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We report the thermal Equation of State (EoS) of the non-magnetic Fe3C phase based on in situ X-ray diffraction (XRD) experiments to 117 GPa and 2100 K. High-pressure and temperature unit-cell volume measurements of Fe3C were conducted in a laser-heated diamond-anvil cell. Our pressure-volume-temperature (P-V-T) data together with existing data were fit to the Vinet equation of state with the Mie-Grüneisen-Debye thermal pressure model, yielding V0 = 151.6(12) Å3, K0 = 232(24) GPa, K0′= 5.09(46), γ0 = 2.3(3), and q = 3.4 (9) with θ0 = 407 K (fixed). The high-T data were also fit to the thermal pressure model with a constant αKT term, PTh = αKT(ΔT), and there is no observable pressure or temperature dependence, which implies minor contributions from the anharmonic and electronic terms. Using the established EoS for Fe3C, we made thermodynamic calculations on the P-T locations of the breakdown reaction of Fe3C into Fe7C3 and Fe. The reaction is located at 87 GPa and 300 K and 251 GPa and 3000 K. An invariant point occurs where Fe, Fe3C, Fe7C3, and liquid are stable, which places constraints on the liquidus temperature of the outer core, namely inner core crystallization temperature, as the inner core would be comprised by the liquidus phase. Two possible P-T locations for the invariant point were predicted from existing experimental data and the reaction calculated in this study. The two models result in different liquidus “phases” at the outer core-inner core boundary pressure: Fe3C at 5300 K and Fe7C3 at 3700 K. The Fe7C3 inner core can account for the density, as observed by seismology, while the Fe3C inner core cannot. The relevance of the system Fe-C to Earth’s core can be resolved by constructing a thermodynamic model for melting relations under core conditions as the two models predict very different liquidus temperatures.

Melting experiments on Fe

The pressure-volume-temperature (P-V-T) measurements of the B2 (CsCl-type) phase of KCl were performed at 9–61 GPa/1500–2600 K and up to 229 GPa at room temperature, based on synchrotron X-ray diffraction measurements in a laser-heated diamond-anvil cell (DAC). The nonhydrostatic stress conditions inside the sample chamber were critically evaluated based on the platinum pressure marker. With thermal annealing by laser after each pressure increment, the deviatoric stress was reduced to less than 1% of the sample pressure even at the multi-megabar pressure range. The obtained P-V-T data were fitted to the Vinet equation of state with the Mie-Grüneisen-Debye model for thermal pressure. The thermal pressure of KCl was found to be as small as ~10 GPa even at 3000 K at any given volume, which is only half of that of common pressure markers (i.e. Pt, Au, or MgO). Such a low-thermal pressure validates the use of a KCl pressure medium as a pressure marker at high temperatures.

The phase transition between a face-centered cubic (fcc) and hexagonal close-packed (hcp) structures in Fe-4wt% Si alloy was examined in an internally resistive heated diamond-anvil cell (DAC) under high-pressure (P) and high-temperature (T) conditions to 71 GPa and 2000 K by in situ synchrotron X-ray diffraction. Complementary laser-heated DAC experiments were performed in Fe-6.5wt% Si. The fcc-hcp phase transition boundaries in the Fe-Si alloys are located at higher temperatures than that in pure Fe, indicating that the addition of Si expands the hcp stability field. The dP/dT slope of the boundary of the entrant fcc phase in Fe-4wt% Si is similar to that of pure Fe, but the two-phases region is observed over a temperature range increasing with pressure, going from 50 K at 15 GPa to 150 K at 40 GPa. The triple point, where the fcc, hcp, and liquid phases coexist in Fe-4wt% Si, is placed at 90–105 GPa and 3300–3600 K with the melting curve same as in Fe is assumed. This supports the idea that the hcp phase is stable at Earth's inner core conditions. The stable structures of the inner cores of the other terrestrial planets are also discussed based on their P-T conditions relative to the triple point. In view of the reduced P-T conditions of the core of Mercury (well below the triple point), an Fe-Si alloy with a Si content up to 6.5 wt% would likely crystallize an inner core with an fcc structure. Both cores from Venus and Mars are currently believed to be totally molten. Upon secular cooling, Venus is expected to crystallize an inner core with an hcp structure, as the pressures are similar to those of the Earth's core (far beyond the triple point). Martian inner core will take an hcp or fcc structure depending on the actual Si content and temperature.

Pressure-volume-temperature (P-V-T) experiments on tetragonal Fe3S were conducted to 126 GPa and 2500 K in laser-heated diamond anvil cells (DAC) with in-situ X-ray diffraction (XRD). Seventy nine high-T data as well as four 300-K data were collected, based on which new thermal equations of state (EoS) for Fe3S were established. The room-T data together with existing data were fitted to the third order Birch-Murnaghan EoS, which yielded, K 0 = 126 ± 2 GPa and K ′ = 5.1 ± 1 with V 0 fixed at 377.0 A3. A constant α K T term in the thermal pressure equation, Pth = α K T ( T -300 ) , fitted the high-T data well to the highest temperature, which implies that the contributions from the anharmonic and electronic terms should be minor in the thermal pressure term. The high-T data were also fitted to the Mie-Gruneisen-Debye model; γ 0 = 1.01 ± 0.03 with θ 0 and q fixed at 417 K and 1 respectively. Calculations from the EoS show that crystalline Fe3S at 4000-5500 K is denser than the Earth's outer core and less dense than the inner core. Assuming a density reduction due to melting, liquid Fe3S would meet the outer core density profile, which however suggests that no less than 16 wt%S is needed to reconcile the observed outer core density deficit. The S-rich B2 phase, which was suggested to be a potential liquidus phase of an Fe3S-outer core above 250 GPa, namely the main constituent of its solid inner core, would likely be less dense than the Earth's inner core. As such, while the outer core density requires as much sulphur as 16 wt%, the resulting liquidus phase cannot meet the density of the inner core. Any sulphur-rich composition should therefore be rejected for the Earth's core.

