Yu Nishihara

In situ X-ray observation of the reaction dolomite = aragonite + magnesite at 900–1300 K

Abstract To determine the reaction boundary dolomite = aragonite + magnesite, in situ X-ray experiments on dolomite decomposition and synthesis were carried out in the temperature range 900 to 1300 K. Dolomite decomposition experiments were conducted with increasing pressure at constant temperature, and the boundary was determined to be 5.3-5.9 GPa in the temperature range 800-1200 K and 5.9-6.3 GPa at 1300 K. Dolomite synthesis experiments were carried out with decreasing pressure at constant temperature or with increasing temperature at constant press load. The dolomite synthesis boundary was determined to be 6.7-6.9 GPa at 1300 K, 3.7-4.4 GPa at 1100 K, and 2-3 GPa at 800 K. Except at 1300K, the synthesis boundary is much lower in pressure and has a steeper dP/dT slope than the decomposition boundary. The difference in the reaction boundary reflects the different kinetics between decomposition and synthesis reactions, and the former may be closer to the equilibrium phase boundary. The experimental results show that the phase boundary between dolomite and aragonite + magnesite is located at ◻6.4 GPa at 1300 K and has a dP/dT slope of 0.001 ± 0.001 GPa/ K in the temperature range 900 to 1200 K.

Phase relation and physical properties of an Al-depleted komatiite to 23 GPa

Abstract Knowledge of the mineralogical constituents of the Earth’s mantle depends heavily on high-P experiments of pyrolite composition. In order to understand the planetary interior better, phase relations and mineral physics properties of compositions other than pyrolite may also be important. High-P and high-T experiments were conducted on an Al-depleted komatiite which can be regarded as a kind of piclogite composition. Fourteen experiments were carried out along a model geotherm between 5 and 23 GPa and 1100 and 1600°C with a multi-anvil apparatus. Analyses of quenched run products showed that, in the studied komatiite, the fractions of olivine and its high-P polymorphs (wadsleyite and ringwoodite) are 40–50 vol%, pyroxene–garnet phase transition occurs at 13–17 GPa (while the respective values for pyrolite are ∼60 vol% and 12–15 GPa). The density and seismic velocity profiles of komatiite mantle were calculated using thermoelastic parameters of related minerals, and they showed quite good agreements with those of seismic observations around the transition zone as well as those for pyrolite.

Effect of chemical environment on the hydrogen-related defect chemistry in wadsleyite

Abstract The effect of chemical environment on the hydrogen-related defect chemistry in wadsleyite was investigated using Fourier-transform infrared (FTIR) spectroscopy. Samples were annealed at P = 14-16 GPa and T = 1230-1973 K using Kawai-type multi-anvil apparatus. The effect of oxygen fugacity (fO₂) was investigated using three metal-oxide buffers (Mo-MoO2, Ni-NiO, and Re-ReO2). The effect of water fugacity (fH₂₂O) was studied using two different capsule assemblies (“nominally dry” and “dry” assemblies). A range of total OH concentration (COH,Total) of studied wadslyeites varies between <50 H/106Si (<3 wt ppm H2O) and 23 000 H/106Si (1400 wt ppm H2O). The observed FTIR spectra were classified into four different classes, i.e., peaks at 3620 (“3620”), 3480 (“3480”), and 3205 cm-1 (“3205”) and the others (Group O), where the Group O includes peaks at 3270, 3330, and 3580 cm−1. The variation in OH concentration corresponding to each peak was analyzed separately. The OH concentrations correspond to “3620,” “3480,” and “3205” were found to be highly dependent on both fH₂₂O and fO₂₂. Assuming COH,Group O = 2[(2H)xM] (COH,Group O is OH concentration of Group O), present data were analyzed by using thermodynamic model for concentration of hydrogen-related defects. Based on analytical results, OH concentration of “3620” and “3480” was found to be reasonably explained by q = 1/2 and r = 1/12 (q and r are fH₂O and fO₂ exponents, respectively), whereas that of “3205” was consistent with q = 1/2 and r = -1/12. These results suggest that “3620” and “3480” correspond to HM′ whereas “3205” corresponds to H·, respectively, under the charge neutrality condition of [FeM′] = 2[VM″ ].

