²³⁰Th–²³⁸U 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 ²³⁰Th–²³⁸U disequilibrium^7–11 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 Gill⁵ and Allègre and Condomines¹⁰ that oceanic arc lavas are distinguished from magmas erupted in virtually all other volcanic environments by activity ratios (²³⁰Th/²³⁸U)⩽1, implying relative enrichment of uranium.