The thermodynamics of the system Fe–Si–O under high pressure (P) and temperature (T) was examined, starting with modelling the phase transition between a face-centred cubic (fcc) and hexagonal close-packed (hcp) structure in Fe–Si alloy which was previously examined by experiment under high P–T conditions. The mixing properties of Fe and Si for the iron phases were found to be approximated by ideal mixing under high P and T conditions. The entropy changes upon melting of the end-members of the system are fairly large, and therefore the melting temperature of the Si-bearing fcc or hcp phases needs to be insensitive to the Si content, to account for the reported close compositions of coexisting liquid and solid ( 50 GPa). The solidus and liquidus temperatures of Fe–Si iron alloy would therefore, not significantly be changed by the presence of Si at the inner core-outer core boundary, which enables us to evaluate the melting curve of Fe–Si fcc and hcp phases. From thus-constrained melting curve, I assessed a thermal equation of state of Si-bearing iron liquid. I then estimated a seismologically consistent outer core composition as a function of Si and O contents using the EoS for liquids constructed in this study and the literature. The best-fit composition is Fe-5.8(0.6) wt%Si–0.8(0.6) wt%O, which however does not precipitate a solid iron phase that is consistent with the inner core density. Therefore, Earth’s core cannot be fully represented by the system Fe–Si–O and it should include another light element.

Hydrogen ions move freely within the crystal structure of a hydrous mineral under lower mantle conditions, resulting in high electrical conductivity that may make it possible to map water in the deep mantle.

LiNbO 3 -type Mg 2.98(2) Al 1.99(2) Si 3.02(2) O 12 (py-LN) was synthesized by recovering a run product from 2000 K and 45 GPa to ambient conditions using a large volume press. Rietveld structural refinements were carried out using the one-dimensional synchrotron XRD pattern collected at ambient conditions. The unit-cell lattice parameters were determined to be a = 4.8194(3) A, c = 12.6885(8) A, V = 255.23(3) A 3 , with Z = 6 (hexagonal, R 3 c ). The average A -O and B -O distances of the A O 6 and B O 6 octahedra have values similar to those that can be obtained from the sum of the ionic radii of the averaged A - and B -site cations and oxygen (2.073 and 1.833 A, respectively). The present compound has the B -site cations at the octahedral site largely shifted along the c axis compared with other LiNbO 3 -type phases formed by back-transition from perovskite (Pv)-structure, and as a result, the coordination number of this site is better described as 3+3. It appears therefore that the B -site cation in the octahedral position cannot be completely preserved during the back-transition because of the small size of Si and Al, which occupy usually a tetrahedral site at ambient conditions. The formation of py-LN can be explained by the tilting of B O 6 octahedra of the perovskite structure having the pyrope composition and formed at high P-T conditions. The tilting is driven by the decrease in ionic radius ratio between the A -site cation and oxygen during decompression. This also explains why there is no back-transition from the Pv-structure to the ilmenite-structure during decompression, since this is a reconstructive phase transition whose activation energy cannot be overcome at room temperature. Py-LN may be formed in shocked meteorites by the back-transformation after the garnet-bridgmanite transition, and will indicate shock conditions around 45 GPa and 2000 K.

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.

The relationship between the sound velocity, density, and temperature of liquid metals is important when one tries to interpret the seismic velocity profile and infer the chemical compositions of the Earth’s outer core. We, therefore, have experimentally measured the longitudinal acoustic (LA) velocity of liquid indium under high P-T conditions. Also, we examined a Hugoniot data of liquid iron by comparing with an existing equation of state (EoS). The LA velocities of liquid and solid indium at pressures up to 6.7 GPa and temperatures mostly at 710 K were measured using inelastic X-ray scattering (IXS) to probe samples in an externally heated diamond-anvil cell. A thermal EoS for liquid indium derived from existing literature was used to calculate the density for the IXS measurements and to provide an independent check on the sound velocities. The IXS data are consistent with the hydrodynamic LA velocity derived from the liquid EoS, implying that the positive dispersion is minimal in liquid indium. The velocity-density relation for liquid indium derived from the EoS has temperature dependence, implying that Birch’s law does not hold for the liquid phase. Similarly we calculated the temperature-velocity-density relation of liquid iron over the Earth’s core range from a recently reported EoS. The resulting velocity-density relation is also temperature dependent, indicating that liquid iron thus does not follow Birch’s law. The violation of Birch’s law implies that the Hugoniot data cannot be directly compared with seismological observations because of the different temperature ranges. Formulation of the temperature-velocity-density of liquid iron-alloys supported by experimental measurements provides better understanding of the thermodynamic state of the Earth’s core.

