Publication Date:
2017-04-04
Description:
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 (H2) 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 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 t of CH4 is annually released into the
atmosphere. The molecular and isotopic compositions of C1–C5 alkanes, CO2, and N2 combined with source
rock maturity data and thermogenic gas formation modelling 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 (δDH2: −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 (N2 millennia) emission of N100 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 (104 or
105 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.
Description:
Published
Description:
96-104
Description:
4.5. Studi sul degassamento naturale e sui gas petroliferi
Description:
JCR Journal
Description:
reserved
Keywords:
abiotic methane
;
seepage
;
serpentinization
;
ophiolites
;
Mars
;
03. Hydrosphere::03.04. Chemical and biological::03.04.05. Gases
;
04. Solid Earth::04.04. Geology::04.04.12. Fluid Geochemistry
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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