Search

Thermal conductivity of bridgmanite (Bdg) is the important physical property controlling the heat transfer inside the Earth. Here we report room temperature lattice thermal conductivity of (Fe,Al)-bearing Bdg with chemical compositions of Mg0.848Fe0.090Al0.206Si0.856O3 and Mg0.718Fe0.123Al0.281Si0.878O3 measured up to 125 GPa and 74 GPa, respectively, using the pulsed light heating thermoreflectance technique in a diamond anvil cell. We found that the lattice thermal conductivity of these Bdg samples show abnormal reduction in the pressure range of 20-40 GPa at 300 K, which is probably due to the spin transition of Fe3+ in octahedral Si-site (B-site). We propose that the lattice thermal conductivity of Bdg is reduced by 46 ± 16% when Fe is in the mixed spin state, which may form a thermal insulating layer in the Earth's mid lower mantle. In addition, we provide a thermal conductivity model of Bdg, taking into account the effect of compositional difference and the spin transition of Fe. Our conductivity model indicates that the thermal conductivity of Bdg in the pyrolitic lower mantle is more than twice as high as that in the descending MORB, which is likely to create heterogeneity of lateral heat flux through the Earth's core-mantle boundary.

The Earth's core is composed mainly of iron. Since the liquid core coexists with solid at the inner core boundary (ICB), the melting point of iron at 330 GPa offers a key constraint on core temperatures. However, previous results using a laser-heated diamond-anvil cell (DAC) have been largely inconsistent with each other, likely because of an intrinsic large temperature gradient and its temporal fluctuation. Here we employed an internal-resistance-heated DAC and determined the melting temperature of pure iron up to 290 GPa, for the first time above 200 GPa by static compression experiments. A small extrapolation of the present experimental results yields a melting point of 5500 ± 220 K at the ICB, higher than 4850 ± 200 K reported by previous laser-heated DAC by Boehler (1993) but is lower than 6230 ± 500 K by Anzellini et al. (2013) . Accounting for the melting temperature depression due to core-alloying elements, the upper bounds for the temperature at the ICB and the core–mantle boundary (CMB) are estimated to be 5120 ± 390 K and 3760 ± 290 K, respectively. Such low present-day CMB temperature suggests that the lowermost mantle has avoided global melting, at least since early Proterozoic Eon.

SiO2 may have been expelled from the core directly following core formation in the early stages of Earth's accretion and onward through the present day. On account of SiO2's low density with respect to both the core and the lowermost mantle, we examine the process of SiO2 accumulation at the core‐mantle boundary (CMB) and its incorporation into the mantle by buoyant rise. Today, if SiO2 is 100–10,000 times more viscous than lower mantle material, the dimensions of SiO2 diapirs formed by the viscous Rayleigh‐Taylor instability at the CMB would cause them to be swept into the mantle as inclusions of 100 m–10 km diameter. Under early Earth conditions of rapid heat loss after core formation, SiO2 diapirs of ∼1 km diameter could have risen independently of mantle flow to their level of neutral buoyancy in the mantle, trapping them there due to a combination of intrinsically high viscosity and neutral buoyancy. We examine the SiO2 yield by assuming Si + O saturation at the conditions found at the base of a magma ocean and find that for a range of conditions, dispersed bodies could reach as high as 8.5 vol % in parts of the lower mantle. At such low concentration, their effect on aggregate seismic wave speeds is within observational seismology uncertainty. However, their presence can account for small‐scale scattering in the lower mantle due to the bodies' large‐velocity contrast. We conclude that the shallow lower mantle (700–1,500 km depth) could harbor SiO2 released in early Earth times.

Bridgmanite (Bdg), iron (Fe)- and aluminum (Al)-bearing magnesium silicate perovskite is the most abundant mineral in the Earth's lower mantle. Thus, its thermal conductivity governs the lower mantle thermal conductivity that critically controls the thermo-chemical evolution of both the core and the lower mantle. While there is extensive research for the lattice thermal conductivity of MgSiO3 Bdg, the effects of Fe and Al incorporation on its lattice thermal conduction are still controversial. Here we report the lattice thermal conductivity of Mg0.832Fe0.209Al0.060Si0.916O3 Bdg measured up to 142 GPa at 300 K using the pulsed light heating thermoreflectance technique in a diamond anvil cell. The results show that the lattice thermal conductivity of Bdg is 25.5 ± 2.2 W/m/K at 135 GPa and 300 K, which is 19% lower than that of Fe and Al-free Bdg at identical conditions. Considering the temperature effect on the lattice conductivity and the contribution of radiative thermal conductivity, the total thermal conductivity of Fe and Al-bearing Bdg does not change very much with temperature at 135 GPa, and could be higher than that of post-perovskite with identical chemical composition.

The sound velocity of liquid Fe47Ni28S25 and Fe63Ni12S25 was measured up to 52 GPa/2480 K in externally-resistance-heated and laser-heated diamond-anvil cells (DACs) using high-resolution inelastic X-ray scattering. From these experimental data, we obtained the elastic parameters of liquid Fe47Ni28S25, KS0 = 96.1 ± 2.7 GPa and KS0’ = 4.00 ± 0.13, where KS0 and KS0’ are the adiabatic bulk modulus and its pressure derivative at 1 bar, when the density is fixed at ρ0 = 5.62 ± 0.09 g/cm3 for 1 bar and 2000 K. With these parameters, the sound velocity and density of liquid Fe47Ni28S25 were calculated to be 8.41 ± 0.17 km/s and 8.93 ± 0.19 to 9.10 ± 0.18 g/cm3, respectively, at the core mantle boundary (CMB) conditions of 135 GPa and 3600 − 4300 K. These values are 9.4 % higher and 17–18 % lower than those of pure Fe respectively. Extrapolation of measurements and comparison with seismological models suggest the presence of 5.8–7.5 wt.% sulfur in the Earth's outer core if it is the only light element.

