FORMATION OF U-SERIES DISEQUILIBRIA IN VOLCANIC ARC MAGMAS: CRUST OR MANTLE CONTROL?

Understanding magmatic processes in volcanic arcs relies upon a fundamental comprehension of the timescales of subduction zone processes such as magma production, transport, and modification. Disequilibria in the uranium-series isotope system is one of the few geochemical tools based on short-lived radionuclides appropriate for probing the short timescales of volcanic processes. However, our knowledge of the partitioning of U and Th in subduction zones is incomplete. Th-excess (²³⁰Th/²³⁸U) >1, typical of MORB, is effectively modeled by decompression melting. In contrast, Th-deficits, typical of arc volcanism, are generally modeled by fluid fluxing the mantle source. Our study investigates the Kamchatka volcanic arc, which, contrary to the above generalizations, preserves a signature of Th-excess. We focus on Bezymianny and Klyuchevskoy volcanoes, two active, adjacent (≈10km apart) volcanic systems. Bezymianny (²³⁰Th/²³⁸U) ranges from 1.04-1.06 and Klyuchevskoy tephras have (²³⁰Th/²³⁸U) between 1.01 and 1.08. These volcanic centers are thought to share the same mantle source but have undergone different extents of differentiation (Klyuchevskoy basalts: 51-56 wt% SiO₂; Bezymianny andesites: 57-63 wt% SiO₂). Therefore, in comparing these systems it is possible to decouple primary mantle source signals from those modified by crustal processing. Th-excess in arcs is generally attributed to interaction of melts with thick continental crust where phases such as garnet can retain high U/Th in crystalline residues. However, in the CKD, we measure low Sr/Y (15.5-19.9) that precludes significant influence of garnet. In addition, our LA-ICP-MS measurements of in situ U and Th mineral-melt partitioning among erupted phases (glass, plagioclase, pyroxene, Fe-Ti oxides, apatite) suggest that U-series disequilibria are transparent to shallow crustal processing. We suggest that Th-excess must, therefore, be a function of either decompression melting or fractionation by phases (e.g., clinopyroxene) not sampled during eruption. Our interpretations are constrained by trace and major element, and Pb-isotope data. Because of the minimal effect of crust-magma interaction on modifying U-series disequilibria in Kamchatka, this is an ideal place to examine U-series behavior in the mantle wedge environment.