Unusual δ26Mg values in oceanic crust basalts from the South China Sea

Whether or not oceanic crust basalts are affected by plate subduction is a hot topic of debate. The South China Sea is one of the largest marginal basins in the western Pacific Ocean and has been surrounded by subduction of the Pacific plate and Indian plate, yet, to date, no study has clearly shown evidence of subduction in the geochemistry of volcanism in the basin due to a lack of sampling of igneous crust basalts on the seafloor. The International Ocean Discovery Program Expedition 349 cored seafloor basalts near the fossil spreading ridges of the eastern (Site U1431) and southwestern (Site U1433 and U1434) subbasins in the South China Sea. The recovered basalt samples indicated a pyroxenite-bearing peridotite mantle source. Here, we report Mg isotopic data from 14 of these oceanic crust basalt samples. The δ²⁶Mg values of most basalts from the three drill holes were higher (up to −0.10‰) than that of the average mantle (−0.25‰). The lack of correlations of δ²⁶Mg with geochemical indices of magmatic processes (e.g., MgO, CaO/Al₂O₃, La/Sm, Nb/Zr) suggests that crystal fractionation and partial melting had insignificant effects on the Mg isotopic compositions of the South China Sea basalts. Thus, the variations in Mg isotopes were inherited from their mantle sources. Considering the highly varied Ce/Pb ratios and elevated ⁸⁷Sr/⁸⁶Sr values but mantle-like ¹⁴³Nd/¹⁴⁴Nd values, we propose that the varied δ²⁶Mg values were likely caused by metasomatism of subduction-released fluids. The coupling of Mg and Sr-Fe isotopes provides robust evidence that the high-δ²⁶Mg values of the South China Sea basalts resulted from mixing among pyroxenite-bearing peridotite mantle, the nearby Hainan plume materials, and subducting serpentinite-released fluids. Therefore, these Mg isotopes suggest that the mantle source of the South China Sea basalts was influenced by subducted materials, providing further evidence of the initial expansion, formation, and evolution of the South China Sea during plate subduction.