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

Turner, S. and Hawkesworth, C. and Van Calsteren, P. and Macdonald, Ray and Heath, Emily and Black, Stuart (1996) U-series isotopes and destructive plate margin magma genesis in the Lesser Antilles. Earth and Planetary Science Letters, 142 (1-2). pp. 191-207. ISSN 0012-821X

Abstract

High precision TIMS230Th232Th 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 low87Sr86Sr in the north, to high K, calc-alkaline to alkalic lavas with higher87Sr86Sr in the south. The northern lavas are characterised by(230Th238U) < 1 whereas those from the south tend towards secular equilibrium and occasionally have(230Th238U) > 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 low87Sr86Sr, which promote partial melting in the wedge.Ta/Zr increases with87Sr86Sr 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 inTh/U that might have been caused by partial melting, and results in lavas with(230Th238U) < 1. Further south, the fluid signature is swamped by partial melts of the subducted sediments, and the (230Th238U) ratios tend towards 1. Assuming that(230Th238U)ratios< 1 reflect the preferential mobility of U in aqueous fluids, the bulk of the LIL element abundances, and the elevated87Sr86Sr ratios in the calc-alkaline and alkalic lavas, were not transported by subduction zone fluids. A three-component mixing model is developed to calculate (230Th232Th) ratios at the time of formation of the measuredU/Th. The time required for radioactive decay from these calculated ratios to the measured (230Th232Th) ratios provides an upper limit on the transport time for the relatively highU/Th fluid component through the wedge of 90000 yr. If 50000 yr UTh 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.