Phosphorites on the Sea Floor

Sedimentary phosphorites sampled from the sea floor off the coasts of Peru and Chile have been investigated to establish their ages and mode of formation. Results of uranium-series dating suggest that phosphate deposition was episodic rather than continuous during the late Pleistocene. Radiometric ages correlate well with periods of high eustatic stands of the sea. Bulk chemical and mineralogical compositions of the phosphate rocks were determined by instrumental techniques. Microanalysis, with an electron probe microanalyzer and a scanning electron microscope suggests that the apatite was authigenic and had formed as direct chemical precipitate rather than by replacement. A model of phosphorite formation is proposed which involves inorganic precipitation of apatite within anoxic pore waters and subsequent concentration of the apatite by physical processes. The concentration of apatite into indurated phosphate rocks is brought about by winnowing and reworking processes, possibly in response to a change in the sedimentary environment caused by eustatic sea-level fluctuations or tectonic movements. (Modified author abstract).