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The chemistry of bridgmanite (Brg), especially the oxidation state of iron, is important for understanding the physical and chemical properties, as well as putting constraints on the redox state, of the Earth’s lower mantle. To investigate the controls on the chemistry of Brg, the Fe3+ content of Brg was investigated experimentally as a function of composition and oxygen fugacity (fo2) at 25 GPa. The Fe3+/∑Fe ratio of Brg increases with Brg Al content and fo2 and decreases with increasing total Fe content and with temperature. The dependence of the Fe3+/∑Fe ratio on fo2 becomes less steep with increasing Al content. Thermodynamic models were calibrated to describe Brg and ferropericlase (Fp) compositions as well as the inter-site partitioning of trivalent cations in Brg in the Al–Mg–Si–O, Fe–Mg–Si–O and Fe–Al–Mg–Si–O systems. These models are based on equilibria involving Brg components where the equilibrium thermodynamic properties are the main adjustable parameters that are fit to the experimental data. The models reproduce the experimental data over wide ranges of fo2 with a relatively small number of adjustable terms. Mineral compositions for plausible mantle bulk compositions can be calculated from the models as a function of fo2 and can be extrapolated to higher pressures using data on the partial molar volumes of the Brg components. The results show that the exchange of Mg and total Fe (i.e., ferric and ferrous) between Brg and Fp is strongly fo2 dependent, which allows the results of previous studies to be reinterpreted. For a pyrolite bulk composition with an upper mantle bulk oxygen content, the fo2 at the top of the lower mantle is −0.86 log units below the iron–wustite buffer (IW) with a Brg Fe3+/∑Fe ratio of 0.50 and a bulk rock ratio of 0.28. This requires the formation of 0.7 wt.% Fe–Ni alloy to balance the raised Brg ferric iron content. With increasing pressure, the model predicts a gradual increase in the Fe3+/∑Fe ratio in Brg in contrast to several previous studies, which levels off by 50 GPa. Oxygen vacancies in Brg decrease to practically zero by 40 GPa, potentially influencing elasticity, diffusivity and rheology in the top portion of the lower mantle. The models are also used to explore the fo2 recorded by inclusions in diamonds, which likely crystallized as Brg in the lower mantle, revealing oxygen fugacities which likely preclude the formation of some diamonds directly from carbonates, at least at the top of the lower mantle.

The electrical conductivity of mineral aggregates depends both on the properties of the constitutive minerals and the ways those minerals are assembled. Mixing, or average models combine the conductivity of single phases to give bulk conductivity of rocks, thereby linking experimental measurements to geophysical observations. In order to compare these mixing models and allow an informed choice, several popular approaches, including bounds and average models, have been used to estimate the conductivity of a typical dry upper mantle and transition zone with a pyrolite composition. All the estimations calculated using the various average models lie between the rigorous constraint that is given by the HS bounds. The average models in this study are found to give similar bulk conductivities with the difference of less than 0.5 orders of magnitude, except the geometric mean, implying that the choice of the average models is insignificant. The effective electrical conductivity of pyrolite mantle has been derived from the conductivity of dry mantle minerals using the effective medium theory, and was found consistent with observed conductivity values for some subsurface regions of the Earth which we expect to be relatively dry. This provides us with baseline conductivity for a dry mantle, which is helpful to understand the water distribution in the deep earth.

The recycling of oceanic crust, with distinct isotopic and chemical signature from the pyrolite mantle, plays a critical role in the chemical evolution of the Earth with insights into mantle circulation. However, the role of the mantle transition zone during this recycling remains ambiguous. We here combine the unique resolution reflected body waves (P410P and P660P) retrieved from ambient noise interferometry with mineral physics modeling, to shed new light on transition zone physics. Our joint analysis reveals a generally sharp 660-km discontinuity and the existence of a localized accumulation of oceanic crust at the bottom mantle transition zone just ahead of the stagnant Pacific slab. The basalt accumulation is plausibly derived from the segregation of oceanic crust and depleted mantle of the adjacent stagnant slab. Our findings provide direct evidence of segregated oceanic crust trapped within the mantle transition zone and new insights into imperfect whole mantle circulation., By combining ambient noise interferometry with mineral physics modeling, this work sheds new light on mantle transition zone physics. Their findings provide new evidence of segregated oceanic crust subducted and trapped within the mantle transition zone, implying complex mantle circulation modes.

Using an iterative numerical approach, we have obtained the self-consistent thermal expansion, heat capacity, and Grüneisen parameters of diopside (MgCaSi2O6) over wide pressure and temperature ranges based on experimental data from the literature. Our results agree well with the published experimental and theoretical data. The determined thermodynamic parameters exhibit nonlinear dependences with increasing pressure. Compared with other minerals in the upper mantle, we found that the adiabatic temperature gradient obtained using the thermodynamic data of diopside is larger than that of garnet while lower than that of olivine, when ignoring the Fe incorporation. Combining our results with thermodynamic parameters of garnet obtained in previous studies, we have estimated the adiabatic temperature gradient and geotherm of an eclogitic upper mantle in a depth range of 200–450 km. The results show that the estimated adiabatic temperature gradient of the eclogite model is ~16% and ~3% lower than that of the pyrolite model at a depth of 200 km and 410 km, respectively. However, the high mantle potential temperature of the eclogite model leads to a similar temperature as the pyrolite model in a depth range of 200–410 km.

Velocity and density jumps across the 410-km seismic discontinuity generally indicate olivine contents of ∼30 to 50 vol.% on the basis of the elastic properties of anhydrous olivine and wadsleyite, which is considerably less than the ∼60% olivine in the widely accepted pyrolite model for the upper mantle. A possible explanation for this discrepancy is that water dissolved in olivine and wadsleyite affects their elastic properties in ways that can reconcile the pyrolitic model with seismic observations. In order to more fully constrain the olivine content of the upper mantle near the 410-km discontinuity, and to place constraints on the mantle water content at this depth, we determined the full elasticity of hydrous wadsleyite at the P-T conditions of the discontinuity based on density functional theory calculations. Together with previous determinations for the effect of water on olivine elasticity, we simultaneously modeled the density and seismic velocity jumps (Δρ, Δ V P , Δ V S ) across the olivine-wadsleyite transition. Our models allow for several scenarios that can well reproduce the density and seismic velocity jumps across the 410-km discontinuity when compared to globally averaged seismic models. When the water content of olivine and wadsleyite is assumed to be equal as in a simple binary system, our modeling indicates a best fit for low water contents (

The core-mantle boundary (CMB) at ~2900 km depth and ~135 GPa is the interface between the solid silicate mantle and the liquid metallic outer core. The temperature at CMB (TCMB) is a key parameter in assessing the heat flow out of the core and is thus important for our understanding of the geodynamo and mantle convection. The upper bound on TCMB is constrained by the solidus temperature (onset of melting) of pyrolite, a chemical proxy of the mantle rock, at ~135 GPa because the lowermost mantle is predominantly solid. Previous laser-heated diamond anvil cell (LH DAC) studies reported pyrolite solidus temperatures of 3430–4180 K (±

