An experimental study of SO2 reactions with silicate glasses and supercooled melts in the system anorthite–diopside–albite at high temperature

Abstract

Sulfur dioxide [SO2(g)] is the most abundant sulfur-bearing volcanic gas species on Earth. From its magmatic origin at depth to expulsion at the surface via either persistent degassing or large explosive volcanic eruptions, SO2(g) interacts with silicate materials at elevated temperatures. Similar high-temperature reactions also occur in the volcanic systems and the atmospheres of Venus, the Galilean moon Io, and in Mars’ past, as well in industrial flue-gas processing. We present an experimental investigation of the reaction between SO2(g), glasses and supercooled melts in the system anorthite–diopside–albite (CaAl2Si2O8–CaMgSi2O6–NaAlSi3O8). The samples were exposed to SO2(g) at 600–800 °C for experimental durations of 10 min to 24 h. The reactions resulted in the formation of sulfate coatings and modified the near-surface composition of the silicate samples. The predominant sulfate reaction product is CaSO4, with hydrated MgSO4 or Na2SO4 also observed in some experiments. In the anorthite–diopside system, the reaction extent strongly depends on the temperature relative to the glass transition temperature (Tg). Above Tg, in reactions with supercooled melts, the reaction forms up to 20 times more sulfate. The overall rate of sulfate formation is controlled by the diffusive flux of Ca, Mg and Na from the increasingly depleted silicate to the surface where the reaction with SO2(g) occurs. The sulfate-forming reaction results in a volume increase relative to the unreacted silicate. When this reaction occurs in the subvolcanic environment it causes an increased molar volume that may close veins, reducing the permeability and decrease the SO2(g) flux at the surface. An increase in the SO2(g) flux would then result in the opening of new veins, which may be accompanied by seismic activity. Additionally, the change in molar volume may itself trigger seismicity. The strong preferential uptake of Ca into the sulfate reaction product results in a Si- and Al-enriched silicate. In the sulfate, the Ca component may be mobilized by secondary processes such as through the interaction with meteoric fluids. We recommend that the products of such gas–solid reactions should be the object of remote and robotic investigations of planetary environments with volcanic histories such as on Mars, Io, Venus and Mercury.