Geochemistry of the Manganese Ore Process in the Ocean: Evidence from Rare Earth Elements

Processes governing the formation of rare earth elements (REE) composition are considered for ferromanganese deposits (nodules, separate parts of nodules, and micronodules of different fractions) within the Clarion–Clipperton ore province in the Pacific Ocean. It is shown that ferromanganese oxyhydroxide deposits with different chemical compositions can be produced in sediments under similar sedimentation conditions. In areas with high bioproductivity, the size of micronodules has a positive correlation with the Mn content and Mn/Fe and P/Fe ratios and a negative correlation with Fe, P, REE, and Ce anomaly. The behavior of REE in micronodules from sediments within bioproductive zones is related to increase of the influence of diagenetic processes in sediments as a response to the growth of the size of micronodules. Distinctions in the chemical composition of micronodules and nodules are related to their interrelations with associated sediments. Micronodules grow in sediments using hydrogenous ferromanganese oxyhydroxides. As they grow, micronodules are enriched in the labile fraction of sediments reworked during diagenesis. Sources of the material of ferromanganese nodules are governed by their formation at the water bottom interface. Their upper part is formed by direct settling of iron oxyhydroxides from the bottom water, whereas the lower part is accumulated due to diagenetic processes in sediments. Differences of REE compositions in ferromanganese deposits are caused by the reduction of manganese during diagenesis and its separation from iron. Iron oxyhydroxides form a sorption complex due to the sorption of phosphate-ion from bottom and pore waters. The sorption of phosphate-ion results in an additional sorption of REE.     

Geochemistry of Rare Earth Elements in Ferromanganese Micro- and Macronodules from the Pacific Nonproductive Zone

Ferromanganese micro- and macronodules in eupelagic clays at Site 35 of the South Basin were examined in order to check the REE distribution during the ferromanganese ore formation in nonproductive zones of the Pacific Ocean. We studied host sediments and their labile fraction, ferromanganese micronodules (fractions 50–100, 100–250, 250–500, and >500 m) from eupelagic clays (horizons 37–40, 105–110, 165–175, and 189–190 cm), and buried ferromanganese micronodules (horizons 64–68, 158–159, and 165–166 cm). Based on phase analysis data, the anomalous REE enrichment of eupelagic clays from Site 35 is related to the accumulation of rare earth elements in iron hydroxophosphates. The Ce concentration, generally linked to manganese oxyhydroxides, is governed by the oxidation of Mn and Ce in oceanic surficial waters. Micronodules (Mn/Fe = 0.7–1.6) inherit compositional features of the labile fraction of sediments. The Ce, Co, and Th concentrations depend on the micronodule dimension. The enrichment of micronodules in hydrogenic or hydrothermal substance is governed by their dimension and the dominant source of suspended oxyhydroxide material. The study of buried ferromanganese micronodules revealed general regularities in the compositional evolution of oxyhydroxide matrices of ferromanganese micro- and macronodules. The compositional variation of micro- and macronodules, relative to the labile fraction of sediments, in the Pacific nonproductive zone dramatically differs from the pattern in bioproductive zones, where micronodule compositions in larger fractions are similar to those in associated macronodules and labile fractions of the host sediment as a result of the more intense suboxidative diagenesis.     

The Refined Wet Sieving Method for the Analysis of Fine-Grained Sediments

A refined wet sieving method for the analysis of nonlithified oceanic and marine sediments is suggested based on comparison of the Petelin (SIO, Russia) and Atterberg (AWI, Germany) methods, which makes it possible to accelerate the gathering and processing of grain-size data and significantly improve the convergence between results obtained in Russian and foreign laboratories.