We examined the density, bulk sound (compressional) velocity, and Gruneisen parameter of liquid pure Fe, Fe100H28 (0.50 wt % H), Fe88H40 (0.81 wt % H), and Fe76H52 (1.22 wt % H) at Earth's outer core pressure and temperature (P-T) conditions (~100 to 350 GPa, 4000 to 7000 K) based on first-principles molecular dynamics calculations. The results demonstrate that the thermodynamic Gruneisen parameter of liquid iron alloy decreases with increasing pressure, temperature, and hydrogen concentration, indicating a relatively small temperature gradient in the outer core when hydrogen is present. Along such temperature profile, both the density and compressional velocity of liquid iron containing ~1 wt % hydrogen match seismological observations. It suggests that hydrogen could be a primary light element in the core, although the shear velocity of the inner core is not reconciled with solid Fe-H alloy and thus requires another impurity element.

The thermodynamics of melting relations in the system Fe-FeO was investigated to the outer core–inner core boundary condition from a self-consistent thermodynamic database which was evaluated from the latest static high-pressure (P) and high-temperature (T) experiments. The evaluated database together with an existing nonideal mixing model for liquids reproduces experimental data on the eutectic composition and temperature to P = 50 GPa. On the other hand at the outer core pressures (136 to 330 GPa), employing an ideal solution model gives calculated eutectic temperatures of T = 2990–4330 K, which are also consistent with experimental data. Hence, the ideal solution model is applied to calculate the liquid property under outer core conditions and yields the eutectic compositions of Fe–7.2–9.1 wt % O. From the Gibbs free energy for the Fe-FeO liquids, I calculated the density, sound velocity, and isentropic temperature gradient of a hypothetical oxygen-bearing outer core. Under the outer core conditions, the addition of oxygen reduces the compressional wave velocity of iron liquid, moving it away from seismologically constrained values. An overall O-rich bulk outer core model is thus excluded. Seismological observations however suggest the presence of a low-velocity layer with a thickness of 60–70 km at the top of the outer core. The origin of such a low-velocity layer can be explained by an enrichment of oxygen which might be a consequence of chemical interactions between the core and mantle.

For life to have dramatically evolved and diversified during the so-called Cambrian explosion, there must have been significant changes in the environmental conditions of Earth. A rapid increase in atmospheric oxygen, which has been discussed as the key factor in the evolution of life, cannot by itself explain such an explosion, since life requires more than oxygen to flourish let alone survive. The supply of nutrients must have played a more critical role in the explosion, including an increase in phosphorus (P) and potassium (K) which are key elements for metabolisms to function. So, what happened at the onset of the Cambrian to bring about changes in environmental conditions and nutrient supply and ultimately evolution of life? An ultimate trigger for the Cambrian explosion is proposed here. The geotherm along subduction zones of a cooling Earth finally became cool enough around 600 Ma to allow slabs to be hydrated. The subduction of these hydrated slabs transferred voluminous water from the ocean to the mantle, resulting in a lowering of the sea level and an associated exceptional exposure of nutrient-enriched continental crust, along with an increase in atmospheric oxygen. This loss of water at the surface of the Earth and an associated increase in exposed landmass is referred to here as leaking Earth. Vast amounts of nutrients began to be carried through weathering, erosion, and transport of the landmass; rock fragments of the landmass would break down into ions during transport to the ocean through river, providing life forms (prokaryote) sufficient nutrients to live and evolve. Also, plume-driven dome-up beneath the continental crusts broadened the surface area providing a supply of nutrients an order magnitude greater than that produced through uplift of mountains by continental collision. Simultaneously, atmospheric oxygen began to increase rapidly due to the burial of dead organic matter by enhanced sedimentation from the emergence of a greater landmass, which ultimately inhibited oxidation of organic matter. Hence, oxygen began to accumulate in the atmosphere, which when coupled with a continuous supply of nutrients, resulted in an explosion of life, including an increase in the size. An enhanced oxygen supply in the atmosphere resulted in the formation of an ozone layer, providing life a shield from the UV radiation of the Sun; this enabled life to invade the land. In addition to a change in the supply of nutrients related to a leaking Earth, the evolution of life was accelerated through mass extinction events such as observed during Snowball Earth, possibly related to a starburst in our galaxy, as well as mutation in the genome due to radiogenic elements sourced from carbonatite magma (atomic bomb magma) in rift valley. There are two requirements to find a habitable planet: (1) the initial mass of an ocean and (2) the size of a planet. These two conditions determine the history of a planet, including planetary tectonics and the birth of life. This newfound perspective, which includes the importance of a leaking planet, provides a dawn of new planetary science and astrobiology.

[1] The stability of the K-rich new aluminous (NAL) phase was examined on the join Na1.00Mg2.00Al4.80Si1.15O12-K1.00Mg2.00Al4.80Si1.15O12 (Na100-K100) up to 144 GPa by X-ray diffraction in a laser-heated diamond anvil cell. Single-phase K100 and Na50K50 NAL were formed up to the lower mantle conditions, and the NAL phase coexisted with the calcium ferrite–type (CF) phase at 120 GPa and 2300 K for the Na75K25 bulk composition. This is a striking contrast to the K-free (Na100) NAL that becomes unstable above 27 GPa at 1850 K, which suggests that potassium stabilizes NAL at significantly higher pressures. K-rich NAL may host potassium in the lower mantle that contains K2O more than 0.09 wt %. In addition, the NAL phase likely formed owing to partial melting in the ultralow-velocity zone or because of a basal magma ocean. Future seismological observations may clarify whether NAL is a radiogenic heat source above the core-mantle boundary.

