Tracing fluid dynamics with noble gas and stable isotope systematics : examples from Krafla, Iceland and Sichuan Basin, China

• Main Authors: Li, Yan, Zhou, Zheng, Holland, Greg
• Format: Thesis
• Language: English
• Published: Lancaster University 2019
• Subjects: Iceland
• Online Access: https://eprints.lancs.ac.uk/id/eprint/131198/; https://eprints.lancs.ac.uk/id/eprint/131198/1/2018yanliphd.pdf

The study of noble gases and reactive gases (i.e., CO2, N2 and CH4) in crustal reservoirs can help better understand the origin, migration and accumulation processes of crustal fluids in the subsurface environment, which provide further insights into fluid dynamics underground. This PhD thesis develops noble gas isotopes as geochemical tools in crustal fluid studies. Study sites have been selected to cover different types of geofluids, including geothermal fluids in Krafla, Iceland and natural gases in the Sichuan Basin, China. Following a short introduction to the research background, objectives and thesis layout are presented in Chapter 1 and Chapter 2 gives a literature review on the application of noble gases as powerful geochemical tools in hydrothermal and hydrocarbon systems. Chapter 3 describes a noble gas extraction and purification system, interfaced to a multi-collector NGX noble gas mass spectrometer (Isotopx), which is constructed particularly for this study. The detailed description of each section in sample preparation system, as well as gas sampling, extraction, purification and separation protocols is provided. Temperatures of 50 K and 95 K can be used as the optimal releasing temperatures for He and Ne on the cryotrap in sample prepline. The temperature of 210 K is tested to be the optimal temperature for releasing Ar and Kr but keeping Xe being trapped onto the charcoal trap in the prepline. Chapter 4 characterizes noble gas and stable isotope data of hydrothermal fluid system in Krafla, Iceland. Stable isotope results indicate that CO2 in the samples is considered to be magmatic in origin, with δ13C (CO2) ranging between -7.99 and -3.86 ‰. Modelling results show that processes of boiling and steam separation have occurred during the circulation of geothermal fluids in the shallow crust in the Krafla field. Air addition, possibly introduced by groundwater re-injection, has had a significant effect on the geochemical signatures of Krafla geothermal fluids as well. Chapter 5 focuses on using ...