Petrographic, X-ray diffraction, and electron spin resonance analysis of deformed calcite: Meteor Crater, Arizona

Item Type Article; text

Authors Burt, J. B.; Pope, M. C.; Watkinson, A. J.

Citation Burt, J. B., Pope, M. C., & Watkinson, A. J. (2005). Petrographic, X￿ray diffraction, and electron spin resonance analysis of deformed calcite: Meteor Crater, Arizona. Meteoritics & Planetary Science, 40(2), 297-306.

DOI 10.1111/j.1945-5100.2005.tb00381.x

Publisher The Meteoritical Society

Journal Meteoritics & Planetary Science

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Link to Item http://hdl.handle.net/10150/655967 Meteoritics & Planetary Science 40, Nr 2, 297–306 (2005) Abstract available online at http://meteoritics.org

Petrographic, X-ray diffraction, and electron spin resonance analysis of deformed calcite: Meteor Crater, Arizona

Jason B. BURT,1†* Mike C. POPE,1 and A. John WATKINSON1

1Department of Geological Sciences, Washington State University, Pullman, Washington 99164, USA †Present address: Department of Geosciences, Virginia Polytechnic and State University, Blacksburg, Virginia 24061, USA *Corresponding author. E-mail: [email protected] (Received 9 June 2003; revision accepted 16 December 2004)

Abstract–Calcite crystals within the Kaibab limestone in Meteor Crater, Arizona, are examined to understand how calcite is deformed during a impact. The Kaibab limestone is a silty fine- grained and fossiliferous dolomudstone and the calcite crystals occur as replaced evaporite nodules with impact-induced twinning. Twinning in the calcite is variable with deformational regimes based on abundances of crystals with twins and twin densities within crystals. The twins are similar to those that are seen in low tectonically deformed regimes. Low levels of shock are inferred from minor peak broadening of the X-ray diffraction patterns (XRD) of the calcite crystals. In addition, electron spin resonance (ESR) spectroscopy data also indicates low shock levels (<3.0 GPa). Quantitative shock pressures and correlation between the XRD and ESR results are poor due to the inferred low shock levels in conjunction with the analytical error associated in calculation of the shock pressures.

INTRODUCTION electron microscopy (TEM) of experimentally shocked samples indicates that dislocations and mechanical twinning Impact-induced shock effects in minerals have been occur in calcite crystals as a result of shock (Ivanov et al. described for over fifty years, but there are few attempts to 2002; Langenhorst et al. 2002). Intense shock-induced categorize the petrologic effects of shock deformation in twinning in carbonate minerals occurs, for example, at calcite crystals. Shock effects in minerals are one of the Meteor Crater and West Hawk Lake Crater, Manitoba (Short primary diagnostic indicators of an . There are and Bunch 1968), Flynn Creek Crater, (Roddy currently over 160 known impact craters, but only 1968), and the Sedan nuclear explosion crater, New Mexico approximately a dozen contain meteoritic material (French (Short 1968). However, these occurrences were not 1998). New craters are being discovered at a rate of 3–5 per systematically analyzed in order to determine if type and year (Grieve and Personen 1992) using indicators such as distribution of twinning correlates with the shock level. geophysical and morphometric anomalies, shatter cones, In the past, calcite was ignored as an indicator of shock PDFs (planar deformation features), and the occurrence of metamorphism because of the ease with which it can anneal high-pressure polymorphs. Of these 160 craters, 60 have and be overprinted by later deformation (Short 1968). Meteor carbonate target rocks and lack quartz-bearing minerals Crater is a good site to examine shock-induced effects on (Grieve and Shoemaker 1994). There are few methods to calcite because of its young age (49,000 ± 3000 a; Sutton quantify the impact-induced deformation in the carbonates. 1985) and the lack of post-impact tectonic events in the area. Deformation mechanisms in calcite are extensively In addition, and occur within the Coconino studied both experimentally (Teufel 1980; Bruhn and Casey Sandstone at Meteor Crater (Chao et al. 1962) indicating the 1997) and in tectonic environments (Burkhard 1992; Van der local presence of shock pressures in excess of 30 GPa. Pluijm 1991). The experimental and regional tectonic studies Recent analytical procedures provide a means of indicate a correlation between the amount of deformation and quantifying the amount of shock applied to rocks and the amount of strain over a , but analysis of minerals. Analysis of experimentally shocked minerals shows shock-induced deformation in calcite crystals from impact that the X-ray diffraction (XRD) peaks of minerals broaden as craters is limited. Recently, analysis using transmission increasing pressure is applied (e.g., Martinez et al. 1995;

297 © The Meteoritical Society, 2005. Printed in USA. 298 J. B. Burt et al.

