More Like this

1. The influence of δ-(Al,Fe)OOH on seismic heterogeneities in Earth’s lower mantle

   https://doi.org/10.1038/s41598-021-91180-9  

   Ohira, Itaru ; Jackson, Jennifer M. ; Sturhahn, Wolfgang ; Finkelstein, Gregory J. ; Kawazoe, Takaaki ; Toellner, Thomas S. ; Suzuki, Akio ; Ohtani, Eiji ( December 2021 , Scientific Reports)

   null (Ed.)

   Abstract The high-pressure phases of oxyhydroxides (δ-AlOOH, ε-FeOOH, and their solid solution), candidate components of subducted slabs, have wide stability fields, thus potentially influencing volatile circulation and dynamics in the Earth’s lower mantle. Here, we report the elastic wave velocities of δ-(Al,Fe)OOH (Fe/(Al + Fe) = 0.13, δ-Fe13) to 79 GPa, determined by nuclear resonant inelastic X-ray scattering. At pressures below 20 GPa, a softening of the phonon spectra is observed. With increasing pressure up to the Fe 3+ spin crossover (~ 45 GPa), the Debye sound velocity ( v D ) increases. At higher pressures, the low spin δ-Fe13 is characterized by a pressure-invariant v D . Using the equation of state for the same sample, the shear-, compressional-, and bulk-velocities ( v S , v P , and v Φ ) are calculated and extrapolated to deep mantle conditions. The obtained velocity data show that δ-(Al,Fe)OOH may cause low- v Φ and low- v P anomalies in the shallow lower mantle. At deeper depths, we find that this hydrous phase reproduces the anti-correlation between v S and v Φ reported for the large low seismic velocity provinces, thus serving as a potential seismic signature of hydrous circulation in the lower mantle. 

   more » « less
2. Vibrational anisotropy of <i>δ</i>-(Al,Fe)OOH single crystals as probed by nuclear resonant inelastic X-ray scattering

   https://doi.org/10.5194/ejm-33-485-2021  

   Buchen, Johannes ; Sturhahn, Wolfgang ; Ishii, Takayuki ; Jackson, Jennifer M. ( January 2021 , European Journal of Mineralogy)

   Abstract. The formation of high-pressure oxyhydroxide phases spanned by the components AlOOH–FeOOH–MgSiO2(OH)2 in experiments suggests their capability to retain hydrogen in Earth's lower mantle. Understanding the vibrational properties of high-pressure phases provides the basis for assessing their thermal properties, which are required to compute phase diagrams and physical properties. Vibrational properties can be highly anisotropic, in particular for materials with crystal structures of low symmetry that contain directed structural groups or components. We used nuclear resonant inelastic X-ray scattering (NRIXS) to probe lattice vibrations that involve motions of 57Fe atoms in δ-(Al0.87Fe0.13)OOH single crystals. From the recorded single-crystal NRIXS spectra, we calculated projections of the partial phonon density of states along different crystallographic directions. To describe the anisotropy of central vibrational properties, we define and derive tensors for the partial phonon density of states, the Lamb–Mössbauer factor, the mean kinetic energy per vibrational mode, and the mean force constant of 57Fe atoms. We further show how the anisotropy of the Lamb–Mössbauer factor can be translated into anisotropic displacement parameters for 57Fe atoms and relate our findings on vibrational anisotropy to the crystal structure of δ-(Al,Fe)OOH. As a potential application of single-crystal NRIXS at high pressures, we discuss the evaluation of anisotropic thermal stresses in the context of elastic geobarometry for mineral inclusions. Our results on single crystals of δ-(Al,Fe)OOH demonstrate the sensitivity of NRIXS to vibrational anisotropy and provide an in-depth description of the vibrational behavior of Fe3+ cations in a crystal structure that may motivate future applications of NRIXS to study anisotropic vibrational properties of minerals. 

   more » « less
3. Elastic Properties of the Pyrite‐Type FeOOH‐AlOOH System From First‐Principles Calculations

   https://doi.org/10.1029/2021GC009703  

   Thompson, Elizabeth C. ; Campbell, Andrew J. ; Tsuchiya, Jun ( May 2021 , Geochemistry, Geophysics, Geosystems)

