Geochemistry of polymetallic nodules from the Central Indian Ocean basin

Abstract

Polymetallic nodules dredged at fourteen stations between 12 degrees-15 degrees S and 76 degrees-80 degrees 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. 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 indicates shifting of the smaller nodules (formed close to the CCD) to deep-sea regions and mixing with larger nodules formed from hydrogenous precipitation