Unveiling the geochemistry of fluids in the Central Aeolian Islands (Italy)

In the last decades, the volcanically active Aeolian Islands have been the focus of numerous geochemical investigations and monitoring activities, primarily focused on the islands of Vulcano, Stromboli and Panarea. However, relatively few studies have explored the geochemical characteristics of other islands, despite evidence of hydrothermal activity. Salina, for instance, hosts a shallow, cold, low-salinity aquifer that overlies a deeper warmer aquifer, with highly saline water. Additional noteworthy features include hydrothermal deposits on the seafloor and offshore submarine gas emissions. Similarly, Lipari hosts a thermal aquifer (e.g. Terme di San Calogero) and exhibits significant hydrothermal emissions along its western coast, particularly in areas of Valle del Fuardo and Caolino quarry. In this study we conducted detailed geochemical surveys on Lipari and Salina to investigate the origins of the fluids and their relationship with the geodynamic framework. The research is part of the Project CAVEAT (Central-southern Aeolian islands: Volcanism and tEArIng in the Tyrrhenian subduction system), which aims to provide a comprehensive understanding of the current geodynamics in the southern Tyrrhenian region, focusing on the interaction between volcanism and tectonic activity within the Tyrrhenian subduction system.

On Salina and Lipari islands, soil CO₂ flux measurement campaigns were conducted to examine the spatial distribution of soil CO₂ emissions. Thermal surveys using an Unmanned Aircraft System were conducted over fumarolic areas to detect thermal anomalies associated with zones of preferential fluid emissions. These measurements helped define preferential pathways for fluid migration and identify active tectonic structures associated with areas of elevated soil CO₂ emissions. At selected sites, isotopic composition of gas was analyzed to infer the gas origins. On Lipari, soil CO₂ emission anomalies revealed a NNW-SSE alignment consistent with the area’s primary tectonic structures. Isotopic analysis confirmed a contribution of deep-origin fluids to these emissions. Thermal (up to 45.8 °C) and cold waters from Salina and Lipari were sampled and analyzed for their chemical and isotopic composition, as well as for dissolved gases. The isotopic composition of the water clearly indicates that the sampled groundwater originates from a mix of meteoric water and seawater, with varying degrees of mixing at each site. Gases dissolved in water exhibit an atmospheric component with a high content of CO₂ in the most brackish samples. At Salina, the isotopic composition of dissolved helium reflects a mantle contribution. Collectively, the findings emphasize the significant influence of mantle and deep-origin origin fluids in shaping the geochemistry of both islands. They further highlight the critical role of geodynamic and tectonic processes in governing fluid emissions across the two islands.