Uranium isotope fractionation during slab dehydration beneath the Izu arc

Fluids released from subducted slabs impart characteristic geochemical signatures on volcanic arc magmas and residual slabs transported into the deeper mantle. Yet, the sources and transport mechanisms of trace elements released from the slab are speculative. We investigate fluids released from subducted slabs from the perspective of U-238/U-235 and radiogenic Pb isotope ratios in lavas from the Izu volcanic arc in the Pacific ocean. Izu arc lavas are fluid-dominated end-member type magmas that allow a close characterisation of slab fluids. The Izu arc lavas have low U-238/U-235 ratios compared to the bulk Earth and mid-ocean ridge basalt (MORB). The low U-238/U-235 (delta U-238 = -0.46 to -0.33 parts per thousand, where delta U-238 = U-238/U-235(sample)/U-238/U-235(CRM145) - 1) is associated with slab-derived fluids low in Th/U that are added to the magma sources. The radiogenic Pb isotope ratios of the lavas form an array between 'Indian' type MORB and subducting sediments that is inconsistent with fluids derived from the altered mafic oceanic crust (AMOC). We infer that 'fluid-mobile' elements, including U and Pb are mobilised from largely unaltered, deeper sections of the mafic crust by migrating fluids that are derived from the dehydration of underlying serpentinites. Uranium is only fluid-mobile as U-VI and needs to be oxidised from predominant U-IV in unaltered magmatic rocks in order to be mobilised by fluids. Uranium isotope fractionation of similar to 0.2 parts per thousand in delta U-238 during this process is required to generate the low U-238/U-235 in the fluids. We propose that channelised fluid flow through the metamorphosed sheeted dyke and gabbroic sections of the mafic crust locally oxidises and mobilises U. We suggest that U isotope fractionation occurs within the fluid channels and is related to equilibrium isotope fractionation during the oxidation of U and the incorporation of U-IV into secondary phases such as epidote, apatite and zircon that grow within the channels. These phases are predicted to carry isotopically heavy U into the deeper mantle beyond subduction zones. The delta U-238 is thus tracing the dehydration process of subducting slabs. Similar observations have been made for other, 'stable isotope' systems in different arcs and subduction-related metamorphic rocks, thus highlighting their potential for studying processes occurring within the slabs during subduction. This information is essential for understanding and the partitioning of elements between subducted slabs and the mantle wedge and constraining the role of subduction zones in global geochemical cycles. (C) 2019 Elsevier B.V. All rights reserved.