A Petro-Physical Model for Serpentinized Mantle and Origin of Natural Hydrogen in the Pyrenees

The relationships between the serpentinized continental mantle in orogens, its geophysical signature at depth and hydrogen seepages are poorly understood. A petro-physical modeling approach accounting for serpentinization shows that a large domain of serpentinized mantle (1,800 km²) is present in the northern Pyrenees. The serpentinization reached a maximum of 40% during the mid-Cretaceous rifting, according to the predicted temperature and pressure. Although high-temperature serpentinization 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 serpentinization inferred by petro-physical modeling. 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.