Hydrothermal gases offshore Milos Island, Greece

Hydrothermal fluids emerge from the seafloor of Paleohori Bay on Miles. 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 parts per thousand) indicates an inorganic carbon source (dissociation of underlying marine carbonates). The carbon and hydrogen isotopes of most CH samples are enriched in the heavy species (delta(13)C = -9.4 to -17.8 parts per thousand; delta D = -102 to -189 parts per thousand) 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 parts per thousand) 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 He-3, is present in the hydrothermal fluids emerging the seafloor of Paleohori Bay. When the isotopic ratio of the excess component is calculated a He-3/He-4(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.