Source: Dissertation Abstracts International, Volume: 77-06(E), Section: B.

Adviser: Kanani K.M. Lee.

Melting behavior of Earth materials and rheology of their respective melts and solids is key to our understanding of Earth's evolution, from early magma ocean to modern plate tectonics. As the second most abundant mineral in Earth's lower mantle, (Mg,Fe)O ferropericlase plays a crucial role in early chemical differentiation and dynamics of the solid Earth today. In this study, several novel experimental approaches are taken to unravel the melting behavior and rheology of (Mg,Fe)O at high pressures.

To achieve this goal, a new technique is developed, combining in situ 2-D temperature measurement and ex situ compositional and textural analysis, to construct the MgO-FeO binary phase diagram up to 40 GPa for the first time. The results of the melting curve of the MgO end-member show consistency with recent theoretical computations and shockwave experiments, hence potentially resolving the long-standing controversy between static experiments and recent first-principles computations.

Rheological properties of (Mg,Fe)O melt are very challenging to study, with very little data at high pressures available despite their geophysical significance. To tackle this, a new method, by analyzing the melt's quenched texture, is proposed to estimate the viscosity of (Mg,Fe)O melt up to 70 GPa as 10-4 - 10-2 Pa·s. We find that this value is near constant between 20-70 GPa at liquidus temperatures, even across the Fe spin transition and suggests that this would remain throughout mantle pressures. This result suggests a melt percolation velocity to 2-3 orders magnitude faster than previous studies, therefore making partial melt unlikely to be the origin of Ultra-Low Velocity Zones.

As equally important, however challenging as well, rheological properties of solid (Mg,Fe)O remain a focal point of high-pressure studies. By examining grain growth at high pressures and high temperatures, diffusivity of (Mg,Fe)O solid up to 70 GPa is estimated. Preliminary results suggests that the Fe spin transition has little effect on the diffusivity of (Mg,Fe)O, consistent with recent computational studies while not ruling out the possibility of 10-30 times enhancement.