Metal mobility in hydrothermal fluids: insights from ab initio molecular dynamics simulations.
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Date
2013
Authors
Mei, Yuan
Editors
Advisors
Brugger, Joel
Sherman, David M.
Liu, Weihua
Sherman, David M.
Liu, Weihua
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Thesis
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Abstract
Aqueous fluids are an important medium for transporting metals in the Earth’s crust, and are responsible for the formation of many ore deposits. The nature and thermodynamic properties of metal complexes in hydrothermal fluids plays a key role in controlling elemental mobility and mineral solubility in natural and man-made systems. The bulk of our knowledge on metal complexation in hydrothermal fluids originates from experimental studies. Experimental studies at extreme conditions (i.e. high temperature and pressure) are challenging; they can be carried on only over limited P-T-x conditions, and require an accurate speciation model for interpretation. Molecular dynamics (MD) simulations are coming of age for studying metals in hydrothermal processes; the simulations can support the interpretation of experiments; explore conditions beyond the range over which experiments are available; and provide a molecular-level understanding of hydrothermal metal mobility. In this thesis, ab initio (first principles) molecular dynamics (MD) simulations based on density functional theory were conducted to predict the stochiometries and geometries of Cu(I), Au(I) and Zn(II) complexes in solutions with different ligands (Cl⁻, H₂O/OH⁻, HS⁻/H₃S, S₃⁻) at temperatures and pressures ranging from ambient to hydrothermal/magmatic conditions. The important complexes related to metal transport in fluids with different temperatures, pressures and ligand concentrations were simulated. The simulations accurately reproduce the identities and geometries of metal complexes derived from experimental studies, where available. The ab initio MD also demonstrates novel complexes which have not yet been observed by experiments (i.e. CuCl(HS)⁻, AuS₃(HS)⁻, AuS₃(OH)⁻, AuS₃(H₂O)). The thermodynamic properties of metal-ligand association/dissociation reactions of Cu(I)- Cl-HS and Zn(II)-Cl complexes were investigated by distance-constrained MD simulations using thermodynamic integration. The predicted equilibrium constants (logK) for the ligand substitution reactions at high temperature (i.e. >= 300 °C) show good agreement (within 1-2 log units) with the experimental values. Although the slow kinetics at lower temperatures (i.e. < 200 °C) leads to a decrease in the accuracy of the predicted logKs, MD simulations can still reproduce the trends of the change of metal mobility successfully. The predictions of the stoichiometry and thermodynamic properties demonstrate the potential of MD simulations in studying metal mobility in hydrothermal fluids.
School/Discipline
School of Earth and Environmental Sciences
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmetal Sciences, 2013
Provenance
Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.