Thermoelastic properties of iron- and aluminum-bearing bridgmanite at high pressures and temperatures.

摘要

Thermoelastic properties of the Earth's forming minerals play an important role in deciphering the tomographic images of seismic observations. In spite of the considerable progress in the experimental measurements of the elastic properties of minerals at high pressures and temperatures, the available data is still quite limited to constrain the composition and thermal structure of the Earth's interior. The first-principles atomistic calculations have often complimented the experimental measurements in the study of minerals under high pressure and temperature conditions. In this work, we present the first-principles investigation of the effect of iron (Fe) and aluminum (Al) on the thermoelastic properties of MgSiO3 perovksite (also known as bridgmanite), the most abundant mineral of the Earth's lower mantle. First, we investigate the pressure induced iron state changes in Fe-bearing MgSiO3 and MgGeO3 perovskite (a low-pressure analog of MgSiO3 ) within the local density (LDA+U) and the generalized gradient approximation augmented by the Hubbard-type correction (GGA+U). We showed that the iron state transitions occur at particular average Fe-O bond-length irrespective of mineral composition (MgSiO3 or MgGeO3 ) or the exchange and correlation functional used in the calculations (LDA+U or GGA+U). We further study the effect of disorder, iron concentration, and temperature on the spin crossover in Fe3+ -bearing bridgmanite using LDA+U calculations. Thermal effects have been addressed within the quasiharmonic approximation using density functional perturbation theory (DFPT). Then, we calculate the aggregate elastic moduli (bulk and shear modulus) and acoustic velocities for the Fe- and Al-bearing bridgmanite to investigate the effect of iron state changes and its possible consequences to the lower mantle composition.