Tin isotope fractionation in arc magmas controlled by degassing and slab input

Arc magmatism plays a critical role in continental crustal growth and the formation of significant metal deposits, including granite-related tin (Sn) systems. However, the mechanisms governing Sn transport and isotopic fractionation at convergent margins remain poorly constrained due to a lack of systematic studies across spatial variations (arc-front to rear-arc) and magmatic-hydrothermal transitions. In this study, we present high-precision Sn isotopic data for lavas, pumices, and hydrothermal products from Whakaari (arc-front) and Taranaki (rear-arc) in the Kermadec system, alongside magmatic H2O concentrations estimated from clinopyroxene. Whakaari lavas exhibit significant variation (δ122/118Sn = –0.241‰ to 0.361‰). The heaviest values are attributed to extensive shallow degassing (>40%), with Rayleigh modeling indicating the preferential partitioning of light Sn isotopes into the vapor phase—a process corroborated by low magmatic water contents (avg. 0.83 wt.%). In contrast, Taranaki samples show limited variation (δ122/118Sn = 0.124 to 0.235‰). While amphibole and titanomagnetite fractionation may lower bulk-rock values, these processes cannot explain why both volcanoes are isotopically lighter than MORB (0.367 ± 0.087‰).We propose that this light Sn signature originates from the subducted slab. Simulations suggest that the addition of 5–20% reduced, Cl-rich fluids derived from altered oceanic crust (AOC) can effectively lower arc magma δ122/118Sn. Regardless of the specific redox mechanism, slab-derived fluids dominate the Sn budget of the mantle wedge and the resulting arc magmas. Our results suggest that widespread light Sn isotope signatures serve as a diagnostic feature of fluid-mediated mass transfer in subduction zones. By combining spatial variations from arc-front to rear-arc, this study provides a robust geochemical framework to decipher slab-mantle interactions and the dynamic cycling of metals at convergent margins.