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

Abstract

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.

A model for the kinetics of high-temperature reactions between polydisperse volcanic ash and SO2 gas

“…By examining the phenomenological results from previous work on high-temperature reactions between glass and SO 2 (Ayris et al 2013;Renggli and King 2018;Casas et al 2019;Renggli et al 2019), a few key starting points can be identified. In most cases where an experimental glass used was iron-bearing, exposure of glass surfaces to high temperatures results in a time-dependent formation of surface phases of a wide range of compositions: CaSO 4 , K 2 SO 4 , FeSO 4 , Na 2 SO 4 , MgSO 4 , Na 2 Ca(SO 4 ) 2 , and Al 2 (SO 4 ) 3 , as well as minor other phases (for a review see Renggli and King 2018).…”

“…Most of this complexity is found in experimental reactions between mafic glasses and SO 2 , such as tholeiitic basalts (Johnson and Burnett 1993;Burnett et al 1997;Palm et al 2018;Renggli et al 2019), not only in terms of the diversity of surface phases that crystallize but also a strong dependence on oxygen fugacity. We posit that more silica-rich rhyolitic particles reacting with SO 2 is a comparatively simple system.…”

“…While experimental evidence appears to confirm that calcium diffusion rates are the limiting processes controlling the kinetics in rhyolite-SO 2 reactions, it clearly also takes a finite time for the nucleation and growth of the crystalline surface phases (Douglas and Isard 1949;Ayris et al 2013;Delmelle et al 2018;Renggli and King 2018;Renggli et al 2019). Therefore, there are two processes at play: (1) calcium moves from the particle bulk to the surface, and (2) the surface reaction(s) must occur.…”

“…In natural systems, high temperature SO 2 -ash reactions involve the formation and growth of calciumbearing salt crystals on the surfaces of ash particles (Ayris et al 2013;King 2017, 2018;Renggli et al 2019). At the range of conditions that have been explored, the dominant salt formed on natural glass is calcium sulfate (Renggli and King 2018), whose generation is rate-limited by the diffusion of calcium to the particle surfaces (Ayris et al 2013;Delmelle et al 2018).…”

“…Cognate processes involving the interaction between SO 2 gas and silicate glass or rocks have been explored for synthetic glasses (Douglas and Isard 1949;Johnson and Burnett 1993;Burnett et al 1997;Renggli and King 2018;Renggli et al 2019), for calcite-SO 2 reactions (Mavrogenes andBlundy 2017a, 2017b;Saadatfar et al 2020), and as a mechanism for porphyry copper deposit formation (Henley et al 2015). In all cases, it is proposed that these reaction pathways represent a key component of the volcanic-or crustal-sulfur cycle on Earth and other planets, and yet there is no clear way to make quantitative predictions of the net contribution of these reaction pathways to the volcanic sulfur budgets across all relevant conditions and input parameters.…”