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

• Published in: Geological Society, London, Special Publications
• Main Authors: Dosseto, Anthony, Turner, Simon
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2014
• Subjects: Kamchatka
• Online Access: https://researchers.mq.edu.au/en/publications/9c84434b-dd88-4147-956f-c982d4e6f986; https://doi.org/10.1144/SP385.5; http://www.scopus.com/inward/record.url?scp=84892652234&partnerID=8YFLogxK

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 ( 226 Ra/ 230 Th) and ( 231 Pa/ 235 U) 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 226 Ra 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 231 Pa excess. Because 238 U- 230 Th 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.