Role of Spin Crossover and Other Physicochemical Transformations in the Lower Mantle

Abstract

Extremely high P–T conditions in the lower mantle affect some basic physicochemical properties of elements in minerals. One of the most important is a spin transition, which significantly changes the properties of iron-containing minerals in lower-mantle associations. The iron high-spin to low-spin transition in ferropericlase occurs at depths of 1000–1500 km. It is accompanied by the reduction of the unit cell volume and corresponding seismic velocity variations. However, the spin crossover in ferropericlase is a seismologically transparent transition owing to its gradual nature. Incorporation of iron in bridgmanite is more complex than in ferropericlase. Fe2+ in the A site remains in the HS state at all mantle conditions. In contrast, Fe3+ undergoes a spin transition in the entire range of lower-mantle conditions. The iron spin transition in bridgmanite does not change the existing seismological model down to the D″ layer. Under high-pressure conditions, chemical elements can obtain dramatic new properties in the lower mantle, including the formation of unexpected crystal structures and completely new counter-intuitive compounds. Some of these compounds are confirmed experimentally. Most of these transformations may occur within the lower mantle in specific compositions, which may produce only accessory mineralization. However, they may play a significant role in the Earth’s balance of light elements, in the formation of the primordial carbonatitic association , and influence some major lower-mantle phases, such as periclase with the formation of magnesium peroxide MgO2.