Geochemistry of re and trace elements in ferromanganese nodules and their genesis

Trace and RE element geochemistry and genesis have been studied with respect to ferromanganese nodules from the sediments of the Pacific, Atlantic and Indian oceans.

1. Geochemical characteristics of major and trace elements The essential composition is Mn and Fe in nodules with 17.88–28.64% of H₂O. Besides δ-MnO₂ and Fe₂O₃, there are hydrous Mn and Fe oxides such as Mn(OH)₄, Fe(OH)₃, MnO₂·nH₂O and Fe₂O₃·nH₂O in the nodules. Fe and Mn are presented in higher valences such as Fe³⁺ and Mn⁴⁺. The Fe²⁺ was not found by chemical analysis and EPR in ferromanganese nodules, but Mn²⁺ was. These nodules contain by weight 1–2 percent of the oxides of nickel (Ni), copper (Cu), and cobalt (Co). The Ba/Sr ratio of the nodules, is usually over 2. It is considered that foraminifera cannot be the material source of the nodule, because foraminifera with calcite structure usually contain a great deat of Sr but almost no Ba.
2. REE data The REE contents in nodules are usually greater than 1000ppm, with ΣCe/ΣY ranging from 3.29 to 3.96. The REE distribution pattern for nodules demonstrates that there is no remarkable anomaly for Ce, with ΔCe ?0.20?+0.48, and ΔEu<0, and a distinct negative slope.
3. Genesis In order to study the geneses of the nodules, an experiment on the seawater-basaltic glass interaction at 250°C and 500 bars has been conducted. It is considered that basalts widely distributed over the ocean floor are perhaps the main source of Mn, Fe, Co, Ni and Cu in the nodules.
4. Conclusion

1. Mn, Fe, Cu, Co, Ni and REE have an important significance in Fe?Mn nodules. Mn and Fe mainly occur in high valence cations, i.e., Mn in Mn⁴⁺, a small part of Mn²⁺ and Mn³⁺, but no Fe²⁺ in the nodules.
2. The Mn/Fe ratio in the nodules is of an indicative character, as the Mn/Fe ratio increases, the contents of Cu and Ni increase, but Ti decreases.
3. The higher ratio of Ba/Sr in the nodules suggests that they are not derived from the carbonaccous bioclast through solution at high pressure below the CaCO₃ compensation depth.
4. The chondrite-normalized REE distribution pattern of the Fe?Mn nodules appear to be a characteristics of LREE enrichment. ΔCe values may be positive or negative. ΔEu values are all negative near zero, which suggests that the REE source is in close relationship with oceanic crust or upper mantle, but that ΣCe/ΣY ratios of 2.29–3.96 in the nodules are approximate to those of the continental crust materials. The facts mentioned above suggest that the REE in the nodules are perhaps of polygenesis.
5. Geochemical experiments and thermodynamical calculations show that Fe and Mn in the nodules are derived principally from volcanic materials. and sea-floor basalts and that the favorable factors for the formation of Fe?Mn nodules are pH values of 6.5 to 8.5 and Eh values of about 4.5–6.5V.