Tetsuya Komabayashi

In situ measurements of the majorite‐akimotoite‐perovskite phase transition boundaries in MgSiO3

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 Andersons gold pressure scale gives slightly higher pressures than the platinum pressure scale proposed either by Jamieson et al. [1982] or Holmes et al. [1989].

Liquid iron‐sulfur alloys at outer core conditions by first‐principles calculations

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 Earths 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.

Thermodynamics of melting relations in the system Fe‐FeO at high pressure: Implications for oxygen in the Earth's core

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.

Toward a mineral physics reference model for the Moon’s core

Significance Iron is the main constituent of terrestrial planetary cores, taking on a hexagonal closed packed structure under the conditions of Earth’s inner core, and a face-centered cubic (fcc) structure at the more moderate pressures of smaller bodies, such as the Moon, Mercury, or Mars. Here we present sound velocity and density measurements of fcc iron at pressures and temperatures characteristic of small planetary interiors. The results indicate that the seismic velocities currently proposed for the Moon’s inner core are well below those of fcc iron or plausible iron alloys. Our dataset provides strong constraints to seismic models of the lunar core and cores of small telluric planets, and allows us to build a direct compositional and velocity model of the Moon’s core. 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.

Carbon-depleted outer core revealed by sound velocity measurements of liquid iron–carbon alloy

The relative abundance of light elements in the Earths 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.

Experimental evidence of superionic conduction in H2O ice

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.

Sound velocity of liquid Fe–Ni–S at high pressure

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 Earths outer core if it is the only light element.

Temperature dependence of the velocity-density relation for liquid metals under high pressure: Implications for the Earth’s outer core

Abstract 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 temperaturevelocity- 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.

Synthesis and crystal structure of LiNbO3-type Mg3Al2Si3O12: A possible indicator of shock conditions of meteorites

Abstract LiNbO3-type Mg2.98(2)Al1.99(2)Si3.02(2)O12 (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) Å, c = 12.6885(8) Å, V = 255.23(3) Å3, with Z = 6 (hexagonal, R3c). The average A-O and B-O distances of the AO6 and BO6 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 Å, respectively). The present compound has the B-site cations at the octahedral site largely shifted along the c axis compared with other LiNbO3-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 BO6 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.