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Seismic anisotropy is a powerful tool to map deformation processes in the deep Earth. Below 660 km, however, observations are scarce and conflicting. In addition, the underlying crystal scale mechanisms, leading to microstructures and crystal orientations, remain poorly constrained. Here, we use multigrain X‐ray diffraction in the laser‐heated diamond anvil cell to investigate the orientations of hundreds of grains in pyrolite, a model composition of the Earth's mantle, at in situ pressure and temperature. Bridgmanite in pyrolite exhibits three regimes of microstructures, due to transformation and deformation at low and high pressure. These microstructures result in predictions of 1.5%–2% shear wave splitting between 660 and 2,000 km with reversals in fast S‐wave polarization direction at about 1,300 km depth. Anisotropy can develop in pyrolite at lower mantle conditions, but pressure has a significant impact on the plastic behavior of bridgmanite, and hence seismic observations, which may explain conflicting anisotropy observations. Plain Language Summary: Seismologists rely on observable data to construct models that describe the dynamic state of the Earth's lower mantle. These models, however, require constraints such as mantle composition and material behavior at high pressures and temperatures, which can be provided through experimental mineral physics. In this study, we use a high pressure devices and X‐rays to impose deformation and image the state of our sample with increasing pressure and temperature. We are able to extract information of individual mineral grains within our assemblage, such as the number of grains per phase and their orientations. Using this experimental data, we identify three regimes of grain orientations in bridgmanite in the lower mantle, corresponding to transformation from lower pressure phases, and deformation at low and high pressure. With this information, we are able to make predictions about how seismic waves travel and behave based on the deformation state of the lower mantle. Key Points: Three microstructure/anisotopy regimes in bridgmanite in the Earth's lower mantle: transformation, low‐P deformation, high‐P deformationEffect of pressure on the active slip systems of bridgmanite with a shift at approximately 50 GPa or 1,300 km depth in the Earth's mantleUp to 2% shear wave splitting predicted at all depth with changes in fast polarization direction for each microstructural regime [ABSTRACT FROM AUTHOR]

The pyrolite model, which can reproduce the upper-mantle seismic velocity and density profiles, was suggested to have significantly lower velocities and density than seismic models in the lower mantle transition zone (MTZ). This argument has been taken as mineral-physics evidence for a compositionally distinct lower MTZ. However, previous studies only estimated the pyrolite velocities and density along a one-dimension (1D) geotherm and never considered the effect of lateral temperature heterogeneity. Because the majorite-perovskite-akimotoite triple point is close to the normal mantle geotherm in the lower MTZ, the lateral low-temperature anomaly can result in the presence of a significant fraction of akimotoite in pyrolitic lower MTZ. In this study, we reported the elastic properties of Fe-bearing akimotoite based on first-principles calculations. Combining with literature data, we found that the seismic velocities and density of the pyrolite model can match well those in the lower MTZ when the lateral temperature heterogeneity is modeled by a Gaussian distribution with a standard deviation of ∼100 K and an average temperature of dozens of K higher than the triple point of MgSiO3. We suggest that a harzburgite-rich lower MTZ is not required and the whole mantle convection is expected to be more favorable globally.

Abstract Seismic anisotropy is a powerful tool to map deformation processes in the deep Earth. Below 660 km, however, observations are scarce and conflicting. In addition, the underlying crystal scale mechanisms, leading to microstructures and crystal orientations, remain poorly constrained. Here, we use multigrain X‐ray diffraction in the laser‐heated diamond anvil cell to investigate the orientations of hundreds of grains in pyrolite, a model composition of the Earth's mantle, at in situ pressure and temperature. Bridgmanite in pyrolite exhibits three regimes of microstructures, due to transformation and deformation at low and high pressure. These microstructures result in predictions of 1.5%–2% shear wave splitting between 660 and 2,000 km with reversals in fast S‐wave polarization direction at about 1,300 km depth. Anisotropy can develop in pyrolite at lower mantle conditions, but pressure has a significant impact on the plastic behavior of bridgmanite, and hence seismic observations, which may explain conflicting anisotropy observations.

