Pressure Destabilizes Oxygen Vacancies in Bridgmanite

Bridgmanite may contain a large proportion of ferric iron in its crystal structure in the forms of FeFeO3 and MgFeO2.5 components. We investigated the pressure dependence of FeFeO3 and MgFeO2.5 contents in bridgmanite coexisting with MgFe2O4‐phase and with or without ferropericlase in the MgO‐SiO2‐Fe2O3 ternary system at 2,300 K, 33 and 40 GPa. Together with the experiments at 27 GPa reported in Fei et al. (2020, https://doi.org/10.1029/2019GL086296), our results show that the FeFeO3 and MgFeO2.5 contents in bridgmanite decrease from 7.6 to 5.3 mol % and from 2 to 3 mol % to nearly zero, respectively, with increasing pressure from 27 to 40 GPa. Accordingly, the total Fe3+ decreases from 0.18 to 0.11 pfu. The formation of oxygen vacancies (MgFeO2.5 component) in bridgmanite is therefore dramatically suppressed by pressure. Oxygen vacancies can be produced by ferric iron in Fe3+‐rich bridgmanite under the topmost lower mantle conditions, but the concentration should decrease rapidly with increasing pressure. The variation of oxygen‐vacancy content with depth may potentially affect the physical properties of bridgmanite and thus affect mantle dynamics.

In this study, we investigated the substitution mechanism of Fe 3+ in Al 3+ -free bridgmanite using a recently developed ultrahigh-pressure (>25 GPa) multianvil technique with tungsten carbide anvils (Ishii et al., 2016(Ishii et al., , 2019 at 33 and 40 GPa at 2300 K. Although bridgmanite in the lower mantle contains Al 3+ , which could affect the Fe 3+ substitution as mentioned above, we investigated the Al 3+ -free system to provide basic understanding of the roles of trivalent cations in bridgmanite chemistry. To maximize the MgFeO 2.5 content in bridgmanite, experiments were performed in the MgO-SiO 2 -Fe 2 O 3 system where bridgmanite coexists with MgFe 2 O 4 -phase, and with/without ferropericlase. Together with our recent work at 27 GPa (Fei et al., 2020), we show that the concentrations of both FeFeO 3 and MgFeO 2.5 in bridgmanite, and thus the total Fe 3+ content, decrease with increasing pressure. Even coexisting with ferropericlase, the formation of oxygen vacancies is completely suppressed at about 40 GPa. Our results provide basic knowledge about the phase relations and Fe 3+ substitution mechanisms in bridgmanite under Fe 3+ -rich conditions. 10.1029/2021JB022437 3 of 18

High-Pressure Multianvil Experiments
The starting materials used in this study were identical to those in Fei et al. (2020), that is, mixtures with compositions of 5MgO +3SiO 2 + 1Fe 2 O 3 (MgO-rich sample) and 4MgO + 3SiO 2 + 2Fe 2 O 3 (Fe 2 O 3 -rich sample) prepared from SiO 2 , MgO, and Fe 2 O 3 oxides ( Figure 1). The purity of each oxide was >99.9%. Platinum chambers with inner diameter of 0.3 mm, outer diameter of 0.4 mm, and length of 0.3 mm were used as sample capsules, which were placed in a Al 2 O 3 sleeve in the LaCrO 3 furnace. A Cr 2 O 3 -doped MgO octahedron with edge length of 5.7 mm was used as the pressure medium ( Figure 2). High pressures were generated by tungsten carbide anvils with truncation edge lengths of 1.5 mm using the 15 MN multianvil press, IRIS-15, at the University of Bayreuth (Ishii et al., 2016). The experimental pressures were 33 and 40 GPa (Table 1). The temperature and annealing duration were 2,300 K and 24 hr, respectively. After annealing, the heating power supplier was switched off, by which the temperature decreased to less than 800 K within 1 s and to less than 400 K within 2-4 s. Afterward, the pressure was decreased to ambient conditions over durations exceeding 15 hr.

