Theoretical Prediction Of Fluids Composition And Properties In The Earth's Upper Mantle

Fluid is the key part the evolution of the Earth system. It plays critical role in degassing, partial melting, magma activity, deep metamorphism and mantle convection. Observation, experiment and calculation are three basic methods to investigate the properties of fluid in the Earth's upper mantle. However, due to the scarcity of sample from the mantle and it is impossible to measure properties in situ, the data from direct observations are very limited. The causticity and flexibility of fluids also make the experimental study to be very hard. So the theoretical predictions become particularly important in the study of fluid in the Earth deep interior.This study synthesizes the quantum mechanics, molecular dynamics, statistical mechanics and thermodynamics method and it gives prediction to the composition, the thermodynamics properties and the mechanism in fluids and melts in geological processes.We first evaluate the interaction force between different species from the ab initio potential surface, then we done extensive molecular simulations to get to PVTx properties in C-O-H system under various TP conditions. After that, we build a general equation of state for major species of C-O-H fluids in the upper mantle. It well reproduce and predict the PVTx properties and fugacity of H₂O, CO2, CH4, C2H6, H₂, O₂, CO and their mixtures. Based on these results and statistical method, we build the thermodynamics model of the composition and properties of C-O-H system in various temperature, pressure and geological environment with Gibbs energy minimization. In an application to the fluid composition in mantle under ancient craton, the model predict the H₂O is the always the dominant fluid species and it is possible to generate methane even ethane under reductive mantle conditions.Besides the macroscopic properties, this thesis gives microscopic picture of the solution of Ar in silica melts under upper mantle conditions. The structure of silica melts under pressure is investigated with an effective Voronoi diagram method, and it indicated the solubility of Ar is closely related to number of the interstitial voids large enough to accommodate Ar. The thermodynamics model based on the molecular simulation data well explains the drop of Ar solubility in silica melts under pressures in the mantle.