Fluid inclusions in halite from the Röt (lower triassic) salt deposit in central Germany: Evidence for seawater chemistry and conditions of salt deposition and recrystallization

The study of fluid inclusions in Lower Triassic Röt halite from the Rockensussra 2/83 borehole in northern Germany showed the presence of primary and secondary gas-liquid inclusions. The gas phase in inclusions indicates their stretching due to salt overheating at some postsedimentary stage. The homogenization temperature of inclusions indicates the overheating temperature of about 60–70°C. The overheating is additionally indicated by trails of migration of large secondary inclusions occurring inside chevrons. The major-ion (K, Mg and SO4) composition of inclusion brines was established with the use of ultramicrochemical analysis (UMCA). The results of chemical analyses of brines of primary inclusions in halite from the Rockensussra 2/83 borehole confirm the earlier results for the Röt halite from the Netherlands and Poland. The bromine content in halite is 78–107 ppm, supporting thus marine origin of halite. Brines of primary gas-liquid inclusions are thus representative samples of trapped evaporite water and they may be used for the reconstruction of the composition of Early Triassic seawater. The comparison of our analytical data with the earlier published data and models of chemical composition of seawater during the Phanerozoic which were constructed with the use of the HMW computer program makes it possible to conclude that the Early Triassic seawater was of the SO4-rich type and considering the ratios of major ions it fully corresponded to the Early Permian (Asselian-Sakmarian) seawater. It differed from the present seawater by a slight decrease of Na and Mg ions, a considerable decrease of SO4 ion (by 31%) and an increase of Ca ion (by 36%).