Abstract

Earth’s mantle convection is powered in part by the radiogenic heat released by the decay of 238U, 235U, 232Th, and 40K. We present ab initio calculations of uranium and thorium incorporation in CaSiO3-perovskite with and without aluminum, and propose that aluminous calcium silicate perovskite is the likely host of uranium and thorium in the lower mantle. At 15 GPa, the enthalpies of solution into aluminum-free CaSiO3-perovskite are 10.34 kJ/mol for U4+ and 12.52 kJ/mol for Th4+ in SiO2 saturated systems, while the enthalpies are 17.09 kJ/mol and 19.27 kJ/mol, respectively, in CaO saturated systems. Coupled substitution of U4+ and Th4+ with aluminum is thermodynamically favored, with the enthalpies of solution negative for U4+ and near 0 kJ/mol for Th4+ throughout the stability field of CaSiO3-perovskite. Therefore, U incorporation into CaSiO3-perovskite is spontaneous in the presence of aluminum while Th forms a near ideal solid solution, implying these elements are potentially compatible with respect to partial melting in the transition zone and lower mantle. Furthermore, the solid solution reactions of U4+ and Th4+ are broadly similar to each other, suggesting a restriction on the fractionation of these actinides between the upper and lower mantle. U and Th compatibility in the presence of Al has implications regarding actinide transport into the deep mantle within subducting slabs and the geochemical content of seismic anomalies at the core-mantle boundary.

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