W. Aeschbachhertig et al., QUANTIFICATION OF GAS FLUXES FROM THE SUBCONTINENTAL MANTLE - THE EXAMPLE OF LAACHER SEE, A MAAR LAKE IN GERMANY, Geochimica et cosmochimica acta, 60(1), 1996, pp. 31-41
Vertical and horizontal distributions of helium and neon isotopes were
measured in the water of Laacher See, a maar lake in the East Eifel v
olcanic field in Germany. Neon is in approximate atmospheric equilibri
um 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 o
f the helium excess, R(ex) = He-3(ex)/He-4(ex), is (7.42 +/- 0.03) . 1
0(-6), i.e., R(ex)/R(a) = 5.36 +/- 0.02, where R(a) = 1.384 . 10(-6) i
s the isotopic ratio in air. This indicates the presence of mantle-der
ived helium. Laacher See is thermally stratified during the summer. It
acts as an almost perfect trap for injected gases. The helium concent
ration in the hypolimnion (deep water) approximately doubled from May
to September 1991 yielding a mean He-4-flux of (10 +/- 2) . 10(12) ato
ms m(-2) s(-1). This value is 30 times smaller than in Lake Nyos, Came
roon, 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 identif
ied as an important source of the helium excess in the lake. If carefu
lly sampled by divers to avoid air contamination, the bubbles exhibit
mantle influenced isotopic signatures not only for helium, but also fo
r 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, t
he 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.