Crustal fluid migration and gas-water-rock interaction processes in a seismic area: the case study of the Contursi hydrothermal system (Southern Appenines)

The geochemical characteristics of fluids that emerge at the Earth's surface are influenced by gas-rock-water interactions in the deep and shallow crustal layers, including mixing, outgassing of volatiles, and precipitation of minerals. The goal of the study was to understand the various interactions that influence the migration and behaviour of fluids within the Earth's crust and how they may change during the process of crustal fluid migration towards a hydrothermal system in the shallow crustal layers and within (Contursi basin, Italy). These processes can make it difficult to identify the source of deep gas by using the classical approach based on mixing processes of fluids and carbonate dissolution. Therefore, alternately the relationship between Total Dissolved Inorganic Carbon (TDIC) and the δ¹³C_TDIC in groundwater from the Contursi hydrothermal system investigating the water-gas-rock interaction at the local scale through the detailed reconstructions of the geological framework at depth have been taken into consideration. We found that both the dissolved and free gas in the hydrothermal system probably originated from a deep CO₂ endmember with a δ¹³C_CO2 value ranging from +2.12‰ to +3.20‰ (PDB) depending on the presence of brine or freshwater in the local aquifers. However, we observed that this CO₂ lost its pristine carbon isotopic signature during its storage in the deep dolomite-composed reservoirs (6-8 km), making it challenging to figure out its deep origin (decarbonation vs mantle/magmatic CO₂). Our calculations also showed that the output of CO₂, taking into account secondary processes (i.e. degassing CO₂ and calcite precipitation) and interactions with water at different salt concentrations, could be at least 40% higher than estimates from the mixing-only approach, such that it is comparable with several active and quiescent worldwide volcanic systems. In order to interpret potential geochemical changes that may occur during future seismic events in sites like Contursi, which are earthquake-prone areas, it is necessary to implement models that can help us understand fluids origin and the processes that influence their chemical and isotopic signature.