LONG MAGMA RESIDENCE TIMES AT AN ISLAND ARE VOLCANO (SOUFRIERE, ST. VINCENT) IN THE LESSER ANTILLES - EVIDENCE FROM U-238 TH-230 ISOCHRON DATING

High-precision (TIMS) measurements of U and Th isotope concentrations have been determined on whole rocks and mineral separates from Soufrie re volcano on St. Vincent in the Lesser Antilles island are. The whole rocks display relatively constant U/Th ratios (similar to 0.5), and a re characterised by excess U-238 relative to Th-230 which is attribute d to the addition of U-rich fluids from the subducting slab to the man tle source of the magmas. Mineral isochrons for four recently erupted (<4 ka) rocks yield ages of 46-77 ka and the combined data have an age of 58 +/- 7 ka with an initial (Th-230/Th-230) ratio of 1.05. Mixing of young magmas, or magma-cumulate mixing, are unlikely to be responsi ble for these isochronous relationships. Contamination of young, pheno crystic magmas with previously crystallised magma could produce the is ochrons, in which case the ages provide maximum and minimum ages, resp ectively, for these two components. However, no lavas with (Th-230/Th- 230) ratios of 1.054 have been found and there is no petrographic evid ence for mixing or phenocryst-magma disequilibrium. Our preferred inte rpretation is that the isochrons reflect long magma residence times wi thin the are crust. Following initial crystallisation, due to heat los s to the wall rocks, the magmas were maintained at a temperature close to their liquidus while Th-230-ingrowth occurred in both the magma an d the crystals. Maintenance of a relatively constant temperature requi res good insulation by cumulate layers and probably heating from below by influxes of fresh magma which either did not mix with the resident magma or else had an essentially indistinguishable composition. Subtr action of the residence times from the similar to 90 ka inferred to ha ve elapsed since fluid addition to the mantle wedge leaves similar to 30 ka for transfer through the wedge. Long residence times may typify more evolved lavas that develop in thicker are crust whereas tholeiiti c are lavas are inferred to have trivial crustal residence times but s imilar transfer times through the mantle wedge. (C) 1998 Elsevier Scie nce B.V. All rights reserved.