395
oxyhydroxides. These concretions occur mainly on
lag deposits in the vicinity of the halocline where
strong bottom currents occur.
Concretions from Kiel Bay in the western Belt Sea
occur in a narrow depth range of 20-28 m at the
boundary between sands and mud in zones of active
bottom currents. They occur as coatings on molluscs
and as spheroidal and discoidal concretions. The
formation of the concretions is influenced by the
development of summer anoxia which leads to the
diagenetic remobilization and lateral transport of Mn.
This accounts for the high Mn/Fe ratios of these
concretions.
Marcus et al. (2004) have studied the speciation of
Mn, Fe, Zn and As in one of the Baltic Sea concretions
described by Hlawatsch et al. (2002) by means of micro
X-ray fluorescence, micro X-ray diffraction and micro
X-ray spectroscopy. The concretion was shown to
consist of thin, alternating Fe- and Mn-rich layers.
The Fe-rich layers consisted of two-line ferrihydrite in
which As was mostly pentavalent and the Mn-rich
layers consisted of birnessite in which Zn is
tetrahedrally coordinated and sorbed in the interlayers
of birnessite. The occurrence of ~15% of As(III) in Ferich layers and of ~15% Mn
3+
in Mn-rich layers was
thought to reflect the stagnation of the bottom waters
at the time of the deposition of the Fe- and Mn-rich
layers.
11.4.6 Hydrothermal Manganese Crusts
At mid-ocean ridges, three types of submarine
hydrothermal minerals are found, sulfide minerals associated with silicates and oxides, sharply fractionated oxides and silicates of localized extent and
widely dispersed ferromanganese oxides. The ferromanganese oxides are generally considered to have
precipitated last in this sequence and are thought to
represent a late-stage, low-temperature hydrothermal
phase with temperatures of deposition estimated to
be in the range 20-5°C (Burgath and von Stackelberg
1995). The hydrothermal Mn deposits are characterized by high Mn/Fe ratios and low contents of Cu,
Ni, Zn, Co, Pb and detrital silicate minerals. They
have growth rates exceeding 1,000 mm Ma -1 in some
cases, more than three orders of magnitude faster
than that of hydrogenous deep-sea nodules and
crusts.
Compared to hydrogenous Mn nodules and crusts,
hydrothermal Mn crusts are relatively restricted in the
marine environment and make up less than 1% of the
total Mn deposits in the world ocean. These crusts
occur in all types of active oceanic environments such
as at active mid-ocean spreading centers in the depth
range 250-5,440 m, in back-arc basins in the depth range
50-3,900 m, in island arcs in the depth range 200-2,800
m (Eckhardt et al. 1997), in mid-plate submarine rift
zones in the depth range 1,500-2,200 m (Hein et al.
1996) and at hot spot volcanoes in the depth range
638-1,260 m (Eckhardt et al. 1997). Fossil submarine
hydrothermal manganese deposits have also been
recovered from sediment cores in the Izu-Bonin Arc
(Usui 1992) and the Central Pacific Basin (Usui et al.
1997).
Submarine hydrothermal Mn crusts have been
reported from Enareta and Palinuro seamounts in the
Tyrrhenian Sea (Eckhardt et al. 1977), along the IzuBonin-Mariana Arc (Usui et al. 1986, 1989; Usui and
Nishimura 1992; Usui and Terashima 1997; Usui and
Glasby 1998) and at the Pitcairn hotspot (Glasby et al.
1997b; Scholten et al. 2004). The Tyrrhenian Sea crusts
consist of porous, black, layered Mn oxides up to 45
mm thick. In some cases, the surface has a black metallic
sheen. The crusts overlie substrates such as calcareous sediment, siltstone and oyster shells. A photograph of a hydrothermal Mn crust from the Tyrrhenian
Sea is shown in Figure 11.17. The crusts consist
dominantly of 10Å manganate and 7Å manganate with
minor quartz, illite, montmorillonite, plagioclase and
goethite. The sample having the highest Mn content
contained 54.2% Mn, 0.07% Fe, 33 ppm Ni, 200 ppm
Cu, 20 ppm Zn, 11 ppm Pb and 910 ppm Ba with a Mn/
Fe ratio of 774. It also had a low REE abundance and a
negative Ce anomaly.
Submarine hydrothermal Mn crusts are also
relatively common along the Izu-Bonin-Mariana Arc.
Recent hydrothermal manganese crusts associated
with active hydrothermal systems tend to occur on
seamounts or rifts located about 5-40 km behind the
volcanic front on the Shichito-Iwojima Ridge. Fossil
hydrothermal Mn crusts associated with inactive
hydrothermal systems occur on seamounts located
on older ridges running parallel to the volcanic front
in both forearc and backarc settings. Fossil hydrothermal Mn crusts are generally overlain by hydrogenetic Mn oxides. The thickness of the overlying
hydrogenetic Mn crust depends on the length of time
since hydrothermal activity ceased. Figure 11.18 shows
the distribution of recent and fossil submarine
hydrothermal Mn crusts across the Izu-Bonin Arc.
Mineralogically, the hydrothermal Mn deposits
consist of 10 Å manganate and/or 7 Å manganate.
The Mn/Fe ratios of these deposits range from 10 to
4,670 and the contents of Cu, Ni, Zn, Co and Pb from
20 to 1,000, 1 to 1,403, 1 to 1,233, 6 to 209 and 0 to 93
ppm, respectively.