We present two-dimensional thermo-mechanical numerical models of coupled lithosphere-mantle deformation, considering the upper mantle down to a depth of 660 km. We consider visco-elasto-plastic deformation and for the lithospheric and upper mantle a combination of diffusion, dislocation and Peierls creep. Mantle densities are calculated from petrological phase diagrams (Perple_X) for a Hawaiian pyrolite. The model generates a 120 Myrs long geodynamic cycle of subsequent extension (30 Myrs), cooling (70 Myrs) and convergence (20 Myrs) in a single and continuous simulation with explicitly modelling convection in the upper mantle. During lithosphere extension, the models generate an approximately 400 km wide basin of exhumed mantle bounded by hyper-extended passive margins. The models show that considering only the thermal effects of upper mantle convection by using an effective thermal conductivity generates results of lithosphere hyper-extension that are similar to the ones of models that explicitly model the convective flow. Applying a lower viscosity limit of 5 × 1020 Pa s suppresses convection and generates results different to the ones for simulations with a low viscosity asthenosphere having minimal viscosity of approximately 1019 Pa s. During cooling without far-field deformation, no subduction of the exhumed mantle is spontaneously initiated. Density differences between lithosphere and mantle are too small to generate a buoyancy force exceeding the mechanical strength of the lithosphere. The extension and cooling stages generate self-consistently a structural and thermal inheritance for the subsequent convergence stage. Convergence initiates subduction of the exhumed mantle at the transition to the hyper-extended margins. The main mechanism of subduction initiation is thermal softening for a plate driving force (per unit length) of approximately 15 TN m−1. If convection in the mantle is suppressed by high effective thermal conductivities or high, lower viscosity limits, then subduction initiates at both margins leading to divergent double-slab subduction. Convection in the mantle assists to generate a single-slab subduction at only one margin, likely due to mantle flow which exerts an additional suction force on the lithosphere. The first-order geodynamic processes simulated in the geodynamic cycle of subsequent extension, cooling and convergence are applicable to orogenies that resulted from the opening and closure of embryonic oceans bounded by magma-poor hyper-extended passive margins, which might have been the case for the Alpine orogeny.

Knowledge of thermal conductivity of mantle minerals is crucial for understanding heat transport from the Earth's core to mantle. At the pressure-temperature conditions of the Earth's core-mantle boundary, calculations of lattice thermal conductivity based on atomistic models have determined values ranging from 1 to 14 W/m/K for bridgmanite and bridgmanite-rich mineral assemblages. Previous studies have been performed at room temperature up to the pressures of the core-mantle boundary, but correcting these to geotherm temperatures may introduce large errors. Here we present the first measurements of lattice thermal conductivity of mantle minerals up to pressures and temperatures near the base of the mantle, 120 GPa and 2500 K. We use a combination of continuous and pulsed laser heating in a diamond anvil cell to measure the lattice thermal conductivity of pyrolite, the assemblage of minerals expected to make up the lower mantle. We find a value of 3.9 + 1.4 − 1.1 W/m/K at 80 GPa and 2000 to 2500 K and 5.9 + 4.0 − 2.3 W/m/K at 124 GPa and 2000 to 3000 K. These values rule out the highest calculations of thermal conductivity of the Earth's mid-lower mantle (i.e. k 6 W/m/K at 80 GPa), and are consistent with both the high and low calculations of thermal conductivity near the base of the lower mantle.

A better understanding of the Earth’s compositional structure is needed to place the geochemical record of surface rocks into the context of Earth accretion and evolution. Cosmochemical constraints imply that lower-mantle rocks may be enriched in silica relative to upper-mantle pyrolite, whereas geophysical observations support whole-mantle convection and mixing. To resolve this discrepancy, it has been suggested that subducted mid-ocean ridge basalt (MORB) segregates from subducted harzburgite to accumulate in the mantle transition zone (MTZ) and/or the lower mantle. However, the key parameters that control basalt segregation and accumulation remain poorly constrained. Here, we use global-scale 2D thermochemical convection models to investigate the influence of mantle-viscosity profile, planetary-tectonic style and bulk composition on the evolution and distribution of mantle heterogeneity. Our models robustly predict that, for all cases with Earth-like tectonics, a basalt-enriched reservoir is formed in the MTZ, and a harzburgite-enriched reservoir is sustained at 660~800 km depth, despite ongoing whole-mantle circulation. The enhancement of basalt and harzburgite in and beneath the MTZ, respectively, are laterally variable, ranging from ~30% to 50% basalt fraction, and from ~40% to 80% harzburgite enrichment relative to pyrolite. Models also predict an accumulation of basalt near the core mantle boundary (CMB) as thermochemical piles, as well as moderate enhancement of most of the lower mantle by basalt. While the accumulation of basalt in the MTZ does not strongly depend on the mantle-viscosity profile (explained by a balance between basalt delivery by plumes and removal by slabs at the given MTZ capacity), that of the lowermost mantle does: lower-mantle viscosity directly controls the efficiency of basalt segregation (and entrainment) near the CMB; upper-mantle viscosity has an indirect effect through controlling slab thickness. Finally, the composition of the bulk-silicate Earth may be shifted relative to that of upper-mantle pyrolite, if indeed significant reservoirs of basalt exist in the MTZ and lower mantle.

We investigated phase relations, mineral chemistry, and density of lunar highland anorthosite at conditions up to 125 GPa and 2000 K. We used a multi-anvil apparatus and a laser-heated diamond-anvil cell for this purpose. In-situ X-ray diffraction measurements at high pressures and composition analysis of recovered samples using an analytical transmission electron microscope showed that anorthosite consists of garnet, CaAl4Si2O11-rich phase (CAS phase), and SiO2 phases in the upper mantle and the mantle transition zone. Under lower mantle conditions, these minerals transform to the assemblage of bridgmanite, Ca-perovskite, corundum, stishovite, and calcium ferrite-type aluminous phase through the decomposition of garnet and CAS phase at around 700 km depth. Anorthosite has a higher density than PREM and pyrolite in the upper mantle, while its density becomes comparable or lower under lower mantle conditions. Our results suggest that ancient anorthosite crust subducted down to the deep mantle was likely to have accumulated at 660–720 km in depth without coming back to the Earth's surface. Some portions of the anorthosite crust might have circulated continuously in the Earth's deep interior by mantle convection and potentially subducted to the bottom of the lower mantle when carried within layers of dense basaltic rocks. Keywords: Anorthosite, Phase transformation, Multi-anvil apparatus, Diamond-anvil cell, Mantle dynamics

The phase relations, mineral chemistry and density of KREEP basalt were investigated at pressures of 12–125 GPa and temperatures up to 2810 K by a combination of large volume multi-anvil press experiments and in situ synchrotron X-ray diffraction measurements in a laser-heated diamond anvil cell. Our results showed that grossular-rich majorite garnet, liebermannite and Al-bearing stishovite are dominant in the upper-to-middle part of the upper mantle while in the lowermost transition zone a dense Ti-rich CaSiO3 perovskite exsoluted from the garnet, which becomes more pyropic with increasing pressure. At lower mantle conditions, these minerals transform into an assemblage of bridgmanite, Ca-perovskite, Al-stishovite, the new aluminium-rich (NAL) phase and the calcium-ferrite type (CF) phase. At pressures higher than 50 GPa, NAL phase completely dissolved into the CF phase, which becomes the main deposit of alkali metals in the lower mantle. The density of KREEP estimated from phase compositions obtained by energy dispersive X-ray spectroscopy (EDS) in scanning (SEM) and transmission (TEM) electron microscopes, was found substantially denser than pyrolite suggesting that the Earth primordial crust likely subducted deep into the Earth’s mantle after or slightly before the final solidification of magma ocean at 4.53 Ga. Radiogenic elements U, Th and 40K which were abundant in the final residue of magma ocean were brought down along the subduction of the primordial crust and generate heat by decay after the settlement of the primordial crust on top of the CMB, suggesting the non-homogeneous distribution of radiogenic elements in the Hadean mantle with implications for the thermal history of the Earth.

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) + magnesiowustite (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.

