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(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]

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.

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]

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]

Phase H (MgSiO4H2) is considered an important carrier of water into the lower mantle by the subduction of slabs. This phase has been reported to decompose into H2O ice VII and MgSiO3 bridgmanite under pressure. However, the dissociation phase boundary under the mantle pressure and temperature conditions has not been determined thus far. In this work, the dissociation phase boundary of phase H is determined by the calculation of Gibbs free energy of H2O ice VII. The stability field of phase H is found to be significantly extended from 52 to 62 GPa by the inclusion of zero‐point vibrational energy. Phase H decomposes into MgSiO3 bridgmanite and H2O ice VII at approximately 60 GPa (at ∼1000 K). This result indicates that the transportation of water by dense hydrous magnesium silicates may be terminated at a depth of approximately 1,500 km in the middle of the lower mantle in a pure Mg‐endmember composition. Key Points: The dissociation phase boundary of phase H MgSiO4H2 is determined by calculation of Gibbs free energy of ice VIIPhase H decomposes into MgSiO3 bridgmanite and H2O ice VII at approximately 60 GPaThe transportation of water by DHMSs may be terminated at a depth of approximately 1,500‐km depth in a pure Mg‐endmember composition [ABSTRACT FROM AUTHOR]

Phase H (MgSiO4H2) is considered an important carrier of water into the lower mantle by the subduction of slabs. This phase has been reported to decompose into H2O ice VII and MgSiO3 bridgmanite under pressure. However, the dissociation phase boundary under the mantle pressure and temperature conditions has not been determined thus far. In this work, the dissociation phase boundary of phase H is determined by the calculation of Gibbs free energy of H2O ice VII. The stability field of phase H is found to be significantly extended from 52 to 62 GPa by the inclusion of zero‐point vibrational energy. Phase H decomposes into MgSiO3 bridgmanite and H2O ice VII at approximately 60 GPa (at ∼1000 K). This result indicates that the transportation of water by dense hydrous magnesium silicates may be terminated at a depth of approximately 1,500 km in the middle of the lower mantle in a pure Mg‐endmember composition. Key Points: The dissociation phase boundary of phase H MgSiO4H2 is determined by calculation of Gibbs free energy of ice VIIPhase H decomposes into MgSiO3 bridgmanite and H2O ice VII at approximately 60 GPaThe transportation of water by DHMSs may be terminated at a depth of approximately 1,500‐km depth in a pure Mg‐endmember composition [ABSTRACT FROM AUTHOR]

Thermal conductivity of the lowermost mantle governs the heat flow out of the core energizing planetary-scale geological processes. Yet, there are no direct experimental measurements of thermal conductivity at relevant pressure–temperature conditions of Earth's core–mantle boundary. Here we determine the radiative conductivity of post-perovskite at near core–mantle boundary conditions by optical absorption measurements in a laser-heated diamond anvil cell. Our results show that the radiative conductivity of Mg 0.9 Fe 0.1 SiO 3 post-perovskite (∼1.1 W/m/K) is almost two times smaller than that of bridgmanite (∼2.0 W/m/K) at the base of the mantle. By combining this result with the present-day core–mantle heat flow and available estimations on the lattice thermal conductivity we conclude that post-perovskite is at least as abundant as bridgmanite in the lowermost mantle which has profound implications for the dynamics of the deep Earth. [ABSTRACT FROM AUTHOR]

