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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]

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]

Phase relations in the system MgSiO3-Al2O3 were investigated at pressures of 27-45 GPa and temperatures of 1700, 2000, and 2300 K using sintered diamond and tungsten carbide anvils in a multianvil apparatus. The bulk compositions in the MgSiO3-Al2O3 binary system crystallize a phase assemblage of pyrope and corundum at pressures below 27 GPa and an assemblage of bridgmanite and corundum at pressures above 27 GPa regardless of temperatures. The solubility of Al2O3 in bridgmanite and that of MgSiO3 in corundum increases significantly with increasing temperature. The solubility of Al2O3 in bridgmanite increases from 6.7 mol % at 1700 K to 21.8 mol % at 2500 K under a constant pressure of 27 GPa. Bridgmanite becomes more aluminous with increasing pressure from 27 to 45 GPa at a given temperature. The MgSiO3 content in corundum increases with increasing pressure at pressure lower than 27 GPa, while it decreases at pressure higher than 27 GPa. Our results suggest that bridgmanite can incorporate a considerably higher Al2O3 content than that of the pyrope composition (25 mol % Al2O3). The present study further suggests that the entire Al2O3 component is accommodated into bridgmanite in the pyrolite lower mantle. However, Al2O3 cannot be fully accommodated into bridgmanite in the coldest parts of subducted slabs in the shallow part of the lower mantle, and therefore, additional phases such as MgAl2O4 with calcium ferrite-type structure are necessary to host the excess Al2O3. [ABSTRACT FROM AUTHOR]

• The grain size exponent for growth of bridgmanite in post-spinel is 5.2 ± 0.3. • The activation enthalpy for grain growth of bridgmanite is 260 ± 20 kJ/mol. • Grain size of bridgmanite is ∼30-45 μm in cold subducted slabs with 1-10 Myr age. The grain size of minerals is an important parameter that controls mantle dynamics. We investigated the grain growth kinetics of bridgmanite that coexists with ferropericlase by multi-anvil experiments at 27 GPa, 1400-2400 K, corresponding to the topmost lower mantle conditions. The experimental results indicate that the grain size of bridgmanite systematically increases with increasing duration, with a grain size exponent of 5.2 ± 0.3. The grain growth rate increases with increasing temperature with an activation enthalpy of 260 ± 20 kJ/mol. The grain size of bridgmanite is ∼30-45 μm in young subducted slabs at the topmost lower mantle conditions. The small grain size in subducted slabs may significantly reduce their creep strength in relatively to ambient mantle, and may cause the slab stagnation at the topmost lower mantle. [ABSTRACT FROM AUTHOR]

• Oxygen fugacity exhibits minimal variation in the upper lower mantle. • Bridgmanite Fe3+/∑Fe ratios decrease with increasing temperature. • Oxygen vacancies in Fe-Al bearing bridgmanite increase with increasing temperature. Bridgmanite, the most abundant mineral in the lower mantle, readily incorporates Fe3+ even under very reducing conditions where it is in equilibrium with metallic iron. This has led to the proposal that charge disproportionation of Fe2+ to produce Fe3+ and metallic iron may occur in the lower mantle. The amount of Fe3+ in bridgmanite is important for understanding the oxidation state of the mantle, as charge disproportionation of Fe2+ to form Fe3+ and metallic iron, followed by loss of metallic iron to the core, could have raised the Fe3+ content of the mantle as a whole after core formation. Here, the Fe oxidation state in bridgmanite was studied as a function of temperature at 27 GPa and an oxygen fugacity of ∼2.0 log units above the iron-wüstite oxygen buffer (ΔIW+2) using multi-anvil techniques. Bridgmanite Fe3+/∑Fe ratios were determined by Mössbauer spectroscopy and decrease from ∼80% at 1800 K to 50% at 2600 K. A decreasing Fe3+/∑Fe ratio with increasing temperature indicates that the level of charge disproportionation would have been lower under the higher temperature conditions of the early Earth. However, if the redox state of the upper lower mantle is buffered by coexisting iron and ferropericlase, then the relative oxygen fugacity will have changed very little during secular cooling. [ABSTRACT FROM AUTHOR]

