Kenji Mibe

Toward an internally consistent pressure scale.

Our ability to interpret seismic observations including the seismic discontinuities and the density and velocity profiles in the earths interior is critically dependent on the accuracy of pressure measurements up to 364 GPa at high temperature. Pressure scales based on the reduced shock-wave equations of state alone may predict pressure variations up to 7% in the megabar pressure range at room temperature and even higher percentage at high temperature, leading to large uncertainties in understanding the nature of the seismic discontinuities and chemical composition of the earths interior. Here, we report compression data of gold (Au), platinum (Pt), the NaCl-B2 phase, and solid neon (Ne) at 300 K and high temperatures up to megabar pressures. Combined with existing experimental data, the compression data were used to establish internally consistent thermal equations of state of Au, Pt, NaCl-B2, and solid Ne. The internally consistent pressure scales provide a tractable, accurate baseline for comparing high pressure–temperature experimental data with theoretical calculations and the seismic observations, thereby advancing our understanding fundamental high-pressure phenomena and the chemistry and physics of the earths interior.

Composition of aqueous fluid coexisting with mantle minerals at high pressure and its bearing on the differentiation of the Earth's mantle

In order to understand the role of aqueous fluid on the differentiation of the mantle, the compositions of aqueous fluids coexisting with mantle minerals were investigated in the system MgO-SiO2-H2O at pressures of 3 to 10 GPa and temperatures of 1000 to 1500°C with an MA8-type multianvil apparatus. Phase boundaries between the stability fields of forsterite + aqueous fluid, forsterite + enstatite + aqueous fluid, and enstatite + aqueous fluid were determined by varying the bulk composition at constant temperature and pressure. The composition of aqueous fluid coexisting with forsterite and enstatite can be defined by the intersection of these two phase boundaries. The solubility of silicate components in aqueous fluid coexisting with forsterite and enstatite increases with increasing pressure up to 8 GPa, from about 30 wt% at 3 GPa to about 70 wt% at 8 GPa. It becomes almost constant above 8 GPa. The Mg/Si weight ratio of these aqueous fluids is much higher than at low pressure (0.2 at 1.5 GPa) and almost constant (1.2) at pressures between 3 and 8 GPa. At 10 GPa, it becomes about 1.4. Aqueous fluid migrating upward through the mantle can therefore dissolve large amounts of silicates, leaving modified Mg/Si ratios of residual materials. It is suggested that the chemical stratification of Mg/Si in the Earth may have been formed as a result of aqueous fluid migration.

Slab melting versus slab dehydration in subduction-zone magmatism.

The second critical endpoint in the basalt-H2O system was directly determined by a high-pressure and high-temperature X-ray radiography technique. We found that the second critical endpoint occurs at around 3.4 GPa and 770 °C (corresponding to a depth of approximately 100 km in a subducting slab), which is much shallower than the previously estimated conditions. Our results indicate that the melting temperature of the subducting oceanic crust can no longer be defined beyond this critical condition and that the fluid released from subducting oceanic crust at depths greater than 100 km under volcanic arcs is supercritical fluid rather than aqueous fluid and/or hydrous melts. The position of the second critical endpoint explains why there is a limitation to the slab depth at which adakitic magmas are produced, as well as the origin of across-arc geochemical variations of trace elements in volcanic rocks in subduction zones.

Connectivity of aqueous fluid in the Earth's upper mantle

The geometrical distribution of the aqueous fluid in textural equilibrium with forsterite has been investigated by measurements of the dihedral angle, θ, at pressures of 3 and 5 GPa and at 1000°C. The measured θ at 3 and 5 GPa are 48° and 40° to 42°, respectively. Since a value of θ smaller than 60° indicates that the aqueous fluid can form an interconnected network along the three-grain and through the four-grain junctions, the existing aqueous fluid in the mantle can migrate by percolation even at the small volume fractions in the pressure range. Combining these results with previously published results at lower pressure, it is suggested that the interconnected network of aqueous fluid is stable only at higher pressure than 2 GPa for the commonly accepted water content of the upper mantle. The present results show that the large scale transport of aqueous fluid is possible above 2 GPa, and that the physical properties of the upper mantle may change drastically at that pressure.

Phase transition of Ca-perovskite and stability of Al-bearing Mg-perovskite in the lower mantle

Abstract Here, using in situ X-ray diffraction combined with a laser-heated diamond anvil cell at high temperatures and high pressures, a phase relationship in KLB-1 peridotite composition samples was investigated from 38 to 106 GPa, and 300 to 2600 K, in order to determine the stability of phases in the lower mantle. We observed that Al-bearing Mg-perovskite and magnesiowüstite remained stable up to 106 GPa, which corresponds to a lower mantle depth of 2400 km depth. By contrast, a phase transition in Ca-perovskite from a cubic to a tetragonal structure was observed. The amount of distortion in this material increases as pressure increases at 300 K. The temperature of the tetragonal to cubic structure transition, therefore, appears to increase with increasing pressure. There is a possibility that this Caperovskite phase transition contributes to unidentified seismic anomalies in the lower mantle.

Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism

Subduction-zone magmatism is triggered by the addition of H2O-rich slab-derived components: aqueous fluid, hydrous partial melts, or supercritical fluids from the subducting slab. Geochemical analyses of island arc basalts suggest two slab-derived signatures of a melt and a fluid. These two liquids unite to a supercritical fluid under pressure and temperature conditions beyond a critical endpoint. We ascertain critical endpoints between aqueous fluids and sediment or high-Mg andesite (HMA) melts located, respectively, at 83-km and 92-km depths by using an in situ observation technique. These depths are within the mantle wedge underlying volcanic fronts, which are formed 90 to 200 km above subducting slabs. These data suggest that sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, react with mantle peridotite to form HMA supercritical fluids. Such HMA supercritical fluids separate into aqueous fluids and HMA melts at 92 km depth during ascent. The aqueous fluids are fluxed into the asthenospheric mantle to form arc basalts, which are locally associated with HMAs in hot subduction zones. The separated HMA melts retain their composition in limited equilibrium with the surrounding mantle. Alternatively, they equilibrate with the surrounding mantle and change the major element chemistry to basaltic composition. However, trace element signatures of sediment-derived supercritical fluids remain more in the melt-derived magma than in the fluid-induced magma, which inherits only fluid-mobile elements from the sediment-derived supercritical fluids. Separation of slab-derived supercritical fluids into melts and aqueous fluids can elucidate the two slab-derived components observed in subduction zone magma chemistry.

Mg/Si ratios of aqueous fluids coexisting with forsterite and enstatite based on the phase relations in the Mg2SiO4-SiO2-H2O system

Abstract Direct observation of aqueous fluids coexisting with MgSiO3 (enstatite) and/or Mg2SiO4 (forsterite) was performed at 0.5-5.8 GPa and 800-1000 °C with an externally heated diamond-anvil cell and synchrotron X-rays. At 1000 °C in the MgSiO3 -H2O system, forsterite crystallizes below 3 GPa but not above that pressure. At 1000 °C in the Mg2SiO4 -H2O system, forsterite congruently dissolves into the aqueous fluids up to 5 GPa. These observations suggest that the aqueous fluids coexisting with enstatite and forsterite have Mg/Si < 1 below 3 GPa and 1 < Mg/Si < 2 above that pressure. Comparison with the previous studies reporting Mg/Si ratios of the aqueous fluid coexisting with enstatite and forsterite indicates that the Mg/Si ratios change rapidly from SiO2-rich to MgO-rich at around 3 GPa and 1000 °C. This change can be related to possible structural changes of liquid water under these conditions. The aqueous fluids coexisting with enstatite and forsterite do have Mg/Si ratios similar to those found in the partial melts of H2O-saturated peridotite. Somewhere within the upper mantle, these two fluids unite to form a single regime and cannot be distinguished from each other.

P-V-T equation of state of platinum to 80GPa and 1900K from internal resistive heating/x-ray diffraction measurements

The P-V-T equation of state (EOS) of Pt has been determined to 80GPa and 1900K by in situ x-ray diffraction of a mixture of Pt and MgO using a modified internal resistive heating technique with a diamond anvil cell. The third-order Birch–Murnaghan EOS of Pt at room temperature can be fitted with K0=273.5±2.0GPa, K0′=4.70±0.06, with V0=60.38A3. High temperature data have been treated with both thermodynamic and Mie–Gruneisen-Debye methods for the thermal EOS inversion. The results are self-consistent and in excellent agreement with those obtained by the multianvil apparatus where the data overlap. MgO is taken as the standard because its thermal EOS is well established and based on a wealth of experimental and theoretical data, and because the EOS at room temperature has been determined by a primary method that is completely independent of any assumptions or measurements by other methods. Improvements to previous internal resistive heating methods were made by using a Re gasket that replaces the original g...

Aqueous fluid connectivity in pyrope aggregates : water transport into the deep mantle by a subducted oceanic crust without any hydrous minerals

Abstract The effect of pressure and temperature on the dihedral angles of aqueous fluid in a pyrope matrix was investigated. Experiments were performed on an H 2 O–pyrope system in a multianvil apparatus over the pressure and temperature ranges of 4–13 GPa and 900–1200°C, respectively. The dihedral angle of the fluid in contact with the pyrope exhibited a significant change at pressures around 8–9 GPa. The dihedral angles increased with increasing pressure up to 9 GPa. At pressures above 9 GPa, the dihedral angles were greater than 60° at temperatures below 1000°C. Therefore, the efficient percolation of aqueous fluid in a pyrope matrix is not feasible in the upper mantle and the transition zone. The fluid released from the breakdown reactions of the hydrous minerals lawsonite and phengite exists in the oceanic crust, which mainly consists of garnet in the upper mantle and transition zone. We conclude that a part of the aqueous fluid released from the hydrous minerals may be retained in the subducted oceanic crust, and transferred into the deep mantle by the subduction process.

An in situ X ray diffraction study of the α‐β transformation kinetics of Mg2SiO4

This is the first report of the direct measurement of α-β transformation kinetics in Mg2SiO4, a major constituent in the Earths mantle, through in situ X ray diffraction experiments at high pressure. The experiments were conducted using the sintered diamond cubic-anvil apparatus with synchrotron radiation (MAX 80). Although the present work is still preliminary, the kinetic data were analyzed using a nucleation and growth model. The growth rate of β-phase is determined to be 3.3 (1.6∼4.9) × 10−10 m/s at 13.0 GPa and 900°C, 7.1 (3.4∼10) × 10−10 m/s at 14.1 GPa and 885°C, and 1.5 (0.7∼2.3) × 10−9 m/s at 14.9 GPa and 875°C.