Grain size data from rock deformation experiments on chlorite, epidote, and amphibole-rich samples

Laboratory shearing experiments were conducted on samples of chlorite schist, epidotite, and a hornblende-rich amphibolite. The samples were powdered to a starting grain size of < 125 µm before experiments were started. The experiments were conducted at room temperature, under fluid-saturated conditions and an applied normal stress of 10 MPa in a single-direct shear device at Marum Research Faculty at the University of Bremen, Germany. This apparatus induces planar shear by displacing the plates relative to each other. This configuration enforces deformation localization onto a thin (up to a few 100 µm), horizontal zone in the centre of the sample. This dataset contains data from these shear zones.

Backscattered scanning electron images were acquired using a Philips/FEI-XL30 field emission gun environmental scanning electron microscope (SEM) in the School of Earth and Ocean Sciences, Cardiff University. The samples were separated along the shear surfaces after experiments, and imaged normal to the slip surface with the SEM in low vacuum mode (0.5 Torr maintained using H₂O vapour), accelerating voltage of 20 kV, and 30 µm diameter final aperture.

Grain size distributions for slip surfaces developed in the end-member rock samples were obtained using ImageJ software (available from https://imagej.nih.gov/ij/) and Jazy macros created by Rüdiger Kilian (downloaded from https://github.com/kilir/Jazy_macros). The workflow for image segmentation follows Heilbronner (2000). After segmentation, we fit ellipses to grains to obtain their long and short axis lengths, and define grain size as the geometrical mean of these lengths (e.g. Shimamoto and Nagahama, 1992). We also use these data to quantify shape-preferred orientations (e.g. Cladouhos, 1999), defined by the orientation of grain long axes measured as an angle from -90˚ to +90˚ from the slip direction.

Research results based upon these data are published at http://doi.org/10.1029/2020JB019487