Ken-ichi Funakoshi

Olivine‐wadsleyite transition in the system (Mg,Fe)2SiO4

Phase relations of the olivine-wadsleyite transition in the system (Mg,Fe) 2 SiO 4 have been determined at 1600 and 1900 K using the quench method in a Kawai-type high-pressure apparatus. Pressure was determined at a precision better than 0.2 GPa using in situ X-ray diffraction with MgO as a pressure standard. The transition pressures of the end-member Mg 2 SiO 4 are estimated to be 14.2 and 15.4 GPa at 1600 and 1900 K, respectively. Partition coefficients for Fe and Mg between olivine and wadsleyite are 0.51 at 1600 K and 0.61 at 1900 K. By comparing the depth of the discontinuity with the transition pressure, the temperature at 410 km depth is estimated to be 1760 ± 45 K for a pyrolitic upper mantle. The mantle potential temperature is estimated to be in the range 1550-1650 K. The temperature at the bottom of the upper mantle is estimated to be 1880 ± 50 K. The thickness of the olivine-wadsleyite transition in a pyrolitic mantle is determined to be between 7 and 13 km for a pyrolitic mantle, depending on the efficiency of vertical heat transfer. Regions of rapid vertical flow (e.g., convection limbs), in which thermal diffusion is negligible, should have a larger transition interval than stagnant regions, where thermal diffusion is effective. This is in apparent contradiction to short-period seismic wave observations that indicate a maximum thickness of <5 km. An upper mantle in the region of the 410 km discontinuity with about 40% olivine and an Mg# of at least 89 can possibly explain both the transition thickness and velocity perturbation at the 410 km discontinuity.

In situ measurements of the phase transition boundary in Mg3Al2Si3O12: implications for the nature of the seismic discontinuities in the Earth’s mantle

Abstract Here we report the phase boundary of pyrope garnet (Mg3Al2Si3O12) to Al-bearing silicate perovskite plus corundum, with the highest transition pressure determined by in situ measurements in a multi-anvil apparatus at high temperature. The consistency of the pressure scales by different standards of Au, NaCl, Pt, W, and Mo at high temperature was also evaluated by in situ X-ray measurements. Our results, together with recent in situ measurements of the post-spinel transition in Mg2SiO4 [Irifune et al., Science 279 (1998) 1698–1700] and the ilmenite–perovskite transition in MgSiO3 [Ono et al., Geophys. Res. Lett. (2000) submitted], show that pressures determined in conventional quench experiments [Ito and Takahashi, J. Geophys. Res. 94 (1989) 10637–10646] could have been overestimated by more than 2 GPa at pressures corresponding to the bottom of the transition zone. On the basis of the in situ measurements, the post-spinel transition occurs at a depth (∼600 km) that is too shallow to match with the 660-km seismic discontinuity in the Earth’s mantle. Therefore, an olivine dominant mantle compositional model may be inconsistent with the seismic observations. Alternatively, we propose a pyroxene–garnet dominant transition zone with an appropriate Al2O3 content (ca. 6–8 mol%), in which majorite garnet transforms to perovskite at the depth of the 660-km discontinuity. Any alternative models would have to consider chemical stratification in the mantle.

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.

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].

High-pressure science with a multi-anvil apparatus at SPring-8

Since first opening its doors to public research in 1997, SPring-8 has seen the accomplishment of many important studies in a wide variety of fields through its stable operation and cutting edge technology. High-pressure experiments have been carried out on a number of beamlines using a diamond anvil cell or a multi-anvil press. Here, we review the multi-anvil presses installed on the SPring-8 beamlines and a few research projects currently utilizing this technology. The significant difference in post-spinel boundary between multi-anvil experiments and diamond anvil studies will also be discussed.

