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Volume 30 Issue 6
Dec.  2019
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Shengxuan Huang, Shan Qin, Xiang Wu. Elasticity and Anisotropy of the Pyrite-Type FeO2H-FeO2 System in Earth's Lowermost Mantle. Journal of Earth Science, 2019, 30(6): 1293-1301. doi: 10.1007/s12583-018-0836-y
Citation: Shengxuan Huang, Shan Qin, Xiang Wu. Elasticity and Anisotropy of the Pyrite-Type FeO2H-FeO2 System in Earth's Lowermost Mantle. Journal of Earth Science, 2019, 30(6): 1293-1301. doi: 10.1007/s12583-018-0836-y

Elasticity and Anisotropy of the Pyrite-Type FeO2H-FeO2 System in Earth's Lowermost Mantle

doi: 10.1007/s12583-018-0836-y
Funds:  Xiang Wu and Shan Qin acknowledge financial support from the National Natural Science Foundation of China (Nos. 41473056 and 41472037)
More Information
  • The pyrite-type FeO2H-FeO2 system has been experimentally confirmed to be stable in Earth's lowermost mantle but there is limited information about its physical properties at high pressures constraining our understanding of its potential geophysical implications for the deep Earth. Here,static calculations demonstrate that the pyrite-type FeO2H-FeO2 system has a high density and Poisson's ratio and ultra-low seismic velocities at conditions of Earth's lowermost mantle. It provides a plausible mechanism for the origin of ultra-low velocity zones at Earth's D″ layer. The incorporation of hydrogen in the pyrite-type FeO2H-FeO2 system tends to decrease the S wave velocity (VS) but increase the bulk sound velocity (VΦ),and can potentially explain the observed anti-correlation of VS and VΦ in the lowermost mantle. Additionally,FeO2H exhibits nearly isotropic whereas FeO2 is highly anisotropic,which may help understand some seismic anisotropies at the core-mantle boundary.
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Elasticity and Anisotropy of the Pyrite-Type FeO2H-FeO2 System in Earth's Lowermost Mantle

doi: 10.1007/s12583-018-0836-y
Funds:  Xiang Wu and Shan Qin acknowledge financial support from the National Natural Science Foundation of China (Nos. 41473056 and 41472037)
    Corresponding author: Xiang Wu

Abstract: The pyrite-type FeO2H-FeO2 system has been experimentally confirmed to be stable in Earth's lowermost mantle but there is limited information about its physical properties at high pressures constraining our understanding of its potential geophysical implications for the deep Earth. Here,static calculations demonstrate that the pyrite-type FeO2H-FeO2 system has a high density and Poisson's ratio and ultra-low seismic velocities at conditions of Earth's lowermost mantle. It provides a plausible mechanism for the origin of ultra-low velocity zones at Earth's D″ layer. The incorporation of hydrogen in the pyrite-type FeO2H-FeO2 system tends to decrease the S wave velocity (VS) but increase the bulk sound velocity (VΦ),and can potentially explain the observed anti-correlation of VS and VΦ in the lowermost mantle. Additionally,FeO2H exhibits nearly isotropic whereas FeO2 is highly anisotropic,which may help understand some seismic anisotropies at the core-mantle boundary.

