Post-eruptive outgassing of heavy metals at Fagradalsfjall, Iceland

Eruptions of basalt are known to emit a gas phase composed of H₂O, CO₂, SO₂, H₂S, HCl and HF, as well as trace elements, including toxic heavy metals. While the gas composition released during an eruption has been well studied, the post-eruptive gas emission remains poorly characterised, and its environmental impact is largely unknown. During the 2021 and 2022 Fagradalsfjall eruptions, gas was collected from an active crater and the post-eruptive gas from actively degassing but extinct crater and from the cooling lava field. A compositional shift is observed from a sulphur-dominated syn-eruptive gas phase (mass ratios S/Cl ~ 20, S/F ~ 50) to a halogen-dominated gas phase from the cooling lava (S/Cl ~ 0.01, S/F ~ 0.1), the extinct crater gas having an intermediate but fluorine-rich composition (S/Cl ~ 0.8, S/F ~ 0.7). The shift in major volatile composition affects the trace element volatility, resulting in a depletion of elements emitted as sulphide or in their elemental form such as Te and Cd. The solidifying lava emits gas rich in chloride-forming species (Sb, Pb and Zn), whereas the gas from the extinct crater is rich in elements forming fluoride species such as Mo and Ru. Estimation of the trace metal emissions reveals significant liberation of a few toxic metals from the crystallising lava, up to 100 tons of Zn over the duration of the lava solidification. Syn- and post-eruptive metal emissions are markedly different and crystallising lava degassing may result in local hazard over extended time.