Recent contribution of sediments and fluids to the mantle's volatile budget

Subduction modifies the cycling of Earth’s volatile elements. Fluid-rich sediments and hydrated oceanic lithosphere enter the convecting mantle at subduction zones. Some of the sediments and volatile components are released from the subducting slab, promote mantle melting and are returned to the surface by volcanism. The remainder continue into the deeper mantle. Quantification of the fate of these volatiles requires an understanding of both the nature and timing of fluid release and mantle melting1. Here we analyse the trace element and isotopic geochemistry of fragments of upper mantle rocks that were transported to the surface by volcanic eruptions above the Batan Island subduction zone, Philippines. We find that the mantle fragments exhibit extreme disequilibrium between their U–Th–Ra isotopic ratios, which we interpret to result from the interaction of wet sediment melts and slab-derived fluids with rocks in the overlying mantle wedge. We infer that wet sediments were delivered from the slab to the mantle wedge between 8,000 and 10,000 years ago, whereas aqueous fluids were delivered separately much later. We estimate that about 625 ppm of water is retained in the wedge. A significant volume of water could therefore be delivered to the mantle transition zone at the base of the upper mantle, or even to the deeper mantle. As fluids and melts rise from subducting slabs they may interact with each other and the surrounding mantle wedge and may be contaminated or modified by shallow processes such as cumulate melting and crystal fractionation. To separate the role of these possible, contributing processes we need to constrain physical processes. A significant number of short-lived (U-series) isotopes studies have been conducted on arc lavas and magma chamber residence times are quite well constrained2. However, conclusions about the timing of source metasomatism have varied (1–100 kyr; refs 2–6) and there has been debate about the extent to which these signals reflect processes of fluid addition and partial melting. Alternative interpretations have included steady-state diffusion in the mantle wedge7 or even shallow-level processes, such as surface contamination8 and crustal melting9. It has also been suggested that correlations between key trace element ratios and U-series disequilibria may largely reflect magma differentiation and ageing10. These issues have predominantly been tackled indirectly by interrogating the geochemical signatures of arc lavas. However, it has also been suggested that some rare, metasomatized, mantle xenoliths, such as those occurring on Batan Island in the Philippines (Fig. 1), may provide direct information on processes occurring in