Anhydrite solubility in differentiated arc magmas

The solubility of anhydrite in differentiated arc magmas was experimentally studied at 200 MPa and 800–1000 °C over a range of oxygen fugacities, from 0.5 log units above the Ni–NiO buffer to the hematite–magnetite buffer. Anhydrite is stable only at oxidizing conditions (fO2 ⩾ Re−ReO2), whereas sulfides only form under reducing conditions. The solubility of anhydrite in the melt ultimately regulates the amount of sulfur available to partition between melt and fluid phase during the eruption. At oxidizing conditions, the solubility product of anhydrite increases with temperature, nbo/t and melt water content. We provide a new calibration of the anhydrite solubility product (KSP = XCaO * XSO3), which reproduces all available experimental data with greatly improved accuracy:lnKSPAnhydrite=8.95-146.5nbot-2.696104T(∘K)+19.72nbot104T(∘K)+0.409·H2O(wt.%)In this equation, the molar fractions XCaO and XSO3 in the melt as well as the number of non-bridging oxygen atoms per tetrahedron (nbo/t) are calculated on an anhydrous basis (H2O refers to the melt water content, T is temperature in Kelvin). We apply our model to estimate the sulfur yield of some recent volcanic eruptions and we show that the sulfur yield of the 1991 Mt. Pinatubo dacite eruption was unusually large, because only a small fraction of the sulfur was locked up in anhydrite. In general, high sulfur yields are expected when anhydrite solubility in the melt is high, i.e. for somewhat depolymerized melts. For rhyolitic systems, most of the available sulfur will be locked up in anhydrite, so that even very large eruptions may only have a small effect on global surface temperatures. Our model therefore allows improved predictions of the environmental impact of explosive volcanic eruptions.