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Hydrous Fe3+‐bearing bridgmanite (Bdg) is potentially a critical water host in the lowermost mantle. The spin transition behaviors of such materials are pivotal for understanding geophysical heterogeneity in the deep Earth but are poorly understood. Here, we investigated the spin transition and related geophysical properties of Fe3+ with associated H defects [Fe3+‐H] at high P‐T conditions using first‐principles simulations. Our calculations predict that the presence of hydrogen reduces the onset pressure of the spin transition of Fe3+ in Bdg, leading to higher fractions of low spin Fe3+. Along standard geotherms, spin transition is predicted to remain incomplete even at the core‐mantle boundary (CMB), and lateral temperature variations would significantly affect the proportions of high and low spin Fe and related properties like elasticity. The thermoelastic property of hydrous Fe3+‐bearing bridgmanite exhibit stronger softening anomalies at the lower mantle conditions compared to dry system, which potentially enhancing the seismic detectability of the hydrous Bdg in the deep earth. Density profile of hydrous Fe3+‐bearing bridgmanite indicates that the [Fe3+‐H] defect modestly increases the system's density, but much less than that caused by incorporating an equivalent amount of iron alone. This is crucial for understanding regions like Large Low Shear Velocity Provinces (LLSVPs), which exhibits large velocity drops but only minor density changes. The co‐adsorption of Fe and H allows for the introduction of Fe to induce velocity drops without the concomitant sharp increase in density, as pure iron would, thus enabling Fe‐H enrichment as a potential source of LLSVPs. Plain Language Summary: Ferric iron (Fe3+)‐bearing bridgmanite is the major mineral in the Earth's lower mantle and a crucial potential host for water, a vital volatile component within Earth's interior. However, the change on the electronic configuration of Fe3+, so called spin state transition in hydrous bridgmanite in lower mantle remains poorly understood. In this work, we calculated the spin transition of Fe3+ and related geophysical properties in hydrous Fe3+‐bearing bridgmanite. We found that water promotes the Fe3+ spin transition and that this transition is present over most of range of the lower mantle. We observed that the distinction in thermoelastic properties arise between hydrous and dry Fe3+‐bearing bridgmanite, potentially providing a way to distinguish between dry and hydrous bridgmanite case in the deep Earth. Finally, we found that the [Fe3+‐H] defect slightly increase the density of bridgmanite, but far less than the addition of pure iron would. This finding helps to explain the unique characteristics of Large Low Shear Velocity Provinces (LLSVPs), which show large velocity reductions but minor density changes. Therefore, [Fe3+‐H] defects could as a promising explanation for the source of LLSVPs. Key Points: Hydrogen shifts the onset pressure of Fe3+ spin transition in bridgmanite to shallower depths in the lower mantleHydrous Fe3+‐bearing bridgmanite exhibit stronger softening anomalies on elasticity compared to H‐free case at lower mantle conditionHydrogen provides a mechanism to incorporate seismic‐softening iron into bridgmanite without geophysically implausible density changes [ABSTRACT FROM AUTHOR]

Bridgmanite, the most abundant mineral in the lower mantle, can play an essential role in deep-Earth hydrogen storage and circulation processes. To better evaluate the hydrogen storage capacity and its substitution mechanism in bridgmanite occurring in nature, we have synthesized high-quality single-crystal bridgmanite with a composition of (M g 0.88 F e 0.05 2 + F e 0.05 3 + A l 0.03) (S i 0.88 A l 0.11 H 0.01) O 3 at nearly water-saturated environments relevant to topmost lower mantle pressure and temperature conditions. The crystallographic site position of hydrogen in the synthetic (Fe,Al)-bearing bridgmanite is evaluated by a time-of-flight single-crystal neutron diffraction scheme, together with supporting evidence from polarized infrared spectroscopy. Analysis of the results shows that the primary hydrogen site has an OH bond direction nearly parallel to the crystallographic b axis of the orthorhombic bridgmanite lattice, where hydrogen is located along the line between two oxygen anions to form a straight geometry of covalent and hydrogen bonds. Our modeled results show that hydrogen is incorporated into the crystal structure via coupled substitution of Al3+ and H+ simultaneously exchanging for Si4+, which does not require any cation vacancy. The concentration of hydrogen evaluated by secondary-ion mass spectrometry and neutron diffraction is ~0.1 wt% H2O and consistent with each other, showing that neutron diffraction can be an alternative quantitative means for the characterization of trace amounts of hydrogen and its site occupancy in nominally anhydrous minerals. [ABSTRACT FROM AUTHOR]

Hydrogen may be incorporated into nominally anhydrous minerals including bridgmanite and post‐perovskite as defects, making the Earth's deep mantle a potentially significant water reservoir. The diffusion of hydrogen and its contribution to the electrical conductivity in the lower mantle are rarely explored and remain largely unconstrained. Here we calculate hydrogen diffusivity in hydrous bridgmanite and post‐perovskite, using molecular dynamics simulations driven by machine learning potentials of ab initio quality. Our findings reveal that hydrogen diffusivity significantly increases with increasing temperature and decreasing pressure, and is considerably sensitive to hydrogen incorporation mechanism. Among the four defect mechanisms examined, (Mg + 2H)Si and (Al + H)Si show similar patterns and yield the highest hydrogen diffusivity. Hydrogen diffusion is generally faster in post‐perovskite than in bridgmanite, and these two phases exhibit distinct diffusion anisotropies. Overall, hydrogen diffusion is slow on geological time scales and may result in heterogeneous water distribution in the lower mantle. Additionally, the proton conductivity of bridgmanite for (Mg + 2H)Si and (Al + H)Si defects aligns with the same order of magnitude of lower mantle conductivity, suggesting that the water distribution in the lower mantle may be inferred by examining the heterogeneity of electrical conductivity. Plain Language Summary: Water or hydrogen may be trapped in the Earth's deep mantle, affecting the state and evolution of our planet. However, the mobility of hydrogen in the lower mantle remains poorly understood. By using advanced machine learning‐driven simulations, we find that hydrogen in lower‐mantle silicates diffuses faster at higher temperatures and lower pressures, and how hydrogen is incorporated into the silicates greatly influences this mobility. Two specific ways of hydrogen incorporation were found to allow hydrogen to move very efficiently. On a geological scale, the transport of hydrogen is slow, suggesting that water may be unevenly distributed in the Earth's lower mantle. The electrical conductivity associated with the movement of hydrogen aligns with previous observations of the Earth's mantle. Variations in electrical conductivity in the lower mantle may inform where water is located in the Earth's deep interior. Key Points: A machine learning potential of ab initio quality has been developed for hydrous MgSiO3 systemHydrogen diffusion is sluggish in both bridgmanite and post‐perovskite, leading to heterogeneous water distribution in the lower mantleProton diffusion may be associated with the variation of electrical conductivity in the deep mantle [ABSTRACT FROM AUTHOR]

Thermal conductivity plays a pivotal role in understanding the dynamics and evolution of Earth's interior. The Earth's lower mantle is dominated by MgSiO3 polymorphs which may incorporate trace amounts of water. However, the thermal conductivity of MgSiO3‐H2O binary system remains poorly understood. Here, we calculate the thermal conductivity of water‐free and water‐bearing bridgmanite, post‐perovskite, and MgSiO3 melt, using a combination of Green‐Kubo method with molecular dynamics simulations based on a machine learning potential of ab initio quality. The thermal conductivities of water‐free bridgmanite and post‐perovskite overall agree well with previous theoretical and experimental studies. The presence of water mildly reduces the thermal conductivity of the host minerals, significantly weakens the temperature dependence of the thermal conductivity, and reduces the thermal anisotropy of post‐perovskite. Overall, water reduces the thermal conductivity difference between bridgmanite and post‐perovskite, and thus may attenuate lateral heterogeneities of the core‐mantle boundary heat flux. Plain Language Summary: MgSiO3 is a major component of the Earth and water may dissolve into it. Even a small amount of water may affect the thermal conductivity of minerals and influence the heat transport in the Earth's interior, which is essential for the dynamics and evolution of our planet. However, the effect of water on the thermal conductivity of MgSiO3 remains largely unknown. In this study, we use computer simulations based on machine learning methods to investigate the thermal conductivity of MgSiO3‐H2O system. We find that adding water reduces the thermal conductivity of MgSiO3 mineral, makes the thermal conductivity less dependent on temperature, and also diminishes the differences in heat conduction in various directions. Our results suggest that the presence of water may reduce the increase in heat flow that is expected when MgSiO3 changes its structure at extreme depths and exerts influences on geological processes in the deep mantle. Key Points: Thermal conductivity of MgSiO3‐H2O system is calculated using Green‐Kubo method with machine learning potentialsWater weakens the temperature dependence and the anisotropy of thermal conductivity of mineralsWater reduces the thermal conductivity enhancement effect due to the phase transition from bridgmanite to post‐perovskite [ABSTRACT FROM AUTHOR]

