Fe-Zn-Cu isotopes reveal spatially variable dual metasomatism in the eastern Neo-Tethyan subduction system

The Yarlung Zangbo ophiolites (YZO), exposed along the southern Tibetan Plateau, represent remnants of the Neo-Tethyan oceanic lithospheric mantle, variably serpentinized during subduction and continental collision. These forearc-type peridotites provide a valuable archive of fluid−rock interactions along the subduction interface during the initiation and evolution of the India−Eurasia convergence. Moreover, such fluid−rock interaction processes may have contributed to the formation of potential natural hydrogen resources. We present an integrated geochemical dataset—including major and trace elements and Fe, Zn, and Cu stable isotopes—for 34 serpentinites and peridotites sampled across the YZO to characterize melt- and fluid-mediated modification processes in mantle rocks. The serpentinites display spoon-shaped rare earth element (REE) patterns, with flat heavy REE, depleted middle REE, and enriched light REE, along with fluid-mobile element (FME) ratios (e.g., low U/Cs, Li/Cs, Rb/Cs and high Li/U, Cs/Th, Rb/U) characteristic of subduction forearc environments. Zn and Cu isotope compositions (δ66Zn = +0.11‰ to +0.39‰; δ65Cu = −0.24‰ to +0.26‰) correlate with Zn depletion, consistent with the loss of isotopically light Zn and heavy Cu via oxidizing slab-derived fluids during serpentinization. In contrast, Fe isotopes (δ56Fe = −0.05‰ to +0.35‰) show systematically heavy values, decoupled from FME proxies. These signatures are inconsistent with metasomatism by fluids alone and instead point to an earlier refertilization stage by sediment-derived melts from the subducting Neo-Tethyan slab, imparting an isotopically heavy signature before serpentinization. Critically, the geochemical and isotopic compositions evolve along-strike: Western and central YZO samples show more mantle-like signatures, whereas eastern segments exhibit stronger metasomatic overprints and more fractionated isotopic values. This east−west gradient likely reflects changes in fluid sources, mantle protoliths, and redox conditions, linked to spatial and temporal variations in subduction zone dynamics. These findings underscore the potential of forearc serpentinites to record complex metasomatic histories and along-strike heterogeneities in subduction-related geochemical cycling.