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Abstract The development of life on Earth has been enabled by its volatile-rich surface. The volatile budget of Earth’s surface is controlled by the balance between ingassing (for example, via subduction) and outgassing (for example, through magmatic and tectonic processes). Although volatiles within Earth’s interior are relatively depleted compared to CI chondrites, the total amount of volatiles within Earth is still substantial due to its vast size. However, the relative extent of diffuse degassing from Earth’s interior, not directly related to volcanism, is not well constrained. Here we use dissolved helium and high-precision argon isotopes combined with radiocarbon of dissolved inorganic carbon in groundwater from the Columbia Plateau Regional Aquifer (Washington and Idaho, USA). We identify mantle and crustal volatile sources and quantify their fluxes to the surface. Excess helium and argon in the groundwater indicate a mixture of sub-continental lithospheric mantle and crustal sources, suggesting that passive degassing of the sub-continental lithospheric mantle may be an important, yet previously unrecognized, outgassing process. This finding that considerable outgassing may occur even in volcanically quiescent parts of the crust is essential for quantifying the long-term global volatile mass balance. 

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 associated⁸⁶Kr 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.