A model of the genesis of stishovite and other SiO2 phases in terrestrial matter is developed; it combines the physicochemical and geodynamic conditions of their formation. Based on the experimental data, a P–T diagram of SiO2 polymorphs in combination with the boundaries of geospheres and geotherm was plotted. Stishovite and other SiO2 phases of cosmic-impact synthesis were buried in the early Earth during the period of meteorite accretion (50 Ma). These SiO2 phases are completely assimilated by melts of the pyrolite global magma ocean that existed for 500 Ma. By 2.0 Ga, the magma ocean crystallized, and the Earth's crust, upper mantle, transition zone, and lower mantle with layer D" (with seismic boundaries between them) were formed. During this period, the main mass of the Earth's core was separated, which completed by 2.7 Ga. As a result, the gravitational field intensified, which contributed to the fractional ultramafic–mafic evolution of mantle magmas with peritectic reactions of ringwoodite–akimotoite in the transition zone and bridgmanite in the lower mantle with melts and the formation of stishovite (shown experimentally at 20 and 26 GPa). These reactions in diamond-forming carbonate–silicate–carbon melts provided the formation of stishovite, which was captured as a paragenetic inclusion by diamonds and transported to the Earth's surface by magmas. The genesis of stishovite under the terrestrial conditions is controlled by global mantle convection as well. The subduction of lithospheric plates to layer D'' near the liquid core was accompanied by the formation of stishovite, and then its transformation into poststishovite phases. When superplumes rise from layer D'' to the Earth's crust, the peritectic reactions of postperovskite and bridgmanite, and then ringwoodite–akimotoite, with melts are likely to form stishovite and cause its subsequent transformation into low-pressure SiO2 phases. With the emergence of the Earth's crust, the impact-meteorite genesis of stishovite resumes. Stishovite that formed under the terrestrial conditions appears as an inclusion in ultradeep diamonds on the Earth's surface. Stishovite of cosmic-impact synthesis is preserved in meteorite craters. In both cases, stishovite is a metastable phase. [ABSTRACT FROM AUTHOR]

(Mg, Fe, Al)(Si, Al)O3 bridgmanite is the most abundant mineral of Earth′s lower mantle. Al is incorporated in the crystal structure of bridgmanite through the Fe3+AlO3 and AlAlO3 charge coupled (CC) mechanisms, and the MgAlO2.5 oxygen vacancy (OV) mechanism. Oxygen vacancies are believed to cause a substantial decrease of the bulk modulus of aluminous bridgmanite based on first‐principles calculations on the MgAlO2.5 end‐member. However, there is no conclusive experimental evidence supporting this hypothesis due to the uncertainties on the chemical composition, crystal chemistry, and/or high‐pressure behavior of samples analyzed in previous studies. Here, we synthesized high‐quality single crystals of bridgmanite in the MgO–AlO1.5–SiO2 system with different bulk Al contents and degrees of CC and OV substitutions. Suitable crystals with different compositions were loaded in resistively heated diamond anvil cells and analyzed by synchrotron X‐ray diffraction at pressures up to approximately 80 GPa at room temperature and 35 GPa at temperatures up to 1,000 K. Single‐crystal structural refinements at high pressure show that the compressibility of bridgmanite is mainly controlled by Al–Si substitution in the octahedral site and that oxygen vacancies in bridgmanite have no detectable effect on the bulk modulus in the compositional range investigated here, which is that relevant to a pyrolytic lower mantle. The proportion of oxygen vacancies in Al‐bearing bridgmanite has been calculated using a thermodynamic model constrained using experimental data at 27 GPa and 2,000 K for an Fe‐free system and extrapolated to pressures equivalent to 1,250 km depth using the thermoelastic parameters of Al‐bearing bridgmanite determined in this study. Plain Language Summary: Earth′s lower mantle, spanning from approximately 660 to 2,890 km depth, is mainly composed of bridgmanite, (Mg, Fe, Al)(Si, Al)O3 with a distorted perovskite‐type structure. In the topmost 500 km of the lower mantle, experiments showed that an MgAlO2.5 oxygen vacancy component may be present in bridgmanite. However, the effect of oxygen vacancies on the thermoelastic properties of bridgmanite is still not well constrained. These parameters are of crucial importance not only to model the thermodynamic stability of bridgmanite components at high‐pressure and high‐temperature but also to calculate the physical properties of lower mantle rocks. Here, we have determined the high‐pressure and high‐pressure‐temperature equations of state of three bridgmanite samples having different concentrations of Al and oxygen vacancies using X‐ray diffraction in diamond anvil cells. These data are complemented by the analysis of the bridgmanite crystal structure as a function of pressure, which revealed that oxygen vacancies have a minor effect on the elastic properties of bridgmanite at concentrations relevant for the lower mantle. Combining our results with experimental data on the stability of bridgmanite in the MgO–AlO1.5–SiO2 system, we have modeled the thermodynamic stability of the MgAlO2.5 component as a function of pressure and bulk Al content. Key Points: The thermal equation of state of Al‐bearing bridgmanite was determined for the first time using single crystalsThe bulk compressibility of Al‐bearing bridgmanite is mostly controlled by Al‐Si substitution in the octahedral siteUsing the obtained equation of state parameters, we modeled the pressure dependency of oxygen vacancies in bridgmanite in MgO–AlO1.5–SiO2 [ABSTRACT FROM AUTHOR]

Water in Earth's mantle plays a critical role in both geodynamic and surficial habitability. Water in the upper mantle and transition zone is widely discussed, but less is known about the water in the lower mantle despite it constituting over half of Earth's mass. Understanding the water storage in Earth's lower mantle relies on comprehending the water solubility of bridgmanite, which is the most abundant mineral both in the lower mantle and throughout Earth. Nevertheless, due to limited access to the lower mantle, our understanding of water in bridgmanite mainly comes from laboratory experiments and theoretical calculations, and a huge controversy still exists. In this paper, we provide a review of the commonly employed research methods and current findings concerning the solubility of water in bridgmanite. Potential factors, such as pressure, temperature, compositions, etc., that influence the water solubility of bridgmanite will be discussed, along with insights into future research directions. [ABSTRACT FROM AUTHOR]

Abstract Bridgmanite formation and amorphization in shocked meteorites constrain the pressure and temperature conditions during planetary impact. However, the effect of the bridgmanite grain size on its amorphization kinetics is still unclear. Here, the amorphization mechanism and kinetics of fine-grained polycrystalline bridgmanite were studied at high temperatures up to 1080 K. High-temperature time-resolved synchrotron X-ray diffraction measurements showed that significant volume expansion due to temperature-induced amorphization caused static stress, which then hindered amorphization progress. Further, the temperature required for the amorphization of fine-grained bridgmanite (~ 1 μm) was found to be approximately 100 K higher than that required for the amorphization of coarse-grained samples (> 10 μm). We also noted that amorphization preferentially commenced at the twin planes and subgrain boundaries of bridgmanite grains, resulting in lower amorphization temperatures for the coarse-grained samples. The limited number of such specific locations in fine-grained natural bridgmanite suggested that grain boundary amorphization may be the dominant mechanism for bridgmanite amorphization in shocked meteorites. This unique amorphization kinetics would support the preservation of bridgmanite during the post-shock annealing in the shocked meteorite. Although bridgmanite amorphization starts easily at temperatures above ~ 420 K, a small amount of bridgmanite grains can survive at temperatures above 800 K by the effect of amorphization-induced stress.

Halogens are powerful tracers for the accretion of Earth's volatiles. Their sensitivity to Earth's differentiation, such as core formation, magma ocean crystallization, and planetary evaporation allows us to track such major events and the origin of Earth's building blocks. Here we examine the role of magma ocean crystallization in fractionating F/Cl ratio of the BSE by experimentally determining the partitioning behavior of F and Cl between silicate melt and crystallized minerals. Previous experimental data, due to technical challenges, are only available at pressures <25 GPa. To simulate relevant conditions for deep magma oceans, we perform laser-heated diamond anvil cell experiments to determine partition coefficients of F and Cl between bridgmanite and silicate melt at temperatures of ∼4000–4500 K and pressures of 33–55 GPa. Our data show that F and Cl are both strongly favored in silicate melt, with partition coefficients of F and Cl in the range of 0.12–0.16 and 0.007–0.017, respectively. Given plausible magma ocean scenarios, our results indicate that F and Cl are not significantly fractionated during crystallization of a deep magma ocean. Neglecting other potential processes that may fractionate F and Cl, the F/Cl ratio in the BSE should reflect that of Earth's building blocks. It indicates Earth's volatiles may be largely derived from inner solar system, in alignment with other isotopic systems such as nitrogen and hydrogen. [ABSTRACT FROM AUTHOR]

Silicate perovskite, with the mineral name bridgmanite, is the most abundant mineral in the Earth’s lower mantle. We investigated crystal structures and equations of state of two perovskite-type Fe3+-rich phases, FeMg0.5Si0.5O3 and Fe0.5Mg0.5Al0.5Si0.5O3, at high pressures, employing single-crystal X-ray diffraction and synchrotron Mössbauer spectroscopy. We solved their crystal structures at high pressures and found that the FeMg0.5Si0.5O3 phase adopts a novel monoclinic double-perovskite structure with the space group of P21/n at pressures above 12 GPa, whereas the Fe0.5Mg0.5Al0.5Si0.5O3 phase adopts an orthorhombic perovskite structure with the space group of Pnma at pressures above 8 GPa. The pressure induces an iron spin transition for Fe3+ in a (Fe0.7,Mg0.3)O6 octahedral site of the FeMg0.5Si0.5O3 phase at pressures higher than 40 GPa. No iron spin transition was observed for the Fe0.5Mg0.5Al0.5Si0.5O3 phase as all Fe3+ ions are located in bicapped prism sites, which have larger volumes than an octahedral site of (Al0.5,Si0.5)O6.