Nucleation and growth kinetics of the α-β transformation in Mg2SiO4 determined by in situ synchrotron powder X-ray diffraction

Abstract The kinetics of the α-β transformation in Mg2SiO4 was studied by in situ synchrotron powder X-ray diffraction (XRD) at nine pressure-temperature conditions in the ranges 13.4-15.8 GPa and 850-1100 °C. The transformation from olivine occurred by grain-boundary nucleation and interface-controlled growth mechanisms. Infrared analysis of the recovered samples indicates that a small amount of water, 750 ± 100 ppm by weight, was present in the samples although the experiments were carried out under nominally dry conditions. Nucleation and growth rates were determined by fitting the rate equation for the grain-boundary nucleated transformation to the kinetic data. The activation energy and activation volume for growth were estimated to be 348 (137) kJ/mol and 1.7 (4.5) cm3/mol, respectively. The growth kinetics determined in this study bear upon the field of metastable olivine in the subducting slab as the water contents of samples studied are well known. Nucleation rates were estimated to be relatively large, even at small overpressure conditions, which is consistent with the small activation energy for nucleation derived in this study.

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.

Pressure generation to 80 GPa using multianvil apparatus with sintered diamond anvils

Experimental techniques for high-pressure generation have been developed using a multianvil apparatus with sintered diamond (SD) anvils. High-pressure cell assemblies have been optimized for SD anvils using a new Al2O3 pressure medium and baked pyrophyllite gaskets, which has enabled generation of pressures up to 80 GPa at room temperature. High-temperature experiments were also performed using cylindrical and graphite-windowed LaCrO3 furnaces in the Al2O3 pressure medium to be suitable for in situ observations. Temperatures were generated up to 1600 K and stably maintained for more than 1 h at pressures up to 60 GPa. Present techniques using SD anvils can now reproduce the middle region of the Earths lower mantle without sacrificing the great advantage of multianvil apparatus in stable and reproducible high-pressure and high-temperature generation.

The system K2CO3-MgCO3 at 6 GPa and 900–1450 °C

Abstract Phase relations in the K2CO3-MgCO3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900-1450 °C temperatures. Subsolidus assemblies comprise the fields K2CO3+K2Mg(CO3)2 and K2Mg(CO3)2+MgCO3 with the transition boundary near 50 mol% MgCO3 in the system. The K2CO3-K2Mg(CO3)2 eutectic is established at 1200 °C and 25 mol% MgCO3. Melting of K2CO3 occurs between 1400 and 1450 °C. We propose that K2Mg(CO3)2 disappears between 1200 and 1300 °C via congruent melting. Magnesite is observed as a subliquidus phase to temperatures in excess of 1300 °C. At 6 GPa, melting of the K2Mg(CO3)2+MgCO3 assemblage can be initiated either by heating to 1300 °C under “dry” conditions or by adding a certain amount of water at 900-1000 °C. Thus, the K2Mg(CO3)2 could control the solidus temperature of the carbonated mantle under “dry” conditions and cause formation of the K- and Mg-rich carbonatite melts similar to those found as microinclusions in “fibrous” diamonds. The K2Mg(CO3)2 compound was studied using in situ X‑ray coupled with a DIA-type multi-anvil apparatus. At 6.5 GPa and 1000 °C, the structure of K2Mg(CO3)2 was found to be orthorhombic with lattice parameters a = 8.8898(7), b = 7.8673(7), and c = 5.0528(5), V = 353.39(4). No structure change was observed during pressure decrease down to 1 GPa. However, recovered K2Mg(CO3)2 exhibited a trigonal R3̅m structure previously established at ambient conditions.

High-pressure phase transitions in SnO2

In situ observations of the phase transition in SnO2 were carried out in both a multianvil and a diamond-anvil cell high-pressure apparatus using synchrotron radiation. The phase transition from the α-PbO2-type (space group: Pbcn) to the pyrite-type (space group: Pa-3) structure was observed at high pressures and high temperatures. The phase boundary between the α-PbO2-type and pyrite-type structures in the temperature range of 800–1500 K was found to be given by the relation P(GPa)=16.7(±0.5)−0.0021(±0.0015)[T(K)−1000]. The negative slope of the transition is consistent with the calculated phase boundary between the α-PbO2-type and pyrite-type structures of silica (SiO2). However, our results do not agree with the slope of the phase boundary of germanium dioxide (GeO2), which was reported to have a positive slope. The Birch–Murnaghan equation of state for the pyrite-type phase of SnO2 was determined from the experimental unit-cell parameters, with K0=307(±10)GPa and V0=128.1(±0.3)A3 with the value of K0′...