86Kr excess and other noble gases identify a billion-year-old radiogenically-enriched groundwater system

Abstract

Deep within the Precambrian basement rocks of the Earth, groundwaters can sustain subsurface microbial communities, and are targets of investigation both for geologic storage of carbon and/or nuclear waste, and for new reservoirs of rapidly depleting resources of helium. Noble gas-derived residence times have revealed deep hydrological settings where groundwaters are preserved on millions to billion-year timescales. Here we report groundwaters enriched in the highest concentrations of radiogenic products yet discovered in fluids, with an associated 86Kr excess in the free fluid, and residence times >1 billion years. This brine, from a South African gold mine 3 km below surface, demonstrates that ancient groundwaters preserved in the deep continental crust on billion-year geologic timescales may be more widespread than previously understood. The findings have implications beyond Earth, where on rocky planets such as Mars, subsurface water may persist on long timescales despite surface conditions that no longer provide a habitable zone.

The Low Permeability of the Earth’s Precambrian Crust

“…However, laboratory testing of core samples have found correlations between permeability and diffusion coefficients [37][38][39] . The lowest diffusion coefficients found in those studies were a few orders of magnitude higher than those required to prevent differential diffusion of Differential transport rates of different noble gases have been noted at Sudbury, Canada 2 and Moab Khotsong, South Africa 6 . In these studies the evidence for diffusive loss of the light noble gases (He, Ne) versus the heavier Ar, Kr, Xe was hypothesized to be related to increased fracturing of the basement due, in both cases, to their history as impact craters 5,6 .…”

“…In this study the overall good coherence between the He-Ne-Ar-Xe derived noble gas residence times at each site support a model of hydrogeologic isolation. However, As Warr et al 2022 6 recently demonstrated, these settings actually represent a spectrum from being fully isolated to fully open. At sufficiently low diffusion coefficients (10 -15 m 2 /s), there will be no appreciable losses of any noble gases 29 .…”

“…At sufficiently low diffusion coefficients (10 -15 m 2 /s), there will be no appreciable losses of any noble gases 29 . At slightly higher diffusion coefficients, noticeable diffusive transport of He and Ne will occur while Ar, Kr, and Xe will be retained 6,29 . Relating these low rates of diffusion to permeability is not straightforward -there is no universally agreed upon relationship between permeability and diffusion coefficient.…”

“…This deep store of mostly saline groundwater accounts for 20 to 30% of total continental groundwater reserves. Estimates of groundwater residence times in Precambrian rocks (Figures 1 and 2a), can exceed 1 billion years 2,[4][5][6] , with the longest residence times found in Archean age rocks. These deep and ancient groundwaters are estimated to contain a substantial portion of the Earth's biomass, with microbial activity found to depths of up to 3 2-3 km [7][8][9][10][11][12] .…”

“…These deep and ancient groundwaters are estimated to contain a substantial portion of the Earth's biomass, with microbial activity found to depths of up to 3 2-3 km [7][8][9][10][11][12] . The degree of hydrogeologic and associated geochemical isolation from nearsurface environments exerts control on the habitability, abundance and diversity of subsurface microbial life 6,9,[11][12][13] .…”