Nano-analytics of natural quartz deformation microstructures at the brittle-viscous transition

• Applicant: Dr. Michel Bestmann
• Subject Area: Palaeontology
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 383288668

Understanding the interplay between brittle and ductile deformation mechanisms at the grain scale in mylonites is essential for understanding shear localization at depth in the continental crust. This interplay also has a strong influence on the length-scale and velocity of fluid transfer below the base of the seismogenic crust, and on the seismic cycle itself. The main goal of the project is to understand the origin of discrete zones of recrystallization (DZR) in quartz as potential indicators of microfracturing during the incipient stages of mylonitization. Such structures are developed in quartz veins from the Schober Group (Hohe Tauern mountains in the Central Eastern Alps), which were deformed at c. 450-500°C, and will be used as a key study. More generally, the project aims to improve the understanding on interaction of different deformation mechanisms (micro-fracturing, subgrain rotation and grain boundary migration, mechanical Dauphiné twinning, dissolution-precipitation, grain boundary sliding) during the initial formation of DZR structures and, how their significance changes with progressive development of the mylonitic and ultramylonitic microstructures. Without an integrated approach, using different up-to-date techniques of high-resolution microstructural and microchemical (trace element) analysis, interpretations of the quartz deformation microstructures detailed above are destined to remain speculative. The project includes integrated micro- and nano-analyses on fine-grained microstructures by means of: electron backscatter diffraction (EBSD), SEM orientation contrast imaging (channeling contrast), SEM cathodoluminescence (CL), transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS and NanoSIMS) for Ti-in-Quartz analysis. Additional, new developments in high resolution analysis (down to atomic scale) by atom probe will be applied to obtain information about (sub)grain-scale diffusion processes (especially of Ti) during localized rock deformation. Furthermore this project will test the applicability of a newly developed Near Field Microscope with NanoFTIR capability to detect intragranular water in quartz at nano- to micrometer scales. This test will be accompanied by (OH- molecular ions) analysis in quartz using NanoSIMS technique. If these two independent methods prove successful, it will open up a new era of measuring water in fine-grained minerals (not only quartz) and, further, could specifically address the measurement of water along grain boundaries, subgrain boundaries and even dislocations structures. Combined with the Ti distribution analysed by the atom probe, this would help in recognizing processes such as dislocation pipe diffusion or diffusion along subgrain boundaries and their effects on the resetting of the Ti-in-Quartz system.

• DFG Programme: Research Grants
• International Connection: Austria, Italy
• Cooperation Partners: Professor Dr. Bernhard Grasemann; Professor Dr. Giorgio Pennacchioni