Hydrothermal Iron and Manganese Crusts from the Pitcairn Hotspot Region

Abstract

Submarine iron and manganese deposits have a widespread occurrence in the oceanic environment. Genetically they can be subdivided into three discrete types (Boström 1983; Usui and Terashima 1997): (1) hydrogenetic, (2) diagenetic, and (3) hydrothermal. Hydrogenetic deposits occur as crusts on seamounts and other volcanic outcrops and as nodules on abyssal sediments via the direct precipitation of ironmanganese oxides and hydroxides from seawater (Koschinsky and Halbach 1995). Since these oxides and hydroxides have high adsorption capabilities, hydrogenetic crusts are characterized by relatively high trace element contents (e.g., Pb, Co, Ni) and slow growth rates (on the order of mm Ma−1; Segl et al. 1989). Mineralogically, they are composed of vernadite (Fe-rich δ-MnO2) and X-ray amorphous iron oxyhydroxides (δ-FeOOH) (Hein et al. 1999). The growth and the composition of diagenetic iron-manganese nodules are controlled by diagenetic element supply from the sediments. These nodules are characterized by high growth rates (on the order of 10–200 mm ka−1) and high Mn/Fe ratios as well as low trace element contents (Reyss 1982).The third type of deposit, the hydrothermal iron-manganese crust, is ubiquitous along Mid-Oceanic Ridges and back arc spreading centers. They are characterized by high growth rates (cm ka−1) and low trace element content. Their origin is closely related to the emanation of metal-rich hydrothermal fluids. These fluids are the result of hydrothermal convection cells that are fueled by the heat of a subsurface magma.