Altered mafic and ultramafic rocks were studied in correspondence with hyperalkaline, CH4-bearing and very low-hydrogen spring waters in the Othrys ophiolite, whose chemical features are typical of present day serpentinisation. The H2 paucity is interpreted as the result of the incorporation of high-silica, aqueous fluids, probably derived from mafic rocks. The vein assemblage of serpentine + magnetite is related to circulation of low-silica fluids whereas serpentine + talc, tremolite after garnet and Fe-rich serpentine in the interior of serpentine veins reflect a late circulation of low-temperature (likely below 120 °C), high silica activity fluids. The highsilica conditions might have limited or interrupted the production of H2, which was subsequently consumed by CO2 hydrogenation to produce CH4. The lack of H2 could also be due to peridotite alteration by CO2-rich fluids. This would imply that the Othrys peridotites, among similar methane-bearing peridotites, may be considered as terrestrial analogues of Martian ultramafic rocks, which are thought to contribute to methane emission in the atmosphere of Mars. Understanding the mechanism of methane abiotic production will likely shed light to the details of some crucial aspects as the greenhouse-gas budget, the production of hydrocarbons and the origin of life on Earth.
Petrological implications for the production of methane and hydrogen in hyperalkaline springs from the Othrys ophiolite, Greece.