Abiotic methane seepage from low temperature serpentinization ophiolites

by Giuseppe Etiope (Istituto Nazionale di Geofisica e Vulcanologia, Rome)

Monday 3 Oct 2011, 11:00 → 11:45 Europe/Stockholm

DeGeersalen, Earth Sciences Building, Y level 2, Stockholm University Frescati Campus

The emission of abiotic methane into the atmosphere from low temperature serpentinization in ophiolitic rocks is documented to date only in four countries, the Philippines, Oman, New Zealand, and Turkey. Methane-bearing ophiolites, however, may be more widespread and indications of active serpentinization on land (e.g., occurrence of H2, hyperalkaline springs) are known in several places (e.g., California, Canada, Italy, Romania). Serpentinization produces large amounts of molecular hydrogen which in theory may react with CO2 or CO to form hydrocarbons. This works reports the first direct measurements of gas (CH4, CO2) flux ever done on onshore ophiolites with present-day serpentinization (Etiope et al, 2011). We investigated the Tekirova ophiolites at Çirali, in Turkey, hosting the Chimaera seep, a system of gas vents issuing from fractures in a 5000 m2 wide ophiolite outcrop. At this site at least 150-190 tons of CH4 are annually released into the atmosphere. The molecular and isotopic composition of C1-C5 alkanes, CO2, and N2 combined with source rock maturity data and thermogenic gas formation modeling suggested a dominant abiotic component (~80-90%) mixed with thermogenic gas. Abiotic H2-rich gas is likely formed at temperatures below 50°C, suggested by the low deuterium/hydrogen isotopic ratio of H2 ("DH2: -720"), consistent with the low geothermal gradient of the area. Abiotic gas synthesis must be very fast and effective in continuously producing an amount of gas equivalent to the long-lasting (>2 millennia) emission of >100 t CH4 yr-1, otherwise pressurised gas accumulation must exist. Over the same ophiolitic formation, 3 km away from Chimaera, we detected an invisible exhalation of abiotic CH4 with fluxes from 0.07 to 1 g m-2 d-1. This is the first documented case of abiotic CH4 microseepage over ophiolites. Although this work provided basic answers about origin and fluxes of the Chimaera gas, it also open new questions, about the actual mechanism of abiotic hydrocarbon generation, the catalysts and the origin of CO2 involved in the reactions. Reference: Etiope G., Schoell M., Hosgormez H. (2011). Abiotic methane flux from the Chimaera seep and Tekirova ophiolites (Turkey): understanding gas exhalation from low temperature serpentinization and implications for Mars. Earth Plan. Sci. Lett, in press.