In order to examine pressure–volume–temperature (P–V–T) relations for CaSiO3 perovskite (Ca-perovskite), high-temperature compression experiments with in situ X-ray diffraction were performed in a laser-heated diamond anvil cell (DAC) to 127 GPa and 2,300 K. We also employed an external heating system in the DAC in order to obtain P–V data at a moderate temperature of 700 K up to 113 GPa, which is the reference temperature for constructing an equation of state. The P–V data at 700 K were fitted to the second-order Birch–Murnaghan equation of state, yielding K 700,1bar = 207 ± 4 GPa and V 700,1bar = 46.5 ± 0.1 A3. Thermal pressure terms were evaluated in the framework of the Mie–Gruneisen–Debye model, yielding γ 700,1bar = 2.7 ± 0.3, q 700,1bar = 1.2 ± 0.8, and θ 700,1bar = 1,300 ± 500 K. A thermodynamic thermal pressure model was also employed, yielding α700,1bar = 5.7 ± 0.5 × 10−5/K and (∂K/∂T) V = −0.010 ± 0.004 GPa/K. Computed densities along a lower mantle geotherm demonstrate that Ca-perovskite is denser than the surrounding lower mantle, suggesting that Ca-perovskite-rich rocks do not rise up through the lower mantle. One of such rocks might be a residue of partial melting of subducted mid-oceanic ridge basalt (MORB) at the base of the mantle. Since the partial melt is FeO-rich and therefore denser than the mantle, all the components of subducted MORB may not return to shallow levels.

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.

The phase transition boundary between the face-centered cubic (fcc) structure and hexagonal close-packed (hcp) structure in an Fe–Ni alloy was determined at pressures from 25 to 107 GPa by using an internally resistive-heated diamond anvil cell (DAC), combined with in situ synchrotron X-ray diffraction measurements. The fcc–hcp phase transition boundary in Fe–9.7 wt% Ni is located at slightly lower temperatures than that in pure Fe, confirming the previous understanding that the addition of Ni expands the stability field of the fcc phase. The dP/dT slope of the boundary was determined to be 0.0426 GPa/K, which is slightly larger than that of pure Fe. The pressure interval of the two-phase region is about 6 GPa at a constant temperature, implying that the previous estimates by laser-heated DAC experiments of 10–20 GPa were overestimated. The two-phase region of fcc + hcp would be limited to a pressure of about 120 GPa even in Fe–15 wt%Ni, excluding the possibility of the existence of the fcc phase in the inner core if the simple linear extrapolation of the two-phase region is applied. The pressure and temperature dependences of the c/a axial ratio of the hcp phase in Fe–9.7 wt% Ni are generally consistent with those in pure Fe, suggesting that Ni has minor effects on the c/a ratio.

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 growth curve of the continental crust shows that large amounts of continental crust formed in the early part of the Earth history are missing. In order to test a hypothesis that the former crust was subducted to the deep mantle, we performed phase assemblage analysis in the systems of mid-oceanic ridge basalt (MORB), anorthosite, and tonalite–trondhjemite–granite (TTG) down to the core–mantle boundary (CMB) conditions. Results show that all these materials can be subducted to the CMB leading to the development of a compositional layering in the D″ layer. We speculate that there could be five layers of FeO-enriched melt from partial melting of MORB, MORB crust, anorthosite, TTG, and slab or mantle peridotite in ascending order. Although the polymorphic transformation of perovskite to post-perovskite in (Mg,Fe)SiO3 may explain the seismic discontinuity at the top of the D″ layer (D″ discontinuity), the effects of solid solution on the sharpness of the transformation suggest that the compositional layering is more plausible for the origin of the D″ discontinuity. The D″ layer can be an “anti-crust” made up mostly of TTG + anorthosite derived from the former continental crust. Tectonic style of the anti-crust at the CMB is similar to that at the surface. At both places, chemically distinct layers are density stratified and are also characterized by the processes of accretion, magmatism, and metasomatism.

The phase transition boundary between the γ (face-centered cubic structure) and e (hexagonal close-packed structure) iron phases was determined up to 69 GPa by using a newly developed internally-heated diamond anvil cell, combined with in-situ synchrotron X-ray diffraction measurements. The improved experimental configuration provides stable heating with reliable temperature and pressure determination and phase identification. A simultaneous laser- and internal-heating system was also used to produce further higher temperature at higher pressures. The sample pressure at any given temperature was calculated using an established thermal equation of state for e iron. The γ-e transition boundary was bracketed with the normal and reversal directions and it is linear from 21 to 69 GPa with a dP/dT slope of 0.0394 GPa/K. There is no evidence for the existence of additional iron solid phase other than the γ and e phases over the P–T range of this study. We inferred that the γ-e-liquid triple point of iron should occur at 88 GPa and 2800 K.

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.

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 physical properties of iron (Fe) at high pressure and high temperature are crucial for understanding the chemical composition, evolution, and dynamics of planetary interiors. Indeed, the inner structures of the telluric planets all share a similar layered nature: a central metallic core composed mostly of iron, surrounded by a silicate mantle, and a thin, chemically differentiated crust. To date, most studies of iron have focused on the hexagonal closed packed (hcp, or ε) phase, as ε-Fe is likely stable across the pressure and temperature conditions of Earth’s core. However, at the more moderate pressures characteristic of the cores of smaller planetary bodies, such as the Moon, Mercury, or Mars, iron takes on a face-centered cubic (fcc, or γ) structure. Here we present compressional and shear wave sound velocity and density measurements of γ-Fe at high pressures and high temperatures, which are needed to develop accurate seismic models of planetary interiors. Our results indicate that the seismic velocities proposed for the Moon’s inner core by a recent reanalysis of Apollo seismic data are well below those of γ-Fe. Our dataset thus provides strong constraints to seismic models of the lunar core and cores of small telluric planets. This allows us to propose a direct compositional and velocity model for the Moon’s core.

