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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.

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 (

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 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.

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.

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.

Elasticity of single-crystal San Carlos olivine has been derived from sound velocity and density measurements at simultaneous high pressure–temperature conditions up to 20 GPa and 900 K using in situ Brillouin spectroscopy and single-crystal X-ray diffraction in externally-heated diamond anvil cells. These experimental results are used to evaluate the combined effect of pressure and temperature on full elastic constants of single-crystal olivine to better understand its velocity profiles and anisotropies in the deep mantle. Analysis of the results shows that the shear moduli display strong concave behaviors as a function of pressure at a given high temperature, while the longitudinal modulus, C 11 , and the off-diagonal moduli, C 12 and C 13 , exhibit greater temperature dependence at higher pressures than at relatively lower pressures. Using a finite-strain theory and thermal equation of state modeling for a pyrolitic mantle composition along an expected mantle geotherm, our results show that the magnitude of the V P and V S jumps at the 410-km depth are 6% and 6.4%, respectively, which are greater than that found in seismic observations, suggesting a mantle olivine content of 40–50 vol%, which is less than what is expected for the pyrolite model. Our modeled velocity profiles for a metastable olivine wedge in the subduction slabs along a representative cold slab geotherm are 6% and 10% lower than those of wadsleyite and ringwoodite, respectively, at corresponding depths of the normal mantle. Our modeled results also show that metastable olivine in the cold slabs could have strong V P and V S anisotropies. The maximum V P anisotropy is estimated to be 19–22% at transition zone depth, whereas the maximum V S splitting is 13–23% and increases with depth. As a result, the presence of a metastable olivine wedge at the transition zone depth would exhibit a seismic signature of low velocity and strong seismic anisotropy which are consistent with recent seismic observations for various locations of the slabs and can be used as mineral physics constraints for future seismic detections of the metastable olivine wedges in the deep mantle.

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.

Knowledge of mineralogy and petrology of unattainable lower mantle material is usually founded on high-pressure experiments with pyrolite (‘ in situ ’ material) and oceanic MORB basalt (subducted material). Primary inclusions in transition zone and lower-mantle ‘super-deep’ diamonds represent heterogeneous fragments of diamond-parental medium (not the unaltered lower mantle material). Inclusions of magnesiowustite and stishovite intergrowths (‘stishovite paradox’) give experimentally-supported evidence that stishovite, similarly to magnesiowustite, is not subducted but in situ lower mantle mineral. Primary Ca-, Mg-, Na-carbonate inclusions are symptomatic for multicomponent carbonatite (carbonate-oxide-silicate) parental melts for the lower-mantle diamonds and inclusions. We investigated melting phase relations of simple carbonates of Ca, Mg, Na and multicomponent Mg-Fe-Na-carbonate up to 60 GPa and 3500–4000 K (using multianvil press and diamond-anvil cell with laser heating) and determined a congruent melting of the carbonates and stability of PT -extended phase fields of the carbonate melts. ‘Super-deep’ diamonds are experimentally crystallized in melts of the lower mantle diamond-parental carbonate - magnesiowustite – Mg-perovskite – carbon system. Based on experimental and mineralogical evidence for the lower mantle diamonds inclusions, genetic links between diamonds and inclusions are determined and a generalized composition diagram of parental media for lower mantle diamonds and inclusions is constructed.

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.

Brillouin scattering spectroscopy was applied to investigate the effect of calcium on the single-crystal elasticity of majorite garnet, Mj42Py50.5Gr7.5, up to 25 GPa, spanning the conditions of the mantle transition zone. The adiabatic bulk and shear moduli, and their pressure derivatives are Ks = 159(2) GPa, μ = 87(1) GPa, KS′ = 4.3(2) and μ′ = 1.2(1). Calcium substitution has a small but resolvable effect on the elasticity of the majorite-pyrope join and lowers both Ks and μ by 6 and 3% respectively compared to the calcium-free counterparts. The elasticity results, along with literature data, were employed to compute shear seismic gradients associated to the exsolution of CaSiO3 perovskite (CaPv) in pyrolite and MORB below 520 km depth. The results show that the exsolution of 18 vol% of CaPv is required to produce gradients comparable to PREM at the bottom of the transition zone, although these amounts are compatible with calcium contents in MORB but too high for pyrolite mineralogy. We conclude that the steep shear seismic gradients in the transition zone are compatible with progressive calcium enrichment likely supplied by subducted oceanic crust accumulated at the bottom of the transition zone by stagnating slabs.

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.

