The “Salinelle of Mt. Etna” Geosite: thermo-physical and geomechanical monitoring of hydrothermal fluids, aimed at understanding both their geothermal potential and their possible correlations with Mt. Etna activity.

Knowledge of hydrothermal fluid emissions can provide detailed information about processes operating at different depths, such as geothermal fluids upwelling with their own characteristics variable over times. In particular, at the “Salinelle of Mt. Etna Geosite (Paternò and Belpasso, Eastern Sicily), natural emissions of fluids have been continuously occurring for at least 12 ka. Their main peculiarity consists of a fluid phase made of salty water, mud, gas and liquid hydrocarbons originated from an admixture of magmatic and hydrothermal gas, leading to the formation of mud volcanoes with the most peculiar morphologies [Giammanco et al., 2016]. Our study mainly focused on the thermophysical and geochemical monitoring of the hydrothermal fluids emissions of the more active site known as “Salinelle dei Cappuccini”, in the Western part of the city of Paterno’. In order to compare this activity with that of other hydrothermal vents in the same area, the “Salinelle del Fiume” (Paternò) and the “Salinelle di San Biagio” (Belpasso) Geosites were also investigated. In particular, field and laboratory analyses were conducted over more than oneyear monitoring. They consisted on: i) continuous water temperature monitoring, using sophisticated thermal sensors (accuracy of 0.1°C) immersed in the pooling waters; ii) InfraRed Thermography (IRT) measures, using a thermal camera (resolution of 320 x 240 pixel) [Chicco et al., 2019]; iii) Thermal Conductivity properties determinations, through a commercial device consisting of a needle probe directly inserted in the investigated material during both field and lab activities; iv) mineralogical compositions through XRay diffraction; v) geochemical analysis, focusing on Electrolytic Conductivity, Salinity, CO2 and Rn determinations, using specific portable field. This was followed by mud/water sampling for VOC (Volatile Organic Content), Alkalinity, Electrical Conductivity, major species in solution and oxygen lab determinations. A further study centered on seismic parameters evaluation such as the Magnitude and its relationship with the frequency of seismic events all around the Mt. Etna area. This revealed useful in verifying how seismic activity linked to volcanic events can affect mud volcanoes activity. Obtained results highlighted a direct correlation between Temperature and Thermal Conductivity data as well as between Electrical Potential and Electrical Conductivity, without any link with CO2 and Rn contents. Daily temperatures showed a constant trend characterized by higher values (up to 35°C and more) within the first monitoring period, followed by a strong decrease (down to 9°C) and then a new gradual increase over the following months. This large variation seems to be linked to magmatic processes occurring at depth below Mt. Etna, likely related to early stages of magma upraise towards the surface. In particular, the higher the frequency of seismic events and then the higher the daily energy released, the higher temperature variations observed. This correlation could be explained by the ascent of new gasrich magma from the deepest magma reservoir of Mt. Etna, producing crustal fracturing along its path. Large release of highenthalpy fluids from the new ascending magma enhanced gaswater interaction in shallow aquifers crossed by major faults acting as gas escape pathways through the flanks of Mt. Etna, with consequent modification of the geochemical and temperature characteristics of the fluids issuing at the surface in the mudpools and gas vents of the Salinelle. Understanding how these fluids blend and what is their relationship with Mt. Etna volcanic activity can be of great importance in forecasting new eruptive cycles of the volcano.