Pressure generation to 25 GPa using a cubic anvil apparatus with a multi-anvil 6-6 assembly

The maximum pressure generated in a cubic anvil apparatus has been extended to approximately 25 GPa with the sample volume approximately one order of magnitude larger than that available in the earlier study reporting the highest pressure of ∼23 GPa. The pressure generation experiment was performed using a newly designed multi-anvil 6-6 (MA 6-6) assembly with tungsten carbide anvils possessing truncated edge lengths of 2.5 and 3.0 mm, operated in a deformation-DIA-type apparatus. The semiconductor-to-metal transitions in GaP, GaAs, ZnS, and ZnTe at room temperature were used as the pressure references. A cubic anvil apparatus has many advantages in high-pressure experiments over the Kawai (or 6-8)-type apparatus, and the extension of both pressure range and sample volume in the former apparatus should greatly contribute to the advancement of the studies relevant to deformation, measurement of physical properties, synthesis, and crystal structure analysis of materials under high pressures and temperatures.

In situ stress-strain measurements in a deformation-DIA apparatus at P-T conditions of the upper part of the mantle transition zone

Abstract We report on technical improvements in experiments with a deformation-DIA (D-DIA) apparatus, which enable the study of the rheology of solid materials at P-T conditions of the Earth’s mantle transition zone. Dimensions of the anvil truncation, pressure medium, and gasket were optimized to achieve deformation experiments above 13 GPa with a relatively low press load (<0.7 MN) to minimize the damage of the X‑ray transparent second-stage anvils. The adoption of low X‑ray absorbing material (e.g., cubic BN anvils, graphite window in a LaCrO3 heater) along the X‑ray path enabled quantitative determination of stress and strain of a sample by means of simultaneous in situ X‑ray radial diffraction and radiography using synchrotron radiation at SPring-8. Based on the new technique, a uniaxial deformation experiment with a strain rate of 3.88 × 10−5 s−1 and strains up to 25.5% was carried out on wadsleyite at a pressure of 14.5 GPa and a temperature of 1700 K.

Mantle dynamics inferred from the crystallographic preferred orientation of bridgmanite

Seismic shear wave anisotropy is observed in Earth’s uppermost lower mantle around several subducted slabs. The anisotropy caused by the deformation-induced crystallographic preferred orientation (CPO) of bridgmanite (perovskite-structured (Mg,Fe)SiO3) is the most plausible explanation for these seismic observations. However, the rheological properties of bridgmanite are largely unknown. Uniaxial deformation experiments have been carried out to determine the deformation texture of bridgmanite, but the dominant slip system (the slip direction and plane) has not been determined. Here we report the CPO pattern and dominant slip system of bridgmanite under conditions that correspond to the uppermost lower mantle (25 gigapascals and 1,873 kelvin) obtained through simple shear deformation experiments using the Kawai-type deformation-DIA apparatus. The fabrics obtained are characterized by [100] perpendicular to the shear plane and [001] parallel to the shear direction, implying that the dominant slip system of bridgmanite is [001](100). The observed seismic shear- wave anisotropies near several subducted slabs (Tonga–Kermadec, Kurile, Peru and Java) can be explained in terms of the CPO of bridgmanite as induced by mantle flow parallel to the direction of subduction.

Towards Mapping the Three‐Dimensional Distribution of Water in the Upper Mantle from Velocity and Attenuation Tomography

A new method is developed to determine the three-dimensional variation in water content, temperature, and other parameters such as major element chemistry or the melt fraction from anomalies in seismic wave velocities and attenuation. The key to this method is mineral physics observations indicating different sensitivity of seismic wave velocities and attenuation to temperature, water content and other parameters such as major element chemistry, melt fraction or grain-size. Our analysis shows that among these parameters, temperature and water content generally have a more important influence on seismic wave velocities and attenuation than other factors such as major element chemistry, which are important only in limited regions. The method is applied to the upper mantle beneath northern Philippine Sea including the Izu-Bonin subduction zone, where high-resolution velocity and attenuation tomographic models are available down to a depth of ∼400 km. We show that the tomographic images of this region can be explained by lateral variations in temperature and water content, with only little influence of major element chemistry. A broad region of high attenuation with modestly low velocities at 300-400 km depth away from the slab in this region is interpreted as region of high water contents. We speculate that this water-rich region may have been formed by the efficient transport of water to deeper mantle by a fast (and cold) subducting slab in this region or water may come from the transition zone.