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''., Comment: 26 pages, 6 figures; submitted to journal 8 June 2015

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 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.

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.

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.

Electrical conductivities of majorite garnet with compositions of pyrolite minus olivine (pyrolite majorite) and mid-ocean ridge basalt (MORB majorite) were measured under physical conditions of the mantle transition zone (18 and 23 GPa and temperatures up to 2000 K) in a Kawai-type multi-anvil apparatus. The samples with MORB composition are mainly composed of majorite, which has higher Fe and Al contents, and contain a small amount of stishovite. The conductivity of the MORB majorite is more than twice higher than those of the pyrolite majorite at the same temperature. The activation energies of these majorites are both 1.4 eV at temperature of 1000–1600 K suggesting that the dominant mechanism of charge transportation is Fe 2+ –Fe 3+ hopping (small polaron) conduction. At higher temperatures (>1600 K), corresponding to temperature conditions of the transition zone, conduction mechanism of the pyrolite majorite would change from small polaron to ionic conduction. The pyrolite majorite has only slightly higher and lower conductivity than dry wadsleyite and ringwoodite, respectively, and will not largely change the conductivity-depth profile predicted for the dry mantle transition zone. The laboratory-based conductivity profile of the mantle transition zone with pyrolitic composition can explain well the current semi-global conductivity-depth profile obtained from electromagnetic study beneath Pacific. On the other hand, the garnetite originating from the oceanic crust has remarkably higher conductivity than the surrounding mantle because the conductivity of MORB majorite is significantly higher than those of wadsleyite and ringwoodite. Conductivity values of MORB majorite agree with those of the stagnant slab beneath the northeastern China.

ARTICLE I NFO Phase relations in α-β-γ Mg2SiO4 have been investigated by first principles quasi-harmonic free energy computations. The computed phase boundaries obtained using the local density approximation (LDA) and the generalizedgradientapproximation(GGA)brackettheexperimentalones,withLDA(GGA)calculationsgivingthe lowest (highest) bound, while the Clapeyron slopes are in good agreement with the experimentally determined ones. This is the same trend displayed by previous similar computations. Further analyses reveal that despite the uncertaintiesinphaseboundary determination, thecalculated discontinuitiesindensity, bulk modulus, and bulk sound velocity are quite insensitive to pressure and have small uncertainties and useful accuracy to discriminate potential sources of discontinuities in the mantle. We verify that ∼3% density discontinuity at 410-km depth can be produced primarily by the α to β transition in an aggregate with pyrolite composition, i.e. ∼60 vol.% of Mg2SiO4. However, the 1.3-2.9% density discontinuity observed in some places at 520-km depth cannot be accounted for solely by the β to γ transition but also requires changes in the coexisting pyroxene/garnet/Ca- perovskite system.

The composition of the mantle transition region, characterized by anomalous seismic-wave velocity and density changes at depths of approximately 400 to 700 km, has remained controversial. Some have proposed that the mantle transition region has an olivine-rich 'pyrolite' composition, whereas others have inferred that it is characterized by pyroxene- and garnet-rich compositions ('piclogite'), because the sound velocities in pyrolite estimated from laboratory data are substantially higher than those seismologically observed. Although the velocities of the olivine polymorphs at these pressures (wadsleyite and ringwoodite) have been well documented, those of majorite (another significant high-pressure phase in the mantle transition region) with realistic mantle compositions have never been measured. Here we use combined in situ X-ray and ultrasonic measurements under the pressure and temperature conditions of the mantle transition region to show that majorite in a pyrolite composition has sound velocities substantially lower than those of earlier estimates, owing to strong nonlinear decreases at high temperature, particularly for shear-wave velocity. We found that pyrolite yields seismic velocities more consistent with typical seismological models than those of piclogite in the upper to middle parts of the region, except for the potentially larger velocity jumps in pyrolite relative to those observed at a depth of 410 km. In contrast, both of these compositions lead to significantly low shear-wave velocities in the lower part of the region, suggesting possible subadiabatic temperatures or the existence of a layer of harzburgite-rich material supplied by the subducted slabs stagnant at these depths.

Imposing a thermal and compositional significance to the outcome of the inversion of seismic data facilitates their interpretation. Using long-period seismic waveforms and an inversion approach that includes constraints from mineral physics, we find that lateral variations of temperature can explain a large part of the data in the upper mantle. The additional compositional signature of cratons emerges in the global model as well. Above 300 km, we obtain seismic geotherms that span the range of expected temperatures in various tectonic regions. Absolute velocities and gradients with depth are well constrained by the seismic data throughout the upper mantle, except near discontinuities. The seismic data are consistent with a slower transition zone and an overall faster shallow upper mantle, which is not compatible with a homogenous dry pyrolite composition. A gradual enrichment with depth in a garnet-rich component helps to reduce the observed discrepancies. A hydrated transition zone would help to lower the velocities in the transition zone, but it does not explain the seismic structure above it. © 2007 by The National Academy of Sciences of the USA.

