Heavy rubidium isotope compositions in intraplate basalts trace recycled carbonates in the deep mantle

Deep carbon cycling modulates Earth’s long-term climate and planetary habitability, yet identifying deeply subducted carbon and discriminating among carbonate-transporting agents remain challenging. Because marine carbonates commonly host authigenic silicates that can concentrate Rb and preferentially incorporate heavy Rb isotope ( 87 Rb), δ 87 Rb may provide a potential tracer of deeply recycled authigenic silicate-bearing carbonates. Here, we analyzed δ 87 Rb of authigenic silicate-bearing sediments from DSDP Site 294/295 and Cenozoic intraplate basalts in eastern China. Site 294/295 sediments have δ 87 Rb ranging from −0.14 ± 0.04‰ to 0.28 ± 0.05‰ (2SD), suggesting possible preferential enrichment of 87 Rb in authigenic silicate phases associated with marine carbonates. The δ 87 Rb of the basalts range from −0.15 ± 0.01‰ to 0.29 ± 0.03‰ with an average of 0.03 ± 0.22‰ (2SD), showing an overall shift toward higher δ 87 Rb relative to typical mantle/MORB values (−0.12 ± 0.08‰). Covariations between δ 87 Rb and δ 26 Mg or δ 66 Zn further show that the high-δ 87 Rb component of the basalts has low δ 26 Mg and high δ 66 Zn, which is consistent with contributions from authigenic silicate-bearing carbonates. Combined Rb-Sr-Nd isotopic systematics identify three source components: depleted MORB mantle (DMM), altered oceanic crust (AOC), and subducted sediments. Monte Carlo estimates based on CO 2 /Ba and CO 2 /Nb ratios, integrated with δ 87 Rb data, indicate that partial melts derived from the subducted AOC and sediments contain elevated CO 2 contents (5–8 wt.%). This implies that both carbonated AOC and subducted sediments can deliver marine carbonates into the deep mantle. Notably, although partial melts from carbonated AOC display elevated δ 87 Rb, those of carbonated sediments have MORB-like δ 87 Rb. This decoupling between δ 87 Rb and CO 2 signatures underscores the need to integrate isotopic and volatile proxies when quantifying deep-carbon recycling. Our results suggest that, when combined with Sr-Nd-Mg-Zn isotopes, trace elements, and CO₂ constraints, δ 87 Rb can provide additional constraints on deep-carbon recycling and may help distinguish different carbonate-transporting agents, including carbonated AOC and subducted sediments.