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Thesis Format

Monograph

Degree

Master of Science

Program

Geology

Collaborative Specialization

Planetary Science and Exploration

Supervisor

Flemming, Roberta L.

Abstract

This work extends on two quantitative methods for evaluating shock metamorphism to include clinopyroxene using X-ray diffraction (XRD). Clinopyroxene samples experimentally shocked to known shock pressures were studied with XRD. Lattice strain (ε) and strain-related mosaicity (SRM) were measured for each sample and plotted against their known shock pressures, creating a ε versus shock pressure calibration curve and an SRM versus shock pressure calibration curve. These calibration curves were applied to three meteorites: Nakhla, Zagami, and ALHA 77005. The SRM calibration curve gave peak shock pressures of 12±8 GPa for Nakhla, 44±8 GPa for Zagami, and 68±8 GPa for ALHA 77005. The SRM calibration curve is inaccurate because high SRM values weren’t adequately sampled. The ε calibration curve gave peak shock pressures of 8±12 GPa for Nakhla, 36±12 GPa for Zagami, and 90±12 GPa for ALHA 77005. The ε calibration curve is effective when sufficient amounts of data are used.

Summary for Lay Audience

IImpact cratering occurs everywhere in the solar system. It changes the affected rocks through shock metamorphism, which alters their mineralogy and textures. The peak shock pressure experienced by a rock during an impact event can be quantitatively determined by measuring properties of the individual minerals. We expand two of these methods to include the mineral clinopyroxene, by using in situ micro X-ray diffraction (μXRD) to measure lattice strain (ε) and strain-related mosaicity (SRM) in experimentally shocked clinopyroxene samples. These samples were shocked either by a light gas gun, vertical gun, or flat plate accelerator, depending on the source of the samples. Lattice strain is a measure of distortion in the crystal lattice due to strain. SRM is a measure of the misorientation of crystal subgrains due to strain. Both the measured ε and SRM from each sample were plotted against their experienced peak shock pressures to create an ε vs shock pressure calibration curve and an SRM vs shock pressure calibration curve. From these calibration curves, the shock pressure experienced by clinopyroxene in rocks can be determined. These calibration curves were applied to clinopyroxene-bearing martian meteorites, which have experienced impact events. These meteorites are Nakhla, Zagami, and ALHA 77005. The SRM calibration produced peak shock pressures of 12±8 GPa for Nakhla, 44±8 GPa for Zagami, and 68±8 GPa for ALHA 77005. Zagami’s calculated peak shock pressure is considerably higher than the 29.2±0.6 GPa peak shock pressure value reported for it in the literature. It is thought that due to the variability of SRM leading to high SRM values not being adequately sampled, the slope of the SRM calibration curve is not as steep as it should be and is thus inaccurate. The ε calibration curve gave peak shock pressures of 8±12 GPa for Nakhla, 36±12 GPa for Zagami, and 90±12 GPa for ALHA 77005. The ε calibration curve is found to be effective when sufficient amounts of data are used. This calibration curve will aid in the evaluation of shock metamorphism in both clinopyroxene-bearing meteorites and terrestrial impact rocks.

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