The anisotropic properties of granites – effects of tectonic emplacement mode on potential crystalline host rocks for nuclear waste deposits in Germany

Abstract. For permanent nuclear waste disposal sites, crystalline rocks, especially granitic/granodioritic batholiths, are considered an appropriate host rock. Principally, three types of granitic plutons occur in the extra-alpine crystalline basement of Germany that were consolidated during the late Paleozoic Variscan orogeny of Central Europe: (i) Pre-Variscan voluminous granodiorites that are hardly affected by the subsequent continent–continent collision; (ii) voluminous granites in various tectonic settings intruded during the late orogenic stage of the Variscides; (iii) post-orogenic granites related to vast Permian intracontinental extension. Thus, in terms of the syn-intrusive tectonic setting and post-intrusive processes there are significant differences. Although it can be expected that different tectonic environments caused significant differences in the material properties, for Germany, however, there is no systematic study regarding the fabric of such plutonites. In order to find the most suitable “granite” we investigate the primary anisotropy of granites evolved during the emplacement and crystallization of the melt. For this we sample rocks of all three principal types and various syn-intrusive tectonic settings, i.e., compression, extension, strike-slip, transtension, and transpression. By means of combined measurements of the “Anisotropy of the Magnetic Susceptibility” and the “Shape Preferred Orientation” we characterize the syn-intrusive flow pattern, i.e., the magmatic foliation and lineation. The Crystallographic Preferred Orientation is analyzed by a combination of neutron time-of-flight experiments and electron backscatter diffraction measurements at the Frank Laboratory of Neutron Physics at JINR, Dubna, Russia, and the TU Bergakademie Freiberg respectively. Furthermore, special attention is given to the systematic mapping of annealed microcracks evolved during late magmatic fluid escape and/or post-crystallization hydrothermal activity. In a second step we compare the primary anisotropy with the post-magmatic fracture pattern of the particular granites. Those fractures constitute probable fluid pathways and, thus, the first-order risk for a potential permanent nuclear waste disposal. All datasets are organized in a Geological Information System allowing for a complete traceability of the different investigation steps. The results of this study will serve as a basis for a future detailed exploration.



THE ANISOTROPIC PROPERTIES OF GRANITES
Effects of tectonic emplacement mode on potential crystalline host rocks for nuclear waste deposits in Germany For permanent nuclear waste disposals sites, crystalline rocks, and especially granite is considered as an appropriate host rock.Three principal types of granitic plutons occur in the extra-alpine crystalline basement of Germany, which were consolidated during the late Paleozoic Variscan orogeny of Central Europe.i) Pre-Variscan voluminous granodiorites that are hardly affected by subsequent continent -continent collision.ii) Voluminous granites in various tectonic settings intruded during the late-orogenic stage of the Variscides.iii) Post-orogenic granites related to vast Permian intracontinental extension.
Although it can be expected that different tectonic environments cause significant differences of the material properties, for Germany, however, there is no systematic study regarding the fabric of such plutonites.In order to find the most suitable "granite" we will investigate the primary anisotropy of granites evolved during the emplacement and crystallization of the melt.For this we sample rocks of all three principal types and various syn-intrusive tectonic settings, i.e., compression, extension, strike-slip, transtension and transpression.
By means of combined measurements of the "Anisotropy of the Magnetic Susceptibility" (AMS) and the "Shape Preferred Orientation" (SPO) we characterize the syn-intrusive flow pattern, i.e., the magmatic foliation and lineation.The Crystallographic Preferred Orientation (CPO) will be analyzed by the combination of neutron time-of-flight (ToF) experiments and electron backscatter diffraction (EBSD) measurements at the Frank Laboratory of Neutron Physics at JINR, Dubna, Russia and the TU Bergakademie Freiberg, respectively.This allows to obtain a wide range of texture information, from volume (~60 cm³) to spatially resolved at sub-grain scale.Special attention lies on the systematic mapping of annealed microcracks evolved during late magmatic fluid escape and/or post-crystallization hydrothermal activity.These datasets will be organized in a geoscientific information system (GIS) allowing for a complete traceability of the different investigation steps.In a second step we compare the primary anisotropy with the post-magmatic fracture pattern of the particular granites.Those fractures constitute probable fluid pathways and, thus, the first order risk for a potential permanent nuclear waste disposal.Conclusions on the (re-)activation conditions of cracks derived from these datasets provide the basis for the future detailed exploration.

Fig. 1 :
Fig. 1: Overview map of the felsic plutonites of Germany which are not covered.

Fig. 7 (
Fig. 7 (left): Calculation of anisotropic seismic properties from texture data (see Mainprice et al. 2011).Displayed are P-and S-wave velocities of quartz crystal orientations of the Niederbobritzsch granite.

Fig. 2 :
Fig. 2: Potential fluid pathways within granitic host rocks resulting from extensional fracturing are highly dependent on the anisotropic fabric of the granitic material.It is determined by various geological processes such as magmatic flow, static crystallisation, plastic deformation, post-intrusive hydrothermal processes, etc.