Atlantic pelagic sediments: REE and PGE partitioning between phosphate and Fe–Mn oxyhydroxide phases

Introduction Rare earth elements and yttrium (REY) are critical for high-technology industries, and deep-sea sediments enriched in these elements are considered a promising future resource. The highest REY concentrations are documented in pelagic clays of the Pacific and Indian Oceans, whereas Atlantic pelagic sediments have substantially lower contents; the reasons for this contrast remain unclear. Furthermore, the distribution of platinum group elements (PGE) among carrier phases in marine sediments remains poorly investigated, particularly with respect to Mn-micronodules, which may represent an important PGE reservoir. Methods Here we present the first quantitative assessment of REY and PGE partitioning among the main mineral phases in pelagic sediments of the Brazil Basin. Bulk contents of REY, PGE, and major elements were determined by ICP-MS and ICP-OES. Sequential leaching was applied to separate carbonate–phosphate (L1), Mn-oxide and easily reducible Fe-oxyhydroxide (L2), Fe-oxyhydroxide (L3), and residual aluminosilicate (L4) fractions. The leachates were directly compared with biogenic apatite and Mn-micronodules handpicked from the same sediments; the micronodules were examined by SEM-EDX and XRD. Results The bulk ΣREY contents range from 323 to 629 μg/g. The phosphate phase is the dominant REY carrier, accounting for 54 to 72% of the bulk ΣREY. The phosphate leachate yields REY concentrations that are an order of magnitude higher than can be accounted for by dissolution of biogenic apatite alone, indicating an additional REY source. Micronodules are of mixed hydrogenetic–diagenetic composition, consisting of hydrogenetic and diagenetic layers. They inherit the REY composition of the dispersed oxyhydroxides, but their Mn/Fe ratio (2.7) is 2–3 times higher than in the initial oxyhydroxide phase (0.7–1.0). Platinum is strongly enriched in the oxyhydroxide phase, with concentrations in micronodules reaching 299 ng/g and exceeding those in the nodule and crust from the same station, whereas palladium remains in the lithogenic fraction. Discussion We suggest that the REY excess in the phosphate leach is largely due to an adsorbed complex, so that the L1 leachates represent a mixture of apatite and adsorbed REY. The lower REY contents in Atlantic sediments compared to the Pacific are attributed to higher sedimentation rates, which dilute bioapatite with terrigenous material and limit the contact time between bioapatite and REY-enriched pore waters.