HR: 0800h
AN: V41E-1521    [Abstracts]
TI: Uranium in the Earth's lower mantle
AU: * Greaux, S
EM: greaux@univ-mlv.fr
AF: Laboratoire des Géomatériaux, FRE 2455 CNRS, Université de Marne-la-Vallée, 5 boulevard Descartes, Champs-sur-Marne, Marne-la-Vallée, 77454 France
AU: Gautron, L
EM: gautron@univ-mlv.fr
AF: Laboratoire des Géomatériaux, FRE 2455 CNRS, Université de Marne-la-Vallée, 5 boulevard Descartes, Champs-sur-Marne, Marne-la-Vallée, 77454 France
AU: Andrault, D
EM: andrault@impmc.jussieu.fr
AF: Institut de Minéralogie et de Physique des Milieux Condensés, UMR 7590 CNRS, Campus Boucicaut, bat7, 140 rue de Lourmel, Paris, 75015 France
AU: Bolfan-Casanova, N
EM: N.Bolfan@opgc.univ-bpclermont.fr
AF: Laboratoire Magmas et Volcans, UMR 6224 CNRS, Université Blaise Pascal, 5 rue Kessler, Clermont-Ferrand, 63038 France
AU: Guignot, N
EM: guignot@esrf.fr
AF: European Synchrotron Radiation Facility, 6 rue Horowitz BP220, Grenoble, 38043 France
AU: Bouhifd, M A
EM: Ali.Bouhifd@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1, 3PR United Kingdom
AB: The Earth's internal activity shows off through, among others, earthquakes, volcanic eruptions and continental drifts. These events are energetically related to the heat flux at the Earth surface (44 TW). About half of this heat flux comes from the radioactive decay of uranium, thorium and potassium, with 9 TW accounting for U alone. Uranium is expected to be mainly present in the mantle and it is assumed that 50 wt % of total U in the Earth is stored in the lower mantle. To locate the heat sources in the mantle is essential for a better understanding of the geodynamics and thermal behaviour of the Earth. Preliminary results show no incorporation of uranium in Mg-perovskite, because of its too large cationic size. On the contrary, calcium is a good candidate for its substitution by uranium. In the lower mantle, calcium is present in the CaSiO₃ perovskite which is believed to be the third important major phase (about 7 wt % or 5 mol %). To determine U-bearing minerals in the deep mantle, we have performed solid-solid reactions between Ca-perovskite and uraninite UO₂ at high pressures (HP) and high temperatures (HT) using both Laser-Heated Diamond Anvil Cell (LHDAC) and Multi-Anvil Press (MAP). LHDAC samples were analyzed in situ by X-Ray Diffraction (XRD) with a synchrotron light source at ESRF (Grenoble, France), while MAP samples were studied using Electron Probe Micro-Analysis (EPMA), Analytical Scanning Electron Microscopy (ASEM) and X-Ray micro-Diffraction (μ-XRD). Here we show the occurrence of a new U-bearing CaSiO₃ perovskite containing up to 35 wt % UO₂ (4 at % U per formula unit). This phase adopts a tetragonal distortion and is stable to pressures up to at least 54 GPa (1300 km depth). The CaSiO₃ perovskite could therefore be the major host of uranium in the lower mantle, and is expected to be present in the bottom half of the lower mantle. The agreement of these results with a recent model of a layered mantle convection is discussed.
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005