Tracing Volcanic Degassing Dynamics During the 2021-2023 Eruptions on the Reykjanes Peninsula, Iceland, via OP-FTIR measurements

The recent eruptions on the Reykjanes Peninsula presented an exceptional opportunity for in-depth analyses of volcanic gas emissions. Utilizing Open-path Fourier Transform Infrared (OP-FTIR) spectroscopy, we analyzed the abundance of major and minor gas molecular species, including H₂O, CO₂, SO₂, HCl, HF and CO, in the gas emissions on more than twenty occasions throughout the eruptions in 2021-2023. Predominantly water-rich emissions (60-95 mol % H₂O) suggest fractional degassing and substantial CO₂ loss at depth. Significant temporal variations in gas composition were observed, with lower H₂O/CO₂ and H₂O/SO₂ ratios and higher SO₂/HCl ratios measured early in the early stages of the Fagradalsfjall eruption and higher H₂O/CO₂ and H₂O/SO₂ ratios and lower SO₂/HCl ratios during later stages and subsequent eruptions. A unique set of measurements, conducted at close range and high temporal resolution during the 2021 lava fountaining phase, provided insights into the dynamics of gas segregation at shallow depths. These findings help explain the pulsatory nature of the lava fountaining, driven by pressure fluctuations in a shallow magma-filled cavity. Furthermore, the gas emission chemistry varied significantly with the degassing style, 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. This study underscores the effectives of OP-FTIR techniques for tracking the spatiotemporal evolution of basaltic magma degassing, offering valuable insights into volcanic processes.