Spin crossover in (Mg,Fe3+)(Si,Fe3+)O3 brigdmanite: effects of disorder, iron concentration, and temperature

Abstract:

The pressure induced spin crossover of Fe³⁺ in (Mg_1-xFe_x)(Si_1-xFe_x)O₃ bridgmanite (br) , the most abundant mineral of the Earth’s lower mantle, is by now a well-established phenomenon, though several aspects of this phenomenon still remain unclear. Here we investigate effects of disorder, iron concentration, and temperature on this crossover using ab initio LDA+U calculations. The effect of concentration and disorder are addressed using complete statistical samplings of coupled substituted ([Fe³⁺]_Mg and [Fe³⁺]_Si in nearest neighbor sites) configurations in 80-, 40-, and 20-atom supercells, respectively, for iron concentration of x = 0.125, 0.25, and 0.50. Vibrational/thermal effects on the crossover are also addressed within the quasi-harmonic approximation for x = 0.125. [Fe³⁺]_Si in the perovskite octahedral site undergoes high (S=5/2) to low-spin (S=1/2) state change in the pressure range ~40-50 GPa at 300 K, consistent with experiments, while [Fe³⁺]_Mg remains in the high-spin state up to core-mantle boundary conditions. The effect of disorder on the crossover seems very small, while increase in the iron concentration (x) results in considerable increase in transition pressure. As in the spin crossover in ferropericlase, this crossover in bridgmanite is accompanied by clear volume reduction and anomalous softening of the bulk modulus throughout the crossover pressure range. These effects reduce significantly with increasing temperature. Though the concentration of [Fe³⁺]_Si in bridgmanite may be small, related elastic anomalies may impact the interpretation of radial and lateral velocity structures of Earth’s lower mantle.