Melting experiments were performed on the Fe–Fe 3 S system at high pressures between 34 and 254 GPa in a laser-heated diamond-anvil cell (DAC), using starting materials of fine-grained homogeneous mixtures of Fe and FeS ( 5.7 ( ± 0.3 ) wt.% S coexisted with both solid Fe 3 S and Fe containing 3.9 ( ± 0.4 ) wt.% S at 254 GPa and 3550 K. The eutectic liquid at inner core boundary (ICB) pressure includes less sulfur than is required to account for the density deficit of the outer core (≥10 wt.% S). Furthermore, the difference in sulfur concentration between coexisting liquid and solid is not sufficient to account for the observed density jump across the ICB. These indicate that sulfur cannot be a predominant light element in the core.

(Mg, Fe)O ferropericlase (Fp) is one of the important minerals comprising Earth's lower mantle, and its thermal conductivity could be strongly influenced by the iron content and its spin state. We examined the lattice thermal conductivity of (Mg, Fe)O Fp containing 19 mol% iron up to 111 GPa and 300 K by means of the pulsed light heating thermoreflectance technique in a diamond anvil cell. We confirmed a strong reduction in the lattice thermal conductivity of Fp due to iron substitution as reported in previous studies. Our results also show that iron spin crossover in Fp reduces its lattice thermal conductivity as well as its radiative conduction. We also measured the electrical conductivity of an identical Fp sample up to 140 GPa and 2730 K, and found that Fp remained an insulator throughout the experimental conditions, indicating the electronic thermal conduction in Fp is negligible. Because of the effects of strong iron impurity scattering and spin crossover, the total thermal conductivity of Fp at the core–mantle boundary conditions is much smaller than that of bridgmanite (Bdg). Our findings indicate that Bdg (and post-perovskite) is the best heat conductor in the Earth's lower mantle, and distribution of iron and its valence state among the lower mantle minerals are key factors to control the lower mantle thermal conductivity.

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.

We carried out melting experiments on Fe–Si alloys to 127 GPa in a laser-heated diamond-anvil cell (DAC). On the basis of textural and chemical characterizations of samples recovered from a DAC, a change in eutectic liquid composition in the Fe–FeSi binary system was examined with increasing pressure. The chemical compositions of coexisting liquid and solid phases were quantitatively determined with field-emission-type electron microprobes. The results demonstrate that silicon content in the eutectic liquid decreases with increasing pressure to less than 1.5 ± 0.1 wt.% Si at 127 GPa. If silicon is a single light element in the core, 4.5 to 12 wt.% Si is required in the outer core in order to account for its density deficit from pure iron. However, such a liquid core, whose composition is on the Si-rich side of the eutectic point, crystallizes less dense solid, CsCl (B2)-type phase at the inner core boundary (ICB). Our data also show that the difference in silicon concentration between coexisting solid and liquid is too small to account for the observed density contrast across the ICB. These indicate that silicon cannot be the sole light element in the core. Previous geochemical and cosmochemical arguments, however, strongly require ∼6 wt.% Si in the core. It is possible that the Earth's core originally included ∼6 wt.% Si but then became depleted in silicon by crystallizing SiO2 or MgSiO3.

The thermal conductivity of the Earth's core can be estimated from its electrical resistivity via the Wiedemann–Franz law. However, previously reported resistivity values are rather scattered, mainly due to the lack of knowledge with regard to resistivity saturation (violations of the Bloch–Gruneisen law and the Matthiessen's rule). Here we conducted high-pressure experiments and first-principles calculations in order to clarify the relationship between the resistivity saturation and the impurity resistivity of substitutional silicon in hexagonal-close-packed (hcp) iron. We measured the electrical resistivity of Fe–Si alloys (iron with 1, 2, 4, 6.5, and 9 wt.% silicon) using four-terminal method in a diamond-anvil cell up to 90 GPa at 300 K. We also computed the electronic band structure of substitutionally disordered hcp Fe–Si and Fe–Ni alloy systems by means of Korringa–Kohn–Rostoker method with coherent potential approximation (KKR-CPA). The electrical resistivity was then calculated from the Kubo–Greenwood formula. These experimental and theoretical results show excellent agreement with each other, and the first principles results show the saturation behavior at high silicon concentration. We further calculated the resistivity of Fe–Ni–Si ternary alloys and found the violation of the Matthiessen's rule as a consequence of the resistivity saturation. Such resistivity saturation has important implications for core dynamics. The saturation effect places the upper limit of the resistivity, resulting in that the total resistivity value has almost no temperature dependence. As a consequence, the core thermal conductivity has a lower bound and exhibits a linear temperature dependence. We predict the electrical resistivity at the top of the Earth's core to be 1.12 × 10 − 6 Ω m , which corresponds to the thermal conductivity of 87.1 W/m/K. Such high thermal conductivity suggests high isentropic heat flow, leading to young inner core age (

Banded iron formations (BIFs), consisting of layers of iron oxide and silica, are far denser than normal mantle material and should have been subducted and sunk into the deep lower mantle. We performed melting experiments on Fe2SiO4 from 26 to 131 GPa in a laser-heated diamond-anvil cell (DAC). The textural and chemical characterization of a sample recovered from the DAC revealed that SiO2 is the liquidus phase for the whole pressure range examined in this study. The chemical compositions of partial melts are very rich in FeO, indicating that the eutectic melt compositions in the FeO SiO2 binary system are very close to the FeO end-member. The eutectic temperature is estimated to be 3540 ± 150 K at the core–mantle boundary (CMB), which is likely to be lower than the temperature at the top of the core at least in the Archean and Paleoproterozoic eons, suggesting that subducted BIFs underwent partial melting in a thermal boundary layer above the CMB. The FeO-rich melts formed by partial melting of the BIFs were exceedingly dense and therefore migrated downward. We infer that such partial melts have caused iron enrichment in the bottom part of the mantle, which may have contributed to the formation of ultralow velocity zones (ULVZs) observed today. On the other hand, solid residues left after the segregation of the FeO-rich partial melts have been almost pure SiO2, and therefore buoyant in the deep lower mantle to be entrained in mantle upwellings. They have likely been stretched and folded repeatedly by mantle flow, forming SiO2 streaks within the mantle “marble cake”. Mantle packages enhanced by SiO2 streaks may be the origin of seismic scatterers in the mid-lower mantle.