The stagnation of ~1000-km deep slabs indicates that dense basalt may be more abundant in the lower mantle than in the upper mantle., Improved constraints on lower-mantle composition are fundamental to understand the accretion, differentiation, and thermochemical evolution of our planet. Cosmochemical arguments indicate that lower-mantle rocks may be enriched in Si relative to upper-mantle pyrolite, whereas seismic tomography images suggest whole-mantle convection and hence appear to imply efficient mantle mixing. This study reconciles cosmochemical and geophysical constraints using the stagnation of some slab segments at ~1000-km depth as the key observation. Through numerical modeling of subduction, we show that lower-mantle enrichment in intrinsically dense basaltic lithologies can render slabs neutrally buoyant in the uppermost lower mantle. Slab stagnation (at depths of ~660 and ~1000 km) and unimpeded slab sinking to great depths can coexist if the basalt fraction is ~8% higher in the lower mantle than in the upper mantle, equivalent to a lower-mantle Mg/Si of ~1.18. Global-scale geodynamic models demonstrate that such a moderate compositional gradient across the mantle can persist can in the presence of whole-mantle convection.

The Earth's upper mantle extends from below the crust to a depth of about 410 km. Here, we review the mineralogical and chemical composition of the upper mantle, especially with respect to the observed heterogeneities, both found in natural mantle rocks and in experiments. We focus on subsolidus phase transitions determined using experimental techniques. Although most upper mantle rocks can be assumed to be peridotite mostly made of olivine and pyroxene, other minerals, including oxides and hydrates, should be taken into account, especially when they participate in transitions or chemical reactions that may affect the physical properties of the mantle rocks.

The fate and amount of granitic materials subducted into the deep mantle are still under debate. The density , elastic property, and phase stability of granitic materials in the mantle pressures are key to clarifying them. Here, we modeled high-pressure properties of the granitic assemblage by using ab initio mineral physics data of grossular garnet, K-hollandite, jadeite, stishovite, and calcium ferrite (CF)-type phase. We find that the ongoing subducting granitic assemblage, such as sediments and average upper crust rocks, is much denser than pyrolite in the pressure range from 9 GPa (~270 km), at which coesite undergoes a phase transition to stishovite, to around 27 GPa (~740 km). Above this pressure, granitic material becomes less dense than a pyrolite. This indicates that the granitic assemblage becomes gravitationally stable at the base of the mantle transition zone (MTZ). Results suggest a possibility that the granitic materials could accumulate around the 740 km depth if carried into the depth deeper than 270 km and segregated at some depth. Comparison of the velocities between granitic and pyrolitic materials shows that granitic materials can produce substantial velocity anomalies in the MTZ and the uppermost lower mantle (LM). Seismic observations such as anomalously fast velocities, especially for the shear wave, around the 660-km discontinuity, the complexity of 660-km discontinuity, and the scatterers in the uppermost LM could be associated with the subducted granitic materials.

Geotherm and water content profiles in the upper mantle above the stagnant slab of the Pacific back-arc were estimated from the electrical conductivity and seismic P-wave velocity ( V p ) structures. The geothermal profiles were determined by using the electrical conductivity and seismic V p structures, which, assuming a dry hartzburgite or a dry pyrolite composition, are designated as electrical and seismic geotherms, respectively. In a deeper part of the upper mantle, neither the dry pyrolite nor the dry harzburgite condition provides consistent electrical and seismic geotherms. This discrepancy can be explained by allowing for a small amount of water (500–1000 ppm H/Si) with the seismic geotherm. In a shallower part of the upper mantle, the electrical and seismic geotherms are consistent with each other within 1500–1700 ° C under the dry harzbutgite condition, whereas they are inconsistent by more than 100 ° C under the dry pyrolite condition. Alternatively, the wet pyrolite condition applied to the deeper part of the upper mantle also satisfies the electical conductivity and seismic V p structures in the shallower part.

