Sub-basin scale inhomogeneity of mantle in the South China Sea revealed by magnesium isotopes

The South China Sea (SCS) is the largest extensional basin in the western Pacific and was formed after rifting of the Euro–Asian continental margin. The nature of its underlying mantle remains enigmatic due to the lack of sampling of the seafloor’s igneous crust. The International Ocean Discovery Program Expedition 349 cored seafloor basalts of the southwestern (Site U1433) and eastern (Site U1431) SCS sub-basins. The recovered basalt samples exhibit different source lithologies and geochemistries. The Mg isotopic compositions of seafloor basalts from these sites were investigated to elucidate the origin of this large-scale mantle inhomogeneity. Results indicate that the Site U1431 basalts have a mantle-like average δ26Mg value of –0.27‰ ± 0.06‰ (2SD; n = 10). Together with inhomogeneous Sr–Nd–Pb–Hf isotopic compositions, the Site U1433 basalts have an average δ26Mg value (–0.20‰ ± 0.06‰; 2SD; n = 8) higher than those of the Site U1431 basalts and normal mantle. Their heavier Mg isotopic compositions and low 206Pb/204Pb ratios (~17.7) indicate that the Site U1433 basalts were affected by the re-melting of detached continental-arc lithosphere in the sub-ridge mantle. The coupling of Mg and Sr–Nd isotopes provides robust evidence that the mantle-like δ26Mg values of the Site U1431 basalts resulted from mixing between detached continental arc lithosphere and the nearby Hainan plume, with respective supra- and sub-normal δ26Mg values. From the perspective of Mg isotope, the mantles of the southwestern and eastern sub-basins are compositionally inhomogeneous, with their mantle evolutionary histories being distinct.