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REE and yttrium contents in Fe-Mn crusts are very high; 100s of ppm for yttrium
and 1000s of ppm for total REE contents (cerium in one bulk crust sample from the
Hawaii EEZ is very high, 1.1%--11,000 ppm). Concentrations of the trivalent REEs
(includes all REEs except cerium) appear to increase in crusts with increasing water
depth of occurrence 70. Irrespective of their specific compositions, Fe-Mn crusts from the
major oceans show remarkably similar REE patterns on normalize plots. Shalenormalized
REE patterns generally show enrichment of the heavy REEs and commonly
show positive anomalies of lanthanum, cerium, europium, and gadolinium, and a
negative yttrium anomaly (Fig. 7). The REEs in crusts are derived from seawater, which
in turn is supplied with REEs from fluvial and eolian input from continental sources.
Except for oxidation-reduction (redox)-sensitive cerium, yttrium and REE distributions in
Fe-Mn crusts represent the exchange equilibrium between yttrium and REEs dissolved in
seawater and yttrium and REEs sorbed on the surface of iron-manganese oxyhydroxide
particles. Cerium is the only REE that in near-surface environments may occur as a
tetravalent ion, and the contrasting charge and size of cerium (IV) leads to its decoupling
from the other REEs. In the marine environment, cerium uptake by manganese and iron
oxyhydroxides is accompanied by cerium oxidation, which is mediated by microbial
processes and/or a surface catalysis, a reaction that would otherwise be very sluggish
(e.g., 73).
Platinum Group Elements (PGEs: platinum, palladium, rhodium, ruthenium,
iridium) are highly enriched in Fe-Mn crusts over their abundances in the Earth’s crust
(with the exception of palladium) and over concentrations in Fe-Mn nodules (Table 6).
Platinum has been considered a potential by-product of cobalt mining of crusts and has
mean values for different areas of up to 0.8 ppm and individual sample values of up to 3
ppm 74. Rhodium, ruthenium, and iridium contents are up to 124, 32, and 54 parts per
billion (ppb), respectively, whereas palladium contents (1-16 ppb) are usually at or just
above its mean content in the Earth’s crust. Most PGEs have a positive correlation with
crust thickness and are concentrated in the inner part of thick crusts. Most PGEs
commonly correlate inversely with water depth of occurrence of the crusts 75.
62 International Seabed Authority