Elastic Properties of the Pyrite‐Type FeOOH‐AlOOH System From First‐Principles Calculations. (11th May 2021)

• Record Type: Journal Article
• Authors: Thompson, Elizabeth C.; Campbell, Andrew J.; Tsuchiya, Jun
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 22:Number 5(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-05-11
• Subjects: AlOOH -- elasticity -- FeOOH -- LLSVP -- pyrite -- ULVZ; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2021GC009703
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: The stability, structure, and elastic properties of pyrite‐type (FeS2 structured) FeO2 H were determined using density functional theory‐based computations with an internally consistent Coulombic self‐interaction term ( U eff ). The properties of pyrite‐type FeO2 H are compared to that of pyrite‐type AlO2 H, with which it likely forms a solid solution at high temperature, as well as the respective lower pressure CaCl2 ‐type polymorphs of both endmembers: ϵ ‐FeOOH and δ ‐AlOOH. Due to substantial differences in the CaCl2 ‐type → pyrite‐type structural transition pressures of these endmembers, the stabilities of the (Al, Fe)O2 H solid solution polymorphs are anticipated to be compositionally driven at lower mantle pressures. As the geophysical properties of (Al, Fe)OOH are structurally dependant, interpretations regarding the contribution of pyrite‐type FeO2 H to seismically observed features must take into account the importance of this broad phase loop. With this in mind, Fe‐rich pyrite‐type (Al, Fe)OOH may coexist with Al‐dominant CaCl2 ‐type δ ‐(Al, Fe)OOH in the deep Earth. Furthermore, pyrite‐type (Al0.5–0.6, Fe0.4–0.5 )O2 H can reproduce the reduced compressional and shear velocities characteristic of seismically observed ultra low velocity zones in the Earth's lowermost mantle while Al‐dominant but Fe‐bearing CaCl2 ‐type δ ‐(Al, Fe)OOH may contribute to large low shear velocity provinces. Plain Language Summary: Hydrogen storage and cycling in the deep Earth may have important implications for the chemistry and dynamics of Earth's mantle. Studies suggest a likely carrier of hydrogen into the Earth's lower mantle is the solid solution formed by ϵ ‐FeOOH, δ ‐AlOOH, and phase H (MgSiO4 H2 ). At extreme pressures, ϵ ‐FeOOH and δ ‐AlOOH are both expected to transform into a pyrite‐type crystal structure. This study provides a detailed examination of the stability and geophysical properties of pyrite‐type FeO2 H at the pressures relevant to the Earth's lower mantle and compares these properties to those of pyrite‐type AlO2 H. Based on these results, we find that Al‐dominant CaCl2 ‐type (Al, Fe)OOH may coexist with Fe‐dominant pyrite‐type (Al, Fe)O2 H in the Earth's lower mantle. Additionally, Fe‐rich pyrite‐type (Al, Fe)O2 H may contribute to the reduced compressional and shear velocities of ultra low velocity zones. Key Points: The elasticity, moduli, and seismic velocities of pyrite‐type FeO2 H and AlO2 H were calculated from 60 to 140 GPa Al‐dominant CaCl2 ‐type (Al, Fe)OOH could coexist with Fe‐rich pyrite‐type (Al, Fe)O2 H in the Earth's lower mantle Fe‐rich pyrite‐type (Al, Fe)O2 H may contribute to the reduced compressional and shear velocities of ULVZs