TERRAPUB Geochemical Journal
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Geochemical Journal, Vol. 44 (No. 6), pp. 519-543, 2010

Noble gases in Lakes Nyos and Monoun, Cameroon

Keisuke Nagao,1 Minoru Kusakabe,2 Yutaka Yoshida3 and Gregory Tanyileke4

1Geochemical Research Center, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
2Department of Environmental Biology and Chemistry, University of Toyama, Gofuku, Toyama 930-8555, Japan
3Yoshida Consulting Engineer Office, Morioka, Iwate 020-0121, Japan
4The Institute for Geological and Mining Research (IRGM), B.P. 4110, Yaoundé, Cameroon

(Received August 4, 2009; Accepted June 4, 2010)

Abstract: He, Ne and Ar isotopic ratios and concentrations of noble gases in different depths of Lakes Nyos and Monoun in Cameroon, West Africa, were investigated. Samples were collected in November 1999, January 2001 and December 2001. Improved sampling method applied in 2001 enabled us to detect detailed isotopic distribution in the lakes. Concentrations of Ne and Xe in the lake waters were low by factors of ~10 and ~3, respectively, compared with those of air saturated water (ASW), whereas in CO2 gas they were depleted by 2 orders of magnitude compared with those in air. In contrast, He was strongly enriched in lake water by more than three orders of magnitude compared to ASW. 3He/4He ratios of 8 × 10-6 (Lake Nyos) and 5 × 10-6 (Lake Monoun) indicated recharge of magmatic gases of mantle origin to the lakes, though the ratios were lowered by the addition of radiogenic 4He from crustal rocks. A small contribution (<2%) of MORB-like Ne was detected in both lake waters. A simple mixing between two end members, i.e., atmosphere and gases from a deep-seated reservoir, can explain the noble gas signatures. Depth profile of He concentration in Lake Nyos showed a clear layered structure with a maximum at 190 m, 20 m above the lake bottom. Below this depth, 3He/4He ratio decreased slightly, suggesting that the fluid from the deep-seated reservoir is supplied to the lake mainly at ca. 190 m. C/3He ratios were 1.5 × 1010 and 0.6 × 1010 in deep and shallow layers of Lake Nyos, respectively. The C/3He ratio was 1.7 × 1010 at Lake Monoun. A gas recharge model to explain the observed layered structure in Lake Nyos is presented.
Key words: noble gases, depth profile of noble gases, Lake Nyos, Lake Monoun, Cameroon volcanic line


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