The thermal conductivity of post-perovskite (ppv), the highest-pressure polymorph of MgSiO3 in the Earth's mantle, is one of the most important transport properties for providing better constraints on the temperature profile and dynamics at the core-mantle boundary (CMB). Incorporation of Fe into ppv can affect its conductivity, which has never been experimentally investigated. Here we determined the lattice thermal conductivities of ppv containing 3 mol% and 10 mol% of Fe at high P-T conditions – of pressures up to 149 GPa and 177 GPa, respectively, and temperatures up to 1560 K – by means of the recently developed pulsed light heating thermoreflectance technique combining continuous wave heating lasers. We found that the incorporation of Fe into ppv moderately reduces its lattice thermal conductivity as it increases the Fe content. The bulk conductivity of ppv dominant pyrolite is estimated as 1.5 times higher than that of pyrolite consisting of bridgmanite and ferropericlase in the lower mantle, which agrees with the traditional view that ppv acts as a better heat conductor than bridgmanite in the Earth's lowermost mantle.

Electrical resistivity and thermal conductivity of iron (Fe)-light element alloys at high pressure and temperature are key parameters to constrain the dynamics and thermal evolution of the Earth's core. We determined the electrical resistivity of hcp Fe-2, 4 and 6.5 wt.% silicon (Si) alloys up to 117 GPa and 3120 K using a four-terminal method in an internally heated diamond-anvil cell. The temperature dependence of electrical resistivity of hcp Fe-Si alloys was suppressed as both Si concentration and temperature increased, which indicates the resistivity saturation phenomenon: the electrical resistivity of metal asymptotically approaches the “saturation resistivity”. Our results are fully reproduced by a highly resistive saturation model, and the obtained saturation resistivities for hcp Fe-Si alloys are comparable to those for hcp pure Fe at around 100 GPa. If Si is a major light element in the Earth's core, the pure Fe like saturation resistivity would keep the core conductivity high enough to induce active dynamics there and rapid growth of the inner core.

We have examined the density and bulk sound velocity of liquid iron alloys, (Fe, Ni)X(H, Si, O, S, C)1-X, at Earth's outer core pressure and temperature conditions based on first-principles molecular dynamics simulations. The nonideal mixing effects on volume and velocity were found to be negligible for all combinations of different liquid alloys examined. By comparing the results with seismological observations, we searched for possible chemical compositions for the outer core. Hydrogen is found to be a primary light element when the inner-core boundary temperature TICB is 4,800 K to 5,400 K. If this is the case, it is suggested that a large amount of water was delivered to the Earth during its accretionary stage and that the present-day core temperature is relatively low. On the other hand, oxygen is the most important light element if TICB = 6,000 K, consistent with the previous calculations by Badro et al. (2014) at TICB = 6,300 K. To further constrain the chemical composition of the outer core, it is necessary to take into account other constraints besides its density and bulk sound velocity; melting temperature, simultaneous solubilities of multiple of light elements, and so forth.

We measured the electrical resistivity of Fe–Ni alloys (iron with 5, 10, and 15 wt.% nickel) using four-terminal method in a diamond-anvil cell up to 70 GPa at 300 K. The results demonstrate that measured resistivity increases linearly with increasing nickel impurity concentration, as predicted by the Matthiessen’s rule. The impurity resistivity is predominant at ambient temperature; the incorporation of 5 wt.% nickel into iron doubles the electrical resistivity at 60 GPa. Such impurity effect becomes minor at high temperature of the Earth’s core because of the resistivity “saturation”. We also calculated that >0.9 TW heat flow is necessary at the top of the inner core for thermal convection in the inner core. It requires the CMB heat flow of ∼30 TW, which is much higher than recent estimates of 5–15 TW. This means that purely thermal convection does not occur in the inner core.

Phase relations of iron–silicon alloy (Fe–6.5 wt.% Si and Fe–9 wt.% Si) were investigated up to 407 GPa and 5960 K in a laser-heated diamond–anvil cell, which likely covers the entire pressure and temperature conditions of the Earth's inner core. Synchrotron X-ray diffraction measurements show that Fe–9 wt.% Si with a hexagonal close-packed (hcp) structure is stable to 4800 K at 330 GPa, corresponding to the pressure at the inner/outer core boundary, and decomposes into a mixture of Si-poor hcp and Si-rich CsCl-type (B2) phases at higher temperatures. We also found that the solubility of silicon in solid iron is relatively insensitive to temperature, decreasing from 9 to >6.5 wt.% over a range of 1500 K at 70 GPa. These suggest that the inner core is composed solely of the hcp phase, when the silicon content is up to 7 wt.% that likely accounts for the inner core density deficit as well as for the Mg/Si ratio and the Si isotopic composition of the mantle. Additionally, the present experiments demonstrate that the incorporation of silicon in iron expands the stability of hcp with respect to that of fcc.

Melting experiments on a pyrolitic mantle material were performed in a pressure range from 34 to 179 GPa based on laser-heated diamond-anvil cell (DAC) techniques. The textural and chemical characterizations of quenched samples were made by using field-emission-type electron microprobe (FE-EPMA). Melts formed by 46 to 77 wt.% partial melting in this study were ultrabasic in composition and became more depleted in SiO2 and more enriched in FeO with increasing pressure. Melting textures indicate that the liquidus phase changed from ferropericlase to MgSiO3-rich perovskite at least above 34 GPa and further to post-perovskite. The first phase to melt (disappear) changed from CaSiO3 perovskite to (Mg,Fe)O ferropericlase between 68 and 82 GPa. The stability of ferropericlase above solidus temperature shrinks with increasing pressure (melting last below 34 GPa and first 82 GPa), resulting in higher (MgO + FeO)/SiO2 ratio in partial melt at higher pressure. Additionally, the Fe-Mg distribution coefficients (KD) between perovskite/post-perovskite and melt decreased considerably with increasing pressure, leading to strong Fe-enrichment in partial melts. It supports dense partial melts in a deep lower mantle, which migrate downward to the core mantle boundary (CMB).

