U-238-Th-230-Ra-226 disequilibria in the Lesser Antilles arc: implicationsfor mantle metasomatism

U-238-Th-230-Ra-226 disequilibria have been determined on historical lava f lows from several active volcanoes of the Lesser Antilles Are. The data rev eal significant correlations between U-238-Th-230 and Th-230-Ra-226 disequi libria 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 (U-238/Th-232) vs. (Th-230/Th-232) isochron diagram are consistent on both global and regional (Lesser Antilles are) scales. This constitutes certain ly a strong argument for Linking the (U-238/Th-230) and (Th-230/Th-232) rat ios variations of the subduction-related lavas to the U-Th-Ba mantle wedge enrichment. For the Lesser Antilles lavas, models relating directly both U- 238-Th-230 and Th-230-Ra-226 disequilibria to a global mantle enrichment pr ocess (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 subd ucted sediments and hydrous fluids derived from the subducted oceanic crust , followed by a late U-Th-Ra fractionation accounting for the U-238-Th-230- Ra-226 correlation. (C) 1999 Elsevier Science B.V. All rights reserved.