Production, redistribution and loss of helium and argon isotopes in a thick sedimentary aquitard-aquifer system (Molasse Basin, Switzerland)

In a rock–water system an apparent residence time for He can be derived from the bulk He concentration in the rock and the porewater and the He production rate provided that (1) the system is at steady state and (2) the He flux from external sources is negligible. This second condition is crucial and needs to be assessed by identifying the various He-sources in a given hydrologic environment. Ar isotope abundances were investigated in whole rock samples and mineral separates of the alternating shale–sandstone rock sequence of the Permo-Carboniferous Trough (PCT) of the Molasse Basin in northern Switzerland. 4He concentrations and 40Ar/36Ar ratios in groundwater in the bounding aquifers are lower than those in PCT groundwaters, thus eliminating a possible external source of radiogenic noble gases. ements producing radiogenic helium isotopes, U, Th and Li, are mainly concentrated in the shales, where they reside in biotite, muscovite, clay minerals and organic matter. Contents of these elements as well as chronological and chemical data allow us to calculate maximum “closed system” He isotope concentrations and the retention coefficients, e.g. 3Hemeasured/3Hecalculated ratios. As deduced from the low retention coefficients, almost all helium-3 that has been produced in these minerals since their deposition has been lost into the complementary reservoir, i.e. the porewater; the same conclusion is valid for 4He. This is also indicated by the similarity between the calculated 4He/3He production ratio of the shales and that observed in PCT groundwater (1.3 × 107). Moreover, similar 4He/3He ratios are measured in quartz and plagioclase mineral grains from the sandstone layers, by far exceeding the production ratios derived for these minerals. Also, the measured concentrations of He isotopes (especially 3He) are higher than the calculated production capacities of these minerals. These observations suggest that some of the He produced in the shale and released from these rocks migrates via the porewater into the sandstone layers and penetrates further into some rock-forming minerals, such as quartz and plagioclase. ements of the migration rate of He atoms through quartz crystals showed that equilibrium between internal (gas–fluid vesicles) and external (porewater) He-concentrations is reached on a short time scale of ~ 104 years at an in-situ temperature of ≈ 70 °C. The equilibrium allows the He concentrations in the porewater across the PCT sequence to be quantified using quartz crystals as detectors. For the PCT segment opened by the Weiach borehole these “quartz-derived” concentrations as well as those directly measured in PCT groundwaters are rather high implying a long residence time for the He-atoms in the PCT rock-porewater system, well exceeding 107 years.