Fluids in the crust

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Figure 1. Hotwater spring site

Almost all geological processes involve fluids. This includes metamorphism, the formation of economic mineral deposits, hydrothermal systems (Figure 1) and the creation of the biosphere to the separation of the planet into the different geochemical reservoirs of the crust, mantle and core. Despite the importance of fluid-related systems, developing a quantitative understanding of role of fluids in many diverse disciplines within the Earth Sciences has been elusive. This is in part because, unlike the solid residue of these processes, the fluids involved have long since been flushed out of the system, have been chemically changed or trapped in fluid inclusion in quantities that are hard to analyse. More recently there has been much focus on the environment, with information from shallow aquifers being used to address a range of issues such as changes in past climate and the age and residence time of water for drinking water resource determination. At Manchester we have an internationally recognized research team who specialize in using noble gas isotopes to investigate different aspects of fluids in the crust. These can broadly be divided into two related groups: hydrocarbons and groundwaters. These are described in outline below.

Noble Gases at Manchester
Noble gas (helium, neon, argon, krypton and xenon) isotopes provide a key tracer in these studies. This is because noble gases from the crust, the mantle and the atmosphere (dissolved in groundwater at recharge) all contain a unique isotopic fingerprint that enables us to resolve and quantify the contribution of fluids from these different sources. In addition, they are chemically unreactive and can preserve the record of a chemically reactive fluid system. A third and key point is that the noble gases are in trace amounts, so that small additions from natural nuclear processes in the crust can be resolved to provide information about the physical conditions in the crust and information about time – a key constraint in all geological and environmental studies. At Manchester we have one of Europe’s leading research laboratories specialising in the analysis of noble gas isotopes  in crustal fluid systems, from fluid inclusions to free groundwaters, natural gases and hydrocarbons.

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Figure 2. Schematic diagram of a gas reservoir, illustrating the different noble gas components that occur in crustal fluids (Fig 1. Ballentine et al., RiMG 47, 2002, 539-614.)

Natural Gases and Hydrocarbons
The application of noble gas isotope studies to understand oil and gas origin and their interaction with the groundwater phase is a new and developing research field. Noble gas isotopes in groundwater are fixed when the water last equilibrated with the atmosphere. The noble gases then partition between the water, gas or oil phases as a function of solubility and relative volume of the different phase. In principle, measurement of the noble gas composition of a groundwater can tell us exactly how much oil or gas has passed through the water column. In practice, obtaining uncontaminated water samples from boreholes of interest is very hard and we have to be content with the concomitant information contained by the water-derived noble gases in the oil or gas phases. These measurements quantify the amount of groundwater the oil or gas phases have ‘seen’. Quantifying the amount of groundwater that has equilibrated with an oil or gas phase allows us to place stringent constraints on the role that groundwater has played in both the secondary migration of oil and gas (where primary migration is expulsion from the source rock) and diagenetic processes associated with groundwater movement during hydrocarbon migration or emplacement. An equally important use of noble gases has been identifying the origin of non-hydrocarbon gases such as CO2 and N2 by relating these species to mantle-derived and crustal-radiogenic gases respectively.

•    Ballentine C. J., Burgess R., and Marty B. (2002) Tracing fluid origin, transport and interaction in the crust. Rev. Min. Geochem. 47, 539-614.

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Figure 3. Diagram showing how noble gases dissolved in the groundwater system can be used. Applications include paleotemperature, 4He and tritium groundwater dating, understanding CH4 and N2 origin and transport and the extent of interaction the groundwater has had with hydrocarbon reservoirs.

With an increased awareness of environmental issues, there has been a great deal of focus on groundwater systems. These include the determination of groundwater age using tritium and 4He techniques to assess water residence time and its origin. Dating water has application in a variety of areas such as water resource management and pollution tracing, but also provides a critical parameter used to assess rates of change in groundwater chemistry.
Tritium is a radioactive isotope of hydrogen found in the atmosphere that is naturally part of the water molecule. Once in the ground there are no significant tritium sources and as the groundwater becomes older, with a half-life of 12.43 yrs, the tritium concentration decreases. By taking a groundwater sample, degassing it and storing it for several months, a portion of any remaining tritium decays in the sample to 3He. This in turn can be precisely measured in the noble gas laboratory, enabling a groundwater tritium date to be calculated. This application is appropriate for groundwaters that are typically less than ~60yrs old, but is also used to identify small additions of young groundwater to older aquifers. 4He in groundwater provides a totally different scale of groundwater ages, from ~1K to 100K years, and is produced by the radioactive decay of naturally occurring U and Th in the sediments and underlying rock strata. The accumulation of 4He in the water is a function of time, and can be used to place relative ages on related groundwaters bodies. Another important use of noble gases in groundwater is the determination of paleotemperature. When groundwater at recharge equilibrates for the last time with the atmosphere, the water temperature determines the concentration of the dissolved noble gases. Knowing the age of the groundwater and its noble gas composition, we can reconstruct how surface temperature has changed in the past.

Some recent publications:
•    Ballentine C. J., Burgess R., and Marty B. (2002) Tracing fluid origin, transport and interaction in the crust. Rev. Min. Geochem. 47, 539-614.

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