Effects of metal protection coils on thermocouple EMF in multi-anvil high-pressure experiments

Abstract Metallic coils (in most cases Cu coils) are often used in high-pressure experiments to protect thermocouple wires. In this paper we show that these coils have important influences on thermocouple EMF and therefore on the temperature measurements. We tested this effect by measuring EMF from Cu coiled single wires of chromel and alumel, and, further, we conducted experiments to compare the EMF from W5Re-W26Re thermocouples with and without Cu coils attached to them. The results show systematic differences in thermocouple readings; the EMFs from W-Re thermocouples with Cu coils give systematically lower values than EMFs from thermocouples without Cu coils. The results were analyzed using a simple model. The difference in thermocouple EMFs between thermocouples with and without protection coils is given by where Ei and Ri are the EMF and the electrical resistance of metal i in the portion of the Cu coil, and the subscripts 1+, 1-, and 2 indicate positive thermocouple metal, negative thermocouple metal, and coil metal, respectively. The addition of a coil with different metal has a large effect . the ΔETC will be close to .(E1+ - E1-) < 0 when the resistance of the coil is significantly smaller than that of a thermocouple wire. For a Cu coil and W-Re thermocouple, R1+, 1- >> R2 and therefore thermocouple readings with a Cu coil will lead to underestimation of the real temperature. Under common experimental conditions with a multi-anvil apparatus, the error in the temperature estimate caused by Cu protection coils is ~100.150 K for a peak temperature of 1600.2000 K.

Rheology of fine‐grained forsterite aggregate at deep upper mantle conditions

High-pressure and high-temperature deformation experiments on fine-grained synthetic dunite (forsterite aggregate) were conducted to determine the dominant deformation mechanism in the deep upper mantle. The sintered starting material has 90% forsterite, 10% enstatite, and an average grain size of ~1 µm. Deformation experiments were performed using a deformation-DIA apparatus at pressures of 3.03–5.36 GPa, temperatures of 1473–1573 K, and uniaxial strain rates of 0.91 × 10−5 to 18.6 × 10−5 s−1 at dry circumstances <50 H/106Si. The steady state flow stress was determined at each deformation condition. Derived stress-strain rate data is analyzed together with that reported from similar but low-pressure deformation experiments using flow law equations for diffusion creep (stress exponent of n = 1, grain-size exponent of p = 2) and for dislocation-accommodated grain-boundary sliding (GBS-disl, n = 3, p = 1). The activation volume for diffusion creep (V*dif) and for GBS-disl (V*GBS) of dunite is determined to be 8.2 ± 0.9 and 7.5 ± 1.0 cm3/mol, respectively. Calculations based on these results suggest that both diffusion creep and dislocation creep play an important role for material flow at typical deformation conditions in the Earths asthenospheric upper mantle whereas the contribution of GBS-disl is very limited, and dislocation creep is the dominant deformation mechanism during the deformation of olivine in sheared peridotite xenolith. Though these conclusions are not definitive, these are the first results on potential deformation mechanisms of forsterite aggregate based on extrapolation in the pressure, temperature, stress, and grain-size space.

Chapter 8 – Development of a rotational Drickamer apparatus for large-strain deformation experiments at deep Earth conditions

In this chapter we describe a new type of torsion apparatus – the rotational Drickamer apparatus (RDA). Large-strain deformation experiments have been performed in the RDA at pressures and temperatures up to ~15GPa and ~1700 K, respectively. The apparatus consists of opposing tungsten carbide anvils that are supported by a pyrophyllite gasket. The sample is sandwiched between the two anvils, and alumina or YAG insulating disks. The sample space is heated by two disk heaters made of a mixture of TiC and diamond. Deformation is achieved by rotating one anvil relative to the other through a Harmonic Drive™ gear box. Deformation experiments with shear strains of up to ~2 have been conducted for samples of (Mg, Fe)O, (Mg, Fe)2SiO4 olivine, and wadsleyite at rotation rates of (3–7) ×10−4 rpm corresponding to shear strain rates of (0–5) ×10−4 s−1. However, the actual shear strain rates are lower presumably due to deformation of portions of the sample assembly other than the sample itself. A conical window is made in the confining cylinder for in situ X-ray stress and strain measurements. In this apparatus, both uniaxial compression and shear deformation occur. To determine the uniaxial stress and the shear stress separately, X-ray diffraction measurements were done at five different angles with respect to the rotation (compression) axis (i.e. 0,45°, 90°). The sample thickness change and shear deformation were monitored by an imaging system during the synchrotron experiments. This apparatus allows quantitative studies of plastic deformation and microstructural development at a prescribed strain rate at pressure and temperature conditions equivalent to the deep mantle (500 km).