The Solubility of Uranium in Bridgmanite in the Earth's Lower Mantle.

The radioactive decay of U and Th is the major heat source in the Earth's lower mantle, driving a range of important geological processes, including mantle convection. U and Th have been believed to be stored in Ca-silicate perovskite (CaPv) because of the large dodecahedral site where Ca exists in perovskite crystal structure with some Al. However, Al is known to strongly partition into bridgmanite (Brg), leaving CaPv with little Al and therefore the solubility of the heat producing elements would be limited in CaPv. Our recent experiments (in prep.) have shown that Ca can be incorporated into Brg above 2,250 K at 40-100 GPa, instead of forming a separate phase of CaPv. The unit-cell volume of Brg was 2-3% larger, suggesting that Ca enters the Brg crystal structure and expands it. These results open up a possibility of the U and Th storage in Brg in the deep lower mantle.

We report the synchrotron X-ray diffraction (XRD) experiments on the mixture of pyrolite and UO₂ at 65-75 GPa and 2,300-3,000 K in the laser-heated diamond anvil cell (LHDAC). We increase the Ca content by a factor of 2 in our pyrolite composition to enhance the compositional effect of Ca for the U solubility in Brg. We observed the formation of Brg and ferropericlase (Fp), but no CaPv, indicating the Ca dissolution in Brg. The unit-cell volume of Brg in pyrolite+UO₂ is smaller by 1.3% than in U-free pyrolite at 1 bar (164.6(3) and 166.7(9) ų , respectively). Also, the octahedral tilting angle of Brg in pyrolite+UO₂ is 7% greater than in U-free pyrolite (19.26° and 17.99°, respectively). These results suggest the different chemical composition of Brg in pyrolite+UO₂, suggesting the possible solubility of U in Brg. If U can be indeed stored in Brg which is the most dominant phase in the lower mantle, it would have important implications for the heat budget and distribution in the lower mantle.