Early evidence of magmatic rise through 3He/4He ratio measurements at Dos Aguas cold mineral spring, La Palma, Canary Islands

A volcanic eruption began at Cumbre Vieja volcano (La Palma, Canary Islands, Spain) on September 19, 2021, and resulted in the longest volcanic event since data are available on the island: it finished after 85 days and 8 hours of duration and 1,219 hectares of lava flows. This volcanic eruption is part of the volcanic evolution of La Palma Island, the fifth in extension (706 km²) and the second in elevation (2,423 m a.s.l.) of the Canarian archipelago. Cumbre Vieja volcano, where the volcanic activity has taken place exclusively in the last 123 ka, forms the southern part of the island. The first geophysical precursory signals of the last eruptive process, started on October 7^th and 13rd, 2017, when two remarkable seismic swarms interrupted a seismic silence of 46 years in Cumbre Vieja volcano with earthquakes located beneath Cumbre Vieja volcano at depths ranging between 14 and 28 km with a maximum magnitude of 2.7. Five additional seismic swarms were registered in 2020 and four in 2021, the last being the one that preceded the eruption, beginning a week before it. ³He/⁴He ratio has been monitored at Dos Aguas cold mineral spring in La Palma Island since 1991 to date as an important volcano monitoring tool able to provide early warning signal of future volcanic unrest episodes, as magmatic helium emission studies have demonstrated to be sensitive and excellent precursors of magmatic processes occurring at depth. A significant increase was observed from 2011 to 2012, when the ³He/⁴He ratio reached the highest value of the period 1991-2019: 10.24 RA (being R_A the ratio in atmospheric helium) in September 2012. At that time, this was the highest ³He/⁴He ratio reported from the Canarian archipelago measured either in the lavas or terrestrial fluids (Padrón et al., 2015). We suggest the occurrence of aseismic magma rising episodes beneath La Palma within the upper mantle towards an ephemeral magma reservoir in the period 2011-2012. Later, in the period 2017-2020, magma rising continued and produced seismic swarms that were accompanied also by the highest ³He/⁴He ratio measured at Dos Aguas (September, 2020). In 2021, ³He/⁴He ratio decreased ~0.32 R_Asince the beginning of the year, including the eruptive period.  ³He/⁴He ratio values suggest that upward magma migration that caused the 2021 Cumbre Vieja eruption likely started in or before 2012.

Padrón et al., (2015). Bull Volcanol 77:28. DOI 10.1007/s00445-015-0914-2