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Methods and materials


Samples discussed in this data report were collected from Section 304-U1309D-64R-2 with the top of the billets at 43 and 46 cm from the top of the core (Fig. F1). This shear zone was chosen because of the clear contact between lithologies along the shear zone boundaries. It is located at 330 m depth (near the top of the total drilled core length) and is 8 cm wide. The bulk of Section 64R-2 is dunitic with a sheared gabbroic intrusion between 41 and 49 cm (although only the bottom contact was preserved in the core). The section exhibits mild greenschist facies alteration, and some chlorite is seen in the thin sections. Sample 64R-2, 43 cm, represents the center of the shear zone and is gabbroic in lithology (Fig. F2A, F2B). The bulk of thin section 64R-2, 43 cm, is plagioclase (~55%) and then clinopyroxene (~25%) with some orthopyroxene (~10%). There are minor amounts of opaques and alteration phases (talc, tremolite, and chlorite; ~10%). Sample 64R-2, 46 cm, exhibits both rock types and represents the edge of the shear zone and the lower contact with the dunite (Fig. F2D, F2E). The bulk of thin section 64R-2, 46 cm, is plagioclase (~50%) and then clinopyroxene (~20%) with some orthopyroxene (~10%) and hornblende (~10%). There are minor amounts of opaques and alteration phases (talc, tremolite, chlorite, and zeolite; ~10%).

Sample preparation

Samples were collected from the working half of the core once the shear zone of interest had been identified. The location for each of the thin section billets was chosen by studying the shear zone and choosing the best example of the microstructure exhibited by the sample. Thin section billets were oriented, where possible, with foliation perpendicular to either the long or short axis of the thin section billet, or the data were rotated subsequent to acquisition (Fig. F2C, F2F). Standard 30 µm thick thin sections were produced from the billets. Thin sections were chemically and mechanically polished using SYTON fluid (Lloyd, 1987) and were carbon coated to prevent charging.

Data acquisition

Sample analysis areas were selected using optical and electron microscopy utilizing orientation contrast (OC) imaging (Prior et al., 1996). OC images show where crystallographic orientations change. Full crystallographic orientation data were obtained from automatically indexed EBSD patterns collected on a CamScan X500 Crystal Probe scanning electron microscope (SEM) fitted with a field emission gun and a FASTRACK stage (Prior et al., 1999). EBSD patterns were collected using a 20 kV acceleration voltage and a beam current of 30 nA. The working distance was 25 mm with an angle of 70° between the beam and the thin section. The step size of 3 µm was chosen as appropriate because of the size and details of the area to be mapped. Samples were mapped utilizing beam movement. EBSD patterns were imaged on a phosphor screen, viewed by a low-light charge-coupled device (CCD) camera, and indexed using the HKL Technology manufacturer’s software package Channel 5 (Schmidt and Olesen, 1989). Average measuring time per point was 0.26 s. The raw data files exhibit indexing between 45% and 74% of the microstructure, leaving 26% to 55% as nonindexed (EBSD patterns with no solution). The nonindexed points correspond to grain boundaries, cracks, holes, and phases which were not mapped for various reasons. The raw data files were manipulated by removing wild spikes (1 pixel that is inconsistent with its surrounding 8 neighbors) and by performing a nearest neighbor extrapolation to fill in nonindexed points. All data manipulation was performed in comparison with the band contrast map to make sure that errors were not introduced. Grain sizes are calculated by the software from the manipulated EBSD maps (where grains <12 µm have been removed as they are considered errors) as the diameter of a circle of equivalent area to the measured grain area.