It is generally accepted, but not experimentally proven, that a quantitative prediction of volcanic eruptions is possible from the evaluation of volcanic gas data. By discussing the results of two years of real-time observation of H2O, CO2, and SO2 in volcanic gases from Mount Etna volcano, we unambiguously demonstrate that increasing CO2/SO2 ratios can allow detection of the pre-eruptive degassing of rising magmas. Quantitative modeling by the use of a saturation model allows us to relate the pre-eruptive increases of the CO2/SO2 ratio to the refilling of Etna's shallow conduits with CO2-rich deep-reservoir magmas, leading to pressurization and triggering of eruption. The advent of real-time observations of H2O, CO2, and SO2, combined with well-constrained models of degassing, represents a step forward in eruption forecasting

The first studies on sulfur isotope geochemistry were performed approximately sixty years ago (e.g., Thode et al. 1949; Trofimov 1949). Since these early measurements on terrestrial and extra-terrestrial materials, S isotopes have been used to highlight and investigate several processes, such as the evolution of the solar system; the oxidation of the Earth’s atmosphere and hydrosphere; the genesis of ore deposits and fossil fuels (coal, oil, and gases); the origin and provenance of S species in different natural fluids, including groundwater, rainwater, as well as present-day and ancient marine waters (as recorded by evaporite deposits); the S isotope...