ELEMENTAL U AND TH VARIATIONS IN ISLAND-ARC ROCKS - IMPLICATIONS FOR U-SERIES ISOTOPES

Isotope and trace element data from well characterised are suites are used to identify contributions from fluids and subducted sediments in are rocks, U-series isotopes are then used to argue that the fluid and sediment components are characterised by different transfer times thr ough the mantle wedge. U, but not Th, is readily mobilised in the flui d component. Th behaves as a high field strength element (HFSE), and t he negative array between Th/Ce and Nd isotopes indicates that signifi cant amounts of the Th and by implication other HFSE, in are rocks are derived from sediments in the subducted slab. These elements may ther efore only be regarded as conservative in the sense that they are not mobilised in the fluid component. As the fluid component contains U, b ut little if any Th, the resultant (U-238/Th-230) disequilibria may be used to estimate transfer times for the fluid component, and these ar e typically 30-120 ka. In contrast, rocks with a greater contribution from subducted sediments (high Th/Ce and low Nd-143/Nd-144) tend to ha ve (U-238/Th-230) similar to 1, and low Be-10, suggesting transfer tim es of several million years. Overall, the Th, LREE and Ta contents of many are rocks are dominated by the sediment component which is probab ly partial melts of sediments in the subducted slab. It is estimated t hat similar to 30% of the Th in subducted sediments is returned to the crust in are magmas. The fractionation of U/Th and Sm/Nd requires tha t average continental crust was generated in the presence of residual garnet, presumably by processes early in Earth history that were diffe rent from those at recent plate margins. (C) 1997 Elsevier Science B.V .