Bridgmanite formation and amorphization in shocked meteorites constrain the pressure and temperature conditions during planetary impact. However, the effect of the bridgmanite grain size on its amorphization kinetics is still unclear. Here, the amorphization mechanism and kinetics of fine-grained polycrystalline bridgmanite were studied at high temperatures up to 1080 K. High-temperature time-resolved synchrotron X-ray diffraction measurements showed that significant volume expansion due to temperature-induced amorphization caused static stress, which then hindered amorphization progress. Further, the temperature required for the amorphization of fine-grained bridgmanite (~ 1 μm) was found to be approximately 100 K higher than that required for the amorphization of coarse-grained samples (> 10 μm). We also noted that amorphization preferentially commenced at the twin planes and subgrain boundaries of bridgmanite grains, resulting in lower amorphization temperatures for the coarse-grained samples. The limited number of such specific locations in fine-grained natural bridgmanite suggested that grain boundary amorphization may be the dominant mechanism for bridgmanite amorphization in shocked meteorites. This unique amorphization kinetics would support the preservation of bridgmanite during the post-shock annealing in the shocked meteorite. Although bridgmanite amorphization starts easily at temperatures above ~ 420 K, a small amount of bridgmanite grains can survive at temperatures above 800 K by the effect of amorphization-induced stress. [ABSTRACT FROM AUTHOR]

Fe,Al-bearing MgSiO3 perovskite (bridgmanite) is considered to be the most abundant mineral in Earth's lower mantle, hosting ferric iron in its structure as charge-coupled (Fe2O3 and FeAlO3) and vacancy components (MgFeO2.5 and Fe2/3SiO3). We examined concentrations of ferric iron and aluminum in the perovskite phase as a function of temperature (1700–2300 K) in the MgSiO3-FeAlO3-MgO system at 27 GPa using a multi-anvil high-pressure apparatus. We found a LiNbO3-structured phase in the quenched run product, which was the perovskite phase under high pressures and high temperatures. The perovskite phase coexists with corundum and a phase with (Mg,Fe3+,□)(Al,Fe3+)2O4 composition (□= vacancy). The FeAlO3 component in the perovskite phase decreases from 69 to 65 mol% with increasing temperature. The Fe2O3 component in the perovskite phase remains unchanged at ~1 mol% with temperature. The A-site vacancy component of Fe2/3SiO3 in the perovskite phase exists as 1–2 mol% at 1700–2000 K, whereas 1 mol% of the oxygen vacancy component of MgFeO2.5 appears at higher temperatures, although the analytical errors prevent definite conclusions. The A-site vacancy component might be more important than the oxygen vacancy component for the defect chemistry of bridgmanite in slabs and for average mantle conditions when the FeAlO3 charge-coupled component is dominant. [ABSTRACT FROM AUTHOR]

Thermoelastic properties of mantle candidate minerals are essential to our understanding of geophysical phenomena, geochemistry, and geodynamic evolutions of the silicate Earth. However, the lower-mantle mineralogy remains much debated due to the lack of single-crystal elastic moduli (Cij) and aggregate sound velocities of (Al,Fe)-bearing bridgmanite, the most abundant mineral of the planet, at the lower mantle pressure-temperature (P-T) conditions. Here we report single-crystal Cij of (Al,Fe)-bearing bridgmanite, Mg0.88Fe0.1Al0.14Si0.90O3 (Fe10-Al14-Bgm) with Fe3+/ΣFe = ~0.65, up to ~82 GPa using X-ray diffraction (XRD), Brillouin light scattering (BLS), and impulsive stimulated light scattering (ISLS) measurements in diamond-anvil cells (DACs). Two crystal platelets with orientations of (–0.50, 0.05, –0.86) and (0.65, –0.59, 0.48), that are sensitive to deriving all nine Cij, are used for compressional and shear wave velocity (νP and νS) measurements as a function of azimuthal angles over 200° at each experimental pressure. Our results show that all Cij of singe-crystal Fe10-Al14-Bgm increase monotonically with pressure with small uncertainties of 1–2% (±1σ), except C55 and C23, which have uncertainties of 3–4%. Using the third-order Eulerian finite-strain equations to model the elasticity data yields the aggregate adiabatic bulk and shear moduli and respective pressure derivatives at the reference pressure of 25 GPa: KS = 326 ± 4 GPa, μ = 211 ± 2 GPa, K s ′ = ′3.32 ± 0.04, and μ′ = 1.66 ± 0.02 GPa. The high-pressure aggregate νS and νP of Fe10-Al14-Bgm are 2.6–3.5% and 3.1–4.7% lower than those of MgSiO3 bridgmanite end-member, respectively. These data are used with literature reports on bridgmanite with different Fe and Al contents to quantitatively evaluate pressure and compositional effects on their elastic properties. Comparing with one-dimensional seismic profiles, our modeled velocity profiles of major lower-mantle mineral assemblages at relevant P-T suggest that the lower mantle could likely consist of about 89 vol% (Al,Fe)-bearing bridgmanite. After considering uncertainties, our best-fit model is still indistinguishable from pyrolitic or chondritic models. [ABSTRACT FROM AUTHOR]

The primary phase of the Earth's lower mantle, (Al, Fe)‐bearing bridgmanite, transitions to the post‐perovskite (PPv) phase at Earth's deep mantle conditions. Despite extensive experimental and ab initio investigations, there are still important aspects of this transformation that need clarification. Here, we address this transition in (Al3+, Fe3+)‐, (Al3+)‐, (Fe2+)‐, and (Fe3+)‐bearing bridgmanite using ab initio calculations and validate our results against experiments on similar compositions. Consistent with experiments, our results show that the onset transition pressure and the width of the two‐phase region depend distinctly on the chemical composition: (a) Fe3+‐, Al3+‐, or (Al3+, Fe3+)‐alloying increases the transition pressure, while Fe2+‐alloying has the opposite effect; (b) in the absence of coexisting phases, the pressure‐depth range of the Pv‐PPv transition is likely too broad to cause a sharp D" discontinuity (<30 km); (c) the average Clapeyron slope of the two‐phase regions are consistent with previous measurements, calculations in MgSiO3, and inferences from seismic data. In addition, (d) we observe a softening of the bulk modulus in the two‐phase region. The consistency between our results and experiments gives us the confidence to proceed and examine this transition in aggregates with different compositions computationally, which will be fundamental for resolving the most likely chemical composition of the D" region by analyses of tomographic images. Plain Language Summary: In 2004, it was discovered that the main phase of the Earth's lower mantle, MgSiO3‐perovskite (Pv) or bridgmanite, undergoes a phase transition to a post‐perovskite phase (PPv) at conditions similar to those expected at the D" discontinuity at the bottom of the lower mantle. The predicted seismic discontinuity across the Pv‐PPv transition suggested that the PPv phase is a major phase in the D" region. This paper presents ab initio predictions of this phase change in Al‐, Fe3+‐, Fe2+‐, and simultaneous (Al3+, Fe3+)‐bearing bridgmanite. Results are consistent with experiments but extend the range of temperature, pressure, and compositions investigated. These alloying elements affect the onset transition pressure differently and broaden the pressure range of the phase change with consequences for the thickness of this transition in the mantle. In the absence of co‐existing phases, the Pv‐PPv transition appears too broad to cause a sharp D" discontinuity (<30 km). The consistency between our results and experiments gives us the confidence to proceed and examine this transition in aggregates with different compositions computationally, which will be fundamental for resolving the most likely chemical composition of the D" region by analyzing tomographic images. Key Points: Ab initio results on the Pv‐PPv transition in Fe‐ and Al‐bearing bridgmanite are consistent with experimental measurements in phases with similar compositionsPlausible compositions display wide transition pressure ranges. A change of aggregate composition across a sharp D" discontinuity is likely to occurThe Pv‐PPv exothermic transition does not contribute significantly to the thermal boundary layer atop the core‐mantle boundary [ABSTRACT FROM AUTHOR]

