Vibrational and Thermodynamic Properties of Hydrous Iron-Bearing Lowermost Mantle Minerals

Jiajun Jiang,Feiwu Zhang,Joshua M R Muir

doi:10.3390/min11080885

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

Introduction

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]

Methods

Results

Conclusion