A method is proposed for determining the temperature of the Earth’s upper mantle from geochemical and seismic data. The data are made consistent by physicochemical simulations, which enable one to derive physical characteristics from geochemical compositional models (direct problem) and to convert seismic velocity profiles into model for the temperature distribution (inverse problem). The methods were used to simulate temperature distribution profiles in the “normal” and “cold” mantle on the basis of profiles for the velocities of P and S waves in the IASP91 model and regional models for the Kaapvaal craton. The constraints assumed for the chemical composition included the depleted material of garnet peridotites and the fertile primitive mantle. The conversion of seismic into thermal profiles was conducted by minimizing the Gibbs free energy with the use of equations of state for the mantle material with regard for anharmonicity and the effects of inelasticity. The sensitivity of the model to the chemical composition and its importance in application to the solution of inverse problems is demonstrated. Temperature profiles derived from the IASP91 and some regional models for depths of 200–210 km display an inflection on geotherms toward decreasing temperatures, which is physically senseless. This anomaly cannot be related to either the presence of volatiles or the occurrence of partial melting, because both of them should have resulted in a decrease, but not an increase, in the seismic velocities. Temperature inversion can be ruled out by the gradual fertilization of the mantle with depth. In this situation, the upper mantle material at depths of 200–300 km should be enriched in FeO, Al2O3, and CaO relative to garnet peridotites and be simultaneously depleted in these oxides relative to the pyrolite material of the primitive mantle. It can be generally concluded that both the lithosphere and sublithospheric mantle of the Kaapvaal craton, as well as the normal mantle, should be chemically stratified.

In this paper, the simplest average physical model of a mantle convecting as a whole (i.e., following an adiabatic temperature gradient) with a single composition (pyrolite with phase transitions) is tested directly against global seismic data, instead of against spherically symmetric seismic models. Constraints from seismic data on average velocities and lower mantle velocity gradients are hard to reconcile with an adiabatic pyrolitic mantle, given the current state of knowledge of elastic and anelastic mineral parameters at high pressure and temperature. This physical model generally gives (a) a stronger baseline offset between upper and lower mantle average travel-time residuals than allowed by the data and (b) an insufficient decrease in velocity with depth in the lower mantle (above 2500 km). We tested 105 upper and 105 lower mantle models that were selected randomly within the mineral parameter uncertainties. Only 2 lower mantle models and 24 upper mantle models yield whole mantle seismic structures that are compatible with global ISC P and S travel times and central frequencies of toroidal and spheroidal fundamental modes with angular order higher than 18. To improve the fit to the seismic data, the physical model would require (a) a lower velocity transition zone composition than dry pyrolite (at least around continents and subduction zones) as well as (b) a gradual change in physical state of the lower mantle that decreases the velocity-depth gradient, e.g., a superadiabatic temperature gradient. © 2005 Elsevier B.V. All rights reserved.

Numerical simulations of thermochemical mantle convection are used to investigate the influence of multicomponent phase changes on mass transport between the upper and lower mantle. The model includes both olivine and pyroxene–garnet components based on a pyrolite composition. The simulations reveal large and focused upwellings through the 660-km phase boundary in the vicinity of subducting slabs. The upwellings are composed of depleted harzbergitic material, which contributes to the depletion of the upper mantle and may generate distinct reservoirs of trace elements in the upper and lower mantle as well. The position and composition of the upwelling is attributed to the development of compositional stratification at the top of the lower mantle, where dense harzbergitic material underlies enriched basaltic material due to a density inversion in the combine phase system. Subducting slabs disrupt the stratification in the vicinity of subduction, permitting a counter flow of harzbergitic material into the upper mantle. This behavior is not observed in a pure olivine phase system, suggesting that realistic mantle compositions are needed to assess the generation of distinct reservoirs in mantle convection simulations. Phase transitions in a multicomponent system provide a plausible mechanism for explaining the formation of a depleted reservoir in the upper mantle without implementing dehydration-induced melting and differentiation in the transition zone.

In situ synchrotron X-ray diffraction experiments were conducted using the SPEED-1500 multi-anvil press at SPring-8 on majoritic garnet synthesized from natural mid-ocean ridge basalt (MORB), whose chemical composition is close to the average of oceanic crust, at 19 GPa and 2200 K. Pressure–volume–temperature data were collected using a newly developed high-pressure cell assembly to 21 GPa and 1273 K. Data were fit to the high-temperature Birch-Murnaghan equation of state, with fixed values for the ambient cell volume (V0 = 1574.14(4) A3) and the pressure derivative of the isothermal bulk modulus (K′TK′T = 4). This yielded an isothermal bulk modulus of KT0 = 173(1) GPa, a temperature derivative of the bulk modulus (∂KT/∂T)P = −0.022(5) GPa K−1, and a volumetric coefficient of thermal expansivity α = a + bT with values of a = 2.0(3) × 10−5 K−1 and b = 1.0(5) × 10−8 K−2. The derived thermoelastic parameters are very similar to those of pyrope. The density of subducted oceanic crust compared to pyrolitic mantle at the conditions in Earth's transition zone (410–660 km depth) was calculated using these results and previously reported thermoelastic parameters for MORB and pyrolite mineral assembledges. These calculations show that oceanic crust is denser than pyrolitic mantle throughout the mantle transition zone along a normal geotherm, and the density difference is insensitive to temperature at the pressures in lower part of the transition zone.

