Secondary fractionation processes of dissolved inorganic carbon and CO2 in thermal waters from active and quiescent volcanic systems

Abstract:

Carbon dioxide is the main component of the dry gas phase in hydrothermal and volcanic fluids, being mainly produced by mantle degassing and thermometamorphic reactions on limestone at which a shallow contribution from microbial activity is commonly added. These three different sources can be recognized on the basis of the d¹³C values, since biogenic CO₂ typically shows an isotopic signature significantly more negative (<-20‰ V-PDB) than that originated at depth (>-7‰ V-PDB). Intermediate d¹³C values are commonly interpreted as due to mixing processes between deep and shallow sources. In this study, the d¹³C values of CO₂ and total dissolved inorganic carbon (TDIC) in thermal waters from distinct hydrothermal/volcanic systems, located in Italy (Campi Flegrei and Vulcano Island) and Chilean Andes (El Tatio), are reported. This dataset includes several carbon isotopic ratios that are not consistent with a pure shallow or deep CO₂ origin. Nevertheless the relatively high CO₂ concentrations and the water chemistry of these samples clearly indicate that they are not resulting by mixing between the deep and shallow end-members. Calcite deposition, which produces a strong isotopic fractionation on the pristine CO₂, seems to represent a reliable alternative explanation for the observed data. It is worth noting that these peculiar isotopic and chemical features have recurrently been recognized in thermal water discharges from different volcanic areas. These results demonstrate that the release of CO₂ from primary sources is strongly affected by secondary processes since they act as sinks of CO₂. As a consequence, they play an important role for the evaluation of the global budget of CO₂ discharged from these natural systems.