We have determined the postspinel transformation boundary in Mg 2 SiO 4 by combining quench technique with in situ pressure measurements, using multiple internal pressure standards including Au, MgO, and Pt. The experimentally determined boundary is in general agreement with previous in situ measurements in which the Au scale of Anderson et al. [1989] was used to calculate pressure: Using this pressure scale, it occurs at significantly lower pressures compared to that corresponding to the 660‐km seismic discontinuity. In this study, we also report new experimental data on the transformation boundary determined using MgO as an internal standard. The results show that the transition boundary is located at pressures close to the 660‐km discontinuity using the MgO pressure scale of Speziale et al. [2001] and can be represented by a linear equation, P (GPa) = 25.12 − 0.0013 T (°C). The Clapeyron slope for the postspinel transition boundary is precisely determined and is significantly less negative than previous estimates. Our results, based on the MgO pressure scale, support the conventional hypothesis that the postspinel transformation is responsible for the observed 660‐km seismic discontinuity.
An internally consistent thermodynamic database for pure iron has been established to pressures ( P ) up to 360 GPa and temperatures ( T ) up to 7000 K from existing static experimental data and thermochemical measurements. The database includes body‐centered cubic (BCC) phases ( α or δ phase), the face‐centered cubic (FCC) phase ( γ phase), the hexagonal close‐packed (HCP) phase (ɛ phase), and the liquid phase. We describe fundamental thermodynamic relations as the Gibbs free energy divided into thermochemical and thermophysical terms. The thermochemical data were evaluated from existing metallurgy databases together with experimentally determined phase relations. The thermophysical term is obtained from the pressure‐volume‐temperature equations of state (EoS) for the phases. We constructed an EoS of the FCC phase from our recent internally‐heated diamond anvil cell (DAC) experimental data and assessed the EoS of the liquid phase from existing laser‐heated DAC experiments together with density data at P = 1 bar, 0.2 GPa, and along the Hugoniot. The HCP‐FCC‐liquid triple point is located at P = 90 GPa and T = 2800 K. The calculated melting temperature of HCP iron at the inner core boundary ( P = 330 GPa) is 4900 K and the density change at melting is −1.2%. The core density deficits at the inner core boundary are 8.1 wt.% and 5.3 wt.% for the liquid outer core and solid inner core, respectively. The calculated melting temperature is much lower than that from dynamic shock wave experiments, suggesting that the HCP structure may not be stable in the inner core. We included a hypothetical high‐pressure BCC phase which could be stabilized above 220 GPa by a solid‐solid transition of high‐ P BCC‐HCP phases. This hypothetical BCC phase should have a large entropy to give a high melting temperature in order to reconcile the existing discrepancies between the static and shock wave experimental studies.
The compression behavior of Al-bearing stishovite was investigated by powder X-ray diffraction up to 40 GPa with the BL13A beamline at the Photon Factory (KEK, Japan). A reliable equation of state for stishovite was obtained using a diamond anvil cell coupled with a yttrium-aluminum-garnet (YAG) laser-heating. A sample containing 2.1 wt% Al2O3 was heated using a YAG laser at each pressure increment to relax deviatoric stress. X-ray diffraction measurements were carried out at 300 K using the angle-dispersive technique. A least squares refinement of the data yielded equation of state parameters where the bulk modulus Ko = 282 (±2) GPa when the first pressure derivative of the bulk modulus Ko' was fixed at 4. The effect of Al is to decrease slightly the bulk modulus of stishovite and increase the density of the subducted oceanic crust. The enhanced compressibility of Al-bearing stishovite certainly has geophysical and geochemical implications for the fate of the subducted slab, as this mineral is the main constituent of subducted mid-oceanic ridge basalt (MORB) in the Earth’s mantle.
The relationship between the sound velocity, density, and temperature of liquid metals is important when one tries to interpret the seismic velocity profile and infer the chemical compositions of the Earth’s outer core. We, therefore, have experimentally measured the longitudinal acoustic (LA) velocity of liquid indium under high P-T conditions. Also, we examined a Hugoniot data of liquid iron by comparing with an existing equation of state (EoS). The LA velocities of liquid and solid indium at pressures up to 6.7 GPa and temperatures mostly at 710 K were measured using inelastic X‑ray scattering (IXS) to probe samples in an externally heated diamond-anvil cell. A thermal EoS for liquid indium derived from existing literature was used to calculate the density for the IXS measurements and to provide an independent check on the sound velocities. The IXS data are consistent with the hydrodynamic LA velocity derived from the liquid EoS, implying that the positive dispersion is minimal in liquid indium. The velocity-density relation for liquid indium derived from the EoS has temperature dependence, implying that Birch’s law does not hold for the liquid phase. Similarly we calculated the temperaturevelocity- density relation of liquid iron over the Earth’s core range from a recently reported EoS. The resulting velocity-density relation is also temperature dependent, indicating that liquid iron thus does not follow Birch’s law. The violation of Birch’s law implies that the Hugoniot data cannot be directly compared with seismological observations because of the different temperature ranges. Formulation of the temperature-velocity-density of liquid iron-alloys supported by experimental measurements provides better understanding of the thermodynamic state of the Earth’s core.
