Residual volcanic activity and deep degassing at Massico-Roccamonfina system (central and southern Italy): New insights from fluid geochemistry

Degassing processes in quiescent volcanic areas provide valuable tools to monitor deep magmatic and tectonic activities. Near Roccamonfina volcanic complex, at the boundary between the Campania-Lazio regions in central-southern Italy, two main degassing areas were identified, suggesting (i) residual volcanic activity and/or (ii) the presence of active faults enabling deep fluids to mix with shallow groundwaters. A comprehensive hydrogeochemical survey was carried collecting waters and gases from deep and shallow wells, springs, and natural gas vents. The investigation aimed to (i) characterize the chemical composition of circulating fluids, (ii) delineate areas influenced by deep degassing, (iii) understanding the origin of the collected fluids and (iv) providing a comprehensive model of the current volcanic/tectonic activity in the area. Our dataset integrates measurements of major ions, dissolved gases, stable isotopes (δ 18 O/δD/δ 13 C-TDIC), and helium isotopic ratios (R/Ra). Results reveal a progressive southward increase in mantle-derived helium, fixing the transition between Roman and Campanian magmatic provinces. Significantly, R/Ra values up to 1.87 in the northeastern caldera sector of the Campanian Plain indicate a significant, if compared to volcanic areas more south (e.g. Campi Flegrei), deep contribution, linked to ongoing regional dynamics. Evidences of deep degassing were also detected in areas recently affected by seismic swarms. The spatial distribution of dissolved gases and isotopic signatures: CO₂, N₂, δ 13 C-TDIC, and R/Ra, points to fluid ascent along regional-fault-controlled pathways. These findings highlight the complex processes and exchanges between (i) fluid circulation, (ii) crustal permeability and (iii) tectono-volcanic activity within this active segment of the Italian volcanic belt. • An extensive hydrochemical characterization of the groundwater for the Massico-Roccamonfina system was performed • Constraints on groundwater origin through water stable isotope (δ 18 O and δD) are highlighted • A novel approach based on dissolved gas analyses and detailed cartography was applied in order to identify localized geochemical anomalies • Robust geochemical tracers, such as noble gases and carbon isotopes, were used to define the origin of fluids