High-pressure aqueous systems: experimental constraints and thermodynamic models of fluid-mineral-melt equilibria

Aqueous fluids are essential agents of mass transport and geochemical cycling in various magmatic, metamorphic orogenic and subduction settings. Thermodynamic properties of aqueous solutes at high pressure have been modelled using several approaches: (1) electrostatic models (Sverjensky et al. 2014), (2) density extrapolations (Wohlers et al. 2011; Manning 2013), or (3) density models (Dolejš and Manning 2010; Dolejš and Salomone 2023). Critical comparison of experimental and predicted mineral solubilities in aqeuous fluids over wide range of temperatures and pressures provides the most accurate constraints on the functional performance of these approaches. This evaluation reveals that (1) electrostatic models offer greater fitting flexibility at costs of extrapolation accuracy and stability across the pressure-temperature space; (2) density models are more robust, require fewer parameters at costs of accuracy at low-temperature conditions. With increasing pressure, the solute concentrations rise and approach the critical transition to hydrous melts. Solute-solvent interaction indicate that mineral-fluid equilibria during critical approach are initially far from congruent and this pressure-temperature region strongly promotes metasomatic effects. The infinite dilution formalism is not physically not suited for reproducing these solubility trends. Instead, thermodynamic description of the critical fluid-melt transition requires consideration of: (1) competing effects of solute polymerization vs. association (hydration) in the dilute region, (2) transition from flexible liquid medium to structured aluminosilicate framework, and (3) speciation mechanism of H₂O in aluminosilicate melt. Using the model system H₂O-SiO₂ we demonstrate coupling between configurational, ideal and excess mixing effects using several molecular and structural formalisms. Use of universal thermodynamic conditions for critical point or curve effectively constrains the location of these phase diagram features and it reduces the number of independent parameters in the mixture equation of state.  In the simplest case, the solubility, melting and critical phase equilibria in the system H₂O-SiO₂ can be reproduced with a four-parameter equation of state for solute and one interaction parameter. These results reveal diverse heuristic aspects arising from classical thermodynamic constraints and indicate predictive capability and practical accuracy of these models for natural solute-rich fluids in high-pressure settings.

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