Experimental evidence for the source of excess sulfur in explosive volcanic eruptions

The amounts of sulfur released in explosive volcanic eruptions are often orders of magnitude larger than those expected to result from the degassing of the erupted SiO2-rich magma. Experimentally measured fluid/melt partition coefficients of sulfur ranged from 47 under oxidizing conditions (where SO2 is the dominant sulfur species in the fluid) to 468 under reducing conditions (where H2S dominates). Therefore, 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 excesses upon eruption.