Abstract:
Earlier studies of individual fluid inclusions in salt minerals of some Devonian evaporite basins showed that the brines in those basins were of chloride type with a high concentration of Ca2+. The genetic significance of this occurrence was subject to different interpretations. We have studied in detail samples taken from the East European Dnipro-Donets and Prypiac' evaporite basins aiming to establish the chemical composition of fluid inclusions in studied evaporites and then, using other geochemical information, to establish a geochemical model of the Devonian evaporite basins. Our study showed that salt deposition in the Devonian Dnipro-Donets and Prypiac' evaporite basins was because of intensive evaporation of mainly marine brines of chloride type with a relatively high Ca2+ content. In turn, the ratios between Na, K, and Mg corresponded to ratios in recent marine brines. Salt minerals precipitated in bottom water conditions. The occurrence of one-phase fluid inclusions indicates the temperatures of halite-precipitating brines <43°C. Gas content varied from 10 to 300 g/L and was controlled by the pressures existing at the basin bottom during halite growth. The recorded high Ca2+ contents, characteristic of brines in the Dnipro-Donets rift basin, is due to intensive discharge of connate highly mineralized Ca2+-rich solutions; however, the parent marine water was also Ca2+ rich.
