Rare-Earth Elements and Uranium in Phosphatic Nodules from the Continental Margins of India
Published:January 01, 2000
B. Nagender Nath, B. Ramalingeswara Rao, K. Mohana Rao, CH. M. Rao, 2000. "Rare-Earth Elements and Uranium in Phosphatic Nodules from the Continental Margins of India", Marine Authigenesis: From Global to Microbial, Craig R. Glenn, Liliane Prévôt-Lucas, Jacques Lucas
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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 <1-5 cm in size, have carbonate flourapatitc (CFA) as a single autliigenic mineral phase, are free of detrital inclusions, and have very high P205 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 higher in eastern continental margin nodules (maximum is 42 ppm). REE abundances relative to shale (ΣREE in sample/ΣREE 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. Light REEs are depleted, with Lan/Ybn r(Lasample/Lashale)/(Ybsample/Ybshale)] values lower than unity, indicating a minimal contribution from terrestrial sources. The cerium anomaly (Cc/Ce*), which is a measure of Ce fractionation relative to the neighboring 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 and LREE depletion collectively indicate either a seawater or porewater source for these elements. Since Ce is mostly stable in its trivalent stale and U precipitates under reducing conditions, the absence of Ce fractionation in association with U enrichment indicates that the nodules may have formed by autliigenic precipitation in reducing porewaters. High productivity in association with upwelling might have driven the accumulation of organic matter, which ill 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.
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Marine Authigenesis: From Global to Microbial
This volume is a collection of 33 state-of-the-art papers focusing on various aspects of authigenic and diagenetic marine minerals and related global elemental cycling. The commingling of the various studies of authigenic minerals in this volume, including the most recent advances in knowledge concerning the occurrence and origins of phosphorites, glauconites, dolomites, siderites, manganese-iron associations, barites, ironstones, and other marine chemical sediments/sedimentary rocks of early authigenic/diagenetic origin, is partly the result of the increasing awareness that there are many overlaps, even direct co-associations, between different authigenic minerals, both in time, space, and genesis. Taken together, this compilation represents a holistic approach towards marine authigenesis that considers the integrated whole more than the simple sum of its parts.