Prakash, and 11 co-authors, reported in 2012 that mineralogy and geochemistry of the ferromanganese oxides collected from two seamounts in the Andaman Sea have been investigated with special reference to the solid-phase partitioning behavior of rare-earth elements (REEs). Bulk elemental compositions indicate that the oxides from a volcanic cratered seamount, CSM, are hydrothermal while those from another seamount, SM2, are hydrogenous in origin.
The distributions of REEs and yttrium in four geochemical phases (viz. easily exchangeable, Mn-oxide, Feoxyhydroxide and residual) of CSM-oxides were distinctly different from that of SM2-oxides. In CSMoxides, the REEs and yttrium are mainly associated with Fe-oxyhydroxide phase while in SM2-oxides, both Mn-oxide and Fe-oxyhydroxide phases contain the major fraction of these elements. The exchangeable and residual phases of both hydrothermal and hydrogenous oxides have extremely low REE-content. The shale normalized negative Ce-anomalies and relative enrichment of heavy REE, characteristic of hydrothermal oxides, are retained in Fe-oxyhydroxide phase. In hydrogenous oxides, the positive Ce-anomalies occurred in both Fe and Mn-oxide phases while negative Y-anomalies were found only in Fe-oxyhydroxide phase.
The phase distribution of REEs and yttrium in hydrothermal oxides is described in terms of co-precipitation and adsorption of their halide complexes and available free ions in hydrothermal solutions. Instead, adsorption of major carbonate species from seawater primarily regulates the partitioning of REEs among the coexisting phases of hydrogenous oxides.
Based on the authors' present investigations on partitioning of REEs and yttrium among the co-existing solid phases of hydrothermal and hydrogenous oxides it can be concluded that:
1) The ferromanganese oxides of hydrothermal and hydrogenous origin have unique and distinctly different distributions of REEs and yttrium in their geochemical phases. The great majority of total REEs and yttrium of these ferromanganese oxides is mainly associated with Mn-oxide and Fe-oxyhydroxide phases rather than with easily exchangeable and residual phases. In hydrothermal oxides, REEs and yttrium are mostly associated with Fe-oxyhydroxide but in hydrogenous oxides, both Mn-oxide and Fe-oxyhydroxide phases are important hosts for the REEs, with the LREE and HREE preferentially adsorbed onto Mn-oxide and Fe-oxyhydroxide, respectively.
2) A major fraction of total REEs and yttrium in hydrothermal oxides co-precipitates along with Fe-oxyhydroxide particulates immediately after hydrothermal emission. With time coprecipitation of REE reduces as the fluid gets diluted with seawater while adsorption of REE on hydrothermal particulates increases away from the source. The hydrogenous oxides have identical solid phase distribution of REEs and yttrium irrespective of location, water depth, tectonic setting and physical nature of the samples. Therefore it can be suggested that adsorption of REE–carbonate species from seawater is the predominant mechanism for REE-partitioning in hydrogenous oxides.
3) The layered structure of vernadite in hydrogenous oxides can accommodate REE species whereas todorokite, birnessite and pyrolusite in hydrothermal oxides allow hardly any REE species within their crystal structure. Thus crystallography of Mn-oxides also effectively contributes to controlling total REE content and their partitioning behavior in two types of ferromanganese oxide.
4) Distribution of yttrium markedly differs from that of REEs in phases of hydrogenous oxides but not in hydrothermal oxides. Mono-carbonate dominated yttrium speciation in seawater makes its partitioning behavior different from that of dicarbonate dominated HREEs in hydrogenous oxides. In contrast, identical speciation of yttrium and REEs in hydrothermal fluid maintains analogous partitioning behavior for the different geochemical phases of hydrothermal oxide.
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