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 pyrolite model, which can reproduce the upper-mantle seismic velocity and density profiles, was suggested to have significantly lower velocities and density than seismic models in the lower mantle transition zone (MTZ). This argument has been taken as mineral-physics evidence for a compositionally distinct lower MTZ. However, previous studies only estimated the pyrolite velocities and density along a one-dimension (1D) geotherm and never considered the effect of lateral temperature heterogeneity. Because the majorite-perovskite-akimotoite triple point is close to the normal mantle geotherm in the lower MTZ, the lateral low-temperature anomaly can result in the presence of a significant fraction of akimotoite in pyrolitic lower MTZ. In this study, we reported the elastic properties of Fe-bearing akimotoite based on first-principles calculations. Combining with literature data, we found that the seismic velocities and density of the pyrolite model can match well those in the lower MTZ when the lateral temperature heterogeneity is modeled by a Gaussian distribution with a standard deviation of ∼100 K and an average temperature of dozens of K higher than the triple point of MgSiO3. We suggest that a harzburgite-rich lower MTZ is not required and the whole mantle convection is expected to be more favorable globally.

• A T-P-X H2O dependent model for ultramafic melt viscosity. • Calibrated on published data at T (900–2700 K), P (0–25 GPa) and X H2O (0–12 mol%). • Model extrapolates well to high T (<6000 K) and P (< 160 GPa). • Model has relevance to petrology, volcanology, geophysics and planetary sciences. A non-Arrhenian model for the Newtonian viscosity (η) of ultramafic melts is presented. The model predicts the viscosity of ultramafic melts as a function of temperature (T), pressure (P), H 2 O content and for a range of melt compositions (70 < Mg# < 100). The calibration consists of 63 viscosity measurements at ambient pressure for 20 individual melt compositions and 5 high-P measurements on a single melt composition, all drawn from the literature. The data span 14 orders of magnitude of η (10−2 to 1011.8 Pa s), a T range of 880 to 2700 K, pressures from 1 atm to 25 GPa, and include measurements on hydrous melts containing 0.2 to 4.4 wt.% H 2 O. The T-dependence of viscosity is modelled with the VFT equation [log η = A + B /(T(K) − C)] whereby A is assumed to be a common, high-T limit for these melt compositions (i.e. log η ∞ = -5.4). The pressure and composition effects are parameterised in terms of 5 adjustable parameters in expanded forms of B and C. The viscosity model is continuous across T - P -composition space and can predict ancillary transport properties including glass transition temperatures (T g) and melt fragility (m). Melt viscosity decreases markedly with increasing H 2 O content but increases significantly with increasing pressure and decreasing Mg# (i.e. higher Fe-content). We show strong systematic decreases in T g and m with increasing H 2 O content whereas an increase in P causes a rise in Tg and decrease in m. The predictive capacity of this model for ultramafic melt viscosity makes it pertinent to the fields of volcanology, geophysics, petrology, and the material sciences. Moreover, it provides constraints on models of magma oceans on terrestrial planets and, the evolution of planetary atmospheres via magmatic degassing on exoplanets. [ABSTRACT FROM AUTHOR]

The early Earth was marked by at least one global magma ocean. Melt buoyancy played a major role for its evolution. Here we model the composition of the magma ocean using a six‐component pyrolite melt, to which we add volatiles in the form of carbon as molecular CO or CO2 and hydrogen as molecular H2O or through substitution for magnesium. We compute the density relations from first‐principles molecular dynamics simulations. We find that the addition of volatiles renders all the melts more buoyant compared to the reference volatile‐free pyrolite. The effect is pressure dependent, largest at the surface, decreasing to about 20 GPa, and remaining roughly constant to 135 GPa. The increased buoyancy would have enhanced convection and turbulence, and thus promoted the chemical exchanges of the magma ocean with the early atmosphere. We determine the partial molar volume of both H2O and CO2 throughout the magma ocean conditions. We find a pronounced dependence with temperature at low pressures, whereas at megabar pressures the partial molar volumes are independent of temperature. At all pressures, the polymerization of the silicate melt is strongly affected by the amount of oxygen added to the system while being only weakly affected by the specific type of volatile added. Key Points: The addition of volatiles to pyrolite melts renders them buoyant at all pressures and temperaturesDepolymerization of the melt increases with the addition of oxygen and is mostly independent of the type of volatile (i.e., CO, CO2, and H2O)Volatiles would have enhanced the circulation in the magma ocean and promoted volatile exchange and equilibration with the early atmosphere [ABSTRACT FROM AUTHOR]

