GEOLOGICAL VOLUME TEXTURES STUDIED BY NEUTRON DIFFRACTION
1Mineralogisch-Petrologisches
Institut, Universität Bonn, D-53115 Bonn, Germany
2Mineralogisches
Institut, Universität Würzburg, D-97074 Würzburg, Germany
email: e.jansen@fz-juelich.de
Keywords: pole
figure, volume texture, neutron diffraction
Neutron diffraction pole figure scanning is a favourable experimental tool for the quantitative analysis of geological volume textures. The high penetration capability of thermal neutrons allows the investigation of large specimens of several cm3 volume and thus sufficiently high grain statistics. Complete pole figures of numerous different lattice planes can be measured simultaneously by using a large linear detector, e.g. the 940 mm JULIOS scintillation detector on the texture diffractometer SV7 at the FRJ-2 reactor in the Forschungszentrum Jülich. Experimental resolution can be varied by using neutron wavelengths between about 1.0 to 2.4 Å, and by using appropriate scanning grids for the sample mounted on a full-circle Eulerian cradle.
Based on longtime experience with the analysis of either individual or overlapped experimental hkl pole figures a program package has been developed for a fast and semi-automatic analysis of the huge amount of diffraction data resulting from each sample scan. The package includes routines for background determination and subtraction, for peak separation by profile fitting procedures, for individual hkl intensity integration and for conversion of non-equidistant j, c -scanning grids into equispaced , a, b -pole figure grids containing pole densities in units of multiples of random distribution (mrd) obtained by interpolation.
Several examples of recent results of geological texture studies are given all being based on the above instrument and analytical software equipment. The investigated sample spectrum extends from high symmetry calcites to triclinic anorthite plagioclases. The discussion is concentrated on the volume texture of a naturally deformed quartzite originating from the Kaoko Belt, Namibia, which previously had been studied by universal stage microscopy. Neutron diffraction reveals a concentration of c-axes on a girdle oblique to the foliation plane in full accordance with the optical findings. The obliquity indicates a component of non co-axial strain in the overall flow. Preferred orientations of the first and second order prisms are visible by the neutron diffraction (100) and (110) pole figures, respectively. The orientations of the first order prisms are strongly concentrated parallel to the lineation direction whereas the orientation density of the second order prisms shows two distinct maxima around the linear fabric and the shear direction. The study reveals that the rock was predominantly deformed by prism slip mechanism under the influence of a simple shear component.
The Bonn institute offers the possibility to measure neutron diffraction pole figures of a broad spectrum of geological materials and to quantitatively evaluate experimental pole densities for subsequent texture analysis on an informal service basis.
The instrument is funded by the BMBF, Bonn, under contract no. KI5BO2.