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

Abstract

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 ∼ 90000 yr. If ∼ 50000 yr U Th isochrons from Soufrière on St. Vincent reflect typical magma chamber residence times for this island arc, this leaves ∼ 40000 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.