Direct evidence for the nature and timing of sub-arc mantle metasomatism

Subduction delivers sediment and hydrated oceanic lithosphere into the convecting mantle. Some of these materials are involved in magma generation and returned to the surface as arc volcanism. The remainder continues into the deeper mantle contributing to long-term heterogeneity that may be later sampled by mantle plumes. In order to understand the global cycling of volatiles in subduction zones it is essential to understand the physical and chemical processes of fluid release and melting. Unique upper mantle samples from Batan Island (Philippines) have incompatible trace element and radiogenic isotope characteristics typical of their host lavas. Here we show that they also preserve extreme U-Th-Ra disequilibria. These do not result from either host magma contamination, steady-state diffusion in the mantle or subsequent crustal level processes. Rather, they provide the first direct evidence that such signatures in arc lavas originate in the mantle and that contributions from both wet sediment melts (between 8 kyr and 10's kyr ago) and aqueous fluids (<< 8 kyr) were separately delivered from the slab. The samples also us to estimate the amounts of water ( (super 3) 625 ppm) that may be returned to the asthenosphere, perhaps to be stored at the seismic transition zone.