Rubidium (Rb) isotope ratio provides a potential means of tracing material recycling involved in subduction zone processes. However, the subduction-related geochemical behavior of Rb isotopes remains enigmatic. We first analyzed Rb isotope ratios of the rear-arc, back-arc, and post-arc basalts (26-2.9 Ma) from Eastern China to investigate subduction zone geochemical processes involving Rb. These basalts are petrogenesically linked and are both related to the subduction of the Paleo-Pacific plate, recording slab-mantle interactions from rear-arc to post-arc. In parallel, subducting sediments from DSDP Sites 294/295/296 that are outside the Ryukyu trench from which the slab had been subducted to have produced the basalts studied were also analyzed. Results show that sediments exhibit homogeneous δ⁸⁷Rb varying from -0.25 ± 0.07 ‰ to -0.11 ± 0.00 ‰ (with an exception of 0.28 ± 0.05‰) with an average of -0.12 ± 0.35‰ similar to that of the UCC (-0.14 ± 0.08‰), suggesting limited Rb isotope fractionation during the seawater alteration of seafloor sediments. The δ⁸⁷Rb of the basalts from Eastern China show clear variations in space, increasing from rear-arc basalts (-0.72‰ to -0.03‰) through back-arc basalts (-0.24‰ to 0.07‰) to post-arc basalts (-0.15‰ to 0.29‰) towards intraplate. Notably, the δ⁸⁷Rb of the rear-arc basalts correlate strongly with Sr-Nd-Pb isotope ratios, suggesting that their light Rb signature arises from the source modification by sediment-derived melts. The constrainedδ⁸⁷Rb of sediment melts is <-0.75‰, substantially lower than those of subducting sediments from DSDP Sites 294/295/296 (-0.25‰ to 0.28‰). This indicates that Rb isotopes have been fractionated during the partial melting of subducted sediments beneath the sub-arc with ⁸⁵Rb being preferentially lost. The back-arc basalts with slightly high δ⁸⁷Rb have OIB-type characteristics, recording the source contributions from residual sediments and igneous oceanic crusts. Well-developed negative correlations between δ⁸⁷Rb with ⁸⁷Sr/⁸⁶Sr_i, Ba/Th, Ba/Nb, Ti/Ti*, Sr/Sr*, and Sr/Nd suggest that melts derived from the partial melting of subducted sediments and igneous oceanic crusts both have relatively low δ⁸⁷Rb. Residual sediments together with the igneous oceanic crusts had further lost their ⁸⁵Rb at back-arc as the slab subducted. The high δ⁸⁷Rb observed in the post-arc intraplate basalts originates from the mixing of three discrete endmembers, namely MORB and melts derived from residual sediments and eclogites, respectively. Slightly elevated δ⁸⁷Rb of sediment-derived melts (-0.04‰) results from the third melting of sediments left behind by the first two melting under the sub-arc and back-arc where ⁸⁷Rb had been preferentially retained. Significantly elevated δ⁸⁷Rb of eclogite-derived melts (>0.3‰) is a result of the second melting of igneous oceanic crusts left behind by the first melting under the back-arc where ⁸⁷Rb had been preferentially retained. This study indicates that Rb isotopes are fractionated during subduction-related processes. The systematic Rb isotope variation of the subduction-related basalts from Eastern China thus documents the slab-mantle interactions from sub-arc to intraplate during the oceanic slab subduction. Our study highlighted that recycled crustal materials had been melted and dehydrated at least twice before being involved into the mantle source of intraplate basalts. Rb isotopes are a potential means of tracing material recycling involved in subduction zone processes.

Rb isotope; Sediments melting; Isotope fractionation; slab-mantle interactions