Geochemistry of La Fossa fumaroles (Vulcano Island, Aeolian Islands) unveils the evolving magmatic sources of the fluid before and during the 2021 unrest

La Fossa volcano (Vulcano Island, Italy) has remained quiescent since its 1888–1890 eruption, punctuated by unrest characterized by increased vapor output and seismic activity. Fumarole vapor composition, a proxy for magmatic and hydrothermal fluid mixing, responds to enhanced magmatic volatile input during unrest, potentially revealing source characteristics. Besides being a marker of the relative contribution of magmatic and hydrothermal fluids, the fumarole composition can reveal some insights, in particular, into the depth of fluid sources and their composition. Hypotheses suggest that varying unrest phases involve different depths within the magma reservoir, influencing fluid input into the hydrothermal system and potentially inducing pressurization. This study presents geochemical and isotopic data from La Fossa's high-temperature fumaroles (2011–2024), including the unrest starting in September 2021. The findings indicate a three-year preparatory phase preceding the 2021 unrest, suggesting the activation of different fluid pathways. The 2021 unrest mobilized previously unobserved gas or magma reservoirs. Identifying geochemical markers linked to specific magma sources can be applied to other volcanoes to decipher degassing histories and hazards. This research underscores the complexity and interconnectedness of volcanic plumbing systems, where activity in one part can influence others, influencing monitoring and interpretation in volcanic regions globally. The interplay of magmatic and hydrothermal fluids, shifts in fluid sources, and the role of degassing in triggering unrest observed at La Fossa are relevant to many active volcanoes worldwide. • Long-term study (1985–2024) tracks the origin of La Fossa fluids. • A 3-year preparatory phase is identified as preceding the 2021 volcanic unrest. • 2021 unrest activated new, previously unseen magma sources.