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

New isotopic values for delta D and delta(18)O (H2O), delta(13)C (CO2) and delta D (H-2) 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 O-18-depleted groundwater, and a magmatic-metamorphic end-member rising from a small magmatic chamber. Comparative analyses of delta D and delta(18)O (H2O), delta D (H-2) values and H-2/H2O ratios suggest that the Solfatara reservoir never attains the reduced conditions typical of other 'mature' geothermal systems; the rise of deep oxidized O-18-enriched fluids is simultaneous with maximum ground uplifts and the highest H-2-H2O isotopic,aeothermometric values. The delta(13)C (CO2) values seem to indicate a mixed magmatic-metamorphic origin also for carbon dioxide: the CO2 magmatic component could be characterized by a low delta(13)C, if there were a closed degassed magmatic system underlying the Phlegrean Fields zone. (C) 1999 Elsevier Science B.V. All rights reserved.