The 2021-22 unrest of La Fossa volcano (Vulcano Island, Aeolian Archipelago) by the side of fumarole chemistry: clues on the magmatic source in the precursory and ongoing phases

In September 2021, the La Fossa volcano entered a new phase of unrest. The monitoring system, operating in the island since late ‘80s, recorded a sudden variation in seismicity, ground deformation, fumarole temperatures, soil and plume degassing. These variations were interpreted as due to the fast vaporization and expansion of the hydrothermal system (Federico et al., submitted), hypothesized at depth > 1.5 km bsl. At the same time, fumarole chemistry showed clear-cut variations, related to the dominant contribution of the magmatic gas over the hydrothermal one. The CO₂ content and the helium isotope composition of the magmatic source revealed the appearance of a more primitive magma, compared to that feeding the fumaroles in the previous period, during the climax of the unrest. The signs of the enhanced contribution of magmatic gases in the fumarolic gases was already evident since 2018, so the 2021 unrest appears to have been the outcome of a long lasting preparatory phase. The systematics of gas species together with C and He isotopes, emitted from fumaroles after the first months of the unrest, revealed the appearance of a different magmatic component, poorer in N₂, ³He and richer in He, S and ¹³C. The magmatic contribution is persistently overwhelming the hydrothermal one by the time of this communication.