CONTRIBUTIONS OF SLAB FLUID, MANTLE WEDGE AND CRUST TO THE ORIGIN OF QUATERNARY LAVAS IN THE NE JAPAN ARC

Abstract

Quaternary lavas from the NE Japan arc show geochemical evidence of mixing between mantle-derived basalts and crustal melts at the magmatic front, whereas significant crustal signals are not detected in the rear-arc lavas. The along-arc chemical variations in lavas from the magmatic front are attributable almost entirely to geochemical variations in the crustal melts that were mixed with a common mantle-derived basalt. The mantle-derived basalts have slightly enriched Sr-Pb and depleted Nd isotopic compositions relative to the rear-arc lavas, but the variation is less pronounced if crustal contributions are eliminated. Therefore, the source mantle compositions and slab-derived fluxes are relatively uniform, both across and along the arc. Despite this, incompatible element concentrations are significantly higher in the rear-arc basalts. We examine an open-system, fluid-fluxed melting model, assuming that depleted mid-ocean ridge basalt (MORB)-source mantle melted by the addition of fluids derived from subducted oceanic crust (MORB) and sediment (SED) hybrids at mixing proportions of 7% and 3% SED in the frontal- and rear-arc sources, respectively. The results reproduce the chemical variations found across the NE Japan arc with the conditions: 0.2% fluid flux with degree of melting F = 3% at 2 GPa in the garnet peridotite field for the rear arc, and 0.7% fluid flux with F = 20% at 1 GPa in the spinel peridotite field beneath the magmatic front. The chemical process operating in the mantle wedge requires: (1) various SED-MORB hybrid slab fluid sources; (2) variable amounts of fluid; (3) a common depleted mantle source; (4) different melting parameters to explain across-arc chemical variations. © 2006 Oxford University Press.