Geochemistry of Early Triassic seawater as indicated by study of the Röt halite in the Netherlands, Germany, and Poland

Abstract

Study of three halite sections in the Röt Basin from the eastern Netherlands (Twente-Rijn 382 borehole), central Germany (Mardorf 6 borehole) and western Poland (Ośno IG2 borehole) shows that the δ 34S values in Röt sulphates of studied samples (27.14–31.98‰) are characteristic of marine sulphates of late Early Triassic. These values fit in the Röt event shift described in the literature. The bromine content in halite is 32–158 ppm. In the Dutch and Polish sections, relics of sedimentary structures—chevron and hopper—are recorded in halite grains. These structures contain fluid inclusions that were analysed using the ultramicrochemical method of Petrichenko (Petrichenko, O.I. 1973. Metody doslidzhennya vkluchen' v mineralakh galogennykh porid. Kiev, Naukova Dumka, 92 pp.). In the German section, no sedimentary halite structures occur. Because the pressure in the inclusions in recrystallized halite is only slightly greater than atmospheric pressure, these inclusions are presumed to be early diagenetic in origin. The analysis of individual brine inclusions, both in sedimentary and diagenetic forms of halite, shows that the Röt brines correspond to the Na-K-Mg-Cl-SO 4 (SO 4-rich) chemical type, i.e. the same type that characterises modern seawater. However, the Röt brines differ from the composition characteristic of modern concentrated seawater by a decrease in the content of sulphate ion and an increase in the content of potassium ion. On a Jänecke diagram, the points of the brine composition during deposition of Röt halite in Poland, Germany and the Netherlands only show a small range. This suggests a stable composition of basinal brines during the deposition of the halite. At the same time, the relative content of sulphate ion in the Lower Triassic Röt Basin was slightly lower than that in Upper Permian evaporite basins, but higher than that in Lower and Middle Permian and Middle and Upper Triassic evaporite basins. The implication is that the content of sulphate ion in seawater was subject to changes in Permian and Triassic times.