Geochemistry Of Major And Minor Elements In Marine Ferromanganese Oxide Deposits

As the two dominant types of the marine ferromanganese oxide deposits, both diagenetic nodules and hydrogenic crusts are enriched in transitional elements, alkaline metals and alkaline earth metal elements as well as rare earth elements. Distribution of above-mentioned elements in the ferromanganese oxide deposits, relationship between the elements and the compositional minerals of the deposits as well as the factors to control the enrichments of the elements in the deposits were investigated systematically in terms of the selective dissolution experiments and adsorption experiments as well as geochemistry, crystal chemistry and mineralogy by using the ferromanganese oxide deposit samples recovered near the equatorial east Pacific.Manganese and iron are present as 10(?)-manganates and ferric oxides in diagenetic nodules, respectively, while they are present asδ-MnO₂ and ferric oxides in hydrogenic crusts, respectively. Little substitution occurred between Fe and Mn in the ferromanganese oxide deposits due to their different crystal chemistry. The diagenetic ferromanganese nodules are relatively rich in Mn, Cu and Ni, because Cu and Ni can enter into the structure of 10(?)-manganates and stabilize the structure. The 10(?)-manganates can be considered as the scavenger of Cu and Ni in the seawater. On the other hand, hdyrogenic crusts are rich in Fe, Co and Ti. The enrichment of Ti in the hdyrogenic crust is attributed to the adsorption and combination of Ti by amorphous ferric oxide/hydroxide, while the enrichment of Co is attributed to the substitution of Co for Mn ofδ-MnO₂. About three factors control the enrichments of Cu, Co, Ni, Ti and Zn in the ferromanganese oxide deposits as follows: 1) chemistry and crystal chemistry of minerals in the marine ferromanganese deposits; 2) states of the transitional elements in the seawater; 3) biological productivity and sedimentation rate of the environments in which the marine ferromanganese oxide deposits formed. The former two factors determine the present phases of the transitional elements in the deposits and the latter one controls the amounts enriched in the deposits.Enrichments of alkali elements and alkali earth elements in marine ferromanganese oxide deposits are mainly attributed to the absorbability of manganese oxides. Alkali and alkali earth elements can enter into the structure of 10(?)-manganates of diagenetic nodules as important constituents despite the fact that different ions occupy varying positions in the structure due to their different effective ionic radius. Sodium ions can make the structure of 10(?)-manganates stable because the absence of sodium ions results in the collapse of 10(?)-manganates to turn into 7(?)-manganates. In addition, it is assumed that magnesium ions also play an important role to brace the structure of 10?-manganates as the transitional elements, such as Cu²⁺, Co²⁺ and Ni²⁺ do. Lithium in the diagenetic nodules is present in the 10(?)-manganates phase rather than in the lithiophorite phase. 10(?)-manganates can be considered as the scavenger of lithium in the seawater and it may play a important role in the mass balance of lithium in the oceans. The enrichments of alkali and alkali earth elements in hydrogenic crusts are different from that of diagenetic nodules. It is assumed that alkali and alkali earth elements ions are adsorbed on the surface ofδ-MnO₂ rather than into its structure, therefore the alkali and alkali earth elements ions is unimportant for the structure ofδ-MnO₂. On the other hand, the alkali and alkali earth elements ions may play a role in the charge balance ofδ-MnO₂ to some extent.The enrichments of rare earth elements (REE) in marine ferromanganese oxide deposits of different origin are attributed to the strong complexing of amorphous ferric oxide/hydroxide for REE in seawater. Rare earth elements in seawater can be more strongly combined by amorphous ferric oxide/hydroxide than by 10(?)-manganates andδ-MnO₂. The amorphous ferric oxide/hydroxide of marine ferromanganese deposits can combine the REE which are complexed by carbonate and bicarbonate in seawater, while 10(?)-manganates andδ-MnO₂ just combine the complex of REE and carbonate/bicarbonate directly from seawater. The pronounced positive Ce anomalies in hydrogenic crusts are not attribute to the oxidation of Ce from soluble Ce³⁺ to insoluble Ce⁴⁺ by eitherδ-MnO₂ or amorphous ferric oxide/hydroxide. The oxidative environments in which the hydrogenic crusts are formed may result in the pronounced positive Ce anomalies. Therefore, the Ce anomaly can be can be used to infer redox conditions. Apatite contribute less to the enrichments of REE in marine ferromanganese oxide deposits than amorphous ferric oxide/hydroxide.