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Uranium-thorium isotope results from island arc volcanic rocks were used to investigate the rates of transfer of fluids and sediments from the downgoing slab. Uranium, but not thorium, is readily mobilized in the fluid. A negative array between thorium/cerium and neodymium-1 43/neodymium-144 indicates that significant amounts of the thorium in arc rocks are derived from subducted sediments, although perhaps only about 30 percent of the thorium in subducted sediments is returned to the crust in this way. The transfer times for fluid through the mantle wedge are about 30,000 to 120,000 years, whereas those for sediment melts may be several million years. The low average uranium/ thorium ratios of bulk crust primarily reflect different crustal generation processes in the Archaean.
Destructive plate margins are major sites of differentiation in the evolution of the crustmantle system. The mantle is cooled by the subduction of cold oceanic crust, yet island arcs contain many of the most active and hazardous volcanoes on Earth. Partial melting occurs in response to the introduction of fluids from the subducted crust, and many arc magmas contain a significant additional contribution from subducted sediments (1-10). The magmatic flux constitutes a major component of new crust, and conversely the subduction of oceanic crust and sediment forms the principal mechanism for recycling of crustal materials into the upper mantle.
A major goal in the earth sciences is to understand the physical processes and element fluxes involved in the generation of arc magmas. A number of chemical tracers for recycled sediments have been developed, most notably loBe (4), but the balance of new and recycled material, the controls on magma composition, and the rates of movement of material from the subducted slab are not well known. Some elements are preferentially enriched, and others are relatively depleted in arc magmas, as compared with magmas generated along mid-ocean ridges and in intra-plate settings. Different elements behave differently; some are preferentially mobilized in fluids from the subducted slab, and the proportions of recycled and new mantle-derived material vary from element to element. Kay ( 1), for example, argued that -90% of the K in island arc magmas was recycled from preexisting continental crust, whereas many estimates for elements such as Ta, Nb, and Ti have suggested that they...