(see Phosphorites below), whereas the higher silicon
and aluminum contents of nodules reflect their origin
within loose, aluminosilicate-rich sediments of the
abyssal seafloor. The mechanism for cobalt enrichment
is oxidative scavenging, whereby dissolved Co
2þ is
oxidized to Co
3þ on the manganese oxide surface. This
mechanism also explains the relative enrichments of
cerium, and possibly some of the lead and titanium
enrichments in crusts. Lower concentrations of iron
(and arsenic, an oxyanion in sea water scavenged by
positively charged FeOOH; MnO 2 surfaces are negatively charged at sea water pH) in nodules reflect a
greater diagenetic component that favors mobilization
of manganese over iron. Because of their extremely
slow growth in areas relatively free of detrital input,
crusts also accumulate large amounts of cosmogenic
debris. Enrichments of platinum and PGE metals can
be partially explained by this source, but several lines
of geochemical evidence indicate that the majority of
platinum and the other PGE (Ir, Os, Pd, Rh, Ru) are
scavenged from sea water. Oxidation and reduction
mechanisms have been proposed for both incorporation and postdepositional remobilization of the PGE
in crusts and nodules.
(A)
(B)
(C)
> 5
2 _ 5
< 1
1 _ 2
> 1%
< 0.25%
0.25 0.5%
−
0.5 1%
−
< 0.25%
> 0.50%
0.25 0.50%
−
Figure 4 (A) Distribution of Mn/Fe ratios in ferromanganese nodules from the Pacific. (B) Distribution of nickel concentrations in
ferromanganese nodules from the Pacific. Copper shows a similar distribution. (C) Distribution of cobalt concentrations in ferromanganese
nodules from the Pacific. (After Calvert SE (1978) In: Sea Floor Development: Moving into Deeper Water. London: The Royal Society.)
AUTHIGENIC DEPOSITS 329
and aluminum contents of nodules reflect their origin
within loose, aluminosilicate-rich sediments of the
abyssal seafloor. The mechanism for cobalt enrichment
is oxidative scavenging, whereby dissolved Co
2þ is
oxidized to Co
3þ on the manganese oxide surface. This
mechanism also explains the relative enrichments of
cerium, and possibly some of the lead and titanium
enrichments in crusts. Lower concentrations of iron
(and arsenic, an oxyanion in sea water scavenged by
positively charged FeOOH; MnO 2 surfaces are negatively charged at sea water pH) in nodules reflect a
greater diagenetic component that favors mobilization
of manganese over iron. Because of their extremely
slow growth in areas relatively free of detrital input,
crusts also accumulate large amounts of cosmogenic
debris. Enrichments of platinum and PGE metals can
be partially explained by this source, but several lines
of geochemical evidence indicate that the majority of
platinum and the other PGE (Ir, Os, Pd, Rh, Ru) are
scavenged from sea water. Oxidation and reduction
mechanisms have been proposed for both incorporation and postdepositional remobilization of the PGE
in crusts and nodules.
(A)
(B)
(C)
> 5
2 _ 5
< 1
1 _ 2
> 1%
< 0.25%
0.25 0.5%
−
0.5 1%
−
< 0.25%
> 0.50%
0.25 0.50%
−
Figure 4 (A) Distribution of Mn/Fe ratios in ferromanganese nodules from the Pacific. (B) Distribution of nickel concentrations in
ferromanganese nodules from the Pacific. Copper shows a similar distribution. (C) Distribution of cobalt concentrations in ferromanganese
nodules from the Pacific. (After Calvert SE (1978) In: Sea Floor Development: Moving into Deeper Water. London: The Royal Society.)
AUTHIGENIC DEPOSITS 329