Understanding of the elastic behavior of minerals under high pressure is a crucial factor for developing a model of the Earth's structure because most information about the Earth's interior comes from seismological data. Therefore, direct measurements of velocities and other elastic properties of minerals at elevated pressures and temperatures are keys to understand the seismic information, allowing us to translate it into chemical composition, mineralogy, and temperature of the minerals inside the Earth. Iron is thought to be the main constituent of the Earth's core and considerable efforts have been made to understand its properties at high pressure and high temperature. However, there are discrepancies between experimental data and theory on the elastic behavior of iron under high pressure and high temperature (HPHT) conditions. To understand the reason for this, the shear and longitudinal wave velocities of iron and iron alloys at high pressure and high temperature should be measured directly. The main goal of this project is to carry out direct measurements of both longitudinal and shear velocities of iron and iron-nickel alloys under high pressures to 100 GPa at room temperature and high temperature up to 3000 K using the laser ultrasonic (LU) coupled with a diamond-anvil cell (LU-DAC), which has been newly developed and successfully tested at the University of Hawaii. Success of the proposed research is of fundamental importance for geophysical science and materials science. For geophysical science, direct measurements of shear and longitudinal velocities of iron and iron-nickel alloys in-situ at elevated conditions similar to the Earth's interior are essential for interpreting the observed seismic anomaly and understanding physical properties of minerals in that region.
The main idea of the LU-DAC system is to use ultra-short nanosecond pulse and continuous wave lasers for remote excitation and detection of acoustical waves in materials under high pressure and high temperature in DAC. The main advantage of the LU-DAC point-source-point-receiver technique for non-transparent amorphous solids and melts is that it does not require any additional data (such as thickness of the specimen under high pressure) to determine the elastic properties of the iron at high pressures. The development of the LU-DAC technique will provide a simple, inexpensive, non-destructive technique for studying the elastic behavior of non-transparent functional materials at extreme conditions; for instance magnetic, superhard, or tribological materials synthesized at high pressure and high temperature.
The results obtained from the proposed research will provide the basis for extending such measurements to study the elasticity of melts and partial melts of iron-rich minerals at high pressure high temperature conditions when the LU-DAC system is combined with laser heating. The knowledge of the elastic properties of iron-rich minerals will allow a more realistic modeling of the Earth's core-mantle interface and shed light on the nature and composition of the D" layer and the Earth's core, and possibly on the formation of the Hawaiian Islands through the hot spot.
