UCLA

In this dissertation, I have conducted physical experiments to investigate the phase relations in the NaAlSi2O6 −SiO2 −H2O system above the critical conditions and the changes in the compositions of the supercritical fluids. I have also conducted a series of molecular dynamics simulations to study the stability fields of the supercritical fluids and their physical properties under subduction zone conditions. The NaAlSi3O8 − H2O system is a simple system for fluids liberating from subducting slabs and triggering mantle melting in subduction zones. The change in the mineral phases and the associated fluids can significantly affect the rheological properties of the crust and mantle in subduction zones, which are responsible for geological processes such as volcanic eruptions and deep earthquakes. The presence of fluids complicates phase diagrams by shifting phase boundaries and dissolving otherwise stable minerals compared to dry systems. Boettcher and Wyllie [1969] first studied the phase relations of NaAlSi3O8 − H2O up to 35 kilobars and provided an overview of mineral reactions and phase transitions. However, they failed to identify supercritical fluids and thus misinterpreted key phase relations. In my experimental approach, I revisited the phase relations of NaAlSi2O6 (jadeite)-H2O, NaAlSi3O8 (albite)-H2O, and NaAlSi3O8 (albite)-H2O-NaCl at 1.7-3 GPa, 600-900 °C, and 10-15 wt% H2O. The run products were carefully examined with X-ray diffraction, binocular microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). My results provide comprehensive phase relations in these systems above the critical pressures and constrain compositional changes of the associated supercritical fluids. In my computational approach, I mainly focused on supercritical fluids in the NaAlSi3O8 (albite)-H2O system above the critical conditions. I applied machine learning molecular dynamics to study the viscosities and electrical conductivities of the fluids. My results challenge the previous viscosity model, support the existence of water-rich subduction zones (∼20 wt% H2O), and rapid ascent of fluids from the subducting slab to the thermal maximum of the mantle wedge. Future work aims to expand the compositions of the fluids with phase changes, the stability fields of the fluids, and how the physical properties of the fluids change under subduction zone conditions.