The distribution of sulfur between haplogranitic melts and aqueous fluids

The distribution of sulfur between haplogranitic melt and aqueous fluid has been measured as a function of oxygen fugacity (Co-CoO-buffer to hematite-magnetite buffer), pressure (0.5-3 kbar), and temperature (750-850 degrees C). Sulfur always strongly partitions into the fluid. At a given oxygen fugacity, pressure and temperature, the distribution of sulfur between melt and fluid can be described by one constant partition coefficient over a wide range of sulfur concentrations. Oxygen fugacity is the most important parameter controlling sulfur partitioning. While the fluid/melt partition coefficient of sulfur is 468 +/- 32 under Co-CoO buffer conditions at 2 kbar and 850 degrees C, it decreases to 47 +/- 4 at an oxygen fugacity 0.5-1 log unit above Ni-NiO at the same pressure and temperature. A further increase in oxygen fugacity to the hematite-magnetite buffer has virtually no effect on the partition coefficient (D-fluid/melt = 49 +/- 2). The dependence of D-fluid/melt on temperature and pressure was systematically explored at an oxygen fugacity 0.5-1 log units above Ni-NiO. At 850 degrees C, the effect of pressure on the partition coefficient is small (D-fluid/melt = 58 +/- 3 at 0.5 kbar; 94 +/- 9 at 1 kbar; 47 +/- 4 at 2 kbar and 68 +/- 5 at 3 kbar) and temperature also has only a minor effect on partitioning. The data show the sulfur excess observed in many explosive volcanic eruptions can easily be explained by the presence of a small fraction of hydrous fluid in the magma chamber before the eruption. The sulfur excess can be calculated as the product of the fluid/melt partition coefficient of sulfur and the mass ratio of fluid over melt in the erupted material. For a plausible fluid/melt partition coefficient of 47 under oxidizing conditions, a 10-fold sulfur excess corresponds to a 17.6 wt.% of fluid in the erupted material. Large sulfur excesses (10-fold or higher) are only to be expected if only a small fraction of the magma residing in the magma chamber is erupted. The behavior of sulfur, which seems to be largely independent of pressure and temperature under oxidizing conditions is very different from chlorine, where the fluid/melt partition coefficient strongly increases with pressure. Variations in the SO2/HCl ratio of volcanic gases, if they reflect primary processes in the magma chamber, therefore provide an indicator of pressure variations in a magma. In particular, major increases in the S/Cl ratio of an aqueous fluid coexisting with a felsic magma suggest a pressure reduction in the magma chamber and/or magma rising to the surface. (c) 2009 Elsevier Ltd. All rights reserved.