Late Cenozoic Hainan OIB-like basalts in SE Asia record the interaction between a deep mantle plume and stagnant slab

Whether Late Cenozoic basalts in SE Asia record plume-slab interactions within the mantle transition zone (MTZ) remains unresolved. To investigate this, we report new data on He isotopes measured in olivine, together with whole-rock major and trace element compositions and Sr–Nd–Hf–Fe–Mo isotope ratios for the Hainan basalts. The Hainan basalts have high MgO and low SiO₂ contents and display trace-element signatures of ocean island basalts (OIBs), such as the enrichments in light rare earth elements and high-field-strength elements. Meanwhile, these basalts are characterized by the depleted Sr–Nd–Hf isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0.7031–0.7043; εNd = +3.3 to +5.5; εHf = +6.8 to +10.3) and yield mid-ocean ridge basalts (MORBs)-like ³He/⁴He ratios (7.0–9.2 Ra). The geochemical characteristics of Hainan basalts illuminate that their mantle source includes depleted and enriched components. Compared with MORBs derived from upper mantle peridotite, the Hainan basalts have higher fractionation-corrected δ⁵⁶Fe values (+0.07 ‰ to +0.17 ‰) and variable δ⁹⁸Mo values (−0.38 ‰ to −0.07 ‰), indicating their source lithology was a pyroxenite formed by the interaction between ambient peridotite and recycled carbonate-bearing oceanic crust. Integrating geophysical constraints, we attribute the depleted and enriched signatures in the mantle source of Hainan basalts to a peridotitic FOZO plume and Pacific carbonate-bearing oceanic crust entrained from the MTZ, respectively. During the Late Cenozoic, an ascending mantle plume from the deep mantle reached the MTZ and reacted with the stagnant Pacific oceanic slab to form pyroxenite. Given the variable thickness and presence of a stagnant slab in the MTZ, the deep mantle plume bifurcated, leading to the formation of secondary mantle plumes. The upwelling secondary mantle plumes from the MTZ transported newly formed mantle pyroxenite into the upper mantle, where pyroxenite underwent partial melting to form the Hainan basalts and other basaltic rocks in SE Asia. Therefore, our model of the interplay between a deep mantle plume and a stagnant slab within the MTZ might be applied to explain the formation of Late Cenozoic basalts in this region. We further propose that MORB-like ³He/⁴He values in basaltic rocks (e.g., Hainan basalts) cannot be simply used as a robust indicator to negate the possibility that they may be sourced from a deep mantle plume. Moreover, the integration of radiogenic (Sr–Nd–Hf) and stable (Fe–Mo) isotope data for basaltic rocks is a robust method to reveal the presence of pyroxenite in the mantle source of basalts and shed light on its genesis.