Sample Analysis
(1) Scanning Electron Microscopy (SEM). Cross sections of the recovered assemblies were prepared and analyzed by SEM. Backscattered electron images (BSE) were taken on the cross sections ( Figure 3). The presented phases on the cross sections were examined by an energy dispersive detector (2) X-ray Diffraction. Microfocus X-ray diffraction analyses were performed using a microfocus X-ray diffractometer (Brucker AXS D8 Discover) with a microfocus source of Co-Kα radiation. The beam diameter was about 100 µm focused on the cross sections of the recovered samples. The acceleration voltage and beam current were 40 kV and 500 µA, respectively. The exposure time was 5-6 hr for each analysis. Examples of the diffraction patterns are shown in Figure 4 (3) Mössbauer Spectroscopy. Synchrotron Mössbauer source (SMS) spectroscopy analyses were performed under ambient conditions on all the samples at beamline BL10XU, SPring-8, Japan. The detailed setup and analytical conditions of SMS spectroscopy are given in Hirao et al. (2020). The spectra were fitted using MossA with Lorentzian doublets (Prescher et al., 2012) ( Figure 5) (4) Electron Microprobe Analysis. The chemical compositions of bridgmanite and coexisting phases were obtained by electron probe microanalyzer (EPMA) at the University of Bayreuth. The acceleration voltage was 15 kV, the beam current was 5 nA, and the counting time was 20 s for each point analysis. An enstatite standard was used for Mg and Si, whereas metallic iron was used for Fe. Tests were also made using a Fe 2 O 3 -standard for analysis of Fe in the samples, which did not show any meaningful difference compared to results using a metallic-Fe standard. Grains near the Pt capsule wall were avoided in the analyses
where the atomic ratio of Mg, Fe, and Si (a: b: c) in bridgmanite was taken from EPMA.

Phase Assemblages in the Recovered Samples
The recovered MgO-rich and Fe 2 O 3 -rich samples contain bridgmanite, a phase close to MgFe 2 O 4 composition (hereafter MgFe 2 O 4 -phase), and either with (MgO-rich samples) or without (Fe 2 O 3 -rich samples) ferropericlase (Table 1 and Figure 1), as demonstrated by the backscattering images ( Figure 3) and X-ray diffraction (Figure 4). No observable inhomogeneity of phase compositions was found throughout the capsules, indicating that chemical equilibrium was reached.
The MgFe 2 O 4 -phase was previously assigned to be a CaMn 2 O 4 -type structure Fei et al., 2020) or CaTi 2 O 4 -type structure (Greenberg et al., 2017), and recently suggested to be a new structure (modified Na-Fe-Ti oxide-type) of post-spinel . Our study primarily focused on the bridgmanite phase without considering the structural complexities of the MgFe 2 O 4 -phase.
Because of the small proportion of ferropericlase, doublets of ferropericlase are not observed within the experimental data scatter.

Composition of Bridgmanite, MgFe 2 O 4 -Phase, and Ferropericlase
By comparison of bridgmanite compositions at 33 and 40 GPa with that at 27 GPa from Fei et al. (2020), it is found that the Fe 3+ content in bridgmanite under MgO-rich conditions decreases dramatically from ∼0.17 pfu at 27 GPa (Fei et al., 2020) to ∼0.11 pfu at 40 GPa ( Figure 7a). As expected, the Fe 2 O 3 -rich samples have higher Fe 3+ content than the MgO-rich samples, and Fe 3+ content decreases from 0.37 to 0.12 pfu at 27-40 GPa (Figure 7a). The Mg/Si ratios in bridgmanite are slightly higher than unity in the MgO-rich samples, whereas they are essentially unity in the Fe 2 O 3 -rich samples ( Table 1).
