Isotopic constraints on the Barium subduction cycle

• Ba in arc lavas is mainly sourced from sediment fluids/melts, not oceanic crust. • Sedimentary Ba has distinct authigenic, lithogenic, and hydrothermal sources. • Barite dissolution during early subduction releases Ba into the forearc mantle. • High δ 138/134 Ba and Ba/Th in Tongan lavas result from forearc mantle recycling. • Crustal δ 138/134 Ba in global arcs, MORBs and OIBs indicates lithogenic Ba input. Subduction processes regulate elemental cycling and consequently the composition of Earth’s geochemical reservoirs - mantle, crust, atmosphere, and hydrosphere, exerting far-reaching effects for the evolution of life. Barium (Ba) is essential for understanding crust-mantle recycling, especially since over 90 % of Ba in arc lavas originates from subducted materials. While Ba enrichments in arc lavas have long been attributed to contributions from oceanic crust derived aqueous fluids, recent studies highlight hydrous sediment melts as a key carrier. Here, we present high-precision Ba isotope data (δ 138/134 Ba) of input and output from the Tongan subduction zone. The results show that Ba is not predominantly supplied by the subducting oceanic crust. Instead, most Ba can be traced back to subducting sediments from which it is released in two stages. During early subduction, sedimentary barite dissolves and releases isotopically heavy Ba via aqueous fluids into the serpentinized mantle wedge, where this Ba is later remobilized during subsequent breakdown. With continued slab descent, remaining lithogenic phengite releases isotopically lighter Ba, which is recycled at subarc depths by hydrous melting. Both mechanisms can explain the entire range of Ba/Th and δ 138/134 Ba in Tongan arc lavas without requiring Ba isotope fractionation between fluids and solids. These results from the endmember setting of Tonga, with minimal subducting sediments, imply higher proportion of sediment-derived Ba in other arcs worldwide. The Forearc Serpentinite Signature (FSS), which is created by the interaction of the mantle wedge with sediment-derived fluids at low temperatures (<600 °C), represents an additional fluid endmember that must be considered in the genesis of arc lavas. Lithogenic Ba that is not completely released at subarc depths can be further subducted into the deeper mantle, where it may later enrich mantle domains that source mid-ocean ridge and hotspot lavas. The relative proportions of sedimentary components in subducted slabs have changed over Earth’s history, highlighting the link between arc magma and mantle compositions and long-term shifts in Earth’s surface (bio)geochemical cycles.