Relevant seepage of natural hydrogen in the Ronda Peridotite Massif (Spain)

Serpentinization of peridotites is a major source of natural hydrogen (H 2 ), a prospective energy resource and presently a focus of geological exploration worldwide. The Ronda Peridotite Massif (Southern Spain) is one of the world's most extensive peridotite outcrops and hosts numerous gas-bearing hyperalkaline springs and seeps related to serpentinization. However, H 2 has been observed only in a few springs at variable, generally low concentrations. Here, we report a new observation of a seep (Vega Escondida) with a large H 2 concentration (∼46 vol%), associated with nitrogen, methane, and heavier hydrocarbons; this represents the highest H 2 concentration ever documented on the land surface in Western Europe. In addition to episodic free-gas (bubbling) manifestations, the spring water associated with the seep contains ∼540 μM of dissolved H 2 and transports to the surface ∼500 L H 2 day -1 . Lower H 2 amounts have been observed in two other springs. The high H 2 seep is located at the intersection of two faults separating peridotite and sedimentary flysch rocks, and acting as a preferential pathway for H 2 migration from serpentinization fronts that, based on available gravimetric data, should be shallower than 8 km, where temperatures are estimated to not exceed 200 °C. In these conditions, H 2 generation rates by serpentinization are not high enough to directly and continuously sustain the relevant H 2 flow at the surface. Higher H 2 production may have occurred, thanks to higher temperatures, during and after the Miocene peridotite emplacement (around 20 My ago). A comparison with previous investigations reveals that the H 2 concentration changes considerably over time at all investigated sites, whereas the bulk and clumped isotope composition of methane associated with H 2 is substantially invariable. All data converge on the hypothesis that the Vega Escondida gas stems from a reservoir within the peridotite massif or the adjacent flysch, where H 2 (and methane) accumulated over geological time, and that variable interactions with hyperalkaline aquifers, microbial consumption, and seepage intensity modulate the H 2 flow to the surface.