Boron, lithium and methane isotope composition of hyperalkaline waters (Northern Apennines, Italy): terrestrial serpentinization or mixing with brine?

Abstract

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.