One of the central challenges in accurately estimating the mantle melting temperature is the sensitivity of the probe for detecting a small amount of melt at the solidus. To address this, we used a multichannel collimator to enhance the diffuse X‐ray scattering from a small amount of melt and probed an eutectic pyrolitic composition to increase the amount of melt at the solidus. Our in situ detection of diffuse scattering from the pyrolitic melt determined an anhydrous melting temperature of 3,302 ± 100 K at 119 ± 6 GPa and 3,430 ± 130 K at the core‐mantle boundary (CMB) conditions, as the upper bound temperature. Our CMB temperature is approximately 700 K lower than the previous estimates, implying much faster secular cooling and higher concentrations of S, C, O, and/or H in the region, and nonlinear, advocating the basal magma ocean hypothesis. Plain Language Summary: The heat stored in the Earth's deep interior has been the primary fuel for a range of global processes from mantle convection to surface tectonics, but quantitative estimation of the heat remains uncertain. The melting temperature of mantle materials is one of the key parameters to understanding the thermal evolution and present‐day state of the Earth's interior, but it has been poorly constrained, with recent measurement discrepancies as large as 600 K. Here, we report melting temperatures of mantle compositions measured over a wide range of pressures expected for the lower mantle. We investigated a eutectic mantle composition using multichannel collimator filtered X‐ray diffraction in combination with the laser‐heated diamond‐anvil cell. Fitting our melting data over the range of 46–145 GPa led to a solidus temperature of 3,430 ± 130 K at the core‐mantle boundary. This temperature is approximately 700 K lower than the previous estimates, implying much faster secular cooling at the lower mantle than previously believed. Furthermore, our solidus curve constrained for a wide pressure range is strongly nonlinear and thus supports the basal magma ocean hypothesis. Key Points: Combined usage of multichannel collimator and laser‐heated diamond‐anvil cell improved detection of silicate meltThis led to the lowest solidus temperature of anhydrous pyrolite, 3,430 ± 130 K, at the core‐mantle boundary conditionsWe advocate Fe‐enriched provinces at the core‐mantle boundary to be originated from the magma ocean [ABSTRACT FROM AUTHOR]

The aluminium incorporation mechanism of perovskite was explored by means of quantum mechanics in combination with equilibrium/off-equilibrium thermodynamics under the pressure-temperature conditions of the Earth's lower mantle (from 24 to 80 GPa). Earth's lower mantle was modelled as a geochemically non-primitive object because of an enrichment by 3 wt% of recycled crustal material (MORB component). The compositional modelling takes into account both chondrite and pyrolite reference models. The capacity of perovskite to host Al was modelled through an Al 2 O 3 exchange process in an unconstrained Mg-perovskite + Mg-Al-perovskite + free-Al 2 O 3 (corundum) system. Aluminium is globally incorporated principally via an increase in the amount of Al bearing perovskite [ Mg-Al-pv (80 GPa)/ Mg-Al-pv (24 GPa) ≈ 1.17], rather than by an increase in the Al 2 O 3 content of the average chemical composition which changes little (0.11–0.13, mole fraction of Al 2 O 3) and tends to decrease in Al. The Al 2 O 3 distribution in the lower mantle was described through the probability of the occurrence of given compositions of Al bearing perovskite. The probability of finding Mg-Al-perovskite is comparable to Mg-perovskites. Perovskite with Al 2 O 3 mole fraction up to 0.15 has an occurrence probability of ∼28% at 24 GPa, increasing up to ∼43% at 80 GPa; on the contrary, perovskite compositions in the range 0.19–0.30 Al 2 O 3 mole fraction drop their occurrence probability from 9.8 to 2.0%, over the same P -range. In light of this, the distribution of Al in the lower mantle shows that, among the possible Al bearing perovskite phases, the (Mg 0.89 Al 0.11)(Si 0.89 Al 0.11)O 3 composition is the likeliest, providing from 5 to 8% of the bulk perovskite in the pressure range from 24 to 80 GPa. The occurrence of the most Al rich composition, i.e. (Mg 0.71 Al 0.29)(Si 0.71 Al 0.29)O 3 , is a rare event (probability of occurrence < 1.7%). This study predicts that perovskite may globally host Al 2 O 3 in terms of 4.3 and 4.8 wt% (with respect to the non-primitive lower mantle mass), thus accounting for ∼90% and 100% of the bulk Al 2 O 3 estimated in the framework of pyrolite and chondrite reference models, respectively. A calcium-ferrite type phase (on the MgAl 2 O 4 -NaAlSiO 4 join) seems to be the only candidate that can compensate for the 10% gap of the perovskite Al incorporation capacity, in the case of a pyrolite non-primitive lower mantle model. [ABSTRACT FROM AUTHOR]

