Journal of Petrology Advance Access originally published online on August 17, 2006
Journal of Petrology 2006 47(11):2185-2232; doi:10.1093/petrology/egl041
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Contributions of Slab Fluid, Mantle Wedge and Crust to the Origin of Quaternary Lavas in the NE Japan Arc
1 DEPARTMENT OF GEOSCIENCE, SHIMANE UNIVERSITY MATSUE 690-8504, JAPAN
2 INSTITUTE OF MINERALOGY, PETROLOGY AND ECONOMIC GEOLOGY, TOHOKU UNIVERSITY AOBAKU, SENDAI 980-8578, JAPAN
RECEIVED NOVEMBER 10, 2004; ACCEPTED JULY 25, 2006
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.
KEY WORDS: arc magma; crustal melt; depleted mantle; NE Japan; Quaternary; slab fluid
*Corresponding author. Telephone: +81 852 32 6462. Fax: +81 852 32 6462. E-mail: jkimura{at}riko.shimane-u.ac.jp and kmrjunichi2006{at}yahoo.co.jp
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