Calcium dissolution in bridgmanite in the Earth's deep mantle

Abstract

Accurate knowledge of the mineralogy is essential for understanding the lower mantle, which represents more than half of Earth's volume. CaSiO₃ perovskite is believed to be the third-most-abundant mineral throughout the lower mantle, following bridgmanite and ferropericlase^1-3. Here we experimentally show that the calcium solubility in bridgmanite increases steeply at about 2,300 kelvin and above 40 gigapascals to a level sufficient for a complete dissolution of all CaSiO₃ component in pyrolite into bridgmanite, resulting in the disappearance of CaSiO₃ perovskite at depths greater than about 1,800 kilometres along the geotherm^4,5. Hence we propose a change from a two-perovskite domain (TPD; bridgmanite plus CaSiO₃ perovskite) at the shallower lower mantle to a single-perovskite domain (SPD; calcium-rich bridgmanite) at the deeper lower mantle. Iron seems to have a key role in increasing the calcium solubility in bridgmanite. The temperature-driven nature can cause large lateral variations in the depth of the TPD-to-SPD change in response to temperature variations (by more than 500 kilometres). Furthermore, the SPD should have been thicker in the past when the mantle was warmer. Our finding requires revision of the deep-mantle mineralogy models and will have an impact on our understanding of the composition, structure, dynamics and evolution of the region.