Elasticity and sound velocity at high-pressures: Geophysics and materials science

For her Ph.D. research, graduate student Zhu Mao carried out a project that focused on characterization of the elastic properties of hydrous olivine polymorphs to better understand the distribution of water in Earth’s upper mantle. Olivine and its high-pressure polymorphs (wadsleyite and ringwoodite) are expected to be major minerals in Earth’s upper mantle and, although nominally anhydrous, can incorporate up to several wt.% H2O in their structure as defects.

At ambient pressure, we have determined the elasticity of wadsleyite (b-Mg2SiO4) as a function of water content, and the elasticity of two hydrous olivine compositions. At high pressures, we have studied the elasticity of hydrous forsterite and hydrous Fe-free and Fe-bearing wadsleyite. One surprising result is the existence of a velocity crossover in forsterite whereby compressional and shear velocities of the hydrous phase exceed those of the anhydrous phase at high pressure. We have used our results to model the effects of H on the seismic velocity structure of Earth’s mantle. This work was carried out in collaboration with S. Jacobsen’s group (Northwestern) and J. Smyth (Colorado).

Other recent projects include examination of the single-crystal elasticity of MgF2, SiO2 (stishovite), AlOOH (diaspore), ettringite, zoisite, alunite, and brucite. There are many further opportunities for new research projects and further technique development in this area.

Figure: Acoustic velocities in selected crystals of hydrous wadsleyite

Figure: Acoustic velocities in selected crystals of hydrous wadsleyite (0.9 wt. % H2O) at 10.2 GPa (From Ph.D. thesis of Z. Mao)