Petrographic and electron microprobe investigation of some deep- and shallow-water manganese nodules

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Abstract
Manganese-rich nodules from the flanks of the Carlsberg Ridge in the Indian Ocean, the Jervis Inlet on the Pacific Coast of Canada, and Loch Fyne, western Scotland have been examined by petrographic, X-ray diffraction, bulk chemical, and electron micro-probe methods. The deep-water nodule from the Carlsberg Ridge is typical of oceanic nodules and is characterised by well developed concentric structures, a high content of Cu, Ni, Co, and Pb and a low Mn/Fe ratio (1.3). The trace elements show associations found in previous studies, although they are less regular. The shallow-water nodules (Jervis Inlet and Loch Fyne) do not show the concentric structures to the same extent, are much poorer in trace elements, and have much higher Mn/Fe ratios (8.8 and 14.9, respectively) than the Carlsberg Ridge nodule. δ-MnO2 is the principal mineral in the deep-sea nodule but 10Å manganite (todorokite) is the principal phase in the shallow-water nodules. Angular detrital quartz and feldspar are common in the shallow-water types but much rarer in the deep-sea nodule. The differences between the nodules are attributed to differences in nucleation and growth rates attendant upon the different concentrations of Mn in the water from which the nodules formed. Other mechanisms of growth are briefly reviewed.