Tin isotope systematics in subduction zones

Abstract

The origin of slab components in arc lavas remains controversial with proposed sources including direct sediment melts, sediment-mantle mélanges, or crustal fluids. Stable isotope distributions of fluid-borne metals and those immobile in subduction components have the potential to trace the respective contribution of these components if placed into the context of additional magmatic differentiation and in conjunction with radiogenic isotopes. Here, we present the first high-precision double-spike tin abundances and isotope compositions in twenty-six arc rocks sampled along the Indonesian Sunda arc, a subduction zone with the highest subducted sediment volume globally. δ^122/118Sn_3161a in these samples ranges from 0.13‰ to 0.46‰ and we find that Sn isotope compositions in Sunda arc rocks were affected by magmatic differentiation, superimposed on additional influences from components derived from the subducting slab. A step change in δ^122/118Sn_3161a at 4–5wt.% MgO suggests the removal of heavier Sn isotopes, likely due to higher compatibility of isotopically heavier Sn⁴⁺ in Fe-Ti oxides along the liquid line of descent. This change of ∼0.1‰, however, is dwarfed by Sn isotope variations consistent with sediment melt contribution coupled with fluid addition from altered oceanic crust, both of which introduce lighter Sn isotopes. Sediment melt contamination increases with depth (temperature), but some exceptions at shallow sub-arc sources (depth ≤ 130 km) indicate additional melting of a mélange-style source. Our findings suggest the coexistence of mélange contributions, and deep fluid-induced melting plus partial sediment melts in the Sunda arc. Mélanges contribute only at shallow depths (≤130 km), consistent with their diapiric nature, whereas deeper levels favor fluid-fluxed melting and, at higher temperatures and pressures, partial sediment melting. This study provides insights into the complex interactions between fluids, sediments, and mantle wedge in subduction zones, highlighting the depth-dependent nature of slab contributions to arc magmatism. It also demonstrates the utility of Sn isotopes for tracing these processes and advancing our understanding of subduction zone geochemistry.