Sk·la and Jakeö 1999; Bell et al. 1998; 2002). Similarly, SAMPLES AND METHODS broadening of the diffraction peaks is observed in naturally shocked minerals (e.g., Hörz et al. 1986; Skála and Jakeö Multiple samples of the same rock section were prepared 1999). For this study, XRD peaks of calcites from the Kaibab by two different laboratories to ensure that twinning or limestone were analyzed for peak broadening using single deformation features were not produced as a function of peak profile fitting and by a Rietveld refinement program to preparation. The 30 µm-thick thin sections were doubly determine the peak shape parameters. polished because singly polished thin sections contained In addition to XRD, electron spin resonance (ESR) has optical artifacts. The thin sections were analyzed to been used to quantitatively determine the shock pressure to determine type and distribution of deformation features in which a carbonate host rock has been subjected. The ESR calcite (e.g., twinning, fractures) within each sample and to technique utilizes Mn2+ ions that fill Ca2+ and Mg2+ positions see if there is a correlation between deformation and the in the crystal lattice of dolomite. The resonance absorption distance from the center of the crater. Four levels of shock spectrum obtained by ESR represents coupling between the deformation are distinguished (not, little, moderately, and electronic and nuclear magnetic moments (Hurd et al. 1954). highly deformed) and categorized according to the size, Shocked samples show a change in the hyper-fine spectral abundance, and distribution of twinning within the crystals peaks, which is correlated to an increase in shock pressure (Table 1). The widths of the twins are categorized as thin (Vizgirda et al. 1980; Polanskey and Ahrens 1994). The (<1.5 mm), medium (1.5–3.5 mm), and thick (>3.5 mm). The quantitative determination of the shock pressure using ESR abundance of twins are categorized as low, medium, and requires a series of experimentally shocked reference high, based on the density of twins within single crystals samples. The samples investigated in this study are (Fig. 2). The type and amount of twinning in each sample is compared to experimentally shocked Kaibab limestone not homogenous with some samples, having crystals that are samples (Polanskey and Ahrens 1994). Combined, XRD and moderately twinned juxtaposed against crystals with no ESR may provide a means of quantifying the optically twinning, so the percentage of crystals with twinning was an defined deformation in a calcite crystal from a meteorite additional criterion for assigning levels of shock crater. deformation. Examination of the calcite by XRD requires careful GEOLOGIC SETTING sample preparation in separating the calcite from the surrounding dolomite. The use of a microdrill or any other Meteor Crater is a 180 m deep bowl-shaped depression instrument with high revolution rates can induce increased located in north-eastern Arizona. The diameter of the crater is peak broadening (Sk·la and Jakeö 1999), so the calcite approximately 1.2 km and the edge of the crater rim has samples were hand-picked from rock chips broken from hand overturned strata rising 30 to 60 m above the surrounding specimens. The calcite crystals were then ground into a fine bedrock surface (Shoemaker 1960). The target rocks at powder under alcohol with a mortar and pestle and sieved Meteor Crater are sedimentary units deposited during the onto a quartz zero background plate coated with petroleum Permian and Triassic periods. The Permian Kaibab limestone jelly. The calcite was continually sieved over the plate until a comprises the majority of the crater wall and is approximately uniform thickness of approximately 300 µm was deposited 80 m of silty dolomudstone and silty fossiliferous with grains <75 µm. The diffraction patterns were collected dolomudstone. The rim of the crater is composed of the using an automated Siemens D500 Kristalloflex overturned stratigraphy within the crater and an overlying diffractometer at Washington State University, with CuKα debris sheet of material ejected during formation of the crater radiation (λ = 1.54059), a graphite diffracted beam (Shoemaker 1960). monochromator, and a scintillation detector. The The calcite crystals within the Kaibab limestone formed diffractometer was operated at 35 kV and 30 mA between as replacement of evaporite nodules (gypsum or anhydrite) 15° and 117° 2θ with a step width of 0.02° 2θ and a count after deposition, yet prior to impact (Roddy 1978). The calcite time of 10 sec per step. is not uniformly distributed in the Kaibab limestone, The full width at half maximum (FWHM) of the occurring primarily in the upper half of this unit. Samples for individual peaks from the diffraction pattern were calculated this study were collected from the debris sheet and the crater using the EVA graphics program DIFFRAC-AT v.3 software walls (Fig. 1). Samples were also collected from Canyon package. The EVA program subtracts out the Kα2 peaks and Diablo (4.2 km west of the crater), an intermittent river smooths the diffraction pattern before calculating the FWHM channel exposing ∼11 m of equivalent Kaibab limestone for values. The patterns are smoothed by fixing the width of the comparison as a unshocked reference. All calcite samples sliding interval over which the data will be smoothed by were analyzed petrographically and with XRD, while samples means of a third-degree polynomial. A sliding interval width of the dolomudstone surrounding the calcite were analyzed of 0.143° 2θ was used so that smoothed data fitted our with ESR. experimental data as closely as possible, while obtaining a Analysis of deformed calcite: Meteor Crater, Arizona 299

