Formula omitted] disequilibria in the Lesser Antilles arc: implications for mantle metasomatism

Abstract

238U– 230Th– 226Ra disequilibria have been determined on historical lava flows from several active volcanoes of the Lesser Antilles Arc. The data reveal significant correlations between 238U– 230Th and 230Th– 226Ra disequilibria suggesting that they share a common and recent origin. The position of subduction zone derived samples in the Th/U vs. Th/Ba diagram and in the ( 238U/ 232Th ) vs. ( 230Th/ 232Th ) isochron diagram are consistent on both global and regional (Lesser Antilles arc) scales. This constitutes certainly a strong argument for linking the ( 238U/ 230Th ) and ( 230Th/ 232Th ) ratios variations of the subduction-related lavas to the U–Th–Ba mantle wedge enrichment. For the Lesser Antilles lavas, models relating directly both 238U– 230Th and 230Th– 226Ra disequilibria to a global mantle enrichment process (sediments + fluids) or only to an hydrous fluids input, are however difficult to reconcile with the available data on U and Th mobility during dehydration of the subducted oceanic crust. Two stage models are therefore discussed. They consist in ancient mantle enrichment processes by both subducted sediments and hydrous fluids derived from the subducted oceanic crust, followed by a late U–Th–Ra fractionation accounting for the 238U– 230Th– 226Ra correlation.