Thermoelasticity of Iron‐ and Aluminum‐Bearing MgSiO₃ Postperovskite

Abstract

We present ab initio (LDA + Usc) studies of high‐temperature and high‐pressure elastic properties of pure as well as iron‐bearing (ferrous, Fe2+, and ferric, Fe3+) and aluminum‐bearing MgSiO3 postperovskite, the likely dominant phase in the deep lower mantle of the Earth. Thermal effects are addressed within the quasiharmonic approximation by combining vibrational density of states and static elastic coefficients. Aggregate elastic moduli and sound velocities for the Mg end members are successfully compared with scarce experimental data available. Effects of iron (Fe) and aluminum (Al) substitutions on elastic properties and their pressure and temperature dependence have been thoroughly investigated. At the observed perovskite to postperovskite transition (P = 125 GPa and T = 2,500 K), compressional and shear velocities increase by 0–1% and 1.5–3.75%, respectively. This observation is consistent with some seismic studies of the D′′ discontinuity beneath the Caribbean, which suggests that our robust estimates of elastic properties of the postperovskite phase will be very helpful to understand lateral velocity variations in the deep lower mantle region and to constrain its composition and thermal structure.

The Post‐Perovskite Transition in Fe‐ and Al‐Bearing Bridgmanite: Effects on Seismic Observables

“…Ab initio results of static, vibrational, and elastic properties of bridgmanite (Pv) and PPv phases of Mg 1‐x Fe 2+ x SiO 3 , (Mg 1‐x Fe 3+ x )(Si 1‐x Fe 3+ x )O 3 , (Mg 1‐x Al x )(Si 1‐x Al x )O 3 and (Mg 1‐x Fe 3+ x )(Si 1‐x Al x )O 3 with the bearing element concentration x = 0 and 0.125 are from Shukla et al. (Shukla et al., 2015, 2016, 2019). These DFT + U SC (LDA + U SC and GGA + U SC ) calculations used the Quantum ESPRESSO software (Giannozzi et al., 2009, 2017) and were performed on supercells with up to 80 atoms.…”

“…x )(Si 1-x Al x )O 3 with the bearing element concentration x = 0 and 0.125 are from Shukla et al (Shukla et al, 2015(Shukla et al, , 2016(Shukla et al, , 2019. These DFT + U SC (LDA + U SC and GGA + U SC ) calculations used the Quantum ESPRESSO software (Giannozzi et al, 2009(Giannozzi et al, , 2017 and were performed on supercells with up to 80 atoms.…”

“…Here we use ab initio thermodynamic results (Shukla et al, 2015(Shukla et al, , 2016(Shukla et al, , 2019 on these Pv and PPv alloys to compute the two-phase boundary regions and test them against experiments. This effort is a fundamental step for methodology validation before computing the phase boundary in aggregates.…”

Elastic Anisotropy of Lizardite at Subduction Zone Conditions

“…This method involves a moderate computational workload, no more than 10% of those of conventional ones. It has been successfully adopted to obtain thermoelastic tensors of numerous minerals (Duan et al., 2019; Hao et al., 2019; Marcondes et al., 2015; Núñez Valdez, Umemoto et al., 2012; Núñez Valdez, Wu et al., 2012; Núñez‐Valdez et al., 2011, 2013; Shukla et al., 2015, 2016, 2019; Wang et al., 2019, 2020, 2021; Wu et al., 2013). Then the adiabatic bulk modulus $${K}_{S}$$ and shear modulus $$G$$ were calculated using the Voigt‐Reuss‐Hill averages of the elastic tensor.…”

cij: A Python code for quasiharmonic thermoelasticity

“…[8,10,11]). In contrast, the SAM-Cij formalism remains predictive up to the 𝑃𝑇 boundary of validity of the QHA and has already been extensively tested for lower mantle minerals at their relevant conditions [12][13][14][15]. It has also been recently extended and validated for low crystal symmetries such as monoclinic and trigonal [16,17].…”