Mantle melting beneath island arcs: U-series constraints from Marianas

U-series disequilibria of erupted lavas are a geochemical means to investigate the geodynamics of melting regimes. At destructive plate margins, U-series isotopes are controlled by two competing processes: i) fluid addition from the subducting plate; ii) melting of the mantle wedge. Depending on the nuclides half-lives and their mobility in the fluids phase, each parent-daughter pair responds differently to the two processes. (230Th/238U) is mainly affected by the fluid addition whilst (231Pa/235U) is more sensitive to melting and requires timerelated processes to ‘in-grow’ the daughter nuclide. The combination of U-Th-Pa-Ra measurements on a single volcanic arc, and in particular the variation of (231Pa/235U) with (230Th/238U) allows investigation of the timing of slab dehydration and its affects on the melting process. The Mariana arc provides the perfect location to study the behaviour of Pa in the arc environment. (230Th/238U) disequilibria suggest that U is fluxed into the mantle during dehydration of the altered oceanic crust inducing significant 230Th-deficits in the most depleted (least sediment enriched) lavas [1]. The more sediment rich lavas are less affected by this U addition, and show minor 230Th. All the measured samples display (231Pa/235U)>1, requiring sufficient 231Pa ingrowth during melting to overcome the initial deficit induced in the source by U-rich fluids. Moreover, the slope of the (231Pa/235U) vs. (230Th/238U) alignment of the erupted lavas, in comparison to that expected for the fluid-fluxed source, suggests that magmas more affected by fluids requires more 231Pa-ingrowth during melting.