Helium Flux from the Craton by Diffusion through Its Pore System

Most radiogenic helium in natural gases is interpreted as a product of helium generation within the crystalline continental crust; however, the pathways to overlying sedimentary basins remain unclear. This study investigates helium diffusion through water-saturated porosity in crystalline rocks as a mechanism for basement helium charge to natural gases in cratonic areas. Diffusive properties of shallow core samples were compiled and extrapolated deeper into the crust. Crystalline rocks in the upper km of basement have a median porosity near 0.12% and median microstructure factor (M) near 3.4 × 10−5. At 10 km, median modeled porosity is about 0.05%, effective helium diffusion coefficient (De) through porous crystalline rock is about 2 × 10−13 m2/s, and effective helium diffusivity (Ðe) is about 4.5 × 10−10 m2/s. Helium surface flux was evaluated by analytical models with helium generation rates decreasing with depth. Helium flux increases with increasing helium diffusivity and time. Given an average Ðe of 4 × 10−10 m2/s and average crust generation rate, surface helium flux in 2 Gy crust is 0.4 moles helium/m2/My, about 25% of total crust generation rate. Most of this helium was generated in the upper crust. Basement helium flux in this range can account for the distribution of most high-helium natural gases. Exhumation increases helium flux, and elevated flux persists long after exhumation. Helium flux measured in lakes and soils above an exhumed basement surface may be enhanced by the exhumation history of the underlying basement. This adds yet another complexity to evaluating noble flux from the deep Earth.