Water chemistry and abiogenic methane content of a hyperalkaline spring related to serpentinization in the Argolida ophiolite (Ermioni, Greece)

Hyperalkaline mineral springs related to active continental serpentinization are a theme of growing interest since they may contain significant amounts of abiotic gas and have important implications for energy resource exploration, subsurface microbiology and astrobiology. We report the discovery of a new hyperalkaline (pH similar to 12) spring issuing in the Agioi Anargyroi monastery at Ermioni (Greece), connected to serpentinization of peridotites of the Argolis ophiolite. Two water samples have been collected from separated springs and analysed for the chemical composition of major, minor and trace elements, and isotopic composition (H-2 and O-18) of water by IC, ICP-OES, ICP-MS and IRMS, and for the chemical (H-2, O-2, N-2, CH4, CO2 and C2H6) and isotopic (He, delta H-2-CH4 and delta C-13-CH4) composition of dissolved gases. The Iliokastron M lange Unit, comprising abundant serpentinized harzburgite, represent the aquifer feeding the hyperalkaline springs. The isotopic composition of water indicates a recent meteoric recharge probably through the close by and stratigraphically higher limestones of the Faniskos Unit. The Ca-OH water composition resulted to be similar to other hyperalkaline waters of Greece and worldwide. Although the concentrations of dissolved H-2 are very low (tens of mu mol/L) compared to other gases collected in similar manifestations, the concentrations of CH4 are considerable (38-314 mu mol/L) and display isotopic compositions indicating a substantial if not exclusive abiogenic origin. Methane oxidation is also hypothesized in one of the two springs. (C) 2017 Elsevier Ltd. All rights reserved.