• The effects of Al and Fe3+on the grain growth rates of bridgmanite are investigated. • The FeFeO 3 , AlAlO 3 , and FeAlO 3 components enhance the growth rate of bridgmanite. • The MgAlO 2.5 and MgFeO 2.5 have minor effects on the growth rate of bridgmanite. • Al and Fe3+ have negligible effect on lower mantle rheology due to their small contents. Trivalent cations such as Al3+ and Fe3+ can be incorporated into the crystal structure of bridgmanite either by the charge-coupled mechanism forming the FeFeO 3 , AlAlO 3 , and FeAlO 3 components or by the oxygen vacancy mechanism forming the MgAlO 2.5 and MgFeO 2.5 components. They may affect the physical properties of bridgmanite and thus affect lower mantle dynamics. In this study, we investigated the effects of Al and Fe3+ on the grain growth kinetics of bridgmanite at a pressure of 27 GPa and temperatures of 2000 – 2300 K by multi anvil experiments. The experimental results indicate that the FeFeO 3 , AlAlO 3 , and FeAlO 3 components enhance the growth rate of bridgmanite, while the MgAlO 2.5 and MgFeO 2.5 components have negligible effects. However, due to the relatively low Fe3+ and Al3+ contents of bridgmanite in the lower mantle, none of them affect the lower mantle rheology significantly. In particular, the mid-mantle viscosity jump interpreted from geoid analysis is unlikely to be caused by the decreasing of MgAlO 2.5 and MgFeO 2.5 concentrations with increasing depth. [ABSTRACT FROM AUTHOR]

Water, in trace amounts, can greatly alter chemical and physical properties of mantle minerals and exert primary control on Earth’s dynamics. Quantifying how water is retained and distributed in Earth’s deep interior is essential to our understanding of Earth’s origin and evolution. While directly sampling Earth’s deep interior remains challenging, the experimental technique using laser-heated diamond anvil cell (LH-DAC) is likely the only method available to synthesize and recover analog specimens throughout Earth’s lower mantle conditions. The recovered samples, however, are typically of micron sizes and require high spatial resolution to analyze their water abundance. Here we use nano-scale secondary ion mass spectrometry (NanoSIMS) to characterize water content in bridgmanite, the most abundant mineral in Earth’s lower mantle. We have established two working standards of natural orthopyroxene that are likely suitable for calibrating water concentration in bridgmanite, i.e., A119(H2O) = 99 ± 13 μg/g (1SD) and A158(H2O) = 293 ± 23 μg/g (1SD). We find that matrix effect among orthopyroxene, olivine, and glass is less than 10%, while that between orthopyroxene and clinopyroxene can be up to 20%. Using our calibration, a bridgmanite synthesized by LH-DAC at 33 ± 1 GPa and 3,690 ± 120 K is measured to contain 1,099 ± 14 μg/g water, with partition coefficient of water between bridgmanite and silicate melt ∼0.025, providing the first measurement at such condition. Applying the unique analytical capability of NanoSIMS to minute samples recovered from LH-DAC opens a new window to probe water and other volatiles in Earth’s deep mantle.

Bridgmanite, the most abundant mineral in the Earth's lower mantle, can be found in meteorites that experienced instantaneous high shock pressure during parent body impact. However, the presence of bridgmanite in meteorites is unusual because bridgmanite grains should be amorphized under residual post‐shock temperatures at ambient pressure. Here, we report the results of time‐resolved synchrotron X‐ray diffraction measurements at high temperatures to analyze the amorphization mechanisms and kinetics of bridgmanite. The thermal expansion coefficient of bridgmanite before the amorphization is 2.1 × 10−5 K−1. At higher temperatures, our results show that the significant volume expansion due to the amorphization induces static stress that can reach up to ∼0.5 GPa, which prevents the progress of the amorphization. This time‐insensitive amorphization kinetics may have enabled the preservation of bridgmanite in the shocked meteorite that fell on Earth. Also, the reaction progress estimated based on the amorphous fraction provides the residual post‐shock temperature. Plain Language Summary: Bridgmanite is the most abundant mineral in the Earth's lower mantle. Natural bridgmanite in the shocked meteorite is used to constrain the impact histories of parent bodies since this mineral can form at very high pressure. However, the presence of bridgmanite in meteorites is mysterious because it should be easily amorphized at atmospheric pressure after the shock events. This study reports the amorphization mechanisms and kinetics of bridgmanite based on time‐resolved synchrotron X‐ray diffraction techniques. We found that the significant volume expansion due to the amorphization induces static stress, which hinders the amorphization from progressing. The preservation of bridgmanite in meteorites can be explained by this unique amorphization kinetics. Key Points: Amorphization kinetics of bridgmanite was studied via time‐resolved synchrotron X‐ray diffraction measurementsSignificant volume expansion due to the bridgmanite amorphization induces static stress, which prevents the progress of the amorphizationPreservation of bridgmanite in the meteorites can be explained by the unique amorphization kinetics [ABSTRACT FROM AUTHOR]

We conducted in situ high‐pressure and ‐temperature X‐ray diffraction measurements of a pyrolitic mantle material up to 4480 K at 122–166 GPa in a laser‐heated diamond anvil cell. Results demonstrate that the phase transition between bridgmanite and post‐perovskite occurs in pyrolite within the lowermost mantle pressure range even at >4000 K. It suggests the ubiquitous occurrence of post‐perovskite above the core‐mantle boundary, which may be consistent with recent high‐quality seismology data that non‐observations of D" reflections are exceptions. Combining with earlier experiments performed at and below the normal lower‐mantle geotherm, our data show that the bridgmanite + post‐perovskite two‐phase region is ∼5 GPa thick and the dP/dT slope of the boundary is +6.5 ± 2.2 MPa/K, slightly smaller than previous theoretical calculations in MgSiO3. The global presence of rheologically weak post‐perovskite at the bottom of the mantle has profound implications in seismology, geodynamics, and heat transfer from the core. Plain Language Summary: (Al, Fe)‐bearing MgSiO3 bridgmanite is a predominant mineral in the lower mantle. While bridgmanite with the MgSiO3 end‐member composition is known to undergo a phase transition to post‐perovskite at lowermost mantle pressures, the pressure, and thickness of the phase boundary in a typical mantle material (pyrolite) have been controversial. The present synchrotron X‐ray diffraction measurements of pyrolite performed up to 4480 K around the core‐mantle boundary (CMB) pressure show that the bridgmanite/post‐perovskite phase transition takes place within the lowermost mantle pressure range even under high temperatures of the CMB region. The bridgmanite + post‐perovskite two‐phase region is found to be about 90 km thick. These results suggest the global presence of post‐perovskite above the CMB, which is consistent with recent seismological observations of D" reflectors not only in the circum‐Pacific high velocity regions but also in many areas away from subduction zones. Post‐perovskite is rheologically weak, and its ubiquitous occurrence in the lowermost mantle has important seismological and geodynamical implications. The large positive dP/dT slope (+6.5 ± 2.2 MPa/K) of the boundary suggests that a phase transition from post‐perovskite to bridgmanite assists upwelling of plumes from hot regions above the CMB. Key Points: We conducted synchrotron X‐ray diffraction measurements of a pyrolitic mantle material up to 4480 K at 122–166 GPa in a laser‐heated diamond anvil cellPhase transition between bridgmanite and post‐perovskite occurs in pyrolite within the lowermost mantle pressure range even at >4000 KUbiquitous occurrence of post‐perovskite above the core‐mantle boundary is consistent with recent high‐quality seismological observations of D" reflections [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Bridgmanite is the most abundant mineral in the Earth's lower mantle. Although its basic chemical formula is MgSiO3, large amounts of Fe3+ can be added in the following two ways: (1) Two Fe3+ replace Mg2+ and Si4+ and form the FeFeO3 component. This is called charge‐coupled substitution because the overall charge does not change. (2) One Fe3+ replaces one Si4+ and forms the MgFeO2.5 component. Here the loss of positive charge is compensated by a loss of oxygen and is therefore called oxygen‐vacancy substitution. In this study, we measured the effect of pressure on the abundance of these two components of bridgmanite. We found that the MgFeO2.5 content decreases greatly with increasing pressure. Some oxygen sites may therefore be vacant in bridgmanite at the top of lower mantle, but the concentration of oxygen vacancies should decrease rapidly in deeper regions. The decrease of oxygen‐vacancy concentration in bridgmanite will change the nature of the lower mantle, for example, rocks will become harder, and electrical conductivity will decrease with increasing depth. Key Points: MgFeO2.5, FeFeO3, and total Fe3+ contents in bridgmanite decrease with increasing pressureFe3+‐linked oxygen vacancies in bridgmanite are destabilized by increasing pressureMgFeO2.5 can be formed in Fe3+‐rich bridgmanite under the topmost lower mantle conditions [ABSTRACT FROM AUTHOR]

