Dating very old pore waters in impermeable rocks by noble gas isotopes

The 4He, 40Ar, and 136Xe content dissolved in the pore water of sedimentary rock samples was measured on samples from borehole cores near the repository for nuclear waste in Morsleben, Germany. Due to the very low permeabilities of the rock formations, conventional groundwater sampling was almost impossible. Hence, the authors developed a new sampling method for noble gases in the pore water of freshly drilled rock cores. This method provides vertical noble gas profiles in high depth resolution, even in impermeable rocks. By application of the new technique quantitative age information of groundwater and pore water have been derived. The authors find palaeowaters from the last glaciation depleted in δD and δ18O with a 4He age of about 55 kyr. The high saline pore solutions below are at least 6 Mio years old. This has been concluded from the profiles of radiogenic 4He and 40Ar close to diffusion in steady-state and from xenon isotopes produced by spontaneous fission of 238U in the rocks. A 4He flux of 2 · 10-7 cc STP/cm2 yr is derived from the profile, which is due to local 4He production within the investigated sediments