Shengxuan Huang, Shan Qin, Xiang Wu. Elasticity and Anisotropy of the Pyrite-Type FeO2H-FeO2 System in Earth's Lowermost Mantle. Journal of Earth Science, 2019, 30(6): 1293-1301. doi: 10.1007/s12583-018-0836-y
Citation: Shengxuan Huang, Shan Qin, Xiang Wu. Elasticity and Anisotropy of the Pyrite-Type FeO2H-FeO2 System in Earth's Lowermost Mantle. Journal of Earth Science, 2019, 30(6): 1293-1301. doi: 10.1007/s12583-018-0836-y
  • Subducting slabs can transport water and large quantities of iron and aluminum to Earth's deep interior (Nishi et al., 2014; Ohira et al., 2014), considerably influencing the phase relations, density, seismic velocities, melting temperatures and so on of materials in the lower mantle, which will consequently have a significant impact on the geochemical, geophysical and geodynamic processes in the deep Earth (Nakagawa, 2017; Panero and Caracas, 2017; Wu et al., 2017; Mashino et al., 2016; Ohtani, 2015; Walter et al., 2015; Andrault et al., 2014). In addition to nominal anhydrous phases, dense hydrous magnesium silicates (DHMSs) are proposed to be important water- carriers to the lower mantle, such as superhydrous phase B and phase D (Yang et al., 2017; Li et al., 2016; Wu et al., 2016; Pamato et al., 2015; Nishi et al., 2014; Ohtani et al., 2003). Recently a new DHMS MgSiH2O4, named phase H, has been experimentally and theoretically detected at lower mantle conditions (Nishi et al., 2014; Tsuchiya, 2013). Phase H (Pnnm) is isostructural with CaCl2-type silica, phase-δ AlOOH and phase-ε FeOOH. Therefore, the structural stability and physical properties of these compounds and their solid solutions have been extensively investigated (Ohtani, 2015; Walter et al., 2015; Bindi et al., 2014; Nishi et al., 2014; Gleason et al., 2013). For examples, the MgSiO2(OH)2-δ-AlOOH system can coexist with bridgmanite (bgm) or post-perovskite (ppv) at lower mantle conditions along the slab geotherm (Ohira et al., 2014; Ohtani et al., 2014). Hu et al. (2016) reported that FeOOH dissociated into the assemblage of the pyrite-type FeO2 and solid H2 at 92 GPa and 2 050 K. However, this kind of assemblage has a higher enthalpy than that of the pyrite-type FeO2H at the bottom of the lower mantle (Nishi et al., 2017). Hu et al. (2017) also observed the pyrite-type FeO2Hx (with x from 0 to 1) (named phase P) from partial dehydrogenation of FeOOH and suggested a solid solution (phase P) at high P-T conditions for the pyrite-type FeO2H-FeO2 system. Using the multigrain crystallography method, a linear relation of unit-cell volume of phase P versus x was also established. Furthermore, water released from hydrous phases can react with iron from the outer core to form phase P at the core-mantle boundary (CMB) (Liu et al., 2017; Mao et al., 2017). With water transported by subduction to the CMB, this phase would accumulate throughout geological times. Based on such observations, Mao et al. (2017) thus proposed that this extremely oxygen-rich phase could be an oxygen reservoir and would cause global climate changes such as the Great Oxidation Event. Compared with the surrounding mantle, phase P could possess a considerably low P wave velocity (VP) based on the Birch's Law (Birch, 1952). With the accumulation of phase P throughout geological times, it can form a compositionally distinct and seismically observable region in the CMB region. Therefore, in addition to its effect on the geochemical processes, phase P has significant geophysical and geodynamic implications for the deep Earth.

    The D″ layer is a thermal, chemical and mechanical boundary layer at the base of the mantle (Trønnes, 2010; Garnero, 2000; Stacey and Loper, 1983). Significant anomalies in seismic wave velocities have been detected in this region. In particular, thin ultra-low velocity zones (ULVZs) with considerable reductions in seismic velocities (~10% for VP and 10%-30% for the S wave velocity, VS) are present within or outside of the large low shear velocity provinces (LLSVPs) in the D″ layer (Garnero et al., 2016; Williams et al., 1998; Garnero and Helmberger, 1996). Moreover, global seismic tomography models have shown an anti-correlation between bulk and shear wave velocities (Masters et al., 2000). However, these enigmatic seismic features can only be partially reconciled by the existing models, such as the bgm-ppv transition, partial melting, and iron-rich post-perovskite or ferropericlase (Wicks et al., 2017, 2010; Mao et al., 2006; Iitaka et al., 2004; Oganov and Ono, 2004; Williams and Garnero, 1996). Since phase P can be stable and accumulate in the D″ layer, it may provide a potential mechanism to understand some of enigmatic seismic features of the D″ layer. Therefore, it is important and urgent to investigate its seismic properties at pressures relevant to the CMB.

