DEFORMING DOLOMITE: HOW SAMPLE PROCESSING CAN AFFECT OUR INTERPRETATIONS OF SHOCK METAMORPHISM IN CARBONATE TARGETS

Impacts from extraterrestrial objects onto Earth’s surface can create shockwaves powerful enough to deform the crystal lattice of most minerals. Silicates have been the subject of most studies of shock metamorphism because of their dominance in the Earth’s crust and the wide variety of shock metamorphic textures (PFs, PDFs, and high pressure polymorphs) they display. Carbonate minerals respond to shock metamorphism in non-unique ways (twinning, cleavage, and fracturing) and have received comparatively lesser attention. Several previous studies have documented the level of shock-related deformation by noting changes in X-ray diffraction (XRD) patterns in powdered shocked vs. unshocked minerals. It has thus far been unclear how much or what portion of broadening of diffraction peaks is due to shock metamorphism versus other phenomena, such as sample processing.

This study assessed the effects of grinding method and grind times on diffraction peaks of unshocked dolostone samples as a means of providing insight into this problem. A sample of Neoproterozoic Beck Springs Dolomite, Inyo County, CA was cut into 6 aliquots. Each aliquot was ground by 3 methods: using a mechanical pulverizer, by hand using a mortar and pestle (dry ground), and by hand in solution with 80% isopropyl alcohol in a mortar and pestle (wet ground). Using these 3 methods of grinding, the aliquots were ground for increasing amounts of time (ranging from 3 to 18 minutes).

Diffraction peaks of samples processed by mechanical pulverization display an overall trend of decreasing intensity and increasing peak width with increased grind time (~62% decrease in intensity from 3 to 18 minutes). Results from the dry hand ground aliquots have variable amounts of peak intensity and peak broadening, though their intensities are all generally higher than the aliquots ground by machine pulverization. Diffraction patterns for the wet hand ground aliquots also have variable peak broadening and intensities, but overall peak intensities are lower than those ground dry.

Results indicate that using the mechanical pulverizer can produce the most predictable and consistent amounts of peak intensity and broadening, and therefore may be the better method to use in studies in order to adequately assess the effects of sample processing on shock metamorphosed materials.