Fluid-undersaturated high-pressure experiments were carried out in the system MgO–SiO2–H2O in a multi-anvil apparatus from 15.5 to 21.5 GPa and from 1000 to 1200 °C in order to constrain fluid-absent solid–solid reactions. From the experimentally determined fluid-absent phase equilibria, thermodynamic parameters for high-pressure hydrous phases were retrieved by a linear programming method. Together with dry mantle minerals from the existing data set and high-pressure hydrous phases from our previous study, an internally consistent set of thermochemical and thermophysical parameters for dense hydrous magnesium silicates (DHMSs) was evaluated up to 35 GPa and 1600 °C. High-pressure hydrous phases involved in the data set are, phase A, phase E, clinohumite, phase D, superhydrous phase B, hydrous wadsleyite, and hydrous ringwoodite. In addition, by calculating water-bearing reactions, we evaluated water activities in the fluid phase coexisting with mantle minerals. The water activity decreases with increasing pressure or temperature, suggesting that the amount of silicate component dissolved in the fluid phase increases with pressure or temperature. The calculated petrogenetic grid consisting of 89 univariant reactions can be used for the discussion of water circulations in the mantle. In a subducting slab peridotite, water in phase A as the post-serpentine phase is transferred to other DHMSs via solid–solid reactions down to the bottom of the transition zone. At around 660-km depth, in the stagnant slab heated from the surrounding mantle, a free fluid will be released by the dehydration of superhydrous phase B or phase D. The released fluid will be trapped by wadsleyite or ringwoodite in the surrounding mantle. In a cold subducting slab without stagnation at 660-km depth, DHMSs will survive into the lower mantle. In an upwelling plume through the water-bearing transition zone, a free fluid will be produced when the plume passes 410-km depth. Finally, water will be released to the Earth's surface by magmatism. The fluid generated in the deep mantle dissolves significant amounts of a silicate component. Therefore, at 410- and 660-km depths, some chemical differentiation will be processed.

Fluid-undersaturated experiments were conducted to determine the phase relations in the simplified peridotite system MgO–SiO 2 –H 2 O (MSH) at 11.0–14.5 GPa and 800–1400 °C. Stability relations of dense hydrous magnesium silicates (DHMSs) under fluid-undersaturated conditions were experimentally examined. From the fluid-absent experimental results, we retrieved thermodynamic data for clinohumite, phase A, phase E, and hydrous wadsleyite, consistent with the published data set for dry mantle minerals. With this new data set, we have calculated phase equilibria in the MSH system including dehydration reactions. The dehydration reactions calculated with lower water activities of 0.68–0.60 match the fluid-present experiments of this study above 11.0 GPa and 1000 °C, indicating that considerable amounts of silicate component were dissolved into the fluid phase. The calculated phase equilibria illustrate the stability of the post-antigorite phase A-bearing assemblages. In the cold subducting slab peridotite, phase A + enstatite assemblage survives into the transition zone, whereas phase A + forsterite + enstatite assemblage forms hydrous wadsleyite at a much shallower depth of about 360-km. The slab is subducted with no dehydration reactions occurring when entering the transition zone. The phase equilibria also show the high temperature stability of phase E. Phase E is stable up to 1200 °C at 13.5 GPa, a plausible condition in the mantle of relatively low temperature, i.e., beneath subduction zones. Phase E is a possible water reservoir in the mantle as well as wadsleyite and ringwoodite.

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.

In this study, a close link between the depth-distribution of subduction-zone earthquakes and dehydration events in the hydrated slab-peridotite is established. A phase diagram up to 27 GPa in the MgO–Al2O3–SiO2–H2O (MASH) system was constructed using a combination of thermodynamic calculation and Schreinemakers analysis on previous experimental data. At intermediate-depths (

We conducted multi-anvil experiments at simultaneous high pressures and temperatures using multiple internal pressure standards including Au, Pt, MgO, W, Mo, Pd, and Ag. Extensive synchrotron X-ray diffraction data for Au, Pt, and MgO were collected at pressures up to 28 GPa and temperatures between 300 and 2173 K. We compare pressures calculated from different pressure scales and demonstrate large discrepancies in pressure determination using different pressure standards or different thermal equations of state for the same standard. The comparison allows us to quantitatively determine the differences in pressure using different pressure scales in the high P–T experiments. Using the MgO scale of [J. Geophys. Res. 106 (2001) 515] as a reference pressure scale, new Au and Pt scales are presented that are consistent with the MgO scale. We further examined the validity of the assumption of constant q value (volume dependence of the Gruneisen parameter in the Mie–Gruneisen relation) for the calculations of thermal pressures, and show that an expression of q as a function of temperature and pressure may be necessary to best fit the simultaneous high P–T data. © 2004 Elsevier B.V. All rights reserved.

