The Impact of High Salinity on Radiolytic H2 Yield and the Evolution of Subsurface Brine

Abstract:

Radiolysis of water has been shown to be a source of H₂ in the marine and terrestrial subsurface that supports chemolithoautotrophic microbial ecosystems. In addition to producing H₂, the loss of water from radiolysis may influence the evolution of salinity in deep subsurface fluids. A brine recently discovered in the South African gold and uranium mine, Moab Khotsong at a depth of >2.9 km in the 3.0-2.8 Ga Witwatersrand Supergroup has high salinity (243 ppt), high abundance of H₂ (~46%), and non-meteoric δ²H/δ¹⁸O values suggesting prolonged hydrogeologic isolation. To examine the effect of high salinity on radiolytic H₂ yield and the potential contribution of water radiolysis to subsurface brine formation, the yields for H₂ and radiolytic oxidants O₂ and H₂O₂ were simulated as a function of Cl^- concentrations. H₂ yield increased from 0.44 molecules/100 eV with increasing salinity, with a resultant molecular yield of 0.48 molecules/100 eV for brine. O₂ and H₂O₂ yields also increased for brine, from 0.29-0.34 and 5.7x10^-3-6.9x10^-3 molecules/100 eV, respectively. Using these results, we estimated the time required to form the Moab Khotsong brine by assuming a dosage rate of ~4x10^-3 Gy/(yr-g rock), derived from prior ³⁶Cl/Cl-based estimations of <100 ppm uranium content in the host rock, the brine stopping power (1.14 MeV/(g-cm^-2)) and low host rock porosity (~1%). The annual yields for principal radiolytic products were ~70 nM/yr (H₂), ~30 nM/yr (O₂), ~10 nM/yr (H₂O₂), equivalent to a loss of ~80 nM/yr (H₂O). In relation to lower dosage rates, these results suggest a 3x increase in H₂O loss (nM/yr) for every 10x increase in uranium (ppm). If the initial subsurface fluid represented Archaean seawater (0.92 M Cl^-) trapped in a closed system then the resulting brine (4M Cl^-) would form within >400 myrs taking into account re-addition of H₂O from abiotic CH₄ production (~20 nM/yr). As such, a major contribution by seawater salinity is not required. If the precursor fluid were 2.0 Ga, low salinity, hydrothermal fluids (similar, but likely lower in starting salinity pre-radiolysis, to fluid inclusions found in fracture minerals of the Witwatersrand Supergroup (~0.4 M Cl^-)), brine formation would occur within >900 myrs. These results indicate that water radiolysis is a major contributing factor to subsurface brine formation.