The structure of a hydrothermal system from an integrated geochemical, geophysical and geological approach: the Ischia Island case study

The complexity of volcano-hosted hydrothermal systems is such that thorough characterisation requires extensive and interdisciplinary work. We use here an integrated multidisciplinary approach, combining geological investigations with hydrogeochemical and soil degassing prospecting, and resistivity surveys, to provide a comprehensive characterisation of the shallow structure of the south-western Ischia’s hydrothermal system. We show that the investigated area is characterised by a structural setting that, although very complex, can be schematised in three sectors, namely the extra caldera sector (ECS), caldera floor sector (CFS), and resurgent caldera sector (RCS). This contrasted structural setting governs fluid circulation. Geochemical prospecting shows, in fact, that the caldera floor sector, a structural and topographic low, is the area where CO2-rich (>40 cm3/l) hydrothermally mature (log Mg/Na ratios <-3) waters, of prevalently meteoric origin (δ18O <-5.5 ‰), preferentially flow and accumulate. This pervasive hydrothermal circulation within the caldera floor sector, being also the source of significant CO2 soil degassing (>150 g∙m-2∙day-1), is clearly captured by electrical resistivity tomography (ERT) and transient electromagnetic (TEM) surveys as an highly conductive (resistivity <3 Ω•m) layer from depths of ~100 m, and therefore within the Mount Epomeo Green Tuff (MEGT) formation. Our observations indicate, instead, that less-thermalised fluids prevail in the extra caldera and resurgent caldera sectors, where highly conductive seawater-like (Total Dissolved Solid, TDS >10,000 mg/l) and poorly conductive meteoric-derived (TDS <4,000 mg/l) waters are observed, respectively. We finally integrate our observations to build a general model for fluid circulation in the shallowest (<0.5 km) part of Ischia's hydrothermal system.