Abstract
U-Th isotope disequilibria can provide constraints on the time elapsed since fluid addition to the mantle wedge beneath island arcs. The Vanuatu arc offers new insights into these processes because Pb isotopes there are not dominated by components from the subducting plate and so preserve the signatures of the mantle wedge. The Pb isotope data document the presence of separate Pacific and Indian mantle domains beneath the arc volcanoes. The Indian mantle was brought beneath the central part of the arc from the backarc by collision with the D'Entrecasteaux Ridge, resulting in a slowing of subduction there. The distinction in the mantle wedge composition is also uniquely apparent in U-Th isotope data, which define two subparallel arrays on the U-Th equiline diagram, one anchored to high U/Th Pacific mantle and the other to lower U/Th Indian mantle. These data provide clear evidence of the effects of variable mantle composition on U-Th isotope disequilibria. We argue that such arrays faithfully record the time elapsed since fluid release from the subducting plate. The data indicate that this occurred ca. 16 ka in the area of collision and slow subduction, but ca. 60 ka where the rate of subduction is substantially faster. This suggests a link between the rate of subduction and the time elapsed since fluid release.
