Raman Spectroscopy and Synchrotron X-ray Diffraction Study of Lunar and Terrestrial Plagioclase Feldspar at High-Pressure and High-Temperature Conditions
Abstract
Plagioclase feldspar is one of the abundant minerals on the surface of the Earth, the Moon and Mars, and is also commonly found in meteorites. This dissertation seeks to better understand the phase changes in plagioclase when subjected to meteorite impact and fundamental information about impact cratering process.
Shock features in plagioclase from Mistastin Lake impact structure, Canada, and Apollo samples returned from the Moon were investigated. A series of progressive optical shock features were observed, and their Raman spectra was collected systematically. Identified shock featurs include deformed twins, planar features, and partially to completely isotropic or diaplectic glass. However, no planar deformation features (PDFs) were observed in plagioclase despite finding PDFs in quartz in the same thin section in Mistastin Lake central uplift samples. Certain features observed in plagioclase (e.g., twinning, needle like inclusions, zeolite alteration and crystalline planar features) may be mistaken as PDFs but can be easily identified by their Raman features. Raman spectra of shock features show that Raman features change progressively as shock level increases. Raman peak broadening, reduction of peak intensities, and peak loss were observed indicating structure deformation, loss of crystallinity and amorphization. Crystalline plagioclase and amorphous diaplectic glass can be distinguished based on their Raman features.
To better understand the deformation of plagioclase under high temperature and high-pressure conditions, static experiments were also conducted using laser heating diamond anvil cell and synchrotron X-ray diffraction. Results reveal pressure/temperature-induced amorphization and crystallization kinetics of high-pressure phase changes. The phase diagram of intermediate plagioclase was constructed up to 64 GPa and 2000 K and a new high-pressure phase was observed. This phase diagram reveals a wide pressure-temperature range for plagioclase to change from crystalline to amorphous state (i.e. diaplectic glass) before melting. Furthermore, the breakdown and phase changes make plagioclase an effective barometer candidate for estimating the pressure-temperature history of shock events and interpreting the mineralogy of meteorites, ultimately of great help to reconstruct the collisional history of asteroids in the early Solar System.