Congratulations to Zeyu Li who successfully defended his dissertation on March 18, 2015.
Advisor: Jie (Jackie) Li
Carbon and hydrogen are two of the most important volatile elements in the Earth's mantle. The deep cycles of carbon and hydrogen induce melting in the mantle and control the material exchange between the surface and interior of the Earth, which could influence the long-term climate change.
Carbon in the mantle is mainly in the form of carbonates and diamond/graphite. The melting of carbonates governs the deep carbon cycle. But current studies about where and how melting of carbonates happen are still inconsistent. In this dissertation these questions were addressed by studying melting temperatures of CaCO3, K2CO3 and Na2CO3 at the mantle pressures. I developed a technique to measure melting temperature of minerals through capacitive current change during melting. Through the new technique, I discovered that the melting temperature of CaCO3 decreased over 7-15 GPa, which indicated carbonate melting may cause low velocity areas in the transition zone. I also found that even starting similarly, the melting temperature of K2CO3 deviated from that of Na2CO3 after a pressure induced phase transition of K2CO3 at 9 GPa. K2CO3 had an exceptionally high melting temperature at the transition zone pressure, which might explain the origin of ultrapostassic rocks.
Hydrogen or specifically water can be stored in a group of minerals called dense hydrous magnesium silicates (DHMS), which can carry water to the lower mantle. So additional work was done to investigate the stability of germanium analogues of minerals (DHMS) and much lower stability pressures of the analogues were found compared with DHMS.