The composition of the MgFe 2 O 4 -phase deviates from the MgFe 2 O 4 endmember ( Figure 1). With increasing pressure from 27 to 40 GPa, the Fe 3+ content increases from 1.9 to 2.3 pfu, and the Mg content decreases from 0.9 to 0.5 pfu. The Si content is low but detectable (∼0.1 pfu) (Figure 7b). Additionally, up to 13.2 mol % Fe was found in ferropericlase ( Figure 7c).

Fe 3+ and Fe 2+ Partitioning Between Bridgmanite and Ferropericlase
The Fe contents [Fe/(Fe + Mg)] in ferropericlase of the MgO-rich samples are 6.5-13.2% (Figure 7c). If all Fe were ferrous in ferropericlase, the partition coefficient of Fe 2+ between bridgmanite and ferropericlase would be nearly zero based on the absence of Fe 2+ in the current bridgmanite samples. The partition coefficient is thus much smaller than that suggested by previous studies (e.g., Nakajima et al., 2012;Prescher et al., 2014;Xu et al., 2017), who reported values of 0.2-0.4 at 25-40 GPa. We emphasize that this discrepancy cannot be caused by undetectable Fe 2+ in our bridgmanite samples. If the Fe 2+ /Mg partition coefficient given by previous   (Frost & Langenhorst, 2002;Frost et al., 2004;Huang, Boffa-Ballaran, McCammon, Miyajima, Dolejš & Frost, 2021;Hummer & Fei, 2012;Lauterbach et al., 2000), all of which are lower than this study because their experimental temperatures are lower, and/or MgFe 2 O 4 -phase did not appear (namely Fe 3+ is not saturated). The data points with blue symbols at 27 GPa are from Fei et al. (2020). The error bars represent one standard deviation of the analyzed points by electron microprobe as shown in Table 1. studies were followed by our samples, Fe 2+ /ΣFe should be 15-50% in bridgmanite. Such a significant fraction of Fe 2+ would definitely be detectable by both in-house (Fei et al., 2020) and synchrotron (this study) Mössbauer spectroscopy because the hyperfine parameters of Fe 2+ doublets in bridgmanite are well known and would not overlap with other components in our spectra (e.g., Huang, Boffa-Ballaran, McCammon, Miyajima, Dolejš & Frost, 2021;Sinmyo et al., 2019;Yoshino et al., 2016, Figure 6). Therefore, the appearance of 6.5-13.2 mol % FeO in ferropericlase does not suggest the presence of Fe 2+ in bridgmanite. Since the experiments in previous studies (e.g., Nakajima et al., 2012;Prescher et al., 2014) were mostly performed under relatively reducing conditions with high Fe 2+ /ΣFe ratios, one explanation for this discrepancy is that Fe 2+ is almost entirely incorporated into ferropericlase when the bulk Fe 2+ content in the system is extremely low, that is, the partition coefficient may have a substantial compositional dependence.

Fe 3+ Content in Bridgmanite
Based on the phase rule, the Fe 3+ content in bridgmanite under MgO-rich conditions should be uniquely constrained because three phases coexist in the system. Although some Fe 2 O 3 might be reduced to FeO in high-pressure experiments as indicated by the presence of Fe in ferropericlase in MgO-rich samples (Figure 7a), the number of components in bridgmanite is still three because the Fe 3+ /ΣFe ratio is close to 100% in bridgmanite as demonstrated by Mössbauer spectroscopy.
