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1. Formation of large low shear velocity provinces through the decomposition of oxidized mantle

   https://doi.org/10.1038/s41467-021-22185-1  

   Wang, Wenzhong ; Liu, Jiachao ; Zhu, Feng ; Li, Mingming ; Dorfman, Susannah M. ; Li, Jie ; Wu, Zhongqing ( March 2021 , Nature Communications)

   Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle are key to understanding the chemical composition and thermal structure of the deep Earth, but their origins have long been debated. Bridgmanite, the most abundant lower-mantle mineral, can incorporate extensive amounts of iron (Fe) with effects on various geophysical properties. Here our high-pressure experiments and ab initio calculations reveal that a ferric-iron-rich bridgmanite coexists with an Fe-poor bridgmanite in the 90 mol% MgSiO₃–10 mol% Fe₂O₃system, rather than forming a homogeneous single phase. The Fe³⁺-rich bridgmanite has substantially lower velocities and a higherV_P/V_Sratio than MgSiO₃bridgmanite under lowermost-mantle conditions. Our modeling shows that the enrichment of Fe³⁺-rich bridgmanite in a pyrolitic composition can explain the observed features of the LLSVPs. The presence of Fe³⁺-rich materials within LLSVPs may have profound effects on the deep reservoirs of redox-sensitive elements and their isotopes.

    

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2. High‐Pressure Phase Stability and Thermoelastic Properties of Iron Carbonitrides and Nitrogen in the Deep Earth

   https://doi.org/10.1029/2021JB021934  

   Huang, Shengxuan ; Wu, Xiang ; Zhu, Feng ; Lai, Xiaojing ; Li, Jie ; Neill, Owen K. ; Qin, Shan ; Rapp, Robert ; Zhang, Dongzhou ; Dera, Przemyslaw ; et al ( June 2021 , Journal of Geophysical Research: Solid Earth)

   Iron‐dominant metallic phases are likely the primary hosts for nitrogen in the reduced deep Earth, hence the storage of nitrogen in the lower mantle and the core is governed by the behavior of the Fe‐N‐C system at high temperatures and pressures. In this study, phase transitions and thermoelastic properties of iron carbonitrides were investigated at high pressure‐temperature conditions by diamond anvil cell experiments and first‐principles calculations. Experimental data revealed no phase transition inε‐type Fe₄(N_0.6C_0.4) or Fe₇(N_0.75C_0.25)₃up to 60 GPa at room temperature. At high temperature, Fe₇(N_0.75C_0.25)₃transforms into the Fe₃C‐type phase at ∼27 GPa, and then into the Fe₇C₃‐type phase at ∼45 GPa, which is also corroborated by our theoretical calculations. We found that the phase stability of iron carbonitrides mainly depends on the N/C ratio, and the elastic properties of iron carbonitrides are dominantly affected by the Fe/(N+C) ratio. Iron carbonitrides with diverse structures may be the main host for nitrogen in the deep mantle. Some iron carbonitride inclusions in lower mantle diamonds could be the residue of the primordial mantle or originate from subducted nitrogen‐bearing materials, rather than iron‐enriched phases of the outer core. In addition, our experiments confirmed the existence of Fe₇C₃‐type Fe₇C₃‐Fe₇N₃solid solutions above 40 GPa. Fe₇C₃‐type Fe₇(C, N)₃has comparable density and thermoelastic properties to its isostructural endmembers and may be a promising candidate constituent of the Earth's inner core.

    

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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.

    

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4. Synthesis, Elasticity, and Spin State of an Intermediate MgSiO 3 ‐FeAlO 3 Bridgmanite: Implications for Iron in Earth's Lower Mantle

   https://doi.org/10.1029/2020JB019964  

   Zhu, Feng ; Liu, Jiachao ; Lai, Xiaojing ; Xiao, Yuming ; Prakapenka, Vitali ; Bi, Wenli ; Alp, E. Ercan ; Dera, Przemyslaw ; Chen, Bin ; Li, Jie ( July 2020 , Journal of Geophysical Research: Solid Earth)

   Fe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg_0.5Fe³⁺_0.5)(Al_0.5Si_0.5)O₃bridgmanite (FA50) with the highest Fe³⁺‐Al³⁺coupled substitution known to date. X‐ray diffraction measurements showed that at ambient conditions, the FA50 adopted the LiNbO₃structure. Upon compression at room temperature to 18 GPa, it transformed back into the bridgmanite structure, which remained stable up to 102 GPa and 2,600 K. Fitting Birch‐Murnaghan equation of state of FA50 bridgmanite yieldsV₀ = 172.1(4) ų,K₀ = 229(4) GPa withK₀′ = 4(fixed). The calculated bulk sound velocity of the FA50 bridgmanite is ~7.7% lower than MgSiO₃bridgmanite, mainly because the presence of ferric iron increases the unit‐cell mass by 15.5%. This difference likely represents the upper limit of sound velocity anomaly introduced by Fe³⁺‐Al³⁺substitution. X‐ray emission and synchrotron Mössbauer spectroscopy measurements showed that after laser annealing, ~6% of Fe³⁺cations exchanged with Al³⁺and underwent the high‐ to low‐spin transition at 59 GPa. The low‐spin proportion of Fe³⁺increased gradually with pressure and reached 17–31% at 80 GPa. Since the cation exchange and spin transition in this Fe³⁺‐Al³⁺‐enriched bridgmanite do not cause resolvable unit‐cell volume reduction, and the increase of low‐spin Fe³⁺fraction with pressure occurs gradually, the spin transition would not produce a distinct seismic signature in the lower mantle. However, it may influence iron partitioning and isotopic fractionation, thus introducing chemical heterogeneity in the lower mantle.

    

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5. The Role of Redox on Bridgmanite Crystal Chemistry and Calcium Speciation in the Lower Mantle

   https://doi.org/10.1029/2020JB020783  

   Creasy, Neala ; Girard, Jennifer ; Eckert, Jr, James O. ; Lee, Kanani K. M. ( October 2020 , Journal of Geophysical Research: Solid Earth)

   The amount of ferric iron Fe³⁺in the lower mantle is largely unknown and may be influenced by the disproportionation reaction of ferrous iron Fe²⁺into metallic Fe and Fe³⁺triggered by the formation of bridgmanite. Recent work has shown that Fe³⁺has a strong effect on the density and seismic wave speeds of bridgmanite and the incorporation of impurities such as aluminum. In order to further investigate the effects of ferric iron on mineral behavior at lower mantle conditions, we conducted laser‐heated diamond‐anvil cell (LHDAC) experiments on two sets of samples nearly identical in composition (an aluminum‐rich pyroxenite glass) except for the Fe³⁺content; with one sample with more Fe³⁺(“oxidized”: Fe³⁺/ΣFe ~ 55%) and the other with less Fe³⁺(“reduced”: Fe³⁺/ΣFe ~ 11%). We heated the samples to lower mantle conditions, and the resulting assemblages were drastically different between the two sets of samples. For the reduced composition, we observed a multiphase assemblage dominated by bridgmanite and calcium perovskite. In contrast, the oxidized material yielded a single phase of Ca‐bearing bridgmanite. These Al‐rich pyroxenite samples show a difference in density and seismic velocities for these two redox states, where the reduced assemblage is denser than the oxidized assemblage by ~1.5% at the bottom of the lower mantle and slower (bulk sound speed) by ~2%. Thus, heterogeneities of Fe³⁺content may lead to density and seismic wave speed heterogeneities in Earth's lower mantle.

    

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