    Previous studies have focused on the stability and structural characteristics of the pyrite-type FeO2H-FeO2 system. Here, we investigate the elastic, seismic and electronic properties of the pyrite-type FeO2H-FeO2 system at high pressures using theoretical calculations based on density functional theory (DFT). These results aim to decipher its geophysical and geodynamic implications in Earth's lowermost mantle.

  • First-principle calculations were performed in Vienna ab-initio simulation package (VASP) using the projected augmented wave (PAW) method (Kresse and Joubert, 1999; Kresse and Furthmüller, 1996; Blöchl, 1994). We chose the generalized gradient approximations (GGA) to treat the exchange correlation potential (Perdew-Burke-Ernzerhof, PBE version) (Perdew et al., 1996). A large kinetic energy cut-off, 1 000 eV, was selected and the energy convergence criterion was 10-6 eV in electronic self-consistent calculations. The total energy difference was converged to 1×10-5 eV/formula unit (f.u.) with respect to the energy cutoff or k-points. The force difference was converged to 1×10-3 eV/Å (less than 0.1 GPa).

    Due to the strong electronic correlation of the target system, a DFT+U method was introduced in the simulation (Dudarev et al., 1998). For the pyrite-type FeO2, the choice of the U parameter was followed by the previous study of Jang et al. (2017), where they found that the combination of U=5 eV and J=0.8 eV could correctly describe the structural and physical properties of FeO2 at high pressures (the U and J parameters are the on-site Coulomb interaction parameter and the Hund coupling constant, respectively). For the pyrite-type FeO2H, we chose the U parameter of 6 eV with J=0.8 eV, similar to the calculations performed by Nishi et al. (2017). We also varied the U parameter at a fixed value of 0.8 eV for J. However, compared with the aforementioned choice, these simulations could not accurately reproduce the experimental results, such as U=5 eV for FeO2H and U=6 eV for FeO2 (Nishi et al., 2017; Hu et al., 2016). It is thus worthwhile to note that we have chosen different U values for FeO2H and FeO2 even if they adopt the same pyrite-type structure considered in the present study. The spin-polarization of iron without spin-orbit coupling was included in calculations to obtain the accurate cell parameters and physical properties. The ferromagnetic and non-magnetic low-spin arrangements were considered for FeO2H and FeO2, respectively at high pressures corresponding to Earth's deep lower mantle as previous studies did (Hu et al., 2017; Jang et al., 2017; Mao et al., 2017; Nishi et al., 2017).

    Calculations were performed within the pyrite-type structure at different volumes, where the atomic positions and the individual magnetic moment of iron were allowed to relax. The Monkhorst-Pack scheme was used with 8×8×8 k-points grids in the Brillouin zone. Energy-volume results were fitted to the third-order Birch-Murnaghan equation of state (EoS) (Birch, 1947; Murnaghan, 1944). The electronic density of states (DOS) of the pyrite-type FeO2H and FeO2 were calculated by the tetrahedral smearing method with Blöchl corrections using 16×16×16 k-points grids. Based on the stress-strain relations, single crystal elastic constants (Cij) of the pyrite-type FeO2H and FeO2 were obtained (Karki et al., 2001). By applying positive and negative strains of magnitude of 0.5%, we derived three independent elastic constants (C11, C12, and C44) for the cubic pyrite-type structure.

  • The corresponding EoS parameters are V0=112.32 Å3, K0= 211.3 GPa, K0'=4.44 for the pyrite-type FeO2H and V0=98.31 Å3, K0=254.6 GPa, K0'=4.51 for the pyrite-type FeO2. The calculated results indicate that the pyrite-type FeO2H is much more compressible than the pyrite-type FeO2 even though the hydrogen bond is symmetric in the pyrite-type FeO2H (Fig. 1). The K0 of the pyrite-type FeO2H is comparable to that of ε-FeOOH with hydrogen bond symmetrization when considering the trade-off between K0 and K0' (Thompson et al., 2017).

