COMPOSITION OF PERMIAN SEAWATER: RESULTS OF STUDY OF FLUID INCLUSIONS IN HALITE

Permian evaporites of marine origin are characterized by the occurrence of giant evaporite basins (including two studied by us: East European and Central European basins) and the presence of potash salts of chloride-sulfate composition. The latter characteristic speaks in favour of concept that these salts have precipitated from marine water that was close to recent one considering its composition. We have examined primary fluid inclusions in sedimentary halite in several well-studied Permian evaporite basins of Ukraine, Russia and Poland. All brine inclusions in studied sections have been done with the use of the UMCA method introduced by Petrichenko. This method allows us to determine the content of major ions in brine inclusions except for Na and Cl. The data on composition of fluid inclusions in Permian sedimentary halite indicate that the brines of all studied evaporite basins were of Na-K-Mg-Cl-SO4 chemical type during halite deposition. The changes of brine concentration during halite precipitation have been recorded by the changes of ion contents in fluid inclusions. In the sections where potash salts occur, there is an upward increase of total brine concentration prior to deposition of the potash, and the decrease of the concentration after potash deposition. In the sections lacking potash salts the changes of total concentration are minor. In almost all cases the changes in the K, Mg and SO4 ion ratios in inclusion brines (and thus in the basin brines) had been relatively small, i.e. there was no change of the principal chemical type of brine. When compared to the recent seawater concentrated to the onset of halite precipitation, all Permian basinal brines had been characterized by the decreased SO4 ion content. The lowest SO4 content was recorded in the Kungurian, the higher content was found in the Asselian and Sakmarian, and the highest content in the Late Permian. In the Late Permian, the sulfate ion content in seawater was close to (but slightly lower than) the value characteristic of modern seawater. The trend of decreasing sulfate ion concentration continued into the Triassic, and during deposition of the Lower Triassic salt, the value of the sulfate ion content in seawater was intermediate between the high values typical of the Late Permian and the very low values typical of the Late Triassic.