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⁺/Fe_total (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.