HR: 17:00h
AN: V24A-05    [Abstracts]
TI: Looking for Critical Endpoints in Aqueous Fluids and Rocks: Input and Then Separation of Supercritical Fluids Underneath Volcanic Arcs
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 Earths Interior, Okayama Univ, Yamada, 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-T and high-P condition. Silicate components dissolved in aqueous fluids coexisting with mantle peridotite change their major element chemistry from andesitic at 1-2 GPa to peridotitic at 3 GPa and higher pressures (Ayers et al., 1997, EPSL; Stalder et al., 2001, CMP; Mibe et al., 2002, GCA; Kawamoto et al., 2004, AM). Observations of unmixing and mixing between aqueous fluid and silicate melts by use of synchrotron X-ray radiography with multi-anvil type high-T and high-P apparatus at SPring-8 allow us to estimate PT conditions of critical endpoint, where critical temperature becomes equal to H2O-saturated solidus temperature (Mibe et al., 2004, GCA). We observe aqueous fluid and a silicate melt coexisting in pressures lower than a pressure, while above the pressure, we observe only one fluid phase, and then interpret it to be the pressure of a critical endpoint. We found a systematic increase in pressure of critical endpoint with decreasing SiO2 concentrations: sediment (63 SiO2 wt. %, 2.6 GPa), high-Mg-andesite (60 SiO2 wt. %, 2.9 GPa), MORB (50 SiO2 wt. %, 3 GPa, Mibe et al., 2007 under review), and peridotite (44 SiO2 wt. %, 3.8 GPa, Mibe et al., 2007 JGR). The chemistry of silicate components in aqueous fluids derived from downgoing slabs can vary from silica-rich in sediment and basalt layers to magnesium-rich in peridotite layers. The present set of data of critical endpoint pressures covers a wide range of chemical compositions and suggests that slab-derived fluids should be under supercritical conditions at the downgoing slabs beneath the volcanic arcs (Tatsumi and Eggins, 1995, Blackwell). This means a continuous change from hydrous melts to aqueous fluids at the base of mantle wedge underneath volcanic arcs. Whether the slab-derived fluids have chemical characteristics like a partial melt or an aqueous fluid depends on the temperature; slab derived-supercritical fluids in relatively warm regions can dissolve more silicate components than slab derived-supercritical fluids in relatively cold regions. The melt-like supercritical fluid formed at the base of the warm mantle wedge will separate into a melt phase and a fluid phase when the supercritical fluid meets its critical curve during its ascent. Partitioning of elements between aqueous fluids and hydrous silicate melts should play a significant role when the supercritical fluid would separate into them. Adakites are characterized by high Sr/Y ratio and low Y, which can be formed by garnet residue through a partial melting of downgoing basaltic crust (Defant and Drummond, 1990, Nature). Partitioning data between alkali chloride fluids and an andesite melt (Keppler, 1996, Nature) indicate high Sr/REE in alkali chloride fluids at 0.3-2 GPa. If this is the case in the nature, Sr/Y of fluids becomes higher than a coexisting silicate melts, and Y remains to be low. Adakites can be produced not by partial melting but by fluids-induced melting; large chemical variations observed in adakites with respect to Sr/Y and Y can be formed by the separation of supercritical fluids into fluids and silicate melts.
DE: 1020 Composition of the continental crust
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1037 Magma genesis and partial melting (3619)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting