Hydrogen generation potential of peridotites and pyroxenites in ophiolitic settings.

The partially serpentinized mantle bodies exposed in the western sector of the Northern Apennines, Italy, represent a favorable environment for natural hydrogen (H2) studies, particularly Mt. Prinzera, which hosts a hyperalkaline spring containing dissolved H2. A geochemical characterization of peridotite, pyroxenite, and serpentinite samples was carried out to reconstruct the past and present stages of serpentinization and to assess their H2 generation potential. This study combined mineralogical, whole-rock compositional, and redox-state analyses, along with thermodynamic modeling of spring waters. The major serpentinization event associated with mantle exhumation in the Middle to Late Jurassic was marked by extensive replacement of olivine, clinopyroxene, orthopyroxene by serpentine and subordinate magnetite. Alteration was more intense in olivine than in pyroxenes, resulting in higher serpentinization degrees in peridotites (∼ 50 vol%), compared to pyroxenites (∼ 10 vol%). Mineralogical evidence suggests temperatures of approximately 220 °C and 300 °C for this stage. Currently, serpentinization and H2 generation are likely active at Mt. Prinzera, with fluid-rock interactions involving particularly the oxidation of a small fraction of Fe2+-serpentine and/or magnetite, rather than the primary mineral assemblage. Thermodynamic equilibria, based on the activities obtained from the aqueous speciation, and the isotope composition of the hyperalkaline water were used to estimate the temperature at depth, which appears to be markedly low. Thus, serpentinization may occur either at the base of Mt. Prinzera or within a deeper, hidden ultramafic body. In addition, trapped H₂ accumulations originating from earlier serpentinization stages can also be considered.