Rare earth element geochemistry of ferromanganese deposits of the Indian Ocean

Abstract

Fourteen manganese nodules and three ferromanganese crusts from the Indian Ocean were analysed for major and minor elements and the 14 naturally occurring rare earth elements (REE). The REE were analysEd. by inductively coupled plasma-mass spectroscopy (ICP-MS). The samples were selected systematically from the Western Indian Ocean and the Central Indian Basin, to represent a seamount top, slope, an abyssal hill, siliceous sediment affectEd. by terrigenous influx, a highly productive siliceous environment, red clay and carbonate sedimentary domains. Although REE zonation is observed in one oriented nodule, with relative enrichment in the top, evidence of top-bottom fractionation appears to have been obliterated as a result of the nodules being turned over. Correlations between Ca, P, Fe and REE in nodules suggest that the REE primarily reside in the iron oxyhydroxide and phosphatic phases. An authigenic origin is attributed to these elements. The nodules and crusts from the Western Indian Ocean and the shallower depths of the Central Indian Basin are delta -MnOd2 rich, and are characterizEd. by high concentrations of REE and higher positive cerium anomalies. These two areas are in the realm of cold Antarctic Bottom Water (AABW), which may enhance the oxidative scavenging of Ce by particles and its subsequent incorporation into manganese nodules. All the nodules and crusts show Gd-Tb anomalies. A diagenetic nodule with a palagonitic, smectite-rich nucleus exhibits an unusual heavy REE (HREE) enrichment with no significant Ce anomaly