Fig. 1. Topographic map of Meteor Crater marking sample locations (adapted from USGS map # TAZ0889). higher accuracy for calculating the FWHM values. The see Young (1993). The values for the FWHM for the range change in the FWHM for the calcite samples is shown in between 20–1450 2θ were calculated and plotted against their Fig. 3. 2θ value (Figs. 3b and 3d). Samples containing accessory Additionally, the diffraction patterns were analyzed by peaks (such as kaolinite and quartz) were not analyzed Rietveld refinement in order to determine the peak shape because the accessory phases affected the calculated U, V, and parameters U, V, and W (Sk·la and Jakeö 1999). The W values. diffraction patterns were refined using the SIROQUANT v.2 For ESR analysis, samples of the Kaibab limestone from Rietveld refinement program. The original parameters for the Meteor Crater and Canyon Diablo were ground into a coarse calcite structure used in this program are a = b = 4.9901, c = powder and placed into Wilmad 710-SQ-250M fused quartz 17.0595, U = V = 0, W = 0.015, preferred orientation using ESR tubes. The samples were run at room temperature in a March-Dollase correction, Pearson’s shape factor m = 1.4, a Bruker EMX spectrometer with a standard TE102 cavity at the zero point parameter taking into account sample displacement University of Washington. The spectra were measured and alignment error, and a non diffraction percent = 0.02. The between a magnetic moment of 3036.5–3688.5 G, which crystal structure parameters were taken from H. D Megaw encompassed the Mn2+ absorption peaks. The spectrometer (1973). The program refined the parameters for overall scale settings had a microwave power of 20 mW, a frequency factor, six order background polynomial parameter, half- between 9.307–9.355 GHz, a modulation amplitude of 3.2 G, width parameters, profile shape function, and unit cell a time constant of 81.92 msec, a conversion time of dimensions. For details about Rietveld refinement techniques 81.92 msec, and were run in the second harmonic mode.

300 J. B. Burt et al.

crater

center of the the of center

Distance from from Distance

(Gpa)

pressure pressure

High field field High

(Gpa)

