Geochemistry of polymetallic nodules from the Central Indian Ocean Basin

Abstract

Polymetallic nodules dredged at fourteen stations between 12°–15°S and 76°–80°E in the Central Indian Ocean Basin (CIOB) display wide variations in shape, size, texture, mineral phases and chemical composition. Among the thirteen different types of nodules present, only spherical and ellipsoidal nodules ranging in size from <2 cm to 2–4 cm with a rough, granular texture are abundant in the siliceous sediments. In the CIOB, the nodules of a particular size range have common morphological, mineralogical and chemical properties. Compared to the larger nodules, the smaller nodules are enriched in Mn, Cu and Ni, have higher growth rates and higher Cu/Ni, Ce/La and Ce/Fe ratios, and todorokite as the major mineral phase. The smaller nodules also show enrichment in heavy REEs and exhibit a negative Ce anomaly. Comparison of the average Cu + Ni + Co content in the different-sized nodules indicates that only smaller nodules have grades (2.61–3.04% for nodules <2 cm and 2.33–2.84% for nodules from 2 to 4 cm) similar to those from the Northeast Pacific Ocean. Different-sized nodules from the CIOB suggest formation at different places and indicate movement due to bottom currents from the place of formation to the deeper water regions. Three models are suggested to explain the observed morphological, mineralogical and geochemical variations with respect to nodule size: (1) In the CCD model, the smaller nodules (< 4 cm) are formed at or near the CCD and are influenced by diagenesis and high biological productivity; they are enriched in Mn, Cu and Ni, and todorokite is present. (2) In the deep-sea environment model, large nodules (> 4 cm) which have high Fe and moisture and low todorokite contents are formed mainly by hydrogenesis. (3) The mixing model indicated shifting of the smaller nodules (formed close to the CCD) to deep-sea regions and mixing with larger nodules formed from hydrogenous precipitation.