We measured the electrical resistivity of iron and iron-silicon alloy to 100 GPa. The resistivity of iron was also calculated to core pressures. Combined with the first geophysical model accounting for saturation resistivity of core metal, the present results show that the thermal conductivity of the outermost core is greater than 90 W/m/K. These values are significantly higher than conventional estimates, implying rapid secular core cooling, an inner core younger than 1 Ga, and ubiquitous melting of the lowermost mantle during the early Earth. An enhanced conductivity with depth suppresses convection in the deep core, such that its center may have been stably stratified prior to the onset of inner core crystallization. A present heat flow in excess of 10 TW is likely required to explain the observed dynamo characteristics.

The composition and state of Earth's core, located deeper than 2,900 km from the surface, remain largely uncertain. Recent static experiments on iron and alloys performed up to inner core pressure and temperature conditions have revealed phase relations and properties of core materials. These mineral physics constraints, combined with theoretical calculations, continue to improve our understanding of the core, in particular the crystal structure of the inner core and the chemical composition, thermal structure and evolution, and possible stratification of the outer core.

Thermal conductivity is essential for controlling the rate of core heat loss and long-term thermal evolution of the Earth, but it has been poorly constrained at the high pressures of Earth's lowermost mantle. We measured the lattice component of thermal diffusivity, heat transport by scattering of phonons, of both MgSiO 3 perovskite (Pv) and post-perovskite (PPv) at high pressures of up to 144 GPa and at room temperature. Lattice thermal conductivity of Pv-dominant lowermost mantle assemblage obtained in this study is about 11 W/m/K, while PPv-bearing rocks exhibit ∼60% higher conductivity. Since such Pv value is comparable to the conventionally assumed lowermost mantle conductivity, our findings do not significantly alter but support the recent notion of high core–mantle boundary heat flow along with a young inner core and high temperatures in the early deep Earth.

We report the measurements of aggregate shear velocity ( V S ) of CaSiO 3 perovskite (CaPv) at high pressure ( P ) between 32 and 133 GPa and room temperature ( T ) on the basis of Brillouin spectroscopy. The sample had a tetragonal perovskite structure throughout the experiments. The measured P – V S data show the shear modulus and its pressure derivative at ambient condition to be G 0 =115.8 GPa and G '=1.20, respectively. The zero-pressure shear velocity is determined to be V S0 =5.23 km/s, in good agreement with the previous estimate inferred from the ultrasonic measurements on Ca(Si,Ti)O 3 perovskite at 1 bar. Our experimental results are broadly consistent with the earlier calculations on tetragonal CaPv but exhibit lower velocity at equivalent pressure. Such tetragonal CaPv is present in cold subducting slabs and possibly in wide areas of the lowermost mantle. While primitive mantle includes certain amount of CaPv, a depleted peridotite (former harzburgite) layer in subducted oceanic lithosphere is deficient in CaPv and enriched in ferropericlase in the lower mantle. Such harzburgite exhibits 0.9% faster V S and 0.7% slower bulk sound velocity ( V Φ ) at the lowermost mantle P – T conditions if CaPv is present in the tetragonal form in the surrounding mantle. The observed fast V S and slow V Φ anomalies in the D” layer underneath the circum-Pacific region might be attributed in large part in the presence of subducted harzburgitic materials.

Acoustic velocities in polycrystalline H 2 O ice have been measured at room temperature in a pressure range 6–60 GPa by a Brillouin scattering method. Synchrotron X-ray diffraction measurements were also conducted simultaneously with the Brillouin scattering measurements in a pressure range 40–60 GPa. The obtained elastic moduli of high-pressure ice indicate that bcc-structured ice undergoes two transitions related to a change in the hydrogen bonding state at approximately 40 GPa and 58 GPa, i.e. transitions of ice VII to the pre-transitional state of ice VII at 40 GPa and to the dynamically disordered ice X at 58 GPa, respectively. This observation is consistent with previous spectroscopic studies and X-ray diffraction studies. Pressure dependencies of adiabatic elastic moduli and specific heat for ice VII and its high-pressure polymorphs were obtained from the acoustic velocity and volume data measured in this study. Isothermal compression data were obtained from previous studies. The relationships between pressure and adiabatic elastic moduli for ice VII were obtained as follows: K s = 4.0(2) + 8.51(4) × P − 0.081(2) × P 2 + 5.2(4) × 10 − 4 × P 3 ; μ s = 14(2) + 1.7(3) × P − 0.04(2) × P 2 + 5(3) × 10 − 4 × P 3 . An empirical relationship between specific heat and pressure at room temperature was obtained for ice VII as follows: C p = 3.3(2) + 22.1(1) × e − 0.058(2) P . This result implies that the transition from ice VII to the dynamically disordered ice X is accompanied with a discontinuous change in several thermodynamic properties of ice. The elasticity difference between ice VII and the dynamically disordered ice X may affect the dynamics of cold subducting slabs in Earth's lower mantle and the interiors of icy planets. The thermoelastic properties of high-pressure polymorphs of ice obtained in this study could contribute to clarifying the dynamics and the evolution of Earth and icy planets and satellites.

