Implications of Reactions Between SO2 and Basaltic Glasses for the Mineralogy of Planetary Crusts

Basalts are ubiquitous in volcanic systems on several planetary bodies, including the Earth, Mars, Venus, and Jupiter's moon Io, and are commonly associated with sulfur dioxide (SO2) degassing. We present the results of an experimental study of reactions between SO2 and basaltic glasses. We examined Fe‐free basalt, and Fe‐bearing tholeiitic and alkali basalts with a range of Fe3+/Fetotal (0.05 to 0.79) that encompass the oxygen fugacities proposed for most terrestrial planetary bodies. Tholeiitic and alkali basalts were exposed to SO2 at 600, 700, and 800 °C for 1 hr and 24 hr. Surface coatings formed on the reacted basalts; these contain CaSO4, MgSO4, Na2SO4, Na2Ca(SO4)2, Fe2O3, Fe3O4, Fe‐Ti‐(Al)‐oxides, and TiO2. Additionally, the SO2‐basalt reaction drives nucleation of crystalline phases in the substrate to form pyroxenes and possible Fe‐oxides. A silica‐rich layer forms between the substrate and sulfate coatings. More oxidized basalts may readily react with SO2 to form coatings dominated by large Ca‐sulfate and oxide grains. On less oxidized basalts (NNO−1.5 to NNO−5), reactions with SO2 will form thin, fine‐grained aggregates of sulfates; such materials are less readily detected by spectroscopy and spectrometry techniques. In contrast, in very reduced basalts (lower than NNO−5), typical of the Moon and Mercury, SO2 is typically a negligible component in the magmatic gas, and sulfides are more likely. Plain Language Summary The primary sulfur‐bearing gas species in volcanic gas is sulfur dioxide (SO2), a very reactive gas species which may modify the chemical and mineralogical properties of magmatic rocks made of minerals and glasses. Here we investigate the reaction between sulfur dioxide and basalt glasses by conducting experiments at high temperatures (600–800 °C). The glass surfaces are altered to form sulfate and oxide coatings. This reaction likely occurred on Venus and Mars and may occur on Jupiter's moon Io. Observations of the surfaces of these planetary bodies may reveal sulfates, oxides, and silica‐rich coatings on the surfaces of volcanic rocks which formed via the investigated gas‐solid reaction. Key Points Basalt glass reacts with SO2 gas to form coatings with Ca‐, Mg‐, Na‐sulfates, Fe‐, Fe‐Ti‐, and Ti‐oxides and silica These reactions may be recorded and detected on Earth, Mars, Venus, and Io