Steeve Gréaux

Uranium in the Earth's lower mantle

[1] The distribution of the radiogenic heat sources strongly influences the geodynamics and thermal behaviour of the Earth. About 11 TW is produced by the radioactive decay of uranium (25% of the total heat flux at Earth surface), and 55% of this energy comes from the lower mantle. Here we report the first experimental evidence that aluminous CaSiO 3 perovskite is the major, or even the only, host of uranium in the Earth lower mantle, since such a phase is able to incorporate up to 35 wt% UO 2 (or 4 at% of U). The aluminous Ca-perovskite could be the main U-bearing constituent of a dense and radiogenic reservoir proposed in a recent model and located in the bottom half of the lower mantle.

Elasticity and sound velocities of polycrystalline Mg3Al2(SiO4)3 garnet up to 20 GPa and 1700 K

Elastic wave velocities of synthetic polycrystalline Mg3Al2(SiO4)3 garnet have been successfully measured to 20 GPa and temperatures up to 1700 K by ultrasonic interferometry combined with energy-dispersive synchrotron x-ray diffraction in a Kawai-type multi-anvil apparatus. Compressional (Vp) and shear (Vs) wave velocities as well as the adiabatic bulk (Ks) and shear (G) moduli exhibit monotonic increase with increasing pressure and decrease with increasing temperature, respectively. Two-dimensional (P-T) linear fittings of the present data yield the following parameters: KS0 = 170.0(2) GPa, əKs/əP = 4.51(2), əKs/əT = −0.0170(1) GPa/K, G0 = 93.2(1) GPa, əG/əP = 1.51(2), and əG/əT = −0.0107(1) GPa/K, which is in good agreement with the earlier results by Brillouin scattering and/or ultrasonic measurements at relatively low P-T conditions. The observed linear pressure and temperature dependence in both Vp and Vs is in contrast to the non-linear behavior of Vp and Vs for majorite garnet with the pyrolite co...

Structural characterization of natural UO2 at pressures up to 82 GPa and temperatures up to 2200 K

Abstract Uranium is one of the main heat sources in the Earth, as about 25% of the total heat is produced by the radioactive decay of U. The location of U in the deep mantle is then essential for a better understanding of the geodynamics and thermal behavior of the Earth. For the first time, the crystal structure of natural simple dioxide UO2 uraninite has been studied by X-ray diffraction with synchrotron radiation (ESRF, Grenoble, France), in situ in a laser-heated diamond-anvil cell at pressures and temperatures relevant to the deep Earth’s mantle. Fluorite-type UO2 displays a new sequence of phase transitions at high P and T, with a cubic modified fluorite Pa3 observed at 18 GPa, and an orthorhombic Pbca structure from 33 GPa up to 82 GPa. Using a second-order Birch-Murnaghan equation of state, we calculated room-pressure bulk modulus K0 = 166(7) GPa with pressure derivative K′0 = 4.0 for the Pa3̄ structure, and K0 = 225(8) GPa with K′0 = 4 for the Pbca structure. The expected Pnma cotunnite structure was not observed but is not excluded at pressures higher than 82 GPa. Since UO2 displays a Pbca structure stable up to 82 GPa and presents a density much higher than the average density of the surrounding mantle, UO2 could be a host of U in the deep lower mantle.

X-ray absorption near edge structure (XANES) study of the speciation of uranium and thorium in Al-rich CaSiO3 perovskite

Abstract X-ray absorption spectroscopy was used to investigate the oxidation state of uranium in various Uand Th-bearing Al-rich CaSiO3 perovskite samples synthesized at high-pressure and high-temperature using a multi-anvil press apparatus. X-ray absorption near edge spectroscopy (XANES) spectra collected at the U LIII- and Th LIII-edges using both micro- and macro-focused beams show U4+ in the Al-rich CaSiO3 perovskite. The structure of the U- and Th-bearing Al-rich CaSiO3 perovskite samples have been cross-checked by XANES spectra collected at the Ca K-, Al K-, and Si K-edges. Al K and Si K spectra suggest that Al incorporates exclusively on the Si site of the CaSiO3 perovskite. Ca K spectra of the (U,Th)-bearing Al-rich CaSiO3 perovskite samples were succesfully compared to FEFF8.2 ab initio models of a tetragonal CaSiO3 perovskite with space group P4/mmm. Our results confirm previous assumptions of the coupled substitution of CaSi2 by UAl2 in CaSiO3 perovskite and that U and Th can be incorporated separately or together in CaSiO3 perovskite by means of this mechanism. The possible occurrence of the U- and Th-bearing Al-rich CaSiO3 perovskite are discussed as a potential candidate to locally host a large amount of actinides in the Earth’s deep mantle. The study of a phase that can act as a storage mineral for heat-producing actinide elements such as uranium and thorium is fundamental to the understanding of the geodynamics and thermal behavior of Earth.

Redox and Speciation of Uranium in Al‐Rich Perovskites from High‐Pressure/High‐Temperature Conditions

Macro‐, and microXANES and EXAFS spectra were collected at the U‐L3, Al‐K, Si‐K and Ca‐K edges in high pressure/high temperature CaSiO3 perovskite that aim to simulate one potential reservoir for the Earth heat flow. In that perovskite, U is tetravalent and appears substituted to Ca thanks to a coupled substitution of Al for Si. A FEFF model of the Ca site occupied by U is consistent with that mechanism but the modeled EXAFS spectrum is markedly different to that measured in various samples at various scales (mm and μm). Structural relaxation of the U site probably explains these discrepancies, suggesting that, if ionic rules can explain qualitatively cation substitutions, more complex mechanisms occur at the A‐scale.