Underground production of ⁸¹Kr detected in subsurface fluids

Radiokrypton dating using the long-lived natural isotope ⁸¹Kr has been developed for determining the age of ancient groundwater. ⁸¹Kr is attractive for this purpose because it is generally thought to be produced solely in the upper atmosphere. Its 229,000-year half-life, a spatially homogeneous distribution in the atmosphere, and the absence of anthropogenic sources makes it an ideal tracer to determine ages up to ∼1.3 million years. As a noble gas, it is inert and thus not subject to interfering geochemical reactions that may alter the age information. We sought to date groundwater samples collected from deep (0.6–1.9 km) rock fractures in the Kaapvaal Craton, South Africa. Previous studies using other dating methods have estimated groundwater ages at these sites in the 1–100 million year range. Surprisingly, three of the four samples collected from flowing boreholes in gold and diamond mines showed ⁸¹Kr isotopic abundances at 2–5 times the atmospheric value. This is the first time that underground production of ⁸¹Kr has been detected, indicating that ⁸¹Kr can be generated underground in measurable quantities in contrast to the long-held paradigm that such production is insignificant in natural rocks. A radionuclide production and release model is proposed in order to quantify the importance of different factors that affect the concentrations of ⁸¹Kr in the groundwater. It is not only the high effective uranium content of the rock but also a higher ⁸¹Kr fission yield than previously anticipated that are likely causing the elevated ⁸¹Kr values in this case. In less extreme environments with average crustal composition (i.e. moderate U concentration), however, we anticipate that the underground production minimally affects groundwater ⁸¹Kr dating as demonstrated.