Takumi Kato

In situ determination of the phase boundary between Wadsleyite and Ringwoodite in Mg2SiO4

The phase boundary between wadsleyite and ringwoodite in Mg2SiO4 composition was determined by in situ observation using synchrotron X-ray and multi anvil apparatus in KEK, Tsukuba, Japan. An energy dispersive method was employed using the Ge solid state detector and the white X-ray beam from the synchrotron radiation source. The pressure was determined by the equation of state of NaCl. The stability field was identified by the change in intensities of diffraction lines of each phases. As a result, the phase boundary is expressed as a linear equation P=10.32(28)+0.00691(9)×T, where P is pressure in gigapascals and T is temperature in degrees Celsius.

Silicon self-diffusion in MgSiO3 perovskite at 25 GPa

Abstract Silicon self-diffusion coefficients in MgSiO3 perovskite were measured under lower mantle conditions. The MgSiO3 perovskite was synthesized and diffusion annealing experiments were conducted at pressure of 25 GPa and temperature of 1673–2073 K using a MA8 type high-pressure apparatus. The diffusion profiles were obtained by secondary ion mass spectrometry. The lattice and grain boundary diffusion coefficients (D1 and Dgb) were determined to be D1 [m2/s]=2.74×10−10 exp(−336 [kJ/mol]/RT) and δDgb [m3/s]=7.12×10−17 exp(−311 [kJ/mol]/RT), respectively, where δ is the width of grain boundary, R is the gas constant and T is the absolute temperature. These diffusion coefficients play a key role for understanding the rheology of the lower mantle.

Early optical emission from the γ -ray burst of 4 October 2002

Observations of the long-lived emission—or ‘afterglow’—of long-duration γ-ray bursts place them at cosmological distances, but the origin of these energetic explosions remains a mystery. Observations of optical emission contemporaneous with the burst of γ-rays should provide insight into the details of the explosion, as well as into the structure of the surrounding environment. One bright optical flash was detected during a burst, but other efforts have produced negative results. Here we report the discovery of the optical counterpart of GRB021004 only 193 seconds after the event. The initial decline is unexpectedly slow and requires varying energy content in the γ-ray burst blastwave over the course of the first hour. Further analysis of the X-ray and optical afterglow suggests additional energy variations over the first few days.

Grain Growth Rates of MgSiO3 Perovskite and Periclase Under Lower Mantle Conditions

The grain growth rates of MgSiO3 perovskite and periclase in aggregates have been determined at 25 gigapascals and 1573 to 2173 kelvin. The average grain size (G) was fitted to the rate equation, and the grain growth rates of perovskite and periclase were G10.6 = 1 × 10−57.4 t exp(−320.8/RT) and G10.8 = 1 × 10−62.3 t exp(−247.0/RT), respectively, where t is the time, R is the gas constant, and T is the absolute temperature. These growth rates provide insight into the mechanism for grain growth in minerals relevant to the Earths lower mantle that will ultimately help define the rheology of the lower mantle.

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.

Metastable garnet in oceanic crust at the top of the lower mantle

As oceanic tectonic plates descend into the Earths lower mantle, garnet (in the basaltic crust) and silicate spinel (in the underlying peridotite layer) each decompose to form silicate perovskite—the ‘post-garnet’ and ‘post-spinel’ transformations, respectively. Recent phase equilibrium studies have shown that the post-garnet transformation occurs in the shallow lower mantle in a cold slab, rather than at ∼800 km depth as earlier studies indicated, with the implication that the subducted basaltic crust is unlikely to become buoyant enough to delaminate as it enters the lower mantle. But here we report results of a kinetic study of the post-garnet transformation, obtained from in situ X-ray observations using sintered diamond anvils, which show that the kinetics of the post-garnet transformation are significantly slower than for the post-spinel transformation. Although metastable spinel quickly breaks down at a temperature of 1,000 K, we estimate that metastable garnet should survive of the order of 10 Myr even at 1,600 K. Accordingly, the expectation of where the subducted oceanic crust would be buoyant spans a much wider depth range at the top of the lower mantle, when transformation kinetics are taken into account.

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.

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.

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.

Quantitative trace element analysis of single fluid inclusions by proton-induced X-ray emission (PIXE): Application to fluid inclusions in hydrothermal quartz

Abstract Single fluid inclusion analogues with known elemental composition and regular shape were analyzed for trace element contents by particle-induced X-ray emission (PIXE)—a nondestructive method for the analysis of single fluid inclusions—to evaluate the accuracy and detection limits of this type of analysis. Elements with concentrations of 10 to 1000 ppm were measured with average estimated relative error of ±7%. For natural fluid inclusions with 30 μm radius and 20 μm depth in quartz, the total analytical errors were estimated to be ±40% relative for Ca, ±16% for Fe, ±13% for Zn, ±12% for Sr, and ±11% for Br and Rb, by considering uncertainties in microscopic measurements of inclusion depths. Detection limits of 4 to 46 ppm for elements of mass numbers 25–50 were achieved for analyses of a spherical fluid inclusion with 30 μm radius and 20 μm depth in quartz, at an integrated charge of 1.0 μC. The trace element compositions of single fluid inclusions in a hydrothermal quartz crystal were also determined. The elemental concentrations in the inclusions varied widely: 0.2–9 wt.% for Ca and Fe, 300–8000 ppm for Mn and Zn, 40–3000 ppm for Cu, 100–4000 ppm for Br, Rb, Sr, and Pb, and less than 100 ppm for Ge. Elemental concentrations of secondary fluid inclusions on the same trail varied over an order of magnitude, even though all these inclusions were formed from the same fluid. Elemental concentrations in inclusions on the same trail are positively correlated with each other, except for Cu and Rb. Ratios of almost all elements in the inclusions on the trail were essentially unchanged; thus, the elemental ratios can provide original information on trace element compositions of a hydrothermal fluid.