MINERAL-SOLUTION EQUILIBRIA AT HIGH PRESSURES

The formulation of the Helgeson-Kirkham-Flowers equation of state for aqueous electrolytes was a groundbreaking advance in geochemistry. It facilitated quantitative investigation of the chemistry of aqueous solutions in a wide-range of geologic settings. However, its limitation to ≤5 kbar prevented study of some of Earth’s important metasomatic environments, such as subduction zones, the upper mantle, and many granulite and Barrovian metamorphic belts. Extension of the HKF-EOS to higher P requires poorly known dielectric constant information; however, accurate and precise H₂O densities (ρ) are available at high P and T. At ρ_H2O above ~0.7 g/cm³, the HKF-EOS predicts isothermal linear correlations between the logarithms of ρ_H2O and of equilibrium constants of homogeneous and heterogeneous equilibria. These correlations have been used to compare predicted and experimental mineral solubility at high P. The extended HKF-EOS accurately predicts measured corundum solubility at ≥5 kbar in H₂O and H₂O-KOH. The predicted solubility of the assemblage albite + paragonite + quartz in H₂O at 10 kbar and ≤500°C also agrees with experiments; however, at 580°C to the solidus at 635±5°C, the extended HKF-EOS significantly underpredicts solubility and does so to an increasing extent with rising T. Similar underprediction occurs for K-feldspar + muscovite + quartz at 700°C, 10 kbar. The thermodynamic data account only for monomeric neutral and charged species and ion pairs. Excess measured solubility therefore points to increasing abundance of aqueous Si, Al-Si, and alkali-Al-Si polymers containing bridging oxygen bonds. Polymerized solutes predominate in all near-solidus solutions, rising to >80% of total dissolved solids at the melting point. The observations support a conceptual model of premelting polymerization of dissolved silicate components, in which these solutes represent precursors that facilitate condensation of the more polymerized silicate liquid at the solidus. The observed high concentrations of alkalis, Al, and Si will yield substantial mass transfer by near-solidus metamorphic and magmatic fluids in the crust and mantle, and likely promote dissolution, transport and precipitation of nominally insoluble, refractory rock components.