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surface-chemical, 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 hardrock
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 coprecipitation,
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 to the less soluble cobalt at the crust surface, possibly through a
disproportionation 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 readily incorporated into iron and manganese metal
oxyhydroxides 105. Zinc contents may be relatively low in crusts compared to nickel
because little zinc may be adsorbed onto crusts after precipitation of the oxyhydroxides,
which follows the order of nickel>cobalt>zinc>copper 106. In contrast, comparable
proportions of manganese:iron:cobalt exist in deep seawater (0.5-1.0:1:0.02-0.05; 107) as
exist in Fe-Mn crusts (0.6-1.6:1:0.02-0.05; Tables 1, 6; 108).
The dominant controls on the concentration of elements in hydrogenetic crusts
are the concentration of each element in seawater; element-particle reactivity; element
residence times in seawater; the absolute and relative amounts of iron and manganese in
the crusts, which in turn are related to their abundance and ratio in colloidal flocs in
seawater 109; the colloid surface charge and types of complexing agents, which will
determine the amount of scavenging within the water column 110; the degree of oxidation
of MnO2 (oxygen/manganese)--the greater the degree of oxidation the greater the
adsorption capacity--which in turn depends on the oxygen content and pH of seawater
111; the amount of surface area available for accretion, which at the surface of growing
crusts is extremely large (mean 300 m 2/g), but which decreases with maturation of
crusts 112; the amount of dilution by detrital minerals and diagenetic phases; and growth
68 International Seabed Authority