Summary and discussion of Chapter 5

Chapter 5 describes the results of neutron texture measurements (chapter 5.1) as well as water content analyses (chapter 5.2) of rock salt samples from the salt deposits Teutschenthal, Morsleben, and Gorleben.

6.1.3.1 Texture measurements

In this study, four rock salt samples from the salt deposits Teutschenthal, Morsleben, and Gorleben were examined. Grain shape preferred orientations and a relatively small grain size (long axes max. several mm) implied that these samples represent mylonitic shear zones.

Such fabric can also be a typical indication for plastic deformation. However, neutron texture measurements did not show any crystallographic preferred orientations (textures) in these samples. These findings indicate that intracrystalline deformation mechanisms such as dislocation creep or dislocation glide can not be the dominating deformation mechanism during the deformation of the investigated rock salt. This, however, is in contrast to literature about experimentally deformed and annealed rock salts from which a variety of well-defined texture types is known. The lack of a texture in naturally deformed rock salts can possibly be explained by the presence of water, as already small amounts of brine (>10 ppm) can change the deformation mechanism from dislocation to solution-precipitation creep (e.g., Urai et al.

1986b, Spiers et al. 1990; Peach et al. 2001; Pennock et al. 2006; Urai & Spiers 2007). Fluid assisted deformation mechanisms such as solution-precipitation creep or fluid assisted grain boundary migration may therefore prevent the development of a crystallographic preferred orientation. This argumentation is in agreement with the findings of studies made on

Particularly the amounts of water present in rock salt as well as the chemical composition of the brine have to be taken into account in the design and the safety assessment of the repository. This is because water can migrate through the repository as a result of thermal gradients induced by the presence of heat generating wastes, corrode the waste containers, and potentially leach the radionuclides present in the waste (e.g., Olander 1982; De Las Cuevas &

Pueyo 1995). The water content is also of considerable importance for the discussion on deformation mechanisms during salt migration, as intergranular water can significantly influence the rheological properties of the rock salt (e.g., Schenk & Urai 2004; Spiers et al.

1986; Urai 1986b). Furthermore, as demonstrated in section 2.2.6, the amount and distribution

of water in rock salt affects the quality of sample preparation and thus the quality of microstructural investigations by, for example, reflected light microscopy or EBSD.

The water content analyses of the present study yielded average values ranging between 0.69 and 1.86 wt % in the Hauptsalz of Teutschenthal, between 0.51 and 1.04 wt % in the Hauptsalz of Morsleben, and between 0.21 and 0.28 wt % in the Hauptsalz of Gorleben (Tables A10-A12). In general, the water contents of the various locations reflect the tectonic setting of the corresponding salt deposits insofar as the average water content is highest in the bedded rock salt of Teutschenthal and comparatively lower in the domal rock salts of Morsleben and Gorleben. The lower amounts in domal salts can be most satisfactorily explained with the emplacement of the salt dome, during which the bulk of the original water is lost. In principle, the results are also in the range of water contents usually expected for domal salts (0.00X to 0.X wt %) and bedded salts (X wt %) (Roedder & Bassett 1981).

However, there is a clear discrepancy to former water content determinations of similar Gorleben rock salt samples, which yielded water contents of averaged 0.014 wt % (Sander &

Herbert 2000), i.e. values one order of magnitude lower than the present results.

A very probable reason for this discrepancy might be the different preparation technique prior to the analysis. In earlier studies, samples have been reduced to small pieces in a jaw crusher and then a representative sample was obtained from the homogenized sample material. However, during this process, a certain amount of brine might have been removed from the samples due to friction heat, and this might explain the consistently lower water contents of these samples. In the present study, such water loss could be largely excluded, since the samples were crushed to small pieces inside a hermetically sealed PE foil. Another difference between the analyses is the analytical method itself. Although the water content of all samples was determined according to Karl-Fischer titration (e.g., Jockwer 1981), the water extraction procedures were carried out in different ways. In former analyses, water was released via heating the sample material to 500°C, whereas in this study water is extracted by using water-free, dried 1,4-dioxane, an organic compound that is miscible with water.

Potentially, the heating rate was not high enough to induce fracturing of the salt and the release of all the intracrystalline water (cf. Roedder & Basset 1981), i.e. the results of earlier analyses might have been generally too low. Unfortunately, both analytical techniques have not been compared with each other yet, therefore it is not clear which of them provides results closest to reality.

6.2 Conclusions

The comparison of the bromide characteristics of rock salt from salt deposits of different tectonic settings aimed to show a possible relationship between deformation intensity and bromide characteristics of the rock salt. This approach was complemented by microstructural investigations that were carried out to show to what extent the different deformation history of the salt deposits is reflected in the microstructural characteristics of the rock salt. From the results it can be concluded that, on the one hand, there are clear indications for the influence of salt migration-related processes on the bromide distribution characteristics insofar that there is a lower dispersion of the bromide contents in the domal salts. It implies that these processes are associated with a redistribution of bromide, which eventually results in a homogenization of the originally varying bromide contents. On the other hand, the characteristic trend in the bromide profiles was preserved in both the bedded and the more intensely deformed domal salts indicating that large-scale brecciation, folding processes, or circulating bromide-rich fluids played only a minor role during the formation of the salt domes. The differences in bromide content between the two halite types do also not considerably influence the general trend of the bromide profile.

Local X-ray texture measurements could clearly evidence that the laminated Kristallbrocken halite type from the Stassfurt Formation rock salt is monocrystalline and that separated pieces of the Kristallbrocken formerly formed one single monocrystalline halite layer. The studied halite type shows brittle as well as ductile deformation behavior. Optical microscopy and SEM analyses revealed that the solid inclusions forming the internal lamination are anhydrite/polyhalite crystals and aggregates. The brittle behavior of Kristallbrocken can be well explained by their monocrystallinity, their formerly large size, and the partially high solid inclusion content. For the formation of the studied halite type, a post-sedimentary process is assumed that is based on coalescence or grain growth by grain boundary migration of a formerly fine-grained, laminated halite sediment. Clear indications for the grain growth mechanism could not be found. This is especially difficult, since also the Kristallbrocken from the bedded salt of Teutschenthal are slightly deformed and therefore the intracrystalline microstructures are overprinted.

The results of neutron texture investigations on naturally deformed rock salt demonstrate a random texture, i.e. a lack of crystallographic preferred orientations, although the samples show clear grain shape preferred orientations. The proof of such a „non-texture‟

strongly indicates that deformation of natural rock salt in the shear zones is not or only subordinately controlled by intracrystalline slip. It is largely controlled by fluid assisted

deformation mechanisms like solution-precipitation creep or fluid assisted grain boundary migration as it is increasingly postulated in the literature for long-term deformations (e.g. Urai

& Spiers 2007). The relatively high water contents of the rock salt samples from the same salt deposits support this argumentation, as already small amounts of water can lead to solution/precipitation or diffusion processes (Urai et al. 1986b; Spiers et al. 1990).The results of this study clearly demonstrate that the deformation mechanisms of natural rock salt still need attention for the setup of rheological models related to the application of using salt structures as hydrocarbon storage caverns or as host rocks for the disposal of radioactive waste.