Post-eruptive emission of metals and metalloids from basaltic lavas

During basaltic eruptions, degassing occurs both syn- and post-eruptively. The atmospheric loading resulting from the latter, taking place as the lava cools down and solidifies, is poorly characterised and its environmental impacts are largely unknown. Characterising post-eruptive degassing can be attempted either by direct sampling of the gas emitted at solidifying lava flow or by analysing the degassing structure (segregation veins and vesicles) from already crystallised lava bodies. The 2021-2023 eruptions at Fagradalsfjall on the Reykjanes Peninsula, Iceland, allowed sampling of both the syn- and post-eruptive gas phases. Segregation veins from prehistorical olivine tholeiite lavas of similar composition to that of Fagradalsfjall have been collected, together with their host lava. Major and trace volatile element concentrations, particularly those of the toxic volatile metals and metalloids (VMM), were analysed in the gas and rock phases. The gas analyses reveal distinct VMM composition of the gas emitted syn- and post-eruptively, with a clear tendency for Zn, Mo and Sb to degas after the eruption, whereas Te, Cd and Bi are predominantly released syn-eruptively. Among the segregation veins sampled, several showed anomalous enrichment in numerous VMM, indicative of the post-eruptive gas phase composition. Molybdenum is largely enriched in these veins whereas Cd enrichment is low, reflecting gas phase rich in the former but poor in the latter. Taken together, both approaches indicate substantial atmospheric loading of As, Pb, Sb and Mo post-eruptively resulting in potential hazard around newly erupted lava bodies.