Summary and discussion of chapter 3

Chapter 3 describes the results of various bromide studies concerning a defined stratigraphic section (Hauptsalz) of the Stassfurt Formation rock salt. A bromide data set is presented for three salt deposits, with bedded salt (Teutschenthal) compared to domal salts (Morsleben, Gorleben). This data set also includes a distinction between the two different halite types of the Hauptsalz (matrix, Kristallbrocken) and is complemented by the results of microstructural investigations. Additionally, chapter 3 contains the results of the detailed sampling of the

Kristallbrocken, the bulk analysis of the drilling cores of the three salt deposits as well as the bromide content determination via electron microprobe. All these investigations have been helpful in answering the following questions:

1. What are the characteristics of the bromide profiles in the three studied locations? How do the three bromide profile trends differ from each other?

2. What are the microstructural characteristics of halite in the studied locations?

3. Does the different deformation history have an influence on the bromide distribution of the different locations?

4. Are the petrographic differences between the two halite types reflected in the bromide content, and if so, does this considerably influence the trend of the bromide profile?

In all studied locations, the bromide profile of the Hauptsalz is characterized by relatively low bromide values in the lower part of the Hauptsalz and more or less steeply increasing bromide values in the upper part. From this follows that the characteristic trend of the bromide profile was generally preserved in both the bedded and the more intensely deformed domal salts. A more detailed comparison shows that, in the bedded salt of Teutschenthal, the dispersion of bromide contents along the running average curve in both the Kristallbrocken and matrix halite is quite high, especially in the upper part of the Hauptsalz. This is in good agreement with the results of detailed sampling of the Kristallbrocken, where the bromide contents vary considerably within particular drilling core slabs. In contrast, the bromide profiles of the domal salts are quite smooth. Furthermore, in the upper half of the Hauptsalz, the average bromide contents are lower in the bedded salt than in the domal salts.

Reflected light examination of etched sample surfaces revealed a quite similar microfabric of matrix halite for the three locations. In all samples from each location, evidence for intracrystalline deformation and recrystallisation can be observed, i.e. the microstructural investigations revealed no significant differences within the Hauptsalz section and between the three studied locations, although they belong to different tectonic settings.

Microstructural investigations have shown that the clearly different regional deformation intensities at the various locations are not reflected in the microfabrics. However, the macrofabric of the Hauptsalz in Morsleben and Gorleben differs considerably from the one in Teutschenthal, as deformation-related processes have destroyed the original sedimentary fabric during salt migration. As a consequence, the Hauptsalz contains less Kristallbrocken and higher amounts of matrix halite, the grain size distribution is more homogeneous, and anhydrite mainly occurs as a fine dispersion in the rock salt or as layer

fragments and nodules. The comparison of the bromide profiles of the bedded salt of Teutschenthal and the domal salt of Morsleben and Gorleben clearly indicates a relationship between salt migration-related processes and bromide distribution. For example, the absence of laminated Kristallbrocken in the lower half of the domal salt and the results of microstructural investigations demonstrate that this halite type must have been recrystallised during salt migration, thereby forming the clear, medium to coarse grained matrix halite. The comparatively smooth bromide profile in the lower half of the Hauptsalz in Morsleben and Gorleben is most likely associated with a redistribution of bromide that eventually results in a homogenization of the varying bromide contents. In contrast, in the upper half of the Hauptsalz, the Kristallbrocken are still present and the dispersion of bromide contents along the running average curve is higher than in the lower part, but still lower than that in the same part of the profile in Teutschenthal. These results can be explained either by an initially higher dispersion of bromide contents in the upper part of the profile (similar to Teutschenthal) or by varying degrees of deformation across the Hauptsalz during salt migration, i.e. the upper half of the Hauptsalz was subjected to less intense deformation than the lower half.

Selective sampling of the Kristallbrocken and matrix halite revealed a disparity in bromide content between the different halite types, e.g. bromide contents in the Kristallbrocken are comparatively lower than those in the matrix halite like in Teutschenthal, or vice versa like in Gorleben. In the bedded salt of Teutschenthal, the average bromide contents of both studied halite types differ significantly from each other, whereas in the studied domal salts, the average bromide contents lie more closely together. However, the trend of the average bromide curve is similar in both halite types, namely characterized by continuously increasing bromide values from bottom to top. Detailed sampling of Kristallbrocken halite in drilling core slabs also shows that, from the lower part of the Hauptsalz section upwards, the average bromide content of the Kristallbrocken increases.

From this follows that the differences in bromide content between the two halite types do not considerably influence the general trend of the bromide profile.

The bromide content determination of the bulk samples yielded values that lie clearly above the running average curve through the bromide values of the samples taken by 1 mm drill bits. This result can be observed in all 3 studied locations and may have different explanations. One hypothesis could be the accidental sampling of the bromide-richer brine in fluid inclusions, as sampling was carried out with a larger drill bit size. As the bromide content of included brine is generally considerably higher than that of the solid (^haliteDBr<1), a contribution of fluid inclusions would result in higher bromide contents. In Teutschenthal, the

values seem to be still higher than those of Morsleben and Gorleben, which may also be due to the fact that the bulk samples were only taken across certain drill core sections (70 cm long), i.e. these samples may therefore not be representative enough. Another difference between bulk and detailed sampling is the amount of material available for the analyses and thus also the used analytical method. From the bulk samples, more material was available and thus the whole sample could be analyzed by ICP-OES, whereas the other samples contained less material making a separate analysis of anions and cations necessary.

The average bromide content of the Kristallbrocken analyzed via electron microprobe agrees with the value obtained by wet chemical analysis. This demonstrates that the electron microprobe is a suitable means for the determination of the bromide content, especially when a more detailed resolution is necessary. In the example of this study, the similar bromide contents within the bent layer indicate that the whole Kristallbrocken crystal was originally parallel to the depositional surface. The preservation of the bromide contents suggests that the deformation of this halite type did not have any influence on the bromide distribution demonstrating that folding probably took place without the presence of brine.