The effect of hydration state, iron, and spin state of iron on the elasticity of mantle minerals at high pressure

The physical properties of candidate mantle minerals provide critical information for understanding seismic observation and the mineralogical composition of Earth's interior. Pressure, temperature, and chemical composition all have strong effects on the elastic properties of minerals. The influences of hydrogen/iron/aluminum incorporations on these properties of mantle minerals (such as olivine, wadsleyite and ferropericlase) are still not well understood. Furthermore, a spin transition of iron in ferropericlase could significantly change its elastic properties; this has only been addressed in a few recent studies that show contradictory results.;My dissertation focuses on the effect of minor elements and a spin transition of iron on elastic properties of mantle minerals using Brillouin scattering at ambient and high-pressure conditions. I found that hydrogen has negligible influence on the elasticity of olivine, while hydrogen decreases the elastic moduli of wadsleyite and increases the pressure derivatives of its elastic moduli. I also measured elasticity of iron-bearing wadsleyite at high pressures, and found that there is no significant of iron on most of the elastic properties. My results suggest that less than 50% olivine content in mantle may provide a satisfactory explanation for 410 km seismic discontinuity in mantle transition zone.;I measured the effect of aluminum incorporation and spin transition of iron on the elasticity of ferropericlase to 61 GPa. I found that aluminum has an insignificant influence on the elastic properties of ferropericlase at low pressures (<30 GPa). At pressures higher than 40 GPa, aluminum significantly decreases the compressional velocity but decreases the shear velocity slightly, if at all, along with a weak iron influence. An iron spin transition was observed between 40-60 GPa, causing softening in the compressional velocity but not in the shear velocity. Al may broaden the transition from a high-spin to a low-spin state. Our results indicate that seismic heterogeneity in the lower mantle could reflect chemical heterogeneity, including variations in Al content and the spin state of iron.;Finally, I addressed the cause of discrepancies in current studies of the effect of water on elastic properties of wadsleyite. To rule out the systematic errors in the determination of water content measured by different methods, I measured water content of wadsleyite using the Fourier Transform Infrared method. The results show that water content of wadsleyite is nearly half of that using Secondary Ion Mass spectroscope. This difference can explain current discrepancies in the reported effects of hydrogen content on the elasticity of wadsleyite.