Geochemical Variations in Vanuatu Arc Lavas: the Role of Subducted Material and a Variable Mantle Wedge Composition

Abstract

New compositional data are presented for recent (<3.5 Ma) basaltic lavas from the Vanuatu arc and North Fiji back-arc, and for material being subducted at the Vanuatu trench. Sr, Nd and Pb isotope data for the lavas vary systematically north–south along the arc and highlight where the D'Entrecasteaux Zone on the subducting plate collides with the fore-arc. Major and trace element data show significant lateral variations in enrichment of the mantle wedge relative to an N-MORB source. New elemental and isotopic analyses of the North Loyalty Basin sediments have allowed the flux of elements in the subducting sediments to be calculated. The wedge has been variably modified by addition of slab-derived material containing a significant subducted sediment component. Merelava in the rear-arc is anomalous and is dominated by a fluid component derived from subducted oceanic crust. Most lavas originate from a Pacific-MORB-like mantle source except for those erupting close to the collision zone in central Vanuatu. Collision of the D'Entrecasteaux Zone has disrupted the arc complex and brought less depleted, isotopically distinct (Indian-MORB-like) mantle material from the back-arc into the arc magma source in this region.