The rare earth element geochemistry of surface sediments in four transects in the South China Sea and its geological significance

摘要

Seventy-five samples of surface sediment were collected from four transects in the South China Sea (SCS) for analyses of rare earth element (REE) geochemistry. Total REE abundance (Sigma REE) in the SCS varies from 45 to 195 ppm (average 128 ppm), lower than that of the upper continental crust. The spatial distribution of REE is strongly dependent upon the geographical location and depositional environment. Low Sigma REE concentrations are observed on the continental slope (e.g., Xisha and Nansha islands), while high Sigma REE concentrations are present in the deep basin. Sediments on the slope are also characterized by lower LREE/HREE, (La/Yb)(N), Eu/Eu*, and Ce/Ce* ratios relative to deep-basin sediments. There is a positive correlation between Al2O3 and Sigma REE, and a negative correlation between CaO and Sigma REE. Conversely, the relatively insignificant Fe-Mn oxide accumulation in the SCS suggests that manganese nodules do not make a significant contribution to the bulk REE composition of sediments. Therefore, the sediment REE geochemistry is mainly controlled by terrigenous and biogenic sources. Biogenic carbonate has a diluting effect on Sigma REE concentrations, but a significant influence on LREE/HREE, Ce/Ce*, and (La/Yb) N values is found only in sediments with especially high biogenic carbonate levels. The REE geochemistry of SCS surface sediments indicates that their source is mainly post-Archean felsic rocks. In the eastern sub-basin, the relatively low LREE/HREE ratios and high Eu/Eu* values of the sediments indicate that significant terrigenous materials are derived from young volcanic rocks (e.g., rocks in the Luzon arc).