Rare earth element distribution and behaviour in buried manganese nodules from the Central Indian Basin

Abstract

Eight buried manganese nodules taken in the upper metre of a siliceous ooze sediment core in the Central Indian Basin at 14°S and 74°E were analysed for major, trace and rare earth (REE) elements. Interelemental correlation yields a strong positive association ( r > 0.75) between Fe, P, Ti and REE suggesting a single carrier phase for REE in these nodules namely a titaniumrich ferriphosphate. This correlation significantly improved ( r > +0.90) when the influence of detrital Fe is eliminated and only authigenic Fe is considered. The buried nodules appear to retain the mineralogical and geochemical characteristics (including positive Ce anomaly) acquired during the accretion at the sediment-water interface prior to their burial at ∼225 kyr B.P. The effect of burial is observed only in the depletion of heavy REE (HREE) in a nodule buried at 23.5 cm depth where the environment is suboxic to anoxic. Middle REE (MREE) in the nodules exhibit enrichment over light REE (LREE) and HREE while LREE exhibit enrichment over HREE compared to the ratios in shale (NASC). This enrichment pattern suggests an uptake sequence of the REE as MREE > LREE > HREE in the manganese nodules.