To help determine the chemical composition of the Earth's mantle, we characterised the high-pressure mineral assemblage of an undepleted natural peridotite—thought to be representative of the Earth's upper-mantle—to 107 GPa using high-resolution X-ray diffraction. At lower-mantle conditions, the peridotite transforms to the assemblage 76 (±2)% (Mg0.88Fe0.052+Fe0.013+Al0.12Si0.94)O3 orthorhombic perovskite by volume (at zero pressure), 17 (±2)% (Mg0.80Fe0.20)O magnesiowustite and 7 (±1)% CaSiO3 perovskite. The measured room-temperature bulk properties of this high-pressure assemblage, together with a range of estimates of thermal properties of the constituent minerals, appear to be inconsistent with seismological constraints on the density and bulk modulus of the lower mantle. Our results suggest that the lower mantle differs in bulk composition (e.g., richer in iron, Mg#∼0.85) from current estimates for the upper mantle, requiring some amount of segregation between the upper and lower mantle over geological history.

The thermal expansions of wadsleyite, ringwoodite, hydrous wadsleyite, and hydrous ringwoodite have been determined by high temperature powder X-ray diffraction at ambient pressure. Wadsleyite and ringwoodite persist up to 700 °C, and their thermal expansion coefficients are measured as 34.0(5)×10 −6 and 30.7(6)×10 −6 K −1 , respectively, values that are slightly larger than those measured by [J. Phys. Earth 27 (1979) 53; J. Phys. Earth 28 (1980) 273]. Hydrous wadsleyite and hydrous ringwoodite were dehydrated at 450 and 400 °C, respectively. Below their dehydration temperatures, their thermal expansion coefficients are 30.1(14)×10 −6 and 27.3(9)×10 −6 K −1 , respectively, which are slightly lower than those of their anhydrous forms. Using the present results combined with the previous thermoelastic data, the seismic velocity contrast at the 410 km discontinuity has been calculated for both dry and wet mantle. If a pyrolite composition is assumed, a hydrous mantle transition zone better explains seismic constraints.

In situ synchrotron X-ray diffraction (XRD) experiments were conducted using the SPEED-1500 multi-anvil press of SPring-8 on (Mg0.91Fe0.09)2SiO4 ringwoodite (Rw), whose composition is similar to that expected in the Earth’s mantle transition zone. Pressure–volume–temperature data were collected using a NaCl or MgO capsule up to 21 GPa and 1273 K. A fit to high-temperature Birch-Murnaghan (HTBM) equation of state (EOS) with fixed values of ambient cell volume V0=527.83(7) A3 and isothermal bulk modulus KT0=187 GPa yielded a pressure derivative of isothermal bulk modulus KT′=4.41(1), a temperature derivative of bulk modulus (∂KT/∂T)P = −0.028(5) GPa K−1, and a volumetric thermal expansivity α=a+bT with values of a=1.9(2)×10−5 K−1 and b=1.2(4)×10−8 K−2. These properties are consistent with the analysis using the thermal pressure (Th-P) EOS. The derived KT′ and (∂KT/∂T)P are consistent with previous studies on Mg2SiO4 ringwoodite. However, consistent with measurements at 0 GPa, the present equation of state yields significantly higher thermal expansivity than derived by diamond anvil experiments on Mg2SiO4 ringwoodite to 30 GPa and 700 K. On the basis of the equation of state, the density jump around 660 km depth expected for pyrolitic homogeneous mantle (caused by ringwoodite → Mg-perovskite (MgPv) + magnesiowustite (Mw) and garnet (GRT)→MgPv transitions) was estimated. The density jump calculated for pyrolite (9.2%) is significantly larger than that across the 660 km discontinuity derived by recent seismological studies (4–6%). One possible explanation for this is that the recent seismic data reflect only the sharp transition concerned with the Rw→MgPv+Mw transition.

