Methana, the westernmost active volcanic system of the south Aegean arc (Greece): Insight from fluids geochemistry

An extensive geochemical survey of the fluids released by the volcanic/geothermal system of Methana was undertaken. Gases were characterized based on the chemical and isotopic [helium (He) and carbon (C)] analysis of 27 samples. Carbon dioxide soil gas concentration and fluxes were measured at 179 sampling sites throughout the peninsula. Forty samples of thermal and cold groundwaters were also sampled and analysed to characterize the geochemistry of the aquifers. Gases of hydrothermal origin gave a preliminary geothermometric estimate of about 210 °C. The He-isotope composition indicated mantle contributions of up to 40%, and the C-isotope composition of CO ₂ indicated that it predominantly (> 90%) originated from limestone decomposition. The groundwater composition was suggestive of mixing between meteoric and hydrothermally modified sea-water endmembers and water-rock interaction processes limited to simple rock dissolution driven by an increased endogenous CO ₂ content. All of the thermal manifestations and anomalous degassing areas, although of limited extent, were spatially correlated with the main active tectonic system of the area. The total CO ₂ output of the volcanic system has been preliminary estimated to be less than 0.05 kg s ^{- 1} . Although this value is very low compared to those of other volcanic systems, anomalous CO ₂ degassing at Methana - which is currently restricted to limited areas and at present is the only volcanic risk of the peninsula - is a potential gas hazard that warrants further assessment in future studies.