Arc magma formation through the fluid-fluxed mélange melting in subduction zones

The transport of subducted slab materials to the overlying mantle plays a crucial role in arc magma formation. However, the contributions of aqueous fluids, hydrous melts, or mélange to the convective mantle remain controversial due to the lack of definitive fingerprints for these materials. Here, we report the Ba isotope composition of lavas from a typical cold-slab arc, the Izu arc, to resolve the recycled materials in the source of arc magmas. δ^138/134Ba of the arc lavas show an across-arc decrease corresponding to increasing depth and positively correlates with both ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd. Across-arc variations of both δ^138/134Ba and Ba/Th ratios support that high Ba/Th in the Izu and other arc magmas originated from slab-derived fluids rather than melts. Quantitative mixing modeling, involving contributions from mélange followed by slab-derived fluids in successive stages, provides a coherent explanation for Ba-Sr-Nd isotopic signatures observed in lavas from cold arcs. Our work reveals the combined role of slab-derived fluids and mélange diapir melting in arc magma formation.