LiNbO3-type Mg2.98(2)Al1.99(2)Si3.02(2)O12 (py-LN) was synthesized by recovering a run product from 2000 K and 45 GPa to ambient conditions using a large volume press. Rietveld structural refinements were carried out using the one-dimensional synchrotron XRD pattern collected at ambient conditions. The unit-cell lattice parameters were determined to be a = 4.8194(3) Å, c = 12.6885(8) Å, V = 255.23(3) ų, with Z = 6 (hexagonal, R3c). The average A-O and B-O distances of the AO6 and BO6 octahedra have values similar to those that can be obtained from the sum of the ionic radii of the averaged A- and B-site cations and oxygen (2.073 and 1.833 Å, respectively). The present compound has the B-site cations at the octahedral site largely shifted along the c axis compared with other LiNbO3-type phases formed by back-transition from perovskite (Pv)-structure, and as a result, the coordination number of this site is better described as 3+3. It appears therefore that the B-site cation in the octahedral position cannot be completely preserved during the back-transition because of the small size of Si and Al, which occupy usually a tetrahedral site at ambient conditions. The formation of py-LN can be explained by the tilting of BO6 octahedra of the perovskite structure having the pyrope composition and formed at high P-T conditions. The tilting is driven by the decrease in ionic radius ratio between the A-site cation and oxygen during decompression. This also explains why there is no back-transition from the Pvstructure to the ilmenite-structure during decompression, since this is a reconstructive phase transition whose activation energy cannot be overcome at room temperature. Py-LN may be formed in shocked meteorites by the back-transformation after the garnet-bridgmanite transition, and will indicate shock conditions around 45 GPa and 2000 K. [ABSTRACT FROM AUTHOR]

Iron- and aluminum-bearing MgSiO3 bridgmanite is the most abundant mineral in the Earth's interior; hence its crystal chemistry is fundamental to expanding our knowledge of the deep Earth and its evolution. In this study, the valence and spin state of iron in well-characterized Al-free Fe3+-rich bridgmanite were investigated by means of Mössbauer spectroscopy to understand the effect of ferric iron on the spin state. We found that a minor amount of Fe3+ is in the low-spin state above 36 GPa and that its proportion does not increase substantially with pressure up to 83 GPa. This observation is consistent with recent experimental studies that used Mössbauer and X-ray emission spectroscopy. In the Earth's deep lower mantle, Fe3+ spin crossover may take place at depths below 900 and 1200 km in pyrolite and MORB, respectively. However, the effect of spin crossover on physical properties may be small due to the limited amount of Fe3+ in the low-spin state. [ABSTRACT FROM AUTHOR]

Melting experiments in a binary system MgO-MgSiO were performed up to 70 GPa using a CO laser heated diamond anvil cell. The quenched samples were polished and analyzed by a dualbeam focused ion beam (FIB) and a field emission scanning electron microscope (FE-SEM), respectively. The liquidus phase and the eutectic composition were determined on the basis of textual and chemical analyses of sample cross sections. Our experimental results show that the eutectic composition is the Si/Mg molar ratio of ~0.76 at 35 GPa and it decreases with increasing pressure. Above 45 GPa, it becomes relatively constant at about 0.64-0.65 Si/Mg molar ratio. Using our experimental data collected at a wide pressure range up to 70 GPa together with previous experimental data, we have constructed a thermodynamic model of the eutectic composition of the MgO-MgSiO system. The eutectic composition extrapolated to the pressure and temperature conditions at the base of the mantle is about 0.64 Si/Mg molar ratio. The modeled eutectic composition is quite consistent with a previous prediction from ab initio calculations (de Koker et al. in Earth Planet Sci Lett 361:58-63, 2013), suggesting that the simple assumption of a non-ideal regular solution model can well describe the melting relation of the MgO-MgSiO system at high pressure. Our results show that the liquidus phase changes from MgO-periclase to MgSiO-bridgmanite at 35 GPa for the simplified pyrolite composition (~0.7 Si/Mg molar ratio), while MgSiO-bridgmanite is the liquidus phase at the entire lower mantle conditions for the chondritic composition (~0.84 Si/Mg molar ratio). [ABSTRACT FROM AUTHOR]