Bridgmanite, the most abundant mineral of the Earth’s lower mantle, has been reported in only a few shocked chondritic meteorites; however, the compositions of these instances differ from that expected in the terrestrial bridgmanite. Here, we report the first natural occurrence of Fe-bearing aluminous bridgmanite in shock-induced melt veins within the Katol L6 chondrite with a composition that closely matches those synthesized in high-pressure and temperature experiments over the last three decades. The Katol bridgmanite coexists with majorite and metal-sulfide inter-growths. We found that the natural Fe-bearing aluminous bridgmanite in the Katol L6 chondrite has a significantly higher Fe3+/ΣFe ratio (0.69 ±0.08) than coexisting majorite (0.37 ± 0.10), which agrees with experimental studies. The Katol bridgmanite is arguably the closest natural analog for the bridgmanite composition expected to be present in the Earth’s lower mantle. Textural observations and comparison with laboratory experiments suggest that the Katol bridgmanite formed at pressures of ∼23 to 25 gigapascals directly from the chondritic melt generated by the shock event. Thus, the Katol L6 sample may also serve as a unique analog for crystallization of bridgmanite during the final stages of magma ocean crystallization during Earth’s formation. [ABSTRACT FROM AUTHOR]

Observations of seismic anisotropy just above the core‐mantle boundary are a powerful way to understand the dynamics of the lowermost mantle. Here we present models of seismic anisotropy in the lowermost mantle beneath Siberia based on new and previously published body wave observations. We compile a set of measurements based on a variety of data types, including the differential splitting of SK(K)S‐PKS and S‐ScS phases and polarities of PdP and SdS reflections off the D″ discontinuity. We carry out ray theoretical forward modeling of these data sets in combination, using a novel approach that allows for tighter constraints on the geometry of seismic anisotropy than would be possible using a single data type. Observations for each seismic phase alone only provide limited information; by combining different data types into one modeling approach, we can constrain D″ seismic anisotropy more tightly. We test a range of plausible mechanisms for seismic anisotropy, including a variety of candidate minerals (bridgmanite, post‐perovskite, and ferropericlase), dominant slip systems, and orientations, and consider both single‐crystal elasticity and elastic tensors based on polycrystalline texture modeling. In general, we find that post‐perovskite (with either a [100](010) or [100](001) dominant slip system) or bridgmanite models provide the best fit to the observations. The best‐fitting models suggest a dominant shear direction to the southwest or northeast direction at the base of the mantle. This is consistent with flow directed toward the Perm anomaly, potentially driven by slab remnants impinging on the core‐mantle boundary. Key Points: Updated modeling and observations of lowermost mantle seismic anisotropy beneath SiberiaWe combine different data types to constrain seismic anisotropy mechanisms and plausible flow directions beneath SiberiaPost‐perovskite is the likely mechanism with a slightly dipping shear plane and shear direction toward the northeast/southwest [ABSTRACT FROM AUTHOR]

High pressure and temperature experiments were carried out on the oxide mixtures corresponding to the bridgmanite stoichiometry under the hydrous shallow lower mantle conditions (24–25 GPa and 1673–1873 K with 5–10 wt. % of water in the starting material). Oxide mixtures investigated correspond to MgSiO3, (Mg, Fe)SiO3, (Mg, Al, Si)O3, and (Mg, Fe, Al, Si)O3. Melting was observed in all runs. Partitioning of various elements, including Mg, Fe, Si, and H is investigated. Melting under hydrous lower mantle conditions leads to increased (Mg + Fe)O/SiO2 in the melt compared to the residual solids. The residual solids often contain a large amount of stishovite, and the melt contains higher (Mg,Fe)O/SiO2 ratio than the initial material. (Mg + Fe)O‐rich hydrous melt could explain the low‐velocity anomalies observed in the shallow lower mantle and a large amount of stishovite in the residual solid may be responsible for the scattering of seismic waves in the mid‐lower mantle and may explain the "stishovite paradox." Since stishovite‐rich materials are formed only when silica‐rich source rock (MORB) is melted (not a typical peridotitic rock [bulk silicate Earth]), seismic scattering in the lower mantle provides a clue on the circulation of subducted MORB materials. To estimate hydrogen content, we use a new method of estimating the water content of unquenchable melts, and also propose a new interpretation of the significance of superhydrous phase B inclusions in bridgmanite. The results provide revised values of water partitioning between solid minerals and hydrous melts that are substantially higher than previous estimates. Plain Language Summary: Melting is a major process by which Earth evolves chemically. However, the consequence of melting is not well understood under the deep mantle conditions. This study focuses on melting under the shallow lower mantle conditions that occurs with the help of water. Nature of water distribution between the melt and remaining solids was studied using new approaches. We discovered that when melting occurs for a material with composition similar to oceanic crust, the residual solid is nearly completely stishovite (SiO2). Stishovite shows anomalous elastic properties below ∼800 km depth depending on the composition. Observed strong seismic scattering in this depth range might be due to the presence of stishovite‐rich materials formed by melting in the lower mantle. Key Points: Hydrous lower mantle meltingStishovite in the lower mantleEstimation of water content in unquenchable melts [ABSTRACT FROM AUTHOR]

The crystal chemistry of ten well‐characterized bridgmanite single‐crystals with Fe and Al contents ranging from 0 to 0.40 atoms per two‐cation formula units were investigated by single‐crystal X‐ray diffraction. Structural refinements indicate that Fe3+ and Al mainly occupy the Mg and Si sites, respectively, when present in similar proportions. Molar volumes of bridgmanite endmember components were refined using data from this and previous studies and found to decrease in the order Fe3+Fe3+O3 > MgFe3+O2.5 > Fe3+AlO3 > MgAlO2.5 > AlAlO3 > Fe2+SiO3 > MgSiO3. Fe3+AlO3 charge‐coupled substitution leads to an anisotropic increase of B‐O bond distances, resulting in more distorted octahedral B sites and in a more significant increase of the c‐axis with respect to the a‐ and b‐axes. Valence bond calculations indicate that the A site is more compressible than the B site for all bridgmanite samples studied, implying that octahedral tilting and distortion will dominate the bridgmanite compression mechanism. Guided by these crystal chemical observations, bulk moduli of bridgmanite endmember components were estimated using results of previous studies. The volume changes of equilibria controlling the speciation of bridgmanite components were then calculated at conditions relevant to the top of Earth's lower mantle. The proportion of oxygen vacancy components is predicted to decrease with pressure. While the stability of the bridgmanite Fe3+AlO3 component will drive charge disproportionation to produce iron metal at the top of the lower mantle, this appears to be much less favorable by 50 GPa. An increase in the proportion of the Fe3+Fe3+O3 bridgmanite component, however, may favor the formation of iron metal at higher pressures. Plain Language Summary: Fe‐Al‐bearing MgSiO3 bridgmanite is the most abundant mineral in the Earth's lower mantle. Its physical and chemical properties, which are closely related to its crystal structure, play a major role in controlling processes in the Earth's lower mantle. In this study, we synthesized several bridgmanite single crystals with various compositions, which allowed the substitution mechanisms and site occupancies of Fe and Al in the structure to be determined. The changes in volume and structure resulting from the different substitution mechanisms were identified and can be used to explain the compressibility of different bridgmanite endmembers. The effect of pressure on the composition of bridgmanite was then predicted. We found that pressure should favor charge‐coupled substitution where 3+ cations enter both the Mg and Si cation sites. The alternative mechanism, where 3+ cations enter the Si site and oxygen vacancies are created, is not favored with pressure. Also Fe3+ and Al may become disordered onto both the Mg and Si sites as pressure increases. The formation of Fe metal through reduction and oxidation of Fe2+ may be suppressed at greater depths in the lower mantle, therefore, the deep lower mantle may be less reduced than the shallower region. Key Points: The molar volumes of bridgmanite endmember components were determined to be Fe3+Fe3+O3 > MgFe3+O2.5 > Fe3+AlO3 > MgAlO2.5 > AlAlO3 > Fe2+SiO3 > MgSiO3Fe3+ and Al mainly occupy the A and B sites, respectively, at 25 GPa when Fe3+ = Al but disordering may be favored at higher pressuresPressure favors charge‐coupled substitution over oxygen vacancy formation and may suppress Fe metal precipitation [ABSTRACT FROM AUTHOR]

