Closed-System Magma Degassing and Disproportionation of SO2 Revealed by Changes in the Concentration and δ34S Value of H2S(g) in the Solfatara Fluids (Campi Flegrei, Italy)

The use of a conceptual model of reference and modelling of relevant processes is mandatory to correctly interpret chemical and isotopic data. Adopting these basic guidelines, we have interpretated the unprecedented increase in the H2S(g) concentration and the concurrent unexpected decrease in the δ34S value of H2S(g) recorded since 2018 in the fumarolic effluents of the Bocca Grande fumarolic vent at Solfatara, Campi Flegrei caldera, in the framework of our conceptual model of the Solfatara magmatic–hydrothermal system. Assuming that the magma chamber situated at depths ≥ 8 km was filled at the end of the 1982–1984 bradyseismic crisis and no refilling episodes took place afterwards, as suggested by gas geochemistry, the concentration and the δ34S value of H2S(g) of the Bocca Grande fumarolic effluents are controlled by closed-system degassing of the melt at depths ≥ 8 km and disproportionation of SO2 in the deep hydrothermal reservoir (6.5–7.5 km depth) hosted in carbonate rocks where H2S equilibrates. These processes have been active during the last 40 years, but 41.1% (±6.4%) of the sulfur initially stored in the melt (2200 mg/kg) was lost in the 4-year period of April 2018–April 2022. This marked loss of S from the melt in 2018–2022 might be due to the high solubility of sulfur in the melt, which caused its preferential separation during the late degassing stages. These findings are of utmost importance for the surveillance of the Solfatara magmatic–hydrothermal system during the ongoing bradyseismic crisis.