Aqueous fluids interaction with ultramafic rocks at low temperatures (<400°C) leads to serpentinization. This hydration reaction is coupled to the oxidation of ferrous iron contained in primary minerals to ferric iron in secondary minerals (magnetite and/or serpentine), and to the release of hydrogen. Serpentinization is a widely studied process in mid-ocean ridges and in subduction contexts but recent studies also report the production of H2 in continental areas via serpentinization reactions. We used an integrated petrological, mineralogical and geochemical approach on 32 variably serpentinized peridotites from three orogenic massifs located in the North-Western Pyrenees, Turon, Montaut and Urdach to characterize their sequence of serpentinization. This study is coupled to a bulk and µ-XANES spectroscopy at the Fe K-edge, to derive the oxidation state of iron and to determine the hydrogen budget for each massif. 
The three massifs expose subcontinental mantle having contrasted serpentine contents (14, 60, and 100 wt.%, respectively) associated to contrasted bulk rock Fe³⁺/ΣFe ratios (0.18, 0.73 and 0.63 respectively). It suggests distinct hydrothermal alteration paths. We show that the pre-serpentinized subcontinental mantle was exposed to heterogenous fluid metasomatism (Montaut) and melt refertilization reactions (Turon), leading to minor compositional heterogeneities between the protoliths. We show the massifs have variable magnetite and Fe³⁺-serpentine contents, decreasing with increasing iron contents in serpentine minerals.