Preliminary observations concerning the genesis of high fluoride contents in the Ethiopian Rift

In the Lakes region of the Ethiopian rift, some lakes and groundwaters are known to have very high fluoride values (10 to 200 mg.1-¹)- In the Afar region near Djibouti,there is also an area with high fluoride values though not as much as around the lakes. Contributions to the evolution of high fluoride contents are made from thermal springs, lacustrine sediments and acidic volcanic rocks. Thermodynamic controls can limit fluoride concentrations, principally the equilibrium constant K = [Ca⁺⁺] [ F⁻] ². In the lake area waters are principally of Na CO₃ type ; the occurence of very high fluoride levels is possible as a result of the rapid precipitation of calcium carbonate during evaporation process.