The compositions of mantle-derived magmas indicate a substantial variety in the abundances of volatiles in the upper mantle. CO2 and H2O depress the melting point of mantle peridotites considerably, delineating a pressure-temperature region of incipient melting where small degrees of melt exist over a large temperature range (~300 °C) before major melting begins. However, the chemical characterization of these melts in high-pressure experiments is challenging at low melt fractions (melt pockets may occupy volumes of only 10–50 μm3) because of analytical uncertainties related to the ubiquitous formation of metastable phases during quenching. This systematic partial melting study presents carefully determined compositions of incipient melts of a range of peridotites in the presence of CO2 + H2O mixtures at 2.5 to 7 GPa. Four different fertile and depleted peridotites were used: Hawaiian pyrolite, K2O-enriched pyrolite, MORB pyrolite and depleted lherzolite. To arrive at accurate melt compositions, we introduce the melt tomography method that integrates multiple area scans of melt pockets polished to several depths. Results confirm that incipient and low degree melts progress abruptly (within 25 °C) from carbonatitic towards melilititic-nephelinitic compositions at 2.5 GPa, whereas they progress gradually from carbonate-rich to carbonated silicate (aillikitic) compositions at 4–5 GPa. Melt compositions at near-solidus conditions are mainly controlled by the breakdown of carbonate, and hydrous phases such as pargasite and phlogopite, and become less siliceous and slightly more magnesian with increasing pressure at given melt fractions. Melts exhibit strong increases in SiO2 (2.75 to 44 wt%) with increasing temperature, whereas TiO2, Na2O and K2O decrease. The generally strongly potassic (K2O ≤ 6.63 wt%) and sodic (Na2O ≤ 3.06 wt%) character of the volatile-rich, incipient and low-degree melts indicate that these would act as reactive metasomatic agents that may transport large amounts of energy and induce chemical changes in large volumes of the upper mantle.

Our current understanding of the electronic state of iron in lower-mantle minerals leads to a considerable disagreement in bulk sound speed with seismic measurements if the lower mantle has the same composition as the upper mantle (pyrolite). In the modeling studies, the content and oxidation state of Fe in the minerals have been assumed to be constant throughout the lower mantle. Here, we report high-pressure experimental results in which Fe becomes dominantly Fe2+ in bridgmanite synthesized at 40-70 GPa and 2,000 K, while it is in mixed oxidation state (Fe3+/∑Fe = 60%) in the samples synthesized below and above the pressure range. Little Fe3+ in bridgmanite combined with the strong partitioning of Fe2+ into ferropericlase will alter the Fe content for these minerals at 1,100- to 1,700-km depths. Our calculations show that the change in iron content harmonizes the bulk sound speed of pyrolite with the seismic values in this region. Our experiments support no significant changes in bulk composition for most of the mantle, but possible changes in physical properties and processes (such as viscosity and mantle flow patterns) in the midmantle.

Current estimates of the carbon flux between the surface and mantle are highly variable, and the total amount of carbon stored in closed hidden reservoirs is unknown. Understanding the forms in which carbon existed in the molten early Earth is a critical step towards quantifying the carbon budget of Earth's deep interior. Here we employ first-principles molecular dynamics to study the evolution of carbon species as a function of pressure in a pyrolite melt. We find that with increasing pressure, the abundance of CO2 and CO3 species decreases at the expense of CO4 and complex oxo-carbon polymers (CxOy) displaying multiple C-C bonds. We anticipate that polymerized oxo-carbon species were a significant reservoir for carbon in the terrestrial magma ocean. The presence of Fe-C clusters suggests that upon segregation, Fe-rich metal may partition a significant fraction of carbon from the silicate liquid, leading to carbon transport into the Earth's core., The amount of carbon stored in closed hidden reservoirs is unknown. Here the authors use a computational approach to study the evolution of carbon species and observe polymerization of carbon atoms at high pressures, illustrating the potential for a significant carbon reservoir in the Earth’s deep interior.

International audience; 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.

The heat flux across the core-mantle boundary (Q(CMB)) is the key parameter to understand the Earth's thermal history and evolution. Mineralogical constraints of the Q(CMB) require deciphering contributions of the lattice and radiative components to the thermal conductivity at high pressure and temperature in lower mantle phases with depth-dependent composition. Here we determine the radiative conductivity (k(rad)) of a realistic lower mantle (pyrolite) in situusing an ultra-bright light probe and fast time-resolved spectroscopic techniques in laser-heated diamond anvil cells. We find that the mantle opacity increases critically upon heating to similar to 3000 K at 40-135 GPa, resulting in an unexpectedly low radiative conductivity decreasing with depth from similar to 0.8 W/m/K at 1000 km to similar to 0.35 W/m/K at the CMB, the latter being similar to 30 times smaller than the estimated lattice thermal conductivity at such conditions. Thus, radiative heat transport is blocked due to an increased optical absorption in the hot lower mantle resulting in a moderate CMB heat flow of similar to 8.5 TW,on the lower end of previous Q(CMB) estimates based on the mantle and core dynamics. This moderate rate of core cooling implies an inner core age of about 1 Gy and is compatible with both thermally- and compositionally-driven ancient geodynamo. (C) 2020 Elsevier B.V. All rights reserved.

The Hawaiian Island chain in the middle of the Pacific Ocean is a well-studied example of hotspot volcanism caused by an underlying upwelling mantle plume. The thermal and compositional nature of the plume alters the mantle phase transitions, which can be seen in the depth and amplitude of seismic discontinuities. This study utilises >5000 high quality receiver functions from Hawaiian island stations to detect P-to-s converted phases to image seismic discontinuities between 200 to 800 km depth. Common-conversion point stacks of the data are used to map out lateral variations in converted phase observations, while slowness stacks allow differentiation between true conversions from discontinuities and multiples. We find that the 410 discontinuity is depressed by 20 km throughout our study region, while the main 660 is around average depth throughout most of the area. To the southwest of the Big Island we observe splitting of the 660, with a major peak at 630 km, and a minor peak appearing at 675 km depth. This is inferred to represent the position of the hot plume at depth, with the upper discontinuity caused by an olivine phase transition and the lower by a garnet phase transition. In the upper mantle, a discontinuity is found across the region at depths varying between 290 to 350 km. Identifying multiples from this depth confirms the presence of a so-called X-discontinuity. To the east of the Big Island the X-discontinuity lies around 336 km and the associated multiple is particularly coherent and strong in amplitude. Strikingly, the discontinuity around 410 km disappears in this area. Synthetic modelling reveals that such observations can be explained by a silica phase transition from coesite to stishovite, consistent with widespread ponding of silica-saturated material at these depths around the plume. This material could represent eclogite enriched material, which is relatively silica-rich compared to pyrolite, spreading out from the plume to the east as a deep eclogite pool, a hypothesis which is consistent with dynamical models of thermochemical plumes. Therefore these results support the presence of a significant garnet and eclogite component within the Hawaiian mantle plume.

Deciphering the origin of seismic heterogeneity has been one of the major challenges in understanding the geochemistry and geodynamics of the deep mantle. Fully anisotropic elastic properties of constituent minerals at relevant pressure-temperature conditions of the lower mantle can be used to calculate seismic heterogeneity parameters in order to better understand chemically- and thermally-induced seismic heterogeneities. In this study, the single-crystal elastic properties of ferropericlase (Mg0.94Fe0.06)O were measured using Brillouin spectroscopy and X-ray diffraction at conditions up to 50 GPa and 900 K. The velocity-density results were modeled using third-order finite-strain theory and thermoelastic equations along a representative geotherm to investigate high pressure-temperature and compositional effects on the seismic heterogeneity parameters. Our results demonstrate that from 660 to 2000 km, compressional wave anisotropy of ferropericlase increased from 4% to 9.7% while shear wave anisotropy increased from 9% to as high as 22.5%. The thermally-induced lateral heterogeneity ratio (RS/P = ∂lnVS/∂lnVP) of ferropericlase was calculated to be 1.48 at ambient pressure but decreased to 1.43 at 40 GPa along a representative geotherm. The RS/P of a simplified pyrolite model consisting of 80% bridgmanite and 20% ferropericlase was approximately 1.5, consistent with seismic models at depths from 670 to 1500 km, but showed an increased mismatch at lower mantle depths below ~1500 km. This discrepancy below mid-lower mantle could be due to either a contribution from chemically-induced heterogeneity or the effects of the Fe spin transition in the deeper parts of the Earth's lower mantle.

