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
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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
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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
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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. As a result, it is possible that the crystal lattices of the minerals could have been deformed enough to broaden XRD peaks and lower their intensities. Sample processing may have induced similar effects. When samples are processed for many
XRD studies they are often ground into fine powders to avoid lattice orientation effects.
This grinding could break down the crystal lattice and result in enhanced disorder that in turn results in more peak broadening (Hanss et al., 1978).
Hypotheses
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This study is an assessment of the following hypotheses and questions using both unshocked and shocked dolomite (dolostone) specimens:
1.) Longer duration grind times will enhance the magnitude of peak broadening
in X-ray diffraction patterns.
2.) Mechanical pulverization of the same sample using variable grind times will
lead to differences in peak broadening that are more consistent and predictable
than comminution by hand.
3.) Samples from an impact structure will show lower intensities and more peak
broadening in the central uplift than in the crater rim when processed using
mechanical pulverization.
A number of other questions arise from these hypotheses. If the grind time does in fact influence the magnitude of the peak broadening, then which method of grinding will have the most peak broadening? This question will be answered by grinding all of the unshocked samples in the same manner using three methods. Could sample processing completely overprint signs of shock metamorphism in naturally shocked samples from an impact structure?
Methods
Part 1: Unshocked Dolostone
Sample collection
Samples of the Neoproterozoic Beck Springs Dolomite were collected from the
Alexander Hills of San Bernandino County, CA (35° 47’ N, 116°, 7’W) from a natural exposure above the contact with the underlying Crystal Springs Formation without the
Page | 9 use of a hammer (Keith Milam pers. comm.) in order to eliminate any artificially induced lattice deformation. The Beck Springs Dolomite is interpreted to be a cap carbonate representing an abrupt change in climate following a global glaciation event (Gutstadt,
1968). Although the Beck Springs now consists entirely of dolomite, it was originally precipitated largely as aragonite and high-Mg calcite that was later dolomitized in a marine setting (Shuster et al., 2018). The Beck Springs Dolomite is approximately 100-
400 m thick in the collection area but thins toward the south where it becomes slightly more siliciclastic. The Beck Springs consists of 3 informal members: the lower laminated member, the middle oolitic-pisolitic member, and the upper cherty member (Gutstadt,
1968). The sample used in this study was collected from the lower laminated member (K.
Milam, pers. comm.). The Beck Springs Dolomite is used in this thesis to represent an unshocked dolostone.
Sample processing
One sample of the Beck Springs Dolomite was cut into 6 pieces using a Hillquist trim saw. The 6 pieces were then cut into less than cm-sized pieces in order to eliminate the need to further crush the sample using a hammer to avoid artificially deforming the crystal lattices in each sample. Each aliquot was then divided into 3 portions, one ground in a mechanical pulverizer (an Angstrom model TE 250 ring pulverizer), one ground, by hand, dry in a mortar and pestle, and the third ground, by hand, in solution with alcohol in a mortar and pestle (as is the usual method for sample processing for XRD analyses)
(Foit, 1992).
Using these three methods of grinding, the samples were then ground into powders and sieved to a particle size of <25μm. This size fraction was used in order to
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minimize bias in X-ray diffraction data that has been shown to occur due to crystal lattice
orientation effects (Warren, 1969). This is important because, if the lattices are not
randomly oriented, the diffraction patterns will only reflect the one or a limited number
of orientations that the crystals may have been in when the rock was formed. Grinding
minerals into powders this fine though may deform the crystal lattice which in turn could
enhance the peak broadening (Hanss et al., 1978).
After all of the aliquots were ground by all 3 methods, 20 diffraction patterns
were obtained for each aliquot in the Rigaku MiniFlex benchtop X-ray diffractometer
(sampling parameters in Table 1) to obtain a mean diffraction pattern. Having twenty Table 1: Sampling parameters for XRD
diffraction patterns also allows for the use of statistical analyses in order to observe if the
differences in intensity values are significant.
Once the diffraction patterns were collected, Reitveld peak refinements were
performed for each aliquot in order to compare the amount of peak broadening. The
Reitveld method is a method of analyzing XRD data where the peaks are refined using a
least-squares approach (McCusker et al., 1999). This method allows for analysis of
patterns containing overlapping Bragg diffraction peaks. Using the Reitveld method, each
peak’s full width at half maximum (FWHM) values was determined (this means, for each
peak the entire width is measured at half the height of the peak) (McCusker et al., 1999).
This method makes it easier to view differences in the amount of shock effects for each
sample and to distinguish impact deformed rocks from tectonically-deformed rocks
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(Huson et al., 2009). In the Huson (2009) study, the tectonically deformed Mission
Canyon Formations of dolomite were compared with shocked samples from the Sierra
Madera impact crater and show less peak broadening than the shocked samples from the central uplift. Figure 6 is an example of a FWHM plot of experimentally shocked vs. unshocked dolomite. Note that the magnitude of peak broadening (as expressed by
FWHM values) appears to increase with increasing peak shock pressures.
Part 2: Shocked Dolostone
Sample collection
The second part of this thesis seeks to determine if sample processing can overprint the effects of shock metamorphism in naturally shocked samples. Samples of the Peebles Dolomite from The Serpent Mound impact structure were collected at the sites shown in Figure 7 also without the use of a hammer. Samples SMPD 17 and SMPD
22 are of the informal Serpent Mound Breccia (SMB) member while SMPD 16 and
SMPD 21 are of the (informal) “Vuggy” member. The Serpent Mound impact structure represents the remnants of a complex impact crater located in southwestern Ohio (Figure
7) at the corner of Pike, Highland, and Adams counties. The age of the structure has been constrained from Upper Devonian/Lower Mississippian (Milam et al., 2010) to Upper
Mississippian/Lower Permian (Watts, 2004). Evidence that Serpent Mound is an impact structure includes: shatter cones (Dietz, 1960; Cohen et al., 1961), shocked quartz
(Carlton et al., 1998; Koeberl et al., 1998; Schedl, 2006) and coesite in samples from the central uplift (Cohen et al., 1961; Gaddis et al., 2001).
Sample processing
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Information about sample processing and its effects from the first part of the thesis played a key role. Diffraction patterns from 4 samples from the Serpent Mound impact structure, 2 from the central uplift (SMPD 21 and 22) and 2 from the outer edge of the structure (SMPD 16 and 17) were obtained in order to identify the major minerals and to evaluate the magnitude of shock metamorphism. The method of sample processing used for the XRD is the same as part one based on the method that produced the least amount and most quantifiable amount of peak broadening (see results section below). In order to assess whether or not there has been diagenetic alteration of the Peebles
Dolomite that would completely erase any shock effects, thin sections were made and petrographic observations were performed to obtain a general comparison of the amount of fractures, cleavage, and twinning in the samples. If post-impact diagenetic alteration has not occurred enough to completely recrystallize the rocks, the samples from the central uplift should hypothetically contain higher amounts of fractures, cleavage, and possibly twinning than the samples from the rim. We do know, however that some amount of diagenetic alteration has occurred and can be seen on the macroscale in the
Peebles Dolomite in features including: vugs created by dissolution, secondary calcite, and a generally pock-marked appearance (Swinford, 1985).
After all of the XRD data and Reitveld peak refinements were obtained for both the unshocked and shocked samples, statistical analyses were used in order to find whether the different grinding times produced diffraction patterns that have significantly different intensities. This was assessed using the Student’s T-test (equation 1). In equation 1, X is the mean intensity value of one angle in an aliquot’s diffraction pattern
(X1), and the intensity at the same angle in the mean diffraction pattern of another aliquot
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(X2), σ is the standard error of an aliquot (σ1), and the standard error of the second aliquot (σ2), n equals 20 (as the means and standard deviations are being compared for 20 diffraction patterns), and t-calc is the measure of the size of the difference relative to the variation in the intensities.
|푥1 − 푥2| 푡푐푎푙푐 = 휎2 휎2 √ 1 + 2 푛1 푛2
The statistical cut off used in this study was p=.005 having a t-value of 3.174, however after a Bonferroni Correction was done, the significance value was .9717. For each aliquot the intensity value at the 2θ angle of 31°, 41°, and 50° plus or minus 1° (as these are the most intense Kα peaks for dolomite) were compared for each amount of time spent grinding. Using the T-test on these data allows the calculation of the differences to be represented in units of standard error (the greater the magnitude of T the greater the evidence that there is significant difference). For the FWHM plots the standard deviation was calculated for all of the curves in order to find whether or not the differences in the values were significant or if they were all within the error of one another. This statistical information is important to take into consideration because, while the results may qualitatively appear to be different for each aliquot and sample ground for the differing amounts of time, the differences may not be statistically significant.
Results
Part 1: Unshocked Dolostone
X-Ray Diffraction of Unshocked Dolostone
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Full XRD diffraction patterns for the unshocked aliquots of Beck Springs
Dolomite are found in Appendix 1 and the FWHM values are in Appendix 2. Figure 8 shows an averaged portion of XRD patterns and the FWHM values for the unshocked aliquots processed in the mechanical pulverizer. These are only depicted for 2θ angles of
20° through 55° in order to show the 2θ range of primary dolomite diffraction peaks (at
31°, 41° and 50°). In Figures 9 and 10, the (constrained) XRD patterns and the FWHM values for the aliquots ground in by mortar and pestle (dry) and in the mortar and pestle in solution with alcohol (wet) can be observed, respectively. These have, again, have been constrained to focus on the 2θ range for primary dolomite diffraction peaks.
Aliquots ground in the mechanical pulverizer show a systematic increase in peak broadening and decrease in peak intensity with increased grind time. Aliquots ground in the mortar and pestle; however, do not display the same systematic trends (Figures 9 and
10).
When the intensities at the 3 main dolomite peaks (31°, 41°, 50°) are compared for each method of grinding, they have very different values. The aliquot ground for 3 minutes in the mechanical pulverizer has a peak intensity at a 2θ angle of 31° of 4,326.55 counts per second (cps), while the aliquot ground for 3 minutes in the mortar and pestle dry at the same angle has a peak intensity of 5,434.4 cps, and the aliquot ground for 3 minutes in the mortar and pestle in solution with alcohol has a peak intensity at that angle of 2,765.85 cps. At the 2θ of 41° the aliquot ground for 3 minutes in the mechanical pulverizer had an intensity of 1166.65 cps, the mortar and pestle dry had an intensity of
1437.2 cps, and the mortar and pestle in solution with alcohol had an intensity of 981.65 cps. At the 2θ angle of 50° the aliquot ground for 3 minutes in the mechanical pulverizer
Page | 15 had an intensity of 840.25 cps, the mortar and pestle dry had an intensity of 1265.65 cps, and the mortar and pestle in solution with alcohol had an intensity of 903.65 cps. These results show that for most, but not all primary diffraction angles, the aliquot ground by the standard method of sample (in solution with alcohol in mortar and pestle) (Foit, 1992) has the lowest peak intensity after being processed for 3 minutes. While this may be observed for aliquots ground for 3 minutes, those ground longer than 3 minutes have variable intensities for each method of grinding and no single method consistently produces the lowest intensities at each time interval.
Statistical Analysis
For each method of grinding at each time interval, 20 diffraction patterns were obtained. This allowed for the mean diffraction pattern to be used in analyses and to avoid any issues that could have arisen due to machine drift. In Appendix 1 the standard deviations for all 20 diffraction patterns have been calculated for each aliquot. Generally, all 20 diffraction patterns have similar intensity values when compared at the same 2θ angles producing low standard deviation. Where drift within the machine has occurred, the intensity values have been shifted ±1° causing the standard deviation to increase at these angles. This shift in intensity values is likely a result of movement within the XRD machine during the 17 hours it takes to obtain all 20 diffraction patterns.
T-test analyses performed on unshocked XRD data can be found in Appendix 1 under the tabs titled t-test’ for each method of grinding. For each method of grinding, 45 t-tests were performed at 31°, 41°, and 50°, respectively. Because of the analyses performed on the standard deviations within the 20 diffraction patterns for each aliquot and the possibility of machine drift during data collection, all of the values around angles
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31°, 41°, and 50° were collected from ± 1° of the angle. Of these 135 t-test analyses performed for the unshocked samples, none of the t-calc values were below the critical value of t=.9717. This shows that 100% of the unshocked XRD intensities are significantly different from each other which indicates that grinding method and grinding time produce different results that are significantly different. This indicates that grind time and method matter when using XRD.
The ±1 sigma standard deviations of the unshocked FWHM values can be found in Appendix 2 under the tabs titled ‘standard deviations’ for each method of grinding.
When all of the FWHM standard deviation values are compared together for the mechanical pulverizer, there is very little overlap between grind times. Aliquots ground in both the mortar and pestle (wet) and the mortar and pestle (dry) show overlap of all of the grind times, making each amount of time indistinguishable from any other. For the unshocked aliquots ground in the mechanical pulverizer, there is a large separation in the
FWHM values for the 3 minutes and the 18 minutes. As observed in Figure 11, the aliquot ground for 3 minutes has substantially less peak broadening than the aliquot ground for 18 minutes. The 3 minute and the 18 minute aliquots ground in the mortar and pestle dry have overlapping standard deviations making the amount of peak broadening between the 2 time intervals indistinguishable (Figure 12). In contrast, the unshocked 3 minute aliquot ground in the mortar and pestle in solution with alcohol exhibits more peak broadening than the 18 minute aliquot with almost no overlap of the standard deviations as observed in Figure 13. Based on the qualitative analysis of the XRD patterns and FWHM values and the statistical analysis of this unshocked data, the 3
Page | 17 minute processing with the mechanical pulverizer was selected as the preferred method for processing Serpent Mound samples to minimize the effects of sample processing.
Part 2: Shocked Dolostone
Petrography and X-Ray Diffraction of Shocked Dolostone
Petrography
Photomicrographs of SMPD 16 (rim), 17 (rim), 21 (uplift), and 22 (uplift) are shown in Figure 14. All 4 samples are dolomitic mudstones. Because these rocks are so fine-grained, the amount of fracturing, cleavage, and twinning was difficult to ascertain.
Twins in carbonate minerals have highly variable sizes depending on the pressures and temperatures reached and can often be thicker than 10µm (Burkhard, 1993). Grains in the Serpent Mound samples are all around this size in diameter, or even smaller (Figure
14), precluding the ability to identify potential twinning. Cleavage was not observable in these thin sections, though it is possible that it is present and was just not able to be observed clearly due to the small grain size. Fracturing within grains was also not observed due to this small grain size even at higher magnification (60x) than that of the photomicrographs. Fracturing across grains should be able to be detected if present, despite the small grain size in these samples because grain boundaries are still able to be seen, however it was not observed.
XRD and FWHM
All of the XRD results for the shocked samples are found in Appendix 3 and the
FWHM values are in Appendix 4. The method of grinding used for sample processing of the shocked samples was the mechanical pulverizer based on the results obtained from the unshocked part of this thesis. Figures 15, 16, 17, and 18 show the XRD patterns and
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FWHM values for Serpent Mound samples SMPD 16, 17, 21, and 22 at different grind times, respectively. These plots are focused at the 2θ range (20° to 55°) for the primary dolomite diffraction peaks in order to observe the patterns more closely. There are no easily identifiable patterns correlating grind time with peak broadening or intensity (this will be discussed further in the following section).
Statistical Analysis
The statistical analyses performed on all of the shocked XRD data can be found in
Appendix 3 under the tabs titled t-test’ with the respective sample names. Again, like with the unshocked data, 20 diffraction patterns were obtained for each grind time allowing a mean diffraction pattern to be used and for standard deviations to be calculated. The standard deviations can be found in Appendix 3 for all samples. Similar to the unshocked data, the shocked samples also generally lie within the ±1 sigma standard deviations. Where they don’t is likely a result of machine drift and this has again been taken into consideration when doing the t-test analyses. When performing the t-tests on the shocked data, all 20 diffraction patterns were used to calculate the mean diffraction pattern at each angle in order to compare the mean at that angle for 1 grind time with the same angle of another grind time. Of the 180 t-tests done (60 at 31°, 60 at
41°, and 60 at 50°) (more t-tests were done at each angle here because there are more shocked samples than there were unshocked samples), 5 t-calc values were found to be not significantly different. This means that for the angles analyzed, the XRD peak intensities for each amount of time spent grinding for all four Serpent Mound samples are significantly different 99% of the time. This indicates that the amount of time spent
Page | 19 grinding the samples and the location that the samples were collected effects the crystal lattice enough to produce different diffraction patterns.
The ±1 sigma standard deviations of the shocked FWHM values can be found in
Appendix 4 under the tabs titled ‘standard deviations’ with the respective sample names.
Results show that for all of the shocked samples, when the peak broadening values for each amount of time are all plotted together, they overlap and are difficult to distinguish from one another. However when only the 3 minute and the 18 minute aliquots are compared they can be easily separated for SMPD 16 (rim) and 17(rim) where the 3 minutes has less peak broadening and the 18 minutes has more seen in Figure 19. SMPD
21(uplift) actually shows 3 minutes to have more peak broadening than 18 minutes. The standard deviations for the 3 minutes and 18 minutes of SMPD 22 (uplift) are completely overlapped showing no difference in FWHM values seen in Figure 20.
Discussion
Hypothesis 1: Grind time influences peak broadening
The aliquots ground for 3 minutes have peak intensities that are approximately
50% higher than the aliquots ground for 18 minutes. This is observed for all methods of sample processing (mechanical pulverization, hand ground), as observed in the XRD patterns in Figures 21, 22, and 23. Although there is a decrease in intensity for the higher grind times, peak broadening is not always increased. Peak broadening is greater for the aliquot ground for 18 minutes than the aliquot ground for 3 minutes by mechanical pulverizer (Figure 21). Samples ground by hand (dry) display the same trend (Figure 22).
In contrast, samples ground by hand in the presence of alcohol display the predicted trend
Page | 20 at lower 2θ values (Figure 23). This increase in peak broadening and decrease in peak intensity for longer duration grind times observed in both the mechanical pulverizer and the dry ground mortar and pestle indicate that more sample processing leads to more deformation within the crystal lattice.
Hypothesis 2: Mechanical vs. hand pulverization
Mechanical pulverization of a sample using gradually increasing grind times leads to decreases in peak intensity and increases in peak broadening that are more consistent than samples that were hand ground by mortar and pestle. A consistent decrease in intensity per 3 minute grind duration interval (ex. from 3 to 6 min, from 6 to 9 min, etc.) of prominent dolomite diffraction peaks of an average of 17% at a 2θ angle of 31⁰, 15% at 41⁰, and 13% at 50⁰ were observed in samples ground by mechanical pulverizer
(Figure 8). For aliquots ground for different durations by hand (Figures 9 and 10) there are no systematic or consistent trends of decreasing peak intensity and increasing peak broadening with increased grind time. Comparing the two methods (mechanical pulverizer vs. mortar and pestle), it is clear that the mechanical pulverizer produces the most consistent decreases in intensity and increases in peak broadening, even though it appears that mechanical pulverization is more likely to produce more deformation that results in a reduction in peak intensities than comminution by hand (as shown by comparing diffraction peak intensities in Figures 8-10). The mechanical pulverizer likely moves with the same motion and experiences consistent peak grind pressures throughout the duration of pulverization, whereas when grinding by hand, peak pressures and motion are thought to be less consistent. The type of motion is also likely a factor in the deformation introduced to the crystal lattice. Both the mechanical pulverizer and the
Page | 21 mortar and pestle will exert a shear force on the rock as it is being ground. The mechanical pulverizer may also have a normal (perpendicular) force as the pieces are being crushed between the rings. In the mortar and pestle, the person grinding the rocks may hit the larger pieces in order to break them down, which could induce similar effects to the crystal lattice as a hammer strike, possibly creating extra lattice deformation.
Hypothesis 3: The potential overprint of shock metamorphic signatures
Sample processing for the shortest duration grind times may not completely overprint the signs of shock metamorphism in naturally shocked samples at lower duration grind times. In Figure 24, all of the Serpent Mound samples ground for 3 minutes are plotted with the unshocked Beck Springs Dolomite sample that was ground for 3 minutes in an effort to compare shocked and unshocked samples. SMPD 17 (rim) appears to have higher intensities than the unshocked sample, whereas SMPD 16 (rim),
21 (uplift), and 22 (uplift) all have much lower intensities than the unshocked sample.
The ±1 sigma standard deviations for SMPD 17 and the unshocked Beck Springs ground for 3 minutes can be observed in Figure 25 and show that these two samples do not overlap and are significantly different. All of the other Serpent Mound samples have intensities that are much lower and easily separable from the unshocked sample. This may suggest two things. First, SMPD 17 may have experienced more recrystallization following impact than the other samples that has effectively erased the shock signatures and second, when ground for 3 minutes in a mechanical pulverizer, shocked samples with little to no post impact diagenesis are differentiable from unshocked samples ground using the same method. SMPD 17 likely has higher intensities than the other shocked samples because of possible post impact diagenetic alteration. It could also have higher
Page | 22 intensities than the unshocked Beck Springs samples because the Beck Springs may have experienced more tectonic deformation than the Peebles Dolomite. The Beck Springs
Dolomite was deposited during the second of three basin-forming events with basin inversion occurring during each event, and syn-sedimentary fault formation (Smith et al.,
2015). This basin-forming event and inversion after deposition could have created more tectonic strain on the Beck Springs producing internal crystal lattice deformation which is why it has lower intensities than SMPD 17.
Figure 26 shows that for 3 minutes of sample processing, SMPD 17 (rim) has higher intensity and less peak broadening than SMPD 22 (uplift); the same is observed for SMPD 16 (rim) and 21(uplift) in Figure 27. Shocked samples are all of the Peebles
Dolomite however; they come from different members of this unit and therefore could have had different diagenetic histories. SMPD 16 and 21 (from the Vuggy member) have much lower intensities and much higher peak broadening than SMPD 17 and 22 (from the SMB member). These differences in intensities and peak broadening may indicate that the SMB member and the Vuggy member could have had different levels of post- impact diagenetic alteration. This is consistent with the macroscale textures observed in the field of the Vuggy member, which has much more dissolution and secondary crystallization than the SMB member appears to have. This is also consistent with the evidence presented in the preceding paragraph that SMPD 17 (SMB member) has higher peak intensities than, not only the other shocked samples, but also the unshocked sample.
While the different members of the Peebles Dolomite are being compared separately because of the possibility of differing diagenetic histories, the photomicrographs in
Figure 14 do not show textures that indicate different levels of deformation, however the
Page | 23 small grain size makes deformation difficult to ascertain with the petrographic microscope. Additional work with higher resolution instruments may shed more light on the potential deformation levels
As discussed in the previous paragraph, when the samples ground for 3 minutes are compared, the rim sample of the SMB member has greater peak intensity and less peak broadening than the SMB sample from the uplift. The same is true for the Vuggy member. This supports the idea that at the low duration grind time of 3 minutes, sample processing may not completely overprint the signs of shock metamorphism in these naturally shocked samples. For grind times higher than 3 minutes though, there is no observable trend of decreased peak intensities with samples closer to the crater center.
This can be observed in Figure 28, which shows the XRD patterns for the shocked samples ground for 6 minutes. Shocked samples ground for longer than 6 minutes have similar XRD patterns to those ground for 6 minutes and show no observable trends. This could be an indication that for longer times spent grinding, sample processing deforms the crystal lattice, effectively overprinting peak broadening which may be attributable to shock metamorphism.
Summary
Results from this study indicate that the method of sample processing and the amount of time spent grinding a sample in preparation for X-ray diffraction studies may result in deformation in the dolomite crystal lattice, which, in turn, results in discernible effects to their associated X-ray diffraction peaks. For unshocked samples, aliquots ground by mechanical pulverization were found to have the highest magnitude of peak intensity reduction and the most consistent decreases in peak intensity and increases in
Page | 24 peak broadening for each increase in time. Unshocked aliquots that were ground by hand had no observable trends in peak intensity reduction or peak broadening with differing durations of grind time. Overall, aliquots ground dry in the mortar and pestle had the lowest values of peak broadening. The aliquots ground using a ring pulverizer and the aliquots ground in the mortar and pestle in solution with alcohol had slightly higher, but very similar values of peak broadening. Therefore, mechanical pulverization using a ring pulverizer for 3 minutes for processing shock metamorphosed samples and was used to process the shocked samples used to test hypothesis 3.
For the second part of this thesis involving shocked samples from Serpent Mound impact structure, samples were processed in the mechanical pulverizer because the error analyses of the FWHM values indicated that the mechanical pulverizer produced values for each grind time that did not overlap and were differentiable. The t-test analyses performed on the XRD data indicated that the intensity values were significantly different. The mechanical pulverizer also produced the most consistent decreases in peak intensity and increase in peak broadening for each increase in time interval which is why it was the method of grinding used on the shocked samples. Results from the shocked samples indicate that at the lower duration grind time of 3 minutes, the shock signatures from the impact may be detectable, however at the longer duration grind times (6-18 minutes), no trends or patterns can be observed indicating that the longer sample processing may be adding unpredictable amounts of deformation to the crystal lattice, resulting in variable diffraction peak intensities and peak broadening values that are reflective of sample processing, not shock metamorphism.
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References
Bell, M.S., Horz, F., and Reid, A., 1998, Characterization of experimental shock effects in calcite and dolomite by X-ray diffraction: Abstract of the Lunar and Planetary Science Conference XXIX, Lunar and Planetary science Institute, Houston, Texas They experimentally shocked limestone and dolomite. They found that the shocked samples were characterized by significant peak broadening. They also found that intensity decreases with increasing shock pressure. Burkhard, M., 1993, Calcite twins, their geometry, appearance and significance as stress-strain markers and indicators of tectonic regime: a review: Journal of Structural Geology, v. 15, nos. 3-5, p. 351-368. They found that thickness of twins is mainly a function of deformation temperature. With increased shear stress, temperature, or confining pressure, enlargement of an existing twin could be easier than the initiation of a new one. Carlton, R.W., Koeberl, C., Baranoski, M.T., and Shumacher, G.A., 1998, Discovery of microscopic evidence for shock metamorphism at the Serpent Mound structure, south central Ohio: Confirmation of an origin by impact: Earth and Planetary Science Letters, v. 162, p. 177-185. Examined drill core from Serpent Mound. They did a general work-up of the petrology and mineralogy in the drill core. They found the presence of shatter cones, and PDFs which they interpreted to indicate a range of shock pressures from ~2 GPa to ~10 GPa. Clarke, F.W., Washington, H.S., 1924, The composition of the Earth’s crust: Washington government printing office, professional paper 127. This book is a write up of the composition of the Earth’s crust. It goes into great detail about the rock types, minerals, and chemical constituents found in the crust. It also details the relative abundances of each of these. Cohen, A.J., Bunch, T.E., and Reid, A.M., 1961, Coesite discoveries establish cryptovolcanics as fossil meteorite craters: Science, v. 134, no. 3490, p. 1624- 1625. Found coesite in the central peaks of multiple impact craters. Formerly “cyrptovolcanic” structures are now accepted as impact craters due to the association of the high pressure polymorph of silica being present. Found coesite present in Serpent Mound shatter cones from the central uplift providing more evidence for Serpent Mound being an impact structure. Dietz, R.S., 1960, Meteorite impact suggested by shatter cones in rock: Science, v. 131, no. 3416, p. 1781-1784. Discussed the need for evidence that can differentiate volcanic or tectonically deformed structures from impact structures. Proposed that the only way to have shatter cones present is the propagation of a shock wave which would require a material to strike the Earth with a velocity of ~15,000m/s. Discussed finding shatter cones at multiple proposed impact structures on Earth. Concluded that shatter cones associated with cryptoexplosional structures are concentrated at ground zero (central uplift). Foit, F.F., 1992, X-Ray and optical data for a vanadium-rich dravite from Silver Knob, Mariposa County, California, USA: Powder diffraction, v. 7, no.4, p. 236. In this paper they were performing X-ray diffraction on a dravite from Silver Knob, Mariposa County, California. The importance of this paper is the technique that was used for grinding the samples for XRD. This was the first paper where alcohol was used to form a paste as samples were being ground and this has become the standard technique for sample processing for XRD.
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French B. M., 1998, Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures: LPI Contribution No. 954, Lunar and Planetary Institute, Houston. 120 pp. This is a review book. It includes explanations and definitions of all aspects of impact cratering. It explains shock metamorphism in both silicates, as well as carbonates. This text goes into great depth on every topic involved in impact cratering. Gaddis, S.J., Angerman, C.E., Wisdom, E., and Hughes, J., 2001, Origin of the Serpent Mound Cryptoexplosion structure, South-Central Ohio: XRD and Re-Os isotope evidence: Abstracts of the Eleventh Annual V.M. Goldschmidt Conference, Roanoke, VA, May, 2001. In this paper they discuss seeing increases in peak with of shatter coned carbonates found at the central peak of Serpent Mound. They found peaks for coesite present in these shatter cones, which indicates that very high pressures and temperatures were experienced here. The Re-Os isotopes, however, did not show clear evidence of meteorite impact. Grieve, R.A.F, Robertson, P.B., and Dench, M.R., 1981, Constraints on the formation of ring impact structures based on terrestrial data, Multi-ring Basins: Formation and Evolution, Proc. Lunar Planet. Sci. p. 37-57. This paper discusses the three different types of complex craters that can form. They describe each type of complex crater and also give diameter constraints for each type. They give examples of the different types of complex craters from Earth. Gutstadt, A.M., 1968, Petrology and depositional environments of the Beck Spring Dolomite (Precambrian), Kingston Range, California: Journal of Sedimentary Research, c. 38, no. 4, p. 1280-1289. This paper discusses the petrology of the Beck Springs Dolomite. It goes into detail about the three different groups that encompass the Beck Springs. This paper also discusses the depositional environment of the Beck Springs. Hanss, R.E., Montague, B.R., Davis, M.K., Galindo, C., and Horz, F., 1978, X-ray diffractometer studies of shocked materials: Proc. Lunar Planet. Scie Conf. 9th, p. 2773-2787. Single crystal quartz, oligoclase, enstatite, orthoclase, granodiorite, and dunite were experimentally shocked. They found that the diffraction peaks decrease in amplitude and increase in line width with increasing peak shock pressure. They also found that component grains in polycrystalline targets appear to suffer the same amount of shock damages as the single crystal equivalents. Hull, D., Bacon, D.J., 2001, Introduction to dislocations: Butterworth-Heinemann Publications. 5th edition. 257 pp. This is a text book discussing dislocations in carbonate minerals. It explains that dislocations form in the crystal lattice due to increase in pressure. It discusses what processes can cause dislocations to form. This text explains how tectonic activities can cause dislocations to form in carbonates, along with the fact that a shock wave could also cause dislocations to form. Huson, S.A., Foit, F.F., Watkinson, A.J., and Pope, M.C., 2009, Reitveld analysis of X- ray powder diffraction patterns as a potential tool for the identification of impact- deformed carbonate rocks: Meteoritics and Planetary Science, v. 44, no. 11, p. 1695-1706. They compare the X-ray diffraction patterns of calcite and dolomite from the Sierra Madera impact structure and those of unshocked equivalent carbonates from outside of the structure. Samples from inside the crater show more peak broadening than those from outside of the crater. They propose that Reitveld refined parameters can potentially be used to distinguish impact-shocked rocks from unshocked equivalents. They conclude
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however, that shock pressures much reach at least 3-10 GPa to distinguish between shocked rocks and those that are deformed by tectonic processes. Koeberl, C., Buchanan, P.C., Carlton, R.W., 1998, Petrography and geochemistry of drill core samples from the Serpent Mound structure, Ohio: confirmation of impact origin (abstract): Abstracts of the 29th Lunar and Planetary Science Conference, Lunar and Planetary Science Institute, Houston, TX. Examined the same two drill cores from Serpent Mound as Carlton et al., 1998 (one from the central uplift and one from the crater floor). They described the petrology and mineralogy present in the drill core from the central uplift. They found PDFs in quartz and using the PDF orientations they were able to determine relatively high shock pressures were experienced here. They concluded that the structure was formed by a hypervelocity impact. Lange, M.A., and Ahrens, T.J., 1986, Shock-induced CO2 loss from CaCO3; implications for early planetary atmospheres: Earth and Planetary Science Letters, v. 77, p. 409-418. They agree with earlier hypothesis that the atmospheres on Earth, Venus, and mars could have been formed by impact-induced release of CO2 from carbonates. The results of thermogravimetric analysis demonstrate a relationship between shock-induced CO2 loss and peak pressure. Some of the release gas gets trapped in partially molten areas where it leads to vesicular textures. Langenhorst, F., 2002, Shock metamorphism of some minerals: Basic introduction and microstructural observations: Bulletin of the Czech Geologic Survey, v. 77, no. 4, p. 265-282. They define a shock wave as an extreme compression wave that propogates with supersonic velocity, abruptly compresses, heats, and plastically deforms solid matter. They explained that a number of shock effects are possible in minerals and which shock effects are present depends on the crystal structure and chemical composition. Martini, J.E.J, 1978, Coesite and stishovite in the Vredefort Dome, South Africa: Nature, v. 272, p. 715-717. In this paper Martini discussed the discovery of coesite and stishovite in very thin peudotachylite veins in the Witwatersrand quartzites of the collar of the Vredefort Dome. Martini explains what coesite and stishovite are and gives pressure and temperature ranges for their formation. McCusker, L.B., Von Dreele, R.B., Cox, D.E., Louer, D., and Scardi, P., 1999, Reitveld refinement guidelines: J. Appl. Cryst, v. 32, p. 36-50. They did studies to assess the accuracy and precision of the parameters obtained in a Rietveld refinement. They defined the 9 specific details that need close attention when doing a Reitveld refinement which are: data collection, back-ground contribution, peak- shape function, refinement of profile parameters, Fourier analysis with powder diffraction data, refinement of structural parameters, use of geometric restraints, calculations of e.s.d.’s, and interpretation of R-values. Melosh, H.J., 1989, Impact Cratering: A Geologic Process: New York, Oxford University Press, 245p. This is a review text book that explains all processes related to impact cratering. It describes the stages of impact cratering, as well as the types of craters that can form. It goes into great detail regarding all of the aspects of impact craters. Milam, K.A., and Gabreski, C., 2010, Evidence of maximum age of the Serpent Mound Impact event from shatter cones: The Ohio Journal of Science, v. 110, no. 3, p. 53-54. They hypothesized that it is possible that what appears to be structural offset in younger Devonian to Mississippian strata at Serpent Mound may relate to post-impact deposition on an uplifted crater rim and excavated crater floor. This would open the possibility of a
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Late Silurian to Early Devonian impact event. Through more study at Serpent Mound, they found this hypothesis to be incorrect, and instead propose a maximum age of the impact structure to be Upper Devonian to Lower Mississippian. Schedl, A, 2006, Applications of twin analysis to studying meteorite impact structures: Earth and Planetary Science Letters, v. 244, p. 530-540. This paper describes twin analyses of dolomite and calcite. This was used to estimate the amount of erosion of the Serpent Mound Impact Structure. Shuster, A.M., Wallace, M.W., Hood, A.vS., and Jiang, G., 2018, The Tonian Beck Springs Dolomite: Marine dolomitization in a shallow, anoxic sea: Sedimentary Geology, v. 368, p. 83-104. This paper discusses the petrology of the Beck Springs Dolomite and goes into detail about the three groups that make up the Beck Springs. This paper also discusses the marine dolomitization of the Beck Springs. Skala, R., Horz, F., Jakes, P., 1999, X-ray powder diffraction study of experimentally shocked dolomite: Abstract of the Lunar and Planetary Science Conference XXX, Lunar and Planetary science Institute, Houston, Texas. They experimentally shocked dolomite in order to determine the amount of peak broadening that can be seen. Figure 9 of this paper comes from this abstract and shows the amount of peak broadening increases systematically with increased shock pressure applied. Skala, R., Matejka, P., Ederova, J., and Horz, F., 2000, Mineralogical investigations of experimentally shocked dolomite: implications for the outgassing of carbonates: Geological Society of America Special Papers, v. 2002, no. 356, p. 571-585. They studied experimentally shocked dolomite at differing pressures to obtain the amount of outgassing that occurs due to shock metamorphism. Their results indicate that variable amounts of CO2 is released during shock loading and associated shock heating. Smith, E.F., MacDonald, F.A., Crowley, J.L., Hodgin, E.B., and Schrag, D.P., 2015, Tectonostratigraphic evolution of the c. 780-730 Ma Beck Springs Dolomite: Basin Formation in the core of Rodinia: Geological Society Publications, v. 424, p. 213-239. This is a paper discussing the tectonic and stratigraphic evolution of the Beck Springs Dolomite. It discusses the processes acting upon this rock unit throughout geologic history. Swinferd, E.M, 1985, Geology of the Peebles Quandrangle, Adams County, Ohio: Ohio Journal of Science, v. 85, i. 5. This paper is a general overview of the geology within Adams County, Ohio. This paper contains a stratigraphic column of all of the units contained within Serpent Mound impact structure and describes each unit. Warren, B.E., 1969, X-ray Diffraction: Addison-Wesley Publishing Company, p. 51-74. This is a foundation book on X-ray diffraction and techniques for using an X-ray diffractometer. For this paper, chapter 5 of this book, on powder diffraction was used to learn the techniques for performing powder diffraction and the reasons why samples need to be ground into fine powder. Watts, D.R., 2004, Paleomagnetic determination of the age of the Serpent Mound structure: The Ohio Journal of Science, v. 104, no. 4, p. 101-108. They used paleomagnetic data to determine the age of Serpent Mound. The direction of the magnetization recorded by rocks can be used to date various geological events. They found that the Brassfield formation has magnetization with a high unblocking temperature that was clearly acquired after the impact structure formed. Because the rocks here were remagnetized after the deformation that created the Serpent Mound impact structure, the structure is likely older than 262 Ma.
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Figures:
Figure 1: Schematic diagram showing the stages of impact cratering: from the Planetary Science Institute (https://www.psi.edu/epo/explorecraters/background.htm)
Figure 2: PDFS seen in a quartz grain from the Sierra Madera impact structure (from French 1998)
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Figure 4: Schematic showing kinking in the lattice and multiple types of mechanical twins that are formed (figure 2 from Burkhard 1993) Figure 3: Schematic showing the appearance of shock effects in different minerals at increasing shock pressures (Figure 6 in Langenhorst 2002)
°2θ
Figure 5: X-ray diffraction patterns showing unshocked dolomite (lower peak) compared to dolomite experimentally shocked to different pressures (from Skala et al 2000)
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Figure 6: Map showing location of Serpent Mound impact structure and sample collection sites.
Figure 7: FWHM values of experimentally shocked dolomite from Skala et al 1999 (Figure 2). FWHM plots are obtained by taking a diffraction pattern and taking the full width of the peak at half of the height for every peak (example of this in the right half of this figure) Page | 32
Unshocked Beck Springs Dolomite Mechanical Pulverizer 4500
4000 0.7 3min 0.65 3500 0.6
6min 0.55
3000
0.5 9min FWHM 0.45 2500 12min 0.4 2000 0.35 15min 0.3 Intensity (cps) Intensity 1500 20 30 40 50 18min ⁰2θ
1000
500
0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 ⁰2θ
Figure 8: XRD patterns and FWHM values for all aliquots ground in the mechanical pulverizer (zoomed in to angles 20-55) showing a very consistent trend of decreased peak intensity and increased peak broadening with increased grind time
Unshocked Beck Springs Dolomite Mortar and Pestle (Dry) 6000 0.6 0.55 5000 3min 0.5
6min 0.45
4000 0.4
9min FWHM 0.35 12 min 0.3 3000 0.25 15min
0.2 Intensity(cps) 20 30 40 50 2000 18min ⁰2θ
1000
0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 ⁰2θ
Figure 9: XRD patterns and FWHM values for all aliquots ground dry in a mortar and pestle showing no trend of decreased intensity or increased broadening with increased grind time
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Unshocked Beck Springs Dolomite Mortar and Pestle (Wet) 6000 0.6
0.55 5000 0.5 3 min
0.45
6 min
4000 0.4
FWHM 9 min 0.35 0.3 3000 12 min 0.25
15 min 0.2 Intensity(cps) 20 30 40 50 2000 ⁰2θ 18 min
1000
0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 ⁰2θ
Figure 10: XRD patterns and FWHM values for all aliquots ground in the mortar and pestle in solution with alcohol showing no trends of decreased intensity or increased broadening with increased grind time
Unshocked Beck Springs Dolomite Mechanical Pulverizer Standard deviations 3min vs 18min 1.2 mean FWHM of 3 min 1.1 Plus 1 sigma (3) Minus 1 sigma (3) 1 mean FWHM of 18min 0.9 Plus 1 sigma (18)
Minus 1 sigma (18) 0.8
FWHM 0.7
0.6
0.5
0.4
0.3 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54
°2θ
Figure 11: Unshocked Beck Springs dolomite mechanical pulverizer standard deviations 3 minutes (red) vs. 18 minutes (pink) showing that the 3 min aliquot has much lower peak broadening than the 18 min aliquot
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Unshocked Beck Springs Dolomite Mortar and
1.2 Pestle (Dry) Standard Deviations 3min vs 18min mean FWHM of 18min MP Plus 1 sigma (18) 1 Minus 1 sigma (18) mean FWHM of 3min MP 0.8 Plus 1 sigma (3)
Minus 1 sigma (3)
0.6 FWHM
0.4
0.2
0 20 25 30 35 40 45 50 55 °2θ Figure 12: Unshocked Beck Springs dolomite mortar and pestle (dry) standard deviations 3 minutes (red) vs. 18 minutes (pink) showing total overlap in the peak broadening of both aliquots
Standard deviations for aliquots ground in the mortar and pestle in solution with alcohol 3min 3 MPA vs 18min 0.7 plus 1 sigma (3min) minus 1 sigma (3min) 0.65 18 MPA plus 1 sigma (18min)
0.6 Minus 1 sigma (18min)
0.55 FWHM 0.5
0.45
0.4 20 25 30 35 40 45 50 55
°2θ Figure 13: Unshocked Beck springs dolomite mortar and pestle in solution with alcohol standard deviations 3 minutes (red) vs. 18 minutes (pink) showing that the 3 min aliquot has higher peak broadening than the 18 min aliquot
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Figure 14: Photomicrographs (XPL) of samples SMPD 16, 17, 21, and 22 at 10x zoom (scale bars are .5mm) (grain size is so small that no deformation features are able to be seen) Shocked Serpent Mound Peebles Dolomite 16 (Crater Rim)
0.7 2500 3 min 0.65
2000 6 min 0.6
9 min FWHM 0.55 1500 12 min 0.5 15 min 0.45 1000 18 min Intensity(cps) 20 30 ⁰2θ 40 50
500
0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54
⁰2θ
Figure 15: XRD patterns and FWHM values for SMPD 16 showing no trend of decreased peak intensity or increased peak broadening with increased grind time (likely a result of the heterogeneity of shock metamorphism)
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Shocked Serpent Mound Peebles Dolomite 17 (crater rim)
5000 0.8 4500 0.7 4000 3min
0.6
3500 6min 0.5
FWHM 3000 9min 0.4
2500 12min 0.3 2000 0.2 intensity (cps) 15min 20 30 40 50 1500 °2θ 18 min 1000
500
0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 °2θ
Figure 16: XRD patterns and FWHM values for SMPD 17 showing a slight trend of decreased peak intensity and increased peak broadening with increased grind time Shocked Serpent Mound Peebles Dolomite 21 (central uplift) 3000 0.7
2500 0.65 3 min 0.6 6 min 2000 0.55
9 min FWHM 0.5 12 min 1500 0.45 15 min
intensity intensity (cps) 0.4 18 min 20 30 40 50 1000 °2θ
500
0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 °2θ
Figure 17: XRD patterns and FWHM values for SMPD 21 showing no trend of decreased peak intensity or increased peak broadening with increased grind time (likely a result of the heterogeneity of shock metamorphism)
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Shocked Serpent Mound Peebles Dolomite 22 (central uplift) 3500
0.75 3000 0.7
0.65
2500 0.6 FWHM 0.55 3 min 2000 0.5 6 min 0.45 9 min 1500 0.4
Intensity(cps) 12 min 20 30 °2θ 40 50 1000 15 min 18 min
500
0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 ⁰2θ
Figure 18: XRD patterns and FWHM values for SMPD 22 showing no trend of decreased peak intensity or increased peak broadening with increased grind time (likely a result of the heterogeneity of shock metamorphism)
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Shocked Serpent Mound Peebles Dolomite 16
0.8 (rim) standard deviations (3min and 18min)
0.7
0.6
0.5
FWHM 0.4
0.3
0.2 20 25 30 35 40 45 50 55 °2θ Poly. (smpd 16 3min) Poly. (plus 1 sigma (3)) Poly. (Minus 1 sigma (3)) Poly. (smpd 16 18min) Poly. (Plus 1 sigma (18)) Poly. (Minus 1 sigma (18))
Shocked Serpent Mound Peebles Dolomite 17 (rim) standard deviations (3min and 18min) 0.8
0.7 0.6
0.5
FWHM 0.4
0.3
0.2 20 25 30 35 40 45 50 55 °2θ
smpd17 3min Plus 1 sigma (3) Minus 1 sigma (3) smpd17 18min Plus 1 sigma (18) Minus 1 sigma (18) Figure 19: FWHM values (dotted lines) with the plus 1 and minus 1 sigma standard deviations (solid lines) for the 3 min and 18 min ground aliquots of SMPD 16 and 17 showing that for both rim samples, the 3 min has lower peak broadening than the 18 min and that they are completely separable with no overlap
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Shocked Serpent Mound Peebles Dolomite 21 (uplift) Standard Deviations 0.65
0.6
0.55
FWHM 0.5
0.45
0.4 20 25 30 35 40 45 50 55 °2θ
smpd 21 3min Plus 1 sigma (3) Minus 1 sigma (3) smpd 21 18min Plus 1 sigma (18) Minus 1 sigma (18)
Shocked Serpent Mound Peebles Dolomite 22 (uplift) Standard Deviations 0.6
0.55
0.5
FWHM 0.45
0.4
0.35 20 25 30 35 40 45 50 55 °2θ smpd 22 3min Plus 1 sigma (3) Minus 1 sigma (3) smpd 22 18min Plus 1 sigma (18) Minus 1 sigma (18)
Figure 20: FWHM values (dotted lines) and the plus 1 sigma and minus 1 sigma standard deviations (solid lines) for the 3 min and 18 min aliquots for SMD 21 and 22 showing that for SMPD 21, 3 min actually has more peak broadening than 18 min, and for SMPD 22, there is complete overlap of the peak broadening for the two times
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Unshocked Beck Springs Dolomite Mechanical Pulverizer 4500
4000 1 0.9
3500 0.8
0.7 3000
0.6 FWHM 2500 0.5 0.4 2000 0.3
Intensity(cps) 20 40 60 80 100 120 1500 ⁰2θ 3min 1000 18min 500
0
24 20 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96
116 100 104 108 112 120 ⁰2θ
Figure 21: XRD patterns and FWHM values for aliquots ground in the mechanical pulverizer showing that the aliquot ground for 18 minutes has peak intensity of approximately half that of the aliquot ground for 3 minutes, the peak broadening also shows this trend
Unshocked Beck Springs Dolomite Mortar and Pestle (Dry) 6000 0.8
5000 0.7
0.6
4000 0.5
FWHM 0.4
3000 0.3
0.2 Intensity(cps) 2000 20 40 60 80 100 120 ⁰2θ 3min 1000 18min
0
52 20 24 28 32 36 40 44 48 56 60 64 68 72 76 80 84 88 92 96
104 108 112 116 120 ⁰2θ 100
Figure 22: XRD patterns and FWHM values for aliquots ground in the mortar and pestle dry showing that the aliquot ground for 18 minutes has peak intensity of approximately half that of the aliquot ground for 3 minutes, however the peak broadening does not show this trend Page | 41
3000 Unshocked Beck Springs Dolomite Mortar and Pestle (Wet)
2500 0.9
0.8 2000
0.7
FWHM 0.6
1500 0.5
Intensity(cps) 0.4 20 40 60 80 100 120 1000 ⁰2θ
3 min 500 18 min
0 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96 100104108112116120 ⁰2θ
Figure 23: XRD patterns and FWHM values for aliquots ground in the mortar and pestle in solution with alcohol showing that the aliquot ground for 18 minutes has peak intensity of approximately half that of the aliquot ground for 3 minutes, however the peak broadening does not show this trend Shocked Serpent Mound samples (3min) compared with the Unshocked Beck Springs sample (3min) 4500 0.6 4000 SMPD 16 3 min 0.5
3500 SMPD 21 3 min
3000 0.4 FWHM 2500 SMPD 17 3min 0.3
2000 SMPD 22 3 min 0.2
Intensity(cps) 20 28 36 44 52 1500 °2θ unshocked 3 min 1000
500
0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 °2θ Figure 24: XRD patterns showing all Serpent Mound Samples ground for 3 minutes with the Beck Springs sample ground for 3 minutes to compare shocked vs unshocked peak intensities
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Mechanical Pulverizer: Shocked SMPD 17 (rim) vs. Unshocked Beck Springs 3 min 0.5
0.45
0.4
0.35 FWHM
0.3 smpd17 3min Plus 1 sigma (3) Minus 1 sigma (3) 0.25 3 min unshocked Plus 1 sigma (3) Minus 1 sigma (3) 0.2 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 °2θ
Figure 25: FWHM plots comparing the SMPD 17 (rim) sample and the Unshocked Beck Springs 3 minutes showing that SMPD 17 (rim) has less peak broadening than the unshocked sample ground for the same amount of time
Shocked Serpent Mound Peebles Dolomite 17 and 22 (3min) 5000 0.55 4500 0.5 4000 0.45 3500
0.4 SMPD 17 3min 3000 FWHM 0.35 2500 SMPD 22 3 min 0.3 2000 Intensity(cps) 0.25 1500 20 30 °2θ 40 50 1000
500
0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 °2θ
Figure 26: XRD patterns and FWHM values for SMPD 17 and 22 (3min) showing that the rim sample (17) has higher peak intensity and lower peak broadening than the uplift sample (22)
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Shocked Serpent Mound Peebles Dolomite 16 and 21 (3min) 2500
0.57
2000 0.55
0.53
1500 FWHM 0.51 0.49 SMPD 16 3 min 0.47 1000 Intensity (cps) Intensity 20 30 °2θ 40 50 SMPD 21 3 min
500
0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 °2θ
Figure 27: XRD patterns and FWHM values for SMPD 16 and 21 (3min) (note the FWHM values are zoomed in more to show the difference better) showing that the rim sample (16) has higher peak intensity and lower peak broadening than the uplift sample (21)
Serpent Mound Samples (shocked) 6 minute comparison 3000
2500 SMPD 16 (rim) SMPD 17 (rim)
2000 SMPD 21 (uplift) SMPD 22 (uplift) 1500
Intensity(cps) 1000
500
0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 °2θ Figure 28: XRD patterns for the Serpent Mound samples ground for 6 minutes showing no trend of higher intensity in the crater rim vs. the central uplift
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