SO2 scrubbing during percolation through rhyolitic volcanic domes

Abstract

Accurate estimates of the atmospheric impacts of large eruptions on the environment are complicated by a paucity of models for gas-magma or gas-rock interactions that can occur in the subsurface. It is in these environments that high-temperature scavenging of magmatic gases, degassed during eruption, may play a major role, resulting in significant time-dependence of the bulk gas budget of the major erupted volatile species. Recent experimental work has identified the principal mechanisms involved in high-temperature scavenging in SO2 and HCl-dominated gas environments, but has neglected the effect of humidity and in-dome gas-magma reactions. Here we present scaled experimental results for the scavenging potential in SO2-H2O mixtures using glassy rhyolitic particles above the acid dew point at 200-800 °C. First, we reproduce previous results for anhydrous SO2 scavenging. Such scavenging is accommodated by the growth of CaSO4 crystals on the ash surface and limited by the temperature- and ash-size dependent diffusion of Ca2+ to the surface of the ash. Subordinate concentrations of Na2SO4 and potassium-bearing salts also grow on the ash surfaces. Na+ and K+ diffusion appear to be the limiting mechanisms for the formation of these salts, but once the bulk glass network is relaxed and equilibrates oxidation state above ∼600 °C, the diffusion of these cations is inhibited by charge compensation with Fe3+. In SO2-H2O mixed-gas atmospheres, the diffusion of Ca2+ appears unaffected by the activity of H2O on the ash surface. In contrast, the diffusion of Na+ and K+ toward the ash surface is enhanced. We speculate that this occurs by alkali exchange with inward diffusing H+. In hydrous atmospheres, the diffusion of Na+ and K+ is also markedly less at the threshold oxidation temperature. This oxidation temperature is reduced to lower values as the water activity at the surface is increased by increasing the partial pressure of H2O. Taken together, these results enable the exploration of scenarios for in-dome processes where either open-system or batch outgassing prevails through fractures filled with populations of welding volcanic ash. Such scenarios predict that a large fraction of the SO2 flowing through the ash-filled fracture networks may be scavenged in permeable fractured domes, which act as scrubbing filters, potentially explaining some unexpectedly low SO2 emissions from rhyolitic dome-forming eruptions.