超高圧下の水 : 第2のマグマ

概要

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The compositions of aqueous fluid coexisting with mantle minerals have been examined experimentally in the system MgO-SiO_2-H_2O at pressures of 3-8 GPa and at temperature of 1100℃ using an MA-8 type apparatus. The composition of the aqueous fluid coexisting with mantle minerals, forsterite and enstatite, was determined chemographically because direct measurements could not be carried out as the aqueous fluid was quenched into a mixture of feathery crystals, glass and nearly pure water in the recovered sample charge. Solubility of silicate components in aqueous fluid coexisting with forsterite and enstatite increases with increasing pressure up to 8 GPa; from about 50 wt% at 3 GPa to 70 wt% at 8 GPa. The Mg/Si wt. ratio of these aqueous fluid is much higher than that at low pressure (0.2 at 1.5 GPa; Nakamura and Kushiro, 1974), and almost constant (1.2) at pressures between 3 to 8 GPa, Aqueous fluid migrating upward through the mantle can, therefore, dissolve large amount of silicates, leaving modified Mg/Si ratios of residual materials. This process could result in differentiation of orjginally homogeneous mantle without melting. The observation that the aqueous fluid has composition similar to hydrous silicate magma may indicate the possibility of continuous change of aqueous fluid to silicate melt with increasing temperature at such high pressures.
1997-04-25