A petro-physical model for serpentinised mantle and origin of natural hydrogen in the Pyrenees

The relationships between the serpentinised continental mantle in orogens, its geophysical signature at depth and hydrogen seepages are poorly understood. A petro-physical modelling approach accounting for serpentinisation shows that a large domain of serpentinised mantle is present in the northern Pyrenees. The serpentinisation reached a maximum of 40% during the mid-Cretaceous rifting, according to the predicted temperature and pressure. Although high-temperature serpentinisation could have generated large quantify of hydrogen during the Mesozoic, the shallow and inactive faulting in Northern Pyrenees make this process unlikely to explain the entire serpentinisation inferred by petro-physical modelling. A combination of low-temperature alteration of mafic and ultramafic rocks in the North Pyrenean Zone, active normal faulting in the North Pyrenean Fault, accumulation in local traps and transport of H₂-rich fluids along inactive but permeable fault may explain the hydrogen seepages observed today.