Melting and Structural Transformations of Carbonates and Hydrous Phases in Earth's Mantle.

Abstract

This dissertation addresses questions about carbon and hydrogen transport and storage in the mantle through experimental investigations of the melting behaviors of carbonates under high pressure and phase stability of dense hydrous germanate. In Chapter II, a new technique was developed to measure melting temperatures at high pressures by monitoring the sudden change of capacitive current through ionic compounds upon melting. In Chapter III, the melting curve of NaCl up to 20 GPa was measured using the capacitive current technique. New results are consistent with previous data on melting temperature of NaCl up to 6.5 GPa, thus validating the accuracy of capacitive current based measurement. In Chapter IV, we measured the melting curve of CaCO3 between 3 and 22 GPa. The melting temperature of CaCO3 was found to decrease between 7 and 15 GPa and then increase with pressure between 15 and 21 GPa. The negative melting slope was attributed to the melt/solid density crossover at 7 GPa and the positive melting slope at higher pressures can be explained by calcite V to aragonite phase transition at 15 GPa. The melting curve of CaCO3 may cross a hot adiabatic geotherm at the transition zone depth in an upwelling setting, producing carbonate melt in the transition zone. In Chapter V, the melting curves of two more carbonates, Na2CO3 and K2CO3, were measured between 3 and 20 GPa. Above 9 GPa the melting temperature of K2CO3 was found to increase with pressure at a much higher rate than Na2CO3. Results from high pressure in situ X-ray diffraction experiments indicated two solid phase transitions of K2CO3 at ~3 and ~9 GPa, respectively. In Chapter VI, the stability of three new dense hydrous magnesium germanate (DHMG) minerals were reported on the basis of phase equilibrium experiments using in situ synchrotron X-ray diffraction and hydrothermal diamond anvil cell. One of them was determined as germanate analogue of phase D, and the other two were likely germanium analogues of superhdrous B and phase H.