Rare-earth elements and uranium in phosphatic nodules from the continental margins of India

Abstract

A sediment core collected from a bathymetric high off Goa on the western continental margin of India has yielded phosphatic nodules at various subsurface depths (at 110, 150, 305 355, 435, 500, 505 and 525 cm). The nodules are hosted by sediments of Pleistocene age. They are less than 1-5 cm in size, have carbonate flourapatite (CFA) as a single authigenic mineral phase, are free of detrital inclusions, and have very high P sub(2) O sub(5) contents (> 30%). In addition, soft and hard phosphatic nodules have also been recovered from the eastern continental margin of India. Eight nodules from various subsurface depths of the sediment core from the western continental margin and two nodules from the eastern continental margin were analyzed for uranium and rare-earth elements (REEs) by inductively coupled plasma-mass spectrometry (ICP-MS). Total REE contents are very low (8-21 ppm) in western continental margin nodules and only slightly in eastern continental margin nodules (maximum is 42 ppm). REE abundances relative to shale (Sigma REE in sample / Sigma REE in shale) are less than 0.22 and are comparable to phosphatic nodules from the Namibian continental margin, and are slightly lower than the nodules from the Peruvian continental margin. The cerium anomaly (Ce/Ce), which is a measure of Ce fractionation relative to the neighbouring REEs, ranges between 0.64 and 1.38 with most close to 1, indicating very little Ce fractionation. Uranium concentrations are very high in nodules from both margins. Low REE contents are LREE depletion collectively indicate either a seawater or porewater source for these elements. Since Ce is mostly stable in its trivalent state and U precipitates under reducing conditions, the absence of Ce fractionation in association with U enrichment indicates that the nodules may have formed by authigenic precipitation in reducing porewaters. High productivity in association with upwelling might have driven the accumulation of organic matter, which in turn would help enrich phosphate in porewaters, eventually leading to the formation of high-grade phosphorites. Ce anomaly values in nodules do not correlate with paleoredox conditions in the water column, thus the REEs for these nodules are interpreted to mainly reflect porewater conditions