A major concern in the analysis of the volatile systematics of the 1991 eruption of Mount Pinatubo in the Philippines is the explanation for the release of such large amounts of SO2 (about 20 million metric tons) into the atmosphere. Although the total amount of sulfur originally in the magmatic system is important to the explanation of this large release, another very significant variable may be what proportion of that sulfur was released into the atmosphere. This paper is a “thought experiment” in which such considerations of elemental distribution are approached by means of an ore-deposit model. Although such models typically are used to evaluate the conditions required for the most effective removal and concentration of a certain desired element, they also can be applied to an undesired element—in this case, sulfur. In particular, Cline and Bodnar's porphyry-copper ore-deposit model reveals several variables that are significant to the evaluation of the Mount Pinatubo eruption, the most important of which is the partitioning behavior of sulfur among the solid and aqueous fluid (liquid + gas) phases of the system. For example, one reason for the release of such large quantities of sulfur from Mount Pinatubo may be that—in contrast to many other oxidized calc-alkalic volcanoes—its melt reached saturation with an SO2-bearing aqueous fluid before the solid phase anhydrite began to precipitate.

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