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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]

As evidenced by isotope geochemistry, the persistence of primitive reservoirs indicates that the earth's lower mantle is likely to be heterogeneous. Such heterogeneity could be a legacy from magma‐ocean (MO) solidification. The viscosity of MO is a key parameter to constrain the solidification type of MO. Here we directly measure the viscosity of peridotite (an analog of MO composition) melt at the pressure‐temperature conditions of the deep mantle, using the in situ falling sphere method. The viscosity of peridotite melt along liquidus is in the range of 38–17 mPa s at pressures from 7 to 25 GPa, which is 0.9–0.4 times of the estimation based on the viscosity of endmember compositions. Low viscosity favors fractional solidification and chemically layering of the early mantle at least to the top lower mantle, which could be a source of heterogeneity for the present mantle. Plain Language Summary: The earth experiences a large‐scale melting and forms a deep magma ocean in its early history. The viscosity of peridotite melt is a key parameter for understanding the solidification type of magma ocean, which leads to the primitive mantle structure. This study directly measured the viscosity of peridotite melt to deep mantle conditions and revealed that peridotite melt has a lower value of viscosity than expected. The low viscosity of peridotite melt suggests a fractional solidification of the magma ocean, which supports a heterogeneous primitive mantle. Key Points: The viscosity of peridotite melt has been measured down to lower‐mantle conditions by in situ falling sphere viscometryPeridotite melt has a lower viscosity than expected from the viscosity of the endmembersThe low viscosity of peridotite melt suggests a fractional solidification in the magma ocean, supporting a heterogeneous primitive mantle [ABSTRACT FROM AUTHOR]

Ferric iron can be incorporated into the crystal structure of bridgmanite by either oxygen vacancy substitution (MgFeO2.5 component) or charge‐coupled substitution (FeFeO3 component) mechanisms. We investigated the concentrations of MgFeO2.5 and FeFeO3 in bridgmanite in the MgO‐SiO2‐Fe2O3 system at 27 GPa and 1700–2300 K using a multianvil apparatus. The FeFeO3 content increases from 1.6 to 7.6 mol.% and from 5.7 to 17.9 mol.% with and without coexistence of (Mg,Fe)O, respectively, with increasing temperature from 1700 to 2300 K. In contrast, the MgFeO2.5 content does not show clear temperature dependence, that is, ~2–3 and < 2 mol.% with and without the coexistence of (Mg,Fe)O, respectively. Therefore, the presence of (Mg,Fe)O enhances the oxygen vacancy substitution for Fe3+ in bridgmanite. It is predicted that Fe3+ is predominantly substituted following the oxygen vacancy mechanism in (Mg,Fe)O‐saturated Al‐free bridgmanite when Fe3+ is below ~0.025 pfu, whereas the charge‐coupled mechanism occurs when Fe3+ > 0.025 pfu. Plain Language Summary: Bridgmanite, the most abundant mineral of the Earth's lower mantle, can contain Fe3+ although the valance of iron is 2+ in general. An important question is how Fe3+ is substituted in the crystal structure of bridgmanite. It may form the MgFeO2.5 component, in which oxygen anions are partly missing. Or it may form the FeFeO3 component, which has no missing cations or anions. Since bridgmanite is present in the lower mantle together with (Mg,Fe)O, we investigated the MgFeO2.5 and FeFeO3 contents in Al‐free bridgmanite that coexists with and without (Mg,Fe)O under the topmost lower mantle conditions. The results show that the presence of (Mg,Fe)O enhances the formation of MgFeO2.5. The solubility of MgFeO2.5 component is about 2.5 mol.% in bridgmanite that coexists with (Mg,Fe)O, whereas it is nearly zero when (Mg,Fe)O is absent. Key Points: The presence of (Mg,Fe)O enhances the formation of the MgFeO2.5 component in Al‐free bridgmaniteFe3+ substitution predominantly follows the oxygen vacancy mechanism in (Mg,Fe)O‐saturated Al‐free bridgmanite when Fe3+ content is lowThe solubility of the MgFeO2.5 component in Al‐free bridgmanite is about 0.025 pfu, and relatively insensitive to temperature [ABSTRACT FROM AUTHOR]

