Hydrothermal Manganese Crusts

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 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 enviromnent and make up less than 1% of the total Mn deposits in the world ocean. These crusts occur in all types of active oceanic enviromnents such 

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 Izn-Bonin Arc (Usni 1992) and the Central Pacific Basin (Usui et al. 

In addition to submarine hydrothermal manganese crasts formed near submarine hydrothermal vents, 

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Moore W. S. and Vogt P. R. (1976) Hydrothermal manganese crusts from two sites near the Galapagos spreading axis. Earth Planet. Sci. Lett. 29, 349-356. 

As shown in Table 11.1, hydrothermal emissions are a major source of soluble iron, manganese, and zinc and a minor source of aluminum, cobalt, copper, and lead. Other elements with significant hydrothermal inputs include lithium, rubidium, cesium, and potassium. Considerable uncertainty also surroimds these flux estimates because they are the result of extrapolations from measurements made at a small number of hydrothermal systems at single points in time. These fluxes appear to vary significantly over short time scales as tectonic activity abruptly opens and closes cracks in the oceanic crust. 

Two types of metal-rich hydrogenous deposits are formed on the seafloor iron-manganese oxides and polymetallic sulfides. The iron-manganese oxides have been deposited as nodules, sediments, and crusts. They are enriched in various trace elements, such as manganese, iron, copper, cobalt, nickel, and zinc, making them a significant repository for some of these metals. Most of the metals in the polymetallic sulfides are of hydrothermal origin. These sulfides have been deposited as metalliferous sediments aroimd hydrothermal systems and as rocks that infill cracks within former... 

Table 18.1 Average Compositions of the Earth s Upper Continental Crust, Shale, Iron-Manganese Oxides, Phosphorite, and Various Types of Marine Sediments (All in Units of ppm. Unless Noted otherwise), along with Seawater and a Hydrothermal Vent Solution from the East Pacific Rise (both in Units of 10 g L ). 

Most of the Mn(IV) oxide minerals listed in table 8.3 occur in weathered continental rocks, and often constitute important manganese ore deposits. However, several of the minerals, notably todorokite, bimessite, vemadite and, perhaps, buserite and asbolane, are major constituents of seafloor hydrothermal crusts near spreading centres and in manganese nodule deposits. 

The best known forms of these precipitates are Bog ore, hydrothermal incrustations of manganese and iron, desert hamish, marine manganese nodules and crusts and marine red or brown clay. 

Another surhcial deposit type includes the manganese-rich crusts and nodules that occur on basalt near mid-ocean ridges, and near the sediment-water interface. Whereas the ultimate source of manganese and iron (as well as the associated nickel, copper, and cobalt) is the hydrothermal alteration of MOR basalt, the fields of manganese-rich encrustations and nodules that cover large areas of sediment-starved ocean floor are the result of surficial authigenic upward remobilization of metals and the fixing of those metals at the sediment-water interface. 

Errors in elemental fluxes derived from crustal estimates are larger than, or similar to, the value of the actual flux estimate for many of the major elements silicon, aluminum, iron, manganese, magnesium, and sodium. The fluxes for these elements are thus poorly constrained, but these estimates do serve as conservative bounds on the fluxes. Unfortunately, these bounds overlap the fluxes derived from hydrothermal fluid data and river data. For this reason, current ocean-crust flux estimates do not provide independent evidence for the magnitude of hydrothermal fluxes in the geochemical cycle for these elements. Within the bounds of these uncertainties, the data indicate that ocean floor hydrothermal processes may balance (or compound ) missing global fluxes of these elements. For these reasons, these elements are not discussed here in any detail. 

Other elements, such as halides, various metals and metalloides (iron, manganese, sodium, magnesium, zinc, mercury), nitrogen and phosphorus were also cycled through main Earth s components as a results of volcanism, hydrothermal venting and tectonic shifting of the crust. We can see the summary ofprebiotic element cycles in Figure 15. 

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