Helium isotope signatures of geothermal fluids across rift-related structures in the Himalaya

Geothermal systems in the Himalayan–Tibetan collision belt reflect recent deformation and associated hot-fluid circulation driven by the ongoing Indian–Eurasian plate collision. The geochemical behavior of these geothermal fluids, particularly helium-isotope signatures (R/Ra), records variable source contributions ranging from crustal radiogenic helium (⁴He) to mantle-derived helium (³He), depending on spatial position relative to the hot Tibetan mantle. The influence of the hot Tibetan mantle is well constrained within rift systems of the Lhasa terrane, varying from mantle-dominated signatures in the north to increasingly crustal-dominated signatures toward the south, thereby constraining the spatial position of Tibetan mantle in the region. However, whether this Tibetan mantle influence propagates farther south via younger rift-related normal faults—associated with Late Miocene dome formation processes and via normal faults near the Main Central Thrust (MCT)—remains poorly constrained. Here, we present helium-isotope measurements from six hot springs, including three from the western Leopargil rift system along the Kaurik–Chango Fault in the Spiti River valley and three from sites near Karcham normal fault, which cross-cuts the Main Central Thrust (MCT) in the Sutlej River corridor of the northwestern Himalaya, to evaluate whether younger rifts permit southward transfer of Tibetan mantle-derived fluids. Measured ³He/⁴He ratios (air corrected R/Ra) range from ~0.02 to ~0.07, indicating dominantly crustal radiogenic helium. These results indicate no resolvable southward influence of the Tibetan mantle across these faults and are more consistent with a collisional geometry involving steep Indian lithosphere  subduction rather than with an intra-crustal Asian mantle configuration.