Rubidium isotopes reveal dehydration and melting of the subducting slab beneath the Mariana arc

Rubidium (Rb) is a strongly incompatible and highly fluid-mobile element and Rb isotopes have the potential to track subducted material recycling to the mantle source of arc magmas. However, the behavior of Rb isotopes during slab subduction and associated processes remains unconstrained. This study for the first time presents Rb isotope data for the altered oceanic crust (AOC) from IODP Site U1365, subducted sediments, and lavas from the Southern Mariana arc. The δ⁸⁷Rb of the AOC varies between –0.16 ± 0.03 ‰ to 0.07 ± 0.02 ‰ with a weighted average of –0.02 ± 0.06 ‰, which is higher than that of fresh mid-ocean ridge basalts (MORB; –0.12 ± 0.08 ‰). Such Rb isotope fractionation may be attributed to the preferential loss of ⁸⁵Rb into seawater during the dissolution of primary phases and the preferential structural incorporation of ⁸⁷Rb into secondary phases and preferential ⁸⁷Rb adsorption by clays. The δ⁸⁷Rb of the sediments varies from –0.20 ± 0.07 ‰ to –0.03 ± 0.02 ‰ with an average of –0.11 ± 0.12 ‰, identical to that of the upper continental crust (UCC; –0.14 ± 0.08 ‰). The correlations observed between δ⁸⁷Rb and sediment depth (and loss on ignition; LOI) suggest that hydrodynamic sorting of sediments which enriches the upper stratigraphic intervals in clays, exerts a first-order control on the Rb isotope fractionation in subducted sediments. The formation of authigenic clays, which act as a sink of ⁸⁵Rb from seawater, could partially account for the elevated δ⁸⁷Rb of seawater (0.14 ± 0.12 ‰) relative to the Bulk Silicate Earth (BSE; –0.12 ± 0.06 ‰). For the Mariana lavas, the δ⁸⁷Rb decreases from fore-arc lavas (–0.03 ± 0.04 ‰ to 0.09 ± 0.02 ‰) to frontal arc lavas (–0.12 ± 0.04 ‰ to –0.01 ± 0.03 ‰), which are all higher than the average δ⁸⁷Rb of fresh MORB (–0.12 ± 0.08 ‰). The elevated δ⁸⁷Rb of the lavas reflects the addition of AOC-derived aqueous fluids with a high δ⁸⁷Rb (∼0.08 ‰) to the magma source, which is higher than the average δ⁸⁷Rb (–0.02 ± 0.18 ‰) of the Site-1365 AOC. The variation of δ⁸⁷Rb with ¹⁴³Nd/¹⁴⁴Nd_i values of the lavas indicates that AOC-derived aqueous fluids are increasingly mixed with a component with a low δ⁸⁷Rb that most likely represents sediment melts. The estimated δ⁸⁷Rb of sediment melts is about –0.27 ‰, lower than the average δ⁸⁷Rb (–0.11 ± 0.12 ‰) of the Mariana sediments. Thus, our study suggests that Rb isotopes can be fractionated during AOC dehydration and sediment partial melting. Overall, the across-arc Rb isotope variation reflects slab dehydration and melting at different subduction depths. Our findings demonstrate that Rb isotopes are a powerful novel tool for probing deep recycling of subducted slabs. Furthermore, they can distinguish contributions from AOC-derived fluids versus sediment-derived melts to the source of arc magmas.