Controls on natural hydrogen generation during serpentinization of mantle rocks

Article https://doi.org/10.1038/s41467-026-73920-5 Controls on natural hydrogen generation during serpentinization of mantle rocks Received: 13 August 2025 Accepted: 23 May 2026 1234567890():,; 1234567890():,; Check for updates Rodolfo Christiansen 1,9 , Mohamed Sobh1, Christian Ostertag-Henning Guido Gianni 3, Nicolas Saspiturry4, Sebastien Chevrot5, Victoria Langenheim 6, Javier García-Pintado7 & Gerald Gabriel 1,8 2 , Mantle rocks undergoing serpentinization can generate significant amounts of natural hydrogen, yet the rates and controlling processes remain poorly understood. Here, we constrain the possible hydrogen generation rates in two distinct mantle rock types, the fertile lherzolites of the Western Pyrenees and the depleted harzburgites of Northern California, to relatively low rates of ~0.1 to ~0.5 tonnes H₂ yr⁻¹ km⁻³ of reactive rock. When integrated over the full reactive volumes, this corresponds to total production rates of ~300 to ~600 tonnes H₂ yr⁻¹. By combining three-dimensional geophysical inversion with numerical modelling of fluid-rock processes, we show that hydrogen generation rates are mainly limited by H₂ saturation in the fluid and reaction kinetics. Under these constraints, hydrogen generation in mantle-derived serpentinization systems proceeds slowly, making rapid large-scale replenishment unlikely and suggesting that large, economically relevant accumulations, would require timescales of thousands to tens of thousands of years to develop. Serpentinization is an abiotic reaction involving the hydration of ultramafic rocks, in which Fe(II) in primary minerals is oxidised to Fe(III) in secondary phases, reducing hydrogen atoms in water molecules to molecular hydrogen (H2). This process is one of the primary natural mechanisms for hydrogen generation1–5. The generation rate of molecular hydrogen (hereafter referred to as hydrogen or H2) and the rate of serpentinization are highly dependent on temperature, with maximum rates between 200 °C and 300 °C2,6. This mineral replacement reaction also affects the petrophysical properties of the rocks by decreasing density and seismic velocities, associated most often with increasing magnetic susceptibility. These contrasts allow serpentinites to be distinguished from fresh mantle rocks based on their geophysical characteristics, such as gravity, magnetic, and seismic properties7–9. In recent years, natural hydrogen has emerged as a potential lowcarbon energy resource10. However, growing evidence suggests that hydrogen generation associated with mantle rocks serpentinization proceeds over long timescales, with limited generation rates, even in the most favourable conditions, such as serpentinization of deeply buried olivine-rich rocks11,12. Major obstacles to quantifying natural hydrogen generation include the inaccessibility of deep crustal environments, a limited understanding of water availability, and uncertainties in reaction kinetics under low water-to-rock ratios (W/R), defined as the fluid-to-rock mass ratio. Serpentinization is nevertheless associated with some of the largest known natural hydrogen occurrences worldwide, with high hydrogen concentrations measured in emanating gases and associated flow-rate estimates of several hundred tonnes per year13,14. In contrast, model-based estimates remain highly uncertain. For example, in the Western Pyrenees, reported values range from several hundred thousand15 to several million tonnes of hydrogen produced per year16. These high estimates largely reflect 1 LIAG Institute for Applied Geophysics, Hannover, Germany. 2Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany. 3Institute of Geophysics, Czech Academy of Sciences, Prague, Czechia. 4Géosciences Montpellier, Université de Montpellier, CNRS, Montpellier, France. 5Géosciences Environnement Toulouse (GET), UMR 5563, Observatoire Midi-Pyrénées, CNRS, Université de Toulouse, Toulouse, France. 6United States Geological Survey (USGS), Moffett Field, California, USA. 7MARUM - Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany. 8Institute of Earth e-mail: System Sciences, Leibniz University Hannover, Hannover, Germany. 9Mantle8, Sassenage, France. Nature Communications | (2026)17:5211 1 Article approaches that quantify cumulative hydrogen generation potential, often assuming efficient and sustained fluid-rock interaction and neglecting explicit constraints related to fluid flows, reaction kinetics, and hydrogen saturation in the fluids. As a result, such values are not directly comparable to observed hydrogen flow rates in similar geological settings. Here, we present a methodological framework for quantifying natural H₂ generation during serpentinization that explicitly constrains geological parameters within well-defined petro-physicochemical boundary conditions. Unlike previous approaches that estimate cumulative generation potential from rock volumes alone, this framework combines fluid-rock thermodynamics with explicit kinetic and saturation limits, representing a fundamental shift from potentialbased to process-resolved hydrogen generation estimates. Hydrogen generation is evaluated using a sequential, quasi-dynamic modelling framework in which thermodynamic equilibrium is assumed locally for each fluid-rock interaction event, and sustained production rates emerge as the serpentinization front advances under explicit kinetic limitations. Fluid transport is parameterised through fracturecontrolled flow and diffusive exchange, while feedbacks from fluid saturation and lithology-dependent alteration are explicitly resolved, together with rock-specific petrophysical properties including mineral-bound water uptake, pressure-dependent hydrogen solubility, and porosity-controlled reaction-front propagation. We apply this model to two different geological areas, the Western Pyrenees and Northern California, selected for their good data availability and contrasting mantle rock compositions. The first case involves relatively fertile mantle rocks rich in olivine and orthopyroxene17,18, whereas the second is dominated by less reactive rocks that represent residues of high degrees of partial melting19. By integrating geological, geophysical, and physico-chemical constraints, we provide first-order estimates of hydrogen production rates and show that, at large scales, mantlederived natural hydrogen behaves as a slowly replenished geological resource. Results Western Pyrenees The Western Pyrenees (Fig. 1a, b) consist of a tectonic wedge, where mantle-derived rocks, mainly spinel lherzolites17,18, were exhumed during middle Cretaceous rifting20,21, and tectonically uplifted to their current shallow position (~ 10 km) as a result of Eocene-Oligocene convergence, which ended about 25 Ma ago22–25. Since then, tectonic conditions have remained relatively stable, which provides a prolonged window that kept the exhumed mantle rocks within optimum pressure-temperature (...truncated)