Satoru Urakawa

The Phase Boundary Between α- and β-Mg2SiO4 Determined by in Situ X-ray Observation

The stability of Mg2SiO4, a major constituent in the Earths mantle, has been investigated experimentally by in situ observation with synchrotron radiation. A cubic-type high-pressure apparatus equipped with sintered diamond anvils has been used over pressures of 11 to 15 gigapascals and temperatures of 800� to 1600�C. The phase stability of α-Mg2SiO4 and β-Mg2SiO4 was determined by taking account of the kinetic behavior of transition. The phase boundary between α-Mg2SiO4 and β-Mg2SiO4 is approximated by the linear expression P = (9.3 � 0.1) + (0.0036 � 0.0002)T where P is pressure in gigapascals and T is temperature in degrees Celsius.

In-situ measurement of viscosity and density of carbonate melts at high pressure

We present the first measurements of carbonate melt viscosity and density at mantle pressures and temperatures and provide important data for modelling carbonatite behaviour within the mantle. Synchrotron radiation was used to observe falling spheres with high atomic number in situ, allowing precise determination of high terminal velocities over short fall distances. The measured viscosities of 1.5 (5) X 10m2 to 5 (2.5) X 10e3 Pas are the lowest of any known terrestrial magma types and these measurements extend the region of measurable viscosity at high pressure by at least 2 orders of magnitude. Accurate measurements of K,Ca(CO,), melt density were performed at atmospheric pressure:

In situ Observation of ilmenite‐perovskite phase transition in MgSiO3 using synchrotron radiation

In situ observations of the ilmenite-perovskite transition in MgSiO3 were carried out in a multianvil high-pressure apparatus interfaced with synchrotron radiation. The phase boundary between ilmenite and perovskite in the temperature range of 1300–1600 K was determined to be P (GPa) = 28.4(±0.4) - 0.0029(± 0.0020)T (K) based on Jamiesons equation of state of gold [Jamieson et al., 1982] and P (GPa) = 27.3(±0.4) - 0.0035(±0.0024)T (K) based on Andersons equation of state of gold [Anderson et al., 1989]. The consistency of our results, using Jamiesons equation of state, with previous studies obtained by quench methods leads us to conclude that the 660 km seismic discontinuity in the mantle can be attributed a phase transition to perovskite phase. However, the phase boundary based on the Andersons equation of state implies that the depth of the 660-km seismic discontinuity does not match the pressure of this transition.

The effect of temperature, pressure, and sulfur content on viscosity of the Fe–FeS melt

Abstract The Fe–FeS melt is thought to be the major candidate of the outer core material. Its viscosity is one of the most important physical properties to study the dynamics of the convection in the outer core. We performed the in situ viscosity measurement of the Fe–FeS melt under high pressure using X-ray radiography falling sphere method with a novel sample assembly. Viscosity was measured in the temperature, pressure, and compositional conditions of 1233–1923 K, 1.5–6.9 GPa, and Fe–Fe 72 S 28 (wt%), respectively. The viscosity coefficients obtained by 17 measurements change systematically in the range of 0.008–0.036 Pa s. An activation energy of the viscous flow, Q =30.0±8.6 kJ/mol, and the activation volume, Δ V =1.5±0.7×10 −6 m 3 /mol, are determined as the temperature and pressure dependence, and the viscosity of the Fe 72 S 28 melt is found to be smaller than that of the Fe melt by 15±10%. These tendencies can be well correlated with the structural variation of the Fe–FeS melt.

Formation of metastable assemblages and mechanisms of the grain‐size reduction in the Postspinel transformation of Mg2SiO4

An in situ X-ray observation of the postspinel transformation kinetics was made using intense synchrotron radiation. We confirmed that Mg2SiO4 spinel transforms into fine lamellae of SiO2 stishovite and periclase, and/or MgSiO3 ilmenite and periclase as an intermediate step in the postspinel transformation. These metastable assemblages eventually disappear and form the stable assemblages of MgSiO3 perovskite and periclase. Initial grain size just after the postspinel transformation drastically changes with overpressure. Viscosity of the subducting slab into the lower mantle, which is thought to be deformed by grain-size-sensitive creep, would depend on overpressure needed for the postspinel transformation at geological time scale.

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.

Radiographic study on the viscosity of the Fe-FeS melts at the pressure of 5 to 7 GPa

Abstract Stokes’ viscometry combined with in situ X-ray radiographic observation, using the 6-8 type multi-anvil press and synchrotron radiation, has been applied to the viscosity measurement of the Fe- FeS melt up to pressures of 7 GPa. The viscosity is found to be about 2 × 10-2 Pa-s at 5 to 7 GPa and temperatures about 1350 K, in marked contrast to previous viscosity measurements, which showed high viscosity, 0.5 to 14 Pa-s, at 2 to 5 GPa (LeBlanc and Secco 1996). Our viscosity data, however, is consistent with all other evidence, which include 1 atm viscosity data, X-ray structure analysis, and ab initio simulations. Recent viscosity measurements (Dobson et al. 2000) also showed the viscosity of Fe-FeS melt to be about 10-2 Pa-s at 2.5 GPa. Thus, we are confident that the viscosity of the Fe- FeS melt is close to a typical value (10-2 Pa-s) of viscosity for liquid metal even at high pressures.

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.

Viscosity of liquid sulfur under high pressure

The viscosity of liquid sulfur up to 9.7 GPa and 1067 K was measured using the in situ x-ray radiography falling sphere method. The viscosity coefficients were found to range from 0.11 to 0.69 Pa s, and decreased continuously with increasing pressure under approximately constant homologous temperature conditions. The observed viscosity variation suggests that a gradual structural change occurs in liquid sulfur with pressure up to 10 GPa. The transition in liquid sulfur proposed by Brazhkin et al (1991 Phys. Lett. A 154 413) from thermobaric measurements has not been confirmed by the present viscometry.