Geological and geochemical approach to natural hydrogen exploration in the Northern Apennines, Italy.

The western sector of Northern Apennines, Italy, presents favorable conditions for the occurrence of natural hydrogen (H2), hosting ophiolitic bodies, hyperalkaline waters, and deep-seated faults. A geological and geochemical investigation was conducted to evaluate the potential for subsurface H2 accumulations. The study involved gas analyses from spring waters, bubbling gas, and soil-gas measurements, along with petrographic analyses of ultramafic rocks. Multiple springs contain dissolved H2 at low (up to 1 μM) to moderate (1 μM - 100 μM) concentrations relative to other springs in serpentinized peridotites worldwide. In the Taro Valley, H2 occurrences (0.28 μM - 0.79 μM) are associated with hyperalkaline springs in proximity to exposed peridotites. However, the limited thickness of the peridotite body at Mt. Prinzera (∼250 m) suggests that the hyperalkaline water and associated H2 likely derive from deeper unexposed ultramafic units. Petrographic analyses of spinel-peridotites reveal varying degrees of serpentinization (45% to 95%), characterized by serpentine mesh textures with olivine relics, pyroxene converted into serpentine, and formation of magnetite and chromite. In the Bobbio Tectonic Window, springs with neutral pH waters, located away from exposed ultramafic bodies, contain higher concentrations of dissolved H2 (0.49 μM - 3.8 μM). Although the origin of this H2 remains unclear, it may be related to hidden ultramafic bodies within the sedimentary sequence undergoing serpentinization. Notably, all the spring-related H2 occurrences are associated with methane (CH4), showing thermogenic isotopic signatures (δ13C: -58.3‰ to -35‰ and δ2H: -200‰ to -145‰). Further research should focus on the characterization of regional hydrocarbon reservoirs, which could also host natural H2.