Microtectonics, deformation mechanisms and rheology of surface reaching salt extrusions
Final Report Abstract
In this project, the methods we developed and used are the following: (1) decoration of microstructures by gamma-irradiation, (2) ion beam beam milling tools (focussed ion beam-FIB and broad ion beam-BIB) to prepare extremely high quality cross-sections suitable for SEM imaging down to nanometer scale, and (3) cryogeny both to preserve specimen from drying damages and quench fluid within pores and grain boundaries. The combination of methods (2) and (3) allow SEM imaging of preserved wet structures on atomically smooth cross-sections at cryo-temperature. Particularly, we developed a novel instrument allowing argon broad ion beam tomography in a cryogenic scanning electron microscope (BIB-cryo-SEM) to study representative microstructures in sedimentary rocks containing pore fluid. The installation of this instrument was difficult because it is a prototype. The final acceptance of this instrument took about 3 years more than initially expected; but now the instrument is performing major and essential capacities as requested in the original tender criteria. This machine forms in deed the complementary tool in term of resolution and cross-section area to FIB-SEM and X-ray tomography methods. We have shown also that the BIB-cryo-SEM instrument is suitable for the investigation of preserved fluid-filled pores in fine-grained clay-rich materials. Microstructural investigations of thin sections decorated by gamma-irradiation on naturally deformed salt rocks point to active pressure solution creep at natural conditions both in extruded salt body (Qom Kuh salt fountain, central Iran and Kum-e-Namak salt glacier, Dashti, Iran) and in salt from underground salt mine (Neuhof mine, Germany). In extruded salt diapirs, dislocation creep is also recognized but it cannot be responsible for relative fast flow of the extruded salt (in contrast to pressure solution creep). Evidences for dislocation creep are interpreted to be the response to the relaxation of stress when the salt diapir reaches the surface. Most recent research on naturally deformed halite rock salt from an active extruded salt glacier (Kum-e-Namak, Dashti, Iran) indicates that second intergranular second phases in “dirty” salt may enhance dominant pressure solution creep at natural conditions, whereas intragranular impurities and initial microstructure acquired in the stem of the diapir in “clean” salts may harden salt aggregates. The key point of salt rock kinetic is the grain boundary mobility related to grain boundary fluids. Development of new tools (FIB-BIB-cryo-SEM) allows investigating in detail preserved grain boundary fluids down to few nanometers in resolution. The study of fluid distribution in naturally salt rock from salt glacier indicates that grain boundaries are healed at the time of observations and, because pressure solution was evidenced in same samples in thin section, are not representative of the deforming grain boundary microstructure suggesting that grain boundary analysis have to be done under stress. BIB-cryo-SEM investigations of static recrystallized synthetic samples show that grain boundaries evolve relatively fast towards healed grain boundaries resulting in isolated fluid inclusions. Grain boundary healing is interpreted to stop the mobility of grain boundaries. Investigations of grain boundary microstructures deforming by pressure solution are still continuing and remain very challenging but not resolved yet.
Publications
- (2008). Cryogenic vitrification and 3D serial sectioning using high resolution cryo-FIB SEM technology for brine-filled grain boundaries in Halite:first results. Geofluids, 8: 60-72
Desbois G. Urai J., Burkhardt C., Drury M., Hayles M. and Humbel B.
- (2010). Deformation and recrystallization mechanisms in naturally deformed salt fountain: microstructural evidence for a switch in deformation mechanisms with increased availability of meteoric water and decreased grain size (Qum Kuh, central Iran). Journal of Structural Geology, 32(4): 580-594
Desbois G., Zavada P., Schléder Z. and Urai J.L.
- (2012). Distribution of brine in grain boundaries during static recrystallization in wet, synthetic halite: insight from Broad Ion Beam sectioning and SEM observation at cryogenic temperature. Contribution to Mineralogy and Petrology,163 (1), 19-31
Desbois G., Urai J.L., Kukla P.A., Wollenberg U., Pérez-Willard F., Radí Z. and Sandor R.
(See online at https://doi.org/10.1007/s00410-011-0656-x) - (2012). Extreme deformation of fine-grained salt by coupled solution-precipitation creep and microfracturation: microstructural evidence from perennial Zechstein sequence (Neuhof salt mine, Germany). Journal of Structural Geology, 37: 89-104
Zavada P., Desbois G., Schwedt A., Lexaand O. and Urai J.L.
(See online at https://doi.org/10.1016/j.jsg.2012.01.024) - (2012). Fluid distribution in grain boundaries of natural fine-grained rock salt deformed at low shear stress: implications for rheology and transport properties. Journal of Structural Geology, 43: 128-143
Desbois G., Urai J.L. and De Bresser H.
- (2013). Argon broad ion beam tomography in a cryogenic scanning electron microscope: a novel tool for the investigation of representative microstructures in sedimentary rocks containing pore fluid. Journal of Microscopy, 249(3): 215-235
Desbois G., J.L. Urai, F. Pérez-Willard, Z. Radi, S. van Offern, I. Burkart, P.A. Kukla, U. Wollenberg
(See online at https://doi.org/10.1111/jmi.12011)
