Hydrothermal circulation on Ischia Island (Southern Italy), revealed by an integrated geochemical, geophysical and geological approach

Abstract

Volcano-hosted hydrothermal systems are complex geological objects, whose thorough characterisation requires extensive and interdisciplinary work. Indeed, even thought geological, geochemical and geophysical observations offer highly significant but independent information, only an integrated multidisciplinary approach can yield a comprehensive characterisation of the chemical/physical structure of hydrothermal systems. Notwithstanding the extensive application of geological, geochemical and geophysical techniques in geothermal research, there are only a few examples in the literature of concurrent use of the three techniques [Finizola et al., 2002; Zlotnicki et al., 2009]; these studies overall demonstrate that only an integrated multidisciplinary approach can yield a comprehensive characterisation of the chemical/physical structure of hydrothermal systems. Ischia Island, in Southern Italy, is an active resurgent caldera [Orsi et al., 1991] that has produced intense volcanism over the past 10 ka, with the last eruption in 1302 A.D. The island is a very good example of an active volcano hosting a large hydrothermal system [Penta and Conforto, 1951]. In particular, its south western sector has long been known for pervasive hydrothermal circulation, as part of the larger hydrothermal system. Thus, this sector of the island is an ideal test site for multidisciplinary geothermal prospecting, since the complex geological and structural setting, the peculiar hydrothermal conditions favouring the formation of remarkably different water types in a relatively small area (̴ 20 km2) [Di Napoli et al., 2009], and the poorly-known but highly contrasted resistivity structure, all contribute to make interpretation of hydrothermal processes challenging. Here, we present the results of a multidisciplinary research performed in the south-western sector of Ischia Island by using geological, geochemical (groundwater composition and soil diffuse CO2 fluxes) and geophysical (Electrical Resistivity Tomography – ERT - and Transient Electromagnetic Method – TEM - soundings) methods. The acquired data are discussed in order to build up a hypothesis for fluid circulation in the shallowest (< 0.5 km) part of the hydrothermal system. The combined investigations highlight that the deep hydrothermal fluid circulation is strongly dependent on the structural setting of the SW Ischia sector, composed by three distinct sectors: Extra Caldera Sector (ECS), Caldera Floor sector (CFS), and Resurgent Caldera Sector (RCS) (Figure 1a). This peculiar tectonic/structural setting of the SW portion of Ischia Island force infiltrating waters to flow and accumulate into the structural and topographic low of the Caldera Floor Sector (CFS in Figure 1a), in which the shallow thermal aquifer has been identified by resistivity measurements at 100-200 m of depth. This aquifer, hosted into the Mt. Epomeo Green Tuff (MEGT) deposits, is characterized by very low resistivity values (< 1 Ω•m), and has a mixed marine-meteoric derivation. Seawater mostly infiltrates in the SW portion of the area (close to A in Figure 1a), whilst a prevalent contribution from cold diluted waters is evidenced in the NE portion of the study area (close to A’ in Figure 1a), along the flanks of Mt. Epomeo relief. Geochemical investigations have evidenced that meteoric (Total Dissolved Solid content ~ 1.5 g/l and δ18O ~ -7‰) and marine (Total Dissolved Solid content ~ 33 g/l and δ18O ~ 0‰) waters accumulate into the aquifer hosted in the CFS and form mixed solutions. However, the collected groundwaters only partially overlap the meteoric-sea water mixing trend, claiming for the existence of additional processes. A number of samples are characterized by Mg poor (Mg down to the detection limit) and CO2-rich (up to 229 cm3/l at STP) compositions, which are indicative for a deep derivation of these fluids. Indeed, Mg-depletion is a typical features of wat