Tatsuya Sakamaki

Stability of hydrous melt at the base of the Earth's upper mantle

Seismological observations have revealed the existence of low-velocity and high-attenuation zones above the discontinuity at 410 km depth, at the base of the Earths upper mantle. It has been suggested that a small amount of melt could be responsible for such anomalies. The density of silicate melt under dry conditions has been measured at high pressure and found to be denser than the surrounding solid, thereby allowing the melt to remain at depth. But no experimental investigation of the density of hydrous melt has yet been carried out. Here we present data constraining the density of hydrous basaltic melt under pressure to examine the stability of melt above the 410-km discontinuity. We infer that hydrous magma formed by partial melting above the 410-km discontinuity may indeed be gravitationally stable, thereby supporting the idea that low-velocity or high-attentuation regions just above the mantle transition zone may result from the presence of melt.

Atomistic insight into viscosity and density of silicate melts under pressure

A defining characteristic of silicate melts is the degree of polymerization (tetrahedral connectivity), which dictates viscosity and affects compressibility. While viscosity of depolymerized silicate melts increases with pressure consistent with the free-volume theory, isothermal viscosity of polymerized melts decreases with pressure up to ~3-5 GPa, above which it turns over to normal (positive) pressure dependence. Here we show that the viscosity turnover in polymerized liquids corresponds to the tetrahedral packing limit, below which the structure is compressed through tightening of the inter-tetrahedral bond angle, resulting in high compressibility, continual breakup of tetrahedral connectivity and viscosity decrease with increasing pressure. Above the turnover pressure, silicon and aluminium coordination increases to allow further packing, with increasing viscosity and density. These structural responses prescribe the distribution of melt viscosity and density with depth and play an important role in magma transport in terrestrial planetary interiors.

Density of dry peridotite magma at high pressure using an X-ray absorption method

Abstract The density of a peridotite magma was measured up to 2.5 GPa and 2300 K using an X-ray absorption method. The method allowed measurement of the density of a peridotite melt under seven different conditions and clarified the pressure and temperature dependence of the density. A fit of the pressure-density-temperature data to the high-temperature Birch-Murnaghan equation of state yielded the isothermal bulk modulus, KT0 = 24.0 ± 1.3 GPa, its pressure derivative, K0′ = 7.3 ± 0.8, and the derivative of bulk modulus (∂KT/∂T)P = -0.0027 ± 0.0017 GPa/K at 2100 K. The large bulk modulus and its pressure derivative of the peridotite melt compared with that of basaltic melt is consistent with previous results from sink-float experiments.

Density measurement of liquid FeS at high pressures using synchrotron X-ray absorption

Abstract The density of liquid iron sulfide (FeS) was measured up to 3.8 GPa and 1800 K using an X-ray absorption method. The compression curve of liquid FeS was fitted using the Vinet equation of state. The isothermal bulk modulus and its temperature and pressure derivatives were determined using a nonlinear least-squares fit. The parameter sets determined were: K0T = 2.5 ± 0.3 GPa at T = 1500 K, (dK0/dT)P = -0.0036 ± 0.0003 GPa/K, and (dK0/dP)T = 24 ± 2. These results suggest that liquid FeS is more compressible than Fe-rich liquid Fe-S.

Stability of hydrous phase H MgSiO4H2 under lower mantle conditions

We report the stability field of a new high-pressure hydrous phase, phase H MgSiO4H2, and its implications for water transport into the deep lower mantle. We observed the existence of hydrous phase H at pressures around 50 GPa, and this phase was stable up to 60 GPa. Our results, together with those of previous works, indicate that pure phase H MgSiO4H2 has a very narrow stability field in the pressure range 35 < P < 60 GPa, equivalent to the uppermost part of the lower mantle. The stability field expands significantly toward higher pressures and temperatures on dissolution of the hydrous AlOOH component. The hydrous phase H-phase δ solid solution (aluminous phase H), (MgSi,Al2)O4H2, is potentially the most important hydrous phase present under the deep lower mantle conditions.

Density of carbonated peridotite magma at high pressure using an X-ray absorption method

Abstract The density of carbonated peridotite magma was measured up to 3.8 GPa and 2100 K using an X-ray absorption method. A fit of the pressure-density-temperature data to the high-temperature Birch-Murnaghan equation of state yielded the isothermal bulk modulus, KT0 = 22.9 ± 1.4 GPa, its pressure derivative, K0′ = 7.4 ± 1.4, and the temperature derivative of the bulk modulus (∂KT/∂T)P = -0.006 ± 0.002 GPa/K at 1800 K. The bulk modulus of carbonated peridotite magma is larger than that of hydrous peridotite magma. The partial molar volume of CO2 in magma under high pressure and temperature conditions was calculated and fit using the Vinet equation of state. The isothermal bulk modulus was KT0 = 8.1 ± 1.7GPa, and its pressure derivative was K0′ = 7.2 ± 2.0 at 2000 K. Our results show that the partial molar volume of CO2 is less compressible than that of H2O, suggesting that, on an equal molar basis, CO2 is more effective than H2O in reducing peridotite melt density at high pressure.

