Melting processes and fluid and sediment transport rates along the Alaska‐Aleutian arc from an integrated U‐Th‐Ra‐Be isotope study

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Abstract
A comprehensive data set for young lavas erupted along the Alaska‐Aleutian arc is used to examine how fluid and sediment transport rates and melting processes vary in response to systematic changes in subduction rate and dip along the arc and across the ocean‐continent boundary. Positive correlations between convergence rate, volcano volume, and238U excesses suggest that magmatic output is closely linked to the size of the fluid flux which occurred 10Be/9Be also increase with convergence rate. However, the inferred10Be/9Be ratio of this component is low relative to that in the incoming sediments, and this could reflect either sediment removal by accretion or storage in the mantle wedge for ∼0.5–1 Myr. Th/Nb and Th/Nd ratios exceed those expected from bulk sediment addition in the center of the arc suggesting that the enhanced sediment signal reflects transfer in a partial melt phase, whereas partial melts from the subducted oceanic crust appear to be restricted to the western tear in the Pacific plate. Therefore the thermal structure at the slab‐wedge interface in the center of the arc must lie close to, and possibly between, the sediment and basalt solidii and is thus constrained to be in the region of 670–740°C at 3 GPa. Elevated (230Th/232Th) ratios, the occurrence of230Th excesses and only modest226Ra excesses in many of the lavas are modeled as the products of dynamic melting effects superimposed upon a fluid‐fluxed mantle wedge. The transition from oceanic to continental lithosphere seems to play an important role in determining the relative effects of fluid addition and partial melting upon U‐Th disequilibria. We infer that the change from fast and steep plate subduction in the Aleutians to shallow and slow subduction, coupled with increasing lithospheric lid thickness, in Alaska together control slab and wedge temperatures and the rate of matrix flow through the melting region.