Quantification of gas fluxes from the subcontinental mantle: The example of Laacher See, a maar lake in Germany

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 (He-4) and 50 (He-3) times the atmospheric equilibrium concentration. The isotopic ratio of the helium excess, R(ex) = He-3(ex)/He-4(ex), is (7.42 +/- 0.03) . 10(-6), i.e., R(ex)/R(a) = 5.36 +/- 0.02, where R(a) = 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 He-4-flux of (10 +/- 2) . 10(12) 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 (Ne-20/Ne-22 = 9.92 +/- 0.03; Ar-40/Ar-36 = 391 +/- 4). Based on the elemental ratios of carbon, neon, and argon to He-3, the fluxes of volatiles from the mantle into Laacher See are determined as 7.4 . 10(7) atoms m(-2) s(-1) for He-3, 13 . 10(7) atoms m(-2) s(-1) for Ne-20, 26 . 10(7) atoms m(-2) s(-1) for Ar-36, and 6.4 . 10(17) molecules m(-2) s(-1) for CO2. Non-atmospheric neon was also found in the well ''Wallenborn'' in the West EifeI volcanic field with a Ne-20/Ne-22-ratio as high as 11.21 +/- 0.06.