ter are given for comparison. pressure

rater walls and debris sheet

Low field field Low

deformation

High High

deformation

Medium Medium

deformation

Low Low No deformation No

of the samples from center cra

extinction Undulose Undulose

Kaibab limestone samples from within the c

densities ±5% densities

with high twin twin high with

% of crystals crystals of %

±5%

twin densities densities twin

with medium medium with

% of crystals crystals of %

densities ±5% densities

with low twin twin low with

% of crystals crystals of %

±10%

samples determined from ESR and distance with large twins large with

lative shock levels for the Meteor Crater

% of crystals crystals of %

twins ±10% twins

with medium medium with

% of crystals crystals of %

twins ±10% twins

with small small with

% of crystals crystals of %

±5% with twins twins with

572 100 80 0 0 20 100 0 0 0 100 100 100 0 0 0 0 0 0 X X X X X X0 0 0 0.00 0.90 0 0.91 0.00 0.24 0.23 0 4.2 km 4.2 km 4.2 km 0 0 X X 0.86 0.22 485 m % of crystals crystals of % 65 20 70 10 40 60 0 X X 0.92 0.24 450 m b b a a a Samples from within crater walls. 2CD6 2CD7 WC2WC4WC5WC6 25WC8 90WC9C 75WC12 100 95WC14 80 50WC15 1 50 35WC17 15 95WC18 0 70 0WC19 40 100 0WC22 70 40 70WC23 20 85 65WC24 40 0 100 100 40 10WC25 25 10 30 0WC27 50 70 80WC28 30 0 50 70 60 50WC31 0 60 95 100 10WC34 30 50 0 0 50WC35 70 80 0 40 20EC3 80 50 20 50 0 50EC4D 50 0 10 0 0EC6 70 60 90 0 30 0EC8 90 30 20 100 80 0 90 20EC9 100 0 50 50EC11A 100 30 0 0 0 30EC13 100 80 5 100 8 0 10 10EC14 20 0 80 0 0 0 0 0 100EC15A 0 0 0 0 100 10EC15B 50 100 100 X 0 0AA12 30 0 30 X 80 X 0 0 20AA10 30 30 0 0 2 0 0 0 100 0 0 60 X 0 0 0 20 0 60 X 90 0 0 20 0 0 0 0 50 X 0 0 0 0 0 X X 0 100 40 0 100 80 10 0 100 0 100 100 X 0 0 X 100 0 0 0 0 0 70 0 X X X 0 30 0 0 X X 0 100 0 X 0 X 0 60 0 0 X X 0 0 40 70 X 80 0 X X 0.98 0 X 0.04 0 1.03 40 0 0 X 100 X 60 0 X 20 0.26 3.05 X 0 100 X 0.13 0.99 0.30 X X 0.57 0.47 X 0 X 1.38 0 0 735 m 0.29 1.20 0 0 990 m 995 m X – X 1.00 X 0 0.14 m 1150 X 0.37 0.91 X X 0.33 1.34 X X 715 m X 1.20 0.24 X X – X 0.93 895 m 0.25 720 m 0.37 X X m 1175 – 0.32 1.60 965 m 0.24 1.29 X m 1170 785 m 0.46 X 800 m 0.41 X 700 m X – 0.71 X 0.37 1.45 X 630 m 0.12 0.77 X 1055 m 0.26 0.39 1310 m 1.55 0.22 740 m 1.90 995 m 1.51 m 1160 2.06 0.40 735 m 0.55 860 m 0.44 0.85 0.76 0.76 0.58 1.14 0.91 585 m 1.25 575 m 0.23 0.17 0.22 585 m 0.31 0.26 625 m 0.34 710 m 610 m 625 m 675 m 670 m 670 m Sample 2CD1 Non-shocked standards. based on petrography. Shock pressures of the based on petrography. Table 1. Characteristics for determining re Table a b Analysis of deformed calcite: Meteor Crater, Arizona 301

Fig. 2: Photomicrographs of coarse Kaibab calcite samples with crossed nicols. (a) Unshocked reference sample from Canyon Diablo. (b) Sample WC17 with medium twin density and medium-sized twins. (c) Sample WC19 with medium density thin twins in the larger crystal and medium density medium-sized twins in the smaller crystal. (d) Sample WC15 with low density thin twins. (e) Sample WC22 with high density thin twins. (f) Sample WC8 with fractures along cleavage planes.

RESULTS A classification based on the differences between calcite samples from inside the crater and the unshocked reference Petrography material is used to infer shock levels (Table 1). Samples with high twin densities with twinning in nearly 100% of the Twinning is the only optically visible deformation feature calcite crystals are categorized as “highly deformed.” within the calcite crystals that was likely produced by the Samples with 65–90% twinned calcite crystals with at least shock. The apparent size of twins is not a good measure for 50% of the crystals having medium or high twin densities the level of shock as it is related to both the orientation of the are categorized as “moderately deformed.” Samples with crystal in the thin section and the orientation of the twins in between 10–70% twinned calcite crystals with mostly low the crystal. Though the size of the twins is variable, the twin densities and low deformation are categorized as “little density and percentage of the twinned crystals is used to infer deformed.” The most distinguishing feature of the samples different levels of shock deformation. with minor deformation is the amount of calcite crystals 302 J. B. Burt et al.

