FORMATION OF HYDROTHERMAL CARBONATE IN LAKE TANGANYIKA, EAST-CENTRAL AFRICA

Hydrothermal activity is observed in the shallow-water environment of Lake Tanganyika. Chimneys and crusts consisting of mixtures of aragoni te and (magnesian) calcite are associated with recent and subrecent hy drothermal fluids emerging from the lake floor. The deltaC-13-values o f the carbonates (mean deltaC-13 = +2.1 parts per thousand indicate ca rbonate precipitation from hydrothermal CO2 (deltaC-13 = -8.6 parts pe r thousand) at temperatures between 16-degrees and 33-degrees-C (monom ineralic samples only). The deltaO-18-values of the carbonates range f rom -1.6 to + 2 parts per thousand. This is in accord with similar low carbonate formation temperatures from 17-degrees to 37-degrees-C if p resent-day hydrothermal solutions (deltaO-18 = +1.4 to +2.7 parts per thousand) would be the source for the carbonates. However, absolute te mperature measurements of the fluids are distinctly higher (50-104-deg rees-C). Thus, the carbonates precipitated at lower temperatures, prob ably from mixtures of hydrothermal fluids with lake water.