Thermodynamic and elastic properties of grossular (Ca3Al2(SiO4)3, the calcium‐aluminium end‐member of garnet) at high pressures and temperatures are obtained using the first‐principles calculations based on the density functional theory. Our calculated results agree well with available experimental data. The elastic moduli and wave velocities of grossular exhibit the nonlinear pressure and temperature dependences. Although the bulk moduli of grossular are similar to those of pyrope (Mg3Al2(SiO4)3) and almandine (Fe3Al2(SiO4)3), its shear moduli are significantly larger than those of pyrope and almandine. Combining our calculated results with previously calculated elasticity of other upper‐mantle minerals, we estimated density and wave velocity profiles for the pyrolitic and eclogitic compositions along different mantle geotherms. We find that a pyrolitic composition along the normal geotherm can well reproduce the reference velocities and densities for the upper mantle, supporting a pyrolitic upper mantle. Although an eclogitic composition also has similar velocities to reference seismic models above 300 km, it is significantly denser than the pyrolitic composition and the ambient mantle. However, the velocities of eclogite along the cold geotherm are relatively larger than those of the pyrolitic composition, indicating that the presence of eclogite in subduction zones could produce high‐velocity anomalies. In addition, the decomposition of grossular to calcium perovskite and corundum at the conditions of the mantle transition zone can produce considerable velocities and density jumps, which probably provides an interpretation for the seismically observed multiple reflections at ~660 km. Key Points: We calculate thermodynamic and elastic properties of grossular at mantle conditions using first‐principles calculationsThe density, VP, and VS of pyrolite and eclogite are estimated along different mantle conditions at the depth range of 220–400 kmIf grossular can be preserved in cold slab, the decomposition of grossular can produce considerable velocities and density jumps at ~660 km [ABSTRACT FROM AUTHOR]

The crystallization of a magma ocean (MO) early in Earth's history shaped the entire evolution of our planet. The buoyancy relations between the forming crystals and the residual melt is the most important but also the most unknown parameter affecting the large-scale structure and evolution of the MO. The accumulation of crystals, near the depth of neutral buoyancy between crystals and the coexisting melt, if happening at mid-depths, can separate convecting regions within the MO. Here we use jointly first-principles molecular-dynamics calculations and diamond-anvil cell experiments to obtain the density relations between the molten bulk silicate Earth and the bridgmanite crystals during the crystallization of the MO. The chemical evolutions of the liquid and the coexisting solid during progressive crystallization were constrained by experiments, and the relevant densities were calculated by molecular dynamics. We find that the first crystal of bridgmanite that is formed in a fully molten mantle is Fe-poor, and becomes neutrally buoyant at 110–120 GPa. Since the cooling of the deep MO is fast, and related convection is vigorous, however, first crystals remain entrained. As crystallization advances, the relative Fe content increases in the melt, and the pressure of neutral buoyancy decreases. At 50% solidification, close to the rheological transition, the pressure of the density crossover moves to ∼50 GPa. At this pressure, crystals form an interconnected network and block global convection currents, which in turn leads to the separation of the partly crystallized MO into a surficial MO and a basal MO through melt-solid segregation. Such a shallow segregation of a crystal mush at mid-mantle depth has important implications for the dynamics and timescales of early mantle differentiation. Moreover, the shallow segregation should have promoted the formation of a voluminous basal MO that evolves into a large geochemically enriched reservoir. Accordingly, the seismically observed residues of basal MO crystallization in the present-day mantle may host an unmixed reservoir for the missing budget of highly incompatible elements. • Density crossover between melt and bridgmanite starts at the base of the magma ocean. • Density crossover moves upwards in the magma ocean. • Magma ocean evolution followed batch crystallization up to 50% by volume. • Rheological transition around the depth corresponding to 50–60 GPa pressure. • Evolution of magma ocean deeper than 1300 km yields basal magma ocean. [ABSTRACT FROM AUTHOR]