The phase transition boundary between stishovite and CaCl2-type structure in pure SiO2 was investigated at 45–75 GPa and 300–2490 K based on the in situ X-ray diffraction measurements in a laser-heated diamond-anvil cell (LHDAC). Results show that the boundary has a positive dP/dT slope of 11.1 MPa/K and the transition occurs about 70 GPa at 2200 K along the typical mantle geotherm. This corresponds to a depth of 1700 km, somewhat shallower than the previous estimate. Seismic heterogeneities have been found in a wide depth range from 800 to 1850 km in the upper to middle layers of the lower mantle. The post-stishovite phase transition is known to be ferroelastic-type, which causes a considerable reduction in shear modulus, leading to a large slow shear velocity anomaly. The almost pure SiO2 phase included in subducted crustal materials may be therefore responsible for the relatively deep mid-lower mantle seismic heterogeneities. The shallow lower mantle anomalies are possibly caused by the similar phase transition in SiO2 phase containing Al2O3 and H2O or by the phase transition in Al-bearing CaSiO3 perovskite from cubic to tetragonal structure.

High-temperature compression experiments with in situ X-ray diffraction of ferropericlase (Fp) with a composition of (Mg 0.81 Fe 0.19 )O were made in a laser-heated diamond anvil cell to pressures ( P ) of 116 GPa at a constant temperature ( T ) of 1600–1900 K. Room-temperature experiments with a laser annealing technique were also carried out on the same material. Anomalous unit-cell volume reductions that can be explained by the spin transition of ferrous iron were observed at P = 63–96 GPa and 45–63 GPa at T = 1600–1900 K and 300 K, respectively, indicating that the spin transition pressure interval expands with increasing temperature. The observed density changes across this spin transition at T = 1600–1900 K and 300 K are about 1.6% and 1.0%, respectively, indicating that the spin transition pressure interval expands with increasing temperature. The thermal expansivity of Fp is large in the mid-lower mantle due to the effect of the spin transition. In a peridotitic composition, the spin transition in Fp increases the rock density by 0.35% at the lowermost mantle conditions. Calculated densities show that both perovskitic and peridotitic mantle models may explain the PREM lower mantle density. However, the peridotitic lower mantle model requires less assumption to satisfy the PREM density and is more self-consistent.

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 extended the pressure range of sound velocity measurements for liquid water to 25 GPa and 900 K along the melting curve using a laser heated diamond anvil cell with a combined system of Brillouin scattering and synchrotron X-ray diffraction. Experimental pressure and temperature were obtained by solving simultaneous equations: the melting curve of ice and the equation of state for gold. The sound velocities obtained in liquid water at high pressures and melting temperatures were converted to density using Murnaghan's equation of state by fitting a parameter of the pressure derivative of bulk modulus at 1 GPa. The results are in good agreement with the values predicted by a previously reported equation of state for water based on sound velocity measurements. The equation of state for water obtained in this study could be applicable to water released by dehydration reactions of dense hydrous magnesium silicate phases in cold subducting slabs at lower mantle conditions, although the validity of Murnaghan's equation of state for water should be evaluated in a wider pressure and temperature ranges. The present velocity data provides the basis for future improvement of the accurate thermodynamic model for water at high pressures.

We have determined the phase transition boundary between NaCl-type (B1) and NiAs-type (B8) structures of FeO up to 208 GPa and 3800 K on the basis of synchrotron X-ray diffraction (XRD) measurements in situ at high pressure and temperature using a laser-heated diamond-anvil cell (DAC). The boundary is located at 200 GPa and 3200 K with positive Clapeyron slope. These results show that B1 phase of FeO is stable along the whole mantle geotherm, whereas B8 phase is stabilized at the inner core condition. Additionally, FeO coexisted with metallic Fe in the present experiments. We found that hexagonal close-packed (hcp) iron is stable over the entire present experimental conditions. Moreover, the direct chemical analyses of the recovered sample demonstrated that solid iron did not contain any detectable oxygen. While extensive solid solution between Fe and FeO has been speculated above 60–80 GPa, the present results strongly suggest that the system Fe–FeO is simple eutectic at least to 200 GPa pressure range.

Structural distortion and the phase transition of CaSnO3 perovskite were examined in situ at high-pressure and -temperature on the basis of synchrotron X-ray diffraction measurement in a laser-heated diamond-anvil cell. The results show that CaSnO3 perovskite transforms into a CaIrO3-type post-perovskite phase above 40 GPa and 2000 K with a high positive Clapeyron slope. It is noted that CaSnO3 post-perovskite is quenchable to ambient condition. Combined with available compression data on other perovskite-type materials, we discuss a mechanism of perovskite to post-perovskite phase transition under pressure. The perovskites, which are distorted largely from the ideal cubic perovskite structure already at ambient condition, become more deformed with increasing pressure. This eventually results in the post-perovskite phase transition at a critical angle of octahedral rotation.

We have measured the electrical resistance of (Mg0.9Fe0.1)SiO3 perovskite with increasing pressure in a diamond-anvil cell (DAC). Results demonstrate that the electrical conductivity increased with pressure to about 70 GPa, then conversely decreased to around 85 GPa, and again increased mildly to 135 GPa. As inferred from the previous studies of (Mg,Fe)O ferropericlase, the observed reduction in the conductivity of perovskite at 70–85 GPa is most likely due to the high-spin to low-spin transition of iron; it decreases the number of unpaired electrons and therefore diminishes the conduction by electron hopping between the ferric and ferrous iron sites. Present conductivity data provide valuable implications for the pressure range of spin transition in (Mg,Fe)SiO3 perovskite, which has been highly controversial. The relatively abrupt reduction in the electrical conductivity above 70 GPa is reconciled with the spin-pairing transition in ferric iron, which may occur rather sharply at this pressure range.

Seismic wave velocity profiles in the Earth provide one of the strongest constraints on structure, mineralogy and elastic properties of the Earth's deep interior. Accurate sound velocity data of deep Earth materials under relevant high-pressure and high-temperature conditions, therefore, are essential for interpretation of seismic data. Such information can be directly obtained from Brillouin scattering measurement. Here we describe an in situ Brillouin scattering system for measurements at high pressure and high temperature using a laser heated diamond anvil cell and synchrotron radiation for sample characterization. The system has been used with single-crystal and polycrystalline materials, and with glass and fluid phase. It provided high quality sound velocity and elastic data with X-ray diffraction data at high pressure and/or high temperature. Those combined techniques can potentially offer the essential information for resolving many remaining issues in mineral physics.

