Across-arc variations in K-isotope ratios in lavas of the Izu arc: Evidence for progressive depletion of the slab in K and similarly mobile elements

In subduction zones, fluids rise from the slab to the mantle, causing metasomatism and flux melting of the mantle to produce arc magmas. The transfer of material from slab to mantle and, in turn, to arc crust is an important control on the long-term chemical evolution of the mantle and continental crust. In this study, we investigate the transport of K in subduction zones by exploring the systematics of K stableisotope variations in lavas of the Izu arc. We find that the Izu lavas have isotopically heavy K relative to estimates for midocean ridge basalt (MORB)-source upper mantle. Moreover, the delta(41)Kvalues of the lavas are clearly heavier than those of subducting sediments and are probably heavier than subducting altered ocean crust. An across-arc decrease in delta(41)Kvalues is apparent. Arc-front lavas are heavier than the mantle by about 0.22 parts per thousand (median), whereas rear-arc lavas are heavier by only about 0.08 parts per thousand (median). The heavy K-isotope compositions of the arc lavas may arise from isotopic fractionation during slab dehydration, where light K is preferentially retained in phases such as phengite in the slab. The across-arc decrease in delta(41)Kvalues may be due to progressive breakdown of these phases, and to associated depletion of the slab in heavy K. Variations in the relative contributions of different source materials-igneous ocean crust, sediment, and mantle peridotite-may also play a role. In particular, we explore a possibility, motivated by radiogenic-isotope studies, that the slab signal in K isotopes may be attenuated in the rear arc as a result of extensive fluid-mantle interaction. If K isotopes do track slab dehydration, then K isotopes provide insight into the transfer of K and similarly mobile elements out of the slab and into the upper mantle and arc crust. Lastly, we observe extreme isotopic variations in some of the lavas, which we interpret to result from crustal-level or Earth-surface processes that affect only a subset of the lavas. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

In subduction zones, fluids rising from the slab to the mantle cause metasomatism and flux melting of the mantle to produce arc magmas. The study investigates the transport of K in subduction zones by analyzing K stable isotope variations in lavas of the Izu arc. Results show isotopically heavy K in Izu lavas relative to midocean ridge basalt-source upper mantle, with delta(41)Kvalues decreasing across the arc, suggesting isotopic fractionation during slab dehydration and progressive breakdown of certain phases. The study also explores the role of different source materials and the possibility of slab signal attenuation in the rear arc due to fluid-mantle interaction.