Abiotic Methane in the Hyperalkaline Springs of Genova, Italy

Serpentinization of ultramafic rocks is considered a major process of production of abiotic methane (CH4) and hydrogen (H2) on Earth, and it may be responsible for CH4 occurrence on other planets. While serpentinization and CH4 synthesis have been widely studied and modeled in high temperature hydrothermal conditions, such as on submarine mid-ocean ridges, the increasing number of discoveries of abiotic CH4 in ophiolites on continents shows the importance of present-day (meteoric water driven) serpentinization in low temperature (<100 °C) gas synthesis. As a new case, we report compositional, isotopic, and flux data of gas dissolved in hyperalkaline Ca-OH waters issuing from serpentinized peridotites at Genova (Italy). CH4 is dominantly abiotic (δ13C: -9 ‰ VPDB; δ2H: -168 to - 225 ‰ VSMOW), similar to that released by ophiolites in Oman, Turkey, the Philippines, and by the submarine Lost City serpentinization system. While the absence of CO2 was expected in this kind of fluids, the absence of H2 is unusual. This could be due to hydration of olivine and pyroxene by CO2-rich fluids, eventually associated with high silica activity, which inhibits H2 formation and produces CH4 directly. Thermodynamic modeling and H2O-CH4 isotope equilibrium confirm the low temperatures (<60 °C) of the serpentinization system, and thus the abiotic methane synthesis.