HR: 0800h
AN: V41A-1435    [Abstracts]
TI: Internally consistent thermodynamic data set for dense hydrous magnesium silicates up to 35 GPa, 1600°C: implications for water circulation in the Earth's deep mantle
AU: * Komabayashi, T
EM: tkomabay@geo.titech.ac.jp
AF: Dept. Earth Planet. Sci., Tokyo Inst. Tech., 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AU: Omori, S
EM: omori@geo.titech.ac.jp
AF: Dept. Earth Planet. Sci., Tokyo Inst. Tech., 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AB: Fluid-absent high-pressure experiments were carried out in the system MgO-SiO₂-H₂O in a multi-anvil apparatus from 11.0 to 21.5 GPa, and from 800 to 1200°C in order to constrain fluid-absent solid-solid reactions. From the determined fluid-absent phase equilibria, thermodynamic parameters of high-pressure hydrous phases were retrieved by linear programming method. Together with dry mantle minerals from the existing data set, one set of thermochemical and thermophysical parameters with internal consistency for dense hydrous magnesium silicates (DHMSs) was evaluated up to 35 GPa and 1600°C. High-pressure hydrous phases involved in the data set are, phase A, phase E, clinohumite, phase D, superhydrous phase B, hydrous wadsleyite, and hydrous ringwoodite. In addition, by calculating water-bearing reactions we have estimated water activities in the fluid phase. The water activity decreases with increasing pressure or temperature, suggesting the amount of silicate component dissolved in the fluid phase increases with pressure or temperature. The calculated petrogenetic grid consisting of 89 univariant reactions can be used for the discussion of water circulation in the mantle. In the subducting slab peridotite, water in phase A as the post-serpentine phase is transferred to other DHMSs only by the solid-solid reactions down to the bottom of the transition zone. At around 660-km depth, in the stagnant slab heated from the surrounding mantle, the free fluid will be released by the dehydration of superhydrous phase B or phase D. The released fluid will be trapped by wadsleyite or ringwoodite in the surrounding mantle. In the case of very cold subduction without stagnation at 660-km depth, DHMSs will survive into the lower mantle conditions. In the upwelling plume through water-bearing transition zone, the free fluid will be produced when the plume passes 410-km depth. The fluid generated in the plume will facilitate the movement of upwelling in the upper mantle. Finally water will be released to the Earth's surface by magmatism. The fluid generated in the deep mantle dissolves significant amounts of silicate component as inferred from the low water activity. Therefore, at 410- and 660-km depths, some chemical differentiation will be processed.
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 1655 Water cycles (1836)
DE: 3630 Experimental mineralogy and petrology
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005