Reappraisal of uranium-series isotope data in kamchatka lavas: Implications for continental arc magma genesis

Uranium-series isotopes can be used to determine constraints on the timescale of slab dehydration and melt production at subduction zones. However, interpretations of U-Th-Ra data suggest very different timescales of slab dehydration. Here, we present new U-Th-Ra data from Kamchatka along with a number of alternative models for production of radioactive disequilibrium. Variations in (²²⁶Ra/²³⁰Th) and (²³¹Pa/²³⁵U) activity ratios are best explained by crystal fractionation with host rock assimilation for a duration of less than c. 6000 years. The association of the largest ²²⁶Ra excesses with high Sr/Th in the most primitive lavas suggests that Ra-Th fractionation is controlled by slab dehydration less than 10 ka ago. We show that U-Th data can be explained by dynamic melting of a recently (>10 ka) metasomatized mantle wedge. Dynamic melting of an oxidized source metasomatized several hundreds of thousands of years ago cannot produce significant ²³¹Pa excess. Because ²³⁸U-²³⁰Th disequilibrium is inferred to be controlled by partial melting, there is no requirement for multi-stage slab dehydration commencing ̃150 ka. We suggest that Ra-Th disequilibria constrain the timing of slab dehydration, whilst U-Th fractionation is dominated by partial melting, at least at the Kamchatka arc.