   The stability, structure, and elastic properties of pyrite‐type (FeS₂structured) FeO₂H were determined using density functional theory‐based computations with an internally consistent Coulombic self‐interaction term (U_eff). The properties of pyrite‐type FeO₂H are compared to that of pyrite‐type AlO₂H, 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 andδ‐AlOOH. Due to substantial differences in the CaCl₂‐type → pyrite‐type structural transition pressures of these endmembers, the stabilities of the (Al,Fe)O₂H 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 FeO₂H 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‐typeδ‐(Al,Fe)OOH in the deep Earth. Furthermore, pyrite‐type (Al_0.5–0.6,Fe_0.4–0.5)O₂H 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‐typeδ‐(Al,Fe)OOH may contribute to large low shear velocity provinces.

    

   more » « less
4. Phase transitions in ε-FeOOH at high pressure and ambient temperature

   https://doi.org/10.2138/am-2020-7468  

   Thompson, Elizabeth C. ; Davis, Anne H. ; Brauser, Nigel M. ; Liu, Zhenxian ; Prakapenka, Vitali B. ; Campbell, Andrew J. ( December 2020 , American Mineralogist)

   null (Ed.)

   Abstract Constraining the accommodation, distribution, and circulation of hydrogen in the Earth's interior is vital to our broader understanding of the deep Earth due to the significant influence of hydrogen on the material and rheological properties of minerals. Recently, a great deal of attention has been paid to the high-pressure polymorphs of FeOOH (space groups P21nm and Pnnm). These structures potentially form a hydrogen-bearing solid solution with AlOOH and phase H (MgSiO4H2) that may transport water (OH–) deep into the Earth's lower mantle. Additionally, the pyrite-type polymorph (space group Pa3 of FeOOH), and its potential dehydration have been linked to phenomena as diverse as the introduction of hydrogen into the outer core (Nishi et al. 2017), the formation of ultralow-velocity zones (ULVZs) (Liu et al. 2017), and the Great Oxidation Event (Hu et al. 2016). In this study, the high-pressure evolution of FeOOH was re-evaluated up to ~75 GPa using a combination of synchrotron-based X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and optical absorption spectroscopy. Based on these measurements, we report three principal findings: (1) pressure-induced changes in hydrogen bonding (proton disordering or hydrogen bond symmetrization) occur at substantially lower pressures in ε-FeOOH than previously reported and are unlikely to be linked to the high-spin to low-spin transition; (2) ε-FeOOH undergoes a 10% volume collapse coincident with an isostructural Pnnm → Pnnm transition at approximately 45 GPa; and (3) a pressure-induced band gap reduction is observed in FeOOH at pressures consistent with the previously reported spin transition (40 to 50 GPa). 

   more » « less
5. Stability and Solid Solutions of Hydrous Alumino-Silicates in the Earth’s Mantle

   https://doi.org/10.3390/min10040330  

   Panero, Wendy R. ; Caracas, Razvan ( April 2020 , Minerals)

   null (Ed.)

   The degree to which the Earth’s mantle stores and cycles water in excess of the storage capacity of nominally anhydrous minerals is dependent upon the stability of hydrous phases under mantle-relevant pressures, temperatures, and compositions. Two hydrous phases, phase D and phase H, are stable to the pressures and temperatures of the Earth’s lower mantle, suggesting that the Earth’s lower mantle may participate in the cycling of water. We build on our prior work of density functional theory calculations on phase H with the stability, structure, and bonding of hydrous phases D, and we predict the aluminum partitioning with H in the Al 2 O 3 -SiO 2 -MgO-H 2 O system. We address the solid solutions through a statistical sampling of site occupancy and calculation of the partition function from the grand canonical ensemble. We show that each phase has a wide solid solution series between MgSi 2 O 6 H 2 -Al 2 SiO 6 H 2 and MgSiO 4 H 2 -2 δ AlOOH + SiO 2 , in which phase H is more aluminum rich than phase D at a given bulk composition. We predict that the addition of Al to both phases D and H stabilizes each phase to higher temperatures through additional configurational entropy. While we have shown that phase H does not exhibit symmetric hydrogen bonding at high pressure, we report here that phase D undergoes a gradual increase in the number of symmetric H-bonds beginning at ∼30 GPa, and it is only ∼50% complete at 60 GPa. 

   more » « less