Helium concentrations and isotope compositions in 10 km deep groundwaters

Abstract

Helium isotopes are useful for tracing terrestrial water movement and calculating residence times, which have important implications for radioactive waste disposal and carbon capture and storage projects. Helium concentrations are generally measured directly in water samples; however, this is not always possible. In this scenario, the He concentration equilibrium between pore waters and He accessible volumes (e.g., fluid inclusions, hereafter HAV) in minerals (e.g., quartz) can be utilized to determine He concentrations. In this study, the He partial pressure was measured in HAVs of quartz grains collected from cores of the Kola Super Deep Borehole (KSDB) using isothermal He extraction/saturation experiments. A large fraction of He was released from these samples during the first interval of isothermal heating, indicating a destruction of the unstable HAVs, likely due to decrepitation of fluid inclusions. We present a new approach to calculate He partial pressures using only the He fraction released from the stable HAVs. This approach gives a He partial pressure pHe of ≅ 1.5 atm at depths of ~ 10 km. When combined with solubility data, this value suggests a high He concentration in the deep pore waters nearby the KSDB, ≅ 1.3 × 10−6 mol (cc H2O)−1, indicating a long residence time of He atoms in a deep water – mineral system, close to the age of the regional metamorphism (≅1700 Ma). The 3He/4He in the deep KSDB quartz samples (and the pore waters) are ~ 3 × 10−8, typical of radiogenic He generated in 10 km deep Archean rocks of the KSDB. Importantly, external fluxes are not needed to explain He isotope abundances in rocks, minerals and pore waters at these depths.