The strain‐dependent spatial evolution of garnet in a high‐P ductile shear zone from the Western Gneiss Region (Norway): a synchrotron X‐ray microtomography study

Abstract

Reaction and deformation microfabrics provide key information to understand the thermodynamic and kinetic controls of tectono‐metamorphic processes, however, they are usually analysed in two dimensions, omitting important information regarding the third spatial dimension. We applied synchrotron‐based X‐ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite–garnet eclogite in four dimensions, where the 4th dimension is represented by the degree of strain. In the investigated samples, which cover a strain gradient into a shear zone from the Western Gneiss Region (Norway), we focused on the spatial transformation of garnet coronas into elongated garnet clusters with increasing strain. The microtomographic data allowed quantification of garnet volume, shape and spatial arrangement evolution with increasing strain. The microtomographic observations were combined with light microscope and backscatter electron images as well as electron microprobe (EMPA) and electron backscatter diffraction (EBSD) analysis to correlate mineral composition and orientation data with the X‐ray absorption signal of the same mineral grains. With increasing deformation, the garnet volume almost triples. In the low‐strain domain, garnet grains form a well interconnected large garnet aggregate that develops throughout the entire sample. We also observed that garnet coronas in the gabbros never completely encapsulate olivine grains. In the most highly deformed eclogites, the oblate shapes of garnet clusters reflect a deformational origin of the microfabrics. We interpret the aligned garnet aggregates to direct synkinematic fluid flow, and consequently influence the transport of dissolved chemical components. EBSD analyses reveal that garnet shows a near‐random crystal preferred orientation that testifies no evidence for crystal plasticity. There is, however evidence for minor fracturing, neo‐nucleation and overgrowth. Microprobe chemical analysis revealed that garnet compositions progressively equilibrate to eclogite facies, becoming more almandine‐rich. We interpret these observations as pointing to a mechanical disintegration of the garnet coronas during strain localization, and their rearrangement into individual garnet clusters through a combination of garnet coalescence and overgrowth while the rock was deforming.

4D porosity evolution during pressure-solution of NaCl in the presence of phyllosilicates

“…4a). The sensitivity of the segmentation and interconnectivity analysis to changes in the threshold values was established by repeating the procedure on volumes that were eroded and dilated by one voxel (see Fusseis et al (2012) and Macente et al (2017) for details of the analysis). This procedure yielded error bars on the porosity measurements.…”

Integrating X-Ray Computed Tomography With Chemical Imaging to Quantify Mineral Re-crystallization From Granulite to Eclogite Metamorphism in the Western Italian Alps (Sesia-Lanzo Zone)

“…To quantitatively investigate the textural and chemical heterogeneities developing during deformation and metamorphism partitioning we propose a novel approach integrating X-ray computed microtomography (µ-CT) with X-ray chemical mapping obtained from an Electron MicroProbe Analyzer (EMPA). This procedure combines (1) the quantitative calibration of the X-ray chemical map for selected microdomains using the Q-XRMA (i.e., Quantitative X-Ray Map Analyzer) statistical approach (Ortolano et al, 2018) to determine different compositional regions within the rock and minerals associated to the superposed metamorphic assemblages; (2) the highresolution thin section optical scanning to determine the mineral grain-size distribution and perform the orientation analysis using the Min-GSD (Mineral-Grain Size Distribution) method (Visalli, 2017); and (3) the X-ray µ-CT capability to resolve the three-dimensional (3D) shape and spatial distribution of minerals marking different fabrics in metamorphic rocks (e.g., Denison et al, 1997;Huddlestone-Holmes and Ketcham, 2010;Voltolini et al, 2011;Baker et al, 2012;Sayab et al, 2015;Macente et al, 2017).…”

Spatial and size distributions of garnets grown in a pseudotachylyte generated during a lower crust earthquake

“…When both methods return similar results, we consider them robust. The procedure of segmentation to extract the garnets and the coronae follows methods from previous studies that image minerals in three dimensions in metamorphic rocks (Denison and Carlson, 1997;Ketcham, 2005;Goergen and Whitney, 2008;Macente et al, 2017). The total volume was processed using only the user-dependent workflow (AvizoFire), as the user-independent algorithm (FiJi) required too much memory on the desktop computer used for the present study.…”