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Evaluation of XRD and Raman Peak Broadening in Shock-Metamorphosed Carbonates from Carbonate-Target Bolide Impact Structures

Introduction: Peak broadening recognized in X-ray diffraction (XRD) and micro-Raman (MRS) spectra of experimentally and naturally shock-metamorphosed carbonates from carbonate-target bo-lide impact structures has been shown to be a reliable indicator of impact-related shock-metamorphism [1-7]. However, studies have not yet shown whether this peak broadening is unique to or diagnostic of all car-bonate-target impact structures. Samples from eight confirmed carbonate-target impact structures that represent a range of potential shock conditions are being analyzed to determine whether diagnostic peak broadening can be recognized in impact structures of various ages and potential magnitudes of impact energy. Samples include matrix and clasts from polymict lithic breccia (Figs. 1-2) and shatter cones (Fig. 3). In addition, XRD spectra from potentially shock-metamorphosed samples are compared to spectra generated from samples that represent other natural terrestrial high pressure-temperature conditions (e.g., fault breccia, carbonatite, marble, etc., Fig. 4) to determine whether peak broadening in carbonates is unique to shock-metamorphism caused by impact-generated pressures. All sample spectra are compared to unaltered Iceland and dolomite spar control standards. Shock-metamorphism in carbonate target rocks should manifest as angstrom-scale reductions in crys-tallite size and an increased crystallite strain (%), which result in the peak broadening observed in XRD generated spectra. Duplicate MRS analyses of samples representative of a variety of potential shock-metamorphic and natural high pressure-temperature conditions will be compared to the XRD results to test the viability of the MRS as a reliable method for analyzing shock-metamorphic effects in carbonates. MRS analysis is more versatile, virtually non-destructive, and requires a smaller sample size than does XRD analysis. Sample Collection and Methodology: All samples were prepared, analyzed, and reduced using procedures developed by [8]. Rock chips were first prepared for each sample using a hammer and chisel to isolate desired sample areas. Chips were then powdered with an agate mortar and pestle under alcohol to reduce the possibility that grinding might artificially alter the crystal structure properties. Sample powders were then sieved through a 45-μm sieve to obtain a uniform fine grain size for optimal XRD analysis re