The Fe 2 O 3 -rich samples in this study and some other studies (e.g., Liu et al., 2018;Wang et al., 2021) show much higher Fe 3+ contents in bridgmanite (up to 1.0 pfu) than the current MgO-rich bridgmanite samples. However, these high Fe 3+ -content bridgmanite samples did not coexist with ferropericlase (bridgmanite + MgFe 2 O 4 -phase in Fe 2 O 3 -rich samples in this study and only bridgmanite in , and Wang et al., 2021. When bridgmanite coexists with ferropericlase, the Fe 3+ content in bridgmanite will be limited because of the formation of the MgFe 2 O 4 -phase from FeFeO 3 and MgO . When bridgmanite does not coexist with MgO, the Fe 3+ content in bridgmanite depends on the starting material. If the bulk Fe 3+ content in the starting material is high, the Fe 3+ content in bridgmanite can accordingly be high based on the phase relations in Figure 1, for example, Fe 3+ can reach 1.0 pfu as shown in Liu et al. (2018) and Wang et al. (2021). This is understandable because the molar volume of hematite (30.5 cm 3 /mol) is slightly larger than the FeFeO 3 component in bridgmanite (29.55 cm 3 /mol, Huang, Boffa-Ballaran, . Therefore, Fe 3+ may tend to be incorporated in bridgmanite by a MgSiO 3 -Fe 2 O 3 solid solution instead of forming hematite, consequently, the Fe 3+ solubility in bridgmanite is high. The upper limit of Fe 3+ content should be obtained in the system with coexistence of bridgmanite and hematite, which was not investigated in this study. Additionally, the formation of the FeAlO 3 component will also increase the Fe 3+ content in Al-bearing bridgmanite, which causes the high Fe 3+ content (about 0.7 pfu) in Liu, Dubrovinsky, et al. (2019) and Liu et al. (2020).
The MgFe 2 O 4 -phase may have structural complexities (e.g., Greenberg et al., 2017;Ishii et al., 2020), which may affect Fe 3+ partitioning between bridgmanite and the MgFe 2 O 4 -phase and thus affect the Fe 3+ content in bridgmanite. Some studies reported phase transitions among polymorphs of MgFe 2 O 4 at high temperatures below 25 GPa (e.g., Ishii et al., 2020;Uenver-Thiele et al., 2017), and at ambient temperature in the pressure range 25-40 GPa (Greenberg et al., 2017). However, no phase transition of MgFe 2 O 4 has been reported at the conditions of our experiments, that is, 2,300 K and 27-40 GPa. The absence of a phase transition of the MgFe 2 O 4 -phase in this study has also been indicated by X-ray diffraction of the recovered samples ( Figure 4). Therefore, the systematic decrease of Fe 3+ content in bridgmanite is not expected to be caused by complex polymorphism of MgFe 2 O 4 .

Pressure Dependence of Fe 3+ Substitution in Bridgmanite
In the MgO-rich samples, the FeFeO 3 content in bridgmanite decreases from 7.7 to 5.3 mol %, whereas the MgFeO 2.5 content decreases from 2.2 to ∼0% at 27-40 GPa (Figure 8a). The extrapolation of data agrees well with the maximum MgFeO 2.5 content of 3.5% reported by Hummer and Fei (2012) at 25 GPa and 1,970-2,070 K ( Figure 8a). In contrast, Fe 2 O 3 -rich samples have FeFeO 3 contents ranging from 17.9% to 5.9%, which is higher than the MgO-rich samples, and have MgFeO 2.5 contents that are essentially zero within error over the entire investigated pressure range (Figure 8b).
Note that smaller Pt capsules were used in the runs at 33 and 40 GPa in this study compared to the 27 GPa runs in Fei et al. (2020). Because Pt capsules may absorb Fe from the samples and thus release O 2 , more O 2 might be released in 33 and 40 GPa runs relative to sample volumes. However, the O 2 formed by Fe dissolution in Pt should not cause MgFeO 2.5 content to decrease with increasing pressure. Since Fe 3+ /ΣFe ≈100% in bridgmanite in all runs, the chemistry of bridgmanite in MgO-rich samples is uniquely constrained with maximized Fe 3+ content and maximized Fe 3+ /ΣFe ratio. Excess O 2 cannot further oxidize bridgmanite. Therefore, the chemistry of bridgmanite from MgO-rich samples in this study will not be affected by excess O 2 . Although the excess O 2 may produce peroxide components (Hu et al., 2016;Zhu et al., 2019), it requires pressures >70 GPa, which is not the case in this study.
