Mineral solubility and hydrous melting relations in the deep earth: Analysis of some binary A-H₂O system pressure-temperature-composition topologies

Phase relations involving hydrous melting, volatile and mineral solubility and supercritical fluid phenomena at high pressure for mineral-H₂O systems are generally not completely constrained by experimental data or adequately treated in thermodynamic models. Here we examine geometric relations in pressure (P)-temperature (T)-composition (X) topologies of simple hydrous A-H₂O binaries, thus avoiding some of the pitfalls associated with other approaches. The relations between mineral solubility surfaces, wet melting and critical L=V behavior are shown explicitly in a series of PT, TX and isopleth contoured PT projections. Our analysis highlights the significance of Clapeyron slopes of melt- and fluid-solubility isopleths for L+V coexistence, supercritical-fluid phenomena and the geometry of phase-equilibrium boundaries. The results are useful for understanding wet melting, magma degassing and fluid behavior in high-pressure metamorphic and subduction-zone environments. The diagrams illustrate the general pattern of mineral solubility in aqueous fluids and volatile solubility in silicate melts. We discuss the significance of the critical-curve geometry for phase relations and fluid/melt densities. We examine a continuum of phase relation topologies for A-H₂O, and show that these can result from subtle but important differences in the compositional behavior of melt coexisting with H₂O-rich fluid. The systems SiO₂(quartz)-H₂O and NaAlSi₃O₈(albite)-H₂O are taken as examples for which there is experimental data available to calibrate a complete phase relation topology.