The occurrence phases and enrichment mechanism of rare earth elements in cobalt-rich crusts from Marcus-Wake Seamounts

To explore the occurrence phases and enrichment mechanism of rare earth elements (REEs) in cobalt-rich crusts, this study analyzes the mineral composition and REE contents of the samples from Marcus-Wake Seamounts by XRD, ICP-OES and ICP-MS. The results show that, (1) the cobalt-rich crusts contain the major crystalline mineral (vernadite), the secondary minerals (quartz, plagioclase and carbonate fluorapatite), and a large amount of amorphous ferric oxyhydroxides (FeOOH). (2) The cobalt-rich crusts contains higher Mn (10.83% to 28.76%) and Fe (6.14% to 18.86%) relative to other elements, and are enriched in REEs, with total REE contents of 1 563-3 238 mu g/g and Ce contents of 790-1 722 mu g/g. Rare earth element contents of the old crusts are higher than those of the new crusts. Moreover, the non-phosphatized crusts have positive Ce and negative Y anomalies, and yet the phosphatized crusts have positive Ce and positive Y anomalies, indicating that cobalt-rich crusts is hydrogenetic and REEs mainly come from seawater. (3) Analytical data also show that the occurrence phases of elements in cobalt-rich crusts are closely related to their mineral phases. In the non-phosphatized crusts, REEs are adsorbed by colloidal particles into the crusts (about 67% of REEs in the Fe oxide phase, and about 17% of REEs in the Mn oxide phase). In contrast, in the phosphatized crusts (affected by the phosphatization), REEs may combine with phosphate to form rare earth phosphate minerals, and about 64% of REEs are enriched in the residual phase containing carbonate fluorapatite, but correspondingly the influence of Fe and Mn oxide phases on REEs enrichment is greatly reduced. In addition, the oxidizing environment of seawater, high marine productivity, phosphatization, and slow growth rate can promote the REE enrichment. This study provides a reference for the metallogenesis of cobalt-rich crusts in the Pacific.