Hydrogen generation and heterogeneity of the serpentinization process at all scales: Turon de Técouère lherzolite case study, Pyrenees (France)

Hydrogen (H 2 ) emanations have been recognized in the south and north of the Pyrenees fold belt, within its two forelands. The proposed source is a mantle wedge quite close to the surface that is currently undergoing serpentinization. The migration pathway seems to be the deep rooting faults, as the H 2 content is higher where the faults reach the surface. The zone of current H 2 generation is around 10 km deep. It is evident from filed observations that kilometric pieces of mantle have been incorporated into the thrusts and outcrop in a few areas along the mountain belt. We studied the Turon de Tecouère, one of these mantle-derived bodies, using various field and laboratory tools that focused on the characterization of its alteration, the degree of serpentinization and its heterogeneity at the kilometre scale. Accordingly, the magnetic field and magnetic susceptibility were mapped, classical optical observations and 3D scans of some samples were performed, and H 2 soil gas content mapping was carried out. The results show a heterogenous degree of serpentinization ranging from 3 to 62% at the kilometre to the micrometre scale. As the temperature and burial history are the same throughout the Turon de Tecouère, these factors were not sufficient to characterize the level of transformation in the H 2 -generating rock. The soil gas measurements show current H 2 emanations in and around the Turon de Tecouère. Near-surface H 2 production of this mantle body is unlikely, based on the current knowledge of H 2 generation kinetics. To explain these emanations, we favour a preferential migration pathway within the root of the Turon and the surrounding faults. Thematic collection: This article is part of the Hydrogen as a future energy source collection available at: https://www.lyellcollection.org/topic/collections/hydrogen