Elasticity of Mantle Minerals at High Pressures by Brillouin Scattering

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Understanding the structure, composition, and evolution of Earth's mantle is a central goal of geophysical research. The most direct information about the interior structure of the Earth comes for analysis of seismic waves. Interpretation of seismic data requires a detailed understanding of how the sound velocities and hence elastic and anelastic properties of minerals vary with pressure, temperature, crystal structure, and composition. The elastic stiffness tensor characterizes the anisotropic elastic response of crystals. Elastic moduli are also fundamental to a range of solid-state phenomena including mechanical stability, inter-atomic interactions, material strength, compressibility, and phase transition mechanisms.In this project, Thomas Duffy of Princeton University is using Brillouin scattering to determine the high-pressure elastic properties of selected minerals of the Earth's crust and mantle. Measurements by Brillouin scattering at high pressures are especially useful because of their ability to constrain not only the bulk and shear modulus but also the variation of elastic anisotropy with compression. Olivine polymorphs and pyroxenes are among the most abundant minerals in the upper mantle. There are a number of fundamental unsolved problems connected to the elasticity of these phases. In recent years, there has been growing recognition that olivine polymorphs have the capacity to accommodate appreciable amounts of hydrogen as structural defects. The presence of hydrogen even in small amounts might strongly influence a number of important properties of the mantle. However, there are no measurements yet to enable us to constrain the effects of hydrogen on the elastic stiffnesses of olivine or its high-pressure polymorph, wadsleyite, at high pressure. Similarly, the effects of H2O incorporation on the elasticity of other upper mantle minerals such as orthopyroxene are poorly known but have potentially important geophysical implications. In this proposal, Duffy is measuring how hydrogen incorporation affects the elasticity of olivine polymorphs up to pressures corresponding to 400-km depth in the Earth. He and his students are also measuring the elastic properties of mantle-relevant pyroxene minerals to similar conditions. The elastic properties of quartz and stishovite are being measured at high pressures. For stishovite, Duffy and his students are extending single-crystal elasticity measurements to as high as 50 GPa, corresponding to pressures of Earth's lower mantle. They are also using polycrystalline Brillouin scattering to study the aggregate elasticity of minerals at 20-100 GPa. Using such data, Duffy and his students are interpreting seismic data for the deep Earth in terms of the composition and crystal structure of the constituent minerals. The results of this research are yielding a better understanding of the structure, composition, and evolution of deep planetary interiors.