A SIMPLE MAGMA-MIXING MODEL FOR SULFUR BEHAVIOR IN CALC-ALKALINE VOLCANIC-ROCKS - MINERALOGICAL EVIDENCE FROM MOUNT-PINATUBO 1991 ERUPTION

The increasing recognition or anhydrite as a primary magmatic mineral in volcanic products, including the important 1991 Pinatubo eruption, places new emphasis on the cycling of sulphur from deep sources including the upper mantle. We present a simple model for the supersaturation of sulphur during mingling and mixing in calc-alkaline magmas due to the quenching and oxidation of the mafic end member, which may have important consequences for predicting the annual flux of igneous sulphur. This model predicts the coexistence of anhydrite and sulphur dioxide gas, and explains the discrepancy between erupted magma volumes and sulphur gas emissions.