Fig. 3. (a) Single peak profile plots for all Kaibab calcite samples. (b) Rietveld refined full-width half-maximum values plotted against their respective 2-theta values for all Kaibab calcite samples. (c) Single peak profile fitting, and (d) Rietveld refined plot for samples WC25, EC6, EC2, 2CD7, and WC23. without twins, because the majority of the samples have multiple crystals with distinct orientations may have been twins in less than 30% of the crystals. Some samples that caused by the , however, the ubiquitous low- have a high percentage of crystals with twins (up to 70%) density fracturing is not likely related to the impact event. are classified as little deformed because the crystals have Undulose extinction also occurs in many of the samples; as low distribution densities of twinning and twins that do not the samples from the Canyon Diablo area also show this cross the entire crystal. Samples with between 0–7% feature, it was formed prior to the impacting event, possibly twinned crystals were classified as “not deformed.” The due to regional stresses. In addition, PDFs were not found in twins in the undeformed samples have very low distribution the quartz grains of the Kaibab limestone, in contrast to their densities and more than 90% of the crystals have no twins. well-documented occurrence in the Coconino sandstone Also, the amount of twinning in the samples does not appear (Kieffer 1971). Thus, twinning is the only deformational to correspond to the distance of the sampling locality from feature, unambiguously related to the impact event. the center of the crater. Samples with abundant twins are as close as 575 m or as far away as 975 m from the center of X-Ray Diffraction the crater (Table 1), whereas samples with few twins occur up to 1310 m or as close as 585 m from the center of the Determination of the shock levels of the Kaibab crater. limestone samples using XRD is examined by peak Other deformational features besides twinning are seen broadening. Peak broadening as a result of shock is analyzed within the calcite, but they cannot be directly related to the through single peak profile fitting and a Rietveld refinement impact event. Fractures are present in all of the calcite of the XRD patterns. The FWHM of diffraction peaks samples, including those from the Canyon Diablo area. Only calculated for all the samples using the single peak profile sample WC8 (Fig. 2f) has a high density of fractures, while fitting method are represented in Figs. 3a and 3c. The sample AA12 has a moderate amount of fracturing. These Rietveld refinement method examines peak broadening by fractures are uniformly distributed within the crystals and calculating the FWHM for the samples (Figs. 3b and 3c) with occur along cleavage planes, whereas fractures in the rest of the refined U, V, and W values, which characterize the widths the samples are not uniformly distributed and have lower of the XRD peaks in relation to the FWHM through the fracture densities. High-density fractures occurring in equation of Caglioti et al. (1958): Analysis of deformed calcite: Meteor Crater, Arizona 303

Fig. 4. X-ray diffraction patterns for samples STO5,A, WC25, WC5, EC6, EC2, 2CD7, WC23, and STO1,B between 55–65° 2θ. Diffraction patterns for a highly shocked sample (STO5,A) and an unshocked sample (STO1,B) are from the Steinheim crater (Sk·la and Jakeö 1999).

