Micro-scale structural and chemical characterisation of deformed rocks with simultaneous in-situ synchrotron X-ray fluorescence and backscatter diffraction mapping
Christoph Schrank, Michael W. Jones, Daryl L. Howard, Alfons Berger, Marco Herwegh
Abstract
In-situ measurements of structural and chemical information on deformed rocks are required to understand microphysical deformation mechanisms, the relative timing of tectono-metamorphic events, and the interplay between deformation, fluid flow, and mineral reactions. Scanning-electron microscopy is commonly used for this purpose, because it can provide compositional information (via energy- or wavelength-dispersive spectroscopy and backscattered electron imaging) and crystallographic orientation data (via electron backscatter diffraction) for large fractions of thin sections down to nanometre resolution. However, elemental and crystal-structural data usually are usually acquired independently, introducing the need for registration of maps with different information and increasing instrument time. In addition, measuring element concentrations to levels below 1000 ppm increases acquisition times significantly, rendering the mapping of centimetre-scale areas with micrometre-resolution or below time-costly. Here, we introduce an alternative method using coeval synchrotron X-ray fluorescence microscopy (XFM) and X-ray backscatter diffraction mapping (XBDM) for the rapid acquisition of structural and compositional information down to the ppb level on whole rock sections with a spatial resolution down to ~1 μm per pixel. We illustrate the principals of XFM/XBDM mapping on a sample of granitic mylonite. As example for the scientific potential of the new method, it shown that the Ti distribution in quartz and feldspar can be accurately correlated with physical structure. A discussion of the limitations and opportunities of XFM/XBDM mapping for studying deformed rocks concludes this article.