Formation and Evolution of Phosphorite Grains and Nodules on the Namibian Shelf, from Recent to Pleistocene
Published:January 01, 2000
G. N. Baturin, 2000. "Formation and Evolution of Phosphorite Grains and Nodules on the Namibian Shelf, from Recent to Pleistocene", Marine Authigenesis: From Global to Microbial, Craig R. Glenn, Liliane Prévôt-Lucas, Jacques Lucas
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Phosphatic grains and nodules related to recent diatom oozes on the inner Namibian shelf as well as Pleistocene grains and nodules from the phosphorite deposit on the adjacent outer shelf have been studied by means of scanning electron microscopy combined with several analytical methods.
Recent grain and nodules are being formed simultaneously in the same diatom oozes. Both consist of poorly, moderately, and well-lithificd varieties that reflect the process of progressive phosphatization, compaction, and crystallization of phosphatic matter along with dissolution and expulsion of nonphosphatic components. Similar processes are going on during the phosphatization of macro- and microcoprolites. The common varieties of apatite mineral morphologies are colloform masses without distinct texture, globules and globular aggregates with more or less pronounced radial crystallization, and micrometer-sized elongated particles and their aggregates of various shape, which are interpreted as diagenetically formed crystallites. All of these textures are related to each other, with the globular one becoming predominant in most lithified accretions. Reworked Pleistocene grains reveal textural features quite similar to the recent grains. They contain similar organic remains, which proves their common origin in the same environment. The chemical evolution of recent accretions in the course of their 1 ithification consists of gradual increases of phosphatic components and decreases of nonapatite components. The most striking feature of their geochemistry is the extremely low concentration of rare Earth elements in recent grains in contrast to their high concentration in Pleistocene grains. This might mean that phosphate absorbs these elements during prolonged contact with the ambient seawater.
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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.