Uranium-series fractionation in mafic extrusives |
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| Institution: | 1. Friedrich-Loeffler-Institut, Federal Research Institute for Animal Health, Institute of Bacterial Infections and Zoonoses, Naumburger Str. 96A, 07743 Jena, Germany;2. Federal Institute for Risk Assessment, Diedersdorfer Weg 1, 12277 Berlin, Germany;3. Ludwig-Maximilians-Universität München, Institut für Infektionsmedizin und Zoonosen, Infektions- und Seuchenmedizin, Veterinärstraße 13, 80539 München, Germany;4. Justus-Liebig-Universität Gießen, Institut für Hygiene und Infektionskrankheiten der Tiere, Frankfurter Str. 87-89, 35392 Gießen, Germany;1. Laboratory for Systems Biology of Gene Regulatory Elements, Berlin Institute for Medical Systems Biology, Max-Delbrück-Center for Molecular Medicine, Robert-Rössle-Strasse10, 13125 Berlin, Germany;2. Department of Pediatrics, Charité University Medical Center, Division of Oncology and Hematology, Augustenburger Platz 1, 13353 Berlin, Germany;1. Department of Materials Science and Engineering, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India;2. Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada;3. Texas Instruments Dallas, TX 75243, USA;4. Department of Electronics & Communications Engineering, Indraprastha Institute of Information Technology, Delhi, Okhla Phase-III, New Delhi 110020, India;5. School of Interdisciplinary Research (SIRe), Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India |
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| Abstract: | Based on U-series disequilibrium arguments there is good evidence for the presence of U-rich accessory minerals in the outer mantle. The very large excesses of 226Ra and 231Pa activity relative to 230Th, 238U, and 235U in most mid-ocean ridge basalts and some non-divergent plate-margin basalts are inconsistent with prevailing incompatibility models of U-series fractionation. Application of a fundamental principle of equilibrium balance reveals that in these instances more than half of the original U and Th remains behind in the residual outer mantle when basaltic magmas separate. One is forced to conclude that, in the outer mantle, U and Th do not occur primarily in major silicate minerals, where they would indeed be incompatible. Rather, they must be occurring as high concentration components in refractory accessory minerals. The precipitous concentration gradients bounding such minerals would allow for the operation of physical processes, such as alpha recoil and daughter diffusion, to produce paired disequilibrated phases. Daughter deficiencies would develop in the high concentration minerals, and daughter excesses in the surrounding low concentration major silicates. This would constitute a steady-state condition existing in the mantle before the onset of melting. Subsequent preferential melting of the matrix silicates would readily result in the disequilibria observed in basalts. |
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