Using noble gases measured in spring discharge to trace hydrothermal processes in the Norris Geyser Basin, Yellowstone National Park, U.S.A.

Dissolved noble gas concentrations in springs are used to investigate boiling of hydrothermal water and mixing of hydrothermal and shallow cool water in the Norris Geyser Basin area. Noble gas concentrations in water are modeled for single stage and continuous steam removal. Limitations on boiling using noble gas concentrations are then used to estimate the isotopic effect of boiling on hydrothermal water, allowing the isotopic composition of the parent hydrothermal water to be determined from that measured in spring. In neutral chloride springs of the Norris Geyser Basin, steam loss since the last addition of noble gas charged water is less than 30% of the total hydrothermal discharge, which results in an isotopic shift due to boiling of ≪ 2.5‰ δD. Noble gas concentrations in water rapidly and predictably change in dual phase systems, making them invaluable tracers of gas–liquid interaction in hydrothermal systems. By combining traditional tracers of hydrothermal flow such as deuterium with dissolved noble gas measurements, more complex hydrothermal processes can be interpreted.

Research Highlights
► Models for noble gas concentration and stable isotopic composition of hydrothermal fluid are developed for single stage and continuous steam in hydrothermal systems.
► Using the residual noble gas concentration we can limit the amount of boiling in the hydrothermal system feeding the spring, which can then be used to limit the isotopic shift due to boiling allowing the isotopic composition of the parent hydrothermal reservoir to be determined from that measured in the spring.
► The average residual noble gas fraction in hydrothermal water in Norris is around 10% and the corresponding isotopic shift is less than 2.5‰ delta Deuterium.
► Thus the isotopic composition of the reservoir feeding the Norris Geyser Basin is very close to that measured in the hydrothermal springs.