Central Pacific Cobalt-Rich Ferromanganese Crusts: Historical Perspective and Regional Variability

Abstract

Interelement correlations coupled with X-ray mineralogy, chemical analyses, and Q-mode factor analysts define five phases that compose ferromanganese crusts: d-MnO2, Fe-phases including Fe oxyhydroxide and Fe silicate, detrital aluminosilicate, biogenic phosphate, and biogenic-nonphosphate. This last phase is derived primarily from the dissolution of biogenic carbonate and silica. These five phases are characterized by the following elements respectively: Co, Mn, Ni Pb; Fe, Si As; Al, Cr, Si Ti K; P, Ca; and Cu, Ni Ba, Zn, Cd. The distribution of the d-MnO2 phase is related to latitude and is controlled by the equatorial zone of higher productivity, which produces a strong and extensive oxygen-minimum zone. The iron phases are similarly distributed, but the overall variability is not as great as for the d-MnO2 phase. The detrital phase has the inverse distribution to the d-MnO2 phase and is greatest at higher latitudes, especially in areas beneath the trade-wind belt. The detrital phase is composed of eolian debris and volcanogenic debris eroded from submarine outcrops by bottom currents. The phosphate phase is not clearly distributed with latitude, longitude, or the equatorial zone of high biological productivity. The biogenic-nonphosphate phase is most prominent in the equatorial zone of high biological productivity. The Co content of bulk crusts in the central Pacific averages 0.79% (308 analyses), Ni averages 0.47% (311 analyses), and Pt averages 0.24 ppm (29 analyses). Cobalt and Ni are richest in crusts from the EEZ of Tuvalu, followed in decreasing order by Kingman-Palmyra Islands, Howland-Baker Islands, Johnston Island, Marshall Islands, Hawaiian Islands, and Kiribati Islands. However, based on 11 geologic, oceanographic, and geochemical criteria, we rank the resource potential of Pacific nations and islands in decreasing order Marshall, Federated States of Micronesia, Johnston, French Polynesia, Kiribati, Hawaiian, Tuvalu, Kingman-Palmyra, and Howland-Baker islands. The knowledge gained from ten cruises devoted to the study of cobalt-rich ferromanganese crusts in the central Pacific has contributed to the understanding of the origin and evolution of crusts and their localized distributions. These ten cruises were undertaken by the Technical University of Clausthal (F.R. Germany), U.S. Geological Survey, and University of Hawaii. The scientific results presented by each group are outlined and put into a historical perspective.