New high pressure phase transition of natural orthoenstatite and sound velocity measurements at simultaneous high pressures and temperatures by laser heating

orthoenstatite;phase transition;high pressure;high temperature;Brillouin spectroscopy;laser heating

Zhang, Jin

关键词: orthoenstatite;    phase transition;    high pressure;    high temperature;    Brillouin spectroscopy;    laser heating;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/50348/Jin_Zhang.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Studies of phase transformations and sound velocities of candidate mantle minerals under high-pressure high-temperature conditions are essential for understanding the mineralogical composition and physical processes of Earth’s interior. Phase transitions in candidate deep earth minerals under high-pressure high-temperature conditions is one of the main causes of the seismically observed discontinuities that define the boundaries between major layers of the Earth. The changes of sound velocities, including the directional dependencies of sound velocities, across phase transitions are still not well constrained. This is largely due to the lack of sound velocity measurements at sufficiently high pressure-temperature conditions and/or using only polycrystalline instead of single-crystal samples. To address the issues mentioned above, my dissertation involves two main topicsFirstly, I discovered a new high-pressure Pbca-P21/c phase transition of natural orthoenstatite (Mg,Fe)SiO3 using synchrotron single-crystal X-ray diffraction. This discovery contradicts with the widely accepted phase diagram of (Mg,Fe)SiO3. I performed additional in situ Raman spectroscopy experiments addressing both compositional and high-temperature effects of this phase transition, providing solid evidences for the stability of orthoenstatite (Pbca) under average upper-most mantle conditions, and a potential stability/metastablity field for the new high-pressure P21/c phase. Furthermore, I measured in-situ the sound velocities change across this transition by single-crystal Brillouin spectroscopy, showing a pronounced increase of elastic anisotropy when transforming from the low to the high pressure phase. Secondly, I built the first laser-heating Brillouin spectroscopy system with the capability of performing single-crystal sound velocity measurements with laser heating to produce simultaneous high pressures and temperatures on samples in a diamond anvil cell. This experimental setup allows us for the first time to measure sound velocities of single-crystals of minerals under realistic P-T conditions of the Earth’s mantle. Sound velocity measurements of water and polycrystalline H2O ice as well as single-crystal San Carlos olivine were made utilizing this system. Our result provide the first experimental constrains on the effect of high temperature on the single-crystal elastic properties of olivine under high pressures. Our results indicate 1. Velocity gradient of olivine is far less than velocity gradient of AK135; 2. Olivine is the slow phase instead of fast phase at deeper than ~250km in depth; 3. Vs of olivine is almost constant or decreases from ~ 300km to ~410km.Based on the above observations, we expected much less olivine in the upper mantle (~ 25%), and much more pyroxene (both opx and cpx) in order to balance the velocity and velocity gradient discrepancy. In addition, this system has been designed so that the scattering angle can be changed continuously to any desired angle up to near back scattering (~141º) via a rotational stage and a compact 532nm diode-pumped solid-state (DPSS) laser. This novel setup allows us to probe a wide range of wave vectors q for investigation of phonon dispersion for crystals with large unit cells (~hundards of nms). This capability is demonstrated by dispersion measurements on cubic-close-packed polystyrene crystals.