Systematic Across‐Arc Variations of Molybdenum Isotopes in a Fluid‐Dominated Subduction Zone System

Abstract

Mass‐dependent Mo isotope variations are a promising new tracer to study magmatic processes in different geological settings. We report the first Mo isotope data for the Kamchatka arc system in the Northwest Pacific, comprising basaltic lavas of a complete Southeast‐Northwest traverse from the volcanic arc front through to the back arc region. The majority of volcanic centers investigated directly override the Hawaii‐Emperor Seamount Chain, which is currently being subducted underneath the arc system. Our Mo isotope data show systematic trends with Ce/Pb, Ce/Mo, Nb/Zr, La/Sm, and 143Nd/144Nd ratios from the volcanic arc front to the back arc. Arc front lavas have higher δ98/95Mo and lower Ce/Pb, Ce/Mo, Nb/Zr, La/Sm compared to back arc lavas. Because the involvement of subducted sediments can be excluded, we attribute the observed variations to a change in the mantle source composition from the arc front to the back arc regions. The isotopic and chemical budget of arc front lavas is dominated by a slab fluid component (high δ98/95Mo, low Ce/Pb, Ce/Mo), whereas mantle‐like Ce/Pb, Ce/Mo, elevated Nb/Zr and La/Sm in the back arc samples suggest an enriched mantle source. Combined δ98/95Mo, Nd, and Pb isotope data in back arc lavas are very similar to those observed for modern ocean island basalts from Hawaii. We thus explore the possibility that the back arc mantle was contaminated by a Hawaii‐type, enriched asthenospheric mantle component from the subducted Hawaii‐Emperor Seamount Chain.

Molybdenum isotopic compositions in volcanic rocks from Northern Izu arc (Izu-Oshima, Niijima, and Kozushima)

“…The δ 98/95 Mo values of Niijima (-0.14 ± 0.05‰, N = 1) measured in this study were consistent with those of the previous study (δ98/95 Mo = -0.15 ± 0.07‰, N=2, 2s;Villalobos-Orchard et al, 2020). The mean δ 98/95 Mo value of Kozushima (δ 98/95 Mo = -0.20 ± 0.21‰, N = 6, 2s) is similar to those of basaltic samples from Kamchatka back arc (δ 98/95 Mo = -0.22 ± 0.09‰, N = 10, 2s;Willbold and Messling, 2023), as well as N-MORB (δ 98/95 Mo ≈ -0.17‰;Bezard et al, 2016;Chen et al, 2022;Hin et al, 2022) and DM (δ 98/95 Mo ≈ -0.20‰;McCoy-West et al, 2019).…”

“…Third, in the δ 98/95 Mo versus Ce/Mo plot (Fig. 9), most of the N-Izu RA basalts lie on the trend defined by the Mariana and Kamchatka arc basalts, to which the contribution of slab melts has been suggested (e.g., Freymuth et al, 2015;Li, H. Y et al, 2021;Li, X. Y et al, 2021;Willbold and Messling, 2023). As suggested by the examination of Nd/Hf ratios, there appears to be a gradual increase in the proportion of slab melt components towards the RA region.…”

“…Recently, the isotopic composition of Mo in volcanic rocks from subduction zones has been recognized as an important geochemical tracer for understanding the dehydration process of the subducting slab and the subsequent material transport in the mantle wedge (e.g., Ahmad et al, 2021;Freymuth et al, 2015;Gaschnig et al, 2017;König et al, 2016;Li, H. Y et al, 2021;Li, X, Y et al, 2021;Rojas-Kolomiets et al, 2023;Villalobos-Orchard et al, 2020;Willbold and Messling, 2023;Yu et al, 2022). Molybdenum has been suggested to be mobile in subduction zone fluids, and experimental constraints indicate that Mo mobility is controlled by the redox state and salinity of the hydrous fluid involved (Bali et al, 2012;König et al, 2008König et al, , 2010Skora et al, 2017).…”

“…Additionally, basaltic samples from RA volcanoes in the Mariana Trough (-0.65 to 0.02‰), Izu arc (-0.17 to -0.12‰), and Kamchatka arc (-0.31 to -0.02‰) have δ 98/95 Mo values lower than those of MORB and DM. Several models have been proposed to explain the relatively low δ 98/95 Mo value in some arc samples, including the contribution of hydrous melt derived from sediment and/or oceanic crust, dehydration of slab components, and diapir of hydrous mantle melting (König et al, 2016;Li, H. Y., et al, 2021;Li, X. H., et al, 2021, Willbold andMessling, 2023).…”

Molybdenum Isotopic Signatures of Forearc Serpentinites: Origin and Contribution to the Subduction Zone Mo Cycle

“…Quantifying the amounts of material inputs and volcanic outputs in a subduction zone requires an accurate evaluation of the elemental and isotopic compositions of subducted materials and arc or backarc magmas. In recent years, the stable Mo isotopic compositions have been used to trace slab-mantle interactions and the contributions of subducted materials to arc or back-arc lavas (e.g., Ahmad et al, 2021;Freymuth et al, 2015;Gaschnig et al, 2017;König et al, 2016;Li et al, 2019Li et al, , 2021aLi et al, , 2021bRojas-Kolomiets et al, 2023;Villalobos-Orchard et al, 2020;Willbold & Messling, 2023;Yu et al, 2022Yu et al, , 2023. Higher δ 98/95 Mo values in lavas than in the depleted mantle have been reported in some arcs, such as the Mariana arc (e.g., Freymuth et al, 2015;Li et al, 2021b), Izu arc (Villalobos-Orchard et al, 2020), and Solomon arc (König et al, 2016), and some isotopically light signatures are found in mid-ocean ridge basalt (MORB)-type eclogites (Chen et al, 2019).…”