U-series isotopes and destructive plate margin magma genesis in the Lesser Antilles

High precision TIMS ²³⁰Th/²³²Th isotope ratios are presented from a suite of lavas which span the length of the Lesser Antilles island arc. Compositions vary from low K, tholeiitic lavas with low ⁸⁷Sr/⁸⁶Sr in the north, to high K, calc-alkaline to alkalic lavas with higher ⁸⁷Sr/⁸⁶Sr in the south. The northern lavas are characterised by (²³⁰Th/²³⁸U) < 1 whereas those from the south tend towards secular equilibrium and occasionally have (²³⁰Th/²³⁸U) > 1. A three-component model is required for magma genesis in the Lesser Antilles. Sediments from the subducting slab melt and variably enrich the mantle wedge, while the subducting oceanic crust dehydrates, releasing fluids enriched in U, Ba, Sr and K, but with negligible Th and low ⁸⁷Sr/⁸⁶Sr, which promote partial melting in the wedge. Ta/Zr increases with ⁸⁷Sr/⁸⁶Sr southwards along the arc and this is inferred to reflect increasing enrichment of the mantle wedge through the addition of partial melts of subducted sediment. Mass balance calculations are consistent with a relatively constant fluid flux (except for U) and little variation in the degree of partial melting (10-15%) along the arc, and they suggest that the sediment contribution to the arc lavas increases from ∼ 2% in the north to ∼ 15% in the south. In the northern parts of the arc, the addition of U by these fluids overprints any increases in Th/U that might have been caused by partial melting, and results in lavas with (²³⁰Th/²³⁸U) < 1. Further south, the fluid signature is swamped by partial melts of the subducted sediments, and the (²³⁰Th/²³⁸U) ratios tend towards 1. Assuming that (²³⁰Th/²³⁸U) ratios < 1 reflect the preferential mobility of U in aqueous fluids, the bulk of the LIL element abundances, and the elevated ⁸⁷Sr/⁸⁶Sr ratios in the calc-alkaline and alkalic lavas, were not transported by subduction zone fluids. A three-component mixing model is developed to calculate (²³⁰Th/²³²Th) ratios at the time of formation of the measured U/Th. The time required for radioactive decay from these calculated ratios to the measured (²³⁰Th/²³²Th) ratios provides an upper limit on the transport time for the relatively high U/Th fluid component through the wedge of ∼ 90 000 yr. If ∼ 50 000 yr U-Th isochrons from Soufrière on St. Vincent reflect typical magma chamber residence times for this island arc, this leaves ∼ 40 000 yr for transport from the slab to the crust. Such constraints require ascent rates > 1 m yr^-1, which imply channelled flow through the mantle wedge.