Toscani, L.

Spring waters issuing from serpentinized ultramafic rocks of the Taro-Ceno Valleys (Northern Apennine, Emilia-Romagna region, Italy) were analyzed for major element, trace element and dissolved gas concentrations and d11B, d7Li, d18O(H2O), d2H(H2O), d13C(CH4) and d2H(CH4) isotope compositions. Similar to other springs worldwide that issue from serpentinites, the chemical composition of the waters evolves with water–rock interaction from Ca-HCO3, through Mg-HCO3 and ultimately to a hyperalkaline Na- (Ca)-OH composition. Most of the Ca- and Mg-HCO3 springs have d11B ranging between +16.3‰ and +23.7‰, consistent with the range of low P–T serpentinites. Very high d11B in two springs from Mt. Prinzera (PR10: +39‰; PR01: +43‰) can be related to isotopic fractionation during secondary phase precipitation, as also inferred from d7Li values. In contrast to typical abiogenic isotope signatures of CH4 from serpentinized rocks, dissolved CH4 from the Taro-Ceno hyperalkaline springs has an apparent biotic (thermogenic and/or mixed thermogenic-microbial) signature with d13C(CH4) ranging from 57.5‰ to 40.8‰, which is similar to that of hydrocarbons from production wells and natural seeps in adjacent hydrocarbon systems. The data suggest that CH4 in the hyperalkaline springs investigated in this study may derive from organic matter of the sedimentary (flysch and arenaceous) formations underlying the ophiolite unit. However, small amounts of H2 were detected in one hyperalkaline spring (PR10), but for two springs with very low CH4 concentrations (PR01 and UM15) the d2H value could not be measured, so the occurrence of some abiotic CH4 cannot be excluded. The occurrence of thermogenic CH4 in ophiolites may be a widespread phenomenon, and thus the characterization of serpentinization-related gases requires accurate evaluation of the regional context including a careful knowledge of the relationships with surrounding sedimentary rocks and their hydrocarbon potential.

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.