Compositional heterogeneity within the mantle beneath the South China Sea

The recycling of crustal materials is widely recognized as the primary mechanism driving compositional heterogeneity within the mantle. Seismic tomography has revealed stagnant slabs in the mantle transition zone (MTZ) beneath the South China Sea (SCS), indicating the significant presence of recycled oceanic crust (ROC) in its upper mantle. However, the extent to which recycled crustal materials contribute to the mantle source of mid-ocean ridge basalts (MORBs) remains a subject of debate, as radiogenic isotopes alone yield ambiguous insights. Here, we present comprehensive data on whole-rock major element, trace element, and Mo–Sr–Nd–Hf–Pb isotopic compositions for MORB samples from International Ocean Discovery Program Expedition 349 sites U1431E and U1433B in the eastern (ESB) and southwestern (SWB) subbasins, respectively, of the SCS. The δ98/95Mo values of the SCS MORBs exhibit a significant range (from −0.80‰ to +0.05‰), in contrast to the restricted composition observed in MORBs (δ98/95Mo = −0.19‰ ± 0.01‰). Specifically, the ESB MORBs display extremely light Mo isotopic compositions with Nd–Hf isotopic compositions similar to those of Pacific MORB, whereas the SWB MORBs show slightly higher δ98/95Mo values and depleted Nd–Hf isotopic compositions. The subbasin-scale mantle heterogeneity in the SCS can be best explained by varying degrees of interaction between a mantle plume and stagnant slabs in the mantle transition zone. The rigid stagnant slab in the mantle transition zone beneath the SWB largely impeded the upwelling of the mantle plume, whereas the slab beneath the ESB was disrupted by the plume and subsequently transported into the upper mantle. Furthermore, the SCS MORBs exhibit enriched Sr and Pb isotopes, indicating the incorporation of terrigenous sediment components in the upper mantle beneath the SCS. Continuous subduction preceding seafloor spreading has facilitated the substantial incorporation of subducted crustal materials into the upper mantle or as stagnant slabs within the MTZ of the SCS. The SCS underwent a rapid transition from continental rifting to seafloor spreading, enabling significant preservation of ROC in the upper mantle or as stagnant slabs in the MTZ. Therefore, mantle recycling in marginal sea basins exhibits distinct characteristics compared to that occurring in open oceans.