EXPLORING METHODS TO DETERMINE THE STRAIN ELLIPSOID IN DUNITES TO PROVIDE CONTEXT FOR EBSD CPO PATTERNS

Crystallographic preferred orientation (CPO) data for olivine, collected using Electron Backscatter Diffraction (EBSD) is typically collected and interpreted in a strain/shape fabric reference frame from thin sections cut parallel to the X-Z strain axes. EBSD data recently collected for several dunite samples from the Buck Creek ultramafic body in southwestern North Carolina show strong CPO patterns that may reveal information about the conditions of deformation of the body. Unfortunately, it has been challenging to visually determine the shape fabrics in these samples, making it difficult to properly orient the samples.

We have used a published method for calculating the vorticity axis (CVA) from EBSD data. Assuming the CVA is parallel to the Y-axis of strain for non-coaxial deformation, axial D-type fabrics seem likely for most samples. To properly orient and make sense of the CPO patterns we have explored methods for measuring the 3D olivine shape fabrics. An ellipsoid can be calculated by determining the strain ellipses for 3 orthogonal thin sections.

Two approaches for 2D measurement of the strain ellipse were tested for accuracy and repeatability. Both relied on use of high resolution thin section photo scans while simultaneously observing the grains in thin section to confirm grain boundaries. The simpler center-to-center method, employed by manually selecting center points of individual olivine grains was tested with multiple observers and a varied number of center picks. Fry plots created using the program EllipseFit resulted in poor and inconsistent ellipses. A more productive approach has been to select the best-fit ellipses for multiple individual olivine grains. Most grains in an area were selected, excluding those that were small or not clearly a single grain. With this approach EllipseFit can produced a repeatable strain ellipse from multiple observers. Thin section quality and alteration effects influence the strength of the ellipses. EllipseFit was used to calculate the shape fabric/strain ellipsoid from the 3 perpendicular slides and we have been able to use this shape fabric to reorient our CPO data. We are in the process of testing this approach with other samples and increasing the numbers of ellipse picks in some slides to confirm the feasibility of this method.