Experimental high pressure and high temperature study of the incorporation of uranium in Al-rich CaSiO 3 perovskite

Abstract

The high ability of the Al-rich CaSiO 3 perovskite to contain large amounts of uranium (up to 4 at.% U) has been studied up to 54 GPa and 2400 K, using laser-heated diamond anvil cell (LH-DAC) and up to 18 GPa and 2200 K using a multi-anvil press (MAP). Both latter HP-HT techniques proved to be complementary and gave similar results, in spite of different heating modes (laser and furnace). Chemical reactions were characterized and described by electron probe microanalysis and analytical scanning electron microscopy while associated structural changes were precisely characterized by synchrotron angle dispersive X-ray diffraction and by X-ray micro-diffraction. The diffusion of uranium into the CaSiO 3 matrix was measured as a function of run duration and temperature. We obtain diffusion coefficients with the same order of magnitude (about 10 −16 m 2 s −1) than for those found in the literature. After this work, coupled cationic substitutions of Ca by U and Si by Al are proposed to generate new interesting crystallographic features for a CaSiO 3 perovskite: a higher compressibility, a tetragonal distortion along the c-axis with c/ a ratio >1, a different compression behaviour of c-axis relative to a-axis, and a perovskite structure quenchable to ambient P and T conditions. The tetragonal U-bearing aluminous CaSiO 3 perovskite is observed to remain stable at pressures up to 54 GPa, then in the ( P, T) range of the upper part of the lower mantle. The influence of the present results, in terms of both uranium and aluminium partitioning related to the coexisting mineral phases as the (Mg,Fe)SiO 3 perovskite, is discussed. Uranium provides approximately 25% of the total energy generated within the deep Earth through its radioactive decay. The location of this source within the deep mantle is fundamental to the understanding of the geodynamics and thermal behaviour of our planet. Since the tetragonal structure of the U-bearing Al-rich CaSiO 3 perovskite is expected to remain stable towards the base of the Earth's mantle, this latter phase is proposed to be the main storage mineral for heat producing actinides of the lower mantle.