Early Precursory Changes in the He-3/He-4 Ratio Prior to the 2021 Tajogaite Eruption at Cumbre Vieja Volcano, La Palma, Canary Islands

We report significant changes in the He-3/He-4 ratio measured at the Dos Aguas CO2-rich cold mineral spring (La Palma, Canary Islands) that represent early precursory signals of the 2021 Tajogaite eruption at Cumbre Vieja volcano. Air-corrected He-3/He-4 ratio has an average value of 9.80 R-A in the last 30 years (1991-2021). Helium isotope ratios higher than the average value were observed during the period 2008-2013 suggesting an input of He-3-rich less-degassed magma beneath La Palma lithosphere. Magma ascended in 2017-2018 toward another reservoir located beneath Cumbre Vieja at 10-15 km depth, causing detectable seismic swarms located between 15 and 25 km, followed in 2020 by additional inputs of He-3-rich less-degassed magma that intruded at depth. Finally, in 2021 the buoyancy of magma and the pressure of the exsolved gases overcame the lithospheric resistance and a volcanic eruption took place between 19 September and 13 December 2021.