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: K_S0 = 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 composition, in particular for Vs.

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 Pa3̅ 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 Pa3̅ 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.