O-18 exchange between steam and carbon dioxide in volcanic and hydrothermal gases: Implications for the source of water

The oxygen isotopic compositions of H2O and CO2 from 31 fumaroles from diff erent volcanoes (Vesuvio, Campi Flegrei, Vulcano and Etna in Italy; Soufrie re of Guadeloupe in Lesser Antilles) prove that O-18 exchange between CO2 a nd steam is fast enough in natural gas phases to allow rapid isotopic reequ ilibration within a wide temperature range (100-1000 degrees C). The isotop ic composition of H2O discharged by volcanic-hydrothermal fumaroles is affe cted by isotopic reequilibration along with cooling of the gas phase and th is secondary process must be taken into account when interpreting the origi n of the water from isotopic data. The use of corrected delta(18)O values c omputed considering the overall oxygen isotopic composition of the H2O-CO2 system is therefore compulsory to overcome interpretative biases. As an exa mple, we apply this method to 100-600 degrees C fumaroles at Vulcano crater , whose isotopic composition is reinterpreted considering the effects of O- 18 exchange between CO2 and H2O. In contrast to most previous interpretatio ns, the corrected isotopic values for H2O clearly show that the Vulcano sys tem is fed by a two-component mixture of magmatic water with a typical "and esitic" composition and either O-18-shifted seawater or magmatic steam part ly condensed within the volcanic ground, with minor contribution of local m eteoric water. The possible additional effects of isotopic reequilibration involving other O- and II-bearing gas species in volcanic fluids, such as S O2, H2S, HCl, HF, and H-2, have been also considered. The influence of SO2 upon the isotopic composition of H2O is subordinate with respect to that of CO2, whereas significant shifts in deuterium composition are expected from hydrogen isotopic exchanges in the H2O-H2S-HCl-HF-H-2 gas system. Copyrigh t (C) 2000 Elsevier Science Ltd.