Formation and growth of ferromanganese oxides on the Nazca plate

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Abstract
Marine ferromanganese oxides form four major types of deposits: hydrothermal crust, ferromanganese coatings on basalt, ferromanganese nodules, and a mixture of micronodules and other dispersed oxyhydroxides within sediments. Hydrothermal crusts grow only near active marine hydrothermal systems that cool newly emplaced basaltic crust. The crusts are characterized by rapid growth rates, extreme fractionation of Mn from Fe, low accessory-element concentrations, and well-crystallized Mn minerals. Ferromanganese coatings on basalt can receive an Fe-rich component from a hydrothermal source, but are mainly composed of ferromanganese oxides grown by direct preceipitation from sea water (hydrogenous formation). They have δ-Mn02 mineralogy, have almost equal Mn and Fe abundances, are relatively enriched in the rare-earth elements, are highly enriched in Ce and Co, and have relatively low Cu, Ni, and Zn abundances. Coatings with a large hydrothermal component are more enriched in Fe and generally have lower trace-element contents. Nodule and micronodule compositional variations resulting from different sources of transition metals may be further modified by diagenetic reactions within the sediments. The most extreme type of diagenesis is preferential reductive mobilization of Mn within the sediment column by oxidation of organic carbon and subsequent Mn diffusion to the sea-water-sediment interface. Ferromanganese oxides formed or altered by this process lie beneath highly productive equatorial waters and near the South American continent, because supply of organic carbon to marine sediments comes primarily from biological productivity in the surface waters. Ferromanganese oxides modified in this manner are characterized by relatively pure and well-crystallized Mn oxides, geneally the 7 Ǻ mineral birnessite, and by rapid growth rates. Mn/Fe weight ratios are 5 or greater, and accessory-element contents are low. The fourth type of ferromanganese deposit is enriched in Mn relative to hydrogenous precipitates and is found in the Bauer Deep and other areas along the fringes of the most highly productive regions. There sediments are not reducing enough to remobilize Mn. Oxic diagenetic reactions, such as Fesmectite formation from biogenic opal, fractionate Fe from ferromanganese hydroxyoxides. The released Mn precipitates as nodules and micronodules. Fe and Mn that consitute by far the dominant components of the ferromanganese deposits have only two ultimate sources: runoff from the continents and hydrothermal interactions between sea water and basalts formed on the mid-ocean ridges. Sea water acts as a reservoir and mixing medium for these two sources as currents carry the introduced metals far from their input point. Hydrothermal input of Mn introduces most of this element to the Nazca plate, although terrigenous input probably dominates worldwide.

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