Oxygen vacancies (OVs), that charge‐balance the replacement of octahedrally coordinated Si4+ by Al3+ in the mineral bridgmanite, will influence transport properties in the lower mantle but little is known about their stability and local structure. Using 27Al nuclear magnetic resonance (NMR) spectroscopy we have characterized OVs within six aluminous bridgmanite samples. In the resulting NMR spectra sixfold, fivefold, and fourfold coordinated Al species are resolved, in addition to near eightfold coordinated Al substituting for Mg. Fivefold coordinated Al is formed by single OV sites but fourfold coordination must result from short range ordering of OVs, producing OV clusters that may form through migration into twin domain walls. Characterizing the occurrence of such OV structures is an important prerequisite for understanding how transport properties change with depth and composition in the lower mantle. Plain Language Summary: The lower mantle encompasses the largest region of the Earth's interior and is mainly composed of the perovskite‐structured mineral (Mg,Fe,Al)(Al,Si)O3 bridgmanite. Its properties, therefore, control both the diffusive transport of elements and solid state flow in the lower mantle, which will be strongly influenced by point defects. We have identified and quantified defects in bridgmanite that arise from the replacement of silicon by aluminum and result in the creation of a vacant oxygen site. These oxygen defects are also found to form clusters in the structure, which in other perovskite structured minerals have been shown to strongly affect physical properties. As defect formation and ordering is dependent on composition and pressure, strong variations in physical properties may be expected within the upper 300 km of the lower mantle. Key Points: Al substitution in bridgmanite is dominated by the oxygen vacancy mechanism for peridotite mantle compositionsShort‐range ordering of oxygen vacancies takes place forming oxygen vacancy clusters possibly due to migration into twin domain wallsOxygen vacancy clusters are expected to have a major influence on transport properties of the lower mantle, such as the electrical conductivity [ABSTRACT FROM AUTHOR]

Pressless split-sphere apparatus (BARS) equipped with an 8-6 type multi-anvil system is used extensively for large (6 carats) gem quality diamond crystal growth and various mineralogical and geochemical studies at shallow mantle conditions (P ≤ 7.5 GPa). In this study, we extend the pressure range for this apparatus to the conditions of the Earth's transition zone (P = 20 GPa) using a 6-8 multi-anvil system. A scaled-up Kawai-cell consisting of 47 mm WC cubic anvils with 10 mm truncation and 18 mm MgO pressure medium was employed for the experiments. A 300 mm spherical multi-anvil block compressed by a hydraulic system with 2500 atm capacity was used to compress the Kawai-cell. A pressure of 15.5 GPa was successfully generated at the oil pressure of 1875 atm. The comparison of the BARS apparatus calibrations with those obtained using a uniaxial split-sphere press USSA-5000 suggests that pressure up to 17 and 20 GPa in compressed cell/sample volumes of 1200/60 and 400/18 mm³, respectively, can be generated using a combination of the BARS apparatus and the scaled-up Kawai-cell. This cell volume is much larger than that in conventional multi-anvil systems, which does not exceed 80 mm3 at the same conditions. [ABSTRACT FROM AUTHOR]

Water has been thought to affect the dynamical processes in the Earth's interior to a great extent. In particular, experimental deformation results suggest that even only a few tens of parts per million of water by weight enhances the creep rates in olivine by orders of magnitude. However, those deformation studies have limitations, such as considering only a limited range of water concentrations and very high stresses, which might affect the results. Rock deformation can also be understood as an effect of silicon self-diffusion, because the creep rates of minerals at temperatures as high as those in the Earth's interior are limited by self-diffusion of the slowest species. Here we experimentally determine the silicon self-diffusion coefficient DSi in forsterite at 8 GPa and 1,600 K to 1,800 K as a function of water content CH2O from less than 1 to about 800 parts per million of water by weight, yielding the relationship, DSi ≈ (CH2O)1/3. This exponent is strikingly lower than that obtained by deformation experiments (1.2; ref. 7). The high nominal creep rates in the deformation studies under wet conditions may be caused by excess grain boundary water. We conclude that the effect of water on upper-mantle rheology is very small. Hence, the smooth motion of the Earth's tectonic plates cannot be caused by mineral hydration in the asthenosphere. Also, water cannot cause the viscosity minimum zone in the upper mantle. And finally, the dominant mechanism responsible for hotspot immobility cannot be water content differences between their source and surrounding regions. [ABSTRACT FROM AUTHOR]