The major element relationships in ferromanganese (Fe-Mn) crusts from Afanasiy-Nikitin seamount (ANS), eastern equatorial Indian Ocean, appear to be atypical. High positive correlations (r = 0.99) between Mn/Co and Fe/Co ratios, and lack of correlation of those ratios with Co, Ce, and Ce/Co, indicate that the ANS Fe-Mn crusts are distinct from Pacific seamount Fe-Mn crusts, and reflect region-specific chemical characteristics. The platinum group elements (PGE: Ir, Ru, Rh, Pt, and Pd) and Au in ANS Fe-Mn crusts are derived from seawater and are mainly of terrestrial origin, with a minor cosmogenic component. The Ru/Rh (0.5-2) and Pt/Ru ratios (7-28) are closely comparable to ratios in continental basalts, whereas Pd/Ir ratios exhibit values (< 2) similar to CI-chondrite (∼ 1). The chondrite-normalized PGE patterns are similar to those of igneous rocks, except that Pd is relatively depleted. The water depth of Fe-Mn crust formation appears to have a first-order control on both major element and PGE enrichments. These relationships are defined statistically by significant (r > 0.75) correlations between water depth and Mn/Co, Fe/Co, Ce/Co, Co, and the PGEs. Fractionation of the PGE-Au from seawater during colloidal precipitation of the major-oxide phases is indicated by well-defined linear positive correlations (r > 0.8) of Co and Ce with Ir, Ru, Rh, and Pt; Au/Co with Mn/Co; and by weak or no correlations of Pd with water depth, Co-normalized major-element ratios, and with the other PGE (r < 0.5). The strong enrichment of Pt (up to 1 ppm) relative to the other PGE and its positive correlations with Ce and Co demonstrate a common link for the high concentrations of all three elements, which likely involves an oxidation reaction on the Mn-oxide and Fe-oxyhydroxide surfaces. The documented fractionation of PGE-Au and their positive association with redox sensitive Co and Ce may have applications in reconstructing past-ocean redox conditions and water masses.

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Co-rich Mn crusts from four different locations of the world ocean have been studied to understand the role of dissolved oxygen of the ambient seawater in the formation of Co-rich Mn crusts. WOCE (World Ocean Circulation Experiment) oxygen profiles of modern seawater in the Equatorial North Pacific Ocean, Equatorial South Indian Ocean and the North East Atlantic Ocean have been evaluated with respect to the occurrence of Co-rich Mn crusts at depths ranging from 1500 to 3200 m. The oxygen content at these depths varied from 90-240 mmol/kg. The oxygen minimum zone (OMZ), with oxygen contents in the range~45-100 mmol/kg, is located in the depth range 800-900 m in these regions. The age of the ocean crust on which seamounts formed is in the range 80.3-180 Ma. Profiles of the oxygen contents of seawater with depth in the oceans are shown to be extremely useful in establishing the optimum conditions for the formation of Co-rich Mn crusts. The use of WOCE oxygen profiles to study geochemical processes in the oceans is highly recommended.