Paroxysmal eruptions tracked by variations of helium isotopes: inferences from Piton de la Fournaise (La Réunion island).

Helium (He) with its isotopes (He, He) is a key tracer enabling the Earth's mantle and dynamics to be characterized. Enrichment in primordial helium (He) has been detected in volcanic gases of numerous magmatic systems in different geodynamic settings. Despite past use to monitor volcano-tectonic unrest, temporal He/He variability in volcanic emissions is still poorly constrained. Here, we investigate noble gas chemistry of Piton de la Fournaise hotspot volcano, where temporal fluctuations of He/He in response to the eruptive activity have never been studied. We compare the He/He signature of volcanic gases and fluid inclusions and we highlight analogous evolution of the He/He signature in both during the last decades of eruptive activity (1990-2017), even during the same eruption. We show that the maximum enrichment in He is found in magmatic fluids that fed the most voluminous eruptions which culminated in caldera collapse events. We argue that this enrichment in He mostly reflects a greater contribution of magmatic fluids from a primitive component of the mantle plume. These results emphasize that He isotopes may provide warnings of increases in deep magmatic contributions that potentially herald paroxysmal eruptions, as documented here at Piton de la Fournaise (2007) and also at Kilauea (2018).