Rapid transition in the mantle composition beneath the nascent mid-ocean ridge in the northern margin of the South China Sea

The geochemical characteristics of the mantle during continental breakup and the initial spreading of marginal sea basins remain poorly understood. Mid-ocean ridge basalt (MORB) samples from Hole U1500B and Hole U1503A in the northern margin of the South China Sea (N-SCS), obtained during IODP Expeditions 367 and 368X, provide crucial insights into mantle evolution of the nascent oceanic basin subsequent to continental breakup. This study analyzes major and trace elements, as well as Mo–Sr–Nd–Hf isotopes, in these MORB samples to explore variations in their mantle sources. MORB samples from Hole U1500B, closer to the continent, exhibit higher ⁸⁷Sr/⁸⁶Sr ratios, along with lower ε_Nd and ε_Hf values compared to the depleted mantle. Additionally, their δ^98/95Mo values correlate positively with Mo/Ce and Mo/Nb ratios, indicating the influence of recycled oceanic crust (ROC) melts in the mantle source. In contrast, MORB samples from Hole U1503A, nearer to the oldest fossil ridge, show a broader range of δ^98/95Mo values, reflecting varying extents of contribution of terrigenous sediment melts alongside ROC melts. The differing trace element and Mo–Sr–Nd–Hf isotope compositions of MORBs from the two sites highlight a significant transition in the mantle beneath the nascent mid-ocean ridge of the SCS. During the initial stages of seafloor spreading in the SCS, the mantle source experienced continuous replenishment from enriched components derived from shallow recycling of metasomatized SCLM. This process significantly contributed to the rapid transition from continental rifting to seafloor spreading in the SCS. The enrichment of the asthenospheric mantle, likely induced by previous subduction processes, facilitated rapid rifting and extensive magmatism in the SCS, distinguishing it from magma-poor margin basins. This research provides critical geochemical insights into the mantle evolution beneath nascent mid-ocean ridges, enhancing our understanding of the early processes in marginal sea basins.