Element geochemistry and genesis of cobalt-rich crust on the Line Seamount of the Central Pacific

Cobalt rich crust is a kind of marine mineral resource with high economic value and significant development prospect. The external morphology, internal structure and geochemical characteristics of the crust recorded the evolution of paleoceanographic environment. In this paper, the genesis or forming process of the cobalt rich crusts on the Line Seamount of the Central Pacific Ocean is studied upon the basis of microstructure and element geochemistry. The cobalt rich crust sample is composed of five layers from top to bottom. The top layer is relatively dense, dark brown in color with smooth surface and little impurities. It has a cedar-like stromatolite structure, indicating a strong hydrodynamic environment; The second layer has similar macroscopic characteristics with the first, characterized by columnar and carrot shaped stromatolite structure, which indicates the strongest hydrodynamic deposition; The third and fourth layers are yellowish brown in color, with patchy structures containing certain amount of sediment impurities suggesting weak hydrodynamics; The fifth is a phosphatic layer with bright black asphalt luster, and rich in foraminifera fossils. Meanwhile, the fifth layer is rich in Sr, P, Ca and other elements, indicating obvious involvement of marine organisms and the weakest hydrodynamics. The geochemical characteristics of Ce anomaly and high Co, high Mn and low Fe indicate that the cobalt rich crust was formed in an oxidizing environment for a long time. The geochemical characteristics of Mn, Fe, Co and other elements reveal that the marine hydrodynamic and oxidizability of the environment gradually increases starting from the fifth on bottom layer up to the second layer, but decreases at the first or top layer. According to the distribution coefficients of the elements, positive anomaly of Ce, positive anomaly of Ho, negative anomaly of Y and the triangle genesis diagram, it is inferred that the cobalt-rich crust of the Line Seamount is formed in seawater environment under the influence of the Antarctic bottom current. Based on the geochronological data of the cobalt rich crusts, the growth process of the cobalt rich crusts is recovered in the paper, and three phosphorylation events are recognized on the element profile.