A substantial amount of subducted basaltic crusts may exist in the lower mantle. It features distinct chemical composition from the peridotitic mantle and plays important roles in the chemical and dynamic evolution of Earth's interior. However, the chemical composition of mineral phases present in basaltic crust in the lower mantle is still poorly constrained. Here, we determined phase relations of normal mid-ocean ridge basalt (MORB) up to 52 GPa at 2000 K by multi-anvil press. Throughout our experiments, the mineral assemblages consist of five phases including bridgmanite, stishovite, calcium perovskite (davemaoite), calcium ferrite and new hexagonal aluminous phases. The density of MORB calculated from our phase assemblages and previously published thermoelastic data is 2-3% higher than that of the average mantle, which is consistent with literature. The new hexagonal aluminous phase is a host of potassium but only occupies 1-2 vol.% due to limited abundance of potassium in normal MORB. This indicates that the new hexagonal aluminous phase doesn't affect the elastic properties of basalt. Electron energy-loss spectra of recovered basaltic Al-rich bridgmanite show significant enrichment of ferrous iron (75-85%) compared with peridotitic Al-poor bridgmanite (∼30%), which is against previous studies showing that ferric iron ratio in bridgmanite increases with Al content. Ferric iron exhibits strong partitioning into the new hexagonal aluminous phase (41-64%), whereas bridgmanite (14-28%) and calcium ferrite phase (5-27%) remain Fe2+-enriched. The oxygen vacancy component of MgAlO 2.5 in bridgmanite is ∼11% up to 40 GPa, which is much higher than that in peridotitic bridgmanite (2-3%), possibly producing a viscosity contrast in the mid-mantle that would explain slab stagnation and plume thinning between 660 km and 1000 km depth. The presence of ferrous iron-rich bridgmanite in the deep lower mantle may contribute to seismic features of large low-shear-velocity provinces. • High pressure-temperature phase relations of MORB were determined up to 52 GPa. • Basaltic bridgmanite has a distinctly low Fe3+/ΣFe of less than 28%. • Basaltic bridgmanite contains much larger MgAlO 2.5 than peridotitic one below 40 GPa. • MORB has higher density than the average mantle in the lower mantle. • Seismological observations around 1000 km depth may be explained by presence of MORB. [ABSTRACT FROM AUTHOR]

• We synthesized high-quality single crystals of bridgmanite up to 300 μm in size. • Bridgmanite crystals contain less than 50 ppm wt. H 2 O at the uppermost lower mantle. • The majority of the top of a pyrolitic lower mantle is nearly dry. Water solubility in the dominant lower-mantle bridgmanite phase remains controversial. Discrepancies between previous results highlight the importance of the growth high-quality single crystals of bridgmanite under high-pressure and high-temperature conditions corresponding to the top of the lower mantle. Here we synthesized high-quality single crystals of aluminous bridgmanite up to 300 μm in size that were saturated with hydrous melt at 24–26 GPa and 1700–1900 K using both stoichiometric and MgO-rich non-stoichiometric hydrous starting materials in a multi-anvil press. Fourier-transform infrared spectroscopy measurements on clear and pure spots of the single-crystal bridgmanites did not detect any pronounced OH-stretching bands, which were prominent in some earlier studies. The present results support that the lower-mantle dominated bridgmanite is nearly dry, at least at the top of the lower mantle, and that Al3+ and Fe3+ cannot enhance water incorporation into the crystal structure even in the presence of oxygen vacancies. Large partition coefficients of water between transition-zone minerals and dry lower-mantle dominated bridgmanite further support dehydration melting at the top of the lower mantle. We suggest that the majority of the top of a pyrolitic lower mantle is nearly dry based on the dry principal minerals and stability of hydrous phases in this region. [ABSTRACT FROM AUTHOR]

The variation of an oxygen vacancy (OV) in the form of an MgAlO 2.5 component in bridgmanite with Mg/Si ratios of bulk compositions was clarified using phase relations in the ternary system MgO–AlO 1.5 –SiO 2 at a pressure of 27 GPa and a temperature of 2000 K using advanced multi-anvil techniques. Both normal and reversed experiments suggest that significant amounts of the OV component exist in bridgmanite synthesized from bulk compositions with Mg/Si ratios above unity. The OV component significantly decreases with decreasing Mg/Si ratio, and it finally becomes negligible for Mg/Si ratios below unity. In contrast, the charge-coupled (CC) component of AlAlO 3 becomes more dominant. The ternary phase relations further indicate that bridgmanite in a pyrolitic or peridotitic lower mantle will contain certain amounts of OV component, while that in mid-ocean ridge basalts (MORB) will not contain any amount of this component. Our study suggests that significant amounts of volatiles such as argon trapped by the OV of bridgmanite may be induced into the ambient lower mantle, while they cannot be brought into basaltic slabs by bridgmanite but other phases such as hydrous phases. The decrease of the OV in bridgmanite with decreasing Mg/Si ratio may offer a simple explanation for the occurrence of some stagnant slabs in the lower mantle. • Clarify the ternary phase relations in the system MgO–AlO 1.5 –SiO 2 at 27 GPa and 2000 K. • Oxygen vacancies in bridgmanite decrease with decreasing bulk Mg/Si ratio. • Oxygen vacancies exist in bridgmanite in the uppermost lower mantle. • Volatiles cannot be transported into the basaltic slabs by bridgmanite but other phases. • Variation of oxygen vacancies may explain some stagnant slabs in the lower mantle. [ABSTRACT FROM AUTHOR]