海産の鉄‐マンガン団塊およびクラストの化学組成

Abstract

The chemical composition of marine ferromanganese nodules and crusts are reviewed in relation to their genesis. 1) Crusts on seamounts are usually of hydrogenous origin and have high contents of Co and low contents of Cu. Their mineralogy is vernadite (δ-MnO2). The high content of Co is owing to the highly oxidizing environment and the low growth rate of hydrogenous crusts. The low content of Cu relative to Ni is attributed to the presence of organically bound copper in seawater, in spite of the fact that the concentration of Cu in deep seawater is similar to that of Ni. 2) Nodules almost buried in pelagic siliceous oozes are commonly of oxic diagenetic origin and have high contents of Ni and Cu. Their mineralogy is todorokite. The high contents of Ni and Cu are owing to their high concentrations in interstitial water relative to deep seawater. Elements associated with settling particles are released into interstitial water during early diagenesis and taken up by manganese nodules. 3) Suboxic diagenetic nodules in hemipelagic environments have low contents of cationic transition elements, especially of Co. The low content of Co is attributed to less oxidizing environments because of a large supply of organic matter by settling particles. The low contents of Ni and Cu are attributed to the high growth rate of suboxic diagenetic nodules. An alternative explanation is the hypothesis of non-steady-state growth. 4) Hydrothermal crusts of which mineralogy is todorokite or birnessite have extremely low contents of cationic transition elements, which are mainly due to the rapid growth of hydrothermal crusts, although the contents of oxyanionic elements in hydrothermal crusts are almost equivalent to those in nodules of other origins. The chemical composition of deep-sea sediments can be simulated by admixtures of aluminosilicates with the average shale composition and Fe-Mn oxides with manganese nodule compositions of four different origins. Therefore, a further understanding of the factors controlling the chemical composition of manganese nodules is necessary to clarify the geochemical cycle of elements in the marine environment.