Hydrogen, oxygen and carbon isotope ratios of Solfatara fumaroles (Phlegrean Fields, Italy): further insight into source processes

New isotopic values for δD and δ18O (H2O), δ13C (CO2) and δD (H2) are combined with chemical and isotope data from the literature to investigate the different fluid sources feeding the biphasic liquid–vapor reservoir underlying the Solfatara crater area. By applying equilibrium treatment to steam isotope data, it has been possible to identify two water components feeding the system: a meteoric end-member, which has been affected by mixing with deuterium and 18O-depleted groundwater, and a magmatic–metamorphic end-member rising from a small magmatic chamber. Comparative analyses of δD and δ18O (H2O), δD (H2) values and H2/H2O ratios suggest that the Solfatara reservoir never attains the reduced conditions typical of other `matureʹ geothermal systems; the rise of deep oxidized 18O-enriched fluids is simultaneous with maximum ground uplifts and the highest H2–H2O isotopic geothermometric values. The δ13C (CO2) values seem to indicate a mixed magmatic–metamorphic origin also for carbon dioxide: the CO2 magmatic component could be characterized by a low δ13C, if there were a closed degassed magmatic system underlying the Phlegrean Fields zone.