Approaches for Noble Gas Isotope Application in Rock Porewater Studies

Abstract

Radioactive decay of U, Th and K contributes to noble gas radiogenic ingrowth in different geological reservoirs, which distinguish mass origin and reveal its transport pathway. Compared with minerals and fluid inclusions, porewater is more relevant in revealing the mobile mass origin and transport in the porous, deep subsurface environment. Hence, the approaches for porewater noble gas extraction and analysis are of great meaning to geochemistry and hydrogeology. However, all five stable noble gases in rock porewater are difficult to acquire because of possible air contamination during storage and difficulty of noble gas separation. This dissertation is dedicated to exploring novel noble gas extraction and analysis methods from rock porewater. Two porewater gas extraction methods were developed for crystalline and sedimentary rocks, respectively. Temperature-controlled heating was applied to crystalline rocks. Out-diffusion in Al-foil bags was used for Ordovician sedimentary rocks. Regarding noble gas methodology, a newly designed pneumatic processing line was built to explore an iterative polished stainless steel wool trapping method to separate Kr from Ar. The iterative trapping method yields > 95% trapping efficiency for Kr and > 99% trapping efficiency for Xe. Simultaneously, comparable and steady noble gas sensitivities and noble gas isotope ratios were attained from air standard aliquots. From heating experiments on crystalline rock porewater, the consistency of noble gas ratios between headspace gas and rock porewater illustrates that this extraction method is valid for crystalline rock. This work provides a benchmark for noble gas extraction from crystalline rock porewater. With room-temperature out-diffusion method in Al-bag, noble gas ratios and concentrations of Ordovician sedimentary rocks reveal crustal features. The measured noble gas ratios in Ordovician sedimentary cores agree with measurements previously made in the Ordovician brine samples from the western flank of the Michigan Basin. The Ordovician porewater residence time is quantitatively estimated with both He and Xe radiogenic ingrowth, yielding an average of 251 million years (m.y.), which is comparable with the previous He accumulation time estimate at the same study site that yielded 260 m.y.. The remarkable preservation of gases in Al-foil bags provides an economic and efficient possibility for noble gas out-diffusion sampling. In summary, the exploration of porewater noble gas extraction and all five noble gas analysis methodology gives satisfying noble gas results and geological information. These original developments are of great meaning to the future work of the noble gas laboratory at the University of Ottawa.