CO2-RICH GASES FROM LAKES NYOS AND MONOUN, CAMEROON - LAACHER SEE, GERMANY - DIENG, INDONESIA, AND MT GAMBIER, AUSTRALIA - VARIATIONS ON A COMMON THEME

Helium (R(A) = 3.0 to 5.6) and carbon (delta-C-13 from -7.2 to -3.4 parts-per-thousand) isotopic compositions, and relative CO2, CH4, N2, He and Ar contents of CO2-rich gases from Lakes Nyos and Monoun, Cameroon; Laacher See, Germany; Dieng Volcanic Plateau, Indonesia, and a well at Mt. Gambier, Australia, point to a common, essentially magmatic origin. Absorption of the original magmatic gases into deeply circulating groundwater and equilibration of the resulting solutions with crustal rock at temperatures of about 300-degrees-C fix CO2 and CH4 contents. On further rise, the solutions start to boil separating gas-rich vapors which, on encountering an impermeable barrier, may accumulate to form gas pockets with steadily increasing pressures. In the case of sufficiently high gas contents, the pressures may exceed lithostatic pressures leading to a blow-out or a "pneumatic" eruption (Dieng). Otherwise, gas may accumulate to form a stable pocket (Mt. Gambier). Minor leakage from such pockets may lead to surface discharges of CO2-rich gases as at Laacher See, re-absorption into shallow groundwater to the formation of the low-salinity, CO2-charged waters encountered at Lakes Nyos and Monoun. The occurrence of these high-CO2, low-temperature systems is likely to be favored in tectonically active regions, allowing deep, possibly mantle gases to rise, but with sufficiently low regional heat flows to prevent the establishment of large-scale geothermal activity.