THE MASONTOWN KIMBERLITE, FAYETTE COUNTY, PENNSYLVANIA: INSIGHTS INTO EMPLACEMENT PROCESSES BY THE CHARACTERIZATION OF XENOCRYST SIZES AND SHAPES USING COMPUTED TOMOGRAPHY

The late Jurassic Masontown dyke in Fayette County, SW Pennsylvania, preserves abundant rounded, mm to cm-diameter masses of olivine and serpentine cemented together in serpentine-rich kimberlite groundmass. Each mass is interpreted to be a partially serpentinized olivine xenocryst or peridotite xenocryst. Each rounded clast is jacketed by a distinct rim of serpentine; probably originally olivine. The (1) ubiquitous roundness of clasts and (2) the presence of distinct serpentine jackets around each clast, supports emplacement of the dyke by a 'kimberlite factory' (Brett et al., 2015). Due to the paucity of available samples, we have used non-destructive imaging by computed tomography (CT) at the National Energy Technology Lab in Morgantown, WV, to construct 3D models of the internal structure of hand samples loaned from the Smithsonian Institute's Museum of Natural History. MicroCT (1-3 micron resolution) and industrial CT (~15 microns resolution) serial scans processed in ImageJ and Blob3D allow for 3D characterizations of individual clasts, including their shape factors (sphericity, roughness, etc.) and sizes (i.e. crystal size distributions).