2 2 because its spectrum is approximately the average of the three ()FWHM = Utan θ ++Vtanθ W (1) Kaibab limestone samples from the Canyon Diablo area Analysis of the single peak profile and Rietveld (Fig. 6). A theoretical integrated difference (ID) value refinement plots (Fig. 3) suggest that samples WC25 and (Polanskey and Ahrens 1994) was then calculated by possibly EC6 are the only samples with XRD patterns that are subtracting the spectra of the shocked samples from the indicative of relatively high shock values. Examination of the unshocked standard sample 2CD1 in order to determine the diffraction peaks (Fig. 4) indicates that sample WC25 shows amount of shock-induced change in each spectrum (Fig. 5). peak broadening and a decrease in intensity compared to The ID value was calculated by the equation: samples WC23 and 2CD7, which are indicated as having n + N relatively low shock histories (Fig. 3), though peak 0 Y ()i – Y ()i broadening in sample WC25 is not as definitive as in calcite ID = ∑ ------standard ------sample------(2) from the Steinheim crater, Germany (Sk·la and Jakeö 1999; N1+ in= 0 see Fig. 4). Examination of peak broadening of the samples STO5,A and STO1,B from the Steinheim crater (Fig. 4), The ID value represents the difference between the indicating a smaller shocked pressure range for the calcite normalized intensities for the standard (Ystandard(i)) and the samples from Meteor Crater compared with the samples from sample (Ysample(i)) spectra, where N is the number of data the Steinheim crater. The XRD analysis cannot provide a points through which the spectra are integrated. The high and quantitative determination of the individual shock pressure low field peaks are integrated over different magnetic field for each sample, but the relative similarity of the FWHM of values due to a difference in the range of the magnetic field the peaks suggests that the samples experienced (i) similar for the peaks. The low field peaks are integrated over a 40 G shock pressures that are (ii) relatively low. The latter range with n0 equal to 20 G below the largest peak; the high statement is based on the similar FWHM for the reference field peaks are integrated over a 60 G range with n0 equal to material and the samples from Meteor Crater. 30 G below the largest peak. The procedure used in calculating the ID values for the high field peak is represented Electron Spin Resonance by Fig. 5. The error involved in this normalization is calculated by determining the ID value corresponding to the Analysis of the pressure history of the samples with ESR difference between the standard and sample absorption values utilizes only the two outermost hyperfine splitting peaks of the baseline outside of the Mn2+ peaks with errors corresponding to the lowest and highest magnetic field averaging ±10% for the low field spectrum and ±20% for the absorption peaks. The resultant spectrum are normalized to high field spectrum. determine the change in peak shape due to shock deformation. For a determination of the respective shock pressures, Each peak is normalized so that the largest peak is equal to calculated ID values are compared with ID values of one and then offset along the horizontal so that the largest experimentally shocked samples given by Polanskey and peak for each sample overlaps the largest peak of the standard Ahrens (1994). As shown in Fig. 7, the difference between the (Fig. 5). The standard used in this analysis is sample 2CD1 spectra for unshocked and shocked samples generally 304 J. B. Burt et al.

Fig. 5. Procedure used for calculating integrated difference (ID) values. (a) Normalization and overlaying of the ESR spectrum for calcite samples 2CD1 (unshocked reference) and WC25. (b) Difference between the normalized ESR spectrum for samples 2CD1 Fig. 6. Difference between the normalized ESR spectrum of the and WC25. reference samples 2CD7, 2CD1, and 2CD6 from the Canyon Diablo area in relation to calculated integrated difference (ID) values for the increases with increasing shock pressure (Polanskey and (a) low field and (b) high field comparisons. Ahrens 1994). The equations given by Polanskey and Ahrens (1994) that correlated shock with calculated ID values are: This may be the reason for the difference in the spectra of the three samples of unshocked Kaibab limestone from the

