2009 Volume 43 Issue 1 Pages 37-47
The distribution of the rare earths and yttrium (REY) in co-existing hydrous Mn oxides and Fe oxides that form marine hydrogenetic ferromanganese crusts is used to better describe the partitioning and fractionation of the REY between these (hydr)oxides and seawater in the natural marine system. Four fractions (easily exchangeable, Mn-oxide-bound, Fe-oxide-bound, and insoluble-residue-bound REY) were separated by an adjusted sequential leaching procedure from two ferromanganese crusts from the Central Pacific. The distribution of the REY differs significantly between these leaching fractions and gives evidence for decoupling of La, Ce, Gd, Y, and Lu from their respective neighbours in the REY series during partitioning between hydrous Fe oxides, Mn oxides and seawater. Both the Mn oxides and the Fe oxides display pronounced positive Ce anomalies of almost similar size. This suggests that in the natural marine system oxidative scavenging of Ce from seawater is not restricted to Mn oxides but also occurs on hydrous Fe oxides. The distribution of Ce between the Mn oxides and the hydrous Fe oxides follows that of the trivalent REY and contrasts sharply with that of tetravalent Zr, Hf and Th. This suggests that preferential Ce removal from seawater does not result from the oxidation of dissolved Ce(III) within the marine water column, but that Ce(III) is oxidized after its sorption at the metal (hydr)oxide surface. Patterns of apparent hydrous Fe oxide/Mn oxide distribution coefficients show a sigmoidal shape and display negative anomalies for La, Gd, Y, and Lu (and the M-type lanthanide tetrad effect), indicating preferential scavenging of these elements by the Mn oxides as compared to the Fe oxides. The pronounced differences between the REY distribution in the Mn oxides and that in the Fe oxides cannot solely be explained by the REY speciation in seawater, but require additional and mineral-specific REY fractionation during surface-complexation.
