RAPID SHOCK LEVEL DETERMINATION IN SILICATES USING POWDER X-RAY DIFFRACTION

Previous workers have demonstrated that higher intensities of shock metamorphism progressively decrease the size of coherent scatter domains (CSDs), and increase the density of X-ray observable defects, associated with specific crystallographic zones in silicates. We investigated the crystallography of quartz, plagioclase, muscovite, and garnet from crystalline target rocks at the Wetumpka impact structure interpreted to have suffered sequentially higher shock from 3 to 20 GPa. Our results suggest that powder X-ray diffraction might be used to rapidly and precisely determine the shock level of impacted minerals, independent of optical techniques.

We interpreted the shock level of crystalline rocks from the Wetumpka structure based on petrographic analyses of quartz microstructures. Quartz in the rim rocks contains fractures and optical discontinuities likely present in the pre-impact schist. Quartz from the central uplift exhibits a range of features from PFs and decorated basal Brazil twins to well-developed feather features. Clasts in the fallback breccias contain a variety of PDF assemblages characteristic of shock levels up to ≥ 20 GPa.

We separated and ground each silicate phase in a ZrO₂ mortar and mounted the powder with an appropriate internal standard. Spectra were collected on a Bruker D8 Advance diffractometer at the University of Georgia. The spectra were analyzed using XPowder and TOPAS software. Mean CSD lengths for each indexed plane were calculated using the Scherrer equation. Although the Scherrer equation strictly is not appropriate for quantifying most of our phases, we suggest that its consistent use throughout the study provides a simple basis for empirical comparisons.

For quartz, plotting 1/domain length (nm^-) vs. 2θ° produces an ascending array of well-resolved M-shaped curves that correspond to each petrographic shock level. Conspicuous gaps occur between curves for PDF assemblages that we did not analyze, and preliminary tests indicate that we can use the plot to predict petrography. Our data also suggest that domain sizes in low-level shocked plagioclase can increase or decrease depending on the crystallographic plane. This may be related to alternate twin deformation. And muscovite may actually become increasingly ordered as a result of low-level shock, producing larger CSDs.