Two mantle domains and the time scales of fluid transfer beneath the Vanuatu arc

U-Th isotope disequilibria can provide constraints on the time elapsed sinc e fluid addition to the mantle wedge beneath island arcs. The Vanuatu are o ffers new insights into these processes because Pb isotopes there are not d ominated by components from the subducting plate and so preserve the signat ures of the mantle wedge. The Pb isotope data document the presence of sepa rate Pacific and Indian mantle domains beneath the are volcanoes. The India n mantle was brought beneath the central part of the are from the backarc b y collision with the D'Entrecasteaux Ridge, resulting in a slowing of subdu ction there. The distinction in the mantle wedge composition is also unique ly apparent in U-Th isotope data, which define two subparallel arrays on th e U-Th equiline diagram, one anchored to high U/Th Pacific mantle and the o ther to lower U/Th Indian mantle. These data provide clear evidence of the effects of variable mantle composition on U-Th isotope disequilibria, We ar gue 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 rat e of subduction and the time elapsed since fluid release.