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There are myriad reasons why we wish to understand the behavior of sulfur in magmatic systems, reasons that vary from pure intellectual curiosity to possible impacts on society and its resources. Since ancient times sulfur has been associated with volcanic activity, and the role of sulfur in the formation of ore deposits has long been recognized because of the necessity of metal ores for our modern life-style (e.g., Barnes 1979; Naldrett 1989; Simon and Ripley 2011, this volume). Recently the mechanisms and quantities of sulfur freed from natural magmas have become an important environmental issue due to their potential effects on global climate change. For example, the average annual volcanic SO2 emission rate of 7.5 to 10.5 teragrams (Tg) per year (Halmer et al. 2002) may contribute 10% of the global atmospheric sulfur input (Halmer et al. 2002; Smith et al. 2004), and individual eruptive episodes can rapidly contribute gigantic sulfur loads to the atmosphere, 100’s to 1000’s of Tg, depending on the scale of the eruption (Self 2006). Such sulfur emissions can produce potentially catastrophic local and global changes (e.g., Fedele et al. 2003; Ward 2009); Courtillot and Rennes (2003) correlated the timing of flood basalts with extinction events in Earth’s history and hypothesized a causal relation. Part of the kill mechanism responsible for extinction may be volcanically derived sulfur creating anoxic oceans and another part of the mechanism may be climatic changes brought about by sulfur injection into the atmosphere (Ward 2009). Indeed, Erwin (2006) advocates that sulfur released from the eruption of the Siberian Flood basalts played a role in the end-Permian extinction.
In light of the evidence that volcanic degassing is a significant source of sulfur to the atmosphere (Stoiber et al. 1987; Symonds et al. 1994; Andres …