    Figure 1.  The pyrite-type structure of FeO2 (a) and FeO2H (b). The blue, red and pink objects represent Fe, O and H, respectively.

    Structural parameters of the pyrite-type FeO2H and FeO2 are plotted in Fig. 2 as a function of pressure and the available experimental and theoretical data are plotted for comparison (Nishi et al., 2017; Thompson et al., 2017; Hu et al., 2016). The volume reduction is ~4.4% at 80 GPa at the structural transition from ε-FeOOH to the pyrite-type FeO2H. There is a ~9.0% volume contrast between the pyrite-type FeO2H and FeO2 at 140 GPa, consistent with very recent calculations (Fig. 2a) (Liu et al., 2017). The FeO6 volume and O-O distance of the pyrite-type FeO2H are comparable to those of ε-FeOOH in the whole calculated pressure range, implying that the volume reduction at the transition is not attributed to the compression of the Fe-O or O-H bonds. The abrupt volume change is likely to result from the reconstruction of the structure through the first-order phase transition. In terms of the pyrite-type FeO2H and FeO2, the FeO6 volume and O-O distance of FeO2H are larger than those of FeO2 by ~3.8% and ~19.0% at 140 GPa, respectively. The existence of hydrogen in the crystal structure can therefore significantly affect the structural character and compressional behavior. Specifically, the pyrite-type FeO2H has a much larger volume and is more compressible, consistent with aforementioned EoS parameters.

    Figure 2.  The evolution of the cell volume (a), FeO6 volume (b) and O-O distance (c) of the pyrite-type FeO2H-FeO2 system as a function of pressure. Previous experimental and theoretical data are plotted for comparison (Nishi et al., 2017; Thompson et al., 2017; Hu et al., 2016).

    The DOS of the pyrite-type FeO2H and FeO2 are calculated at high pressures at 20 GPa intervals from 80 to 140 GPa (Fig. 3). For the pyrite-type FeO2H, there exists a gap of 0.6-0.8 eV in the spin-down state, whereas in the spin-up state the total DOS crosses the Fermi level. This implies that the pyrite-type FeO2H is half-metallic from 80 to 140 GPa. The pyrite-type FeO2 exhibits metallic in the calculated pressure range, in agreement with previous calculations (Jang et al., 2017). Figure 3 also illustrates that the 3d orbitals of iron cations mainly constitute the bottom of conducting bands and the top of valance bands in two compounds. The 2p orbitals of oxygen anions are mainly responsible for the valance bands from -10 to -2 eV. The contribution of 2p electrons of oxygen anions to the bands near the Fermi level (-1 to 2 eV) is more pronounced in FeO2 than that in FeO2H. This difference can lead to different electronic properties of two compounds as mentioned above. Previous simulations have predicted that ε-FeOOH exhibits insulating with nonzero band gaps from 0 to 140 GPa (Thompson et al., 2017). Our results further indicate that there is an electronic transition from insulator to half-metal in FeOOH through the structural transition at ~80 GPa (Nishi et al., 2017).

    Figure 3.  Calculated DOS of the pyrite-type FeO2H-FeO2 system under different conditions. The total DOS, partial DOS of Fe, O and H at each equivalent atomic site are marked. The Fermi level is indicated by the vertical dashed line. (a) FeO2H at 80 GPa; (b) FeO2H at 140 GPa; (c) FeO2 at 80 GPa; (d) FeO2 at 140 GPa.

