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

Alistair C. Hack^*,**,, Jörg Hermann^** and John A. Mavrogenes^**,

^* Institute for Mineralogy and Petrology, ETH Zürich, Zürich 8092, Switzerland
^** Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia
Department of Earth and Marine Sciences, The Australian National University, Canberra, ACT 0200, Australia

Corresponding author: alistair.hack{at}erdw.ethz.ch

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.

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