The aggregate shear wave velocities of MgO (periclase) have been determined throughout Earth's lower mantle pressure regime approaching 130 GPa using Brillouin spectroscopy in conjunction with synchrotron X-ray diffraction technique in a diamond anvil cell apparatus. We found that the extrapolations of the high-pressure shear wave velocities and shear moduli to ambient pressure are highly consistent with earlier studies. However, the measurements over a wide pressure range revealed that the pressure derivative of the shear modulus (d G /d P = G 0 ′) of MgO is 1.92(2), which is distinctly lower than that of previous lower-pressure experiments. Compared with the previous results on (Mg,Fe)O ferropericlase, there is no clear correlation between iron content and G 0 ′. We calculate that the shear wave velocity profile of lower mantle along the adiabatic geotherm applied by the lower G 0 ′ value of periclase can remarkably well reproduce the global seismological 1-D velocity profile model with uniform composition model. The best-fitting result indicates the possibility of a lower mantle mineralogy with ~ 92 vol.% silicate perovskite phase, implying that the bulk composition of lower mantle is likely not to be pyrolitic but more chondritic. The present acoustic measurements performed over the large pressure range have thus led us to a better understanding of compositional model of the Earth's lower mantle.

We have determined the post-perovskite phase transition boundary in MgSiO3 in a wide temperature range from 1640 to 4380 K at 119–171 GPa on the basis of synchrotron X-ray diffraction measurements in-situ at high-pressure and -temperature in a laser-heated diamond-anvil cell (LHDAC). The results show a considerably high positive Clapeyron slope of + 13.3 ± 1.0 MPa/K and a transition temperature of about 3520 ± 70 K at the core–mantle boundary (CMB) pressure. The thermal structure in D″ layer can be tightly constrained from precisely determined post-perovskite phase transition boundary and the depths of paired seismic discontinuities. These suggest that temperature at the CMB may be around 3700 K, somewhat lower than previously thought. A minimum bound on the global heat flow from the core is estimated to be 6.6 ± 0.5 TW.

We have investigated the phase relations of iron and iron–nickel alloys with 18 to 50 wt.% Ni up to over 300 GPa using a laser-heated diamond-anvil cell. The synchrotron X-ray diffraction measurements show the wide stability of hcp-iron up to 301 GPa and 2000 K and 319 GPa and 300 K without phase transition to dhcp, orthorhombic, or bcc phases. On the other hand, the incorporation of nickel has a remarkable effect on expanding the stability field of fcc phase. The geometry of the temperature–composition phase diagram of iron–nickel alloys suggests that the hcp–fcc–liquid triple point is located at 10 to 20 wt.% Ni at the pressure of the inner core boundary. The fcc phase could crystallize depending on the nickel and silicon contents in the Earth's core, both of which are fcc stabilizer.

We have determined the unit-cell volumes of Au and MgO by synchrotron X-ray diffraction (XRD) measurements at pressures between 11 and 140 GPa and temperatures from 300 to 2330 K in a diamond-anvil cell (DAC). Since the MgO pressure scale appears the most practical at present, it is useful to obtain the P – V – T equation of state of Au that is consistent with the MgO scale. Here we present the new Au pressure scale based on the simultaneously measured volume data of Au and MgO in this study and in Fei et al. [Fei, Y., Li, J., Hirose, K., Minarik, W., Van Orman, J., Sanloup, C., Westrenen, W.V., Komabayashi, T., Funakoshi, K., 2004. A critical evaluation of pressure scales at high temperatures by in situ X-ray diffraction measurements. Phys. Earth Planet. Inter. 143–144, 515–526] using MgO as a reference pressure scale. The new scale predicts larger thermal pressure than previously proposed thermal equations of state of Au and therefore gives higher pressure above 1500 K in a pressure range exceeding 35 GPa.

Subduction of mid-oceanic ridge basalt (MORB) gives rise to strong chemical heterogeneities in the Earth's mantle, possibly extending down to the core–mantle boundary. Phase relations in both pyrolite and MORB compositions are precisely determined at high pressures and temperatures corresponding to lowermost mantle conditions. The results demonstrate that the post-perovskite phase transition occurs in pyrolite between 116 and 121 GPa at 2500 K, while post-perovskite and SiO2 phase transitions occur in MORB at ∼ 4 GPa lower pressure at the same temperature. Theory predicts that these phase changes in pyrolite and MORB cause shear wave velocity increase and decrease, respectively. Near the northern margin of the large low shear velocity province in the lowermost mantle beneath the Pacific, reflections from a negative shear velocity jump near 2520-km depth are followed by reflections from a positive velocity jump 135 to 155-km deeper. These negative and positive velocity changes are consistent with the expected phase transitions in a dense pile containing a mixture of MORB and pyrolitic material. This may be a direct demonstration of the presence of accumulations of subducted MORB crust in the deep mantle.

Simultaneous volume measurements of MgSiO 3 post-perovskite (PPv) and perovskite (Pv) were performed in a diamond anvil cell (DAC) combined with synchrotron X-rays. An externally-heated DAC was used in addition to a laser-heated DAC for the volume measurement experiment at high temperatures. The volume data were collected in the stability field of post-perovskite from 115 to 130 GPa. The temperature generated in the externally-heated and the laser-heated DACs for the volume measurement were up to 832 and 2330 K, respectively. Using two different but complementary heating techniques, we collected the data at a wide temperature range from 300 to 2330 K. The obtained P-V-T data for PPv and Pv were fitted to a third-ordered Birch-Murnaghan equation of state (EOS). For a precise comparison of the volume between the two phases, the EOSs were constructed based on the same pressure scale of MgO. The simultaneous volume measurements and the volumes calculated from the determined EOSs demonstrate that the volume difference between PPv and Pv of about 1.5% is almost constant with increasing temperature to 4000 K at the transition. At the base of the mantle, this density difference corresponds to a temperature anomaly of 1300 K without the phase transition due to the very small thermal expansivity of minerals, which has a significant effect on mantle dynamics. The thermal expansivity contrast between the top and the bottom of the mantle is a factor of 3.6. From a mantle convection study, this value suggests that huge and hot plumes are formed at the core–mantle boundary.

