The 2021–2023 unrest of Vulcano Island volcanic system (Aeolian Islands, Italy): geochemical evidence from fumarolic gas discharges and well waters

Vulcano Island (Aeolian Archipelago, southern Italy) is an active volcanic system exhibiting persistent fumarolic activity over decades, sustained by magmatic degassing. Geophysical and geochemical monitoring, carried out since the 1980s, intensified in autumn 2021 due to volcanic unrest. This study reports original geochemical data including the chemical composition of (i) fluid discharges from La Fossa crater and Baia di Levante coastal area, and (ii) water and dissolved gases from Vulcano Porto village. Using these data we reconstruct the physicochemical processes controlling the compositional evolution of fluids discharged from the hydrothermal/magmatic system. Over 2013–2023, the temporal patterns in the SO2/H2S and (HCl + HF)/H2S ratios of emitted gases (which are sensitive to the relative influence of magmatic degassing vs. hydrothermal activity), showed that in January–November 2021 a remarkable pulse of magmatic fluids were emitted from the fumaroles of the summit crater. In contrast, gas discharges located in the inner flank of the crater displayed no notable compositional changes and a decrease of outlet temperatures, indicating a partial sealing of the fumarolic conduits. These results suggest a differential response of the fumarolic field to the stress caused by the magmatic fluid input. Geothermometric computations in organic gas compositions (the light alkenes-alkanes systems) indicate increasing temperatures at depth starting in 2018 up to the beginning of the unrest. These results suggest that organic gases are useful geoindicators for the geochemical monitoring of this volcano. The results of this study have shown that fluids from the Baia di Levante responded to the magmatic fluid pulse with a delay of several months that was due to heat buffering and high-temperature gas dissolution within the hydrothermal reservoir feeding the fluid discharges. From early 2022, magmatic gases (SO2, HCl, HF) and estimated temperatures based on H2, CO, and light hydrocarbons from the crater fumaroles indicated a decrease in the input of magmatic fluids, mimicking the events of the 2004–2005 aseismic volcanic crisis at Teide volcano (Canary Islands, Spain). For the peripheral gas discharges, temperature-sensitive gas species registered a remarkable decrease in reservoir temperatures almost 1 year later than the crater fumaroles, in agreement with the delayed response to the magmatic pulse that affected the hydrothermal-dominated system of Baia di Levante. During the 2012–2016 eruptive cycle at Copahue volcano (Argentina), distal geothermal gas emissions recorded a similar delayed response to a magmatic pulse of distal fluid discharges relative to those from the main active crater. These results suggest that peripheral hydrothermal discharges are less useful for volcano monitoring. Although gas emissions are not back to pre-crisis background, the temporal trends of the geochemical parameters sensitive to magmatic fluid inputs, evidenced at both the summit of La Fossa crater and the Baia di Levante, suggest that the 2021 unrest at Vulcano appears to be weaker than that recorded in the late 1980s. Therefore, unless significant magma injection occurs, we anticipate that Vulcano Island will return to a relatively quiet status of activity typical of the decade prior to 2021.