Lattice thermal conductivity of ferropericlase and radiative thermal conductivity of iron bearing magnesium silicate perovskite (bridgmanite) – the major mineral of Earth’s lower mantle– have been measured at room temperature up to 30 and 46 GPa, respectively, using time-domain thermoreflectance and optical spectroscopy techniques in diamond anvil cells. The results provide new constraints for the pressure dependencies of the thermal conductivities of Fe bearing minerals. The lattice thermal conductivity of ferropericlase Mg 0.9 Fe 0.1 O is 5.7(6) W/(m * K) at ambient conditions, which is almost 10 times smaller than that of pure MgO; however, it increases with pressure much faster (6.1(7)%/GPa vs 3.6(1)%/GPa). The radiative conductivity of a Mg 0.94 Fe 0.06 SiO 3 bridgmanite single crystal agrees with previously determined values for powder samples at ambient pressure; it is almost pressure-independent in the investigated pressure range. Our results confirm the reduced radiative conductivity scenario for the Earth’s lower mantle, while the assessment of the heat flow through the core-mantle boundary still requires in situ measurements at the relevant pressure–temperature conditions. [ABSTRACT FROM AUTHOR]

We have investigated the electronic configuration of iron in Fe-, Al-containing magnesium silicate perovskite, i.e., bridgmanite, the main component of the lower mantle, at conditions of the deep Earth's interior using the energy domain Synchrotron Mössbauer Source technique. We show that the high ferric iron content observed previously in quenched samples is preserved at high temperatures and high pressures. Our data are consistent with high-spin to intermediate-spin (HS-IS) crossover in Fe 2+ at high pressures and ambient temperature. We see no evidence of spin crossover in Fe 3+ occupying the A-position of bridgmanite. On laser heating at pressures above ∼40 GPa we observe a new doublet with relative area below 5% which is assigned to Fe 3+ in the octahedral (B-site) position in bridgmanite. We conclude that at lower mantle conditions Fe 3+ remains predominantly in the HS state, while Fe 2+ occurs solely in the IS state. [ABSTRACT FROM AUTHOR]

• We report the opacity of major mantle minerals at core-mantle boundary (CMB) P-T. • Bridgmanite remains semi-transparent while ferropericlase is nearly opaque at the CMB. • Radiative contribution to the total thermal conductivity at the CMB is < 10%. • The electrical conductivity of ferropericlase at the CMB is > 45 S/cm. Earth's lowermost mantle displays complex geological phenomena that likely result from its heterogeneous physical interaction with the core. Geophysical models of core-mantle interaction rely on the thermal and electrical conductivities of appropriate geomaterials which, however, have never been probed at representative pressure and temperature (P-T) conditions. Here we report on the opacity of single crystalline bridgmanite and ferropericlase and link it to their radiative and electrical conductivities. Our results show that light absorption in the visible spectral range is enhanced upon heating in both minerals but the rate of change in opacity with temperature is a factor of six higher in ferropericlase. As a result, bridgmanite in the lowermost mantle is moderately transparent while ferropericlase is highly opaque. Our measurements support previous indirect estimates of low (< 1 W/m/K) and largely temperature-independent radiative conductivity in the lowermost mantle. This implies that the radiative mechanism has not contributed significantly to cooling the Earth's core throughout the geologic time. Opaque ferropericlase is electrically conducting and mediates strong core-mantle electromagnetic coupling, explaining the intradecadal oscillations in the length of day, low secular geomagnetic variations in Central Pacific, and the preferred paths of geomagnetic pole reversals. [ABSTRACT FROM AUTHOR]

Constraining the spin state of Fe in Earth’s lower mantle is critical to understanding the chemistry and dynamics of Earth’s interior. In the October 2015 issue of American Mineralogist, Dorfman et al. present an experimental study of the effect of iron concentration on the spin transition in bridgmanite. Their experiments involved two different bridgmanite compositions (38 and 74% FeSiO3). Based on the total spin moment determined by synchrotron-based X‑ray emission spectroscopy, they show that Fe2+ in bridgmanite is in the high-spin state in the lower mantle but transition pressure decreases within highly enriched iron concentrations. [ABSTRACT FROM AUTHOR]