Rare earth elements in cold seep carbonates from the southwestern Dongsha area, northern South China Sea

摘要

The circulation of methane-rich fluids at cold seeps often leads to the precipitation of seep carbonates close to the seafloor along continental margins, which can be used as records of past fluid seepage. Rare earth element (REE) concentrations in seep carbonates have been used to trace fluid sources and provide information on associated biogeochemical processes at cold seeps. The REE concentrations of a series of carbonates collected from cold seeps in the southwestern Dongsha area of the northern South China Sea are analyzed in this study. The total REE contents (Sigma REE) of the seep carbonates analyzed show a wide variation from 17 ppm to 523 ppm with an average Sigma REE value of 54 ppm, which are higher than the carbonates from other cold seep areas. A positive correlation between Fe-Mn content and Sigma REE was observed. These results suggest that the seep carbonates of this study were primarily controlled by the methane-derived fluid from which they precipitated. The Fe-rich dolomite and siderite, which are the main components of the carbonates, are responsible for the enrichment of the REE. A slight positive Ce anomaly observed in the shale-normalized REE patterns of the studied seep carbonates suggests that they formed in anoxic conditions, and the correlations between Ce/Ce* and La-N/Sm-N, Ce/Ce* and Dy-N/Sm-N, Ce/Ce* and Sigma REE further reveal that the REE characteristics of most seep carbonate samples preserve the original redox conditions in which they precipitated and late diagenesis has had little effect on the REE. However, the REE characteristics of sub-samples DS2-2B, DS1-6A and DS1-7A are very different from those of the other sub-samples, indicating a greater impact of late diagenesis and post-oxidation favored REE enrichment. (C) 2014 Elsevier Ltd. All rights reserved.