Compressional and shear wave velocities of ringwoodite (γ-Mg 2 SiO 4 ) were measured on a polycrystalline specimen to 12 GPa at room temperature using ultrasonic interferometry techniques. Velocity measurements at ambient conditions yielded V p = 9.86(3) and V s = 5.78(2) km/s. Finite strain analysis of the high pressure velocity data yields K = 185(2) GPa, G = 120(1) GPa, K ’ = 4.5(2), and G ’ = 1.5(1) for the elastic moduli and their pressure derivatives, respectively. The velocities and elastic moduli at ambient conditions are indistinguishable from aggregate properties of polycrystalline ringwoodite calculated using single crystal elastic constants ( C ij ). Comparison of the current results on pressure derivatives of bulk and shear moduli with previous acoustic data on iron free ringwoodite to 3 GPa and iron bearing sample to 16 GPa indicated that the discrepancy can be explained by the pressure range of the experiments rather than iron content. Current results suggest that ringwoodite and wadsleyite posses very similar pressure dependence in elastic properties to the transition zone depth. In a pyrolite mantle with 1400 °C adiabatic foot temperature, the wadsleyite to ringwoodite phase transition is characterized as a seismic reflector spreading about 20 km in width with a density jump of 2.1% and impedance jumps of 2.4% and 3.1% for P and S waves near 520 km depth, which is consistent with seismic observations in long period data.

The water storage capacity of the major constituent of the lower mantle, Mg-perovskite, is a matter of debate. Here we report water solubility of Mg-perovskites with different compositions observed in peridotite and MORB systems. IR spectra of pure MgSiO3-perovskite show bands at 3397, 3423, 3448, and 3482 cm−1 and suggest about 100 ppm H2O. The H2O content in Al-Mg-perovskite (4–7 wt% Al2O3; Mg#=100) is 1000–1500 ppm (major band at 3448 cm−1), whereas Al-Fe-Mg-perovskite in MORB (Al2O3=13–17 wt%; Mg#=58–61) contains 40–110 ppm H2O (major band at 3397 cm−1). The H2O content in Al-Fe-Mg-perovskite observed in peridotite (Al2O3=5–6 wt%; Mg#=88–90) is 1400–1800 ppm (major band at 3397 cm−1). Al-Fe-Mg-perovskite from the MORB system has a high Fe3+ content, Fe3+/∑Fe=0.6, determined by electron energy loss spectroscopy measurements. Water can enter into the perovskite structure with oxygen vacancies originating from the substitution of Si by Al and Fe3+. Oxygen vacancy incorporation is favored for aluminous perovskite synthesized from the MgO-rich peridotite system. The substitution of Si4++Mg2+=2(Al,Fe)3+ prevails however in the Al-Fe-Mg-perovskite from the MORB system (MgO-poor, Al- and Fe-rich), explaining its restricted water solubility. The maximum amount of water stored in the lower mantle is estimated to be 3.42×1021 kg, which is 2.5 times the present ocean mass. Comparison of the phase relations in hydrous pyrolite and hydrous MORB indicates that pyrolite is more important as water container and water carrier in the mantle. Pyrolite contains: (1) dense hydrous magnesium silicates, existing under conditions of subducting slabs, and (2) hydrous wadsleyite, hydrous ringwoodite and water-bearing perovskite under the normal mantle and hotter conditions. Distribution of water to the MORB is restricted at the conditions of the transition zone and lower mantle.

We carried out a series of melting experiments with hydrous primitive mantle compositions to determine the stability of dense hydrous phases under high pressures. Phase relations in the CaO–MgO–Al2O3–SiO2 pyrolite with ˜2 wt% of water have been determined in the pressure range of 10–25 GPa and in the temperature range between 800 and 1400 °C. We have found that phase E coexisting with olivine is stable at 10–12 GPa and below 1050 °C. Phase E coexisting with wadsleyite is stable at 14–16 GPa and below 900 °C. A superhydrous phase B is stable in pyrolite below 1100 °C at 18.5 GPa and below 1300 °C at 25 GPa. No hydrous phases other than wadsleyite are stable in pyrolite at 14–17 GPa and 900–1100 °C, suggesting a gap in the stability of dense hydrous magnesium silicates (DHMS). We detected an expansion in the stability field of wadsleyite to lower pressures (12 GPa and 1000 °C). The H2O content of wadsleyite was found to decrease not only with increasing temperature but also with increasing pressure. The DHMS phases could exist in a pyrolitic composition only under the conditions present in the subducting slabs descending into the lower mantle. Under the normal mantle and hot plume conditions, wadsleyite and ringwoodite are the major H2O-bearing phases. The top of the transition zone could be enriched in H2O in accordance with the observed increase in water solubility in wadsleyite with decreasing pressure. As a consequence of the thermal equilibration between the subducting slabs and the ambient mantle, the uppermost lower mantle could be an important zone of dehydration, providing fluid for the rising plumes.

