A simple magma-mixing model for sulphur behaviour in calc-alkaline volcanic rocks: mineralogical evidence from Mount Pinatubo 1991 eruption

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Abstract
The increasing recognition of 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. Although sulphur may exist as a number of species in volcanic emissions, one of its most important phases in volcanic rocks is primary anhydrite (CaS0 4 ). A number of important historic eruptions of calc-alkaline volcanoes have now been found to involve anhydrite-bearing magmas. These include Mount Lamington in Papua New Guinea (Arculus et al. 1983), Nevado del Ruiz in Colombia (Fournelle 1990), and El Chichon in Mexico ( Rose et al. 1984 ). The Ruiz eruption involved documented magma mixing, and both the Nevado del Ruiz and El Chichon eruptions were associated with very high sulphur emissions (both approximately 3.3 x 10 6 tonnes; Krueger 1983 ; Williams et al. 1990 ). We have observed magma-mixing textures in detailed studies of two neighbouring calc-alkaline volcanoes in northern Chile, at Lascar ( Matthews 1992 ) and Tumisa ( Gardeweg 1988 ). Anhydrite has been found associated with these textures in pumice from Lascar Volcano. Both anhydrite and similar disequilibrium textures indicative of magma