Effect of alkalinity on sulfur concentration at sulfide saturation in hydrous basaltic andesite to shoshonite melts at 1270 °C and 1 GPa

We have measured the effect of alkalis on S concentration at sulfide saturation (SCSS) in an under-explored compositional space of natural hydrous arc melts (basaltic andesite to shoshonite) at 1270 °C and 1 GPa. At an oxygen fugacity approximately 2.5 log units below the fayalite-magnetite-quartz (FMQ) buffer, SCSS increases with Na₂O (562 ppm S/wt% Na₂O), K₂O (98 ppm S/wt% K₂O), and total alkalis (88 ppm S/wt% Na₂O+K₂O) over the compositional range we have studied (1.6–3.1 wt% Na₂O; 0–6.5 wt% K₂O; 1.9–6.3 wt% FeO^tot). Experiments with ∼1.3 wt% H₂O show approximately half the increase in SCSS with alkalinity compared to those with ∼3.0 wt% H₂O. Our results show a possible limit to the increase in SCSS solely by increasing alkali concentration at ∼7.5 wt% total alkali concentration. Using our results and published data, we retrained earlier SCSS models to provide a better fit to test data. We also developed a new empirical model using theoretical optical basicity as a compositional parameter that predicts SCSS in the overall data set with slightly better accuracy compared to previous models:

with temperature (T) in Kelvin, pressure (P) in GPa, the optical basicity (Λ) and mole fractions (X) of FeO (calculated from Kress and Carmichael 1991), and H₂O in the melt. The discrepancies between observed and predicted SCSS for our experiments of varying alkalinity reflects the heavy bias toward anhydrous, alkali-poor basalt compositions in the underlying data sets on which most models are developed.