Chapter 3 - The Influence of Pressure on the Properties and Origins of Hydrous Silicate Liquids in Earth's Interior

Abstract

Hydrous silicate liquids are produced in a wide range of deep-Earth environments. I review the influence of pressure on their compositions and properties, a topic about which recent experimental advances are yielding important new insights. Pressure increases the solubility of H2O in silicate liquids. At upper-mantle pressures, the primary dissolution products are molecular H2O and OH−, but extended forms become important at higher pressures. H2O dissolution promotes depolymerization, regardless of speciation. Liquid properties are functions of composition, conditions of formation, and evolutionary path, all of which are strongly influenced by H2O. All silicate–H2O systems exhibit full miscibility at high pressure and temperature, which arises from the efficacy of polymerization/depolymerization of liquid and coexisting solute-rich H2O. Supercritical fluids may have compositions intermediate between silicate liquid and H2O, yet they possess very high H2O activity, which endows them with great metasomatic power. Hydrous silicate liquids are less dense but more compressible than their anhydrous equivalents, translating to a high likelihood of neutral buoyancy at depths where geophysical anomalies suggest melts may be present. Where density differences dictate migration, high-pressure H2O-bearing silicate liquids are quite reactive with the mineral matrix, and likely form high-permeability channels leading to rapid transport. Enhanced mobility is also favored by decreased viscosity with pressure. Electrical conductivity and diffusivity decrease with pressure, yielding values consistent with magnetotelluric data from a range of deep environments.