W. AESCHBACH-HERTIG, R. KIPFER, M. HOFER, AND D. M. IMBODEN
Environmental Physics, Swiss Federal Institute of Technology (ETH), Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-8600 Dübendorf
R. WIELER AND P. SIGNER
Isotope Geology, Swiss Federal Institute of Technology (ETH), NO C61, CH-8902 Zurich
Vertical and horizontal distributions of helium and neon isotopes were measured in the water of Laacher See, a maar lake in the East Eifel volcanic field in Germany. Neon is in approximate atmospheric equilibrium throughout the lake, but the concentrations of both helium isotopes increase with depth and reach values of up to 10 (4He) and 50 (3He) times the atmospheric equilibrium concentration. The isotopic ratio of the helium excess, Rex = 3Heex/4Heex , is (7.42 ±0.03).10-6 , i.e. Rex/Ra = 5.36 ±0.02, where Ra = 1.384.10-6 is the isotopic ratio in air. This indicates the presence of mantle-derived helium. Laacher See is thermally stratified during the summer. It acts as an almost perfect trap for injected gases. The helium concentration in the hypolimnion (deep water) approximately doubled from May to September 1991 yielding a mean 4He-flux of (10 ±2).1012 atoms m-2 s-1. This value is 30 times smaller than in Lake Nyos (Cameroon), but 20 times larger than in Crater Lake (USA). Release of bubbles of nearly pure CO2 (> 99 %) observable in shallow waters at the eastern shore as well as detected by divers in about 30 m depth, was identified as an important source of the helium excess in the lake. If carefully sampled by divers to avoid air contamination, the bubbles exhibit mantle influenced isotopic signatures not only for helium, but also for neon and argon (20Ne/22Ne = 9.92 ±0.03; 40Ar/36Ar = 391 ±4). Based on the elemental ratios of C, Ne, and Ar to 3He, the fluxes of volatiles from the mantle into Laacher See are determined as 7.4.107 atoms m-2 s-1 for 3He, 13.107 atoms m-2 s-1 for 20Ne, 26.107 atoms m-2 s-1 for 36Ar and 6.4.1017 molecules m-2 s-1 for CO2. Non-atmospheric neon was also found in the well "Wallenborn" in the West Eifel volcanic field with a 20Ne/22Ne-ratio as high as 11.21 ±0.06.
Map of the Eifel region (a) and the Laacher See (b)
Depth profiles of 4He (a) and the 3He/4He-ratio (b) from Laacher See.
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