Abstract: Large thenardite crystals have been sampled at New Agorgott, in the Taoudenni area of northern Mali. They are still in equilibrium with a pressurized NaCl saturated brine capped by a halite layer. Clays located about 1 m above the thenardite occurrence have been dated at 6760 y.BP. The crystals contain numerous, large, brine and solid inclusions. Microcryscopic studies show that the fluids can be explained by the addition of MgCl₂ to the Na₂SO₄-NaCl-H₂O system (eutectic temperature: −31 to −35°C; possible bloedite Na₂Mg(SO₄)₂.4H₂O formed after freezing). The homogenization temperatures of primary fluid inclusions are in the range 28 to 50°C. In order to understand the significance of the highest T_h values, overheating experiments under 1 bar pressure were performed at different heating rates up to 170°C. The results are as follows: When the temperature of stretching (T_Oh) is higher than about 10°C, overheating is recorded and fossilized (identical T_h after some hours, several days or 8 months storage at 5°C). The lowest T_h values (28°C) are probably near the formation temperature of thenardite; the highest ones reflect stretching under present desert conditions. With T_Oh lower than about 60°C, a fair correlation is observed between T_h and T_Oh.

Finally, taking into account recent natural overheating, the fluid inclusion data are compatible with the formation of thenardite from underground brines later than the beginning of desert conditions in the Taoudenni area (i.e. about 3000 y.BP).