WHY ARE SO MANY ARC MAGMAS CLOSE TO U-238 TH-230 RADIOACTIVE EQUILIBRIUM

New analyses of U-238-Th-230 disequilibria are reported for four active volcanoes: Merapi and Krakatoa in the Sunda arc (Indonesia), Masaya in Central America (Nicaragua), and Ambrym in the New Hebrides island arc. Despite a large range in (Th-230/Th-232) ratios (from 0.65 in Merapi andesites to 2.5 in Masaya basalts), U-238 and Th-230 are close to radioactive equilibrium, as in many other arc magmas. In several mantle sources, Th/U ratios have clearly been modified by metasomatic processes associated with subduction. This is demonstrated in Central America by the correlation between (Th-230/Th-232) and Be-10/Be-9 ratios for several active volcanoes along the arc. It is proposed that the U-238-Th-230 radioactive equilibrium found in many arc magmas is the result of disequilibrium melting involving an easily melted, slab-derived, metasomatic component which dominates the uranium and thorium budget of the mantle sources. The departure from equilibrium may be either due to mixing with Th-230 enriched melts derived from unmetasomatized mantle sources or to a late stage uranium addition by fluids. This latter process, producing uranium enriched magmas, has a greater influence in uranium and thorium poor magmas.