Phase relations and melt compositions in the CaO–MgO–Al 2 O 3 –SiO 2 –pyrolite under hydrous (2% of H 2 O) and anhydrous conditions have been determined at 10–25 GPa and temperatures from 1400 to 2400 °C. At the pressure of 16 and 25 GPa the apparent solidus temperatures for the hydrous system are about 200 and 100 °C lower than the solidus temperatures for the dry system, respectively. The apparent solidus temperature changes drastically from 1600 °C at 13.5 GPa to about 1850 °C at 16 GPa. Majorite is a liquidus phase in the hydrous pyrolite from 10 to 25 GPa. The liquidus phases in the dry pyrolite are olivine at 10–14 GPa, majorite at 14–20 GPa, and periclase at pressures above 20 GPa. We observed expansion of the stability field of anhydrous phase B and its complex relations with the other phases in the CMAS–pyrolite system. We suppose it is formed not only by incongruent melting of wadsleyite, but also by the subsolidus reaction of wadsleyite and majorite in the dry condition. Compositions of partial melts formed by low degree of melting (10–20%) have Al 2 O 3 -depleted and CaO-rich compositions up to 22 GPa and enriched in Al 2 O 3 (5–7 wt.%) and CaO at 25 GPa. At 10 GPa, dry and hydrous melts formed by low degree of melting have high SiO 2 (48–51 wt.%) and relatively high Al 2 O 3 (3.5–5.2 wt.%). Their compositions are generally consistent with those of aluminum-depleted komatiite. At 13–22 GPa, dry and hydrous melts have low SiO 2 (44–49 wt.%) and Al 2 O 3 (1.7–3.0 wt.%) and high CaO contents (9–15 wt.%). An essential depression of the apparent solidus temperature in the hydrous deep upper mantle compared to that in the transition zone provides the dehydration melting of the hydrous plume. The present results support a model of komatiite genesis by dehydration melting of rising wet plumes at the base of the upper mantle.

Melting Moments The boundary between Earth's core and mantle defines where the iron-rich liquid outer core meets the more chemically heterogeneous solid lower mantle and is marked by a sharp thermal gradient of nearly 1500 kelvin. The precise relationship between temperature and melting of the lowermost mantle constrains the structure and heat flow across the core-mantle boundary. In order to identify trace amounts of liquid as melting initiates, Nomura et al . (p. 522 , published online 16 January) performed x-ray microtomographic imaging of rocks of a primitive mantle composition that had been subjected to high pressures and temperatures in a diamond anvil cell. The experimentally determined maximum melting point of 3570 kelvin suggests that some phases typically thought to lose stability in the lowermost mantle, such as MgSiO 3 -rich post-perovskite, may be more widely distributed than expected.

The compressional (P) and shear wave (S) velocities for Mg2SiO4 wadsleyite have been measured to 7 GPa and 873 K using simultaneous ultrasonic interferometry and in situ X-ray diffraction techniques. From the velocity measurements we obtained the pressure and temperature derivatives for the elastic shear (G) and adiabatic bulk (KS) moduli, (∂G/∂P)T=1.5(1), (∂G/∂T)P=−0.017(1) GPa/K, KS=173(2) GPa, (∂Ks/∂P)T=4.2(1), and (∂Ks/∂T)P=−0.012(1) GPa/K; for the P and S waves, we obtained (∂Vs/∂P)T=0.021(1) (km/s)/GPa, (∂Vs/∂T)P=−0.035(2) (km/s)/K, (∂VP/∂P)T=0.065(2) (km/s)/GPa, and (∂VP/∂T)P = −0.038(2) (km/s)/K (values in parentheses are standard deviations, e.g., 1.5(1)=1.5±1). Independent equation of state analysis of P-V-T data provided an estimation of the temperature dependence for the isothermal bulk modulus of (∂KT/∂T)P=−0.022(12) GPa/K and thermal expansion (α=a+bT) coefficients of a=2×10−5 K−1 and b=2.5×10−8 K−2. Using these data along with elastic properties for other mantle phases, a velocity-depth profile for a pyrolite model to 670 km depth is constructed using a finite strain method along a 1673 K adiabat. In the transition zone the pyrolite model has a smaller gradient between 410 and 660 km than the body wave models from synthetic waveform analyses but converges with the seismic profiles at the bottom of the transition zone just above the 660-km discontinuity. The pyrolite model has velocity jumps of 6.9% and 7.9% for P and S waves, respectively, over a thickness of ∼10 km for the phase transformation from olivine to wadsleyite, which is in good agreement with short-period P wave reflection data and a recent fine structure model (C4) for both the velocity jumps and the thickness of the 410-km seismic discontinuity.

