Noble gas measurements in groundwater are an established tool to investigate groundwater systems including magnitude and timescales of fluid exchanges. In formations with very slow to no advective groundwater flow (e.g., aquitards and shales), noble gases, especially helium, are excellent tracers for confirming diffusion-dominated transport. In “dry” formations of extremely low porosity and permeability, such as igneous rocks, studying noble gases within fluid inclusions is the only means to assess fluid migration in the matrix. Fluid inclusions in mineral grains evolve mainly during crystallization or are enclosed during rock deformation. However, fluid inclusions are not necessarily completely isolated from surrounding pore water. For example, in the case of quartz grains in sedimentary rock, helium can diffuse from pore water into the fluid inclusions which may influence total helium concentration in the porous quartz grains, and hence add uncertainty to the derivation of aquitard permeability based on the quartz-helium method. Information on these very slow fluid migration time scales is paramount to confirm the effectiveness of the sealing capacity of deep formations for carbon sequestration, nuclear waste disposal, and as a potential natural gas resource
The Environmental Tracer Laboratory (ETL) at the Waite Campus, SA, operates one of only a few High-Resolution Noble Gas Mass Spectrometers in Australia. This device can purify noble gases and measure rare isotope ratios like136Xe/132Xe, 21Ne/20Ne and 3He/4He in groundwater and pore fluids. In fluid inclusions, these noble gas isotopes are the only means to evaluate how long fluids were isolated from the water cycle, due to the small amount of fluid and the large time scales.
The presentation will review and compare methods reported in literature to extract fluid inclusions for noble gas measurement and is further informed by laboratory visits in Australia, Germany, the UK, and the USA. The most common approach is to crush mineral grains in vacuum to release noble gases from the fluid inclusions. The findings will underpin the design for a new crushing system at CSIRO.