HELIUM AND ARGON ISOTOPES IN ROCKS, MINERALS, AND RELATED GROUNDWATERS - A CASE-STUDY IN NORTHERN SWITZERLAND

To address the problems related to the transfer of helium and argon is otopes from rocks into related aquifers, the concentrations of U, Th, Li, and K and helium and argon isotopes were measured in a sedimentary sequence from Tertiary to Permo-Carboniferous and in crystalline rock s in northern Switzerland. In addition to whole-rock samples, mineral separates have also been investigated. The observed concentrations of He-3, He-4, and Ar-40 in rocks, minerals and groundwaters are compared to calculated values which would result from in situ radiogenic produ ction in a closed system. This comparison shows that the rocks and min erals have almost completely lost radiogenic in situ produced helium. Loss of 3He and He-4 has been controlled by different retention capaci ties of the rock-forming and accessory minerals and fillings. Therefor e, measured He-3/He-4 ratios in these rocks can differ from the calcul ated production ratios. The deviation is generally less than a factor of 3. However, in certain chemical sediments, e.g., anhydrites, elevat ed ratios of He-3/He-4 up to 15 x 10(-8), which is 20 times the produc tion ratio, were observed. It is postulated that H-3, the precursor of He-3, might be chemically bound in such sediments during the 12.3 yea rs half-life of 3H and that the beta decay energy of H-3 is too small to liberate He-3. The calculated closed-system concentrations of heliu m in groundwater exceed the measured ones by up to three orders of mag nitude, implying that the water/rock system has not been a closed syst em since the time Of sedimentation and that movable waters have remove d helium to a discharge area and to the atmosphere. Stagnant old water s, however, could contain high concentrations of helium and supply it to younger movable waters. Aquitards with low permeability (Permian sh ales) appear to be a more important source of helium isotopes than wat er-bearing rocks (Permian sandstones). High abundances of all parent e lements in the shales, almost complete loss of radiogenic helium, and the enhanced production ratio of He-3/He-4 = 7.2 x 10(-8) in these roc ks, similar to the ratio observed in the neighbour Permian aquifer, 9. 4 x 10(-8), suggest the shale as a major-source of helium isotopes for this aquifer and diffusion as a principal process for helium transfer from the stagnant porewaters in shales into the movable waters in san dstones. Some sedimentary rocks, e.g., Permian sandstones add shales, contain more helium; than could have been produced since the time of s edimentation. While Permian sandstones mainly have retained this exces s rare gas component, they have released radiogenic helium. The differ ence in the retention of these two components most probably demonstrat es that migration of radiogenic helium from damage tracks along crysta l imperfections to grain boundaries is an important process controllin g its loss. The abundances of excess He-3 and He-4 in rocks cannot be predicted by in situ production Calculations; studies of rocks and min erals are required. The average ratio of Ar-40/Ar-36 = 385 in groundwa ters collected from Permian sediments from the Weiach borehole is much higher than that in the aquifers from the crystalline basement below the sediments. Radiogenic Ar-40 in Permian sediments is mainly libera ted by diagenetic water-rock interactions. The low contribution of rad iogenic Ar-40 in deep aquifers of the crystalline basement and the fa ct that chlorine concentrations in groundwaters from the basement are also much lower than in the overlaying Permian aquifer eliminates a hy pothetical upwards fluid flux. From this study we envisage intrabasin sources for helium and argon isotopes in groundwaters with mainly late ral migration of waters through sedimentary layers at different rates and diffusion of helium from ''slower-velocity'' older waters in more stagnant zones (shales) into flow-passes with ''higher-velocity'' youn ger waters in sections with higher permeability (sandstones). An exter nal source for noble gases is not required in such a model.