    The elastic constants of the pyrite-type FeO2H and FeO2 are calculated and displayed in Fig. 4a. All Cij of FeO2H and FeO2 increase with pressure monotonically. And they both present mechanically stable up to at least 140 GPa (Born and Huang, 1954). The C11 of FeO2H is much smaller than that of FeO2 but the C12 of FeO2H is larger. The incorporation of hydrogen only slightly affects the C44. Based on the Voigt-Reuss-Hill averages, we have calculated the adiabatic bulk (KS) and shear (G) moduli under high pressure (Fig. 4b) (Hill, 1952). The KS and G of FeO2H and FeO2 increase with pressure. Either KS or G of FeO2 is larger than that of FeO2H. Calculated results of some hydrous phases at 0 K, which are likely to transport water into Earth's lower mantle, are also displayed in Fig. 4b (Thompson et al., 2017; Tsuchiya and Mookherjee, 2015; Tsuchiya and Tsuchiya, 2009). The KS of the target system is generally larger than that of ε-FeOOH, phase H or δ-AlOOH. The G of phase H and δ-AlOOH lie between FeO2H and FeO2 whereas that of ε-FeOOH is much smaller.

    Figure 4.  Calculated single-crystal elastic constants (Cij) (a), bulk and shear moduli (KS and G) (b), variation of P wave anisotropies (AVP) (c), and shear wave splitting factors (AVS) and anisotropy factors of two polarized S waves (AVS1 and AVS2) (d) of the pyrite-type FeO2H-FeO2 system as a function of pressure. Previous theoretical results and PREM data are plotted for comparison (Thompson et al., 2017; Tsuchiya and Mookherjee, 2015; Tsuchiya and Tsuchiya, 2009; Dziewonski and Anderson, 1981).

    We further derive the magnitude of velocity anisotropies and their distributions of the target system (Figs. 4c, 4d, 5, 6) (Mainprice et al., 2000; Mainprice, 1990). The P wave anisotropy factor (AVP) of FeO2H is 3.1% at 60 GPa and decreases to 0.3% at 130 GPa. In contrast, the AVP of FeO2 only slightly decreases from 10.7% at 60 GPa to 9.1% at 130 GPa. In both FeO2H and FeO2, the slowest P wave distributes in the (111) plane and the fastest P wave propagates along (001) direction from 80 to 130 GPa (Fig. 5). The propagation directions of the fastest and slowest P waves reverse in FeO2H at 140 GPa (Fig. 5b). The shear wave splitting factor (AVS) of FeO2H decreases to 1.0% at 130 GPa from 9.1% at 60 GPa whereas the AVS of FeO2 slightly changes by 1%. The AVS of FeO2H and FeO2 are both mainly from the AVS1 (Figs. 4d, 6). The present results demonstrate that the external pressure significantly affects the AVP and AVS of FeO2H and the pyrite-type FeO2H exhibits nearly isotropic at the CMB pressures. In contrast, the AVP and AVS of FeO2 almost remain unchanged and have high values at the CMB pressures. It is thus reasonable to suggest that the incorporation of hydrogen plays a significant role in the anisotropy of the target system.

    Figure 5.  P wave velocity distributions for the pyrite-type FeO2H-FeO2 system under different conditions. All pole figures are the upper hemisphere projections. (a) FeO2H at 80 GPa, (b) FeO2H at 140 GPa, (c) FeO2 at 80 GPa, (d) FeO2 at 140 GPa.

    Figure 6.  The shear wave splitting factor and polarized S wave velocity distributions for the pyrite-type FeO2H-FeO2 system under different conditions. All pole figures are the upper hemisphere projections. (a) FeO2H at 80 GPa; (b) FeO2H at 130 GPa; (c) FeO2 at 80 GPa; (d) FeO2 at 130 GPa.

  • We have evaluated the density profile of the pyrite-type FeO2H and FeO2 as a function of pressure based on our EoS parameters (Fig. 7a). The density of either δ-AlOOH or phase H is lower than the Preliminary Reference Earth Model (PREM) up to 13% (Dziewonski and Anderson, 1981). The density of the pyrite-type FeO2H and FeO2 are much higher than PREM.

    Figure 7.  Calculated density (a), aggregate velocities (VP and VS) (b), and Poisson's ratio (c) of the pyrite-type FeO2H-FeO2 system as a function of pressure. Previous theoretical results and PREM data are plotted for comparison (Thompson et al., 2017; Tsuchiya and Mookherjee, 2015; Tsuchiya and Tsuchiya, 2009; Dziewonski and Anderson, 1981).