We determined phase relations of harzburgite and basalt (mid‐ocean ridge basalt, MORB) at 12–28 GPa and 1600–2200 °C with a large number of experiments using a multianvil high‐pressure apparatus. These phase relations were precisely compared with those of pyrolite simultaneously determined by multisample cell technique. The post‐spinel (pSp) transition of harzburgite occurs at 23 GPa and 1600 °C with the boundary slope of −3 ± 1 MPa/°C. The post‐garnet (pGt) transition boundary of MORB, defined as the beginning of the majorite garnet‐bridgmanite transition, is located at 25 GPa and 1600 °C, with a slope of 1.5 ± 1 MPa/°C. The pSp transition of harzburgite occurs at higher pressure by 0.5 GPa at 1600 °C and has the more negative slope than that of pyrolite (−1 ± 1 MPa/°C). The pGt transition of MORB occurs at higher pressure by 3 GPa than the pSp transition of harzburgite. At 1600 °C, the density of pyrolite is smaller than those of harzburgite and MORB before the pSp transition of pyrolite (pyrolite–harzburgite/MORB = −0.17 g/cm3). After the pSp transition of pyrolite, harzburgite has lower densities than that of pyrolite at 22–27 GPa (pyrolite–harzburgite = 0.31 and 0.08 g/cm3 before/after the pSp transition in harzburgite, respectively). On the other hand, the density of MORB becomes the highest among the three rocks above 25 GPa after the pGt transition of MORB (pyrolite–MORB = −0.18 g/cm3). The present density contrasts suggest that harzburgite and MORB may stagnate and accumulate in the transition zone by slab subduction, resulting in that the pyrolitic part of slabs is subducted into the lower mantle. Key Points: The post‐spinel transition boundary of harzburgite is located at higher pressure and has a more negative slope than that of pyroliteThe post‐garnet transition of mid‐ocean ridge basalt occurs at higher pressure than the post‐spinel transitions of harzburgite and pyroliteHarzburgite is less dense than pyrolite at 1000 to 1600 °C at depth >660 km, suggesting accumulation of harzburgite in the transition zone [ABSTRACT FROM AUTHOR]

The viscosity of molten Earth mantle has been determined for various redox states using very high temperature viscometry at 1 bar. The viscosity-temperature relationship of the most oxidised pyrolite studied here is comparable to that of a previous determination of the viscosity of a peridotite melt. For the first time, the effect of iron redox state on molten mantle viscosity has been determined by calorimetric analysis of the glass transition on extremely carefully characterised glassy samples quenched from conditions of variable f O 2 using gas-mixing levitation. There is a clear trend of decreasing viscosity with increasing redox state over the range of Fe3+/ΣFe. = 0.07–0.29. We also observe an increase in glass transition temperatures with increasing melt depolymerisation (i.e., higher NBO/T ratios). Both the negative oxidation dependence of viscosity and glass transition behaviour upon depolymerisation are contradictory to the trends observed from redox viscometry of more silica-rich melts but may be consistent with the broadly diminishing positive redox viscometry trends of melts with increasing basicity. • First determination of the effect of Fe redox state on the viscosity of pyrolite melts. • Increasing oxygen fugacity (Fe3+/ΣFe) lowers the viscosity. • The observed trend conflicts with viscosity observations of more Si-rich melts. • Glass synthesis using aerodynamic levitation ensures sample purity. [ABSTRACT FROM AUTHOR]

