility of silicate materials in aqueous fluid increase with increasing pressure. This could suggest that, above a certain critical pressure and temperature (the second critical endpoint), silicate melts and aqueous fluid become indistinguishable from each other. Before discussing the movement of these two mobile phases, therefore, the stability field of each phase should be investigated. In order to determine the second critical endpoint in peridotite-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 1.7 to 4.0 GPa and at temperatures up to about 1400 deg. C. Pressure is applied first, and then temperature is increased. At around 1000 deg. C and 1.7 GPa, a dark gray sphere appeared in the light gray matrix. The light gray matrix that absorbed less X-ray is considered to be an aqueous fluid phase, whereas the dark gray sphere is silicate melt. With further increasing temperature, the drastic overturn was observed. In the experiments up to 3.6 GPa, two phases (fluid and melt) were observed. At 4.0 GPa, we could not distinguish two phases in the radiographic images. Our experimental results indicate that aqueous fluid and silicate melt can coexist up to 3.6 GPa and there is no difference between these two phases above 4.0 GPa. It could be concluded that the second critical endpoint in the system peridotite-H2O occurs at pressure between 3.6 and 4.0 GPa. At pressures below the second critical endpoint, chemical differentiation could be occurred by the fluid-liquid immiscibility. On the other hand, no such differentiation could happen above the second critical endpoint because the only supercritical fluid is stable. Our experimental results suggest important implications for the magmatism under hydrous conditions, such as the subduction zone magmatism.
DE: 8434 Magma migration
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting