This thesis has been approved by The Honors Tutorial College and the Department of Geological Sciences ______________________________ Dr. Keith Milam Associate Professor, Geological Sciences HTC Thesis Advisor Director of Studies, Geological Sciences ______________________________ Cary Roberts Frith Interim Dean, Honors Tutorial College Page | 1 The Effect of Sampling Processing on X-Ray Diffraction Peaks of Dolomite: Implications for Studies of Shock Metamorphosed Materials Emily N. Simpson, Department of Geological Sciences, Honors Tutorial College, Ohio University April 2019 A Thesis Presented to the Honors Tutorial College, Ohio University In Partial Fulfillment of the Requirements for Graduation from the Honors Tutorial College with the degree of Bachelor of Science in Geological Sciences Page | 2 Table of Contents: Abstract: p. 4 Introduction: p. 4 Hypotheses: p. 8 Methods: p. 9 Results: p. 14 Discussion: p. 19 Summary: p. 23 References: p. 25 Figures: p. 30 Page | 3 Abstract Impact craters form when shock waves propagate through target rock on the surface of a planet during an impact event. Shock waves produced during such collisions commonly affect the crystal lattices of existing minerals. Carbonate minerals, such as dolomite, are changed in non-unique ways during impacts. For example, the amount of twinning, fracturing, and cleavage may increase as a result of deformation. Previous work has shown that the X-ray diffraction (XRD) peaks of minerals may be affected as a result of crystal lattice deformation during an impact event. With increased shock metamorphism, peak broadening increases and peak intensity decreases. Prior studies, however, have done little to assess the effects of sample processing on X-ray diffraction patterns, and whether or not such effects might be misinterpreted to represent the effects of shock metamorphism. This study investigates the effects of sample processing and how it changes the X-ray diffraction peaks of unshocked materials that have been processed for differing lengths of time and with different techniques. Shocked samples were also studied to determine if there is enough of an effect from sample processing that the potential effects of shock metamorphism are overprinted completely. Results indicate that the method of grinding, as well as the length of time spent grinding can have measureable effects on the diffraction patterns obtained. (Note, to view Appendices 1-4, email [email protected] to obtain copies) Introduction Impact cratering and shock metamorphism Impact craters on Earth are formed when an object (an asteroid or comet) penetrates the atmosphere with little or no deceleration and then collides, or nearly Page | 4 collides, with the Earth’s surface (French, 1998). The formation process of an impact crater is generally considered in three stages: 1) contact and compression, 2) excavation, and 3) modification (Figure 1). The contact and compression stage occurs when the projectile first encounters the target; a shock wave is generated and transmitted outward and downward. This stage lasts no longer than a few seconds (French, 1998). The impact crater forms during the excavation stage as a result of the complex interactions between the expanding shock wave and the original ground surface. This stage lasts on the order of a couple minutes or less (French, 1998). The modification stage occurs when the transient crater becomes immediately affected by the gravitational response of the crater rim and floor. This stage has no clearly marked end (French, 1998). Multiple types of impact craters may be formed as a result of impact: simple craters, complex craters, and multi-ring basins (French, 1998). Simple craters, the smallest of types, are bowl-shaped, and example of a simple crater is Meteor Crater (Barringer Crater) in Arizona. During modification of larger craters, the final crater diameter may be enhanced due to collapse of the steep upper walls into the crater cavity and uplift of the crater center producing a complex crater (Melosh, 1989). Complex craters are characterized by a centrally-uplifted region surrounded by a relatively flat floor with a collapsed crater rim, an example of a complex crater is the heavily-eroded Serpent Mound impact structure located in Ohio. There are three types of complex craters (Grieve et al., 1981): central peak structures, central-peak basin structures, and peak-ring basin structures. The final and largest type of impact structure is the multi-ring basin; an example of this type of crater is Sudbury impact structure in Canada. These structures are huge geologic bull’s-eyes composed of multiple concentric, uplifted rings and Page | 5 intervening down-faulted valleys (ring grabens) (French, 1998). They are typically produced by the impact of projectiles tens to hundreds of kilometers in diameter (Melosh, 1989). Impact structures this large formed mainly during the early formation of the Solar System and they represent the most energetic and catastrophic impact events (Melosh, 1989). The ability to confirm the identity of impact craters relies not only on the recognition of a circular landform, but on the identification of the shock effects in the minerals within and/or ejected from the crater itself. A shock wave is a high-pressure, supersonic wave caused by a rapid change in pressure within a medium (French, 1998). When a shock wave travels through target rock, it releases enough pressure to permanently deform crystal lattices of minerals. Different minerals respond differently to shock waves depending on their crystal structures and the geometry of the shock wave propagating through them (Langenhorst, 2002). Shock metamorphism of silicates Silicates, specifically quartz, have been the focus of more studies on shock metamorphism than any other group of minerals because they make up 90% of the crust (Clarke and Washington, 1924), which therefore makes them the target of more impacts. Silicates are also deformed by shock waves in unique ways, so identifying impact craters by observing the features in silicates has been commonly done. Shocked silicates commonly contain planar fractures (PFs), or parallel sets of planar cracks or cleavages in grains, and PDFs (planar deformation features) (Langenhorst, 2002). PDFs are narrow planes arranged in parallel sets that have distinct orientations with respect to a grain’s crystal structure (Figure 2). Diaplectic glass may also form at higher peak pressures and Page | 6 temperatures in silicates (French, 1998). Diaplectic glass is formed when the pressures and temperatures are so high during impact that a melt is formed, then following the release of the high pressure and temperature, the melt cools so quickly that there is not enough time for a crystal structure to form (French, 1998). High pressure polymorphs, such as coesite and stishovite, are formed at moderately high pressures (>2-8 GPa) and relatively high temperatures (~500-700°C) (Martini, 1978). While silicates dominate the target rocks of most terrestrial impacts, other mineral types, such as carbonates, are also commonly affected by shock metamorphism. However, they have not been the focus of a significant body of scientific research, so comparatively less is known about them. Shock metamorphism of carbonates Carbonate minerals are affected by impact in different ways than silicates, (Figure 3). Carbonates often experience mechanical twinning when exposed to high pressures due to kinks in the crystal lattice (Figure 4) (Burkhard, 1993). Lattice dislocations within the crystal are also common in carbonates as a result of impact (Langenhorst, 2002). Dislocations are similar to mechanical twinning, but are formed by slipping of the crystal lattice, rather than kinking of it (Hull and Bacon, 2001). Decomposition occurs in shock metamorphosed dolomite at pressures above ~60-70 GPa (Skala et al., 2000). Most of these effects are not unique of shock metamorphism and can be the result of regular pressures experienced during tectonic episodes on Earth. Because of this, other methods have to be used to identify shock metamorphism in carbonates Shock metamorphism may also result in disorder within the crystal lattice that can be measured by X-ray diffraction (XRD) (Figure 5; Hanss et al., 1978). Diffraction peaks of shocked minerals show a decrease in intensity of X-rays, and a broadening of Page | 7 the width of individual peaks (Hanss et al., 1978). It has been suggested that peak broadening may be a good indicator of what shock pressures were experienced by target rocks in different parts of an impact structure (Skala et al., 1999) and may also aid in documenting shock wave dissipation (Bell et al., 1998). Understanding how minerals are deformed by shock pressure is vital in order to understand what has occurred in the target rocks of terrestrial impact craters. One issue with XRD peaks that has not been considered in previous studies is the potential effects that post-impact processes (natural and otherwise), such as sample history, collection, and processing can have on peak broadening. It is important, when doing XRD studies of shock materials, to know where exactly the samples have been collected from. If samples were collected from a quarry or by the side of the highway on a road cut, they may have been blasted by dynamite at peak pressures reaching the gigapascal range. Or if samples were collected from a site that has a history of tectonic deformation, there is the potential that the rocks have experienced enhanced crystal lattice deformation.