Applications of Nuclear Resonant Scattering to Further Our Understanding of Earth’s Interior

Citation

Zhang, Dongzhou (2015) Applications of Nuclear Resonant Scattering to Further Our Understanding of Earth’s Interior. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9SQ8XD5. https://resolver.caltech.edu/CaltechTHESIS:12152014-195321239

Abstract

The elastic and the thermodynamic properties of minerals under extreme P-T conditions are of general importance to the geodynamic modeling and the interpretation of seismic-wave observations. An accurate laboratory determination of these properties provides constraints to determine the composition and temperature of Earth's interior. In this thesis, I focus on the application of nuclear resonant scattering, an advanced synchrotron based spectroscopic technique, to further our understanding of candidate materials in Earth's interior. Specific examples include enstatite, which is an abundant mineral in the mantle, and iron-nickel alloy, which is believed to be the major component of the core. Our nuclear resonant scattering experiments is complemented with other synchrotron based techniques, such as diffraction.

Nuclear resonant scattering is capable of detecting subtle changes in the mineral's hyperfine parameters, and can therefore be sensitive to the transitions occurring in minerals under pressure. For example, we explore the site-specific hyperfine behavior of iron in a ⁵⁷Fe-enriched powdered enstatite sample using nuclear resonant scattering and diamond-anvil cells in two independent experiments. The (Mg_0.980Fe_0.020(5) )(Mg_0.760Fe_0.240)Si₂O₆ sample is pressurized up to 36 GPa at ambient temperature. In one experiment, NaCl is used as the pressure-transmitting medium, and in the other experiment, Ne surround the sample. Analyses of both data sets reveal a change in the trend or discontinuity in the hyperfine parameters around 10 GPa, indicative of a structural transformation in enstatite. However, the detailed behaviors of the iron sites with pressure appear to depend on the local stress conditions provided by the different pressure media.

Nuclear resonant scattering is also used to measure the elastic properties of iron-bearing enstatite at high pressures. The behavior of synthetic powdered ⁵⁷Fe-enriched (Mg_0.980Fe_0.020(5) )(Mg_0.760Fe_0.240)Si₂O₆ is explored by X-ray diffraction (XRD) and nuclear resonance inelastic X-ray scattering (NRIXS). The Pbca-structured enstatite sample is compressed in fine pressure increments for our XRD measurements. One structural transition between 10.1 and 12.2 GPa is identified from the XRD data. The XRD reflections observed for the high-pressure phase are best matched with space group P2₁/c. The partial phonon density of states (DOS) is derived from the raw NRIXS data up to 17 GPa, and from the low-energy region of the DOS, the Debye sound velocity is determined. We use the equation of state determined from XRD and Debye sound velocity to compute the isotropic compressional and shear wave velocities of enstatite at different pressures. We combine density-functional theory with nuclear resonant scattering to understand the local site symmetry of the Fe atoms in our sample. We compare our experimental results with seismic observations to understand large lateral variations in shear wave velocities in the upper mantle.

Recently, nuclear resonant scattering has been shown to be a powerful probe in determining the solid-liquid boundary of iron-bearing materials. To capture the sample's transient temperature fluctuations and reduce uncertainties in melting temperatures, we have developed a Fast Temperature Readout (FasTeR) spectrometer in-line with nuclear resonant scattering measurements under extreme conditions at Sector 3-ID-B of the Advanced Photon Source at Argonne National Laboratory. Dedicated to determining the sample's temperature near its melting point, FasTeR features a fast readout rate (about 100 Hz), high sensitivity, large dynamic range and well-constrained focus. FasTeR is capable of reading out temperatures about 1 to 2 magnitudes faster than the conventional CCD spectrometer, without sacrificing accuracy, and is especially suitable for dynamic measurements at extreme conditions.

By combining nuclear resonant scattering with the laser heated diamond anvil cell and the FasTeR spectrometer, we have determined the melting temperatures of fcc-structured iron and iron-nickel alloy at high pressures. We find that the melting curve of Fe is slightly higher than the melting curve of Fe_0.9Ni_0.1, but the difference is smaller than the uncertainty. We calculate the fcc-hcp-l triple point of Fe_0.9Ni_0.1 to be 117±3 GPa and 3285±200 K, and 111±3 GPa and 3390±200 K for Fe. With the fcc-hcp-l triple points of Fe and Fe_0.9Ni_0.1, and the thermophysical parameters of hcp-Fe determined from a NRIXS measurement, we compute the high pressure melting curves of hcp-structured Fe and Fe_0.9Ni_0.1. We estimate the upper bound of Earth's inner core-outer core boundary temperature as ~5600±200 K, and we compute the upper bound of outer core temperature with an adiabatic model. We discuss the implications of these temperatures on the phase relations of deep Earth materials.

Thesis Files