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<p>The 2021 eruption of Cumbre Vieja volcano (La Palma Island) is one of the largest natural disasters in Europe in recent times, but also a unique opportunity for monitoring the evolution of a volcanic system and its underlying mantle source.</p><p>Geophysical and geochemical evidence suggests that volcanism in Canary Islands is driven by the presence of a mantle plume, even though helium isotopes highlight this lower mantle component (<sup>3</sup>He/<sup>4</sup>He>9 Ra) only in the Dos Aguas spring gases and the older lavas from the Taburiente caldera (north of La Palma). Conversely, fluid inclusions in lavas and spring gases from the recent Cumbre Vieja system have a MORB-like signature (8±1 Ra). These distinct signatures were ascribed to the mixing between different mantle components (Day and Hilton, 2020). In this framework, the 2021 Cumbre Vieja eruption opens new avenues to investigate the current composition of the local mantle and test the pre-existing models.</p><p>Here, we present the first insights into the <sup>3</sup>He/<sup>4</sup>He signature of volcanic gases and phenocryst-hosted fluid inclusions from lavas erupted by the Cumbre Vieja in September-November 2021. For comparison, we analyzed the poorly evolved lavas from 1677 San Antonio eruption bearing mantle xenoliths (South of Cumbre Vieja) and a 3 Ma old picrite cropping out in the Taburiente caldera, close to the Dos Aguas spring (Day et al., 2010).</p><p>The 2021 lavas belonging to the October 27<sup>th</sup> and November 9<sup>th</sup> flows are basanite tephrites, with an average Mg# of 58.6, being more mafic than those from the September opening phase (Mg# = 50.3; Pankhurst et al., 2022). Olivine phenocrysts have Fo content mostly of mostly 78-83, and elevated Al and Cr contents. The estimated T based on the Cr and Al in olivine thermometers (DeHoog et al., 2010) is 920-960°C.</p><p>The <sup>3</sup>He/<sup>4</sup>He ratio in phenocryst-hosted fluid inclusions from the 2021 products is 7-7.5 Ra, confirming the MORB-like signature of the volcanic products and gases dissolved in water of the Cumbra Vieja system (Day and Hilton, 2020; Torres-Gonzalez et al., 2020). Instead, the olivines in the Taburiente picrite yield 9.4±0.1 Ra, comparable to values in the Dos Aguas spring, confirming the existence of a lower mantle component below this sector of the island.</p><p>The distinct <sup>3</sup>He/<sup>4</sup>He signature observed at Taburiente and Cumbre Vieja products is preliminary interpreted as due to either (i) small-scale heterogeneities in the local mantle, and/or (ii) a plumbing system effect that lowers the <sup>3</sup>He/<sup>4</sup>He of the recently erupted magmas. In the latter case, magma differentiation and degassing at the crust-mantle boundary or even deeper in the mantle, coupled to the production and accumulation of radiogenic <sup>4</sup>He, would play a central role.</p><p>REFERENCES</p><p>Day, J.M.D., et al. 2010, Geochimica et Cosmochimica Acta, v. 74, p. 6565–6589.</p><p>Day, J.M.D., Hilton, D.R., 2020. Geology.</p><p>De Hoog, J. C., Gall, L., & Cornell, D. H., 2010. Chemical Geology, 270(1-4), 196-215.</p><p>Pankhurst, M. J., et al., 2022. Volcanica, 5, 1-10.</p><p>Torres-González, P. A. et al., 2020. J. Volcanol. Geotherm. Res. 392, 106757.</p>

<p>We collected samples from the lava flows from the Cumbre Vieja 2021 eruption, the 1677 San Antonio eruption and picrites from the 3 Ma old Taburiente calderaand in La Palma (Canary Islands) with the aim to investigate the geochemistry of the lavas and their He and CO2 isotopic composition in fluid inclusions. The above information is crucial to better understand the evolution in time of the volcanic system and the nature of the local mantle source.</p> <p>Our results suggest that during the Cumbre Vieja eruption there was a systematic increase in the volatile concentrations (particularly He and CO<sub>2</sub>) between late September and early October. The above is accompanied by an increase of the whole rock Mg#, CaO/Al<sub>2</sub>O<sub>3</sub> and the Nb/La ratios from 50.5 to 58.7, from 0.78 to 0.87, and from 0.88 to 1.01, respectively (we also observed that these variations coincide with the occurrence of deeper earthquakes; 30-40 km; D’Auria et al., 2022), which likely indicate the intrusion of a more primitive less-degassed magma rising from the asthenosphere at the end of September. Regarding the mineral chemistry, the composition of olivine and pyroxene do not show any noticeable heterogeneity in all analyzed samples. The <sup>3</sup>He/<sup>4</sup>He ratios are homogeneous over time and exhibit MORB-like signatures between 7 and 7.5 Ra. In comparison the whole rock composition and the <sup>3</sup>He/<sup>4</sup>He ratios of the San Antonio lavas are very similar to the lavas erupted in late October-early November 2021, whereas the picritic lavas from the Taburiente caldera show higher <sup>3</sup>He/<sup>4</sup>He ratios equal to 9.4±0.1 Ra that are comparable with the signatures measured in the Dos Aguas cold spring (Pérez et al., 1994; Padrón et al., 2022) indicating plume origin.</p> <p>In conclusion, the He isotopic differences between lavas form northern and southern La Palma could be advocated to the small-scale heterogeneities in the mantle, and/or a plumbing system responsible for the lower <sup>3</sup>He/<sup>4</sup>He of the 2021 Cubre Vieja magmas.  Moreover, considering the more radiogenic helium ratios reported in mantle xenoliths from La Palma (6.5 - 7.2Ra; Sandoval Velasquez et al., 2022) we propose that the isotopic signatures observed at Cumbre Vieja and San Antonio are likely the result of a mixing between a plume component (highlighted by the <sup>3</sup>He/<sup>4</sup>He in the picrites and the Dos Aguas Spring of the Taburiente caldera) and a more radiogenic reservoir located in the shallower lithospheric mantle.</p>