Pyrope is an Mg end-member garnet, which constitutes up to ~15% of the upper mantle. Thermodynamic and thermoelastic properties of garnet at high pressure and high temperature are important for understanding the composition and structure of the Earth. Here we report the first-principles calculations of vibrational properties, thermodynamic properties, and elasticity of pyrope over a wide pressure and temperature range. The calculated results exhibit good consistency with the available experimental results and provide values up to the pressure and temperature range that are challenging for experiments to achieve and measure. Pyrope is almost isotropic at high pressure. The elastic moduli, especially the shear modulus of pyrope, exhibit nonlinear pressure and temperature dependences. Density and seismic velocities of pyrope along with piclogite, pyrolite, and eclogite are compared with seismic models along the normal upper mantle and the cold subducting lithospheric plate geotherms. Pyrope has the highest density and seismic velocities among the major minerals of the upper mantle along these geotherms. Eclogite with 30% pyrope and 70% clinopyroxene is denser than the surrounding mantle even along the normal upper mantle geotherm and becomes still denser along the cold slab geotherm. Seismic velocities of eclogite with 30% pyrope along the normal upper mantle geotherm are slower than the surrounding mantle but along the cold slab geotherm are faster than the surrounding mantle.

The composition of the Earth's lower mantle (LM) is critical in understanding the Earth's interior and dynamics. Previous reports on the composition of the LM are controversial. The composition of LM, constrained here using high-temperature and high-pressure data of the velocities and density of minerals from first-principles calculations, spans a large range: from pyrolite with ~ 15% ferropericlase (Fp) in weight (wt %) to perovskitic-rich composition with ~ 10 wt % Fp. Any composition well constrained by preliminary reference Earth model (PREM) has a sufficient amount of Fp to exhibit the positive temperature dependence of VΦ (= Kρ) at the middle LM, leading to the insensitivity of VP to temperature. The depth at which VP is insensitive to temperature variation deepens significantly with increasing temperature because the spin transition in Fp has the positive Clapeyron slope (~17 MPa/K). The Large Low Shear Velocity Provinces below Africa is estimated to be ~ 750 K higher than the ambient mantle.

Constraining Earth's bulk composition is fundamental to understanding our planet's formation and evolution. While the lower mantle accounts for a majority of the bulk silicate Earth, it is also the least accessible. As experimental and theoretical mineral physics constraints on mineral elasticity at lower mantle temperatures and pressures have improved, comparisons between predicted seismic velocity and density profiles for hypothesized bulk compositions and 1D seismic models have become commonplace. However, the degree to which a given composition is a better or worse fit than another composition is not always reported, nor are the influences of the assumed temperature profile and other uncertainties discussed. Here we compare seismic velocities and densities for perovskitite, pyrolite, and harzburgite bulk compositions calculated using advanced ab initio techniques to explore the extent to which the associated uncertainties affect our ability to distinguish between candidate compositions. We find that predicted differences between model compositions are often smaller than the influence of temperature uncertainties and therefore these comparisons lack discriminatory power. The inability to distinguish between compositions is largely due to the high sensitivity of seismic properties to temperature accompanied by uncertainties in the mantle geotherm, coupled with diminished sensitivity of seismic velocity to composition toward the base of the mantle. An important exception is the spin transition in (Mg,Fe)O-ferropericlase, which is predicted to give a distinct variation in compressional wave velocity that should distinguish between relatively ferro-magnesian and silica-rich compositions. However, the absence of an apparent spin transition signature in global 1D seismic profiles is a significant unresolved issue in geophysics, and it has important geochemical implications. The approach we present here for establishing discriminatory power for such comparisons can be applied to any estimate of seismic velocities and associated uncertainties, and offers a straightforward tool to evaluate the robustness of model comparisons.

As the major component of garnet, the second most abundant phase in Earth's transition zone, MgSiO3-majorite plays a fundamental role in controlling the state and dynamics of Earth's mantle. However, due to challenges of experiments and simulations, there are still very limited data on the elastic properties of MgSiO3-majorite at simultaneously high temperatures and pressures. In this study, we have carried out extensive first principles calculations to determine the thermoelastic properties of MgSiO3-majorite up to 2000 K and 40 GPa. We find that the elastic constants of MgSiO3-majorite change significantly over the temperature and pressure range studied, with noticeable non-linearities in their pressure dependences. The seismic anisotropy of MgSiO3-majorite is high and generally increases with pressure. It is much higher than that of the other end-members of garnet and ringwoodite, which makes it the most anisotropic mineral in assemblages expected in the lower transition zone. Based on our calculated elastic moduli and with careful elimination of systematic errors, we establish a third-order Birch-Murnaghan-Mie-Gruneisen model for MgSiO3-majorite with the parameters: V0 = 114.1 cm3/mol, K0 = 163.6 GPa, G0 = 86.4 GPa, K0′ = 4.44, G0′ = 1.16, γ0 = 1.08, q0 = 0.48, ηS0 = 0.76, and θ0 = 822.5 K. Integrating our results into a thermodynamic model able to predict the properties of mantle assemblages, we find that a pyrolite composition produces velocities that agree with the seismic model AK135 in the upper transition zone. In the lower transition zone, a pyrolite composition fits well with some specific local observations, but a mechanical mixture with 18% basalt and 82% harzburgite is in better agreement with the global seismic model PREM. The much larger abundance of MgSiO3-majorite in the garnet phase of harzburgite suggests that the anisotropy in the lower transition zone may not be negligible and would be observable at least in the heterogeneous zones near subducting slabs.

A better understanding of the Earth's compositional structure is needed to place the geochemical record of surface rocks into the context of Earth accretion and evolution. Cosmochemical constraints imply that lower-mantle rocks may be enriched in silica relative to upper-mantle pyrolite, whereas geophysical observations support whole-mantle convection and mixing. To resolve this discrepancy, it has been suggested that subducted mid-ocean ridge basalt (MORB) segregates from subducted harzburgite to accumulate in the mantle transition zone (MTZ) and/or the lower mantle. However, the key parameters that control basalt segregation and accumulation remain poorly constrained. Here, we use global-scale 2D thermochemical convection models to investigate the influence of mantle-viscosity profile, planetary-tectonic style and bulk composition on the evolution and distribution of mantle heterogeneity. Our models robustly predict that, for all cases with Earth-like tectonics, a basalt-enriched reservoir is formed in the MTZ, and a harzburgite-enriched reservoir is sustained at 660∼800 km depth, despite ongoing whole-mantle circulation. The enhancement of basalt and harzburgite in and beneath the MTZ, respectively, are laterally variable, ranging from ∼30% to 50% basalt fraction, and from ∼40% to 80% harzburgite enrichment relative to pyrolite. Models also predict an accumulation of basalt near the core mantle boundary (CMB) as thermochemical piles, as well as moderate enhancement of most of the lower mantle by basalt. While the accumulation of basalt in the MTZ does not strongly depend on the mantle-viscosity profile (explained by a balance between basalt delivery by plumes and removal by slabs at the given MTZ capacity), that of the lowermost mantle does: lower-mantle viscosity directly controls the efficiency of basalt segregation (and entrainment) near the CMB; upper-mantle viscosity has an indirect effect through controlling slab thickness. Finally, the composition of the bulk-silicate Earth may be shifted relative to that of upper-mantle pyrolite, if indeed significant reservoirs of basalt exist in the MTZ and lower mantle.

