『Abstract
Co-rich Fe-Mn crusts occur throughout the Pacific on seamounts,
ridges, and plateaus where currents have kept the rocks swept
clean of sediments at least intermittently for millions of years.
Crusts precipitate out of cold ambient sea water onto hard-rock
substrates forming pavements up to 250 mm thick. Crusts are important
as a potential resource for Co, Ni, Pt, Mn, Tl, Te, and other
metals, as well as for the paleoclimate signals stored in their
stratigraphic layers. Crusts form at water depths of about 400
to 4000 m, with the thickest and most Co-rich crusts occurring
at depths of about 800 to 2500 m, which may vary on a regional
scale. Gravity processes, sediment cover, submerged and emergent
reefs, and currents control the distribution and thickness of
crusts on seamounts. Crust occur on a variety of substrate rocks
that generally decrease in the order, breccia, basalt, phosphorite,
limestone, hyaloclastite, and mudstone. Because of this wide variety
of substrate types, crusts are difficult to distinguish from the
substrate using remotely sensed data, such as geophysical measurements,
but are generally weaker and lighter-weight than the substrate.
Crusts can be distinguished from the substrates, however, by their
much higher gamma radiation levels. The mean dry bulk density
of crusts is 1.3 g/cm3, the mean porosity is 60％, and
the mean surface area is extremely high, 300 m2/g.
Crusts generally grow at rates of 1 to 10 mm/Ma. crust surfaces
are botryoidal, which may be modified to a variety of forms by
current erosion. In cross-section, crusts are generally layered,
with individual layers displaying massive, botryoidal, laminated,
columnar, or mottled textures. Characteristics layering is persistent
regionally in the Pacific. Crusts are composed of ferruginous
vernadite (δ-MnO2) and X-ray amorphos Fe
oxyhydroxide, with moderate amounts of carbonate fluorapatite
(CFA) in thick crusts and minor amounts of quartz and feldspar
in most crusts. Elements most commonly associated with the vernadite
phase include Mn, Co, Ni, Cd, and Mo, whereas those most commonly
associated with Fe oxyhydroxide are Fe and As. Detrital phases
are represented by Si, Al, K, Ti, Cr, Mg, Fe, and Na; the CFA
phase by Ca, P, Sr, Y, and CO2; and a residual
biogenic phase by Ba, Sr, Ce, Cu, V, Ca, and Mg. Crusts contain
Co contents up to about 2.3％, Ni to 1％, and Pt to 3 ppm, with
mean Fe/Mn ratios of 0.6 to 1.3. Fe/Mn decreases, whereas Co,
Ni, Ti, and Pt increase in central Pacific crusts and Fe/Mn, Si,
and Al increase in continental margin crusts and in crusts with
proximity to west Pacific volcanic arcs. Vernadite and CFA-related
elements decrease, whereas Fe, Cu, and detrital-related elements
increase with increasing water depth of crust occurrence. Cobalt,
Ce, Tl, and maybe also Ti, Pb, and Pt are strongly concentrated
in crusts over other metals because of oxidation reactions. Total
rare earth elements (REEs) commonly vary between 0.1％ and 0.3％
and are derived from sea water along with other hydrogenetic elements,
Co, Mn, Ni, etc. Platinum, Rh, Ir, and some Ru in crusts are also
derived from sea water, whereas Pd and the remainder of the Ru
derive from detrital minerals. The older parts of thick crusts
were phosphatized during at least two global phosphogenic events
during the Tertiary, which mobilized and redistributed elements
in those parts of the crusts. Silicon, Fe, Al, Th, Ti, Co, Mn,
Pb, and U are commonly depleted, whereas Ni, Cu, Zn, Y, REEs,
Sr, and Pt are commonly enriched in phosphatized layers compared
to younger nonphosphatized layers. The dominant controls on the
concentration of elements in crusts include the concentration
of metals in sea water and their ratios, colloid surface charge,
types of complexing agents, surface area, and growth rates. Crusts
act as closed systems with regard to the isotopic ratios of Be,
Nd, Pb, Hf, Os, and U-series, which in part have been used to
date crusts and in part used as isotopic tracers of paleoceanographic
and paleoclimatic conditions. Those tracers are especially useful
in delineating temporal changes in deep-ocean circulation. Research
and development on the technology of mining crusts are only in
their infancy. Detailed maps of crust deposits and a better understanding
of small-scale seamount topography are required to design the
most appropriate mining equipment.』
9.1 Introduction
9.1.1 Classification
9.1.2 Distribution
9.1.3 Historical perspective
9.2 Fe-Mn crust characteristics
9.2.1 Textures and physical properties
9.2.2 Mineralogy
9.2.3 Ages and growth rates of Fe-Mn crusts
9.2.4 Chemical composition
9.2.4.1 Rare earth elements and yttrium
9.2.4.2 Platinum group elements
9.2.5 Phosphatization of Fe-Mn crusts
9.2.6 Local and regional variation in composition
9.3 Fe-Mn crust formation
9.4 Paleoceanographic and paleoclimate studies
9.5 Resource, technology, and economic considerations
9.5.1 Mining systems
9.5.2 Economics
9.6 Research for the 21st century
References