Electron backscatter diffraction

The EBSD data were collected using an SEM either coupled to an EDAX Pegasus system at ISTO/BRGM, or using an e⁻Flash^HR detector at Bruker Nano Analytics GmbH (Berlin, Germany). Crystallographic information was also combined with EDX (Energy-Dispersive X-ray) spectroscopy to identify phases. The EBSD measurements were performed at a working distance of 15 or 20 mm with an accelerating voltage of 20 kV and a probe current of ∼6 nA on polished thin sections (diamond paste of 0.25 μm followed by colloidal silica). In order to avoid indexing errors, we constructed pole figures using one measurement per grain collected manually. The iso-contours and grey shadings on pole figures were plotted using the Unicef careware software package written by David Mainprice (www.gm.univ-montp2.fr/PERSO/mainprice/W_data/CareWare_Unicef_Programs). We further used the open-source matlab-based MTEX toolbox to construct IPF of misorientation axes and to calculate the texture (J) and misorientation (M) indices^30,60. To plot pole figures and calculate the J index, we used a Gaussian half-width angle of 10°.

For acquisition data processing and analysis of EBSD maps, we used the OIM Analysis 7 software (EDAX) and/or HKL Tango channel 5 system (Oxford Instruments). The indexing hit rates were higher than 80% for all maps, including cavities. We used a minimum of six indexed bands and a maximum band mismatch of 2°. The outlier pixels and zero solutions were removed up to a maximum pixel count of 10. Grain detection was performed using correlated misorientation angles higher than 10° for grain boundaries and between 2 and 10° for sub-grain boundaries. Each grain contains a minimum of five pixels considering a hexagonal grid. The average grain size corresponds to the mean equivalent diameter calculated using the grain ellipsoid fit. EBSD maps were used only to calculate the grain size in the protomylonites (edge), mylonites and ultramylonites. In the protolith and centre of protomylonites, we applied the length-intercept method on BSE images because of their larger grain sizes.