地球深部物質学から見たアストロバイオロジー

抄録

I review the circulation style, storage capacity, and current amount of water in the mantle. Possible water subduction and circulation processes in the deep mantle are discussed on the basis of calculated phase relations of simplified hydrous peridotite in the system MgO-SiO₂-H₂O. For water transportation by subduction of lithospheric peridotites, important phase relations are (1) of serpentine at lower pressures less than 10 GPa and (2) of seven different high-pressure hydrous phases including dense hydrous magnesium silicates at higher pressures. Along the cold slab geotherm, large fluid fluxes are predicted at shallow (〜300km depth) and deep (〜700km depth) levels, depending on the slab temperature. The whole mantle may have the storage capacity of water more than ten times as much as the ocean. Nevertheless, the whole mantle seems not to be saturated with water. This would give rise to a mystery of the existence of the ocean at the surface, because the solid Earth should have drained all the water during its accretion period. A solution to this question will contribute to the astrobiology, particularly to the conditions for the habitable planet.