72 • Marine Minerals: Exploring Our New Ocean Frontier 



Table 2-6.— Average Chemical Composition for Various Elements of Crusts From < 8,200 Feet Water Depth 



From the EEZ of the United States and Other Pacific Nations (all data are in weight percent) 



Areas n Mn Fe Co Ni Cu Pb Ti SiOa PaOs Fe/Mr7 



Hawaii and Midway (on axis) 2^38 24 16.0 0.91 045 005 — ll 7^9 ^^ 073 



Hawaii and Midway (off axis) 4-15 21 18.0 0.60 0.37 0.10 0.18 1.3 16.0 1.0 0.88 



Johnston Island 12-40 22 17.0 0.70 0.43 Oil 0.17 1.3 12.0 1.2 0.81 



Palmyra Atoll-Kingman Reef 7-8 27 16.0 1.1 0.51 0.06 0.17 1.1 5.5 1.9 0.61 



Howland-Baker Islands 3 29 18.0 0.99 0.63 0.08 0.14 1.2 6.2 1.7 0.64 



Marshall Islands 5-13 26 14.0 0.94 0.56 0.13 0.25 1.1 5.6 0.90 0.56 



Average central Pacific crusts 34-117 23 17.0 0.81 0.45 0.09 0.18 1.2 9.1 1.3 0.75 



Northern Marianna Islands 



(and Guam) 6-7 12 16.0 0.09 0.13 0.05 0.07 _ _ _ 1.41 



Western U.S. borderland 2-5 19 16.0 0.30 0.30 0.04 0.15 0.31 17.0 — 0.95 



Gulf of Alaska Seamounts 3-6 26 18.0 0.47 0.44 0.15 0.17 0.57 — 0.87 0.72 



Lau Basin (hydrothermal) 2 46 0.60 0.007 0.005 0.02 0.006 0.005 — 0.05 0.01 



Tonga Ridge and Lau Basin 



(hydrogenous) 6-9 16 20.0 0.33 0.22 0.05 0.16 1.0 - 1.0 1.26 



South China Sea 14 13 13.0 0.13 0.34 0,04 0.08 — 14.0 — 1.07 



Bonin Island area (Japan) 1-10 21 13.0 0.41 0.55 0.06 0.12 0.67 4.9 0.82 0.70 



French Polynesia 2-9 23 12.0 1.2 0.60 0.11 0.26 1.0 6.5 0.34 0.56 



Average for Pacific hydrogenous 



crusts (all data from figures 2-8) .. . 55-319 22 15.0 0.63 0.44 0.08 016 0.98 11.0 1.1 0.81 



n = Number of analyses for various elements. 



— = No data, 



SOURCE; J.R. H«in, L.A- Morgenson, D.A. Clague, and R.A. Koski, "Cobalt-Rioti Ferromanganese Crusts From ttie Exclusive Economic Zone of tfie United States and 



Nodules From ttie Oceanic Pacific," D. Scholl, A. Grantz, and J. Vedder (eds.), Geology and Resource Potential of the Continental Margins of Western Norttt 



American and Adjacent Ocean Basins: American Association of Petroleum Geologists, Memoir, in press. 



Pacific Mountains and the Hawaiian Archipelago.'' 

 Further, high cobalt values of 2.5 percent were 

 found in the top inch or so of crusts from the S.P. 

 Lee Seamount at 8° N. latitude. These deposits oc- 

 cur at depths coincident with a water mass that con- 

 tains minimum concentrations of oxygen, leading 

 most investigators to attribute part of this cobalt 

 enrichment to low oxygen content in the seawater 

 environment. However, high cobalt values (greater 

 than 1 percent) have also been found in the Mar- 

 shall Islands, the western part of the Hawaiian 

 Ridge province, and in French Polynesia, all of 

 which are outside the well-developed regional 

 equatorial oxygen-minimum zone but which ap- 

 pear to be associated with locally developed oxygen- 

 minimum zones. Oxygen-minimum zones are also 

 associated with low iron/manganese ratios. Figure 

 2-12 illustrates the zone of cobalt enrichment in fer- 

 romanganese crusts on seamounts and volcanic is- 

 lands. In general, while progress is being made to 

 understand more fully the physical and geochemi- 

 cal mechanisms of cobalt-manganese crust forma- 



"J.R. Heine, et al., "Geological and Geochemical Data for Sea- 

 mounts and Associated Ferromanganese Crusts In and Near the Ha- 

 waiian, Johnston Island, and Palmyra Island Exclusive Economic 

 Zones," U.S. Geological Survey, Open File Report 85-292, 1985, 

 p. 129. 



tion, the cobalt enrichment process is still uncer- 

 tain. Investigations to gain insight in this area will 

 be of considerable benefit in identifying future re- 

 sources. 



Surface texture, slope, and sediment cover also 

 may influence crust growth rates. For example, 

 sediment-free, current-swept regions appear to be 

 favorable sites for crust formation. 



Nodules are also found associated with cobalt- 

 rich manganese crusts in some areas. These nod- 

 ules are similar in composition to the crusts and, 

 consequently, differ from their deep ocean coun- 

 terparts. Another difference between crust- 

 associated nodules and deep ocean nodules is the 

 greater predominance of nucleus material in the 

 crust-associated nodules. The cobalt-rich nodules 

 generally occur as extensive fields on the tops of 

 seamounts or within small valleys and depressions. 

 While of much lesser extent overall than crust 

 occurrences, these nodules may prove more easily 

 recoverable and, hence, possibly of nearer term eco- 

 nomic interest. 



Prospects for Future Development 



The geblogic considerations mentioned previ- 

 ously are important determinants in assessing the 



