Source of excess sulfur in volcanic eruptions

The amounts of sulfur released in explosive volcanic eruptions are often orders of magnitude larger than those expected from the degassing of the erupted SiO₂-rich magma. Hans Keppler (Bayreuth University, DE) reports a study to test whether small amounts of hydrous fluid in the magma chamber could extract most of the sulfur out of the entire reservoir, the test involving measurement of the partition coefficient of sulfur between hydrous fluids and water-saturated SiO₂-rich silicate melts.
The author reports that the experimentally measured partition coefficients indicate that a few weight percent of hydrous fluid accumulated in the top of a magma chamber may extract most of the sulfur out of the entire magma reservoir and generate sulfur excess upon eruption. The author suggests these results emphasize the importance of considering the amount of sulfur stored in the entire magma chamber in estimating the sulfur released in explosive volcanism. Estimates based on the sulfur content of the erupted melt alone would severely underestimate the environmental impact of explosive volcanism in the geologic past.
(Science 284:1652)