The deep interior of the Earth is not directly accessible to study. The most instructive information about its structure is mainly from seismological studies of earthquake waves. The speed at earth quake vibrations or waves travel is different for different types of rocks and strongly depends on the elastic properties of the rocks. The seismological data provides variations of compressional (Vp) and shear (Vs) wave velocities as a function of depth, and data taken for different regions of the Earth have revealed unusual rapid increases in the compressional and shear wave velocities (also called velocity discontinuities or velocity jumps) at depths of 410-km and 660- km in the Earth. The data also show that both seismic velocities increase rapidly with depth in the region between the two discontinuities, or the transition zone. Laboratory petrological studies demonstrate that olivine [α-(Mg,Fe)2SiO4], an iron-magnesium silicate and a major Earth mineral, transforms to denser phases (different crystal structures), wadsleyite [β-(Mg,Fe)2SiO4] and ringwoodite [γ-(Mg,Fe)2SiO4 at high pressures. Moreover, the seismically observed 410-km velocity jump has been ascribed to the change of olivine to the wadsleyite crystal structure, and the transition of wadsleyite to the ringwoodite crystal structure has been attributed to a minor velocity jump at 520-km. Current laboratory velocity measurements on the mantle minerals have mainly utilized dry or anhydrous samples to study and match the seismic data. However, experimental and theoretical studies indicate that wadsleyite and ringwoodite can incorporate up to 2-3 wt. % of H2O as hydroxyl (OH-) in their crystal structures that affect their physical properties such as thermal and electrical conductivities including the speed at which elastic waves travel through the minerals. The proposed study is to fabricate synthetic rock samples of wadsleyite and ringwoodite containing controlled structural water, and to measure the elastic wave velocities of the hydrous specimens, as a function of temperature and pressure similar to the conditions inside the Earth?s transition zone. Data from the study will be compared with the seismic velocity profiles of the Earth?s mantle, to address persistent questions related to the precise depth and magnitude of the seismic velocity jumps, as well as the velocity gradients between the discontinuities. Combined with petrological and geochemical data, the results of the study could significantly enhance our knowledge of the composition and structure of the Earth?s interior.
The proposal is to conduct systematic measurements of the elastic wave velocities, to constrain the elastic properties of polycrystalline specimens of hydrous olivine (α-Mg2SiO4) and its high pressure polymorphs, wadsleyite (β-Mg2SiO4) and ringwoodite (γ- Mg2SiO4), as a function of the content of structurally bound water (OH-) in the mineral, pressure (P) and temperature (T), by acoustic ultrasonic interferometry techniques. Experimental and theoretical studies indicate that wadsleyite, and ringwoodite can incorporate up to 2-3 wt. % of H2O as hydroxyl (OH-) in their structures, and thus affecting many physical properties of the phases, including their elastic properties. However, despite their abundance in the Earth?s upper mantle and transition zone (410?660 km depth), there are currently very few data on the elasticity of the hydrated phases of the nominally anhydrous mantle minerals. Two primary activities are proposed: (1.) Hot-pressing of optimum acoustic-quality polycrystalline specimens characterized in detail by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron microprobe analysis, immersion density and bench-top acoustic velocity measurements, and IR spectroscopy and SIMS for quantifying the water content prior-to and after the high P and T ultrasonic studies. (2.) Initial measurement of the elasticity of the materials at high pressure up to 10 GPa, and room T, in a 1000-ton uniaxial split-cylinder apparatus (USCA-1000) of the Kawai-type, to obtain accurate pressure dependences of the elastic bulk (K) and shear (G) moduli for the hydrated phases, followed by measurement of the elastic properties at simultaneous high-pressure to 15 GPa and moderate temperature to 650 K, in conjunction with X-ray diffraction analysis of the sample, at the 13ID beam line of the Advanced Photon Source (APS), Argonne National Laboratory. It is proposed to apply the elastic properties and their variations with pressure (P) and temperature (T) for the Mg end-member hydrated mantle phases, to provide tighter constraints on the depth and sharpness of the 410-km discontinuity, to re-define the velocity jumps associated with the olivine to wadsleyite and the wadsleyite to ringwoodite phase transitions associated with the 520-km discontinuity in the transition zone, to assess the role of water in the lateral inhomogeneity observed from seismic tomographic studies of the Earth?s mantle, and in general to improve our understanding of the Earth?s mineralogical and chemical composition.