Using recent results from high-pressure experiments and thermodynamic calculations, phase diagrams of simplified hydrous peridotite in the system MgO-SiO2-H2O have been constructed to lower mantle conditions. Possible water subduction and circulation processes in the deep mantle are discussed on the basis of the constructed phase relations. For water transportation by subduction of peridotites, important phase relations are : (1) of antigorite (serpentine) at lower pressures (less than 10 GPa), and (2) of seven different high-pressure hydrous phases at higher pressures. In cold subducting slabs, water in antigorite is partially transferred to the hydrous phase A at depths greater than 160 km. With further subduction, water in phase A may be transported to the bottom of the transition zone via solid-solid reactions among seven high-pressure hydrous phases. If the slab temperature at 30 GPa is lower than 1000°C, hydrous phase D will carry water into the deep lower mantle. Along the cold slab geotherm, large fluid fluxes are predicted at shallow (300-km depth) and deep (700-km depth) levels, depending on the slab temperature. The depth distributions of dehydration reactions suggest that observed subduction zone seismicity could be related to dehydration reactions in the slab. The lower water activities in the fluid phases at deep mantle conditions imply that such fluid phases could dissolve significant amounts of silicate components. Therefore, fluid phases released by dehydration reactions at deeper levels should have different physical properties from those at shallower levels.

Ionic conductivity and molar volume measurements were performed on H(2)O ice at high pressure (P) and temperature (T) in a resistive-heated diamond anvil cell. The conductivity data obtained at P = 20-62 GPa, T = 304-930 K are well fitted with a single Arrhenius equation. Isothermal volume measurements at T = 873 K, P = 30-101 GPa indicate that H(2)O ice undergoes phase transitions at P = 50 GPa and 53 GPa due to hydrogen-bond symmetrization. Combining these results, we suggest that the conduction mechanism does not change with pressure-induced hydrogen-bond symmetrization. Along the Arrhenius behavior of conductivity data, the experimental evidence for superionic conduction (10(-1) S/cm) was found at T = 739 K, P = 56 GPa and T = 749 K, P = 62 GPa, which is significantly low temperature compared with earlier theoretical estimates resorted to the observation of a drastic rise of the melting curve. We infer that the sudden increase of the melting temperature is not related to the onset of superionic conduction, but is attributed to the phase change regarding to the symmetrization.

[1] The crystal structure of Fe-10%Ni was investigated up to 340 GPa and 4700 K, corresponding to the Earth's inner core conditions by synchrotron X-ray diffraction measurementsin-situat ultrahigh pressure and temperature in a laser-heated diamond-anvil cell. The results show that hexagonal closed-packed (hcp) structure is stable throughout the experimental conditions investigated with no evidence for a phase transition to body-centered cubic (bcc) or face-centered cubic (fcc) phases. The axialc/a ratio of the hcp crystal obtained around 330 GPa has a small temperature dependence similar to the axial ratio of pure Fe. Iron alloy with less than 10% Ni crystallizes to an hcp structure with c/a ratio of ∼1.61 at inner core conditions, although the effect of small amount of light elements remains to be examined by experiments.

International audience; We performed sound velocity and density measurements on polycrystalline hexagonal close-packed (hcp) iron at simultaneous high pressure and high temperature, up to 93 GPa and 1100 K, by inelastic x-ray scattering and x-ray diffraction. Our experimental results indicate that high-temperature anharmonic corrections are negligible at least up to 1100 K and that the aggregate compressional velocity VP scales linearly with density over the pressure and temperature range of the investigation (Birch's law). The new results are compared with literature studies and we discuss the extrapolation schemes commonly used in experimental mineral physics, with specific regard to extrapolations to the Earth's core conditions

We conducted simultaneous high-pressure $(P)$ and high-temperature $(T)$ unit-cell volume $(V)$ measurements of ${\text{H}}_{2}\text{O}$ ice VII in an externally heated diamond anvil cell using in situ synchrotron x-ray diffraction technique with an angle-dispersive system. Isothermal unit-cell volume of ice VII was collected at $P=33--50\text{ }\text{GPa}$ with $T=873\text{ }\text{K}$. Nonisothermal experiments at $T=430--740\text{ }\text{K}$ and $P=19--37\text{ }\text{GPa}$ were also conducted to further constrain $P\text{\ensuremath{-}}V\text{\ensuremath{-}}T$ properties of ice VII. Using the existing 300 K compression data for an equation of state (EoS) at the reference temperature, a thermal EoS of ice VII was constructed from the collected high-$P\text{\ensuremath{-}}T$ data. The melting temperature of ice VII was computed up to $P=40\text{ }\text{GPa}$ by thermodynamic calculations with the newly constructed EoS of ice VII. The resulting melting temperatures are higher than that of previous calculations since the new EoS gives a smaller molar volume of ice VII. In contrast, the calculated melting curves are consistent with recent experimental estimates.

[1] An internally consistent thermodynamic database for pure iron has been established to pressures (P) up to 360 GPa and temperatures (T) up to 7000 K from existing static experimental data and thermochemical measurements. The database includes body-centered cubic (BCC) phases (α or δ phase), the face-centered cubic (FCC) phase (γ phase), the hexagonal close-packed (HCP) phase (ɛ phase), and the liquid phase. We describe fundamental thermodynamic relations as the Gibbs free energy divided into thermochemical and thermophysical terms. The thermochemical data were evaluated from existing metallurgy databases together with experimentally determined phase relations. The thermophysical term is obtained from the pressure-volume-temperature equations of state (EoS) for the phases. We constructed an EoS of the FCC phase from our recent internally-heated diamond anvil cell (DAC) experimental data and assessed the EoS of the liquid phase from existing laser-heated DAC experiments together with density data at P = 1 bar, 0.2 GPa, and along the Hugoniot. The HCP-FCC-liquid triple point is located at P = 90 GPa and T = 2800 K. The calculated melting temperature of HCP iron at the inner core boundary (P = 330 GPa) is 4900 K and the density change at melting is −1.2%. The core density deficits at the inner core boundary are 8.1 wt.% and 5.3 wt.% for the liquid outer core and solid inner core, respectively. The calculated melting temperature is much lower than that from dynamic shock wave experiments, suggesting that the HCP structure may not be stable in the inner core. We included a hypothetical high-pressure BCC phase which could be stabilized above 220 GPa by a solid-solid transition of high-P BCC-HCP phases. This hypothetical BCC phase should have a large entropy to give a high melting temperature in order to reconcile the existing discrepancies between the static and shock wave experimental studies.