    At 80 GPa, the density contrast between the pyrite-type FeO2H (FeO2) and PREM is ~30.6% (~43.5%). Using present moduli and densities, we have also derived the aggregate VP, VS and Poisson's ratio of the pyrite-type FeO2H and FeO2 (Figs. 7b, 7c). The calculated results of the present system are basically consistent with previous theoretical data (Liu et al., 2017; Zhang et al., 2017). However, the calculated VS of FeO2H and FeO2 are much larger than experimental results (Liu et al., 2017). The VP and VS of FeO2 are greater than those of FeO2H. In contrast to δ-AlOOH and phase H, the VP and VS of the pyrite-type FeO2H and FeO2 are much lower than PREM (Dziewonski and Anderson, 1981). At 135 GPa, the VP and VS of FeO2H (FeO2) are ~12.8% (~11.2%) and ~22.3% (~16.1%) smaller than PREM, respectively. The Poisson's ratio of FeO2H and FeO2 are larger than PREM by ~19.3% and ~11.3%, respectively.

    Zhang et al. (2017) have investigated the effect of temperature on the density and seismic velocities of the pyrite-type FeO2 by ab-initio molecular dynamics calculations. At 120 GPa and 3 200 K, the density of FeO2 decreases by ~4.6% compared with the static calculation. Assuming that the temperature dependence of the density of FeO2H is similar to that of FeO2, the density of the present system can be still 25%-35% higher than PREM at high P-T conditions. In terms of seismic velocities of FeO2, the VP and VS of FeO2 decrease by ~6.9% and ~3.2%, respectively, from 0 to 4 000 K at 120 GPa (Zhang et al., 2017). On the other hand, Liu et al. (2017) suggested that the reductions of VP and VS are approximately ~4% and ~6% for FeO2H and ~3% and ~4% for FeO2, respectively, from 300 to 3 000 K at 130 GPa. Though the effect of temperature on the reductions of VP and VS is different in previous calculations, these results demonstrate that seismic velocities of phase P can decrease by at least 3%-4% from 0 K to high temperature relevant to Earth's CMB. In addition, according to the character of the shear wave splitting shown in Fig. 6, the AVS of the cubic pyrite-type FeO2 can be specifically expressed as

    The derived AVS of FeO2 is ~11.5% at 0 K and 120 GPa from the data obtained by Zhang et al. (2017), which is lower than the present result. Zhang et al. (2017) have neglected the strongly-correlated effects in FeO2, which may explain this difference. At 4 000 K and 120 GPa, the AVS of FeO2 is calculated to be ~11.7%, very similar to that at 0 K. That is the high temperature has a slight effect on the AVS of FeO2. Considering the aforementioned analysis and our results, the AVS of FeO2 can have a high value at conditions of the CMB. However, this certainly requires the experimental verification.

    In addition to the formation of phase P at the CMB (Liu et al., 2017; Mao et al., 2017), ε-FeOOH undergoes a transition to the pyrite-type structure at ~80 GPa (Nishi et al., 2017). It can further contribute to a negative buoyancy force for the subducted slabs in the deep lower mantle due to its high density. This minor denser phase may carry a small amount of lighter hydrous phases down to the CMB. Considering the reaction between water and iron at the CMB (Liu et al., 2017; Mao et al., 2017), the formation of the pyrite-type FeO2H through the subducting process can further contribute to the formation and accumulation of phase P at the CMB. In contrast, Liu et al. (2017) suggested that phase P forming from goethite in the subducted oceanic crust might not reach the CMB. It is of great importance to consider the effect of the viscosity of the surrounding mantle and the subducted slab on the subducting process.