We determined phase relations of pyrolite at 12–28 GPa and 1600–2200 °C using a 6–8 Kawai-type multianvil apparatus. A large number of high pressure and high temperature experiments were conducted to constrain the phase relations especially around 660-km depth, compared with previous studies. The phase relations drastically change between 19 and 23 GPa with temperature as follows. At 19–22 GPa, a phase transition of ringwoodite (Rw) to garnet (Gt) + magnesiowüstite (Mw) occurs at 1800 °C, and no Rw is observed at 2200 °C. The mineral proportion of Gt increases with increasing temperature due to this decomposition reaction and becomes larger than that of Rw above 1800 °C. At 23 GPa, pyrolite crystallizes to a mineral assemblage of bridgmanite (Bm) + Mw + Gt + calcium perovskite by the post-spinel (pSp) transition at 1600–2000 °C and the post-garnet (pGt) transition at 2100–2200 °C, the latter of which is defined as a phase transition of a part of Gt to Bm. Clapeyron slopes of the pSp transition and the pGt transition are determined to be −1 and 5 MPa/°C, respectively, in this study. Gt is stable at any temperatures around the 660-km discontinuity (D660), and Gt-disappearance reaction occurs at 25 GPa with no temperature dependence. Densities of pyrolite were calculated in various pressure and temperature conditions by using the mineral proportions and available data of equations of state. The density at 1600 °C is higher than that at 1800–2200 °C at any pressure-temperature conditions in this study. In particular, the drastic density drop occurs at 19–24 GPa between 1600 and 1800–2200 °C because of increase of Gt proportion with increasing temperature. These suggest that upwelling hot plumes composed of pyrolite from the lower mantle do not stagnate and their ascent is promoted across D660. The mineral proportions and the density changes also suggest that the phase transition which causes D660 changes around 1800 °C from the pSp to the pGt transitions because of the mineral proportion inversion between Rw and Gt. [ABSTRACT FROM AUTHOR]

The concentrations of Fe, Al, and Ni and their distributions were determined for all known natural assemblages of ferropericlase (fPer) and bridgmanite (Bridg), coexisting as inclusions in deep-mantle diamonds from Brazil, Canada, Guinea, and South Australia. Based upon these data, it is likely that some areas within the deep lower mantle are iron-rich and differ markedly from a pyrolitic composition. In the lowermost lower mantle, Bridg is Al-rich and fPer is Ni-poor, witnessing the presence of a free metallic phase in the mineral-forming environment. The iron partitioning in the Bridg + fPer association … in juvenile diamond inclusions is as low as 0.1-0.2. During ascent of the diamonds with their inclusions to the surface, the … eventually increases to values of 0.4-0.5 and even as high as 0.7. The details of the element partitioning between natural Bridg and fPer in the lower mantle are as follows: iron in Bridg is ferrous Fe2+ in the A site, substituting for Mg2+; almost all iron in fPer is ferrous Fe2+; the share of ferric Fe3+ iron in fPer is Fe3+/ΣFe = 8-12 at %; iron concentrations in both Bridg and fPer increase with depth (pressure), reflecting the increasing Fe content in the lower part of the lower mantle, different from that of a pyrolitic model. Al in Bridg is mainly in the cation site B and partly in the cation site A, in both cases substituting for Si, Mg, and Fe with vacancy formation; and in the case of Al positioning into both B and A sites, a charge-balanced reaction occurs. The natural samples show very diverse … values and elemental distribution that cannot be simply explained by our current understanding on alumina dissolution in Bridg and the spin transition of Fe2+ in fPer. These major differences between experimental results and observations in natural samples demonstrate the complex, inhomogeneous iron speciation and chemistry in the lower mantle. [ABSTRACT FROM AUTHOR]

Vaporization is a major outcome of giant impacts during planet formation. The last giant impact marked a major stage in the early history of our planet, with the formation of a highly vaporized protolunar disk, that condenses onto the final Earth and Moon. The thermodynamic state of the disk and its condensation path are still uncertain, as most impact simulations have not used accurate material models. In this study, we compute the critical point and liquid spinodal of the bulk silicate Earth composition. We find that the thermal profiles through portions of the protolunar disk and the post-impact Earth exceeded the mantle critical point of 80-130 MPa kbars and 6500-7000 K. We find that Earth, and most rocky planets, will traverse a temporary state that lacks a surface defined by a magma ocean-atmosphere boundary. Furthermore, the atomic structure of the silicate fluid varies with the radius within the disk due to strong pressure and temperature gradients. Fluffy short-lived chemical species dominate the outer parts of the disk, and long-lasting dense polymers abound in the deeper parts. During cooling, the silicate vapor condenses and the composition of the post-impact atmosphere is dominated by species along the mantle vapor curve. • Giant impacts release enough energy to bring portions of the bodies to supercritical states. • Cooling and condensation of some of the post-impact system takes place at supercritical conditions. • There is no magma ocean - atmosphere interface after most giant impacts between rocky planets. [ABSTRACT FROM AUTHOR]

