DESTRUCTIVE PLATE MARGIN MAGMATISM - GEOCHEMISTRY AND MELT GENERATION

Subduction related magmas are characterised by distinctive minor and trace element ratios which are widely attributed to the introduction of a hydrous component from the subducted crust. In general, high LIL/HFSE ratios are best developed in low HFSE rocks, and the variation in LILE is less than that in HFSE. Moreover, the size of the contributions from subducted material estimated on the basis of minor and trace element variations are consistently greater than those inferred from radiogenic isotopes. The Nd isotope ratios of island arc rocks are lower than those of N-MORB, and there is a broad negative correlation between Nd-143/Nd-144 and Pb-208*/Pb-206* consistent with a small contribution (2-4%) from subducted sediment in arc magmas. However, the isotope and trace element compositions of IAB cannot be modelled by two component mixing of sediment and N-MORB from the mantle wedge, and they require an additional high LIL/HFSE component which is typically attributed to the release of hydrous fluids from the subducted slab. A number of models are considered for the effects of fluid percolation in the mantle wedge. D(wedge/fluid) values are not well constrained but, if local equilibrium occurs, the available data on arc rocks are consistent with D(wedge/fluid) approximately 10(-2) for the LILE. Several authors have proposed that some of the trace elements in the subduction component were scavenged from the mantle wedge, in order to reconcile the different contributions from subducted materials inferred from radiogenic isotope and trace element data. In the models outlined here that suggests that D(slab/fluid) for the LILE is > 0.1. Within low Ce/Yb arc rocks there is a general increase in average Ce/Sm, but not K/Ce, with increasing crustal thickness. The observed range in Ce/Sm in the low Ce/Yb rocks is consistent with 3-18% melting of slightly LREE depleted source rocks, and it appears that the degree of melting varies with crustal thickness.