Phosphorite deposits from the sea floor off Peru and Chile: radiochemical and geochemical investigations concerning their origin
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1974
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University of Hawaii at Manoa
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Abstract
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. Uranium-series disequilibrium studies verify that phosphate deposits are currently forming in that area. The distribution of radiometric ages over the past 150,000 years implies 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. The fractionation of uranium isotopes between oxidation states (IV) and (VI) in these relatively young phosphorites is low as compared with that in older deposits. The relative amount of U(IV) contained in phosphate deposits appears to be a function of the extent of the reducing environment during deposition and how much, if any, of the uranium had been oxidized since incorporation into the apatite structure. The bulk chemical and mineralogical compositions of the phosphate rocks reflect varying degrees of dilution of the phosphatic material, apatite, by other authigenic minerals and various allogenic components. Electron probe microanalysis shows that the composition of the phosphate rocks is complex, i.e., the rocks are derived from more than one phase, even within extremely small areas. Examination with the scanning electron microscope (SEM) of freshly fractured surfaces of phosphate rocks and small pellets from associated diatomaceous ooze suggests that the apatite was authigenic and had formed as a direct chemical precipitate rather than by replacement. Some surfaces of siliceous biogenic materials appear to act as sites for apatite nucleation. The model of phosphorite formation hypothesized here involves inorganic precipitation of apatite within anoxic pore waters and subsequent concentration of the apatite by physical processes. Oxidation of organic materials (mainly diatoms) during SO42- reduction is the main source of dissolved phosphate. Apatite precipitation is favored by the high phosphate concentration in the interstitial waters, especially where the sediments have been deposited in highly oxygen-deficient waters, and by diagenetic reactions which remove interfering Mg2+ ions within the sediments. The common association of apatite with Mg-bearing phases (chlorite, sepiolite, dolomite) within the sediments from the Peru shelf supports the view that reactions resulting in Mg2+ depletion in pore waters are essential for apatite precipitation. Reactions such as dolomitization, replacement of Fe3+ by Mg2+ in clays (Drever, 1971), and the authigenic formation of Mg-si1icates are proposed as the most likely controls of the Mg2+ content in anoxic pore waters from this region. 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.
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Phosphate rock, Marine sediments, Ocean bottom, Submarine geology
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Pacific Ocean
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Theses for the degree of Doctor of Philosophy (University of Hawaii at Manoa). Geology and Geophysics; no. 619
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