11.4
Manganese Nodules and Crusts
oxyhydroxides. These concretions occur mainly on
lag deposits in the vicinity of the halocline where
strong bottom currents occur.
Concretions from Kiel Bay in the western Belt Sea
occur in a narrow depth range of 20-28 m at the
boundary between sands and mud in zones of active
bottom currents. They occur as coatings on molluscs
and as spheroidal and discoidal concretions. The
formation of the concretions is influenced by the
development of summer anoxia which leads to the
diagenetic remobilization and lateral transport of Mn.
This accounts for the high Mn/Fe ratios of these
concretions.
Marcus et al. (2004) have studied the speciation of
Mn, Fe, Zn and As in one of the Baltic Sea concretions
described by Hlawatsch et al. (2002) by means of micro
X-ray fluorescence, micro X-ray diffraction and micro
X-ray spectroscopy. The concretion was shown to
consist of thin, alternating Fe- and Mn-rich layers.
The Fe-rich layers consisted of two-line ferrihydrite in
which As was mostly pentavalent and the Mn-rich
layers consisted of birnessite in which Zn is
tetrahedrally coordinated and sorbed in the interlayers
of birnessite. The occurrence of ~15% of As(III) in Ferich layers and of ~15% Mn
3+
in Mn-rich layers was
thought to reflect the stagnation of the bottom waters
at the time of the deposition of the Fe- and Mn-rich
layers.
11.4.6 Hydrothermal Manganese Crusts
At mid-ocean ridges, three types of submarine
hydrothermal minerals are found, sulfide minerals associated with silicates and oxides, sharply fractionated oxides and silicates of localized extent and
widely dispersed ferromanganese oxides. The ferromanganese oxides are generally considered to have
precipitated last in this sequence and are thought to
represent a late-stage, low-temperature hydrothermal
phase with temperatures of deposition estimated to
be in the range 20-5°C (Burgath and von Stackelberg
1995). The hydrothermal Mn deposits are characterized by high Mn/Fe ratios and low contents of Cu,
Ni, Zn, Co, Pb and detrital silicate minerals. They
have growth rates exceeding 1,000 mm Ma -1 in some
cases, more than three orders of magnitude faster
than that of hydrogenous deep-sea nodules and
crusts.
Compared to hydrogenous Mn nodules and crusts,
hydrothermal Mn crusts are relatively restricted in the
marine environment and make up less than 1% of the
total Mn deposits in the world ocean. These crusts
occur in all types of active oceanic environments such
as at active mid-ocean spreading centers in the depth
range 250-5,440 m, in back-arc basins in the depth range
50-3,900 m, in island arcs in the depth range 200-2,800
m (Eckhardt et al. 1997), in mid-plate submarine rift
zones in the depth range 1,500-2,200 m (Hein et al.
1996) and at hot spot volcanoes in the depth range
638-1,260 m (Eckhardt et al. 1997). Fossil submarine
hydrothermal manganese deposits have also been
recovered from sediment cores in the Izu-Bonin Arc
(Usui 1992) and the Central Pacific Basin (Usui et al.
1997).
Submarine hydrothermal Mn crusts have been
reported from Enareta and Palinuro seamounts in the
Tyrrhenian Sea (Eckhardt et al. 1977), along the IzuBonin-Mariana Arc (Usui et al. 1986, 1989; Usui and
Nishimura 1992; Usui and Terashima 1997; Usui and
Glasby 1998) and at the Pitcairn hotspot (Glasby et al.
1997b; Scholten et al. 2004). The Tyrrhenian Sea crusts
consist of porous, black, layered Mn oxides up to 45
mm thick. In some cases, the surface has a black metallic
sheen. The crusts overlie substrates such as calcareous sediment, siltstone and oyster shells. A photograph of a hydrothermal Mn crust from the Tyrrhenian
Sea is shown in Figure 11.17. The crusts consist
dominantly of 10Å manganate and 7Å manganate with
minor quartz, illite, montmorillonite, plagioclase and
goethite. The sample having the highest Mn content
contained 54.2% Mn, 0.07% Fe, 33 ppm Ni, 200 ppm
Cu, 20 ppm Zn, 11 ppm Pb and 910 ppm Ba with a Mn/
Fe ratio of 774. It also had a low REE abundance and a
negative Ce anomaly.
Submarine hydrothermal Mn crusts are also
relatively common along the Izu-Bonin-Mariana Arc.
Recent hydrothermal manganese crusts associated
with active hydrothermal systems tend to occur on
seamounts or rifts located about 5-40 km behind the
volcanic front on the Shichito-Iwojima Ridge. Fossil
hydrothermal Mn crusts associated with inactive
hydrothermal systems occur on seamounts located
on older ridges running parallel to the volcanic front
in both forearc and backarc settings. Fossil hydrothermal Mn crusts are generally overlain by hydrogenetic Mn oxides. The thickness of the overlying
hydrogenetic Mn crust depends on the length of time
since hydrothermal activity ceased. Figure 11.18 shows
the distribution of recent and fossil submarine
hydrothermal Mn crusts across the Izu-Bonin Arc.
Mineralogically, the hydrothermal Mn deposits
consist of 10 Å manganate and/or 7 Å manganate.
The Mn/Fe ratios of these deposits range from 10 to
4,670 and the contents of Cu, Ni, Zn, Co and Pb from
20 to 1,000, 1 to 1,403, 1 to 1,233, 6 to 209 and 0 to 93
ppm, respectively.
11.4
Manganese Nodules and Crusts