Microstructures and mapping of deformation mechanisms in a rock salt fold from the Neuhof Mine

 

Supervisors: Dr. Guillaume Desbois and Prof. Janos Urai


Please send your application to: Dr. Guillaume Desbois.

Summary

The properties of rock salt are extensively discussed in the literature. Its multiple role in sedimentary basin evolution (Littke et al., 2008; McClay et al., 2003; Rowan et al., 1999), fluid sealing capacity for gas storage (Li et al., 2005), drilling problems in petroleum industry (Muecke, 1994; Wilson et al., 2002), the possibility of waste disposal in salt (Salters and Verhoef, 1980) and salt mining give the topic enormous economic importance and emphasize the need for the proper knowledge of the mechanical and transport properties of halite rocks deforming in nature (Drury and Urai, 1990; Urai and Spiers, 2007; Schoenherr et al., 2007, 2009; Urai et al., 2008).
Deforming rock salts in nature  (Talbot and Jackson, 1987; Talbot, 1998) offers an unique opportunity to reconstruct deformation conditions in slow creep for rock salt based on identification of deformation mechanisms and constitutive equations from laboratory calibrations, experiments and theory. Dislocation creep and grain boundary dissolution-precipitation processes, such as solution-precipitation (SP) creep and dynamic recrystallisation, play a significant role in the rheology of salt diapiric systems (Talbot and Rogers, 1980; Urai et al., 1986; Schléder and Urai, 2007; Urai and Spiers, 2007; Schoenherr et al., 2009; Desbois et al., 2010; Zavada et al., submit). These deformation mechanisms are strongly controlled by inter-granular fluid phases and grain boundary microstructures.
Although the relative contribution and activity of these mechanisms can strongly vary in different layers of salt, the general scheme is following: high differential flow stresses in the source layer and salt stock/wall are compatible with dynamically recrystallized fabric and combined mechanisms of grain boundary migration (GBM) and subgrain rotation (SGR). In contrast, extrusive salts that are associated with relatively low flow stresses (Schléder and Urai, 2008) are characterized by fine grained fabric of elongated grains and dominant activity of solution-precipitation (SP) creep coupled with grain boundary sliding (GBS) (Desbois et al., 2010b). An exception from this simplified summary of identified deformation mechanisms in rock salt represents the Zechstein salt in the Werra and Fulda basin, Neuhof, Germany, where the intense folding of a narrow fine-grained perennial-lake sequence was accommodated by SP creep and GBS (Schléder et al., 2008).
Picture_1465 - MScPropGuillaumeNeuhofRockSaltFigure 1: (A) Location of the Neuhof Mine, Germany. (B) Photograh of typical folding structures visible on walls of the mine. (C) Micrograph showing the grain fabric overview in unfolded regions. (D+E+F) Details from (C): (D) core-mantles microstructures interpreted as two recrystallization events; (E) truncated fluid-inclusions-rich cores interpreted as pressure solution evidence; (F) microcracking of the salt aggregate. (G) Overview of the studied fold before sub sampling. (H) A typical optical microscope.


