GNGTS 2022 - Atti del 40° Convegno Nazionale

94 GNGTS 2022 Sessione 1.2 THE AEOLIAN ISLANDS: A POTENTIAL GEOTHERMAL ENGINE C.G. Caruso, A. Gattuso, G. Lazzaro, M. Longo, M. Meschis, S. Scirè Scappuzzo, F. Italiano Istituto Nazionale di Geofisica e Vulcanologia, (INGV), Palermo, Italy The Sicilian territory is characterized by a geothermal potential among the highest in Italy, with a variety of thermal manifestation spread over both onshore and offshore areas and including the volcanic islands where geothermal potential looks to be even higher. In spite of this manifest geothermal vocation, this resource has never been exploited for energy purposes. The areas with the largest availability of high-enthalpy fluids are the island of Pantelleria and the Aeolian arc. Here we focus the attention on the geochemical features of the submarine hydrothermal fluids vented over the Aeolian islands. It is well known that the Aeolian Archipelago is located less than 40 km off the north- east coast of Sicily and includes seven subaerial volcanic edifices (Alicudi, Filicudi, Salina, Lipari, Vulcano, Panarea and Stromboli) and a number of submarine volcanoes (at least 5 main edifices: Eolo, Sisifo, Enarete, Alicudi and Filicudi North). The Aeolian arc represents the tectonically extending backarc basin associated with both the ongoing southeastwardly Ionian subduction process and the opening of Tyrrhenian Extensional Basin. It is one of the most active volcanic areas of the Mediterranean basin, affected by volcanic/hydrothermal activity. Structural trends and volcanic activity in the area are strongly controlled by the regional stress fields. Indeed, Quaternary NW-SE oriented crustal extension affecting the overriding plate of the Ionian subduction zone is seismically identified by historical damaging earthquakes, rupturing active normal faults, mostly NE-SW to N-S oriented such the likely tectonic link between Stromboli and Panarea Islands. Moreover, the shallow magma chambers, alongside the tectonic activity due to plate subduction and overriding plate crustal extension, deformation and cooling of erupted lavas results in convective circulation of dense, cold seawater through the cracked and fissured upper portions of the lithosphere; this circulation promotes the formation of venting sites that release hot hydrothermal fluids and dissolved elements. A widespread and intense submarine hydrothermal activity marked by long-lasting recharge and high-temperature/high-pressure characteristics, are clues for potentially exploitable geothermal energy sources. Indeed, the Aeolian arc is affected by numerous and distributed heat sources, generally represented by hot magmatic bodies at shallow depths (<10 km) in an extensional geodynamic setting. Vulcano, Panarea, Salina and Lipari exhibit some features that make them a potential site hosting exploitable submarine geothermal systems. All the observations done since late 90 s over the Aeolian islands, confirm that these areas are affected by hydrothermal fluid circulation; triggered by cold seawater entering into fractured rocks and getting superheated by magmatic bodies at crustal depths. The wide spread presence of submarine hydrothermal fluids, vented at temperatures in the range 40-150°C represents a challenge for a feature development of geothermal plants over the Aeolian arc. Clues of active venting of hydrothermal fluids have been further provided by the injection of magmatic-type volatiles into the sea water, as demonstrated by the anomalous 3 He content and the whole chemical composition of the gases. In this study we describe and discuss the geochemical features of the submarine hydrothermal fluids discharged over the Aeolian offshore. The results show how thermal energy of magmatic origin is still available over the archipelago providing significant perspectives for geothermal energy exploration. Following the assumption that the submarine hydrothermal fluids may equilibrate inside a geothermal reservoir at some level beneath the seafloor, we propose the existence of reservoirs kept at boiling conditions by the thermal energy released by cooling magma batches located at shallow crustal levels. We constrained the chemical-physical conditions (pressure, temperature and redox) that buffer the chemical composition of the hydrothermal fluids as well as the processes (i.e. mixings, fractionations) governing their composition at the venting sites. Preliminary geothermometric and geobarometric estimations indicate equilibrium temperatures in the range of 150-400°C besides H 2 O pressures in the range of 70 bar and 250 bar.