230 Th– 238 U disequilibrium in island arcs: evidence from the Aleutians and the Marianas

摘要: 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, example, refs 1–6). These involve partial melting of metasomatized mantle. The metasomatizing liquids derive subducted slab are large-ion-lithophile (LIL) element-bearing fluids or themselves melts. Further modification chemical composition lavas may occur by processes such as shallow-level fractional crystallization. Because Th/U ratio is quite different each sources mentioned above, Th U behave differently during fractionation processes, particularly if volatile transport involved, studies ^(230)Th–^(238)U disequilibrium great potential investigations arc lavas. Our data, given here, imply that Mariana Aleutian Island Arc similar to ocean ridge basalt sources, although there a hint somewhat lower component source. data also confirm extend observations Gill Allegre Condomines distinguished magmas erupted virtually all other volcanic environments activity ratios (^(230)Th/^(238)U)⩽1, implying relative enrichment uranium.