Sulfur in Degassing Volcanoes: Thermochemistry and Experiments

Very high temperature fumaroles unambiguously represent samples of magmatic gas expanding to the surface from sub-volcanic magma bodies. Here we present the results of thermochemical modelling of measured fumarole gas compositions that confirm that magmatic gases are SO2-dominant when redox is controlled by homogeneous gas reactions involving SO2, H2S and other species, i.e. the 'gas buffer'. In subsurface volcanic environments, SO2 also dominates when oxygen fugacity, fO2, is greater than the value imposed by the Ni-NiO buffer; a common situation in arc environments. These results indicate that to understand the systematics of arc magma degassing, it is necessary to investigate how SO2, the principle sulfur gas, reacts directly with magmatic materials. Thermochemical modelling of the interaction between SO2-bearing gas mixtures and common rock-forming minerals such as anorthite and calcite indicates that SO2 spontaneously disproportionates to form anhydrite and a reduced sulfur species. Experimental investigation of these reactions in a gas-mixing tube furnace at 600-800 °C, 1 bar, demonstrates extremely rapid anhydrite formation on the surface of crystalline anorthite through chemisorption. In the presence of H2O, the reduced sulfur species is H2S, which may subsequently react with co-transported metals such as Fe and Cu in the magmatic gas to form metal sulfides. It is proposed that this mechanism provides a straightforward explanation for the massive amounts of coexisting sulfate and sulfide cogenerated at depths of 1 to 4 km inside volcanic systems and now exposed as porphyry copper deposits. Anhydrite dissolution may contribute to the fragility of volcanic structures.