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Evaluation of XRD and micro-raman peak broadening in shock-metamorphosed carbonates from selected carbonate-target bolide impact structures
Bliss, Kean M.
Identification of shock metamorphism in minerals is an important line of evidence for confirming the origin of potential impact-generated structures and associated deposits. Unlike common quartz and feldspar occurring in crystalline target rocks, shock-metamorphosed carbonates are not readily identified petrographically. Studies have indicated that X-ray diffraction (XRD) and micro-Raman spectrometry (MRS) peak broadening is a reliable indicator of shock metamorphism in carbonates. Samples from seven confirmed carbonate-target impact structures and other high temperature-pressure regimes (i.e., carbonatite, marble, and fault breccia) were analyzed in order to determine whether peak broadening is unique to impact structures. In addition, MRS is further tested for comparability to XRD analyses, and for the effects of different sample preparation (i.e., thin section vs. powder). XRD analyses reveal that microstructural effects (i.e., crystallite size and lattice strain) contributing to peak broadening in potentially shocked samples follow expected trends of decreased crystallite size and increased lattice strain with increasing shock. However, carbonatite and fault breccia show microstructural effects comparable to those recorded for shocked samples, indicating that peak broadening is not unique to impact-related deposits. Although XRD and MRS sample trends are similar, machine- and sample-derived variability is high in MRS analyses, contributing to uncertainty and less accurate peak width measurements. Large discrepancies exist between thin section and powder MRS analyses of identical samples. Estimated peak shock pressures calculated from previously established calibration lines for dolomite are much higher than expected; indicating that calibration curves should be specific to each impact structure and sample preparation protocol.
San Diego State University
Master of Science (M.S.) San Diego State University, 2009
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