Hydrothermal gases offshore Milos Island, Greece

Hydrothermal fluids emerge from the seafloor of Paleohori Bay on Milos. The gases in these fluids contain mostly CO 2 but CH 4 concentrations up to 2% are present. The stable carbon isotopic composition of the CO 2 (near 0%) indicates an inorganic carbon source (dissociation of underlying marine carbonates). The carbon and hydrogen isotopes of most CH 4 samples are enriched in the heavy species ( δ 13 C = −9.4 to −17.8‰ ; δD = −102 to −189‰) which is believed to be characteristic for an abiogenic production of CH 4 by CO 2-reduction (Fischer-Tropsch reactions). Depletions in the deuterium content of three CH 4 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. We show 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 3He 4He excess 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.