[sup 230]Th-[sup 238]U disequilibrium in East Scotia backarc basalts: Implications for slab contributions.

New U-series isotope data for lavas from the East Scotia backarc spreading center span a large range in ([sup 230]Th/[sup 232]Th) and ([sup 238]U/[sup 232]Th). Most of the backarc lavas have ([sup 238]U/ [sup 230]Th) < 1, similar to the composition of mid-oceanic-ridge basalts (MORB). Lavas from two segments have ([sup 238]U/[sup 230]Th) > 1 and are enriched in fluid-mobile elements, implying a recent addition of a U-rich slab-derived component. The data from one segment suggest an influence from an aqueous fluid from altered MORB, whereas samples from the other slab-influenced segment show addition of a sediment melt. The slab-influenced samples form a distinct trend in the equiline diagram between aqueous fluid and sediment melt that is suggested to be a mixing line rather than an isochron.