Supra-subduction zone ophiolites retain hydrogen generation potential after 300 million years

The Ural Mountains host some of the earliest documented natural hydrogen (H₂) emissions, yet the Paleozoic ultramafic complexes are 250–400 Ma old and would normally be expected to have exhausted their serpentinization potential. We revisit overlooked Soviet observations from the Kempirsay chromitite district and compare them with recent data from the younger, Jurassic Bulqizë ophiolite in Albania. In both massifs, H₂-rich seeps (>80–90 vol% H₂) occur within ~300 m of podiform chromitite bodies. At Kempirsay, degassing takes place at low temperatures (14–30 °C), and experiments on Kempirsay rocks show that Fe-bearing minerals can generate H₂ at near-ambient conditions. New radiocarbon data from Bulqizë methane (3.76 ± 0.06 pMC, apparent age ~26 kyr) demonstrate that associated CH₄–H₂ inventories are renewed on 10⁴-year timescales. We interpret chromitite bodies and their damage zones as catalytic and hydraulic hubs embedded in a supra-subduction-zone ophiolitic architecture that localizes serpentinization and preserves reactive peridotite. This chromitite-centered architecture implies that chromitite-bearing mantle slabs can retain hydrogen generation potential for hundreds of millions of years and constitute priority targets for natural hydrogen exploration and stimulated geological hydrogen production.