    Considering our results and the potential effect of temperature on seismic velocities of the present system analyzed above, a mixture containing 60%-70% phase P can reproduce the seismic observations in ULVZs. The accumulation of phase P at the CMB may be one of origins of ULVZs at Earth's D″ layer. Previous experiments have confirmed that the temperature and the heating time significantly affect the x value of phase P (Hu et al., 2017). The hydrogen in phase P possibly gradually releases at the CMB throughout geological times. The seismic velocities of phase P may increase with decreasing x value based on our results and thus the seismic observations in the corresponding ULVZs will also change. Liu et al. (2017) have further suggested that only one-tenth of the mass of water in Earth's oceans transported to the CMB is sufficient to form ULVZs. It demonstrates that the formation and accumulation of phase P can be a plausible origin of ULVZs. Lay et al. (1998) have attributed ULVZs to partial melting. This mechanism can only explain ULVZs within LLSVPs but in fact, there are many ULVZs outside of the LLSVPs. For example, there exist some ULVZs beneath subduction regions, where the temperature is much lower than that in LLSVPs (McNamara et al., 2010). We suggest that the accumulation of phase P accounts for ULVZs beneath subduction regions. Indeed, recent three-dimensional geodynamical calculations have indicated a compositionally distinct origin for most ULVZs (Li et al., 2017).

    We have plotted the bulk sound velocity (VΦ) of the target system as a function of pressure (Fig. 8). In contrast to the VS, the VΦ of FeO2H is slightly larger than that of FeO2 at pressures corresponding to the lowermost mantle. That is the release of hydrogen tends to increase VS but decrease VΦ, leading to the anti-correlation between VS and VΦ. This may provide an alternative mechanism to understand the anti-correlation of VS and VΦ in addition to the bgm-ppv transition (Iitaka et al., 2004; Oganov and Ono, 2004). However, whether the bulk and shear wave velocities will keep anti-correlated at P-T conditions at the CMB requires further experimental and theoretical explorations. We have also found that the pyrite-type FeO2H and FeO2 exhibit half-metallic and metallic, respectively, which are quite different from electronic properties of other subducting materials or the surrounding mantle. This may provide a clue to differentiate the present mechanism from previously proposed ones. Moreover, previous calculations have indicated that the seismically detected VSH > VSV at the bottom of the lower mantle can potentially be partially attributed to the lattice preferred orientation of iron-rich solid solutions from the FeOOH-AlOOH-MgSiO2(OH)2 system (Thompson et al., 2017). Both experiments and calculations have confirmed the structural transition of ε-FeOOH to the pyrite-type FeO2H at ~80 GPa (Nishi et al., 2017). Our results have further found that the anisotropy of the pyrite-type FeO2H-FeO2 system is significantly dependent of the amount of hydrogen incorporated in the system. As hydrogen gradually releasing throughout geological times, the anisotropy of the target system would increase. These results imply that, if phase P can orientate along a particular direction, it may cause a seismically visible anisotropic feature by the heating of the outer core throughout geological times.

    Figure 8.  Calculated bulk sound velocity (VΦ) of the pyrite-type FeO2H-FeO2 system as a function of pressure. The VS of the pyrite-type FeO2H-FeO2 system is plotted for comparison.

  • In summary, we have performed first-principle calculations to investigate the elastic, seismic and electronic properties of the pyrite-type FeO2H-FeO2 system at high pressures. Our results show that the incorporation of hydrogen significantly affects the structural character, compressional behavior and anisotropy of the pyrite-type FeO2H-FeO2 system under high pressure. The pyrite-type FeO2H and FeO2 have a high density and Poisson's ratio and ultra-low seismic velocities compared with PREM. At the CMB, the pyrite-type FeO2H is nearly isotropic whereas FeO2 is highly anisotropic. The pyrite-type FeO2H and FeO2 exhibit half-metallic and metallic, respectively in the lower mantle, considerably different from those of other subducting materials or the surrounding mantle. The present study thus proposes that the accumulation of phase P at the CMB can be one of origins of ULVZs at the D″ layer, and provides a clue to differentiate the present mechanism from previously proposed ones.

  • Xiang Wu and Shan Qin acknowledge financial support from the National Natural Science Foundation of China (Nos. 41473056 and 41472037). Thanks go to the reviewers and the editors for their helpful suggestions. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0836-y.

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