Chemical and isotopical characterisation of fluid manifestations of Ischia Island (Italy)

Between 1994 and 1995, gas samples from fumaroles and thermal waters were collected on Ischia Island. The chemical composition of the shallow and deep natural hydrothermal fluids discharged is related to the main hydrological and lithological characteristics of the rock formations present in the reservoir. A biphase reservoir (liquid+gas) is identified, where the dominant liquid has a temperature of about 280°C. On the basis of δ13CTDC values it was possible to hypothesize a deep source characterised by carbon isotopic values varying from 0 to −3δ‰. These values are noticeably more positive with respect to those attributed to magmatic CO2 (δ13CCO2 ranging from −5 to −8δ‰), thus suggesting a magmatic source modified by crustal contamination. This hypothesis is supported by the carbon isotopic composition of CO2 in sampled gases, which varied from 0 to −5δ‰. The inferred isotopic value of carbon of magmatic CO2 would then be about −2δ‰. The observed differences in C isotopic composition between fumarolic and magmatic gases would be caused by kinetic and/or equilibrium fractionation processes. These processes would cause a fractionation of δ13C of deep CO2 towards more negative values (down to −5‰). Actually, CO2 removal or addition processes caused by the interaction between deep gases and shallow hydrothermal waters are likely to be responsible for the different chemical and isotopic compositions of gaseous emissions. For these reasons, and on the basis of the homogeneity of geothermometric values, the existence of a single, large reservoir that feeds all of the fluids discharged at Ischia Island can be hypothesised. Based on acquired data, a new geochemical model of the geothermal system of Ischia Island is proposed.