PVTX and Raman Spectral Properties of Fluids at Elevated Pressures and Temperatures
Sublett Jr, David Matthew
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Fluids are associated with a wide range of physical and chemical processes in the Earth, including transporting and concentrating important ore elements such as Cu, Au, Zn, and Pb. Significant amounts of fluid may be generated as a result of dehydration or decarbonation reactions, and the volatile content of a magma is directly linked to the explosivity of eruptions. In most cases, small amounts of the fluids involved in the formation or alteration of rocks are trapped within minerals in the form of fluid inclusions. These fluid inclusions may be studied to understand the composition and pressure and temperature of the original fluid involved in the geologic process of interest, however, an understanding of the composition of the fluid as well as how the fluid behaves under changing pressure and temperature conditions is essential to reconstruct the fluid evolution path based on data obtained from fluid inclusions. Several analytical techniques are involved in the study of fluids, including fluid inclusion microthermometry and Raman spectroscopy. Microthermometry is the heating/cooling of fluid inclusions to observe and record temperatures of phase changes which, in turn, are used to determine properties such as salinity (based on the freezing point depression of liquid), or density based on the temperature at which all phases within the fluid inclusion homogenize to a single phase. Raman spectroscopy is a non-destructive analytical technique that measures the vibrational frequency of molecules in a given material. The Raman spectral properties of fluids act as a "fingerprint" of the chemical species within the fluid and serve to identify both the presence of chemical species, such as H2O, N2, CO2, and CH4, and the density of the fluid. Microthermometric and Raman spectroscopic experiments involving synthetic fluid systems are necessary to elucidate the pressure-volume-temperature-composition (PVTX) and Raman spectral behavior of the fluid systems, which then aids in the study and characterization of natural fluids. In chapter 1, the partitioning of NaCl and KCl between coexisting immiscible fluid phases during boiling is experimentally determined at temperatures and pressures relevant to magmatic-hydrothermal systems using synthetic fluid inclusions. The partitioning behavior is then combined with literature data to calculate the Na/K ratio of the original silicate melt phase in a magma body before the exsolution of a fluid phase. In chapter 2, we explore the Raman spectral behavior of N2, CO2, and CH4 in pure, single-component systems from PT conditions corresponding to the liquid-vapor curve to elevated temperatures and pressures, and relate the changes in the spectral behavior to changes in the bonding environment of the molecules through intermolecular attraction and repulsion. In chapter 3, the observations and relationships determined for pure fluids and described in chapter 2 are used to explore the Raman spectral properties of N2, CO2, and CH4 in the N2-CO2-CH4 ternary system and the manner in which the spectral behavior of each component in the system varies with changing temperature, pressure, molar volume, and fugacity.
General Audience Abstract
Water and other fluids play an important role in the formation of mineral deposits that are the source of the many metals, such as copper, silver, gold, and others, that are needed by a modern technological society. In addition, water and other fluids affect the way rocks behave under stress and can promote earthquakes and influence the explosivity of volcanoes. When minerals in a rock form, often small amounts of the fluid will be trapped within the minerals in the form of fluid inclusions. These fluid inclusions contain samples of the fluid involved in the geologic process of interest and can be studied using a variety of methods to determine the chemistry and the temperature and pressure conditions of rock formation. Two of the many methods used to study fluid inclusions are microthermometry and Raman spectroscopy. Microthermometry involves heating and/or cooling the fluid inclusion while it is being observed on a microscope, and this method can be used to determine the salinity of water in the inclusion and the fluid density. The density of the fluid may then be used to determine the pressure or temperature at which the fluid was encapsulated into the rock, and by extension the temperature and pressure at which the rock formed. Raman spectroscopy is an analytical technique in which a rock or fluid is illuminated using a laser. The laser light interacts with the rock or fluid and gains or loses energy, and this change in energy serves as a "fingerprint" to identify the molecules in the rock or fluid. The Raman spectrum can also be used to determine fluid density because the signal generated when the laser interacts with the fluid depends on the density of the fluid. Experiments on fluids at carefully-controlled laboratory conditions are necessary to understand the behavior of fluids trapped in natural samples. In chapter 1, the preference of sodium and potassium to go into either a liquid or a gas phase during boiling at high pressures and temperatures is determined. In chapter 2, gases containing only nitrogen, carbon dioxide, or methane are studied using Raman spectroscopy and the changes in the Raman behavior of the gases with changing pressure and temperature are related to molecular interactions. In chapter 3, the results from chapter 2 are used to understand the Raman behavior of nitrogen, carbon dioxide, and methane in gas mixtures as pressure and temperature are changed and how this relates to the interactions of the molecules.
- Doctoral Dissertations