An accurate model to predict sulfur concentration at anhydrite saturation in silicate melts

Abstract

Magmatic sulfur is an essential constituent for the genesis of porphyry-type ore deposits and also significantly affects the Earth’s climate when emitted into the atmosphere. As an increasing number of studies indicate that anhydrite commonly occurs as primary igneous mineral in arc magmas, any quantitative model aiming to assess the efficiency of sulfur degassing from magmas must take the potential presence of anhydrite into account. To facilitate this, we present a new empirical model to predict the solubility of anhydrite in silicate melts as a function of pressure (P), temperature (T) and silicate melt composition. The model is based on 189 experimental data points, encompassing T from 750 to 1325 °C, P from 30 to 3000 MPa and basaltic to rhyolitic melt compositions. Fifteen of these experiments were conducted as part of this study in rapid-quench Molybdenum – Hafnium Carbide pressure vessel assemblies to obtain tighter constraints on the effect of water concentration, P and T on anhydrite solubility. These experiments show that anhydrite solubility in the silicate melt rapidly increases with increasing T following Arrhenius relationship. The solubility of anhydrite also increases with increasing dissolved water concentration, with the relative increase being the most significant for felsic melt compositions. The new model predicts sulfur concentration at anhydrite saturation (SCAS) in the silicate melt with median and mean absolute percentage errors of 19 and 25 relative%, respectively. The model is equally successful over the entire P, T and compositional space included in its calibration. This is a significant improvement over most previously published models, which were calibrated based on a more limited P, T and compositional range, and predict SCAS with significantly larger errors when applied to the entire dataset utilized in this study. The implementation of the new anhydrite saturation model in an example scenario of the crystallization of a hydrous andesite magma at P = 200 MPa suggests that anhydrite saturation may severely limit the amount of S that can be transferred into the exsolving magmatic volatile phase during crystallization-driven degassing of upper crustal magma reservoirs.