Hydrothermal gases offshore Milos Island, Greece

Hydrothermal fluids emerge from the seafloor of Paleohori Bay on Milos. The gases in these fluids contain mostly CO2 but CH4 concentrations up to 2% are present. The stable carbon isotopic composition of the CO2 (near 0%) indicates an inorganic carbon source (dissociation of underlying marine carbonates). The carbon and hydrogen isotopes of most CH4 samples are enriched in the heavy species (δ13C = −9.4 to −17.8‰; δD = −102 to −189‰) which is believed to be characteristic for an abiogenic production of CH4 by CO2-reduction (Fischer-Tropsch reactions). Depletions in the deuterium content of three CH4 samples (to −377%) are probably caused by unknown subsurface rock alteration processes. Secondary hydrogen isotope exchange processes between methane, hydrogen and water are most likely responsible for calculated unrealistic methane formation temperatures. w that excess helium, slightly enriched in 3He, is present in the hydrothermal fluids emerging the seafloor of Paleohori Bay. When the isotopic ratio of the excess component is calculated a 3He4Heexcess of 3.6 · 10−6 is obtained: This indicates that the excess component consists of about one third of mantle helium and two thirds of radiogenic helium. We infer that the mantle-derived component has been strongly diluted by radiogenic helium during the ascent of the fluids to the surface.