A Model for the Solubility of Minerals in Saline Aqueous Fluids in the Crust and Upper Mantle

Abstract

Quantifying and predicting the dissolution of minerals in complex (multicomponent) aqueous fluids across wide ranges of P-T space is critical for interpreting geologic processes that involve water-rock interactions in the Earth’s crust and upper mantle. Here, we define a new thermodynamic model for mineral solubility in saline aqueous fluids. The model is based on the coupling of two previous models: one for the solubility of minerals in pure H2O fluids as function of temperature and pressure (Dolejš and Manning, 2010; Geofluids, 10, 20-40), with the additional effects of fluid composition (salinity) modeled in part based on Akinfiev and Diamond (2009; Geochimica et Cosmochimica Acta, 73, 1597-1608), with some additional modifications. Specifically, the new model adopts the approach of Akinfiev and Diamond (2009) to incorporate the effect of reduced H2O activity in saline brines for reactions that involve hydration, and also adds new expressions for the equilibrium constants of reactions involving explicit sodium and/or chloride species. As such, the generic model is applicable to the solubility of minerals that dissolve as hydrous species, sodium and/or chloride species, and combinations thereof. The model has been calibrated against experimentally determined solubilities for five common rock-forming minerals – quartz, calcite, corundum, fluorapatite, and fluorite – in H2O-NaCl solutions at temperatures up to 1100 °C and pressures up to 20 kbar. Data and trends observed in experimental measurements are well reproduced by our model predictions. In the case of pure H2O fluids (zero salinity), the model is implicitly equivalent to the Dolejš and Manning (2010) model. The accuracy of the model is within both experimental uncertainties and accuracy ranges of the two models on which it is built. This thermodynamic model, accounting for dissolution reactions in multi-component fluids over an extreme range of P-T-x conditions, will allow for robust modeling of reactions and mass transport in natural systems.