Diffuse He degassing monitoring of Cumbre Vieja volcano, La Palma, Canary Islands

Cumbre Vieja volcano, at the southern part of La Palma Island, is the last stage on the geological evolution of the island, the fifth in extension (706 km²) and the second in elevation (2,423 m asl) of the Canarian archipelago. The recent volcanic activity in La Palma Island has taken place exclusively in Cumbre Vieja volcano in the last 123 ka, being the last volcanic activity the Tajogaite eruption, from 19 September to 13 December, 2021. We report herein the results of several intensive soil gas studies, focused on the non-reactive and highly mobile gas helium (He) in Cumbre Vieja. He has unique characteristics as a geochemical tracer: it is chemically inert and radioactively stable, non-biogenic, highly mobile and relatively insoluble in water. The geochemical monitoring of soil He emission at Cumbre Vieja started in 2002. Soil gas samples have been regularly collected at ~40 cm depth using a metallic probe at 600 sites for each survey and later the He content has been analysed by means of a quadrupole mass spectrometer. Spatial distribution maps have been constructed following the sequential Gaussian simulation (sGs) procedure to quantify the diffuse He emission from the studied area. The time series of diffuse He emission rate has shown significant increases before and during the occurrence of seismic swarms that took place in the period 2017-2021. During the eruptive period, a significant increase was also observed, with good temporal agreement with the increase of the volcanic tremor. Increases in diffuse He preceded peaks of diffuse CO₂ emission as expected by the characteristics of these gases. The absence of visible volcanic gas emissions (fumaroles, hot springs, etc.) at the surface environment of Cumbre Vieja, makes this type of studies in an essential tool for volcanic surveillance purposes.