GRAIN SIZE AND SHAPE CHARACTERISTICS OF XENOCRYSTS IN THE MASONTOWN (SW PENNSYLVANIA) AND STOCKDALE (NE KANSAS) KIMBERLITES: INSIGHTS FROM OPTICAL MICROSCOPY AND COMPUTED X-RAY TOMOGRAPHY

Kimberlite is an ultramafic, potassic and carbonate-rich igneous rock that originates deep within the mantle. As the magma propagates upwards through the lithosphere it entrains mantle- and crustal-derived xenoliths. Much of the crystal-load within kimberlite is thought to come from the entrainment of mantle peridotite xenoliths and their disaggregation to olivine xenocrysts, as described in the “kimberlite factory” model of Brett et al. (2015). Our study examines grain size and grain shape data from two non-diamondiferous kimberlites (Masontown and Stockdale), to allow comparison with Brett et al. (2015)’s observations from diamondiferous kimberlites in Canada and Tanzania, and to further test their model. Non-destructive, three-dimensional grain size and shape (sphericity) measurements of samples of the Masontown kimberlite loaned from the Smithsonian National Museum of Natural History were made using computed X-ray tomography (CT) data collected at the National Energy Technology Lab – Morgantown, and image-processed in ImageJ / FiJi and Blob3D. Additional grain size and shape measurements from both the Masontown and Stockdale kimberlites were made on hand specimens and thin sections. Optical microscopy allows examination of textural features characteristic of the kimberlite factory model, including regrowth rims, healed-cracks within the olivine xenocrysts, rounding, and pitted xenocryst surfaces. Hand samples have been photographed and turned into shareable three-dimensional models using AgiSoft Metashape and SketchFab.com.

CT imaging of kimberlite has numerous advantages for quantitative analysis: it is non-destructive, large numbers of grains are identified, rapid analysis; but is made challenging by attenuation of the X-rays due to the density of the kimberlite and the complex and time-consuming image-processing. A lack of consistent contrast between silicate phases (olivine and serpentine, in particular), and the presence of incredibly bright, dense phases (e.g., perovskite), makes grain delineation and separation challenging, and requires more user input and control than normal. However, despite these issues, hundreds of thousands of grains can be detected and measured in three dimensions in a single sample, greatly surpassing analyses that can be done by optical microscopy.