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Effect of Water on Thermoelasticity of Majoritic Garnet: Implications for the Seismic Structure at the Top of the Lower Mantle

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High-pressure and high-temperature laboratory experiments on the physical properties of mantle minerals provide a window into the Earth’s interior chemistry and geodynamics. The measurement of material density, compressibility, and elasticity at high P-T conditions provides thermoelastic parameters necessary to model seismic velocities in the Earth’s mantle at regional and global scales. In the first study, I determined the influence of hydration on thermoelasticity of majoritic garnet, an important silicate phase in the mantle transition zone (MTZ, 410-660 km depth). The MTZ is thought to potentially contain a large geochemical reservoir of H2O, bound in the crystal structures of nominally anhydrous minerals as hydroxyl (OH-) defects. My results show little or no effect of hydration on seismic velocities in or below the MTZ, suggesting that low-velocity anomalies recently observed below the transition zone beneath North America by Schmandt et al. (2014) and others are caused by dehydration melting of garnet as it transitions to bridgmanite at ~780 km depth. In the second study I perform a high-pressure synchrotron X-ray diffraction study of clinoenstatite (Mg2Si2O6) up to 45 GPa to determine its structure and compressibility. From 9.5 to 35.5 GPa I observed the high-pressure clinoenstatite (HPCEN) phase and measured its P-V equation of state and structural evolution over an expanded pressure range relevant to pyroxene metatstability. At 45 GPa, I observed a phase transition to a new monoclinic form of Mg2Si2O6, called HPCEN2. Observation of HPCEN2 in Mg2Si2O6 is the third apex of the pyroxene quadrilateral to adopt this structure type at high-pressure, which has also been observed in Fe2Si2O6 (Pakhomova et al. 2017) and in MgCaSi2O6 (Plonka et al., 2012; Hu et al., 2016). In the third study I developed an optical microscope at Los Alamos National Laboratory to quantify the tensor of refractive indices (i.e. the indicatrix) in molecular crystals, and applied the new setup to determining the indicatrix and chromatic dispersion of acetaminophen (p-hydroxyacetanilide form I, commonly known as Tylenol®) as well as the orientation of the optical indicatrix in a monoclinic basis. Results indicate the optical indicatrix of acetaminophen form 1 is optically negative and biaxial. The results will ultimately be applied towards determining the full elastic tensor of acetaminophen from ongoing Brillouin spectroscopy experiments. Knowledge of the elastic tensor of molecular crystals such as acetaminophen will improve models of mixtures of pharmaceuticals with binders under compression, potentially reducing the amount of binder necessary to produce tablets (Anderson, 2008; Toms et al., 2008; Ong et al., 2010). Chapter 5 presents development of a 2-dimensional X-ray diffraction system in the Northwestern University High-Pressure Science Laboratory. Built onto an existing 4-circle Huber diffractometer, addition of the area detector system expands the capabilities of the instrument to include powder diffraction, rapid determination of crystal orientation matrices, and X-ray diffraction studies of minerals and materials at simultaneous high-pressure and temperature conditions.

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  • 04/03/2019
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