Because of the uncertainties in the Mg, Si, and Fe contents obtained from EPMA analysis (Table 1), the error bars are relatively large for the relatively small MgFeO 2.5 contents (Figure 7a). Additionally, the reproducibility of MgFeO 2.5 content obtained in different runs under the same pressure and temperature conditions is about ±0.3-1.25 mol % (Table 1 and Fei et al., 2020), which is not negligible. These problems make it challenging to obtain a definite conclusion regarding the pressure dependence of the MgFeO 2.5 content. However, plots of Mg and Si contents in bridgmanite versus Fe 3+ content show that data at 27 GPa and 1,700-2,300 K under MgO-rich conditions clearly deviate from the theoretical Mg and Si contents of pure FeFeO 3 substitution, whereas they essentially follow the trend of the pure FeFeO 3 substitution mechanism in Fe 2 O 3 -rich samples (Figure 9 and Fei et al., 2020). This behavior demonstrates the presence of MgFeO 2.5 components at 27 GPa under MgO-rich conditions. In contrast to the 27-GPa data, the 33-GPa data are closer to the pure FeFeO 3 substitution, and the 40-GPa data are exactly on the trend of pure FeFeO 3 substitution even in MgO-rich samples (Figure 9), which suggests a decrease of MgFeO 2.5 content with increasing pressure. Therefore, the reduction of MgFeO 2.5 content with increasing pressure is convincing despite relatively large uncertainties of absolute MgFeO 2.5 contents.
The suppression of Fe 3+ -linked oxygen vacancies with increasing pressure can be understood by the volume increase associated with MgFeO 2.5 formation. Partial molar volume of the MgFeO 2.5 component in bridgmanite is estimated to be 27.65 cm 3 /mol , whereas the molar volume of the MgFe 2 O 4 -phase is 41.4 cm 3 /mol , and that of MgO is 11.25 cm 3 /mol at ambient conditions (Dorogokupets, 2010;Tange et al., 2012). Thus, the volume change in the reaction, is about +2.7 cm 3 /mol at ambient conditions. This volume change will be even larger (4.3-4.7 cm 3 /mol) by adjusting the pressure to 27-40 GPa using reported equation of states (Dorogokupets, 2010;Ishii et al., 2020;Tange et al., 2012), because the MgFe 2 O 4 -phase has a much smaller bulk modulus (164 GPa) than bridgmanite (257 GPa) Tange et al., 2012). The reaction should thus be significantly suppressed by increasing pressure. As a result, the concentration of MgFeO 2.5 content decreases rapidly with increasing pressure.  Table 1.
The FeFeO 3 content in our samples decreases with increasing pressure as well. The reaction of Fe 3+ between bridgmanite and MgFe 2 O 4 -phase can be written as: The FeFeO 3 component has a molar volume of 29.55 cm 3 /mol . Although the volume change of the above reaction is negative at ambient conditions (ΔV = −0.6 cm 3 /mol), it becomes positive (+0.5 to +0.8 cm 3 /mol) after adjusting to 27-40 GPa using the equation of state for each phase (Dorogokupets, 2010;Ishii et al., 2020;Tange et al., 2012). As a result, the FeFeO 3 content in bridgmanite decreases with pressure.
