Chemical and Isotopic Composition of Thermal Waters and Dissolved Gases from Campi Flegrei (southern Italy): Insights into Isotopic Fractionation Processes Affecting the CO2 Isotopic Signature

In the framework of a study aimed to investigate the interactions between superficial aquifers and uprising deep-originated hydrothermal fluids at the Campi Flegrei volcanic district (southern Italy), water and dissolved gases from wells, springs and Lake Averno for chemical and isotopic (delta13C in CO 2) analyses were recently sampled and analyzed. This area is characterized by an intense hydrothermal activity that at the surface originates hot springs, steam-heated pools, sub-aerial fumaroles, mostly located within two craters (Solfatara and Agnano), along with a number of submarine vents in the Pozzuoli Bay. The hydrothermal circulation pattern is likely fed by: (i) a Na-Cl deep brine with high salinity and T and rich in CO 2 , whose isotopic signature is relatively heavy (-1 ÷ -2 ‰ vs. V-PDB); (ii) a HCO 3 -rich component, also displaying high salinity and T; (ii) meteoric water; (iv) seawater, mostly in the coastal area. N 2 -rich dissolved gases, with relatively high O 2 and Ar contents, are commonly associated with cold water wells characterized by relatively low CO 2 contents (<10 % by vol.), likely deriving from microbial activity, as suggested by the delta13C-CO 2 values, which range from -23.7 to -19.7 ‰ vs. V-PDB. Thermal waters show significant enrichments in dissolved CO 2 (24.1 to 89.8 % by vol.), whose origin is likely associated with the deep source, as the delta 13 C-CO 2 values are from -7.56 to -5.03 ‰ vs. V-PDB. This study has individuated the occurrence of a third group of waters characterized by relatively high concentrations of CO 2 (21 to 90% by vol.) with delta 13 C values in between the other two types of waters: -15.2 and -11.8 ‰ vs. V-PDB. These waters (Lake Averno, Stufe di Nerone, Esposito and Damiani wells) are all located in a narrow zone close to Monte Nuovo, where the latest historical eruption at Campi Flegrei occurred (1538). By combining the CO 2 concentrations and delta 13 C-CO 2 values, a simple mixing process between deep and shallow end-members is not able to reproduce such intermediate isotopic signatures, rarely recognized in fluids from volcanic and geothermal areas. Therefore, the carbon isotopic composition of these waters should reflect a different deep source or, more likely, secondary fractionation processes altering the original isotopic signature of the deep CO 2 . This process would be favored by chemical-physical conditions prevailing in a very restricted area. Calcite precipitation can cause an isotopic fractionation able to modify the original magmatic delta 13 C-CO 2 values into those measured in the four anomalous samples. This process seems to be efficient only for those fluids collected in the area of Monte Nuovo, which prior the 1538 eruption was covered by the sea. Occurrence of a seawater-like aquifer at shallow depth is also suggested by the chemistry of the Stufe di Nerone and Lake Averno waters. It is worth noting that anomalous delta 13 C-CO 2 values were also recognized in the fluid discharges inside and close to the village of Sake, which is located in the southernmost part of Nyamulagira volcano (DRC), as well as at the bottom of the adjacent Kabuno Bay (-11.48 ‰ V-PDB) that is the north-westernmost extension of Lake Kivu. These isotopic values are in contrast with the typical magmatic isotopic signature characterizing the fluid emergences of the whole area and are not in agreement with that measured at the bottom of Lake Kivu Main basin (-6.79 ‰ V-PDB). In analogy with what observed at Campi Flegrei, it is realistic to suppose that the deep-originated fluids discharging in the Sake area can be affected by secondary process, induced by local and peculiar chemical-physical conditions, able to cause such a dramatic isotopic fractionation of CO 2 . Further studies are needed to deeply investigate on this uncommon geochemical feature.