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• 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]

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]

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]