URANIUM AND THORIUM DECAY SERIES ISOTOPIC CONSTRAINTS ON THE SOURCE AND RESIDENCE TIME OF SOLUTES IN THE YELLOWSTONE HYDROTHERMAL SYSTEM

Hydrothermal fluids in Yellowstone National Park have widely varying chemical composition. Heat and volatile flux from the hydrothermal system can be estimated by monitoring the composition and volume of emitted hydrothermal fluid, but the source of solutes in hydrothermal fluid is often nebulous and the geochemical processes that affect the nuclides are poorly understood.

Measurements of ²²⁰Rn and ²²²Rn activity in hydrothermal fluids and of CO₂ flux from fumaroles and hot springs were carried out in Yellowstone National Park during the summer of 2010. We observed a weak relationship between (²²⁰Rn/²²²Rn) and CO₂ flux, which indicates that CO₂ acts as a carrier gas to bring radon to the surface, but the radon is sourced from aquifer rocks rather than magma. If radon reaching the surface were sourced from magma below Yellowstone, there would be a stronger correlation between (²²⁰Rn/²²²Rn) and CO₂ flux.

Measurements of ²²³Ra, ²²⁴Ra, ²²⁶Ra, ²²⁸Ra, and major solute chemistry in hot spring waters support the hypothesis that the time scale of solute transport from the deep hydrothermal reservoir is long compared to the half lives of ²²⁰Rn and ²²²Rn, which are useful for processes operating on the time scale of 5 minutes to 20 days. Radium isotope activities in hot springs indicate that the solute transport time varies significantly from region to region, indicating that circulation in some areas operates on the time scale of ²²⁴Ra/²²³Ra (20-55 days) and circulation in other areas operates on the time scale of ²²⁸Ra/²²⁶Ra (25-1600 years). The radium isotope composition of hot spring water is also influenced by differences in regional aquifer rocks and geochemical processes such as sorption and mineral precipitation. In summary, geochemical and hydrothermal processes in Yellowstone operate on many different time scales and in diverse geologic conditions, but radionuclide activities possess excellent potential to study these complex phenomena.