Deep Earth Water (DEW) model for predicting aqueous species thermodynamic properties to 6 GPa and 1,200 °C: Preliminary applications and the need for fundamental data (Invited)

Deep aqueous fluids appear to play an important role in the long-term geologic cycling of many chemical elements. However, quantitative theoretical evaluation of water-rock interactions in the Earth involving aqueous ionic speciation has long been restricted to a pressure of 0.5 GPa only. We present recent progress extending the revised Helgeson-Kirkham-Flowers (HKF) equations of state for aqueous species to 6 GPa and 1,200°C based on (1) Calculation of the dielectric constant of water (ɛH2O) needed in the HKF equation for calculating changes in the standard partial molal Gibbs free energies of aqueous species as functions of pressure and temperature and (2) New predictive linear correlations for key equation of state coefficients governing the pressure dependence of aqueous species. The latter are needed because of the paucity of experimental volumes and compressibilities as functions of pressure and temperature. Estimated equation of state coefficients are available for hundreds of aqueous species in SUPCRT92 [1]. However, most of these require revision for application to the elevated pressures under consideration. The new DEW model [2] has been applied to analysis of the experimental solubilities of quartz and corundum which extend to 2.0 GPa and 1,100°C to develop equations of state for the aqueous Si monomer and dimer, and aqueous Al-species. From 3.0 to 6.0 GPa, the DEW model has been applied to Raman spectroscopic measurements of carbonate/bicarbonate speciation in equilibrium with aragonite [3]. Revised estimated equation of state coefficients for aqueous ions, neutral species and complexes enable a beginning to the investigation of water-rock reactions in the upper mantle. Preliminary results include the following: (1) The pH values of low Cl aqueous fluids in equilibrium with model mantle peridotite at 3.0 to 6.0 GPa, pH's are mildly alkaline, whereas with eclogite they are strongly alkaline. (2) Mantle metasomatic reactions forming diamond may depend on changes in pH rather than logfO2. (3) Metastable equilibria between aqueous organic species at crustal pressures and temperatures may replaced in the mantle by full equilibrium involving the entire range of C oxidation states. Despite the new modeling capability for aqueous fluids, much rests on predictive correlations. There are huge gaps in our experimental databases for fundamental properties of aqueous species. Experimental and theoretical studies of solubilities and speciation, as well as volumes, compressibilities and heat capacities of aqueous species are needed to test and refine the predictive equations of state. In this way, predictive geochemical aqueous speciation models could be integrated with geophysical data and models as part of an overall approach combining experiments, theory and the study of natural samples relevant to the deep Earth. [1] Johnson, J. W. et al., Computers and Geoscience 18, 899 (1992). [2] Sverjensky, D. A. et al., Geochim. Cosmochim. Acta (in rev.). [3] Facq, S. et al., Geochim. Cosmochim. Acta (in rev.).