Further studies of the nature,composition and ages of contemporary phosphorite from the Namibian Shelf |
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| Authors: | J Thomson SE Calvert S Mukherjee WC Burnett JM Bremner |
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| Institution: | 1. Institute of Oceanographic Sciences, Wormley, Godalming, Surrey , GU8 5UB England;2. Department of Oceanography, University of British Columbia, Vancouver, B.C. V6T 1W5 Canada;3. Department of Oceanography, Florida State University, Tallahassee, FL 32306 U.S.A.;4. Department of Geology, University of Cape Town, Rondebosch 7700, Cape Town South Africa |
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| Abstract: | Major and minor element and radiochemical data are examined for a set of phosphorite concretions and other phosphatic material from the Namibian Shelf. The concretions were separated on a morphological basis from grab samples taken in a transect of shallowing depth over the diatomaceous ooze belt. Compositional data indicate that the samples are fairly pure carbonate-fluorapatite, and trend towards a maximum 32% P2O5 content. A range of compositional variability similar to other samples from the Namibian Shelf is observed.Radiochemical systematics confirm that pelletal phosphorite and glauconitized pelletal phosphorite from depths > 170 m are at radioactive equilibrium, and are therefore old and reworked. The concretions, collected from depths shallower than 78 m, are entirely Recent, with maximum ages in the range 500–3800 years. They may be much younger. Despite distinct textural and small compositional differences, similar ages are found for separates from the same grab. Samples of fish scales and an unidentified high-magnesian phosphate mineral are also shown to have incorporated uranium recently. |
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