Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Geophysics

Supervisor

Sean Shieh

Abstract

Rheological properties of the Earth control most of the important geological processes, such as mantle convection, plate tectonics, earthquakes and nature of thermal evolution. Most parts of the Earth consist of multi-phase polycrystalline aggregates with various composition and compressibility. Therefore, deformation studies on multi-phase materials are important to understand the rheological properties and convection of the Earth. NaCl and MgO with large contrast in elastic properties are excellent analogue materials for modelling the Earth that is generally made of both strong and weak materials. In addition, NaCl and MgO are widely used as pressure transmitting medium and pressure calibration standard for high pressures research. Therefore, study of NaCl and MgO at high pressure should provide meaningful information of stress, strain and strength for mantle dynamics and also help to construct a two-phase deformation model.

In this study, four different starting materials, pure NaCl, MgO-NaCl (1:3), MgO-NaCl (1:2), MgO-NaCl (1:1) mixture, were investigated in situ at mantle pressures using radial X-ray diffraction technique at beamline X17C, National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL) and pressure gradient method at Department of Earth Sciences, Western University. Pure NaCl was studied by radial energy dispersive X-ray diffraction (EDXD) at a pressure up to 43.7 GPa. MgO-NaCl (1:3), MgO-NaCl (1:2) and MgO-NaCl (1:1) were investigated at pressure to 57.6, 43.1 and 44.4 GPa respectively by radial angular dispersive X-ray diffraction (ADXD). Pressure gradient method was only applied to pure NaCl to 55.8 GPa.

It is well known that NaCl undergoes a phase transition from B1 (rock-salt structure) to B2 (CsCl structure) under high pressure. In this study, the B1 to B2 phase transition in the mixed systems varies with pressures, including both initial and completion pressures. The experimental results show that the higher the volume ratio of MgO, the higher pressure is needed for NaCl to start and complete the phase transition. Therefore, the involvement of strong materials (MgO) extends the stability of NaCl B1 phase to higher pressures. The differential stresses (lower bound of yield strength) of NaCl are also varied with different starting materials. The highest differential stresses occurred in the mixture with highest volume ratio of “strong” material MgO. It therefore can be inferred that in a mixture, strong material could strengthen the “soft” material. Conversely, the stresses supported by MgO in the MgO-NaCl mixture are lower than that of pure MgO, indicating that “soft” material may also has influence on strong material.

Generally, the strength of NaCl B1 phase increase gently with pressure, suggesting that NaCl B1 phase is a good pressure transmitting medium, whereas B2 is no longer regarded as “soft” material due to the abruptly increment of its differential stress. Results from peak broadening study show that deformation of NaCl B1 phase remains in elastic regime, whereas B2 phase undergoes a plastic deformation. The elastic constants of B1 phase calculated based on lattice strain theory show reasonable agreement with previous reports within B1 phase regime, whereas the elastic constants of B2 phase appear to deviate largely from the theoretical predictions. Differential stresses supported by different crystal planes show that B1 200 has the lowest value, suggesting B1 200 may be responsible for the pressure induced initial deformation.

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