Simultaneous structure and elastic wave velocity measurement of SiO2 glass at high pressures and high temperatures in a Paris-Edinburgh cell

An integration of multi-angle energy-dispersive x-ray diffraction and ultrasonic elastic wave velocity measurements in a Paris-Edinburgh cell enabled us to simultaneously investigate the structures and elastic wave velocities of amorphous materials at high pressure and high temperature conditions. We report the first simultaneous structure and elastic wave velocity measurement for SiO(2) glass at pressures up to 6.8 GPa at around 500°C. The first sharp diffraction peak (FSDP) in the structure factor S(Q) evidently shifted to higher Q with increasing pressure, reflecting the shrinking of intermediate-range order, while the Si-O bond distance was almost unchanged up to 6.8 GPa. In correlation with the shift of FSDP position, compressional wave velocity (Vp) and Poissons ratio increased markedly with increasing pressure. In contrast, shear wave velocity (Vs) changed only at pressures below 4 GPa, and then remained unchanged at ~4.0-6.8 GPa. These observations indicate a strong correlation between the intermediate range order variations and Vp or Poissons ratio, but a complicated behavior for Vs. The result demonstrates a new capability of simultaneous measurement of structures and elastic wave velocities at high pressure and high temperature conditions to provide direct link between microscopic structure and macroscopic elastic properties of amorphous materials.

Structure of jadeite melt at high pressures up to 4.9 GPa

The structure of jadeite (NaAlSi2O6) melts has been studied using multiple-angle energy-dispersive x-ray diffraction up to 4.9 GPa and 1923 K. The first sharp diffraction peak in the structure factor shifts toward higher momentum transfer as pressure increases, indicating the shrinkage of the intermediate network in the melt. The radial distribution function shows a monotonous decrease in average T-T length and T-O-T angle with increasing pressure, but displays no detectable change in the average bond length between tetrahedrally coordinated cations and oxygen (T-O length, where T = Si4+, Al3+). Our observations indicate that the dominant structural changes occur in the intermediate range order at pressures up to 4.9 GPa. The changes in T-O length, T-T length, and T-O-T angle appear to correlate with the viscosity anomaly in this pressure range.

Constraints on Earth’s inner core composition inferred from measurements of the sound velocity of hcp-iron in extreme conditions

Experimental determination of VP of hcp-Fe can show potential candidates for major light elements in Earth’s inner core. Hexagonal close-packed iron (hcp-Fe) is a main component of Earth’s inner core. The difference in density between hcp-Fe and the inner core in the Preliminary Reference Earth Model (PREM) shows a density deficit, which implies an existence of light elements in the core. Sound velocities then provide an important constraint on the amount and kind of light elements in the core. Although seismological observations provide density–sound velocity data of Earth’s core, there are few measurements in controlled laboratory conditions for comparison. We report the compressional sound velocity (VP) of hcp-Fe up to 163 GPa and 3000 K using inelastic x-ray scattering from a laser-heated sample in a diamond anvil cell. We propose a new high-temperature Birch’s law for hcp-Fe, which gives us the VP of pure hcp-Fe up to core conditions. We find that Earth’s inner core has a 4 to 5% smaller density and a 4 to 10% smaller VP than hcp-Fe. Our results demonstrate that components other than Fe in Earth’s core are required to explain Earth’s core density and velocity deficits compared to hcp-Fe. Assuming that the temperature effects on iron alloys are the same as those on hcp-Fe, we narrow down light elements in the inner core in terms of the velocity deficit. Hydrogen is a good candidate; thus, Earth’s core may be a hidden hydrogen reservoir. Silicon and sulfur are also possible candidates and could show good agreement with PREM if we consider the presence of some melt in the inner core, anelasticity, and/or a premelting effect.

Hydrogenation of FeSi under high pressure

Abstract Hydrogen is the most abundant element in the solar system, suggesting that hydrogen is one of the plausible light elements in the planetary cores. To investigate the solubility of hydrogen into FeSi and phase relations of the FeSi-H system under high pressure, we performed in situ X-ray diffraction experiments on the FeSi-H and FeSi systems at high pressure and high temperature. Hydrogen starts to dissolve in FeSi (hydrogenation) and form FeSiHx with cubic B20 structure above 10 GPa. Hydrogen content (x), estimated from the volume difference between the FeSi-H and FeSi systems, increases from 0.07 to 0.22 with increasing pressure for P > 10 GPa. Comparing the present results with hydrogenation pressure of Fe, presence of Si in metal increases the minimal pressure for H incorporation. Hydrogen, therefore, can only incorporate into the Fe-Si core at the deeper part (P > 10 GPa) in the planetary interior.