International audience; Fe-S-Si immiscibility has been investigated using in situ X-ray methods at high pressure and high temperature. An in situ X-ray diffraction study of immiscible liquids for P 5 GPa and T/Tm 1.1 has been performed, showing differences in structural properties between S-rich and Si-rich coexisting liquid phases. Moreover, the respective role of S and Si on Fe alloys has been quantitatively investigated in Fe-X liquids (X = S, Si) with 20%wt of light elements. The transition from immiscible to miscible textures has been observed in the ternary mixture by in situ X-ray radiography for the Fe-18wt%S-8.5wt%Si (Fe-28.8at%Si-11.9at%S) sample composition between 12 and 16 GPa. Closure of the miscibility gap occurs in the same pressure range as the Fe-S eutectic liquid evolves toward a compact structure. By working within the framework of the hard sphere (HS) fluid commonly used in liquid state theory, we show that the equation of state (EOS) for additive HS reproduces very well the compressibility of Fe-Si alloys measured in the 0- 5 GPa pressure range, whereas that of Fe-S alloys behave quite differently, with a high degree of covalency. However it is also stressed that at higher pressures (P > 15 GPa) liquid Fe-S adopts a structure close to that exhibited by Fe-Si alloys, a feature which suggests that the compressibility of the two alloys should behave similarly at very high pressure due to transition of S behaviour from covalent to interstitial. Hence for Fe-S liquid under core conditions, we conclude that the sound velocity in Fe-S alloy is compatible with PREM model.

[1] A semiquantitative petrogenetic grid in the system MgO-SiO2-H2O up to 30 GPa and 1600°C was constructed using Schreinemakers analysis on previous experimental data. The grid includes stability relations of hydrous wadsleyite, hydrous ringwoodite, and dense hydrous magnesium silicates (DHMSs): phase A, phase D, phase E, and superhydrous phase B. A sequence of chemical reactions among these hydrous phases was clarified. In the mantle transition zone (410–660-km depth), hydrous wadsleyite and hydrous ringwoodite are stable even at a standard mantle temperature of 1600°C, whereas in the other depths, nominal hydrous phases including DHMSs are stable below 1400°C. Newly found phase transitions in hydrous wadsleyite and hydrous ringwoodite are multiphase and multireaction because their compositions are on neither MgO-SiO2 nor Mg2SiO4-H2O tie lines, different from single transition in the dry Mg2SiO4 system. The grid indicates that water in the subducting slab peridotite would be transported to the bottom of the upper mantle by several DHMSs and hydrous polymorphs of olivine, via solid-solid reactions after antigorite decomposition. Finally, DHMSs would dehydrate to liberate free water at the upper mantle-lower mantle boundary layer or deeper level. We also examined a relation between the predicted depth distributions of dehydration reactions in slab peridotite along several possible pressure-temperature paths and the mode of seismic frequency in each subduction zone. The result suggests a possible origin of intermediate to deep seismicity by the dehydration of hydrous phases in the subducting slab.

[1] We have determined the postspinel transformation boundary in Mg2SiO4 by combining quench technique with in situ pressure measurements, using multiple internal pressure standards including Au, MgO, and Pt. The experimentally determined boundary is in general agreement with previous in situ measurements in which the Au scale of Anderson et al. [1989] was used to calculate pressure: Using this pressure scale, it occurs at significantly lower pressures compared to that corresponding to the 660-km seismic discontinuity. In this study, we also report new experimental data on the transformation boundary determined using MgO as an internal standard. The results show that the transition boundary is located at pressures close to the 660-km discontinuity using the MgO pressure scale of Speziale et al. [2001] and can be represented by a linear equation, P(GPa) = 25.12 − 0.0013T(°C). The Clapeyron slope for the postspinel transition boundary is precisely determined and is significantly less negative than previous estimates. Our results, based on the MgO pressure scale, support the conventional hypothesis that the postspinel transformation is responsible for the observed 660-km seismic discontinuity.

Here we report the phase boundaries between majorite, akimotoite (ilmenite), and perovskite in MgSiO3 determined by in situ measurements in a multi-anvil apparatus. We used both gold and platinum as internal pressure standards at high pressure and temperature. Our results obtained at 1400-2000°C, together with previous results obtained at 1000-1200°C by Ono et al. [2001], precisely locate the akimotoite-perovskite transition boundary at P (GPa) = 25.09 - 0.0027 × T (°C), based on the P-V-T equation of state of gold [Anderson et al., 1989]. Our new experimental data show the position of triple point to be 20.0 GPa and 1920°C. The present measurements reconfirm our earlier study [Hirose et al., 2001] that Anderson's gold pressure scale gives slightly higher pressures than the platinum pressure scale proposed either by Jamieson et al. [1982] or Holmes et al. [1989].

