Minerals Report - International Seabed Authority

4. Iron-Manganese Crust Formation
Even though Fe-Mn crusts form by hydrogenetic precipitation, the
exact mechanisms of metal enrichments in the water column and at the crust
surface are poorly understood. The ultimate sources of metals to the oceans
are river and eolian input, hydrothermal input, weathering of ocean-floor
basalts, release of metals from sediments, and extraterrestrial input. Elements
in seawater may occur in their elemental form or as inorganic and organic
complexes. Those complexes may in turn form colloids that interact with each
other and with other dissolved metals (e.g., 99). Thermodynamic, surfacechemical,
and colloidal-chemical models show that most hydrogenetic
elements in crusts occur as inorganic complexes in seawater (100). Hydrated
cations (cobalt, nickel, zinc, lead, cadmium, thallium, etc.) are attracted to the
negatively charged surface of manganese oxyhydroxides, whereas anions and
elements that form large complexes with low charge-density (vanadium,
arsenic, phosphorus, zirconium, hafnium, etc.) are attracted to the slightly
positive charge of iron hydroxide surfaces.
Mixed iron and manganese colloids with adsorbed metals precipitate
onto hard-rock surfaces as poorly crystalline or amorphous oxyhydroxides,
probably through bacterially mediated catalytic processes. Continued crust
accretion after precipitation of that first molecular layer is autocatalytic, but is
probably enhanced to some degree by bacterial processes (101). Additional
metals are incorporated into the deposits either by co-precipitation, or by
diffusion of the adsorbed ions into the manganese and iron oxyhydroxide
crystal lattices. Cobalt is strongly enriched in hydrogenetic crusts because it is
oxidized from cobalt (II) to the less soluble cobalt (III) at the crust surface,
possibly through a disproportionate reaction (102). Lead, titanium, tellurium,
and thallium, as well as cerium are also highly enriched in hydrogenetic
deposits, probably by a similar oxidation mechanism (103).
Concentrations of elements in seawater are generally reflected in their
concentrations in crusts, although there are many complicating factors. For
example, copper, nickel, and zinc occur in comparable concentrations in
seawater (104), yet nickel is much more enriched in crusts than either copper
or zinc. Copper contents may be relatively low in hydrogenetic crusts because
it occurs mostly in an organically bound form in deep seawater, which is not
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