CONTROLS ON THE CONCENTRATION OF CO IN FERROMANGANESE CRUSTS FROM THE MAGELLAN SEAMOUNTS, WEST PACIFIC

Ferromanganese crusts are known as a potential cobalt resource in the deep ocean, and mainly occur on the seamounts of the northwest Pacific, such as the Magellan seamounts. In order to clarify the controls on Co concentration of hydrogenetic unphosphatized ferromanganese crusts, an equation was deduced based on Fick’s First Law:

Co(%) = MnO₂(%)∙D_sw∙ (C_sw/δ) ∙ (z/GR) ∙S_sp, and eight potential controls are gleaned from this equation, including dilution, diffusivity of Co ions in seawater (D_sw), temperature which controls the D_sw, Co ion concentration in seawater (C_sw), the diffusion distance of Co ions near the interface of seawater and Fe-Mn crusts (δ), thickness of one molecular layer (z), growth rate (GR), and specific surface area of Fe-Mn crusts (S_sp). In order to constrain the value of δ, we determined the Co concentrations, growth rates, and specific surface area of the outermost layer of Fe-Mn crusts from the Caiwei seamount of the Magellan seamounts, calculated diffusivity of Co²⁺, and analyzed Co concentration in the ambient seawater. The δ for the Fe-Mn crusts from Caiwei seamount was determined to be 69-279 μm. According to the equation established in this study, the trend of decreasing Co concentration in Fe-Mn crusts with increasing water depth is controlled mainly by dilution effects and to a lesser extent by seawater Co ion concentration, temperature of seawater, and consequently the diffusivity of Co ions in seawater.