Magmatic noble gases in the crater lakes Nyos and Monoun
Keisuke Nagao1), Minoru Kusakabe2), Yutaka Yoshida3) and Gregory Tanyileke4)
1) Laboratory for Earthquake Chemistry, Graduate School of Science, University of Tokyo, Bunkyo-ku,
Tokyo 113-0033, Japan (E-mail: nagao@eqchem.s.u-tokyo.ac.jp)
2) Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori-ken 682-0193, Japan3) Yoshida Consulting Engineer Office, Morioka-shi 020-0121, Japan
4) IRGM, B.P. 4110, Yaoundé, Cameroon

Depth (m)Introduction: After the gas disasters at Lake Nyos in 1986 and Lake Monoun in 1984,
geochemical studies have been carried out to find the cause of the gas burst and to clarify
depth profiles of CO2 concentration, pH, temperature etc. [1]. The CO2 is being
accumulated at the bottom layers of the lakes from an underlying magmatic source, asrevealed from the carbon isotopic ratios of the dissolved CO2 gas (δ13C= -3.5‰) [1].
Helium isotopic ratios 3He/4He=8x10-6 and 4x10-6 for Lake Nyos and Lake Monoun,
respectively [2], also support the magmatic origin. We collected gas and water samples
from various depths at both lakes in October 1999, January 2001 and December 2001.
Isotopic compositions of He, Ne and Ar as well as concentrations of all noble gases weremeasured to figure out depth profiles of noble gases and to compare them with othergeochemical data obtained for both lakes.
Experimental: Two collection methods were employed for gases dissolved in lake water:
1) gas exsolved from water in a Niskin water sampler was collected in a plastic bag andthen transferred into a glass bottle with high vacuum stop cocks (in 1999), and 2) gases
exsolved from various depths were directly collected in glass bottles through plastic hosesthat had various intake depths (in January and December of 2001). The gas was sealed in a
glass ampoule with breakable seal on a vacuum line in laboratory, and the ampoules wereattached to a noble gas purification line connected to a modified-VG5400 (MS-III) noble
gas mass spectrometer. After purification of noble gases, Ar, Kr and Xe were retained on acharcoal trap cooled at the temperature of liquid nitrogen, then Ne was trapped on asintered stainless steel trap cooled at 15K and He was measured. After the He analysis, Nereleased from the trap at 45K was measured. Finally, Ar, Kr and Xe were released from thecharcoal trap at 200°C and measured for Ar isotopic ratios and abundances of 40Ar, 84Krand 132Xe.
0air
Results and discussion: Depth profiles of34He/ He ratio for Lakes Nyos and Monoun-50
are presented in Fig. 1. The ratios higher
Monounthan the atmospheric value of 1.4x10-6-100
BottomofLake Monoun
confirm the magmatic gas input to the lakewater as already reported in [1, 2]. Though-150
the ratios are almost constant in the depth
Nyos
ranges of 80-210m and 30-100m for Nyos
-200
Bottom of Lake Nyosand Monoun, respectively, the highest-6-6-250
ratios of 8.2x10 and 5.2x10 were12345678910
observed at 20-40m above the bottoms of
3He/4He ( E-6 )Nyos and Monoun, respectively. NoFig. 1. 3He/4He depth profiles for the Lakes
systematic differences among the profiles
Nyos and Monoun.
for 1999 and 2001 were observed,
suggesting magmatic He input of constant
3He/4He ratios for both lakes.

040Ar/36Ar

Depth (m)Fig. 2 compares 40Ar/36Ar
1999
ratios for Nyos samples collected-50
in 1999, January of 2001 andDecember of 2001. The ratios for-100
samples of 1999 collected using
plastic bags are systematically-150
lower than those collected using
2001Dec
pendent hoses. The low ratios are-200
attributed to contamination from asmall amount of air that remained
2001 Jan-250
in the plastic bag attached to the250
300
350
400
450
500
550
600
650
Niskin bottle, whereas 40Ar/36Ar40Ar/36Ar
ratios in gases from the hoses weremuch higher with a maximum Fig. 2. Depth profiles of 40Ar/36Ar ratios in Lake Nyos.
value of 600. Noble gas The ratios for samples collected by different methods areconcentrations in CO2 gas exsolved compared.
from waters are very low, e.g.,
Lake Nyos≤40ppm 40Ar in CO2 gas.
700
4036
The highest Ar/ Ar ratio of650
600, observed in the bottom waterin January 2001 just prior to the600
2001 Dec
degassing operation at the Lake,550
seems to have disappeared in500
December 2001, 10 months after
190-208 m
450
the degassing operation. Almost
187.5muniform 40Ar/36Ar ratios (530-540)
400
420and He/ Ne ratios (∼1100)
350
between the depths of 195m and2001 Jan
300
208m in December 2001 are quite
250
a contrast to those in January 2001.
051015202530354045
This may have resulted from the1/C(36Ar) (1/ppm)
gas removal from bottom layer ofthe lake (203m). This interpretation
Fig.3. Ar isotopic compositions for samples of January
is confirmed in Fig. 3, where the and December in 2001 are compared.
Ar data points for January 2001 lie
roughly on a single straight line, suggesting a simple mixing of atmospheric and magmatic
Ar. While the data points for the depths 190-208m in December 2001 are plotted in anarrow area away from the line, showing low Ar concentration with almost constant
40Ar/36Ar of 530 in the bottom layer. The depletion of Ar in the bottom water might havebeen caused by the degassing with CO2.
The gases from Lake Nyos with high 40Ar/36Ar ratios are accompanied by a small
contribution of MORB-type Ne. These results indicate that magmatic gas is now being
supplied to the bottom layer of these crater lakes with the incoming CO2 gas.
References: [1] M. Kusakabe, G. Z. Tanyileke, S. A. McCord and S. G. Schladow (2000) Recent pH andCO2 profiles at Lakes Nyos and Monoun, Cameroon: implications for the degassing strategy and its
numerical simulation. Journal of Volcanology and Geothermal Research 97, 241-260. [2] Y. Sano, M.
Kusakabe, J. Hirabayashi, Y. Nojiri, H. Shinohara, T. Njine and G. Tanyileke (1990) Helium andcarbon fluxes in Lake Nyos, Cameroon: constraint on next gas burst. Earth and Planetary Science
Letters 99, 303-314.