On the other hand, in the case that bridgmanite coexists with MgAl 2 O 4 and MgO, the AlAlO 3 component in bridgmanite can be formed by, which has a small, but positive volume change (ΔV = +0.5 cm 3 /mol at ambient conditions) using the molar volume of each component reported previously Kojitani, Hisatomi, & Akaogi, 2007;Liu, Akaogi, & Katsura, 2019;Sueda et al., 2009). Thus, the small but positive volume changes of reactions (5) and (6) suggest that the AlAlO 3 content in bridgmanite should slightly decrease or be nearly constant with increasing pressure, which contradicts the tendency reported by Liu et al. (2016) and Liu, Nishi, et al. (2017). A possible cause for this discrepancy could be a large uncertainty in the reported molar volume of AlAlO 3 component in bridgmanite since it is extrapolated from the volume of bridgmanite with relatively low AlAlO 3 content (up to 14 mol %) Liu, Akaogi, & Katsura, 2019;Liu, Boffa-Ballaran, et al., 2019), or that the AlAlO 3 component has a much smaller bulk modulus than MgSiO 3 -bridgmanite since Al 3+ has a smaller ionic radius than Mg 2+ (0.50 Å versus 0.65 Å), leading to negative ΔV for reactions (5) and (6)

Fe 3+ -Bearing and Al-Free System
Because bridgmanite coexists with ferropericlase and MgFe 2 O 4 -phase in the MgO-SiO 2 -Fe 2 O 3 ternary system under MgO-rich conditions, the experimentally determined MgFeO 2.5 , FeFeO 3 , and total Fe 3+ contents should represent their maximum contents in Al-free bridgmanite when ferropericlase is present. Fei et al. (2020) demonstrated that the concentration of FeFeO 3 component increases with increasing temperature, whereas the MgFeO 2.5 content has no clear temperature dependence when bridgmanite coexists with ferropericlase at 27 GPa. On the other hand, the MgFeO 2.5 , FeFeO 3 , and total Fe 3+ contents are found to decrease with increasing pressure in the present study (Figures 7a and 8a). By combining the pressure and temperature effects following the geotherm in the lower mantle (Katsura et al., 2010), the MgFeO 2.5 content in Al-free bridgmanite coexisting with ferropericlase decreases rapidly by more than two orders of magnitude from 4 to 5 mol % at the topmost lower mantle to nearly zero at 1,000-1,200 km depth. In contrast, the FeFeO 3 content decreases from 8% at 700 km depth to a minimum of ∼4% at about 800 km depth, and is nearly constant or slightly increases to 5% at 1,200 km depth because of the negative and positive pressure and temperature dependences, respectively ( Figure 10).
Although the Fe 3+ content in bridgmanite under deep lower mantle conditions is considered to be relatively low in comparison with Al 3+ (Frost & McCammon, 2008;Irifune & Ringwood, 1987a;Kurnosov et al., 2017;Lauterbach et al., 2000;Liu et al., 2020;Nakajima et al., 2012;Prescher et al., 2014;Shim et al., 2017;Sinmyo et al., 2019) and therefore Fe 3+ should mainly form FeAlO 3 , Fe 3+ /Al 3+ could be larger than unity in some regions. For example, bridgmanite in the topmost lower mantle has relatively high Fe 3+ solubility (Fei et al., 2020;Liu et al., 2018;Wang et al., 2021) but low Al 3+ solubility (Liu et al., 2016;Panero et al., 2006), subducted slabs may have relatively high oxygen fugacity conditions and thus should be Fe 3+ -enriched (Zhao et al., 2021), and harzburgitic rocks are depleted in Al 3+ (e.g., Irifune & Ringwood, 1987b). All of these regions may have Figure 10. FeFeO 3 and MgFeO 2.5 contents as a function of depth according to the pressure dependences determined in this study, temperature dependences given by Fei et al. (2020), and geotherm from Katsura et al. (2010). The MgAlO 2.5 content in Fe 3+ -free bridgmanite is based on the pressure and temperature dependences given by Liu, Ishii, and Katsura (2017) and Liu, Akaogi, & Katsura (2019). relatively high Fe 3+ content in bridgmanite, and thus MgFeO 2.5 and FeFeO 3 components could be formed. Their concentrations should decrease rapidly with increasing pressure because of the negative pressure dependence of their solubilities as determined in this study.

