NOVEL AND EFFICIENT CHARACTERIZATION OF CRYSTAL POPULATIONS FOR PETROLOGY AND GEOCHRONOLOGY

We introduce and demonstrate new workflow and analysis capabilities for X-ray CT data sets in Blob3D software that have been enhanced to allow routine processing of grain mounts containing tens to thousands of individual crystals. An efficient, single-pass watershed algorithm based on the rainfall approach allows separation of touching grains to be conducted much more efficiently than previously possible. Algorithm inputs can be changed interactively on a case-by-case basis while processing a sample to optimize separations, and problematic cases can be dealt with using more manual methods, allowing the user to ensure that all grains are correctly identified and isolated. Using these methods, a 1000-grain mount can be processed in about an hour. Grain shapes can then be quantified with a number of standard metrics, including caliper dimensions, and a new method has been introduced to automatically quantify grain shape in terms of whether they are euhedral, rounded, or broken. Using stereographic projection of smoothed surface normals, it is possible to detect flat faces based on point concentrations, rounding by point dispersion, and symmetric crystal faces based on comparison of upper- and lower-hemisphere projections. In an attempt to determine whether these observations can replicate human judgement in describing grain shape, we evaluated over 2000 grains of zircon and 100 grains of apatite, coding each based on shape and roughness. These values are compared to numbers derived from the Blob3D analysis, including the correlation of upper- and lower-hemisphere stereograms to identify crystal faces, and the skewness of the point concentration distribution to gauge rounding. These data will help to categorize mineral grains used for geochronology studies, providing information on their transport properties and history. The stereographic projection method may eventually be useful for automatically characterizing the habit of euhedral grains, aiding in mineral identification or elucidation of crystallization conditions.