Geochemistry and sources of rare earth elements in cobalt-rich crusts from the Caiwei and Xufu seamounts, West Pacific Ocean

Using the testing methods of XRD, ICP-OES and ICP-MS, the mineral composition, major and minor elements contents of cobalt-rich crusts collecting from the Caiwei Guyot and Xufu Guyot in the West Pacific Ocean have been determined. Based on the data, we discussed in this paper the geochemical characteristics and material sources of the REE. It is observed that the cobalt-rich crust is dominated by the crystalline mineral of vernadites, accompanied by the auxiliary minerals of quartz, plagioclase, potassium feldspar and carbon fluoride apatite. Amorphous ferric minerals also occur in certain amounts in the crusts. In terms of chemical composition, Mn contents change within 16.20%～26.62%, and Fe contents 8.56%～18.19%, which are the highest among the others. Phosphatization is observed in the old crust layers. REE are enriched in the cobalt-rich crusts. LREE are higher than HREE. REE contents are as high as 1 842～2 854 µg/g, in which Ce accounted for nearly 50%. Moreover, it is found that REE contents in the old layers are higher than that in the new layers, and it is believed that phosphatization in the old layers might play an active role in the REE distribution pattern. And REE diagrams show that there are positive Ce anomalies but no Eu anomalies, so Ce is relatively enriched. Meanwhile, REE show positive correlation with Ce, Y, CaO, P₂O₅, Ba and Sr, negative correlation with Fe, Al₂O₃, Na₂O, K₂O, MgO, TiO₂, Pb and V, but no correlation with Mn, Co, Cu, Ni and Zn. By the way, the elements of the cobalt-rich crusts may be classified into four groups by clustering analysis. ①Phosphate group including REE, Ce, Y, CaO, P₂O₅, Ba and Sr. ② Mn group including Mn, Co, Cu, Ni and Zn. ③ Fe group including Fe, TiO₂, Pb and V. ④ Detritus group including Al₂O₃, Na₂O, K₂O and MgO. In conclusion, cobalt-rich crusts from the Caiwei Guyot and Xufu Guyot of the West Pacific Ocean are hydrogenetic in origin, and REE are precipitated together with phosphate group in the seawater, that caused the enrichment of REE in the cobalt-rich crusts.