The composition of the lower mantle $-$ comprising 56% of Earth's volume $-$ remains poorly constrained. Among the major elements, Mg/Si ratios ranging from $\sim$0.9$-$1.1, such as in rocky solar-system building blocks (or chondrites), to $\sim$1.2$-$1.3, such as in upper-mantle rocks (or pyrolite), have been proposed. Geophysical evidence for subducted lithosphere deep in the mantle has been interpreted in terms of efficient mixing and thus homogeneous Mg/Si across most of the mantle. However, previous models did not consider the effects of variable Mg/Si on the viscosity and mixing efficiency of lower-mantle rocks. Here, we use geodynamic models to show that large-scale heterogeneity with viscosity variations of $\sim$20$\times$, such as due to the dominance of intrinsically strong (Mg,Fe)SiO$_3-$bridgmanite in low-Mg/Si domains, are sufficient to prevent efficient mantle mixing, even on large scales. Models predict that intrinsically strong domains stabilize degree-two mantle-convection patterns, and coherently persist at depths of $\sim$1,000$-$2,200 km up to the present-day, separated by relatively narrow up-/downwelling conduits of pyrolitic material. The stable manifestation of such "bridgmanite-enriched ancient mantle structures" (BEAMS) may reconcile the geographical fixity of deep-rooted mantle-upwelling centers, and fundamental geophysical changes near 1,000 km depth (e.g. in terms of seismic-tomography patterns, radial viscosity increase, lateral deflections of rising plumes and sinking slabs). Moreover, these ancient structures may provide a reservoir to host primordial geochemical signatures.

Scientific reports 8(1), 6358 (2018). doi:10.1038/s41598-018-24832-y, The 660-km seismic discontinuity, which is a significant structure in the Earth’s mantle, is generally interpreted as the post-spinel transition, as indicated by the decomposition of ringwoodite to bridgmanite + ferropericlase. All precise high-pressure and high-temperature experiments nevertheless report 0.5–2 GPa lower transition pressures than those expected at the discontinuity depth (i.e. 23.4 GPa). These results are inconsistent with the post-spinel transition hypothesis and, therefore, do not support widely accepted models of mantle composition such as the pyrolite and CI chondrite models. Here, we present new experimental data showing post-spinel transition pressures in complete agreement with the 660-km discontinuity depth obtained by high-resolution in situ X-ray diffraction in a large-volume high-pressure apparatus with a tightly controlled sample pressure. These data affirm the applicability of the prevailing mantle models. We infer that the apparently lower pressures reported by previous studies are experimental artefacts due to the pressure drop upon heating. The present results indicate the necessity of reinvestigating the position of mantle mineral phase boundaries previously obtained by in situ X-ray diffraction in high-pressure–temperature apparatuses., Published by Springer Nature, Cham

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.

The sound velocities and single-crystal elastic moduli of iron-bearing wadsleyite with [Fe]/[Fe + Mg] molar ratio of 0.075 have been measured by Brillouin scattering experiments at high pressures up to 17.7 GPa. Pressure derivatives for the adiabatic bulk modulus ( K S0 ) and shear modulus ( μ 0 ) are 4.1(1) and 1.45(4), respectively. A comparison of our results with previous Brillouin scattering results on the Mg end-member wadsleyite shows that incorporating 7.5 mol% iron in wadsleyite at high-pressure conditions decreases the shear moduli by ∼4–5%, but does not have a discernable effect on the bulk modulus. The effects of iron on the elastic moduli of wadsleyite at ambient pressure persist to high-pressure conditions at a relatively constant level. Using our results on iron-bearing wadsleyite at high pressure, we conclude that less olivine than in the pyrolite model of mantle composition provides a satisfactory explanation for 410 km seismic discontinuity at the top of the transition zone.

To elucidate the water cycles in iron-rich Mars, we investigated the phase relation of a water-undersaturated (2 wt.%) analog of Martian mantle in simplified MgO-Al2O3-FeO-SiO2-H2O (MAFSH) system between 15 and 21 GPa at 900&ndash, 1500 &deg, C using a multi-anvil apparatus. Results showed that phase E coexisting with wadsleyite or ringwoodite was at least stable at 15&ndash, 16.5 GPa and below 1050 &deg, C. Phase D coexisted with ringwoodite at pressures higher than 16.5 GPa and temperatures below 1100 &deg, C. The transition pressure of the loop at the wadsleyite-ringwoodite boundary shifted towards lower pressure in an iron-rich system compared with a hydrous pyrolite model of the Earth. Some evidence indicates that water once existed on the Martian surface on ancient Mars. The water present in the hydrous crust might have been brought into the deep interior by the convecting mantle. Therefore, water might have been transported to the deep Martian interior by hydrous minerals, such as phase E and phase D, in cold subduction plates. Moreover, it might have been stored in wadsleyite or ringwoodite after those hydrous materials decomposed when the plates equilibrated thermally with the surrounding Martian mantle.

We investigate the velocity and density structure of the mantle beneath the Northern Atlantic at a depth of 410 km where the olivine to wadsleyite phase transformation occurs. Measuring polarities of precursor arrivals to PP seismic waves that reflect at this transition we analyze over 1700 teleseismic seismograms from 13 different source-receiver combinations using the advantages of high-resolution array seismology methods and crossing ray paths. The final dataset consists of 45 events with Mw ≥ 5.8 with a good coverage of reflection points beneath the investigation area. We find several events where the polarity of the precursor signal is opposite to that of the PP wave, however, events with same polarity of precursor and PP wave are also observed. We test several causes for changing polarity of reflections at the 410 km discontinuity and find out that there is a dependence of the polarity on epicentral distance. We computed the reflection coefficients of the precursor wave using the Zoeppritz equations with the initial values of density, VP and VS of a pyrolite model for the olivine-wadsleyite phase transition and varied the values by ±10% above and below 410 km depth. More than 729 million combinations of density, compressional and shear wave velocities for olivine and wadsleyite layers were tested to find the combinations that can explain our polarity-distance observations. While the best-fitting models have a smaller positive contrast than pyrolite in density, a small negative contrast for P and S wave velocities is needed. Possibilities to explain these values include combination of a hydrous wadsleyite layer beneath, and anhydrous or Fe-enriched olivine above the boundary.

We have determined the post-spinel transition boundary in anhydrous and hydrous Mg2SiO4 in a temperature range from 1173 to 2023 K at 19.3–25.4 GPa using synchrotron in situ X-ray diffraction measurements. The phase boundary in Mg2SiO4 is located at 22 GPa and 1800 K and 22.1 GPa and 1500 K, which is slightly lower (~0.3–0.5 GPa) than that determined in the previous in situ measurements using the same pressure scale [e.g. Katsura et al., 2003 , Post-spinel transition in Mg2SiO4 determined by high P–T in situ X-ray diffractometry. Phys. Earth Planet. Inter. 136, 11–24]. The Clapeyron slope of Mg2SiO4 was found to be gentle i.e. between −0.4 and −0.7 MPa/K, which is also consistent with previous in situ measurements, but inconsistent with diamond anvil cell experiments and theoretical estimations. The phase boundary in Mg2SiO4+2 wt% H2O which is relevant to Fe free-depleted harzburgitic composition is located between 23.4 and 23.6 GPa and 1500 K, which shifts the hydrous boundary to the higher pressures relative to anhydrous Mg2SiO4 from 1.3 to 1.0 GPa. The result for hydrous Mg2SiO4 shows steeper Clapeyron slope between −3.2 and −3.1 MPa/K compared with anhydrous Mg2SiO4 and hydrous pyrolite system. The present data suggest that water has a strong influence on 660 km discontinuity and the depressions observed at this boundary in several regions, especially related to subduction zones, can be explained by the presence of water in depleted harzburgite component.

