Volcanic degassing during the recent Fagradalsfjall and Merardalir eruptions, Iceland

The recent eruptions of Fagradalsfjall and Meradalir (Iceland) marks the first eruptive episode on the Reykjanes Peninsula in nearly 800 years. Open-path Fourier Transform Infrared (OP-FTIR) measurements of major and minor gas molecular species (including H₂O, CO₂, SO₂, HCl, HF and CO) in the gas emissions have been performed on more than twenty occasions throughout the eruptions in 2021 and 2022. Generally, the gas emissions are water-rich (60-95 mol % H₂O) and show CO₂/SO₂ molar ratios of ~4, consistent with magma generation at >15 km depth. Comparison of measured gas emissions with geochemical models of degassing of the Fagradalsfjall basaltic melt suggest that fractional degassing is necessary to explain the high-water contents of the fountaining gas at Fagradalsfjall, implying that a significant fraction of the CO₂ that has exsolved from the magma is lost at depth prior to eruption. The measured vent gas emissions display enigmatic changes as a function of time, with lowest H₂O/CO₂ and H₂O/SO₂ ratios measured early in the eruption at Fagradalsfjall in 2021 and higher ratios during later stages and during the Meradalir eruption in 2022. The chemistry of the gas emissions is significantly affected by the style of degassing, with gas emitted by surface lava flows characterized by higher H₂O/CO₂ and H₂O/SO₂ and lower SO₂/HCl and SO₂/HF ratios compared to gas emitted at actively erupting vents. Moreover, the data record significant short-term temporal changes in chemistry on the timescales of minutes associated with intermittent fountaining and cooling/solidification of lava flows. This study highlights the utility of OP-FTIR techniques for tracing basaltic magma degassing in space and time.