Seminars Archive

Sulfur in Volcanic Systems: Some new results and research in progress

Abstract
Sulfur is commonly the third most abundant volatile in volcanic systems; it plays a major role in affecting Earth\'s climate and in methodologies used to predict volcanic eruptions. Thus, understanding its behavior in natural melts and volcanic gases is imperative. Additionally, more refined quantifications of sulfur emissions from volcanoes will better constrain the global sulfur cycle. The amount of sulfur dissolved in most natural silicate melts is controlled by the saturation of the melt with a sulfide phase. Therefore, experiments were performed to measure the maximum amount of sulfur a silicate melt could hold when saturated with a sulfide phase (the SCSS, sulfur concentration at sulfide saturation); these data formed the basis for an empirical model that predicts the SCSS in natural melts. The model works well for most melts, but fails for high-silica melts with low iron and high sulfur concentrations, whose measured concentrations are significantly greater than predicted. To investigate this failure, sulfur XANES measurements were made on a suite of S-bearing silicate glasses that differed only in Fe/Mg ratio. The spectra demonstrate significant differences between glasses with Fe/S ratios above and below 2. The spectra of glasses with Fe/S >2 can be fit by combinations of reference spectra from sulfide, sulfite and sulfate minerals; however, glasses with Fe/S < 2 cannot. These results suggest that a different sulfur solubility mechanism occurs in low-iron melts, but it remains unknown and is still under investigation. Despite our ignorance of the S dissolution mechanism in melts with Fe/S < 2, the empirical model for the SCSS can be applied to predict sulfur flux at almost all individual volcanoes and the global sulfur flux from volcanoes, which appears to be 4 times greater than previously believed.