Linking serpentinization, hyperalkaline mineral waters and abiotic methane production in continental peridotites: an integrated hydrogeological-bio-geochemical model from the Cabeço de Vide CH4-rich aquifer (Portugal)

15 páginas.--10 figuras.-- 2 tablas.-- 87 referencias.-- Supplementary data related to this article can be found at https://doi.org/10.1016/j.apgeochem.2018.07.011.Continental active serpentinization of ultramafic rocks is today recognized as a key process triggering a sequence of phenomena involving the passage from inorganic, to organic and metabolic reactions. These may have a role in the origin of life, and may explain the occurrence of abiotic hydrocarbons on Earth and other planets. Production of hyperalkaline waters and abiotic methane (CH4) are two critical steps in this sequence. They were described independently by specific hydrogeological and geochemical models. Here, we update and combine these models into a unified scheme using and integrating geological, hydrogeological, hydrogeochemical, gas-geochemical and microbial analyses acquired from 2002 to 2014 in the Cabeço de Vide (CdV) study site, Portugal. The hyperalkaline (pH > 10.5), Na-Cl/Ca-OH mineral water of CdV evolve from groundwater-peridotite interaction (serpentinization) generating hydrogen (H2), which, according to multiple theoretical, laboratory and field evidence, likely reacted with CO2 within metal- (catalyst) rich rocks, abiotically producing CH4 (up to 1.2 mg/L; -24.4°/oo < δ13C-CH4 < -14.0°/oo and -285°/oo < δ2H-CH4 < -218°/oo). The hyperalkaline water hosts hydrogen oxidizing bacteria “Serpentinomonas” which may explain the paucity of H2 observed in the dissolved gas. The CdV gas-rich mineral waters ascend along a fault at the boundary of the peridotite intrusion. Temporal changes of pH and CH4 concentration result from episodic mixing with shallower Mg-HCO3-type waters. Soil-gas analyses show that methane migrates to the surface along the fault, also independently from the water emergences, consistently with non-aqueous abiotic CH4 production. Our integrated model is generally compatible with observations from other gas-bearing continental serpentinization sites.This study was supported by FCT/MEC (PIDDAC) funds under the scope of the Project PTDC/AAG-MAA/2891/2012, CERENA/IST acknowledges the FCT (the Portuguese Science and Technology Foundation) support through the UID/ECI/04028/2013 Project and C2TN/IST author gratefully acknowledge the FCT support through the UID/Multi/04349/2013. The study on gas origin and emission, performed by INGV, was supported by the Deep Carbon Observatory – Deep Energy community. Work performed at the Jet Propulsion Laboratory, California Institute of Technology, was done under contract with NASA and funded by the NASA Astrobiology Institute (grants 08- NAI5-0021 and 13-13NAI7_2-0024). The authors would like to thank Benjamin Tutolo and an anonymous reviewer for their comments and suggestions in order to improve a previous draft of the manuscript. The authors also would like to thank José Teixeira and Helder Chaminé for redrawing Figs. 1 and 12. The authors also would like to thank L. Rocha, Technical Director of Cabeço de Vide thermal spa, for his support during field work campaigns. Special thanks to the Junta de Freguesia of Cabeço de Vide for all the logistical support during this study. No competing financial interests existPeer reviewe