Compositional variation and genesis of ferromanganese crusts of the Afanasiy—Nikitin Seamount, Equatorial Indian Ocean

摘要

Eight ferromanganese crusts (Fe-Mn crusts) with igneous and sedimentary substrates collected at different water depths from the Afanasiy-Nikitin Seamount are studied for their bulk major, minor and rare earth element composition. The Mn/Fe ratios < 1.5 indicate the hydrogenetic accretion of the Fe-Mn hydroxides. These Fe-Mn crusts are enriched in Co (up to 0.9%, average ～0.5%) and Ce. The Ce-content is the highest reported so far (up to 3763 ppm, average ～2250 ppm) for global ocean seamount Fe-Mn crusts. In spite of general similarity in the range of major, minor, and strictly trivalent rare earth element composition, the dissimilarity between the present Fe-Mn crusts and the Pacific seamount Fe-Mn crusts in Co and Ce associations with major mineral phases indicates inter-oceanic heterogeneity and region-specific conditions responsible for their enrichment. The decrease in Ce-anomaly (from ～8 to ～1.5) with increasing water depth (from ～1.7 km to ～3.2 km) might suggest that the modern intermediate depth low oxygen layer was shifted and sustained at a deeper depth for a long period in the past.