Eruptive style controls the formation of silicon hexafluoride salts on volcanic ash: The case of the 2010 eruption of Eyjafjallajökull volcano, Iceland

Abstract The presence of fluoride (F−)-bearing compounds in volcanic ash fallout is recognised to pose significant environmental and health hazards. Nevertheless, understanding of the formation and composition of these compounds remains limited, or even inconsistent. This hampers full comprehension of the risks and impacts elicited by fluorine-rich ash emissions after an explosive eruption. The 2010 activity of Eyjafjallajokull volcano in Iceland was characterised by contrasted eruptive styles - phreatomagmatic and magmatic - which produced ash deposits with distinct soluble F− contents. Based on X-ray photoelectron spectroscopy and leachate analyses of Eyjafjallajokull ash materials, we infer the existence of silicon hexafluoride salts, particularly Na2SiF6, dominantly if not exclusively on the magmatic ash, and suggest a reaction mechanism for their formation. The presence of Na2SiF6 explains the much greater F− concentration and higher acidity measured in leachates of the magmatic ash compared to those of the phreatomagmatic ash. River waters affected by ash deposition during the magmatic activity showed a temporary increase in F− and decrease in dissolved silicon isotope values (δ30Si). We contend that the formation of silicon hexafluoride salts on ash leads to a surface enrichment of the ash's silicate glass and mineral components in 28Si, with the release of this light Si from the magmatic ash during the early stage of weathering causing a depression in the riverine δ30Si. This supports existing evidence of silicate dissolution processes being influenced by the presence of F. Our study also points to the likely important role of Na2SiF6 in modulating the environmental and health effects of ash emissions from fluorine-rich magmatic eruptions.