Various geophysical constraints on the deep Earth point to a chemically heterogeneous mantle. Based on such constraints, Bulk Earth compositions inferred from Enstatite chondrites (E-Earth composition) predict that, whereas the Primitive Upper mantle (PUM) had a pyrolitic composition, the Primitive Lower mantle (PLoM) was enriched in Fe and Si. In E-Earth formalism, this chemical heterogeneity is related to the formation and differentiation of the Early Earth, and mantle Si and Fe variations reflect variations in the efficiency of Si and FeO dissolution in the metal phase during core formation. In the simplest scenario of homogeneous accretion, we calculate by mass balance the composition and the mass fraction of the metallic extract in equilibrium with a pyrolite. The O, Si and Ni contents of this metal extract correspond to a silicate-metal equilibrium at high pressure (50±5 GPa) and high temperature (3500±500 °C), in line with a giant impact scenario. Mass balance calculations then yield the composition of the proto-core and the proto-mantle prior to the giant impact. We obtain that the core of the proto-Earth was almost devoid of oxygen, hence formed under lower pressure and temperature conditions, in agreement with an early differentiation of planetesimals in the early solar system. In such a two-stage scenario of Earth's core formation, no massive silicate differentiation is required to create a pristine mantle heterogeneity. The concentration of lithophile elements in the Primitive Lower mantle can then be constrained using RLE ratios in E-chondrites and in the upper mantle.

The Mg-(Fe + Ti)-Al melting diagram for pyrolite based on experimental data from literature shows the composition of the liquid as a function of pressure and the degree of pyrolite melting. Three mechanisms of liquid separation from a mantle source material are discussed: (i) gravitational mechanism, which works at a degree of source material melting of 25%, (ii) filter pressing mechanism, which is efficient at degrees of melting lower than 2%, and (iii) nearly complete local melting of mantle material. Garnet in the solid residue is thought to play an important role by affecting the chemistries of mantle magmas. The comparison of petrochemical and experimental data in a Mg-(Fe + Ti)-Al ternary plot shows that picrite and ferropicrite alcaline primary magmas are segregated at depths of 120 and 210 km, respectively, in the garnet stability zone, at degrees of melting lower than 2%; and tholeiite basalt magmas are segregated above this zone. At degrees of melting of 25%, picrobasalt, komatiite-basalt, picrite, and ferropicrite primary magmas of the tholeiite series are derived at depths of 80, 130, 240, and 300 km, respectively. Ultrabasic komatiite magma is generated at high degrees of mantle source melting, with the solid residues devoid of garnet. The tholeiite basalt series can be produced by two parental melts: aluminous and magnesian basaltic, both separated from the mantle sources via the filter pressing mechanism: the former at depths shallower than 30 km in ocean spreading zones (TOR-2), and the latter at depths of 50–60 km in oceanic spreading zones (TOR-1) and in the subcontinental lithosphere. Primary magnesian basalt magmas of the calc-alkaline and tholeiite series are derived in the lithospheric mantle at the same depths and low degrees of melting. Different evolutionary trajectories of compositionally similar primary magmas are controlled by the conditions of their further fractional crystallization: in compressional environments and with fluids saturating the melts in subduction zones for the former and in extensional environments and free magma ascent to the surface for the latter. Ultrapotassic rock series, such as lamprophyres, leucitites, kamafugites, lamproites, and kimberlites, are most probably generated via the melting of the metasomatized subcratonic mantle.

In order to place better constraints on the conditions of core formation on Earth and other planetary bodies we have performed experiments to determine the partitioning of Mo and W between liquid Fe-rich metal and liquid silicate at pressures of 1.5–24 GPa and temperatures of 1803–2723 K. Experiments performed in MgO capsules at 1.5 GPa/1923 K indicate that Mo is in the +4 oxidation state in the silicate at oxygen fugacities >2 log units below the IW (Fe–FeO) buffer. In contrast W6+ is the dominant tungsten oxidation state in the silicate at 1.5 GPa/1923 K and 1.8–3.3 log units below the IW buffer. When our 15 data for pressures between 6 and 24 GPa are combined with those of Cottrell et al. (2009) we find evidence neither for a change in oxidation state of W above 6 GPa nor for a change in pressure dependence of partitioning in the experimental fO2 range. Metal–silicate partitioning of both Mo and W shows strong dependence on silicate melt composition with both elements becoming more siderophile as the melt becomes more SiO2-rich. Although the trends in the partitioning data can be related to silicate melt composition in terms of the ratio of nonbridging oxygens to tetrahedral cations NBOT we find that use of a regular solution model for the silicate melt results in a significantly better fit to the data. We combined our results with those in the literature to obtain partitioning equations applicable to the Earth. In terms of weight partitioning we define Diwt and (KDi)wt as follows: (DMowt)=[Mo]met[Mo]sil;(DFewt)=[Fe]met[Fe]sil;(KDMo)wt=(DMowt)(DFewt)2;(KDW)wt=(DWwt)(DFewt)3 The experimental data, when corrected for compositional effects, yield the following expressions for a pyrolite mantle: log(KDMo)wt=1.44-143T-167PT(0.19) log(KDW)wt=1.85-6728T-77PT(0.24) The value in brackets corresponds to 1 standard error of the fit. These expressions were combined with the continuous accretion model of Wade and Wood (2005) to investigate the constraints which they place on the accretionary process. We find, however, that, for accretionary paths consistent with the silicate Earth contents of Ni, Co, V, Cr and Nb, W should partition twice as strongly into the core as Mo. This is in stark contrast to the estimated core–mantle partition coefficients of ∼40 for W and 90–140 for Mo. Neither changes to the accretionary path nor the assumption of partial disequilibrium can readily alter this result. The answer appears to reside with the identity of one of the light elements in the core. We investigated the effect of S on our accretionary model by adding 2% of this element (consistent with cosmochemical estimates) to the core. If S is added at constant S/Fe ratio throughout accretion the net effect is negligible. If, however, S is added exclusively during the last 10–20% of accretion DMo and DW become consistent with the silicate Earth contents of these elements. Only small additional adjustments to the model are required to accommodate changes in partitioning of Ni, Co, V, Cr and Nb. We conclude that the Mo and W contents of the silicate Earth indicate that S (and other moderately volatile elements) was added to the Earth during core formation but only during the last ∼20% of accretion. This conclusion is the same as that reached by Schonbachler et al. (2010) from the Ag isotopic composition of silicate Earth.

Density of ultramafic silicate melts was determined using the sink/float technique at high pressures. Seven melt compositions were studied, among which three were dry compositions with different Mg's (molar MgO/(MgO + FeO) x 100) and the other four were hydrous compositions synthesized by adding 2-7 wt.% H{sub 2}O to the anhydrous ones. Experimental conditions range from 9 to 15 GPa and from 2173 to 2473 K. The sinking and floatation of density markers were observed for all melt compositions. Melt density data were analyzed by applying the Birch-Murnaghan equation of state and a newly developed equation of state for silicate melts based on the model of hard sphere mixtures. The presence of water can significantly reduce the density of melts due to its small molecular mass. On the other hand, water makes hydrous silicate melts more compressible than anhydrous melts and therefore the effect of H{sub 2}O on melt density is less significant at high pressures. The density of hydrous melts was then calculated as a function of H{sub 2}O content at the conditions of the bottom of the upper mantle, and was compared with the density of the dominant upper mantle minerals. Results show that the conditions for amore » negatively buoyant melt that coexists with a pyrolite mantle atop the 410 km discontinuity are marginally satisfied if H{sub 2}O is the only volatile component to facilitate melting, but such conditions will be satisfied by a broader range of conditions when other heavier volatile elements (C, K, etc.) are also present.« less

The discovery of a phase transition in Mg-silicate perovskite (Pv) to postperovskite (pPv) at lowermost mantle pressure-temperature ( P - T ) conditions may provide an explanation for the discontinuous increase in shear wave velocity found in some regions at a depth range of 200 to 400 km above the core-mantle boundary, hereafter the D ′′ discontinuity. However, recent studies on binary and ternary systems showed that reasonable contents of Fe 2+ and Al for pyrolite increase the thickness (width of the mixed phase region) of the Pv - pPv boundary (400–600 km) to much larger than the D ′′ discontinuity (≤ 70 km). These results challenge the assignment of the D ′′ discontinuity to the Pv - pPv boundary in pyrolite (homogenized mantle composition). Furthermore, the mineralogy and composition of rocks that can host a detectable Pv → pPv boundary are still unknown. Here we report in situ measurements of the depths and thicknesses of the Pv → pPv transition in multiphase systems (San Carlos olivine, pyrolitic, and midocean ridge basaltic compositions) at the P - T conditions of the lowermost mantle, searching for candidate rocks with a sharp Pv - pPv discontinuity. Whereas the pyrolitic mantle may not have a seismologically detectable Pv → pPv transition due to the effect of Al, harzburgitic compositions have detectable transitions due to low Al content. In contrast, Al-rich basaltic compositions may have a detectable Pv - pPv boundary due to their distinct mineralogy. Therefore, the observation of the D ′′ discontinuity may be related to the Pv → pPv transition in the differentiated oceanic lithosphere materials transported to the lowermost mantle by subducting slabs.