A recent microstructural study (Zavada et al., submit) confirmed that the ductile flow of intensely folded fine-grained perennial sequence exposed in an underground mine (Zechstein-Werra salt sequence, Neuhof mine, Germany) was accommodated by coupled activity of solution precipitation (SP) creep and microcracking of the halite grains. The grain cores of the halite aggregates contain remnants of sedimentary microstructures with straight and chevron shaped fluid inclusion trails (FITs) and are surrounded by two concentric mantles reflecting different events of salt precipitation. Numerous intra-granular or transgranular microcracks originate at the tips of FITs and propagate preferentially along the interface between sedimentary cores and the surrounding mantle of reprecipitated halite. These microcracks are interpreted as tensional Griffith cracks. Microcracks starting at grain boundary triple junctions or grain boundary ledges form due to stress concentrations generated by grain boundary sliding. Solid or fluid inclusions frequently alter the course of the propagating microcracks or the cracks terminate at these inclusions. Because the inner mantle containing the microcracks is corroded and is surrounded by microcrack-free outer mantle, microcracking is interpreted to reflect transient failure of the aggregate. Microcracking is argued to play a fundamental role in the continuation and enhancement of the SP-GBS creep during halokinesis of the Werra salt, because the transgranular cracks (1) provide the ingress of additional fluid in the grain boundary network when cross-cutting the FITs and (2) decrease grain size by splitting the grains. More over, the ingress of additional fluids into grain boundaries is also provided by non-conservative grain boundary migration that advanced into FITs bearing cores of grains. The described readjustments of the microstructure and mechanical and chemical feedbacks for the grain boundary diffusion flow in halite–brine system are proposed to be comparable to other rock-fluid or rock-melt aggregates deforming by the grain boundary sliding (GBS) coupled deformation mechanisms.
At the opposite, grain boundary healing process, which may occur under conditions of salt diapiric systems, could anneal active deformation. Observations pointing to this phenomenon were reported by Hickman and Evans (1992), Visser (1999), Schenk and Urai (2004), Schenk (2005), Schleder and Urai (2005), Schmatz (2006) and Desbois et al. (2011).
The microstructural analysis of rock salt is facilitated by combination of several techniques such as gamma-irradiation and etching of thin sections. Gamma-irradiation induces damage on atomic scale in the halite lattice, which can decorate halite microstructure in blue (Urai et al., 1987; Garcia Celma and Donker, 1996; Schléder and Urai, 2007; Schoenherr et al., 2009). The physical basis of the blue decoration of halite microstructures is the production of F-center defects during irradiation, which aggregate into sodium colloids, while H-centers, representing the Cl2 molecules remain white (Van Opbroek and den Hartog, 1985; Garcia Celma and Donker, 1994). The intensity of the blue coloration is a function of sodium colloids concentration at solid-solution impurities and crystal-defect sites (Jain-Lidiard model). The gamma-irradiation together with the etching technique reveals a range of microstructures characterizing the different deformation and fluid transport mechanisms in rock salt; syn-sedimentary cores of grains, growth bands, subgrains, dissolution-precipitation features, etc. (Przibram, 1954; Urai et al., 1986, 1987, 2008; Schléder and Urai, 2007; Schléder et al., 2008; Schoenherr et al., 2010; Desbois et al., 2010b). Although high gamma-irradiation doses can induce solid-state deformation and migration of fluid inclusions (Garcia Celma and Donker, 1994), these effects are minor at relatively low gamma-irradiation doses (as used in this project) and can be distinguished from the microstructures produced by natural deformation of the investigated specimens (Anthony and Cline, 1973; Holdoway, 1973; Garcia Celma and Donker, 1994; Schléder et al., 2007).

Objectives

In Zavada et al. (submit), microstructural observations were done in selected parts of the rock salt from Neuhof mine (Germany) without paying attention of the evolution or/and the predominance of deformation mechanisms related microstructures within a single fold system. This project proposes to study microstructures in a pluri-centimetric-scale single fold collected in the Neuhof underground Mine (Germany) with optical microscope. The fold is already sampled and sub-sampled into about 30 samples of 5 x 5 cm2 in size before they have been gamma-irradiated to decorate microstructures.
The main objectives of the Msc. project are as follow:
  1.  Detailed microstructural studied of the sample set made of 30 samples covering the fold
  2.  Grain and sug-grain size statistics, and estimation of paleostress
  3.  Interpretation of microstructures in term of deformation mechanisms
  4.  Mapping of deformation mechanisms
  5.  Impact of second phase (anhydrite) layering on local microstructures and deformation mechanisms
  6.  Healing of microcracks ?

Work plan

  1. Review of the relevant literature
  2. Selection of regions of interest and preparation of samples: The studied fold is sub-sampled into 34 samples. The first step will be to select only few samples to be investigated in priority. These samples will be selected in regards to apparent grain size, location within the fold structure and the presence of second phase. Selected samples, both already gamma-irradiated and non-irradiated, have to be prepared as thin sections (~ 40 µm) suitable for optical microscopy. At longer term, the 34 samples covering the whole fold have to be all thin-sectioned for a complete microstructure/deformation mechanisms mapping.
  3. Microstructural study and microstructures digitizing:The combination of chemical etching and gamma irradiation used to decorate halite microstructures will provide a rich detail of microstructures like sub-grain rich grain, strain free/new grains, growth bands, syn-sedimentary chevrons, grain boundary indentation/truncation, edgewise propagation of sub-grains, dissolution-precipitation features, core-mantle structure, pressure fringes. Grain boundaries and sub-grain boundaries will be segmented.
  4. Microstructures interpretation: Microstructures in identified in section III will be used to infer qualitatively the deformation, recrystallization and fluid transport mechanisms in studied samples. Segmented grain and sub-grain boundaries will be used to infer. Axial ratio of grains, together with shape preferred orientation, and differential stress using the piezometric equation D (μm) = 215 σ -1.15 (MPa)  (Schléder and Urai, 2005; Urai and Spiers, 2007). The whole fold will be mapped in term of deformation mechanisms. Results will be compared to previous microstructural studies of naturally deformed rock salt. Depending on time few EBSD measurement can be done to precise some of deformation mechanisms.

Methods

  • Transmitted and reflected light microscopy
  • Thin sectioning
  • Piezometry
  • Gamma irridiation
  • EBSD
  • Chemical etching
  • Software used: ArcGIS, Matlab, Poly X Matlab toolbox, Adobe Photoshop

References