The '660' discontinuity is often associated with a phase transitions in pyrolite at about 24 GPa. There are ubiquitous reports of seismic anisotropy below the '660' which are difficult to explain from a mineralogical point of view. In this study, we implement multigrain crystallography X-ray diffraction in the laser-heated diamond anvil cell in order to track microstructures induced by phase transitions at the pressure and temperature conditions of the discontinuity. Before the onset of transformation, garnet is isotropic with ringwoodite displaying a weak 001 texture. After the transformation, bridgmanite displays strong 001 transformation textures which we attribute to growth under stress. Davemaoite exhibits weak maxima in 101 and 111 orientations with ferropericlase displaying no texture. The results are used to model anisotropy in a subducting slab, with a prediction of no anisotropy above the '660' and up to 1.28% (0.08 km/s) shear wave splitting below the '660'. In addition, we predict V S V > V S H for horizontally traveling waves and near-vertical subduction and V S V < V S H in the case a slab impinging below the boundary layer. • Experimental study of transformation microstructures in polycrystalline pyrolite. • Multigrain crystallography to follow microstructures in-situ at relevant P and T. • Anisotropy observations below 660 may be due to growth of bridgmanite under stress. [ABSTRACT FROM AUTHOR]

Subduction zones exhibit a wide range of behaviour, from slab stagnation at the base of the transition zone, to direct sinking into the lower mantle. Numerical simulations have utilised a range of approximations to improve computational efficiency, such as limiting the phase transitions or compositional effects included in a model: the net effect of these approximations on slab dynamics remains unclear. With the goal of developing more complete, self-consistent, and less idealised simulations, we analyse the importance of various modelling choices on slab evolution: the presence of shear, adiabatic and latent heating, compositional layering and composition-dependent phase transitions. We find that slabs in models with additional heating terms (extended Boussinesq approximation) are warmer, weaker and exhibit more folding and stretching than in models without these terms (Boussinesq approximation). The presence of compositional layers and the stress-dependence of rheology acts to strengthen the slab, decreasing slab dip and leading to less folding and lateral migration of the slab. We also find that the extent of slab folding and stagnation is overestimated by modelling only the olivine phase transitions at 410 and 660 km. A more realistic subduction zone with compositional layers and all the pyrolite phase transitions exhibits almost no slab stagnation at 660 km and is instead characterised by broad, periodic, slab folding. This slab behaviour is more similar to a model with no phase changes or compositional layers. Therefore, for studies in which a complete treatment of compositional layers and phase transitions is not possible, rather than including an incomplete approximation that over-predicts folding, the large-scale deformation of slabs is best approximated by a model with no phase transitions and no layers. [ABSTRACT FROM AUTHOR]

Fe and Al are two of the most important rock-forming elements other than Mg, Si, and O. Their presence in the lower mantle's most abundant minerals, MgSiO 3 bridgmanite, MgSiO 3 post-perovskite and MgO periclase, alters their elastic properties. However, knowledge on the thermoelasticity of Fe- and Al-bearing MgSiO 3 bridgmanite, and post-perovskite is scarce. In this study, we perform ab initio molecular dynamics to calculate the elastic and seismic properties of pure, Fe 3+ - and Fe 2+ -, and Al 3+ -bearing MgSiO 3 perovskite and post-perovskite, over a wide range of pressures, temperatures, and Fe/Al compositions. Our results show that a mineral assemblage resembling pyrolite fits a 1D seismological model well, down to, at least, a few hundred kilometers above the core–mantle boundary, i.e. the top of the D ″ region. In D ″ , a similar composition is still an excellent fit to the average velocities and fairly approximate to the density. We also implement polycrystal plasticity with a geodynamic model to predict resulting seismic anisotropy, and find post-perovskite with predominant (001) slip across all compositions agrees best with seismic observations in the D ″ . [ABSTRACT FROM AUTHOR]

