Tracking of degassing pathways in the COHS-silicate melt helps to understand phenomena leading towards either high- or low-energy eruptions at Mt. Etna

At Mt. Etna, high-energy Plinian eruptions took place in the past (e.g., 122 BC), and still represent the major volcanic threat for the town of Catania and its surroundings. By investigating the H2O-CO2-SO2- H2S-silicate melt system, we could track the pathlines of magmatic degassing and give insights into the phenomena determining the evolution of volcanic activity towards either high- or low-energy eruptions. Thermodynamic modeling of liquid-gas properties allowed us to describe gas evolution up to surface under various conditions as long as information was provided in order to estimate a) the redox buffer holding over the PTX range of interest, b) the bulk composition, including total (dissolved+exsolved) volatile contents, c) indicators for closed vs. open system degassing. The results we present give further insights into the deep-roots features of the magmatic feeding system, and confirm the large amount of carbon dioxide that have been already found in the literature. We compared the information retrieved from melt inclusions of the 122 BC plinian event to those from more recent eruptions (e.g. February 1999). From a volatile point of view, the deeper part of the plumbing system shows almost unchanged chemical and thermobarometric features. However, the focus on the conjugated chemical and isotopic features of sulfur allowed us to investigate the differing sulfur degassing patterns drawn by these two eruptions. It seems that elucidating the processes controlling S chemistry during the evolution of Etna magmas, from classic Strombolian or fire-fountain based activity to plinian eruptions may provide useful indications to discriminate the evolution to either high or low-energy eruptions.