SDSU - Department of Geological Sciences - Thesis Defense - Spring 2009

ABSTRACT

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, the MRS method 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. However, fault breccias and carbonatites 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-derived 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.