New hexagonal aluminous (NAL) phase and orthorhombic calcium-ferrite (CF) type phase are considered to be major mantle components of the mid-ocean ridge basalt (MORB) at pressure and temperature conditions in the lower mantle, which can potentially host alkali elements with large ionic radii. The high-pressure stability and elastic properties of both NAL and CF phases are therefore of fundamental importance for understanding the fate of subducted MORB. Here we report those properties of the NaMg2Al5SiO12 system studied by means of the first-principles computation method. NAL was found to transform to the CF phase at 39.6 GPa, accompanied by discontinuities in density (+1.8%), as well as compressional wave (−0.2%), shear wave (+0.9%), and bulk sound (−1.0%) velocities. The property of subducted MORB was evaluated using these results, and the velocity contrast between pyrolite and MORB of ~−5% was found to be quite comparable to the shear velocity anomaly observed for seismic scatterers at depths from 1100 to 1800 km. However, since the transformation of the NAL to CF phase within MORB produces insignificant increases in the seismic velocities, it would be seismologically undetectable. On the other hand, the anisotropy change associated with the phase transition is significant and could be seismically detectable using observations such as shear wave splitting measurements since the CF phase is considerably more anisotropic compared to the NAL phase.

SUMMARY Current interpretations of seismic observations typically argue for significant chemical heterogeneity being present in the two large low shear velocity provinces under Africa and the Pacific. Recently, however, it has been suggested that large lateral temperature variations in the lowermost mantle resulting from a strong thermal gradient across D″ may provide an alternative explanation. In case of a high heat flux from the core into the mantle, the magnitude of shear wave velocity variations in tomographic models can be reconciled with isochemical whole mantle flow and a pyrolite composition. So far, the hypothesis of strong core heating has been tested in a consistent manner only against tomographic S-wave velocity models, but not against P-wave velocity models. Here, we explore a new approach to assess geodynamic models and test the assumption of isochemical whole mantle flow with strong core heating directly against the statistics of observed traveltime variations of both P and S waves. Using a spectral element method, we simulate 3-D global wave propagation for periods down to 10 s in synthetic 3-D elastic structures derived from a geodynamic model. Seismic heterogeneity is predicted by converting the temperature field of a high-resolution mantle circulation model (MCM) into seismic velocities using thermodynamic models of mantle mineralogy. Being based on forward modelling only, this approach avoids the problems of limited resolution and non-uniqueness inherent in tomographic inversions while taking all possible finite-frequency effects into account. Capturing the correct physics of wave propagation allows for a consistent test of the assumption of high core heat flow against seismic data. The statistics of long-period body wave traveltime observations show a markedly different behaviour for P and S waves: the standard deviation of P-wave delay times stays almost constant with turning depth, whereas that of the S-wave delay times increases strongly throughout the mantle. Surprisingly, synthetic traveltime variations computed for the isochemical MCM reproduce these different trends. This is not expected from a ray-theoretical point of view and highlights the importance of finite-frequency effects. Most importantly, the large lateral temperature variations in the lower mantle related to strong core heating are able to explain most of the standard deviation of observed P- and S-wave delay times. This is a strong indication that seismic heterogeneity in the lower mantle is likely dominated by thermal variations on the length scales relevant for long-period body waves.

Mapping the thermal and compositional structure of the upper mantle requires a combined interpretation of geophysical and petrological observations. Based on current knowledge of material properties, we interpret available global seismic models for temperature assuming end-member compositional structures. In particular, we test the effects of modelling a depleted lithosphere, which accounts for petrological constraints on continents. Differences between seismic models translate into large temperature and density variations, respectively, up to 400 K and 0.06 g cm(-3) at 150 km depth. Introducing lateral compositional variations does not change significantly the thermal interpretation of seismic models, but gives a more realistic density structure. Modelling a petrological lithosphere gives cratonic temperatures at 150 km depth that are only 100 K hotter than those obtained assuming pyrolite, but density is similar to 0.1 g cm(-3) lower. We determined the geoid and topography associated with the density distributions by computing the instantaneous flow with an existing code of mantle convection, STAG-YY. Models with and without lateral variations in viscosity have been tested. We found that the differences between seismic models in the deeper part of the upper mantle significantly affect the global geoid, even at harmonic degree 2. The range of variance reduction for geoid due to differences in the transition zone structure (i.e. from 410 to 660 km) is comparable with the range due to differences in the whole mantle seismic structure. Since geoid is dominated by very long wavelengths (the lowest five harmonic degrees account for more than 90 per cent of the signal power), the lithospheric density contrasts do not strongly affect its overall pattern. Models that include a petrological lithosphere, however, fit the geoid and topography better. Most of the long-wavelength contribution that helps to improve the fit comes from the oceanic lithosphere. The signature of continental lithosphere worsens the fit, even in simulations that assume an extremely viscous lithosphere. Therefore, a less depleted, and thus less buoyant, continental lithosphere is required to explain gravity data. None of the seismic tomography models we analyse is able to reproduce accurately the thermal structure of the oceanic lithosphere. All of them show their lowest seismic velocities at similar to 100 km depth beneath mid-oceanic ridges and have much higher velocities at shallower depths compared to what is predicted with standard cooling models. Despite the limited resolution of global seismic models, this seems to suggest the presence of an additional compositional complexity in the lithosphere.

We have measured the effects of the coupled substitution of Fe3 + and Al on the density and compressibility of mantle silicate perovskite (Pv) up to 95 GPa. X-ray emission spectroscopy and synchrotron Mossbauer spectroscopy reveal a rapid increase in the population of low-spin Fe3 + in Fe3 +, Al-bearing Pv over a narrow pressure range near 70 GPa, which is in sharp contrast with Al-free Fe3 +-bearing Pv, where Fe3 + undergoes a gradual spin transition, and with Al-free Fe2 +-bearing Pv, where Fe2 + does not become low spin. At low pressure, Fe3 + and Al expand the perovskite lattice. However, near the pressure range of the abrupt increase in the low-spin population, the unit-cell volume of Fe3 +, Al-bearing Pv becomes similar to that of Mg-endmember Pv, while those of Al-free Fe3 +-bearing Pv and Al-free Fe2 +-bearing Pv remain larger throughout the lower mantle. Consequently, Pv in Al-rich systems should have lower density in the shallow lower mantle but similar or greater density than Pv in pyrolite in the deep lower mantle, affecting the buoyancy and mechanical stability of heterogeneities. Although the Fe3 + spin transition in Pv is unlikely to cause a seismic discontinuity at mantle temperatures, it may result in a large change in bulk sound speed at 1200–1800 km depth, such that a vertically extending structure with an elevated amount of Fe3 + would generate slower and faster anomalies above and below the depth of the spin transition, respectively, relative to the surrounding mantle. This may have important implications for bulk sound speed anomalies observed at similar depths in seismic tomography studies.

