Paper No. 122-8
Presentation Time: 9:00 AM-6:30 PM
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.