^(230)Th-^(238)U disequilibrium in island arcs: evidence from the Aleutians and the Marianas

Abstract

Complex models for magma generation in island arcs have recently been proposed which require varying contributions from three possible sources: (1) the mantle; (2) subducted, altered oceanic crust; and (3) sediments (see, for example, refs 1–6). These models involve partial melting of metasomatized mantle. The metasomatizing liquids derive from the subducted slab and are large-ion-lithophile (LIL) element-bearing fluids or are themselves partial melts. Further modification of the chemical composition of the lavas may occur by processes such as shallow-level fractional crystallization. Because the Th/U ratio is quite different for each of the three possible magma sources mentioned above, and as Th and U may behave differently during fractionation processes, particularly if volatile transport is involved, studies of ^(230)Th–^(238)U disequilibrium have great potential in investigations of arc lavas. Our data, given here, imply that the Th/U ratio in the sources of Mariana and Aleutian Island Arc lavas is similar to ocean ridge basalt sources, although there is a hint of a somewhat lower Th/U component in the Aleutian source. The data also confirm and extend the observations by Gill and Allègre and Condomines that oceanic arc lavas are distinguished from magmas erupted in virtually all other volcanic environments by activity ratios (^(230)Th/^(238)U)⩽1, implying relative enrichment of uranium.