Unusual 826Mg values in oceanic crust basalts from the South China Sea

Whether or not oceanic crust basalts are af-fected by plate subduction is a hot topic of de-bate. 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 subduc-tion 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 spread-ing ridges of the eastern (Site U1431) and southwestern (Site U1433 and U1434) sub -basins 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 626Mg values of most ba-salts from the three drill holes were higher (up to -0.10%0) than that of the average mantle (-0.25%0). The lack of correlations of 626Mg with geochemical indices of magmatic pro-cesses (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 var-ied 626Mg values were likely caused by meta-somatism of subduction-released fluids. The coupling of Mg and Sr-Fe isotopes provides robust evidence that the high-626Mg 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, pro-viding further evidence of the initial expan-sion, formation, and evolution of the South China Sea during plate subduction.