Atmospheric and radiogenic gases in groundwaters from the Stripa granite

Abstract

Groundwaters from depths of 350 m to 1250 m in the Stripa granite contain dissolved radiogenic He in amounts up to 50,000 times that due to air-saturation. The groundwater He-contents increase with depth and lie close to the expected profile for He loss by aqueous diffusion ( D = 0.032 m 2 a −1). Measurements on core samples show that the rock has retained about 10% of the possible cumulative radiogenic He and that this component is lost by matrix diffusion ( D = 5 × 10 −7 m 2 a −1). Diffusive equilibrium between He in fracture fluids and in the adjacent rock matrix is rapidly established for the narrow fracture widths of the flow system. A major loss of stored He by both diffusion and advection along fluid-filled fractures is attributed to the proximity of a major fraction of uranium to the aqueous flow system because of its deposition within an interconnective microfracture system. The crustal flux of He is limited by its diffusion coefficient in the matrix of a granitic crust but may be supplemented by transport due to fluid circulation. The 3He 4He ratio of the excess He present in the Stripa groundwaters, corresponds to that expected for radiogenic He production within the granite. The 40Ar 36Ar ratio of dissolved Ar shows that radiogenic 40Ar has been released from the rock matrix, especially for groundwaters from greater than 450 m depth. Slow alteration reactions are the most probable cause of this radiogenic 40Ar release which has occurred in the more saline groundwaters. Groundwater recharge temperatures, estimated from their noble gas contents, are about 3°C lower than those for modern shallow groundwaters in the locality and are related to the stable isotope composition of the groundwater. Most groundwater age measures at Stripa are the result of mixing between recent recharge waters (<100 a) and a much older fracture-stored brine.