grain parameters

Published: 29 October 2020| Version 1 | DOI: 10.17632/3kthrpngct.1
Contributor:
leonardo casini

Description

EBSD data for grain parameters of mylonite developed from troctolite. The sample has been dredged at 16.5°N along the Mid Atlantic Ridge during the The RV Knorr cruise 210 Leg 5. Each sample and sub-sample contain consists of several grain parameters such as: i) grain area, ii) grain size, iii) Axial ratio, iv) convexity, v) tortuosity, vi) shape factor and vii) solidity. All these parameters are provided for all minerals of the assemblage plagioclase + olivine + pyroxene + Ti-pargasite.

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Olivine, Plagioclase, pyroxene and amphibole grain parameters of selected samples were measured by indexation of electron backscattered diffraction (EBSD) patterns using the SEM-EBSD facility at Geosciences Montpellier. Areas of data acquisition (Fig. 4) were chosen to represent textural pattern of increasing deformation zones: from porphyroclastic texture (ASF60_27F) to mylonite (ASF60-27C3-4) and ultra-mylonite (ASF60-27C1-2). Measurements were performed using a CamScan X500FE Crystal Probe equipped with an Oxford/Nordlys EBSD detector, run with an accelerating voltage of 17 kV, a working distance of 25 mm and a step size of 1.5 µm in the ultra-mylonite, and 6 µm in the mylonite and proto-mylonite. The first stage of data processing was done using the Tango software of the Channel 5 suite (hkl software) to remove isolated pixels that are either non-indexed or indexed as a given phase and surrounded by pixels indexed for another phase, and fill non-indexed pixels that have a minimum of 6 neighbour pixels with the same orientation. The Matlab toolbox MTEX (version 5.2.8; e.g., Hielscher and Schaeben, 2008; Bachmann et al., 2010; Mainprice et al., 2014) was then used to plot EBSD maps, and extract grain size and shape parameters. Pixels with a mean angular deviation (i.e., the angle between the acquired diffraction pattern and the indexing solution proposed) > 1° were excluded from the data set. Grain reconstruction was done using a 10° misorientation threshold. Grains < 3 pixels were considered as potential measurement errors and removed. Twins in Pl and clinopyroxene were distinguished from grain boundaries by filtering out the boundaries with the relevant misorientation (180°) and rotation axes.