Characterization of ferromanganese crusts from the Central and South West Indian ridges: Evidence for hydrothermal activity

Ferromanganese (FeMn) crusts with oxides ranging from a thin coating to a maximum of 5 cm thickness were recovered from the South West Indian Ridge (SWIR) and Central Indian Ridge (CIR) segments. The substrate for the oxides are altered mafic, ultramafic, and serpentinite rocks. Twenty FeMn crusts were studied to determine their mineralogical compositions, microstructures, and chemical compositions. The crusts display rough botryoidal or smooth surface textures. Todorokite, hematite, goethite, and nontronite are the major minerals followed by vernadite, magnetite, and maghemite. Electron micrographs show framboids of FeMn oxides, fossil worms, foraminifera nuclei, and euhedral mineral crystals. The oxides have higher FeO wt% (27-47; average similar to 43) than MnO (13-45, average similar to 30), and the average wt% of SiO2 is similar to 40, Al2O3 similar to 3, MgO similar to 2, CaO similar to 4, Na2O similar to 0.4, K2O, NiO similar to 0.5, CuO similar to 0.3, CoO similar to 0.8, and P2O5 similar to 1.5. Our studies indicate a hydrothermal origin for the crusts and this fact is corroborated by low MnO/FeO ratio (<= 1) and occurrence of framboids and hydrothermal minerals. Negative Ce anomaly and the plot of Mn-Fe-10x(Ni + Co + Cu) indicate characteristic signatures of hydrogenous-hydrothermal origin for most of the studied FeMn crusts.