A new method for quantitative analysis of helium isotopes in sediment pore-waters

Abstract

Owing to their inertness and contrasted composition in the various earth reservoirs, helium isotopes are powerful tracers of a number of processes pertaining to geophysics and geochemistry. Because sediments cover a large portion of the earth's surface, helium isotope geochemistry of sediment pore-waters is of particular interest. In spite of this potential, its development has been hampered by the difficulty of collecting samples without gas loss and/or contamination problems. We developed a new method for the sampling and the quantitative extraction of dissolved helium from sediment pore-waters, leading to the determination of 3He and 4He concentration profiles. Core sampling is non-destructive (no squeezing). The principle of the method is to use standard copper tubes (1.2 cm OD/25 cm in length), subsequently sealed with clamps, to take mini-cores along the sediment core immediately following its retrieval. In the lab, the sediment is transferred from the copper tube to a noble gas extraction line by applying pressurized helium-free water at one end of the copper tube. This technique allows dissolved helium to be recovered and analyzed using standard procedures for water samples. Tests were carried out successfully on an artificial core equilibrated with air to check the extraction efficiency in the same conditions as for real cores. The validity of the method was further confirmed by acquiring a vertical helium profile from a real marine core from the Zaire deep-sea fan, illustrating some possible applications.