U-TH ISOTOPES IN ARC MAGMAS - IMPLICATIONS FOR ELEMENT TRANSFER FROM THE SUBDUCTED CRUST

Uranium-thorium isotope results from island are volcanic rocks were us ed to investigate the rates of transfer of fluids and sediments from t he downgoing slab. Uranium, but not thorium, is readily mobilized in t he fluid. A negative array between thorium/cerium and neodymium-143/ne odymium-144 indicates that significant amounts of the thorium in are r ocks are derived from subducted sediments, although perhaps only about 30 percent of the thorium in subducted sediments is returned to the c rust in this way. The transfer times for fluid through the mantle wedg e are about 30,000 to 120,000 years, whereas those for sediment melts may be several million years. The law average uranium/thorium ratios o f bulk crust primarily reflect different crustal generation processes in the Archaean.