Short communicationMethane in serpentinized ultramafic rocks in mainland Portugal

We report a new case of methane (CH4) of apparent abiotic origin in continental serpentinized ultramafic rocks. Multiple analytical techniques, on-site and in the laboratory, revealed methane and ethane degassing from hyperalkaline (pH > 11) Ca2+-OH− mineral waters in boreholes drilled in the Alter-do-Chão igneous intrusion, at Cabeço de Vide, in mainland Portugal. The C and H isotopic composition of CH4 (δ13C ∼ −20‰; δ2H: −283‰) suggests a dominant abiotic origin, although minor thermogenic contributions cannot be excluded. Similarly, low methane-to-ethane ratios suggest a predominantly non-microbial source, consistent with previous microbiological data showing the lack of methanogenic archaea in these waters. Heavier hydrocarbons, CO2 and H2 are below detection limits. This case study confirms that CH4 from serpentinized ultramafic rocks can be transported by hyperalkaline fluids linked to deep circulation of meteoric waters. Maximum depth of Cabeço de Vide serpentinized rocks is less than 1 km, and present temperatures are likely lower than 50 °C. Serpentinization and related gas formation may have occurred at any time during thermal evolution of the igneous intrusion, so gas formation temperature cannot be easily determined. This case is an opportunity to test thermometry provided by CH4 isotopologue analyses. The existence of methane in continental serpentinized igneous rocks is more widespread than previously thought and petroleum systems with similar serpentinized ultramafics in reservoir rocks may have traces of the observed 13C-enriched CH4.