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Recent advances in techniques for high-pressure and high-temperature experiments enable us to measure the velocity of sound in liquid Fe alloys. However, reported velocities in liquid Fe–S differ among research groups (e.g., by >10% at 5 GPa), even when similar methods are used (i.e., the ultrasonic pulse–echo overlap method combined with a large volume press). To identify the causes of the discrepancies, we reanalyzed previous data and conducted additional sound velocity measurements for liquid Fe–S at 2–7 GPa, and evaluated the potential error sources. We found that the discrepancy cannot be explained by errors in the sound velocity measurements themselves, but by inaccuracies in determining the temperature, pressure, and chemical composition in each experiment. Of particular note are the significant errors introduced when determining pressures from the unit-cell volume of MgO, which is a temperature-sensitive pressure standard, using inaccurate temperatures. To solve the problem, we additionally used h-BN as a pressure standard, which is less sensitive to temperature. The pressure dependence of the sound velocity became smaller than that of the original data because of the revised pressure values. Our best estimate for the seismic velocity of the Moon’s liquid outer core is 4.0 ± 0.1 km/s, given a chemical composition Fe 83 S 17 .

To understand the origin of observed low velocities in the crustal portion of subducting plates, we performed in situ measurements of elastic wave velocities of anorthite at temperatures up to 1373 K at pressure of ~1 GPa and up to 773 K at 2.0–7.0 GPa. A fine-grained polycrystalline anorthite, which was synthesized using a gas pressure apparatus, was used for the measurements. The high-pressure experiments were performed using the multi-anvil apparatus installed on beamline BL04B1 at SPring-8. The elastic wave velocity was measured by the ultrasonic pulse method with synchrotron X-ray radiographic imaging and X-ray diffraction techniques. At ~1.0 GPa, elastic wave velocities exhibited a sharp temperature-induced kink at ~500 K. Below 500 K, the elastic wave velocities decrease with increasing temperature. In contrast, above 500 K, the elastic wave velocities show an increasing trend in the range of 500–900 K, and then revert back to a decreasing trend at above 900 K. We also found a pressure-induced velocity anomaly of anorthite. At 300–373 K, v P is constant up to 4 GPa, but decrease above 4 GPa with increasing pressure, while v S decreases monotonously with increasing pressure. These elastic anomalies are considered to be attributable to the tilting behavior of the corner-sharing TO4 (T = Al, Si) tetrahedra in three-dimensional frameworks of anorthite. Our results suggest the presence of plagioclase feldspar has the potential to causes low-velocity anomaly in the subducting oceanic crust when it survives as a metastable phase in the slab at higher pressure and lower temperature conditions.

To investigate the partitioning coefficient of water between orthopyroxene (Opx) and olivine (Ol) (D(Opx/Ol)) under low-water concentrations ( 3 ∼ 387 wt . ppm ) similar to the Earth's mantle conditions, high-pressure experiments have been conducted at pressures of 1.5–6 GPa and a temperature of 1573 K. The experiments were performed with Kawai-type multi-anvil and piston-cylinder apparatus by using starting materials of natural Ol and synthetic Opx with various Al contents. The water contents were obtained with a vacuum type Fourier transform infrared spectrometer (FT-IR6100, IRT5000). IR-spectra of Ol and Al-bearing Opx in this study are similar to those obtained by high-pressure experiments ( Bai and Kohlstedt, 1993 ) and natural rocks ( Grant et al., 2007 ), respectively. It is believed that broad bands in IR spectra of natural Opx are due to effect of crystal distortion by large Al substitution. On the contrary, IR-spectra of Al-free Opx are not consistent with those reported by Rauch and Keppler (2002) likely because of the large difference of water fugacity. D(Al-free Opx/Ol) is ∼1 at all pressure conditions or decreases with increasing pressure. However, the water contents of Al-bearing Opx are significantly larger than those of Ol at the same conditions. In addition, the effect of Al concentration in Opx on D(Opx/Ol) becomes larger with increasing pressure. The high Al content in Opx significantly increases D(Opx/Ol) and the trend increases with increasing pressure. D(Opx/Ol) drops sharply at the pressure at which the Al concentration of Opx becomes nearly 0 in the Earth's mantle conditions. This conclusion indicates that viscosity of the upper mantle decreases sharply at depths larger than those in which orthopyroxene contains no Al. The dramatic change of D(Opx/Ol) may explain the lithosphere–asthenosphere boundary beneath oceans and continents.