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 delta Rb-87 of the AOC varies between -0.16 +/- 0.03 parts per thousand to 0.07 +/- 0.02 parts per thousand with a weighted average of -0.02 +/- 0.06 parts per thousand, which is higher than that of fresh mid-ocean ridge basalts (MORB; -0.12 +/- 0.08 parts per thousand). Such Rb isotope fractionation may be attributed to the preferential loss of Rb-85 into seawater during the dissolution of primary phases and the preferential structural incorporation of Rb-87 into secondary phases and preferential Rb-87 adsorption by clays. The delta Rb-87 of the sediments varies from -0.20 +/- 0.07 parts per thousand to -0.03 +/- 0.02 parts per thousand with an average of -0.11 +/- 0.12 parts per thousand, identical to that of the upper continental crust (UCC; -0.14 +/- 0.08 parts per thousand). The correlations observed between delta Rb-87 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 Rb-85 from seawater,could partially account for the elevated delta Rb-87 of seawater (0.14 +/- 0.12 parts per thousand) relative to the Bulk Silicate Earth (BSE;-0.12 +/- 0.06 parts per thousand). For the Mariana lavas, the delta Rb-87 decreases from fore-arc lavas (-0.03 +/- 0.04 parts per thousand to 0.09 +/- 0.02 parts per thousand) to frontal arc lavas (-0.12 +/- 0.04 parts per thousand to -0.01 +/- 0.03 parts per thousand), which are all higher than the average delta Rb-87 of fresh MORB (-0.12 +/- 0.08 parts per thousand). The elevated delta Rb-87 of the lavas reflects the addition of AOC-derived aqueous fluids with a high delta Rb-87 (similar to 0.08 parts per thousand) to the magma source, which is higher than the average delta Rb-87 (-0.02 +/- 0.18 parts per thousand) of the Site-1365 AOC. The variation of delta Rb-87 with Nd-143/Nd-144(i) values of the lavas indicates that AOC-derived aqueous fluids are increasingly mixed with a component with a low delta Rb-87 that most likely represents sediment melts. The estimated delta Rb-87 of sediment melts is about -0.27 parts per thousand, lower than the average delta Rb-87 (-0.11 +/- 0.12 parts per thousand) 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.