Insights into magma degassing processes during the 2021 Cumbre Vieja eruption, La Palma, from open-path FTIR spectroscopy

On September 19^th 2021 a fissure eruption started on the Cumbre Vieja rift on La Palma, Canary Islands. The fissure eruption rapidly evolved into a cone-forming eruption, with several summit vents producing explosive activity and lava jetting, while lava spattering and effusive activity occurred at/from lower flank vents.

We used open-path Fourier transform infrared spectroscopy (OP-FTIR) to measure the chemical composition of degassing associated with both explosive and effusive activities, using absorption spectra of the radiation emitted by molten lava and incandescent ash. Measurements were performed daily since October 2nd until the end of the eruption (mid-December), from different sites and at distance range of 0.6 to 5 km from the vents. They allowed us to retrieve the molar proportions of H₂O, CO₂, SO₂, HCl and CO in gas emissions from the different vents and different activities.

In this work, we report the main results obtained for the gas compositions, their spatial and temporal evolution during the eruption, the influence of fragmentation (ash) on the degassing of HCl, and the gas-magma redox state during the La Palma 2021 eruption.