THE EFFECT OF GRIND TIME ON THE CRYSTAL LATTICE OF DOLOMITE AND THE IMPLICATIONS FOR SHOCK METAMORPHIC STUDIES OF IMPACTS INTO CARBONATE TARGETS

Impacts from asteroids or comets onto the Earth’s surface create shockwaves strong enough to deform the crystal lattice of most minerals. Because of their dominance in the Earth’s crust, silicates have been the subject of most empirical and experimental shock metamorphic studies. Whereas silicates display a wide variety of shock metamorphic textures (e.g. PFs, PDFs, high pressure polymorphs), the response of carbonate minerals are not as diverse and resultant fabrics (twinning, cleavage, and fracturing) are not uniquely generated by shock metamorphism. 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 is 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 potential effects of grind times on diffraction peaks in unshocked dolostone samples as a means of providing insight into this problem. A sample of the Neoproterozoic Beck Spring Dolomite, Inyo County, CA was cut into 6 aliquots, ground by using a mechanical pulverizer in variable grind times (ranging from 3 to 18 minutes) in order to characterize potential variations in X-ray diffraction peaks with grind time.

Diffraction peaks display an overall trend of decreasing intensity and increasing peak width with increasing grind time. There is a 55% decrease in intensity observed between aliquots ground for 3 minutes and 15 minutes respectively. However, this trend is not systematic among all aliquots, as there is only a 29% decrease observed from 3 to 18 minutes, which may be due to compositional variations within portions of the sample itself. Preliminary results suggest that sample processing using a mechanized pulverizer does have the potential to deform the dolomite lattice, which is represented by broadened X-ray diffraction patterns and to affect our ability to assess the magnitude of shock metamorphism in carbonate rocks.