Trace element fractionation processes in the generation of island arc basalts

Subduction-related magmas are characterized by distinctive minor and trace element ratios which are widely attributed to the introduction of a hydrous component from the subducted crust. Island arc rocks may usefully be subdivided into high and low Ce/Yb groups, and the latter are characterized by relatively restricted radiogenic isotope ratios. In general, high LIL/HFSE ratios are best developed in low HFSE rocks, and the variation in LILE is less than that in HFSE. A local equilibrium model is developed in which the distinctive minor and trace element feature of arc rocks are the result of fluid percolation in the mantle wedge. Peridotite/fluid distribution coefficients are inferred to vary systematically with ionic radius in the range 69-167 $\times $ 10$^{-12}$ m. However, in practice the calculated olivine/fluid partition coefficients are too high to develop an arc signature in the wedge peridotite in reasonable timescales, and for acceptable fluxes from the slab. The available geochemical data would suggest that realistic distribution coefficients are 2-3 orders of magnitude less than those presently available from experimental data, presumably because the fluid compositions are different, or that local equilibrium is not appropriate. Average compositions from the low Ce/Yb arc suites exhibit a positive correlation between Ce/Sm, but not K/Sm, and crustal thickness. It is argued that the degree of melting varies with crustal thickness, but not in any simple way with the magnitude of the fluid contribution. The observed range in Ce/Sm in the low Ce/Yb rocks is consistent with 3-18% melting of slightly LREE depleted source rocks.