Implications for Lower Mantle Dynamics
The presence of MgFeO 2.5 and FeFeO 3 components in bridgmanite may affect its physical and chemical properties as predicted from the effects of MgAlO 2.5 and AlAlO 3 components (e.g., Andrault et al., , 2007Boffa-Ballaran et al., 2012;Brodholt, 2000;Daniel et al., 2004;Frost & Langenhorst, 2002;Saikia et al., 2009;Xu et al., 1998;Yagi et al., 2004;Zhang & Weidner, 1999). Because the MgFeO 2.5 component contains oxygen vacancies, the atomic diffusivity, which is proportional to the defect concentration, is expected to be enhanced. On the other hand, although the FeFeO 3 component does not produce vacancies, it should strongly distort the crystal structure of bridgmanite by substitution of Fe 3+ on the Si site compared to other components such as Mg-SiO 3 , AlAlO 3 , FeSiO 3 , and FeAlO 3 due to the much larger ionic radius of Fe 3+ compared to Si 4+ and Al 3+ . As a result, the FeFeO 3 component is expected to enhance element diffusivities as well. Therefore, the decrease of both FeFeO 3 and MgFeO 2.5 content with pressure may cause decreasing atomic diffusivities in bridgmanite, which may affect diffusion-controlled physical and chemical processes and thus affect the mantle dynamics.
One example is mantle rheology. The creep of minerals is controlled by diffusion of the slowest species (e.g., Herring, 1950;Nabarro, 1967). Although the viscosity of bridgmanite is controlled by Mg and Si diffusion rather than O because Mg and Si diffuse slower than O (e.g., Dobson et al., 2008;Holzapfel et al., 2005;Xu et al., 2011;Yamazaki et al., 2000), both Mg and Si are fully surrounded by O in polyhedrons. The hopping of Mg and Si ions from/into the polyhedron should become easier when an oxygen ion is missing. Hence, oxygen vacancies may enhance the diffusion of Mg and Si and thus reduce the viscosity. Therefore, it is predicted that the decrease in both FeFeO 3 and MgFeO 2.5 contents in bridgmanite from 700 to ∼1,000-1,200 km depth could suppress Mg and Si diffusivities in bridgmanite, which may contribute to the large viscosity increase in the midmantle inferred from geoid analysis (Rudolph et al., 2015).
Another example is electrical conductivity in the lower mantle. The electrical conductivity of bridgmanite is dominated by the ionic conduction mechanism at relatively high temperatures (e.g., Dobson, 2003;Xu & Mc-Cammon, 2002;Yoshino et al., 2016), which is controlled by atomic diffusion of the fastest species, that is, O in bridgmanite (Dobson et al., 2008). Therefore, based on the Nernst-Einstein relation, the ionic conductivity of bridgmanite should be enhanced by the presence of the MgFeO 2.5 component. The decreasing of MgFeO 2.5 content with depth may contribute to the decrease in observed conductivity at >800 km depth based on magnetotelluric sounding (e.g., Civet et al., 2015;Civet & Tarits, 2013).
The above examples are based on qualitative interpretation. To constrain the role of MgFeO 2.5 and FeFeO 3 components on mantle dynamics in more detail, further investigations about their effects on the physical and chemical properties of bridgmanite are required. Additionally, as mentioned above, bridgmanite in the lower mantle contains Al 3+ , which could affect the substitution mechanism of Fe 3+ (e.g., Huang, Boffa-Ballaran, McCammon, Miyajima, Dolejš & Frost, 2021;Liu et al., 2020). More experimental studies on the pressure and temperature dependences of Fe 3+ substitution in both Fe 3+ and Al 3+ bearing bridgmanite are therefore necessary following the pattern of the detailed investigated in Huang, Boffa-Ballaran, McCammon, Miyajima, Dolejš and Frost (2021) ;; at a single condition (25 GPa, 1970 K) corresponding to the topmost lower mantle.