Texture, plastic deformation, and phase transformation mechanisms in perovskite and post-perovskite are of general interest for our understanding of the Earth's mantle. Here, the perovskite analogue NaCoF3 is deformed in a resistive-heated diamond anvil cell (DAC) up to 30 GPa and 1013 K. The in situ state of the sample, including crystal structure, stress, and texture, is monitored using X-ray diffraction. A phase transformation from a perovskite to a post-perovskite structure is observed between 20.1 and 26.1 GPa. Normalized stress drops by a factor of 3 during transformation as a result of transient weakening during the transformation. The perovskite phase initially develops a texture with a maximum at 100 and a strong 010 minimum in the inverse pole figure of the compression direction. Additionally, a secondary weaker 001 maximum is observed later during compression. Texture simulations indicate that the initial deformation of perovskite requires slip along (100) planes with significant contributions of f110g twins. Following the phase transition to post-perovskite, we observe a 010 maximum, which later evolves with compression. The transformation follows orientation relationships previously suggested where the c axis is preserved between phases and hh0 vectors in reciprocal space of post-perovskite are parallel to [010] in perovskite, which indicates a martensitic-like transition mechanism. A comparison between past experiments on bridgmanite and current results indicates that NaCoF3 is a good analogue to understand the development of microstructures within the Earth's mantle. [ABSTRACT FROM AUTHOR]

Clues to a planet’s geologic history are contained in its interior structure, particularly its core. We detected reflections of seismic waves from the core-mantle boundary of Mars using InSight seismic data and inverted these together with geodetic data to constrain the radius of the liquid metal core to 1830 ± 40 kilometers. The large core implies a martian mantle mineralogically similar to the terrestrial upper mantle and transition zone but differing from Earth by not having a bridgmanite-dominated lower mantle. We inferred a mean core density of 5.7 to 6.3 grams per cubic centimeter, which requires a substantial complement of light elements dissolved in the iron-nickel core. The seismic core shadow as seen from InSight’s location covers half the surface of Mars, including the majority of potentially active regions—e.g., Tharsis—possibly limiting the number of detectable marsquakes. [ABSTRACT FROM AUTHOR]

In this study, we report for the first‐time shock‐induced incongruent melting of olivine in an ordinary chondrite. Several olivine grains (Fo74), entrained in the shock‐melt vein of the Kamargaon L6 chondrite, were dissociated into magnesiowüstite (XFe = 0.71) and orthoenstatite (XFe = 0.22). We propose that the breakdown of olivine took place as a result of incongruent melting to produce magnesiowüstite and Mg‐rich liquid. We suggest that bridgmanite may have crystallized as the second phase from the olivine melt which back‐transformed to a low‐pressure phase of orthoenstatite from subsequent high‐temperature and low‐pressure events. In this case, olivine grains may have experienced pressure and temperature of ∼25 GPa and ∼2500°C, respectively. Our results suggest that the incongruent melting of olivine may possibly operate as one of the alternative mechanisms of dissociation reaction driving the phase transformation of olivine in the natural systems. Plain Language Summary: The planet Earth was formed from the similar material that constitutes present‐day asteroids which is mostly made up of olivine. Therefore, it is important to study olivine at high pressure and high temperature to understand its behavior. Olivine breaks down into bridgmanite and magnesiowüstite in the Earth's lower mantle which is one of the most important reactions that largely controls the physical and chemical properties of the Earth's interior. This breakdown may occur where the olivine remains in the solid state or may also form by melting of the olivine. The breakdown assemblage of bridgmanite and magnesiowüstite formed by both of these mechanisms has been reported in few Martian meteorites. Recently, this breakdown assemblage by the solid‐state has been reported in the Suizhou meteorite. However, no such assemblage formed by melting has been found in meteorites originated from the asteroid belt. We, for the first time, report the possible occurrence of bridgmanite and magnesiowüstite formed by incongruent melting of olivine in an ordinary chondrite. This assemblage may have formed at pressure and temperature of ∼25 GPa and ∼2500°C. These observations suggest that the dissociation of olivine in the natural systems can also take place by the melting of olivine. Key Points: We report shock‐induced incongruent melting of olivine in an ordinary chondrite for the first timeOlivine first dissociated to magnesiowüstite and liquid followed by crystallization of bridgmanite from the residual liquidOlivine grains may have experienced pressure and temperature of ∼25 GPa and ∼2500°C [ABSTRACT FROM AUTHOR]

The chemistry of bridgmanite (Brg), especially the oxidation state of iron, is important for understanding the physical and chemical properties, as well as putting constraints on the redox state, of the Earth's lower mantle. To investigate the controls on the chemistry of Brg, the Fe3+ content of Brg was investigated experimentally as a function of composition and oxygen fugacity (f o 2) at 25 GPa. The Fe3+/∑Fe ratio of Brg increases with Brg Al content and f o 2 and decreases with increasing total Fe content and with temperature. The dependence of the Fe3+/∑Fe ratio on f o 2 becomes less steep with increasing Al content. Thermodynamic models were calibrated to describe Brg and ferropericlase (Fp) compositions as well as the inter-site partitioning of trivalent cations in Brg in the Al–Mg–Si–O, Fe–Mg–Si–O and Fe–Al–Mg–Si–O systems. These models are based on equilibria involving Brg components where the equilibrium thermodynamic properties are the main adjustable parameters that are fit to the experimental data. The models reproduce the experimental data over wide ranges of f o 2 with a relatively small number of adjustable terms. Mineral compositions for plausible mantle bulk compositions can be calculated from the models as a function of f o 2 and can be extrapolated to higher pressures using data on the partial molar volumes of the Brg components. The results show that the exchange of Mg and total Fe (i.e., ferric and ferrous) between Brg and Fp is strongly f o 2 dependent, which allows the results of previous studies to be reinterpreted. For a pyrolite bulk composition with an upper mantle bulk oxygen content, the f o 2 at the top of the lower mantle is −0.86 log units below the iron–wüstite buffer (IW) with a Brg Fe3+/∑Fe ratio of 0.50 and a bulk rock ratio of 0.28. This requires the formation of 0.7 wt.% Fe–Ni alloy to balance the raised Brg ferric iron content. With increasing pressure, the model predicts a gradual increase in the Fe3+/∑Fe ratio in Brg in contrast to several previous studies, which levels off by 50 GPa. Oxygen vacancies in Brg decrease to practically zero by 40 GPa, potentially influencing elasticity, diffusivity and rheology in the top portion of the lower mantle. The models are also used to explore the f o 2 recorded by inclusions in diamonds, which likely crystallized as Brg in the lower mantle, revealing oxygen fugacities which likely preclude the formation of some diamonds directly from carbonates, at least at the top of the lower mantle. [ABSTRACT FROM AUTHOR]

The combination of seismic observations and mineral physics data represents a unique tool to understand the structure and evolution of the deep Earth's interior. However, to date, elasticity data on both compressional (vP) and shear (vS) wave velocities of MgSiO3 bridgmanite are limited to shallow mantle conditions, hampering the resolution of mineral physics models. Here, we report the first single‐crystal measurements of vP and vS of MgSiO3 bridgmanite up to ∼79 GPa using high‐pressure Brillouin scattering and single‐crystal X‐ray diffraction in a diamond anvil cell. At shallow lower mantle pressures, the elastic anisotropy of MgSiO3 bridgmanite was found to be similar, albeit smaller than that of Fe,Al‐bearing bridgmanite of Kurnosov et al. (2017) but differed significantly from that proposed in the recent study of Fu et al. (2019). Using the elastic stiffness coefficients of bridgmanite obtained in this study at different pressures, we calculate the pressure dependence of the adiabatic bulk modulus, KS0 = 257.1(6) GPa, K'S0 = 3.71(4), and of the shear modulus, G0 = 175.6(2) GPa, G'0 = 1.86(1). These elastic parameters are included in a self‐consistent thermodynamic model to calculate seismic wave velocities along a lower mantle adiabat for a primitive upper mantle bulk composition in the FeO‐CaO‐MgO‐SiO2 system, which is currently the most complex system for which sufficient data exist. This preliminary model provides a good match to the vS and vP of 1D seismic models, implying that the composition of the lower mantle may be closer to pyrolite, rather than being more bridgmanite rich. Key Points: We measured both compressional and shear wave velocities of MgSiO3 bridgmanite up to mid‐lower mantle pressure for the first timeThe effect of chemistry on the elastic moduli and elastic anisotropy of bridgmanite at lower mantle pressures is discussedWe compare the seismic velocities for a simplified primitive upper mantle composition with 1D seismic models of Earth's lower mantle [ABSTRACT FROM AUTHOR]

