The origin of helium in deep sedimentary aquifers and the problem of dating very old groundwaters

Abstract

Abstract Noble gases, inert elements having isotopes produced by the decay of long half-life radionuclides, offer a powerful approach for tracing fluid circulation and dating groundwaters. The (U + Th)- 4 He water ages — calculated from the accumulation rate in water of radiogenic 4 He produced by decay of U and Th contained in the aquifer rocks — is frequently higher than the hydrological ages. This discrepancy is generally interpreted by two contrasting models: (i) heterogeneities of the aquifers, which allow water stagnation and accumulation of large amounts of radiogenic 4 He, or (ii) addition of 4 He produced in deeper regions of the continental crust. In this contribution, we propose that the apparent contrast between (U + Th)- 4 He ages and hydrological ages in the Paris Basin reflects the mixing of different types of water, having different residence times. We show, using the helium isotopic signatures of waters, that this mixing occurs between three aquifers, the Middle Jurassic, the Triassic and the Palaeozoic basement, which have contrasting helium contents and heterogeneous chemical compositions and permeabilities. The difference of radiogenic 4 He/ 40 Ar* ratios between the aquifers of Triassic and Middle Jurassic strongly suggests that a significant fraction of helium is produced in the aquifer rocks. This implies residence times for groundwaters circulating in the Middle Jurassic carbonate aquifer much longer than those obtained from hydrological studies. Independent fluid age estimates, based on the ground palaeotemperatures recorded in the same groundwaters by the atmosphere-derived noble gases, seem to confirm the presence of very old groundwaters in the Paris Basin.