Mantle evolution of the nascent oceanic basin subsequent to continental breakup constrained by Mo–Sr–Nd–Hf isotopes in MORBs from the northern margin of the South China Sea

Abstract

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 Holes U1500B and U1503A in the northern margin of the South China Sea (N-SCS), obtained during International Ocean Discovery Program (IODP) Expeditions 367 and 368X, provide crucial insights into mantle evolution of the nascent oceanic basin subsequent to continental breakup. MORB samples from Hole U1500B, closer to the continent-ocean transition zone, 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) in the mantle source. In contrast, MORB samples from Hole U1503A, nearer to the oldest fossil ridge of the steady-state ocean, show a broader range of δ^98/95Mo values, higher ⁸⁷Sr/⁸⁶Sr ratios, and lower ε_Nd and ε_Hf values compared to those from Hole U1500B, due to varying extents of subcontinental lithospheric mantle (SCLM) metasomatized by terrigenous sediment input into the mantle source. The distinct trace element and Mo–Sr–Nd–Hf isotope compositions of MORBs from the two sites in the N-SCS highlight a significant transition in the mantle source of the nascent oceanic basin subsequent to continental breakup. During the initial stages of seafloor spreading in the SCS, the mantle source beneath nascent mid-ocean ridges may have been replenished with enriched components, likely derived from subduction-related processes since the Mesozoic. This study offers critical geochemical insights into the evolution of the mantle in nascent oceanic basins. The prolonged subduction led to substantial amounts of enriched components into the mantle, potentially lowering its solidus temperature, thereby facilitating rapid lithospheric rupture and extensive magmatism during the early development of the SCS.