Experiments in laser‐heated diamond anvil cells (LH DACs) are conducted to assess phase diagrams of planetary materials at high pressure‐temperature (P‐T) conditions; thus, reliable determination of temperature in LH DAC experiments is essential. Radiometric temperature determination in LH DACs relies on the assumption of sample's wavelength‐independent optical properties (graybody assumption), which is not justified for major lower mantle materials. The result is that experimental phase diagrams contain systematic unconstrained errors. Here we estimate the systematic error in radiometric temperature of nongray polycrystalline bridgmanite (Bgm; Mg0.96Fe2+0.036Fe3+0.014Si0.99O3) in a LH DAC by modeling emission and absorption of thermal radiation in a sample with experimentally‐constrained optical properties. A comparison to experimental data validates the models and reveals that thermal spectra measured in LH DAC experiments record the interaction of radiation with the hot nongray sample. The graybody assumption in the experiments on translucent Bgm (light extinction coefficient, k < ∼250 cm‐1 at 500–900 nm) yields temperatures ∼5% higher than the maximum temperature in the sample heated to ∼1900 K. In contrast, the graybody temperature of dark Bgm (k > ∼1500 cm−1), such as that produced upon melt quenching in LH DACs, underestimates the maximum temperature by ∼10%. Our experimental results pose quantitative constraints on the effect of nongray optical properties on the uncertainty of radiometric temperature determination in Bgm in the LH DACs. Evaluating nongray temperature in the future would enable a revision of the Bgm to post‐perovskite phase transition and the high‐pressure melting curve of Bgm. Plain Language Summary: We modeled how the color of Earth's most abundant mineral (bridgmanite) affects temperature measurements in laser‐heated diamond anvil cell experiments with implications to the phase diagram of bridgmanite. We show that, depending on the optical properties of bridgmanite, temperature determination in such experiments may either systematically overestimate or underestimate the maximum temperature by up to 5%–10% due to the wavelength‐dependent emission and absorption in the hot sample. Overall, this study shows the phase boundaries in the bridgmanite system need revision. Key Points: Thermal radiation spectra record a nongray interaction with bridgmanite in laser‐heated diamond anvil cellsRadiometric temperature of nongray bridgmanite in laser‐heated diamond anvil cells is up to 5%–10% off the maximum sample temperatureExtant melting curves and experimental phase transition boundaries in the bridgmanite system contain this systematic error in temperature [ABSTRACT FROM AUTHOR]

The temperature dependence of the Al 2 O 3 solubility in bridgmanite has been determined in the system MgSiO 3 –Al 2 O 3 at temperatures of 2750–3000 ​K under a constant pressure of 27 ​GPa using a multi-anvil apparatus. Bridgmanite becomes more aluminous with increasing temperatures. A LiNbO 3 -type phase with a pyrope composition (Mg 3 Al 2 Si 3 O 12) forms at 2850 ​K, which is regarded as to be transformed from bridgmanite upon decompression. This phase contains 30 ​mol% Al 2 O 3 at 3000 ​K. The MgSiO 3 solubility in corundum also increases with temperatures, reaching 52 ​mol% at 3000 ​K. Molar volumes of the hypothetical Al 2 O 3 bridgmanite and MgSiO 3 corundum are constrained to be 25.95 ​± ​0.05 and 26.24 ​± ​0.06 ​cm3/mol, respectively, and interaction parameters of non-ideality for these two phases are 5.6 ​± ​0.5 and 2.2 ​± ​0.5 ​KJ/mol, respectively. The increases in Al 2 O 3 and MgSiO 3 contents, respectively, in bridgmanite and corundum are caused by a larger entropy of Al 2 O 3 bridgmanite plus MgSiO 3 corundum than that of MgSiO 3 bridgmanite plus Al 2 O 3 corundum with temperature, in addition to the configuration entropy. Our study may help explain dynamics of the top lower mantle and constrain pressure and temperature conditions of shocked meteorites. Image 1 • Phase relations in the system MgSiO 3 –Al 2 O 3 were determined up to 3000 ​K ​at 27 ​GPa. • Bridgmanite can contain 30 ​mol% Al 2 O 3 at 27 ​GPa and 3000 ​K. • The solubility of Al 2 O 3 in bridgmanite and that MgSiO 3 in corundum increase with temperatures. • We constrain the molar volume and non-ideality of Al 2 O 3 bridgmanite and MgSiO 3 corundum. • Temperature dependence of Al 2 O 3 solubility in bridgmanite may explain the dynamics of the top lower mantle. [ABSTRACT FROM AUTHOR]

The isotopic compositions of iron in major mantle minerals may record chemical exchange between deep-Earth reservoirs as a result of early differentiation and ongoing plate tectonics processes. Bridgmanite (Bdg), the most abundant mineral in the Earth's lower mantle, can incorporate not only Al but also Fe with different oxidation states and spin states, which in turn can influence the distribution of Fe isotopes between Bdg and ferropericlase (Fp) and between the lower mantle and the core. In this study, we combined first-principles calculations with high-pressure nuclear resonant inelastic X-ray scattering measurements to evaluate the effects of Fe site occupancy, valence, and spin states at lower-mantle conditions on the reduced Fe partition function ratio (β -factor) of Bdg. Our results show that the spin transition of octahedral-site (B-site) Fe3+ in Bdg under mid-lower-mantle conditions generates a +0.09‰ increase in its β -factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smaller β -factors relative to Fe3+-bearing varieties, especially those containing B-site Fe3+. Our models suggest that Fe isotopic fractionation between Bdg and Fp is only significant in the lowermost mantle due to the occurrence of low-spin Fe2+ in Fp. Assuming early segregation of an iron core from a deep magma ocean, we find that neither core formation nor magma ocean crystallization would have resulted in resolvable Fe isotope fractionation. In contrast, Fe isotopic fractionation between low-spin Fe3+-bearing Bdg/Fe2+-bearing Fp and metallic iron at the core-mantle boundary may have enriched the lowermost mantle in heavy Fe isotopes by up to +0.20‰. [ABSTRACT FROM AUTHOR]

Plastic deformation and texture development in minerals of the lower mantle can result in seismic anisotropy, and studying deformation of lower mantle materials is therefore important for interpreting lower mantle flow. Most previous deformation experiments documenting texture development at lower mantle pressures have been conducted on single-phase samples and/or at room temperature. However, real rocks deform at high temperature and are poly-phase and deformation is therefore likely different from that of a single phase. Here we report on high temperature diamond anvil cell deformation experiments on a multiphase assemblage of bridgmanite, ferropericlase, and ringwoodite compressed from ∼28 to ∼39 GPa and resistively heated at a constant temperature of 1,000 K. We employ the elasto-viscoplastic self-consistent method to model both texture and lattice strain of bridgmanite as a function of deformation mechanisms. Simulations indicate deformation of bridgmanite is accommodated by about half of slip activity on (100)[010] with the remainder split between (100)[001] and/or (100)〈011〉. Texture in bridgmanite is consistent with most seismic observations in the lowermost mantle. Although there is texture development in both bridgmanite and ringwoodite, ferropericlase does not develop coherent texture throughout the course of the experiment. Analysis of lattice strains suggests that the lack of coherent texture development in ferropericlase is due to heterogeneous plastic deformation resulting from microstructural interactions imposed by other phases. Variations in texturing of bridgmanite and ferropericlase could therefore cause laterally varying, complex anisotropy. Our models for binary mantle-like mixtures of bridgmanite and ferropericlase show that changes in strain and texture partitioning can explain the range of observed lower mantle anisotropies.

Abstract The rheological properties of Earth's lower mantle play a key role for global mantle dynamics. The mineralogy of the lower mantle can be approximated as a bridgmanite‐ferropericlase mixture. Previous work has suggested that the deformation of this mixture might be dramatically affected by the large differences in viscosity between bridgmanite and ferropericlase. Here, we employ numerical models to establish a connection between ferropericlase morphology and the effective rheology of the Earth's lower mantle using a numerical‐statistical approach. Using this approach, we link the statistical properties of the two‐phase composite to its effective viscosity tensor using analytical approximations. We find that ferropericlase develops elongated structures within the bridgmanite matrix that result in significantly lowered viscosity. While our findings confirm previous endmember models that suggested a change of mantle viscosity due to the formation of interconnected weak layers, we show that significant rheological weakening can thus be already achieved even when ferropericlase does not form an interconnected network. Additionally, the alignment of weak ferropericlase leads to a pronounced viscous anisotropy that develops with total strain, which may have implications for understanding the viscosity structure of Earth's lower mantle as well as for modeling the behavior of subducting slabs. We show that to capture the effect of ferropericlase elongation on the effective viscosity tensor (and its anisotropy) in large‐scale mantle convection models, the analytical approximations that have been derived to describe the evolution of the effective viscosity of a two‐phase medium with aligned elliptical inclusions can be used.

