Grain size dependent large rheology contrasts of halite at low deviatoric stress: evidence from microstructural study of naturally deformed gneissic Zechstein-2 rock salt (Kristallbrockensalz) from the Northern Netherlands

Abstract. Constitutive laws of rock salt are required for the prediction of long-term deformation of radioactive waste repositories and solution mined caverns, which are used for energy storage and play an important role in the energy transition. Much of this deformation is at differential stresses of a few MPa. The vast majority of laboratory measurements of salt creep are at much higher differential stress and require extrapolation over many orders of magnitude. This extrapolation can be made more reliable by including microphysical information on the deformation mechanisms in the laboratory samples, integrated with microstructural analysis of samples deformed in natural laboratories at low differential stress.

Rock salt can deform at widely different rates at the same temperature and deviatoric stress, depending on state variables such as grain size, solid solution- and second phase- impurities, crystallographic preferred orientation, water content and grain boundary structure. Both dislocation creep and dissolution-precipitation creep processes are common, but dissolution-precipitation creep (pressure solution) is not commonly included in current engineering predictions.

Here we show evidence for large grain size-dependent differences in halite rheology based on microstructural observations from Zechstein rock salt cores of the Northern Netherlands that experienced different degrees of tectonic deformation. We studied the relatively undeformed Z2 (Stassfurt Formation)‚ horizontal-layered salt from Barradeel, and compare it with much stronger deformed equivalent in diapiric salt form Winschoten, Zuidwending, and Pieterburen. We used optical microscopy of Gamma-irradiated thin sections for microtectonic analysis, recrystallized grain size measurements and subgrain size piezometry, SEM-EDX and XRD for second phase mineralogy. Subgrain size piezometry shows that this deformation took place at differential stress between 0.5 and 2 MPa, providing a natural laboratory.

In the undeformed, layered salt from Barradeel we find cm-thick layers of single crystalline halite (Kristalllagen) alternating with fine-grained halite and thin anhydrite layers. The domal salt samples are typical of the well-known "Kristallbrocken" salt, and consist of cm-size tectonically disrupted megacrystals surrounded by fine-grained halite with grain size of a few mm. We infer high strains in the fine-grained halite as shown by folding and boudinage of thin anhydrite layers, as compared to the megacrystals, which are internally much less deformed and develop subgrains during dislocation creep. Subgrain size shows comparable differential stresses in Kristallbrocken than in matrix salt. The fine-grained matrix salt is dynamically recrystallized, has few subgrains and microstructures indicating deformation by solution-precipitation processes. We infer that the finer grained halite deformed dominantly via pressure solution and the megacrystals dominantly by dislocation creep.

This provides evidence that the fine-grained matrix salt is much weaker than Kristallbrocken because of different dominant deformation mechanisms. This is in agreement with microphysical models of pressure solution creep in which grain size has a significant effect on strain rate at these low differential stress. Our results on the operation of pressure solution creep in rock salt at differential stress of a few MPa point to the importance of this mechanism at low differential stresses around engineered structures but also in most salt tectonic settings. We suggest that including results of microstructural analysis can strongly improve engineering models of rock salt deformation.

We recommend that this mechanism of grain size dependent rheology is included more consistently in the constitutive laws describing deformation of engineered structures in rock salt.