DESTRUCTIVE MARGIN MAGMATISM AND THE CONTRIBUTIONS FROM THE MANTLE WEDGE AND SUBDUCTED CRUST

Destructive plate margin magmas may be considered in two groups on the basis of their REE ratios. Many island arc suites have low Ce/Yb, and remarkably restricted isotope ratios of Sr-87/Sr-86 = 0.7033 (+/- 0.0002), Nd-143/Nd-144 = 0.51302 (+/- 0.00004), Pb-206/Pb-204 = 18.76 (+/- 0.13), Pb-207/Pb-204 = 15.57 (+/- 0.02) and Pb-208/Pb-204 = 38.4 (+/- 0.18). However, they also have Rb/Sr (0.027 +/- 0.025), Th/U (2.18 +/- 0.5) and Ce/Yb (8.4 +/- 2.7) ratios that are significantly less than accepted estimates for the bulk continental crust. The high Ce/Yb suites have higher incompatible element contents, more restricted HREE and much more variable isotope ratios, and these have been attributed to both contributions from subducted sediment, and old, trace element-enriched material in the mantle wedge. The Th isotope ratios of destructive margin rocks are similar to those in mid ocean ridge basalt (MORB) and ocean island basalt (OIB). However, while many MORB and OIB are displaced to low (U-238/Th-230) on the (Th-230/Th-232)-(U-238/Th-232) diagram. approximately 60% of the arc rocks studied are close to secular equilibrium. and the rest tend to be displaced to high rather than low (U-238/Th-230). Despite the large range of Th isotope ratios in sediments and altered MORB in the subducted slab, the majority of destructive margin rocks plot within the Th-Sr and Th-Nd isotope arrays defined by MORB and OIB. Both measured Th/U and the source Th/U inferred from Th isotopes correlate with Pb-208*/Pb-206*, indicating that the major (six-fold) difference in Th/U between different arc suites was established several 100 Ma prior to subduction, and it does not appear to reflect element fractionation processes associated with recent subduction. In contrast, many arc rocks have high Be-10/Be and B/Be ratios which require a contribution from young sediments, and also indicate that B/Be arc likely to have been fractionated during the dehydration of the subducted slab. It is argued that for isotope and trace element models the slab component can usefully be taken to consist of subducted sediment and altered MORB, since these may contain approximately 80% of the water in the subducted slab, and the distinctive trace element features of arc magmas are generally attributed to the movement of material in hydrous fluids. The isotope data indicate that less-than-or-equal-to 15% of the Sr and Th in an average arc magma were derived from subducted material and that the rest are derived from the mantle wedge. The fluxes of elements that cannot be characterized isotopically are much more difficult to constrain but for most minor and trace elements, the slab-derived contribution in arc magmas is too small to have a noticeable effect on the residual slab.