U-series isotope data on Lau Basin glasses: The role of subduction-relatedfluids during melt generation in back-arc basins

New thermal ionisation mass spectrometry U-series disequilibrium data are p resented for 24 basaltic to dacitic glasses from active spreading centres i n the back-arc Lau Basin (SW Pacific), together with additional inductively coupled plasma mass spectrometry trace element analyses and Sr-Nd-Pb isoto pe data. Valu Fa Ria ge samples, adjacent to the arc front, have high U/Th and (Th-230/U-238) < 1, implying a recent (much less than 350ka) addition o f a U-rich slab-derived fluid. The Valu I, data can be combined with existi ng Th-230-U-238 data for the Central Tonga arc to infer a fluid addition ev ent at similar to 50ka. The, similar sources and time scales for fluid tran sfer beneath the Valu Fa Ridge and beneath the an, itself suggest that the Valu Ea Ridge is propagating into the arc-front region. Central Lau Basin s amples, further behind the me, have lower U/Th, and (Th-230/U-238) greater than or equal to 1, similar to typical mid-ocean ridge basalts (MORB). With in the Central Lau Basin, a water-rich subduction component is seen only in samples closest to the an,, and this fluid does not have the high-U/Th com position of the fluid at Valu Fa. Melt generation in the Central Lau Basin appears to be dominated by normal ridge-type processes, but the relatively low, (Th-230/U-238) for these shallow ridges compared with global MORB coul d be a consequence of increased melt productivity, as a result of the eleva ted water contents. The transition from U-238 to Th-230 excesses within the back-aw basin is not a smooth function of decreasing addition of a U-rich fluid moving away from the an,front, but also reflects the complex dynamics between two major mantle domains within the mantle wedge ('Pacific' beneat h Valu Fa Ria ge, 'Indian' beneath the Central Lau Basin).