Fractional degassing of S, Cl and F from basalt magma in the Bárðarbunga rift zone, Iceland

The composition of gas emitted from a volcano producing basalt magma can vary during an eruption and according to the volcano-tectonic setting of the degassing vents. Post-eruptive filter-pack gas samples from the 2014–2015 Holuhraun crater in the Bárðarbunga rift zone have lower ratios of S over halogens (Cl and F) and elevated F/Cl (~ 50 times lower S/Cl and ~ 5 times higher F/Cl; mass ratios) compared with samples of the syn-eruptive gas plume. The compositional changes are readily explained by Rayleigh distillation with decreasing sulphur concentrations and increasing concentrations of halogens and F relative to Cl in the final gas phase. For Cl, the vapour-melt partition coefficient (D^V/M) decreased from 13–85 to 2.2 during residual degassing, whereas that of F remained uniform at approximately 1.8. Distinctly different degassing behaviour is observed for Cl and F. High D for Cl may indicate an important influence of sulphur and water on Cl volatility in basaltic melt, whereas that of F remains unaffected. The primary gas of the Holuhraun magma had similar ratios of S over Cl and F as observed at the Kilauea rift zone which, together with lower S/halogens in the residual gas in both cases, suggests similar degassing mechanism. By inference, initial CO₂ degassing is likely to have occurred subglacially close to the Bárðarbunga central volcano before and during the 2014–2015 eruption on the rift-related fissure swarm.