Rare Earth Element Characteristics and Its Geological Implications for Seawater from Cobalt-Rich Ferromanganese Crust Exploration Contract Area of China

The contents of rare earth elements and yttrium (REY) are determined using ICP-MS spectrometry in 34 seawater samples obtained from various depths from Caiwei Guyot and Weijia Guyot in western Pacific. Based on the data, the vertical profile of ∑REY content of seawater column are established in cobalt-rich ferromanganese crust exploration contract area of China. The contents of rare earth element in seawater increase gradually with the water depth. The ∑REY contents range from 14.0×10-12 to 65.5×10-12, with an average of 31.94×10-12. Both the absolute content and relative content of Y are high, ranging from 6.0×10-12 to 24.1×10-12, with mean (Y/Ho)N value of 1.98, followed by La, with content ranging from 1.8×10-12 to 11.6×10-12. Ce is relatively low, with contents ranging from 2.4×10-12 to 8.8×10-12. And the δCe is from 0.33 to 1.03, with mean value of 0.66. The NASC-normalized REY patterns show left mode, with mean (La/Yb)N value of 0.71, obvious negative Ce anomalies, positive Y anomaly and no obvious Eu anomaly. Aqueous cobalt-rich ferromanganese crust is widely distributed in the study area, whose rare earth elements and other components are derived from seawater. The rare earth content of cobalt-rich crusts is 6-7 orders of magnitude higher than that of seawater, while NASC-normalized REY patterns of cobalt-rich ferromanganese crust show remarkable positive Ce and negative Y anomaly. This coupling relationship indicates that cobalt-rich ferromanganese crust components are selective for the cleaning rare earth elements of sea water, causing seawater rare earth anomaly. Both the phosphate rock developed on the seamounts and the phosphate components in the deep mud of the surrounding basin have high rare earth contents and a rare earth pattern similar to that of seawater. These phenomena indicate that the marine phosphate is not fractionated but inherited the REE patterns of seawater. The unique characteristics of REE patterns of seawater are the result of the balance between supply and consumption. Ferromanganese oxide and marine phosphate are two typical marine autogenic components, which are essential for the formation of REE patterns of seawater. © 2019, Editorial Department of Earth Science. All right reserved.