The geological activity that manifests itself so dramatically at the Earth's surface ultimately originates from processes in the deep interior. To improve our understanding of the interior dynamics and structure of the Earth, it is necessary to combine geophysical observations such as those from seismology with direct experimental measurements of the physical properties of the relevant geological materials. The aim of this study is to measure the sound velocities of minerals at high pressure and to use the results in combination with seismic data to construct improved models of the composition and structure of the Earth's upper mantle.
Using Brillouin spectroscopy we will carry out new measurements of the high-pressure elastic properties of major phases of the upper mantle including olivine and its high-pressure polymorphs. The elastic properties of minerals are essential for the interpretation of seismic data for the Earth. The bulk and shear moduli along with density determine compressional and shear wave speeds which can be directly compared to seismic data. In combination with our previous work on hydrous olivine polymorphs, these studies will yield new understanding of how iron and hydrogen affect the elastic properties of the major phases of the upper mantle. In particular, our studies will shed new light on the velocity crossover between hydrous and anhydrous forsterite observed in our previous work. Another goal for this project is to develop and apply techniques for Brillouin spectroscopy at simultaneous high pressures and temperatures (to as high as 600oC) using an externally heated diamond anvil cell. This work will provide the first experimental constraints on the high P-T elastic tensor for some key mantle minerals. Lastly, we will undertake a comprehensive synthesis of our current understanding of chemical and structural controls on mineral elastic properties and their importance for interpreting seismic data.
