Insights into magma dynamics from daily OP-FTIR gas compositions throughout the 2021 Tajogaite eruption, La Palma, Canary Islands

From September 19 to December 12, 2021, a mixed explosive-effusive eruption impacted La Palma Island, in the Canary archipelago, leading to the growth of Tajogaite volcanic cone. Daily Open-Path Fourier Transform Infrared (OP-FTIR) measurements from October 3 until the end of the eruption allowed us to capture the molar proportions of H 2 O, CO 2 , SO 2 , HCl and CO (plus COS occasionally) in magmatic gases emitted from summit and flank vents of the new cone. Our results reveal high CO 2 /SO 2 ratios ranging from 11 to 53 in explosive gas emissions throughout most of the eruption, with a time-averaged value of ~30. In contrast, effusive degassing at lower flank vents systematically displayed lower CO 2 /SO 2 ratios (from 3 to 11) and enhanced proportions of H 2 O, S and Cl. Combined with solubility data and the magma eruption rates, this chemical contrast suggests gas fractionation in a very shallow conduit branching beneath the Tajogaite cone, were most of the pre-exsolved CO 2 -rich gas phase but a minor fraction of the magma explosively escaped through the main eruptive conduit, while CO 2 -depleted gas and most of the magma were effusively discharged through lateral branches. The CO/COS/CO 2 ratios for explosive degassing are consistent with petrological evidence of oxidized magmatic conditions (FMQ +1.7 ± 0.3), which enhanced sulfur solubility and late-stage SO 2 outgassing. The high oxidation state, as well as low HCl/HF ratios, retrieved from solar occultation across the volcanic plume, are both typical of Ocean Island Basalt (OIB) magmatism. The apparent increase of CO 2 /SO 2 and SO 2 /HCl ratios at summit vents during the first half of the eruption is consistent with the influx of progressively more mafic, deeper-derived, basanitic magma, though we cannot entirely rule out artefacts due to more challenging measurement of the pure explosive gas in that period. Our study presents the very first detailed data set for gas geochemistry during a subaerial eruption in the Canary archipelago and highlights the remarkable potential of OP-FTIR spectroscopy for real-time monitoring and study of volcanic eruptions.