Nano-Analytik von natürlichen Quarz-Deformationsmikrostrukturen am spröd-viskosen Übergang
Zusammenfassung der Projektergebnisse
Understanding the interplay of different deformation mechanisms at the grain and lattice scale in mylonites is essential for understanding shear localization and strain accommodation at depth in the continental crust. Deformation in the Earth’s crust is commonly accommodated in quartz-rich rocks. Especially pure quartz veins localize a final increment of strain under retrograde deformation. Therefore, analysing the deformation processes and estimating the temperature of quartz deformation is crucial to unravel the deformation history and provides an important contribution with respect to the tectonic and rheological evolution in the regional geological context. In this project, we analysed deformed quartz veins from the Schobergruppe in the Eastern Alps applying a combination of microstructural, spectroscopic and geochemical analyses in order to understand the processes of geochemical exchange of Ti in quartz, which is used as a proxy to calculate the deformation temperatures. Initially nano-scale secondary ion mass spectrometry (nanoSIMS) and atom probe tomography (APT) was planned to understand small-scale variation in the Ti distribution (nanoSIMS) and exchange process (i.e. pipe diffusion) of Ti from the quartz lattice (APT). However, SIMS analysis revealed very low Ti concentration in the quartz veins (highest concentration: 3.0-4.7 ppm Ti; lowest concentration: 0.2-0.6 ppm Ti). These concentrations are far too low for nanoSIMS and also for APT analysis. Initially, Ti concentration [Ti] were expected to be one magnitude higher at around 30-40 ppm. However, correlated high-resolution optical and electron microscopy analyses revealed that the initial [Ti] (3.0-4.7 ppm) and cathodoluminescence (CL) signature of the vein crystals decrease during deformation mainly depending on the availability of fluids across the microstructure. The amount of strain played a subordinate role in resetting to lower [Ti] and corresponding darker CL shades. We find that the most complete re-equilibration in recrystallized aggregates ([Ti] of 0.2-0.6 pm) occurred (i) in strain shadows around quartz porphyroclasts, acting as fluid sinks, and (ii) in localized microshear zones that channelized fluid percolation. The application of the TitaniQ geothermometer gives deformation temperatures in the range of 320-370°C consistent with the temperatures estimated from the chlorite thermometry and from the regional geological context. As a second part of the project, unexpected microstructures were found during processing of the electron backscatter diffraction (EBSD) data of localized microshear zones (MSZ). These MSZ are characterized by a nearly 90° misorientation angle between the c-axes of the host and new grains, which also share one {m} and one {11-22} pole, compatible with Japan twinning. So far, Japan twinning has only been described as growth feature. However, we were able to show for the first time that deformation-induced Japan twinning in quartz can play an important role for initiation of crystal-plastic deformation within the crust. And as a third part of the project, also unexpected, spectacular kinked quartz microstructures were analysed within deformed quartz veins. In general, kinked microstructures mainly develop in strongly anisotropic material or within lamellar mineral, i.e. micas. CL images and EBSD analyses reveal a very high density of short-wavelength undulatory extinction microstructures (SWUE). These microstructures can be interpreted as primary deformation lamellae developed during coseismic loading at stress which were subsequently overprint by ongoing creep at lower stresses. Kinked micas in the host rock next to the deformed quartz veins and nearby pseudotachylytes give evident for seismic events. The densely spaced subplanar microstructure causes a high anisotropy of the quartz grains, which finally were kinked during ongoing deformation at lower greenschist facies conditions under dextral sense of shear.
Projektbezogene Publikationen (Auswahl)
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2021. Deformation-induced Japan twinning in quartz during incipient mylonitization. Geology, 49, 1267-1271
Bestmann, M., Pennacchioni, G., Grasemann, B.
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2021. Influence of deformation and fluids on Ti exchange in natural quartz. Journal of Geophysical Research: Solid Earth, 126
Bestmann, M., Pennacchioni, G., Grasemann, B., Huet, B., Jones, M. W. M., & Kewish, C. M.