Models of seismic velocity as a function of depth through the upper mantle provide some of the strongest constraints on the mineralogy and composition of the mantle. Although receiver function studies have provided new information on the depths of upper mantle discontinuities they do not provide as much information on seismic gradients between discontinuities and absolute velocities within the upper mantle. The waveforms and travel times of upper mantle turning waves provide the strongest constraints on vertical variations in upper mantle velocity although in the past they suffered from the lack of dense profiles of data sampling a single part of the upper mantle that would minimize effects of 3D variations in velocity. Here we model three dense profiles of triplicated upper mantle broadband S and SS waves recorded by USArray, Canadian and NARS-Baja stations located in western North America. Earthquakes along the East Pacific Rise were recorded along profiles within 5° back azimuth windows and with stations at a maximum of 0.5° separation. The distance range covered is from 35° to 60° and thus the waves sample the mantle from the shallow upper mantle to depths near 1000 km. The data were inverted using a conjugate gradient algorithm that utilizes the reflectivity synthetic technique. The results show a much smaller gradient within the transition zone than the PREM model with larger jumps in velocity across the 410 km and 660 km depth discontinuities. These results are consistent with velocities predicted for a pyrolite composition mantle transition zone. Compositional models with lower olivine content, such as piclogite, are not consistent with our seismic model. [ABSTRACT FROM AUTHOR]

Abstract: Elastic properties of MgSiO3-perovskite are of fundamental importance for our understanding of the thermal and chemical state of the lower mantle. However, the elastic moduli and especially their derivatives with respect to temperature and pressure at lower mantle conditions are still uncertain. In this study, we have carried out extensive first principles molecular dynamics simulations to determine the equation of state, elastic constants, moduli and velocities of MgSiO3-perovskite over a wide temperature and pressure regime (from static conditions to 3500 K and from 36 GPa to 140 GPa). Systematic errors arising from approximations to the exchange–correlation functional in density functional theory have been essentially eliminated with a generalized re-scaling method. Molecular dynamics trajectories were carefully converged with respect to duration and system size and analyzed to distinguish the effects of anharmonicity. Based on the new elastic properties derived in this study, the pyrolite mineralogical model is found to predict wave velocities in close agreement with those of seismic observations in most of lower mantle regime. [Copyright &y& Elsevier]

Abstract: We precisely compared phase boundaries of post-spinel transition in pyrolite and Mg2SiO4 and of akimotoite–perovskite transition in MgSiO3 at 21–28GPa and 1400–1800°C by detailed phase relation experiments using a multi-anvil apparatus. We used a multi-sample cell technique, in which pyrolite, Mg2SiO4 and MgSiO3 were kept simultaneously at the same pressure–temperature conditions in each run. The experiments were performed in pressure and temperature intervals of 0.3GPa and 100°C, respectively. The post-spinel transition boundary in Mg2SiO4 is located at higher pressure by about 0.8GPa than the akimotoite–perovskite transition boundary in MgSiO3. Both the transition boundaries have the same slope of −0.002GPa/°C. In pyrolite, the post-spinel transition occurs in a pressure interval within 0.4GPa at lower pressure by about 0.2–1.0GPa than that in Mg2SiO4 at 1400–1800°C. The Clapeyron slope of the post-spinel transition boundary in pyrolite is −0.001GPa/°C, which is half of −0.002GPa/°C of Mg2SiO4. When we assume that both the transition zone and the uppermost lower mantle have approximately pyrolitic composition, the above results imply that the Clapeyron slope of the transition boundary in pyrolite is more appropriate than that of Mg2SiO4 to evaluate effects of the post-spinel transition on mantle dynamics and the 660-km discontinuity topography. In pyrolite, the akimotoite–perovskite transition and the post-spinel transition occur at the same pressure at 1400°C. Above 1700°C, a part of ringwoodite in pyrolite transforms to garnet+magnesiowüstite at pressure below the post-spinel transition, and abundances of garnet and magnesiowüstite increase with increasing temperature. [Copyright &y& Elsevier]