The response of forsterite, Mg2SiO4, under dynamic compression is of fundamental importance for understanding its phase transformations and high‐pressure behavior. Here, we have carried out an in situ X‐ray diffraction study of laser‐shocked polycrystalline and single‐crystal forsterite (a‐, b‐, and c‐orientations) from 19 to 122 GPa using the Matter in Extreme Conditions end‐station of the Linac Coherent Light Source. Under laser‐based shock loading, forsterite does not transform to the high‐pressure equilibrium assemblage of MgSiO3 bridgmanite and MgO periclase, as has been suggested previously. Instead, we observe forsterite and forsterite III, a metastable polymorph of Mg2SiO4, coexisting in a mixed‐phase region from 33 to 75 GPa for both polycrystalline and single‐crystal samples. Densities inferred from X‐ray diffraction data are consistent with earlier gas‐gun shock data. At higher stress, the response is sample‐dependent. Polycrystalline samples undergo amorphization above 79 GPa. For [010]‐ and [001]‐oriented crystals, a mixture of crystalline and amorphous material is observed to 108 GPa, whereas the [100]‐oriented forsterite adopts an unknown phase at 122 GPa. The first two sharp diffraction peaks of amorphous Mg2SiO4 show a similar trend with compression as those observed for MgSiO3 in both recent static‐ and laser‐driven shock experiments. Upon release to ambient pressure, all samples retain or revert to forsterite with evidence for amorphous material also present in some cases. This study demonstrates the utility of femtosecond free‐electron laser X‐ray sources for probing the temporal evolution of high‐pressure silicate structures through the nanosecond‐scale events of shock compression and release. Key Points: The phases occurring along the Hugoniots of forsterite are identified from 19 to 122 GPaBetween 33 and 75 GPa, forsterite and forsterite III coexist for both single‐crystal and polycrystalline samplesAt higher shock stress, variable degrees of amorphization are observed with different onset stresses for polycrystals and single crystals [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Iron primarily exists in three oxidation states within the mantle: metallic iron (Fe0), ferrous iron Fe2+ (more reduced form), and ferric iron Fe3+ (more oxidized form); however, the amount of Fe3+ in the lower mantle is largely unknown. Recent work has shown that Fe3+ has a strong effect on rock density and speed at which seismic waves travel through lower mantle minerals. In order to further investigate these effects, we conducted experiments on samples nearly identical in composition except for the Fe3+ content. We compressed the samples between two diamonds and heated the sample to the conditions of Earth's lower mantle. For the reduced sample (low Fe3+ content), we observed a complex assemblage of minerals, primarily composed of bridgmanite (the most abundant mineral in the Earth) and calcium silicate perovskite (a common secondary phase in the lower mantle). In contrast, the oxidized sample (high Fe3+ content) yielded a single phase of these two minerals combined together as one mineral—a Ca‐bearing bridgmanite. The resulting oxidized sample is lighter and seismic waves travel faster through it than the reduced sample. Thus, changes in Fe3+ content may lead to density and seismic wave speed variations in Earth's lower mantle. Key Points: Al‐rich pyroxenite mineralogy, density, and seismic velocity are tied to ferric iron content at lower mantle conditionsFerric iron and aluminum content may affect the formation of calcium‐bearing bridgmanite [ABSTRACT FROM AUTHOR]

To understand the effects of H2O on the mineral phases forming under the pressure-temperature conditions of the lower mantle, we have conducted laser-heated diamond-anvil cell experiments on hydrous ringwoodite (Mg2SiO4 with 1.1 wt% H2O) at pressures between 29 and 59 GPa and temperatures between 1200 and 2400 K. Our results show that hydrous ringwoodite (hRw) converts to crystalline dense hydrous silica, stishovite (Stv) or CaCl2-type SiO2 (mStv), containing 1 wt% H2O together with Brd and MgO at the pressure-temperature conditions expected for shallow lower-mantle depths between approximately 660 to 1600 km. Considering the lack of sign for melting in our experiments, our preferred interpretation of the observation is that Brd partially breaks down to dense hydrous silica and periclase (Pc), forming the phase assembly Brd + Pc + Stv. The results may provide an explanation for the enigmatic coexistence of Stv and Fp inclusions in lower-mantle diamonds. [ABSTRACT FROM AUTHOR]

The valence and spin states of Fe in (Fe,Al)-bearing bridgmanite (bdg) affect its physical properties, which is important for the interpretation of geophysical observations. Currently, tens of studies on the compressibility and spin states of Fe-bearing bdg have been reported. A consensus is that Fe-bearing bdg shows spin transition, which affects its elastic parameters. However, there is a conflict between reports on the compressibility and spin states of (Fe,Al)-bearing bdg in experiments using samples pre-synthesized in a multi-anvil apparatus (MA), and samples directly synthesized in a diamond anvil cell (DAC). There are no reports showing evidence of spin transition of Fe in compression experiments using (Fe,Al)-bearing bdg samples pre-synthesized in a MA, while those synthesized at relatively high pressure (at least above 45 GPa) in a DAC all exhibited the spin transition. Here, we performed synchrotron X-ray diffraction measurements on Mg0.85Fe0.09Al0.21Si0.86O3 and Mg0.85Fe0.14Al0.05Si0.96O3 bdg synthesized at relatively high pressure in a laser-heated DAC from amorphous starting material up to 47 and 56 GPa, respectively, at room temperature. The obtained pressure (P)–lattice volume (V) relations show noticeable softening at 22–30 GPa and 35–45 GPa, respectively, which is probably due to the spin transition of Fe. Combining our results and previous reports, we suggest that the lower mantle bdg is capable of containing low-spin Fe3+, which questions the general view. Such a transition changes density and may affect the physical properties of bridgmanite such as thermal conductivity and iron partitioning coefficient, thus having profound implications for mantle dynamics, and the chemical composition of the Earth. [ABSTRACT FROM AUTHOR]

Electronic states of iron in the lower mantle's dominant mineral, (Mg,Fe,Al)(Fe,Al,Si)O3 bridgmanite, control physical properties of the mantle including density, elasticity, and electrical and thermal conductivity. However, the determination of electronic states of iron has been controversial, in part due to different interpretations of Mössbauer spectroscopy results used to identify spin state, valence state, and site occupancy of iron. We applied energy-domain Mössbauer spectroscopy to a set of four bridgmanite samples spanning a wide range of compositions: 10–50% Fe/total cations, 0–25% Al/total cations, 12–100% Fe3+/total Fe. Measurements performed in the diamond-anvil cell at pressures up to 76 GPa below and above the high to low spin transition in Fe3+ provide a Mössbauer reference library for bridgmanite and demonstrate the efects of pressure and composition on electronic states of iron. Results indicate that although the spin transition in Fe3+ in the bridgmanite B-site occurs as predicted, it does not strongly affect the observed quadrupole splitting of 1.4 mm/s, and only decreases center shift for this site to 0 mm/s at ~70 GPa. Thus center shift can easily distinguish Fe3+ from Fe2+ at high pressure, which exhibits two distinct Mössbauer sites with center shift ~1 mm/s and quadrupole splitting 2.4–3.1 and 3.9 mm/s at ~70 GPa. Correct quantification of Fe3+/total Fe in bridgmanite is required to constrain the efects of composition and redox states in experimental measurements of seismic properties of bridgmanite. In Fe-rich, mixed-valence bridgmanite at deep-mantle-relevant pressures, up to ~20% of the Fe may be a Fe2.5+ charge transfer component, which should enhance electrical and thermal conductivity in Fe-rich heterogeneities at the base of Earth's mantle. [ABSTRACT FROM AUTHOR]

Fe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3 bridgmanite (FA50) with the highest Fe3+‐Al3+ coupled substitution known to date. X‐ray diffraction measurements showed that at ambient conditions, the FA50 adopted the LiNbO3 structure. 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 yields V0 = 172.1(4) Å3, K0 = 229(4) GPa with K0′ = 4(fixed). The calculated bulk sound velocity of the FA50 bridgmanite is ~7.7% lower than MgSiO3 bridgmanite, 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. Plain Language Summary: Fe‐Al‐bearing bridgmanite may be the dominant mineral in the lower mantle, which occupies more than half of Earth's volume. A subject of much debate is whether spin transition of Fe in bridgmanite produces an observable influence on the physics and chemistry of the lower mantle. In this study, we synthesized a new (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3 bridgmanite with the highest Fe3+‐Al3+ coupled substitution known to date. We studied its structure, elasticity, and spin state by multiple experimental and theoretical methods. The high Fe content allowed us to better resolve a pressure‐induced spin transition of Fe3+ caused by Fe‐Al cation exchange at high temperature. Our results suggest that the spin transition is enabled by cation exchange but has a minor effect on the seismic velocity, although it may introduce chemical heterogeneity in the lower mantle. Our study helps resolve existing discrepancies on the nature of spin transition of Fe‐Al bridgmanite and its influence on the physics and chemistry of the lower mantle. Key Points: Bridgmanite may contain 50% trivalent cations through Fe3+‐Al3+ coupled substitutionThe bulk sound velocity of (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3 bridgmanite is 7.7% lower than MgSiO3Through Fe‐Al cation exchange, Fe3+ in (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3 bridgmanite undergoes gradual spin transition at lower mantle conditions [ABSTRACT FROM AUTHOR]