URANIUM AND THORIUM STORAGE IN CASIO3-PEROVSKITE IN THE EARTH’S LOWER MANTLE

The Earth’s mantle convection is predominantly powered by the radiogenic heat released by the breakdown of ²³⁸U, ²³⁵U, ²³²Th and ⁴⁰K. Aluminous calcium silicate perovskite, Ca(Si,Al)O₃, is a candidate to host uranium and thorium in the lower mantle. We present results from ab-initio and thermodynamic calculations to model pure and solid solutions of Ca(Si,Al)O₃-perovskite, ThSiO₄, USiO₄, ThO₂ and UO₂. The enthalpies of solution of U⁴⁺ and Th⁴⁺ species in CaSiO₃-perovskite are large, approximately 2.6 eV at 0 GPa and increasing to 8.3 eV at 100 GPa, but Th⁴⁺ defect enthalpies are ± 1 eV of zero up to 110 GPa in Ca(Si,Al)O₃-perovskite, and U⁴⁺ defect enthalpies are negative. Therefore, U and Th are unlikely to substitute into pure CaSiO₃, but are significantly more likely to substitute when associated with aluminum defects, with uranium more compatible than thorium. Furthermore, substitution into aluminous CaSiO₃ is preferred for both actinides compared to minor pure oxide or silicate phases. The difference in compatibility suggests fractionation of the actinides upon cooling from a deep magma ocean or partial melt generation at depth, preferentially sequestering more uranium in the lower mantle than thorium, leading to variable heat production as a function of depth in the mantle.