The discovery of a post-perovskite phase transition in MgSiO 3 has significant implications for seismological observations in the D ″ region at the bottom of Earth's mantle. The D ″ discontinuity, which is manifested as a sharp positive seismic-wave velocity jump 200–300 km above the core–mantle boundary (at pressure of 119∼ 125 GPa), is one of the most enigmatic seismic features in this region. Whether this velocity increase may be due to the formation of a post-perovskite phase at the D ″ discontinuity has not, however, been directly addressed by experiments. Here we present the results of aggregate sound velocity measurements of the MgSiO 3 post-perovskite phase by Brillouin spectroscopy in the diamond anvil cell (DAC) up to a pressure of 172 GPa, in combination with infrared laser annealing of the sample. Based on these results and our recent high-pressure velocity measurements on perovskite, the aggregate shear wave velocity contrast across the perovskite to post-perovskite phase transition is at most 0.5%. This contrast is much smaller than typically observed across the D ″ discontinuity, indicating that the formation of an isotropic aggregate of the post-perovskite phase provides an insufficient velocity increase to explain the D ″ discontinuity. Lattice preferred orientation (LPO) of post-perovskite is likely to be crucial for explaining the D ″ discontinuity.

Phase relations in Mg 0.5 Fe 0.5 SiO 3 and Mg 0.25 Fe 0.75 SiO 3 were investigated in a pressure range from 72 to 123 GPa on the basis of synchrotron X-ray diffraction measurements in situ at high-pressure and -temperature in a laser-heated diamond-anvil cell (LHDAC). Results demonstrate that Mg 0.5 Fe 0.5 SiO 3 perovskite is formed as a single phase at 85–108 GPa and 1800–2330 K, indicating a high solubility of FeO in (Mg,Fe)SiO 3 perovskite at high pressures. Post-perovskite appears coexisting with perovskite in Mg 0.5 Fe 0.5 SiO 3 above 106 GPa at 1410 K, the condition very close to the post-perovskite phase transition boundary in pure MgSiO 3 . The coexistence of perovskite and post-perovskite was observed to 123 GPa. In addition, post-perovskite was formed coexisting with perovskite also in Mg 0.25 Fe 0.75 SiO 3 bulk composition at 106–123 GPa. In contrast to earlier experimental and theoretical studies, these results show that incorporation of FeO stabilizes perovskite at higher pressures. This could be due to a larger ionic radius of Fe 2+ ion, which is incompatible with a small Mg 2+ site in the post-perovskite phase.

Lakshtanov et al. (2007) showed that incorporation of aluminum and some water into SiO2 significantly reduces the post-stishovite transition pressure in SiO2. This discovery suggested that the ferroelastic post-stishovite transition in subducted MORB crust could be the source of reflectors/scatterers with low shear velocities observed in the mid to upper lower mantle. A few years later, a similar effect was observed in anhydrous Al-bearing silica. In this paper, we show by first principles static calculations and by molecular dynamics using inter-atomic potentials that hydrogen bonds and hydrogen mobility play a crucial role in lowering the post-stishovite transition pressure. A cooperative redistribution of hydrogen atoms is the main mechanism responsible for the transition pressure reduction in hydrous aluminous stishovite. The effect is enhanced by increasing hydrogen concentration. This perspective suggests a potential relationship between the depth of seismic scatterers and the water content in stishovite.

Here, we precisely determined the low-pressure stability limit of phase A in the Mg-end-member antigorite bulk composition defined as the reaction forsterite + water = phase A + enstatite (“water-line” or “water-storage line”) in a multi-anvil apparatus. Pressures were determined by in situ synchrotron X-ray diffraction measurements using NaCl as an internal pressure standard. Results demonstrate that the water-line is located at 800 °C and 8.5 GPa and at 550 °C and 5.1 GPa with a Clapeyron slope of 13.6 MPa/°C. We also examined the conditions for the formation of phase A by the decomposition of antigorite on the basis of the phase relations in the systems MgO–SiO 2 –H 2 O (MSH) and MgO–Al 2 O 3 –SiO 2 –H 2 O (MASH). A descending slab peridotite retains water in phase A beyond the stability of antigorite only when temperature in the slab is lower than 550 or 660 °C at 5.1 GPa (corresponding to 160 km depth), in the system MSH or MASH, respectively. The double seismic planes observed within the slabs may be caused by the dehydration reaction of antigorite or chlorite. Their merging depths, which could represent the high-pressure stability limit of antigorite or chlorite, are 160 km or deeper beneath northeast Japan, Aleutian, Kamchatka and Kuril. Water included in hydrous slab peridotite is transported into the transition zone and deeper mantle in these areas but recycled to the surface in other subduction zones.

Phase relations, mineral chemistry, and density of a natural mid-oceanic ridge basalt (MORB) composition were investigated up to 134 GPa and 2300 K by a combination of in-situ X-ray diffraction measurements and chemical analyses using transmission electron microscope (TEM). Results demonstrate that the MORB composition consists of MgSiO 3 -rich perovskite, stishovite, CaSiO 3 perovskite, and CaFe 2 O 4 -type Al-phase in the upper part of the lower mantle. The most abundant mineral of MgSiO 3 -rich perovskite undergoes phase transition to a CaIrO 3 -type post-perovskite phase above 110 GPa and 2500 K. Post-perovskite phase is similar in composition to perovskite except considerably high Na 2 O content. Stishovite transforms to CaCl 2 -type SiO 2 phase above 62 GPa and 2000 K and further to α-PbO 2 -type phase above 110 GPa. α-PbO 2 -type SiO 2 phase includes large amount of Al 2 O 3 , which significantly expands its stability relative to CaCl 2 -type phase. Phase transition of CaSiO 3 perovskite from tetragonal to cubic was also observed with increasing temperature. CaFe 2 O 4 -type Al-phase is stable to the bottom of the mantle. The density of MORB crust was calculated using volume data, combining with measured chemical compositions and calculated mineral proportions. The former MORB crust is denser than the average lower mantle at all depths greater than ∼ 720 km, contrary to earlier predictions. The subducted basaltic crust may have accumulated at the base of the mantle.

