Examining Canonical Theories of U-series Disequilibria in Volcanic Arcs in Light of a More Comprehensive, Global Database

Abstract

Disequilibrium in the short-lived U-series isotopic system occurs during partial melting, differentiation, and volatile transport; therefore, the U-series decay chain is a unique tool to examine the magnitude and timing of magmatic processes. However, our understanding of U-series fractionation in subduction zones is incomplete. We use published data from volcanoes around the world and new data from volcanic systems in the Kamchatka Arc (Bezymianny, Klyuchevskoy, and Karymsky) to examine two theories regarding the behavior of U-series nuclides: 1) that Th-excess in arc magmas, (230Th)/(238U) >1, is a function of arc-thickness/garnet in the melting region, or magnetite fractionation and 2) that 210Pb deficits, (210Pb)/(226Ra) <1, are the result of continuous magma degassing. Our results show that neither of these theories explains the complete dataset produced by the U-series community. Th-excess is generally observed in MORB and attributed to decompression melting; however, global data also record Th-excess in fluid-fluxed subduction zones. Limited experimental data suggest preferential U transport over Th in subduction zone fluids and, therefore, U-excess rather than Th-excess should exist in arcs. The common explanation for arc Th-excess is melt interaction with thick continental crust where phases such as garnet retain high U/Th in crystalline residues. We record Th-excess at Bezymianny, (230Th)/(238U) from 1.04-1.06 and Klyuchevskoy, (230Th)/(238U) from 1.01 and 1.08, volcanoes, which are located on relatively thin (~35 km), primitive crust. These magmas have low Sr/Y (15.5-19.9) that preclude a significant influence of garnet. In addition, LA-ICP-MS measurements of in-situ U and Th mineral-melt partitioning on erupted mineral phases (plagioclase, pyroxene, Fe-Ti oxides, apatite) suggest that U-series disequilibria are transparent to shallow crustal processing. Th-excess at Klyuchevskoy inversely correlates with Ba/Th, Sr/Th, Dy/Yb, and Ce/Pb. We suggest that Th-excess is a function of decompression mantle melting beneath arcs and/or phase fractionation by lower crustal mineral assemblages. 210Pb deficits are interpreted as the result of 222Rn gas loss in magmatic systems. With 222Rn loss, the daughter product, 210Pb, cannot grow into magma. Two published models (Gauthier and Condomines 1999 and Condomines et al. 2010) simulate this process where the magnitude of 210Pb deficit increases with the duration of degassing. We record 210Pb deficits at Bezymianny and Karymsky volcanoes (0.846-0.966). However, for known eruption intervals, these 210Pb deficits are larger than current models predict. If physical constraints for gas behavior in magma are added to these models, the problem is magnified. Our results highlight a need for studies quantifying the fractionation of U-series nuclides among all phases (crystals, melts, and volatiles) relevant to subduction zones. Primitive, thin volcanic arcs that have a well-characterized volcanic history through time, similar to the Kamchatka Arc, are ideal places to begin