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

Abstract

Isotope and trace element data from well characterised arc suites are used to identify contributions from fluids and subducted sediments in arc rocks. U-series isotopes are then used to argue that the fluid and sediment components are characterised by different transfer times through the mantle wedge. U, but not Th, is readily mobilised in the fluid component. Th behaves as a high field strength element (HFSE), and the negative array between Th/Ce and Nd isotopes indicates that significant amounts of the Th and by implication other HFSE, in arc rocks are derived from sediments in the subducted slab. These elements may therefore only be regarded as conservative in the sense that they are not mobilised in the fluid component. As the fluid component contains U, but little if any Th, the resultant (238U/230Th) disequilibria may be used to estimate transfer times for the fluid component, and these are typically 30-120 ka. In contrast, rocks with a greater contribution from subducted sediments (high Th/Ce and low 143Nd/144Nd) tend to have (238U/230Th) ~ 1, and low 10Be, suggesting transfer times of several million years. Overall, the Th, LREE and Ta contents of many arc rocks are dominated by the sediment component which is probably partial melts of sediments in the subducted slab. It is estimated that ~ 30% of the Th in subducted sediments is returned to the crust in arc magmas. The fractionation of U/Th and Sm/Nd requires that average continental crust was generated in the presence of residual garnet, presumably by processes early in Earth history that were different from those at recent plate margins.