- © The Mineralogical Society Of America
Volatile constituents dissolved in magmatic fluids control fundamental geologic processes including magma ascent, degassing, and eruption; metasomatism; and hydrothermal mineralization in the shallow crust, and volatile compounds involving sulfur are vigorous agents of these fluids. Volcanic outgassing of magmatic volatiles, including SO2 and H2S, to the atmosphere has influenced atmospheric chemistry throughout Earth’s history (Arthur 2000), and of particular importance has been the effectiveness of SO2- and H2SO4-laden stratospheric aerosols to reflect sunlight and cool Earth’s surface (Siggurdson et al. 1990; Oppenheimer et al. 2011, this volume; Wallace and Edmonds 2011, this volume). In addition, many ore metals exhibit chalcophile behavior, i.e., a strong affinity to S. As a result, magmatically sourced S that is typically present as reduced sulfide species plays an essential role in processes of generating nickel sulfide, porphyry copper-gold-molybdenum, high-sulfidation precious metal, and volcanogenic massive sulfide deposits (Hedenquist and Lowenstern 1994; Burnham 1997; Vaughan and Craig 1997; Seo et al. 2009; Simon and Ripley 2011, this volume). Crucial and poorly understood aspects common to each of these processes are determining how and when multi-component fluids exsolve, evolve, and escape from magmas and how significantly S dissolves in such chemically complex fluids. The results of hydrothermal experiments, summarized herein, provide important new insights into these processes.
The silicate melt phase (herein abbreviated si-mt) in magmas may contain variable concentrations of S, and S-isotopic data indicate that this S is mostly of magmatic origin (Rye et al. 1984; Wallace 2001). The melts of mafic and intermediate-silica-content magmas generally exhibit greater S concentrations (e.g., > several thousands of ppm S) than felsic melts (Ducea et al. 1994; Wallace and Anderson 2000; Cervantes and Wallace 2003; Métrich and Wallace …