Internal lamination

The results of this study show that the internal lamination of the Kristallbrocken can consist of several types of solid inclusions: anhydrite aggregates with surrounding brine, anhydrite aggregates or individual crystals without surrounding brine, polyhalite aggregates as well as polyhalite-anhydrite aggregates, both with surrounding brine (Tab. 2). Especially the presence of the sulfate aggregates with surrounding brine is quite unusual in a single crystal for two reasons. First, the aggregates cannot represent the daughter crystals of fluid inclusions, because the calcium or potassium concentration of the inclusion would be too low to form that large amount of anhydrite or polyhalite, respectively, which can be observed within the inclusions. Second, due to their large size and amount, they can hardly be the result of accidental trapping during single crystal growth. The question arises how and especially when did the sulfate aggregates form and how can the surrounding brine and the irregularity of the whole inclusion be explained.

Tab. 2: Different types of solid inclusions that have been observed in Kristallbrocken samples taken from different salt sites of the Zechstein Basin (Fig. 4.20B). In Teutschenthal, Morsleben, and Gorleben, several samples were investigated throughout the entire Hauptsalz section, whereas of the other locations only one sample was studied exemplarily.

Salt site

Gorleben Entire Hauptsalz Upper parts of

the Hauptsalz primary precipitate of these anhydrite crystals. Evaporation experiments (Usdowski 1973) as

well as data from gypsum precipitates (e.g., Geisler 1982, Rosell et al. 1998) show that the strontium content of seawater increases with progressive evaporation. Strontium is incorporated into the crystal lattice of calcium sulfates instead of calcium (Noll 1934).

According to Usdowski (1973) the partition coefficient for strontium in anhydrite is significantly higher (b=1210) than that for strontium in gypsum (b=50); therefore the anhydrites directly precipitated from seawater are expected to have significantly higher strontium contents. Strontium contents in anhydrites from German Zechstein evaporites range mainly between 800 and 2700 ppm (e.g., Jung & Knitzsche 1960; Herrmann 1961; Kramm &

Wedepohl 1991), which matches the expected strontium contents in gypsum at sodium chloride saturation of seawater. In this study, the strontium contents ranging between 824 and 3329 ppm (Tab. 2) are in good agreement with previous studies, and thus indicate that the anhydrite inclusions originate from the conversion of primary gypsum.

The most probable explanation for the formation of the irregular inclusions filled with anhydrite and brine is the initial incorporation of gypsum aggregates during the precipitation of the halite crystals (cf. chapter 5.3). Later, during diagenesis and when the halite crystal mush transformed into the monocrystalline Kristallbrocken, these gypsum crystals converted into anhydrite plus water, with anhydrite and water having volumes of ~62% and ~38%

respectively. As the released water was undersaturated with respect to sodium chloride, parts of the halite surrounding the anhydrite crystals (= inclusion wall) were dissolved until the water was in equilibrium with halite. In order to saturate the water with respect to sodium chloride, about 6% of the surrounding halite has to be dissolved, yielding a brine volume of

~44%. The presence of undersaturated water inside the inclusion leading to dissolution processes is also evidenced by the porous appearance of the inclusion walls. When splitting the Kristallbrocken before SEM analysis, the previously enclosed brine flows out of the opened inclusions and immediately evaporates, thereby forming new halite crystals on the anhydrites or around the inclusion (Figs. 4.24A-C). Therefore, the cavity that can be observed during SEM analysis results primarily from the solid volume reduction of about 38% in the course of a complete gypsum-anhydrite conversion and secondarily from the partial dissolution of halite of about 6% through the undersaturated water released during the gypsum-anhydrite conversion. This estimation is in agreement with the volume of anhydrite crystals of approximately 50-60%, when it is considered that the solid volume of the complement of the inclusion is unknown and that some anhydrite crystals may have fallen out of the inclusion during the preparation.

Basin 113

Individual anhydrite crystals and aggregates without surrounding brine are also typical for the Kristallbrocken. The individual anhydrite crystals occur in low quantities all over the Hauptsalz of Teutschenthal, Morsleben and Gorleben and are supposed to have been included as anhydrite, i.e. the conversion from gypsum took place earlier. For instance, warmer surface seawater (more than 20°C at halite saturation; Holser 1979) saturated with respect to calcium sulfate enabled early transformation from gypsum to anhydrite. The brine-free anhydrite aggregates were only observed in the Hauptsalz of Gorleben (Tab. 2). They may have also been included as anhydrite, as supposed for the individual anhydrite crystals. Another possibility is that they result from a similar formation process as the anhydrite aggregates with surrounding brine, only that brine was later removed by deformation processes during diapirism. However, at least in the upper parts of the Hauptsalz of Gorleben, brine-free anhydrite aggregates occur together with brine-surrounded anhydrite aggregates, which raises the question why the brine of these inclusions was not removed.

Polyhalite inclusions

Polyhalite crystals as solid inclusions in Kristallbrocken were observed in the samples from Gorleben, Etzel, Lesum, and the Netherlands (Tab. 1). Their occurrence in these locations is restricted to the upper parts of the Hauptsalz, representing the higher evaporation level of Permian seawater. Polyhalite (K2MgCa2[SO4]4•2H2O) is a common mineral in many ancient (e.g., Schulze 1960b; Simon & Haltenhof 1970; Peryt et al. 1998; Chong Diaz et al. 1999;

Roy et al. 2006) as well as recent evaporite deposits (e.g., Holser 1966b; Irion & Müller 1968, Perthuisot 1971; Camur & Mutlu 1996).

Like for the anhydrite crystals, the question arises when these polyhalite crystals formed. There are some petrographic evidences like a partial replacement of anhydrite by polyhalite (Figs. 4.25C, 4.25E) and the presence of swallow-tail twins (Fig. 4.25F) demonstrating that the polyhalite within the inclusions most likely did not precipitate directly from the seawater brine but most likely converted from gypsum or anhydrite (cf. Braitsch 1962). The polyhalite crystals cannot be the daughter crystals of fluid inclusions.

Furthermore, it is also less probable that the conversion of gypsum or anhydrite into polyhalite took place inside the inclusion, because the volume of the included brine is too small and therefore does not contain enough potassium as well as magnesium to form the amount of polyhalite that can presently be observed in the inclusions. Therefore, it seems to be more plausible that the polyhalite crystals already existed before they were included in the Kristallbrocken halite type.