Helium exploration in European rift systems: Regional framework and new data from the Upper Rhine Graben

Helium (He) is a non-renewable natural resource critical for medical imaging, electronics, and scientific research, yet its availability is highly dependent on hydrocarbon production. In the context of the energy transition, alternative low-carbon sources like U-Th-rich crystalline basement rocks are needed. He can migrate along deep fault networks, potentially accumulating in hydrocarbon-free sedimentary reservoirs alongside nitrogen. However, such systems are poorly documented in Europe. Here, we present a continent-scale compilation of He data from European thermal and geothermal fluids, and provide new dissolved and free gas He data from 25 thermal water sites sampled across the Upper Rhine Graben (URG). Our measurements reveal He concentrations of up to 2.4 vol% in free gases and flow rates for dissolved He reaching 4 × 10 4 mol.yr −1 , highlighting active crustal He generation and migration. He levels exhibit a significant north-south gradient with elevated concentrations near basement-rooted border faults and intersecting Variscan transverse fault systems. This pattern reflects efficient upward transport under elevated geothermal gradients. To the south, mantle CO₂ contributions increase, diluting the crustal He signal, particularly near the Kaiserstuhl volcanic complex. Dissolved hydrogen (H 2 ) concentrations remain negligible, indicating decoupling between He and H 2 during migration. The URG is among the highest He-degassing provinces in Europe. Although active degassing prevents near-surface accumulation, identifying deep-seated fault systems as primary He conduits is crucial to guide exploration in adjacent basins. This framework shows that combining surface He data with geological and thermal analyses can effectively identify high-potential radiogenic He systems in Europe and analogous intracontinental rifts worldwide.