We investigated the phase transitions of FeO at high pressure and high temperature up to 87 GPa and 1730 K by in situ synchrotron X-ray diffraction measurements in a laser-heated diamond anvil cell (DAC). The results demonstrated that the NaCl-type (B1) structure of FeO undergoes transition to the NiAs-type (B8) structure above 70 GPa at 1600 K. The density increase across this transition is about 2%. Present B1–B8 transition pressure is consistent with the previous shock-induced transition pressure. The relative intensities of diffraction peaks of the B8 phase may suggest a polytype normal/inverse NiAs structure, which has a metallic nature that could promote the oxygen incorporation into the iron-rich core. Transition to the metallic B8 phase above 70 GPa is also consistent with the earlier electrical resistance measurements. The hexagonal close-packed (hcp) e-phase of Fe coexisted with FeO both at room temperature and high temperatures above 44–87 GPa.

We measured the dihedral angle of molten iron in (Mg,Fe)SiO3-perovskite aggregate with increasing pressure and temperature using laser-heated diamond anvil cell. Results demonstrate that it decreases from 94° at ∼27 GPa and ∼2400 K to 51° at ∼47 GPa and ∼3000 K. This value is smaller than the critical angle of 60°, thus allowing iron melt to wet the grain boundaries of silicate perovskite and develop an interconnected melt network within the perovskite-dominant matrix, even at very small melt fractions. The quenched liquid iron contained substantial amounts of oxygen and silicon as pressure and temperature increase. Such a decrease in the dihedral angle is likely due to a reduction in the iron-perovskite interfacial energy by dissolving oxygen and silicon into the liquid iron from coexisting silicate perovskite. These suggest that a wetting behaviour of core melts in the solid silicate mantle changes above ∼40 GPa, corresponding to ∼1000 km depth in the present Earth, and efficient metal segregation may have proceeded by permeable flow in a late stage of accretion. Oxygen and silicon incorporated during core formation and later by core–mantle boundary process may be important light elements in the Earth's core.

Trace element partitioning data between CaSiO3-perovskite (CaPv), MgSiO3-perovskite (MgPv), calcium–aluminum silicate (CAS-phase), and coexisting melts in peridotite and mid-ocean ridge basalt (MORB) compositions were obtained at 25–27 GPa and 2400–2530 °C using multi-anvil apparatus and ion microprobe. Results clearly show that CaPv is the predominant host for large ion lithophile elements (LILE) in the lower mantle. Because of the overwhelmingly high CaPv/melt partition coefficients (>10 for many of the LILE), partial melting in the lower mantle causes strong enrichment of LILE in the CaPv-bearing solid phase residue. CaPv has the following partitioning characteristics: (1) uniformly high partition coefficients for heavy rare earth elements (HREE) (e.g. 15 for Yb), decreasing toward light REE (e.g. 7 for La), (2) systematically lower partition coefficients for high field strength elements (Nb, Zr, Ti) and Sr relative to neighboring REE, (3) high Th and U, and systematically low Pb partition coefficients. Previous high-pressure studies have shown that the stability field of CaPv above solidus temperature is much wider in basaltic composition than in peridotite, indicating that melting of subducted oceanic crust in the lower mantle could produce significant geochemical CaPv signatures. Strong enrichment in Th and U relative to Pb in CaPv would result in radiogenic Pb isotopic compositions of the CaPv-bearing solid residue. Some clinopyroxenes in plume mantle peridotite xenoliths possess trace element patterns closely resembling those of natural CaPv found in diamonds and CaPv from the present experiments, suggesting that they were inherited from the CaPv-bearing precursor. In contrast, CaPv is the first phase to disappear during partial melting of peridotite above 24 GPa, and its geochemical signature may not be observable in nature. (MgPv + CaPv) fractional crystallization from a magma ocean has previously been put forward as a mechanism for Si depletion of the upper mantle. However, our data show that the magma ocean Sm/Ba ratio would deviate significantly from observed chondritic values when fractionation exceeds only 6%, if the crystallizing mixture were 10% CaPv and 90% MgPv. A significant hidden perovskite reservoir in the lower mantle can therefore be excluded.

Phase transitions in CaSiO 3 (+5.9 wt.% Al 2 O 3 ) and pure CaSiO 3 perovskites were investigated at 24–75 GPa and 300–2250 K on the basis of in situ X-ray diffraction measurements in a laser-heated diamond anvil cell (LHDAC). Results demonstrate that CaSiO 3 (+5.9 wt.% Al 2 O 3 ) has a GdFeO 3 -type orthorhombic perovskite structure at low temperatures and undergoes phase transition to a cubic structure above 1840 K at 50 GPa. The transition boundary determined in a pressure range from 32 to 75 GPa shows strong temperature-dependence with a slightly positive P/T slope. The structure of end-member CaSiO 3 is slightly distorted from cubic symmetry at low temperatures and similarly transforms to a cubic structure with increasing temperature. The transition occurs at about 580 K and 52 GPa, which is a much lower temperature than for the Al 2 O 3 -bearing composition at equivalent pressure. These distorted CaSiO 3 -rich perovskites may be ferroelastic and have large elastic anomalies near the structural transition to the cubic phase. Part of the seismic discontinuities in the upper- to middle-layers of the lower mantle, which are often observed beneath convergent margins, may be attributed to the phase transition in Al 2 O 3 -bearing CaSiO 3 perovskite included within relatively cold subducting slabs.