Chemical and isotopic characteristics of fluids along the Cameroon Volcanic Line, Cameroon

Abstract

Results of the chemical and isotopic analysis of the water and gases discharged from volcanic crater lakes and soda springs located along the Cameroon Volcanic Line were used to characterize and infer their genetic relationships. Variations in the solute compositions of the waters indicate the dominant influence of silicate hydrolysis. Na + (40–95%) constitutes the major cation in the springs while Fe 2+ + Mg 2+ (70%) dominate in the CO 2-rich lakes. The principal anion is HCO 3 (>90%), except in the coastal springs where Cl-predominates. Lakes Nyos and Monoun have FeMgCaHCO 3 − type signatures; the soda springs are essentially NaHCO 3 − type, while all other lakes show similar ionic compositions to dilute surface waters. Dissolved gases show essentially CO 2 (>90%), with small amounts of Ar and N 2, while CH 4 constitutes the principal component in the non-gassy lakes. Active volcanic gases are generally absent, except in the Lobe spring with detectable H 2S. Stable isotope ratio evidence indicates that the bicarbonate waters are essentially of meteoric origin. CO 2 ( δ 13C = −2 to −8% 0 and He ( 3He/ 4He = 1 to 5.6R a) infer a mantle contribution to the total CO 2. CH 4 has a biogenic source, while Ar and N 2 are essentially atmospheric in origin, but mixing is quite common.