Abstract
The vibrational and thermodynamic properties of minerals are key to understanding the phase stability and the thermal structure of the Earth’s mantle. In this study, we modeled hydrous iron-bearing bridgmanite (Brg) and post-perovskite (PPv) with different [Fe3+-H] defect configurations using first-principles calculations combined with quasi-harmonic approximations (QHA). Fe3+-H configurations can be vibrationally stable in Brg and PPv; the site occupancy of this defect will strongly affect its thermodynamic properties and particularly its response to pressure. The presence of Fe3+-H introduces distinctive high-frequency vibrations to the crystal. The frequency of these peaks is configuration dependence. Of the two defect configurations, [FeSi′+OH·] makes large effects on the thermodynamic properties of Brg and PPv, whereas [VMg″+FeMg·+OH·] has negligible effects. With an expected lower mantle water concentrations of <1000 wt. ppm the effect of Fe3+-H clusters on properties such as heat capacity and thermal expansion is negligible, but the effect on the Grüneisen parameter γ can be significant (~1.2%). This may imply that even a small amount of water may affect the anharmonicity of Fe3+-bearing MgSiO3 in lower mantle conditions and that when calculating the adiabaticity of the mantle, water concentrations need to be considered.
Highlights
Iron-bearing bridgmanite (Brg) and its high-pressure phase post-perovskite (PPv) are thought to be the most abundant mineral phases in the Earth’s lower mantle and D” layer along with ferropericlase (Mg, Fe)O and Calcium silicate perovskite CaSiO3 [1,2]
Water can be incorporated into a cationic vacancy (Mg site or spin Fe3+ in the B site (Si site)) in a MgSiO3 lattice as a hydrogen defect via a charge-coupled substitution mechanism [53,54,55]
Two different hydrous states were considered in this study: a [Fe3+-H]Si defect that simulates the interaction of water with a Fe3+ in a Si octahedral site (B-site), and an [Fe3+-H]Mg-Mg defect which simulates the interaction of water with Fe3+ in an Mg dodecahedral site (A-site)
Summary
Iron-bearing bridgmanite (Brg) and its high-pressure phase post-perovskite (PPv) are thought to be the most abundant mineral phases in the Earth’s lower mantle and D” layer along with ferropericlase (Mg, Fe)O and Calcium silicate perovskite CaSiO3 [1,2]. The lower mantle is reductive, Fe can be incorporated into bridgmanite and post-perovskite in both the +2 (ferrous) and +3 (ferric) state, with the latter being favored in the presence of Aluminum at the high pressures and temperatures near the D” [12]. This is created through a disproportionation reaction of Fe2+ into Fe3+ and metallic Fe [13,14,15]. Experimental and theoretical studies demonstrate that Fe3+ can be incorporated into the lattice of lower mantle minerals either as Fe3+-Fe3+ pairs on the Mg (A) and Si (B) sites or as Fe3+-Al3+ pairs with Fe3+ on the A and Al3+ on the B though above ~80 GPa these preferences can be changed as some Fe3+ swaps to the B site and Al3+ to the A site [2,16,17,18,19]
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