Volcanic gases and the reaction of sulfur dioxide with aluminosilicate glasses

Abstract

Volcanic gases are an important part of the volatile cycle in active planetary systems and contribute significantly to the mobilization and transport of metals to planetary surfaces. On Earth, Venus, Mars and Io, SO2 is the most abundant corrosive species in volcanic gases, and basalts are ubiquitous on these planetary bodies. The reaction between SO2 and silicate rocks forms oxidized sulfate and reduced sulfide. This reaction is a key process in the formation of porphyry deposits. In volcanic eruption plumes SO2 reacts with volcanic ash and is scavenged onto the surface of the ash particles. Knowledge of the reaction mechanisms between volcanic gas and rocks, minerals and glasses, and processes controlling the metal mobilization and transport in volcanic gas can constrain models of volatile and metal budgets of planetary crusts and surfaces. Using thermochemical modelling, I present a new model for the composition of volcanic gas on the Moon and compare it to a terrestrial volcanic gas from Erta Ale volcano (Ethiopia). The main species in lunar volcanic gas are H2, S2 and CO. This finding is in contrast to previous studies which suggested that CO was the sole driver of explosive volcanic eruptions on the Moon. This lunar volcanic gas has a lower capacity for metal transport compared to the Cl- and H2O-rich volcanic gas from Erta Ale volcano. To identify how SO2-glass reactions occur at high temperature and to investigate what might promote and limit these reactions, I present results from an experimental study. Pure SO2 was reacted with silicate glasses in the system anorthite-diopside-albite and with Fe-bearing natural basaltic glasses. The sulfate reaction products are relatively enriched in Ca compared to the silicate glass composition, in particular in experiments with Fe-free anorthite-diopside glasses. On these Fe-free glasses CaSO4 is the sole observed phase in the coatings at 800 °C, whereas at 600 °C minor amounts of MgSO4 were detected. At 800 °C, the flux of Ca from the silicate glass to the surface exceeds that of Mg by a factor of up to 330, whereas at 600 °C this factor is only 3. The rate of reaction is not constant, decreasing by an order of magnitude from 1 h to 24 h at 800 °C. The reaction of SO2 with tholeiitic basalt glasses produces coatings of CaSO4, MgSO4, Na2SO4 and oxides including Fe2O3 and TiO2. In addition, the reaction modifies the basalt glass because Ca, Mg and Na are lost to the coating. This results in the nucleation of crystalline spherulites and needles including SiO2, Al2O3, as well as Fe-Na-rich and Mg-rich pyroxenes. VIII The results suggest that the structural properties of the silicate glass substrate control the diffusive transport of Ca, Na, Mg, Fe and Ti to the surface which in turn controls the overall reaction rate and the formation of sulfates, oxides and silicates. These findings can be applied to predicting reactions on planetary surfaces and at shallow levels within their crusts. Show more