We precisely compared phase boundaries of post-spinel transition in pyrolite and Mg2SiO4 and of akimotoite–perovskite transition in MgSiO3 at 21–28 GPa 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 0.3 GPa and 100 °C, respectively. The post-spinel transition boundary in Mg2SiO4 is located at higher pressure by about 0.8 GPa than the akimotoite–perovskite transition boundary in MgSiO3. Both the transition boundaries have the same slope of − 0.002 GPa/°C. In pyrolite, the post-spinel transition occurs in a pressure interval within 0.4 GPa at lower pressure by about 0.2–1.0 GPa than that in Mg2SiO4 at 1400–1800 °C. The Clapeyron slope of the post-spinel transition boundary in pyrolite is − 0.001 GPa/°C, which is half of − 0.002 GPa/°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 + magnesiowustite at pressure below the post-spinel transition, and abundances of garnet and magnesiowustite increase with increasing temperature.

A comparative study of the equation of states of hydrous (0.4 wt% H2O) and anhydrous San Carlos olivine (

The calculated bulk attenuation due to phase transition from mineral physics data is reported here. With relaxation time less than 1 s, the calculated value for a pyrolite mantle is consistent with the inverted bulk attenuation of the upper mantle from seismic observations. The two important mechanisms of phase transitions, diffusion-controlled and kinetics-controlled, have different relaxation time as indicated by the models here. The diffusion controlled is more likely to contribute to the observed seismic bulk attenuation than the kinetic-controlled process based on the available diffusivity and kinetics data. The correlation between the bulk attenuation and relaxation time emphasizes the importance of a number of parameters in the mineral physics database such as Fe–Mg diffusivity and kinetics in olivine–wadsleyite–ringwoodite–perovskite, Mg–Ca–Al–Si diffusivity and kinetics in pyroxene–garnet–Ca perovskite, some of which are still unknown.

Minerals of various mantle-derived xenoliths from the Hannuoba basalt in Hebei Province have been studied by means of IR spectroscopy. The results show that all xenoliths from the mantle at depths

The electrical conductivities of natural pyrolitic mantle and MORB materials were measured at high pressure and temperature covering the entire lower mantle conditions up to 133 GPa and 2650 K. In contrast to the previous laboratory-based models, our data demonstrate that the conductivity of pyrolite does not increase monotonically but varies dramatically with depth in the lower mantle; it drops due to high-spin to low-spin transition of iron in both perovskite and ferropericlase in the mid-lower mantle and increases sharply across the perovskite to post-perovskite phase transition at the D″ layer. We also found that the MORB exhibits much higher conductivity than pyrolite. The depth–conductivity profile measured for pyrolite does not match the geomagnetic field data below about 1500-km depth, possibly suggesting the existence of large quantities of subducted MORB crust in the deep lower mantle. The observations of geomagnetic jerks suggest that the electrical conductivity may be laterally heterogeneous in the lowermost mantle with high anomaly underneath Africa and the Pacific, the same regions as large low shear-wave velocity provinces. Such conductivity and shear-wave speed anomalies are also possibly caused by the deep subduction and accumulation of dense MORB crust above the core–mantle boundary.

We investigate the bulk density variations of some representative compositions for the lower mantle based on the pressure–volume–temperature equation of state of the constituent mineral phases. The density variations of pyrolite, harzburgite, mid-ocean ridge basalt (MORB), tonalite–trondhjemite–granodiorite (TTG), and anorthosite are studied at a temperature of 300 K and at lower mantle pressures. The density of MORB is greater than that of pyrolite throughout the lower mantle, while the density of harzburgite is slightly lower than that of pyrolite. The density of anorthosite is comparable to that of pyrolite in the lower mantle in general, and greater in the lowermost mantle, while the density of TTG is lower than pyrolite throughout the lower mantle. The above results have important implications for the fate of primordial continents, TTG and anorthosite crust. While subducted TTG might be stagnant in the mantle transition zone, dense subducted anorthositic crust could be expected to sink to the core–mantle boundary (CMB) and thus might be a major component of the D" layer immediately above the CMB. Thus, we propose that significant bodies of continental material could be present in the mantle in the transition zone and immediately above the CMB, in addition to the continents on the Earth's surface.

A new method of reconstruction of the temperature profile in the lunar mantle from the velocities of seismic P- and S-waves for different models of chemical composition is developed. The procedure of the solution of an inverse problem is realized with the help of the minimization of the Gibbs free energy and the equations of state of a mantle substance, taking into account phase transformations, anharmonicity, and the effects of inelasticity. The geophysical and geochemical constraints on composition and temperature distribution in Moon’s mantle are established. The upper mantle can be composed of olivine pyroxenite, depleted by low-volatile oxides (∼2 wt % of CaO and Al2O3). On the contrary, the lower mantle must be enriched by low-volatile oxides (∼4–6 wt % of CaO and Al2O3). Its composition can be represented by a mineral association of the olivine + clinopyroxene + garnet or olivine + orthopyroxene + clinopyroxene + garnet type, which is close in composition to pyrolite. The temperature distribution at depths 50–1000 km are approximated by the equation: T(°C) = 351 + 1718[1–exp (−0.00082H)]. The constraints inferred make it possible to conclude that the published values of the velocities of P- and S-waves for the lunar mantle, obtained by processing the data of seismic experiments of the Apollo lunar mission are inconsistent with each other at depths below 300 km. Otherwise, the variations in the velocities of P- and S-waves disturb the symmetry between the petrological model (composition), the temperature profile, and the seismic profile.

Density of silicate melts in the Earth's deep interior is important to understand the material circulations in the mantle. We study the role of chemical composition on melt density based on the ideal-mixing model and the third-order Birch–Murnaghan equation of state (EOS). Using this EOS, the role of composition can be understood through its effect on three EOS parameters: room-pressure density, room-pressure bulk modulus, and the pressure derivative of bulk modulus at room pressure. The dependences of these parameters on compositional variables such as Mg# (molar MgO/(MgO + FeO) × 100) of the melt, SiO2 content in the dry part of the melt, and H2O content in the melt are estimated from available experimental data in the literature using the ideal-mixing model. Results show that H2O content and Mg# of the melt are the most important factors that control the melt density at high pressure. Calculated densities of silicate melts are compared to the density of the ambient upper mantle from the PREM model near 410 km depth where dehydration-induced melting may occur. The relative density between the melt and the surrounding solid minerals is sensitive to the water content, Mg# and temperature at the conditions of melting. For an Mg# that is consistent with the pyrolite model, the conditions for density crossovers are marginally satisfied for a system when water (hydrogen) is the only volatile component to promote melting.

P waves from earthquakes south of Taiwan, recorded by the BATS seismic array and CWB seismic network, were used define the P wavespeed structure between depths of 100 and 800 km below the Philippines region. The presence of a low wavespeed zone in the upper mantle is inferred, although the details are unclear. Wavespeeds in the uppermost mantle are low, as expected for seismic energy propagating within an oceanic plate. The estimated depths of the 410- and 660-km discontinuities are 325 and 676 km respectively. The unusually shallow depth of the upper discontinuity below and to the east of Luzon is inferred by clearly resolving the travel-time branch produced by refraction through the transition zone. A possible explanation for the northern part of the region covered is that seismic energy reaches its maximum depth within or close to the cool, subducted oceanic South China Sea slab where subduction has been slow and relatively recent. Further south, however, the presence of a broken remnant of the South China Sea slab, formed during a period of shallower subduction, is suggested at depths below 300 km to explain the broad extent of the elevated 410-km discontinuity. The 660-km discontinuity is slightly deeper than usual, implying that low temperatures persist to lower mantle depths. The wavespeed gradients within the transition zone between depths of 450 and 610 km are higher than those predicted by both the pyrolite and piclogite models of the mantle, possibly due to the presence of water in the transition zone.