The behaviour of sulfur during volcanic degassing
A Department of Geology Seminar presented by Ery Hughes (GNS Science)
Abstract
Sulfur is the third most abundant volatile in volcanic systems. Volcanic SO2 emissions are used for volcano monitoring, can impact climate, and sulfur is important during ore-formation. Sulfur is multivalent in silicate melts (dissolves as S2- and S6+), which causes complex behaviour during degassing as it is intimately linked to the evolution of oxygen fugacity.
One such example is the "sulfursolubility minimum" observed experimentally, which exerts an important control on the behaviour of sulfur in natural processes. The sulfur solubility minimum results in a minimum in the amount of sulfur than can dissolve in a silicate melt at intermediate oxygen fugacity. We have built a thermodynamic COHS degassing model to explore the behaviour of sulfur.
We find that sulfur begins to degas deeper at the sulfur solubility minimum than at lower or higher oxygen fugacities. Additionally, the silicate melt reduces during degassing if the system begins more reduced than the sulfur solubility minimum, but oxidises if the system begins more oxidised. Water also influences the behaviour of sulfur, with water-rich melts degassing sulfur deeper than water-poor melts.
This has implications for interpreting volcano monitoring data (e.g., CO2/S ratios), interpreting fO2 data from melt inclusions and fumaroles, and using volatile contents as barometers.
