Fluid inclusions in halite are widely used in research to determine the conditions of sedimentation in salt basins and reconstruct the chemical composition of seawater during a specific geological period. However, previous preliminary studies of the genetic types of inclusions, considered in the present research project, have not received due attention. Consequently, we decided to take into account the main distinguishing features of fluid inclusions in halite, belonging to various genetic types. The ultramicrochemical analysis (UMCA) method is one of the several methods that are used for the quantitative determination of the chemical composition of the primary fluid inclusions in halite. We have upgraded that technique, and that allowed us to reduce the analytical error rates of each component determination. The error rates were calculated in the study of Ca-rich and SO4-rich types of natural sedimentary brines.

It was established that in order to avoid errors in the interpretation of paleotectonic conditions of salt formation based on fluid inclusions in halite, the primary stage of the research should be the genetic identification of the sedimentation textures of halite and fluid inclusions in this mineral. For the thermometric study of inclusions and to determine the depth of the sedimentation basin based on the obtained data, only thermal test chambers are suitable which provide the possibility of observing groups of inclusions in different zones of sedimentary halite, as, for example, in the micro thermal test chamber designed by Prof. V. A. Kalyuzhny. In the course of the research, the equipment of the thermometric method, which is based on the use of a microthermal test chamber designed by V. A. Kalyuzhny, was modernized. In particular, the material of the thermal chamber (stainless steel) was replaced with copper, which made it possible to avoid excessive thermal gradients into chamber and to increase the permissible heating rate by 20 times due to the higher thermal conductivity of copper. For the same purpose, the glass optical windows of the camera were replaced with leukosapphire windows, which have a much higher thermal conductivity. The measuring system of the installation is made on a miniature platinum resistance thermometer with an electronic measuring unit. These improvements made it possible to achieve high system stability and good reproducibility of measurement results. Using the thermometric method, it was established that the temperature of sedimentation at the bottom of the Badenian salt basin of the Carpathian region was 19.5–20.5; 20.0–22.0; 24.0–26.0 °C, and on the surface of the brine was 34.0–36.0 °C. On this basis, a model of the basin with a pronounced thermocline and a total thickness of the water column of up to 30 meters was built, which is the most likely to establish the features of sedimentation. Crystallization of halite at different depths in basins with a thermocline can explain the presence of so-called “low-temperature” (24.0–25.0 °C) and “high-temperature” (37.8–42.6 °C) bottom halite in a number of ancient salt-bearing basins.

The fluid inclusions in the marine Middle Ordovician halite of the Majiagou Salt Formation of the Ordos Basin (China) have been investigated. In addition to the primary inclusions the secondary ones of several generations were also detected. The fluid inclusions brine chemistry of halite was studied using an ultramicrochemical (UMCA) method, and the homogenization temperature of fluid inclusions was determined in a special thermal chamber designed by V. A. Kalyuzhny At the post-sedimentation stage, the studied salt strata were exposed to high temperature (58–72 °C) and high (up to several tens of MPa) pressure. Although there are opinions of the inability of primary inclusions in such halite to determine the physical and chemical conditions of sedimentation, however, the informative value of primary inclusions in halite of the Majiagou Formation has remained. The preservation of the integrity (and thus the informative value) of primary inclusions in halite is evidenced by the same chemistry of their brines, which differs from that of secondary inclusions The sedimentation brines of the basin were concentrated to the middle of halite stage and points to the Na-K-Mg-Ca-Cl seawater. The physical and chemical conditions of evaporites formation are not known enough. Currently, the results of the brine chemistry of primary fluid inclusions in marine halite are the best indicators of seawater composition in the Phanerozoic. It is established that the magnesium content in the brines of the Lower Paleozoic basins is lower comparing to modern seawater of the corresponding concentration, and the potassium ion concentration is higher. The chemical composition of the concentrated seawater from which the halite was crystallized in the Ordovician salt basin of Ordos, with the exception of the calcium ion content, is similar to the seawater chemistry of the Cambrian and Silurian basins, which indicates the relative constancy of Early Paleozoic seawater chemistry. Age-related changes in the chemical composition of seawater are always consistent with many quantitatively or qualitatively characterized processes of the Earth’s crust evolution. So we believe that the causes that led to more than twice the potassium content of Riphean-Devonian clays, unlike the younger ones, it were also the reason for the increase in potassium content in the Lower Paleozoic marine brines. The studies conducted also clarify the limits of oscillation of calcium ion content, which determines the type of seawater. Its content in the sedimentary brines of the Ordos basin of the Middle Ordovician reaches 66 g/l at the middle of halite stage. Therefore, at the beginning of the stage of halite precipitation, its content should be approximately 20 g/l (considering its theoretical content of 10 g/l with the modern composition of the atmosphere). Apparently, the cause of the abnormally high calcium content in the early Paleozoic Ocean was the direct flow of it with hydrothermal solutions into the ocean during the activation of global tectonics of the Earth and the increase of solubility of carbonates of continents and ocean floor due to high carbon dioxide atmospheric content.