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.