Abstract
Recurring discoveries of abiotic methane in gas seeps and springs in ophiolites and peridotite massifs worldwide raised the question of where, in which rocks, methane was generated. Answers will impact the theories on life origin related to serpentinization of ultramafic rocks, and the origin of methane on rocky planets. Here we document, through molecular and isotopic analyses of gas liberated by rock crushing, that among the several mafic and ultramafic rocks composing classic ophiolites in Greece, i.e., serpentinite, peridotite, chromitite, gabbro, rodingite and basalt, only chromitites, characterized by high concentrations of chromium and ruthenium, host considerable amounts of 13C-enriched methane, hydrogen and heavier hydrocarbons with inverse isotopic trend, which is typical of abiotic gas origin. Raman analyses are consistent with methane being occluded in widespread microfractures and porous serpentine- or chlorite-filled veins. Chromium and ruthenium may be key metal catalysts for methane production via Sabatier reaction. Chromitites may represent source rocks of abiotic methane on Earth and, potentially, on Mars.
Highlights
Over the last twenty years, a long series of discoveries revealed considerable amounts of methane (CH4) manifesting in surface fluids or aquifers in continental, serpentinized ultramafic rocks, within ophiolites or peridotite massifs[1,2,3,4]
We have documented that among the several mafic and ultramafic rocks composing classic ophiolites in Greece, i.e., serpentinite, peridotite, chromitite, gabbro, rodingite and basalt, only chromitites host considerable amounts of 13C-enriched methane, hydrogen and heavier hydrocarbons with inverse isotopic trend, which is typical of abiotic gas origin
Ruthenium is the only known catalysts capable of producing CH4 at temperatures below 150 °C9, which are the CH4 generation temperatures in ultramafic rock settings on land, as suggested by geological models and geothermometry based on isotopologue analyses[10]
Summary
Over the last twenty years, a long series of discoveries revealed considerable amounts of methane (CH4) manifesting in surface fluids or aquifers (seeps, hyperalkaline springs, boreholes) in continental, serpentinized ultramafic rocks, within ophiolites or peridotite massifs[1,2,3,4]. These gas occurrences are more common than assumed in the past and are today documented in at least 17 countries[4]. The study includes analysis of gas liberated by rock crushing (62 samples from four ophiolite complexes), confocal high-resolution Raman mapping, microscopic and petrographic observations, and analysis of chromium and ruthenium, which may represent potential catalysts for abiotic methane production
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