Manganese crusts

Sometimes, the precipitates do not form maganese nodules but manganese crusts. Usually, manganese crusts are growing on the hard substrate, i.e. on the fresh basalt not yet covered with sediment, or at positions where hard rock is in direct contact with sea-water because strong currents make sedihientary deposition impossible. In such cases crusts are simply overdimensioned nodules. 

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. 

Segl M., Mangini A., Bonani G., Hofmann H. J., Nessi M., Suter M., Wolfli W., Friedrich G., Plilger W. L., Wiechowski A., and Beer J. (1984) Be-dating of a manganese crust from central North Pacific and implications for ocean palaeocirculation. Nature 309, 540-543. 

The enrichment of trace elements in iron-manganese crusts and nodules is well-known and has been described, for example, by Hlawatsch et al. (2002), Marcus et al. (2004), Kersten and Kulik (2005). Besides the diffusion from the underlying sediments, the main pait of trace elements in crusts is derived from their dissolved forms in sea water. Even if the accumulation process is natural, the available amount of dissolved heavy metals is affected... 

Perhaps the most comprehensive explanation of the role of hydrogenetic and diagenetic processes on manganese nodule accretion has been presented by Dymondetal (1984). On their classification, hydrogenous deposition involves the direct precipitation or accumulation of colloidal metals oxides from seawater. Strictly, this involves deposition of manganese oxides on surfaces in contact with seawater such as involved in the formation of manganese crusts. In practice, manganese nodules formed on red clays have charac-... 

Table 11.5 Variation of the composition of manganese crusts from the Mid-Pacific Mountains with water depth (after Mangini et al. 1987). Analyses expressed as wt. % of dried material.

For Co-rich manganese crusts, the crusts should contain >0.8% Co and be >40 mm thick to be considered economically viable. The crusts occur in shallower water depths (1,000-2,500 m) than deep-sea nodules (>4,000 m) and could be mined within national Exclusive Economic Zones (EEZs) and therefore imder national jurisdiction. Excellent accounts of the distribution and composition of Co-rich crusts in the world ocean have been presented by Andreev and Gramberg (1998) and Hein (2000) and several authors have attempted to assess of their economic potential (Wiltshire etal. 1999 Hein2000,2004 Wiltshire 2000). Andreev and Gramberg (2002) have estimated the total abundance of Co-rich crasts in the world ocean to be about 21-10%. 

Knie, K., Korschinek, G, Faestermann, T., Dorfi, E.A., Rugel, G and Wallner, A., 2004. Ee anomaly in a deep-sea manganese crust and implications for a nearby supernova source. Physical Review Letters, 93 171103. 

Meylan, M.A., Glasby, G.P., Hill, P.J., McKelvey, B.C., Walter, P. and Staffers, R, 1990. Manganese crusts and nodules from the Manihiki Plateau and adjacent areas Results of HMNZS Tui cruises. Marine Mining, 9 43-72. 

The expected source of Pu is the interstellar medium (ISM) from fresh r-process nucleosynthesis or in direct ejecta from supemovae. Deposition onto Earth may result in traces of live Pu in suitable reservoirs. Ofan et al. (2006) developed a method for Pu detection based on RIMS. Gd was chosen as a proxy, because its chemical properties, excitation and ionization schemes, and wavelengths are similar to those of Pu. The Pu detection efficiency achieved for by RIMS was found to be smaller than that obtained by AMS (10 -10 ) by a factor of 1,000. However, a value of the order of 0.5%, as obtained here for Gd, is believed to be feasible for Pu detection as well. This efficiency would allow the determination of the abundance of Pu continuously accreted from the ISM or deposited in a single supernova event such as the one occurred 3 Ma before present, as evidenced by the Fe anomaly reported in a deep-sea manganese crust. 

Yamazaki, T., Park, S.-H., Shimada, S., Yamamoto, T. (2002). Development of technical and economical examination method for cobalt-rich manganese crusts. Proceedings, 12th international offshore and polar engineering conference, Kita-Kyushu, 454-461. 

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