Compression and decompression experiments on face-centered cubic (fcc) γ′-Fe4N to 77 GPa at room temperature were conducted in a diamond-anvil cell with in situ X-ray diffraction (XRD) to examine its stability under high pressure. In the investigated pressure range, γ′-Fe4N did not show any structural transitions. However, a peak broadening was observed in the XRD patterns above 60 GPa. The obtained pressure-volume data to 60 GPa were fitted to the third-order Birch-Murnaghan equation of state (EoS), which yielded the following elastic parameters: K0 = 169 (6) GPa, K′ = 4.1 (4), with a fixed V0 = 54.95 Å at 1 bar. A quantitative Schreinemakers' web was obtained at 15–60 GPa and 300–1600 K by combining the EoS for γ′-Fe4N with reported phase stability data at low pressures. The web indicates the existence of an invariant point at 41 GPa and 1000 K where γ′-Fe4N, hexagonal closed-packed (hcp) ε-Fe7N3, double hexagonal closed-packed β-Fe7N3, and hcp Fe phases are stable. From the invariant point, a reaction γ′-Fe4N = β-Fe7N3 + hcp Fe originates toward the high-pressure side, which determines the high-pressure stability of γ′-Fe4N at 56 GPa and 300 K. Therefore, the γ′-Fe4N phase observed in the experiments beyond this pressure must be metastable. The obtained results support the existing idea that β-Fe7N3 would be the most nitrogen-rich iron compound under core conditions. An iron carbonitride Fe7(C,N)3 found as a mantle-derived diamond inclusion implies that β-Fe7N3 and Fe7C3 may form a continuous solid solution in the mantle deeper than 1000 km depth. Diamond formation may be related to the presence of fluids in the mantle, and dehydration reactions of high-pressure hydrous phase D might have supplied free fluids in the mantle at depths greater than 1000 km. As such, the existence of Fe7(C,N)3 in diamond can be an indicator of water transportation to the deep mantle.

The relative abundance of light elements in the Earth's core has long been controversial. Recently, the presence of carbon in the core has been emphasized, because the density and sound velocities of the inner core may be consistent with solid Fe7C3. Here we report the longitudinal wave velocity of liquid Fe84C16 up to 70 GPa based on inelastic X-ray scattering measurements. We find the velocity to be substantially slower than that of solid iron and Fe3C and to be faster than that of liquid iron. The thermodynamic equation of state for liquid Fe84C16 is also obtained from the velocity data combined with previous density measurements at 1 bar. The longitudinal velocity of the outer core, about 4% faster than that of liquid iron, is consistent with the presence of 4–5 at.% carbon. However, that amount of carbon is too small to account for the outer core density deficit, suggesting that carbon cannot be a predominant light element in the core., The composition of the Earth's core, particularly the light elements present, is not well constrained. Here, the authors report sound velocities of liquid iron-carbon alloy as measured at very high pressures using inelastic X-ray scattering and suggest that carbon cannot be predominant in the outer core.

The compression behavior of Al-bearing stishovite was investigated by powder X-ray diffraction up to 40 GPa with the BL13A beamline at the Photon Factory (KEK, Japan). A reliable equation of state for stishovite was obtained using a diamond anvil cell coupled with a yttrium-aluminum-garnet (YAG) laser-heating. A sample containing 2.1 wt% Al 2 O 3 was heated using a YAG laser at each pressure increment to relax deviatoric stress. X-ray diffraction measurements were carried out at 300 K using the angle-dispersive technique. A least squares refinement of the data yielded equation of state parameters where the bulk modulus K o = 282 (±2) GPa when the first pressure derivative of the bulk modulus K o′ was fixed at 4. The effect of Al is to decrease slightly the bulk modulus of stishovite and increase the density of the subducted oceanic crust. The enhanced compressibility of Al-bearing stishovite certainly has geophysical and geochemical implications for the fate of the subducted slab, as this mineral is the main constituent of subducted mid-oceanic ridge basalt (MORB) in the Earth’s mantle.

Melting experiments on Fe3S were conducted to 75 GPa and 2800 K in laser-heated and internally resistive-heated diamond anvil cells with in-situ x-ray diffraction and/or post-mortem textural observation. From the constrained melting curve, we assessed the thermal equation of state for Fe3S liquid. Then we constructed a thermodynamic model of melting of the system Fe–Fe3S including the eutectic relation under high pressures based on our new experimental data. The mixing properties of Fe–S liquids under high pressures were evaluated in order to account for existing experimental data on eutectic temperature. The results demonstrate that the mixing of Fe and S liquids are nonideal at any core pressure. The calculated sulphur content in eutectic point decreases with increasing pressure to 120 GPa and is fairly constant of 8 wt% at greater pressures. From the Gibbs free energy, we derived the parameters to calculate the crystallising point of an Fe–S core and its isentrope, and then we calculated the density and the longitudinal seismic wave velocity (Vp) of these liquids along each isentrope. While Fe3S liquid can account for the seismologically constrained density and Vp profiles over the outer core, the density of the precipitating phase is too low for the inner core. On the other hand, a hypothetical Fe–S liquid core with a bulk composition on the Fe-rich side of the eutectic point cannot represent the density and Vp profiles of the Earth’s outer core. Therefore, Earth’s core cannot be approximated by the system Fe–S and it should include another light element.