Minerals Report - International Seabed Authority

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. Shale-normalized 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 absorbed 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).
INTERNATIONAL SEABED AUTHORITY 213