Knowledge of the mineralogical constituents of the Earth’s mantle depends heavily on high-P experiments of pyrolite composition. In order to understand the planetary interior better, phase relations and mineral physics properties of compositions other than pyrolite may also be important. High-P and high-T experiments were conducted on an Al-depleted komatiite which can be regarded as a kind of piclogite composition. Fourteen experiments were carried out along a model geotherm between 5 and 23 GPa and 1100 and 1600°C with a multi-anvil apparatus. Analyses of quenched run products showed that, in the studied komatiite, the fractions of olivine and its high-P polymorphs (wadsleyite and ringwoodite) are 40–50 vol%, pyroxene–garnet phase transition occurs at 13–17 GPa (while the respective values for pyrolite are ∼60 vol% and 12–15 GPa). The density and seismic velocity profiles of komatiite mantle were calculated using thermoelastic parameters of related minerals, and they showed quite good agreements with those of seismic observations around the transition zone as well as those for pyrolite.

There is a growing consensus that the Earth's lower mantle possesses a bulk composition broadly similar to that of the upper mantle (known as pyrolite)1,2,3. But little is known about lower-mantle mineralogy and phase chemistry4,5, especially at depth. Here we report diamond-anvil cell experiments at pressures of 70 and 135 GPa (equivalent to depths within the Earth of about 1,500 and 2,900 km, respectively) which show that pyrolite would consist solely of magnesian-silicate perovskite (MgPv), calcium-silicate perovskite (CaPv) and magnesiowustite (Mw). Contrary to recent speculation6,7, no additional phases or disproportionations were encountered and MgPv was found to be present at both pressures. Moreover, we estimate that, at ultra-high pressures where thermal expansivities are low, buoyancy forces inherent in subducted slabs because of their lithology will be of similar magnitude to those required for thermally driven upwelling. So slabs would need to be about 850 °C cooler than their surroundings if they are to sink to the base of the mantle. Furthermore, initiation of plume-like upwellings from the core–mantle boundary, long attributed to superheating, may be triggered by lithologically induced buoyancy well before thermal equilibration is attained. We estimate that ascent would commence within ∼0.5 Gyr of the slab reaching the core–mantle boundary, in which case the lowermost mantle should not be interpreted as a long-term repository for ancient slabs.

Pyrolite1 is a hypothetical mixture of distinct minerals which iswidely believed to represent the composition of the Earth's mantle. The main pressure-induced phase transformations of the olivine component of pyrolite occur at about 13.5 GPa (α to β) and 24 GPa (γ to MgSiO3-rich perovskite + magnesiowustite)2,3, which are thought to be responsible for the seismic discontinuities at 410 and 660 km depths in the mantle. Recent seismological studies, however, have demonstrated that the 410-km seismic discontinuity is sharper in some areas than that expected from the α to β transformation in mantle olivine with a fixed composition4,5,6,7. Moreover, some mineral physics studies suggest that the seismic velocity jump at the 410-km discontinuity is inconsistent with that associated with the α to β transformation in olivine8,9. Here we present a phase equilibria study of a material having pyrolite composition at pressures of 6–16 GPa. We found that the iron content in olivine changes significantly with increasing pressure, as a result of the formation of a relatively iron-rich majorite phase at these pressures. This variation in iron content can overcome, or at least reduce, both of the above difficulties encountered with the pyrolite model of mantle composition, by showing that the component mineral systems cannot be treated as separate.

The recent increasing number of experimental works leads us to review the elastic properties and mineralogical transformations of mantle minerals. The updated data set is used to compute seismic profiles for two petrological models along three adiabatic temperature profiles. These profiles are chosen to stress out the influence of non-olivine minerals on seismic parameters, and to represent cold and horizontally averaged temperature profiles of the Earth's mantle. In a first part, starting compositions of pyrolite and piclogite and a single layer convection are assumed. The results clearly point out the importance of the non-olivine part of the mineralogy. Two scenarios are found to explain the 660 km depth discontinuity, .