Congratulations to Jiachao Liu who successfully defended his dissertation on March 16, 2015.
Advisor: Jie (Jackie) Li
Iron (Fe) is the most abundant element in the Earth and it adds a variety of influences onto the physical properties and chemical behavior of the Earth and planetary interiors. This dissertation addresses a number of issues concerning the nature and dynamics of the Earth’s lower mantle and planetary cores through experimental investigations of the crystal chemistry, elastic properties and melting behaviors of iron-bearing materials. I will focus on two projects, the first providing a new hypothesis for the origin of the Earth's Ultra-Low Velocity Zone (ULVZ), and the second offering an explanation for the disappearance of the Moon's magnetic field at about 3.6 billion years ago.
ULVZ are anomalous regions above the core-mantle boundary (CMB) with reduced shear wave velocity (Vs) and compressional wave velocity (Vp) and they are important for understanding the thermochemical evolution of the deep mantle. Existing hypotheses for the origin of the ULVZ often require a very hot core, which may not be consistent with independent constraints for sustaining the geodynamo. Recent studies predicted that when the subducted slab entered the lower mantle, a mixture of about 1 wt.% iron and a small amount of carbon existed in the grain boundaries. To evaluate the fate of such mixture, I conducted melting experiments of the Fe-C system up to the lowermost mantle pressures, using the laser-heated diamond anvil cells and synchrotron x-ray diffraction techniques. Our results show that the melting curve of the Fe-C system intersects with mantle geotherm at 3068 kelvin and 128 GPa, implying that the onset of melting of iron-carbon mixture occurs within the D’’ layer and that about 1 wt.% Fe-C melt associated with subducted slabs would be present at depths of 20 ~ 120 km above CMB. If such mixture melts at the lowermost mantle, it would completely wet grain boundaries and lower Vs more than Vp. Therefore, such metallic melt from subducted slabs provides a plausible explanation for the nonubiquitous occurrence of ULVZ.
Paleomagnetic studies on lunar rocks revealed that ancient lunar dynamo probably existed 4.2 billion years ago and lasted for more than 500 million years. Recent dynamic simulations suggested that the lunar dynamo might have stopped when the solidification regime of the lunar core changed from freezing at the bottom of the molten core to crystallizing at the top of the liquid layer. To test this hypothesis, I conducted experiments using the multi-anvil apparatus to determine the melting behavior of simplified Fe-S binary and Fe-Ni-S ternary model compositions for the lunar core at relevant pressure conditions. Our results show that for the Fe-S binary system, the solidification regime of the core depends strongly on its sulfur content, and it is expected to switch from the Earth-like "frozen heart" scenario to "snowing" if the initial sulfur content falls between 5 and 11 wt.%. Applying a thermal evolution model, we find that a core with the initial sulfur content of about 7 wt.% can explain the timing and duration of the ancient lunar dynamo.