HR: 14:25h
AN: V33C-04    [Abstracts]
TI: Direct Observation of Critical Behaviors Between Aqueous Fluids and an Andesitic Melt: Major Element Chemistry of Supercritical Fluids in Mantle Wedge
AU: * Kawamoto, T
EM: kawamoto@bep.vgs.kyoto-u.ac.jp
AF: Inst Geothermal Science, Kyoto Univ, Noguchibaru, Beppu, 874-0903 Japan
AU: Kanzaki, M
EM: mkanzaki@misasa.okayama-u.ac.jp
AF: Inst Study of the Earth's Interior, Okayama Univ, Onsen, Misasa, 682-0193 Japan
AU: Mibe, K
EM: mibe@eri.u-tokyo.ac.jp
AF: Earthquake Research Inst, Univ of Tokyo, Yayoi, Bunkyo, Tokyo, 113-0032 Japan
AU: Matsukage, K N
EM: kmatsu@mx.ibaraki.ac.jp
AF: Dept of Environmental Science, Ibaraki Univ, Bunkyo, Mito, 310-0056 Japan
AU: Ono, S
EM: sono@jamstec.go.jp
AF: IFREE, JAMSTEC, Natsushima, Yokosuka, 237-0061 Japan
AB: Aqueous fluids dissolve significant amounts of silicates under high-temperature and high-pressure conditions. Silicate components dissolved in aqueous fluids coexisting with mantle peridotite change their major element chemistry from silisic at 1-2 GPa to peridotitic at 3 GPa and higher pressures (Ayers et al. 1997 EPSL; Stalder et al. 2001CMP; Mibe et al. 2002 GCA; Kawamoto et al 2004 Am Min). In the present study, we show direct observations of complete mixing between aqueous fluids and a calc-alkaline andesitic melt (61.5 weight percent SiO2) at around 1 GPa by use of Bassett type externally heated diamond anvil cell (Bassett et al 1993, Rev Sci Instrum). Aqueous fluids and andesitic melts can mix completely in 2 - 3 GPa. Mibe et al. (2004 Fall Meeting) reported a possible second critical endpoint between aqueous fluids and a peridotitic melt at 3.6 _| 4 GPa by use of synchrotron X-ray radiography using multi-anvil type high-T and high-P apparatus at SPring-8. These experiments suggest that the slab-derived component can be under supercritical conditions. Whether the component has chemical characteristics like a partial melt or an aqueous fluid depends on temperature. In wedge mantle, such supercritical fluids can lose water by forming hydrous minerals as reactions with mantle minerals, and get rich in silicate components. In this case, those fluids can become gradually water-rich melts. Critical temperature, which is a boundary between two-fluids and a single supercritical fluid regions, increases as decreasing pressure (Shen and Keppler, 1997 Nature; Bureau and Keppler 2001 EPSL). Therefore, supercritical fluids separate into silicate melts and aqueous fluids along its migration to the surface. In this case, an elemental fractionation should occur between an aqueous fluid and a silicate melt.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005