Abiotic methane flux from the Chimaera seep and Tekirova ophiolites (Turkey): Understanding gas exhalation from low temperature serpentinization and implications for Mars

作者：Etiope Giuseppe^*; Schoell Martin; Hosgormez Hakan

来源：Earth and Planetary Science Letters, 2011, 310(1-2): 96-104.

DOI：10.1016/j.epsl.2011.08.001

• abiotic methane
• seepage
• serpentinization
• ophiolites
• Mars

The emission of abiotic methane (CH4) 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. Serpentinization produces large amounts of hydrogen (H-2) which in theory may react with CO2 or CO to form hydrocarbons (Fischer-Tropsch Type synthesis, FTT). Similar mechanisms have been invoked to explain the CH4 detected on Mars, so that understanding flux and exhalation modality of ophiolitic gas on Earth may contribute to decipher the potential degassing on Mars. This work reports the first direct measurements of gas (CH4, CO2) flux ever done on onshore ophiolites with present-day serpentinization. We investigated the Tekirova ophiolites at Cirali, in Turkey, hosting the Chimaera seep, a system of gas vents issuing from fractures in a 5000 m(2) wide ophiolite outcrop. At this site at least 150-190 t of CH4 is annually released into the atmosphere. The molecular and isotopic compositions of C-1-C-5 alkanes, CO2, and N-2 combined with source rock maturity data and thermogenic gas formation modelling suggested a dominant abiotic component (similar to 80-90%) mixed with thermogenic gas. Abiotic H-2-rich gas is likely formed at temperatures below 50 degrees C, suggested by the low deuterium/hydrogen isotopic ratio of H-2 (delta D-H2: -720%4, 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 microseepage of abiotic CH4 with fluxes from 0.07 to 1 g m(-2) d(-1). On Mars similar fluxes could be able to sustain the CH4 plume apparently recognised in the Northern Summer 2003 (10(4) or 10(5) t yr(-1)) over the wide olivine bedrock and outcrops of hydrated silicates in the Syrtis Major and Nili Fossae; just one seep like Chimaera or, more realistically, a weak, spatially sporadic microseepage, would be sufficient to maintain the atmospheric CH4 level on Mars.

• 出版日期2011-10-1