PKaibab(Gpa) = 17.2 (ID) − 0.494 Low field (3a) Canyon Diablo area (Fig. 6). Shock pressures calculated for PKaibab(Gpa) = 9.77 (ID) + 0.391 High field (3b) samples 2CD6 and 2CD7 (Table 1) differ by ∼0.9 GPa using the low field pressure equation (4a) and ∼0.2 GPa using the In order to correlate the data for the samples studied for this high field pressure equation (4b). In addition, samples WC6, work with those of their experimentally shocked counterparts, WC28, WC17, and 2CD7 have larger peaks than the new equations were determined that better fit the data. The unshocked reference sample 2CD1, erroneously indicating revised equations for a best fit line for the high and low field that the latter has experienced a higher shock level. data also incorporate the ID value for the Kaibab sample that Despite the fact that ESR is not useful for quantitatively was experimentally shocked to 0.9 GPa by Polanskey and determining the shock pressures of the Meteor Crater Ahrens (1994). The revised equations are: samples, a qualitative result is obtained, indicating that all studied Kaibab limestone samples were subjected to only low PKaibab(Gpa) = 13.247 (ID) Low field (4a) levels of shock (Table 1). Though ESR only yielded PKaibab(Gpa) = 8.054 (ID) High field (4b) qualitative results for the samples from Meteor Crater, this technique may be a useful tool for quantitatively determining Shock pressures for the Kaibab limestone samples derived the shock level of samples from other impact craters. For from Equations 4a and 4b are compiled in Table 1. Samples example, the calibrated regression line of ESR data for WC15 and WC24 were not analyzed using this technique due stronger shocked carbonate samples from the OAK nuclear to abnormal absorption peaks. crater had a considerably smaller error, and thus reliable Quantitative determination of the shock pressures for the shock barometry was possible (Polanskey and Ahrens 1994). samples from the Kaibab formation at Meteor Crater could In addition, sample WC25, which according to the XRD data not be achieved. Broadening of the ESR spectra in the experienced a relatively high shock level, yielded one of the samples may be due to factors other than shock that may highest shock pressures calculated with ESR data (Table 1). severely increase the error in the shock pressure measurement. For instance, the limestone is not DISCUSSION homogeneous over the 80 m thick Kaibab unit and the concentration of Mn2+ may vary, causing the spectra for An in-depth analysis of deformation features in impact samples that experienced the same shock level to be different. craters is critical in understanding processes that occur during Analysis of deformed calcite: Meteor Crater, Arizona 305 cratering events, how different lithologies are affected by impacts, and for identifying new craters. Systematic studies of calcite twinning produced in orogenic environments indicate temperature zones that correspond with different types of twinning (Burkhard 1992). In this study, we classified calcite samples of the Kaibab Limestone from Meteor Crater into the categories of not, little, moderately, and highly (shock) deformed (Table 1) based on the petrographic features. Kaibab samples of the group “high deformation” would be categorized as “low to medium deformed” in an orogenic environment, reflecting the different deformational mechanisms. Calcite twinning, which is produced in shock recovery experiments, is initiated at pressures greater than 8.2 GPa and undulose extinction Fig. 7. Integrated difference (ID) values determined from appears at pressures above 6.2 GPa (Hackbarth and Stˆffler experimentally shocked Kaibab limestone by Polanskey and Ahrens (1994). Trend lines correspond to equations 3a, 3b, 4a, and 4c, which 1995). This would suggest that most of the investigated relate unknown shock pressures to calculated ID values determined calcite samples of the Kaibab limestone experienced from the samples ESR spectrum. pressures greater than 8.2 GPa, which is in contrast to our XRD and ESR data. The different threshold pressure for also did not result in a quantitative determination of the shock-induced twinning may represent a matrix effect due to respective pressure histories. As shock is applied to a sample, the Kaibab calcite crystals being encased within a fine- the ESR absorption peaks should decrease, thereby increasing grained dolomite matrix, whereas pure single crystal calcites the difference between spectra for the shocked and reference were used in the experiments. Twinning within calcite sample. For most of the Kaibab limestone samples, shock- crystals may also be dependent on the distribution, induced changes in the ESR spectrum were smaller then the orientation, and/or amount of crystals in the rock and not analytical error. The differences in the ESR spectra between solely on the pressures applied. Additionally, this suggests the unshocked Canyon Diablo reference samples equal the that petrographic data from experimentally shocked crystals difference between an unshocked sample and one shocked to may not be wholly representative for naturally shocked 0.7 GPa. The differences between the unshocked samples crystals. Thus, the development of twinning in calcite crystals could be caused by respective differences in the concentration within Meteor Crater may be dependent on the orientation of of Mn2+. This indicates that the ESR technique may be useful the crystals with respect shock stresses and/or the type of for characterization of the shock history if the carbonatic material surrounding the crystals. target sequence has a homogeneous Mn2+ distribution. It The increase in twin densities, widths, and distributions should also be noted that sample WC25 had one of the largest in the shocked Kaibab calcites should correspond to a shock pressures calculated with ESR and also the largest qualitative peak broadening in the XRD patterns. However, broadening of its FWHM values indicating a correlation the apparent low levels of shock experienced by these between these techniques. samples may have masked the relationship between peak broadening and the level of deformation. In the future and CONCLUSIONS with a more refined data set, it may become possible to quantitatively determine reliable shock pressures based on Calcite crystals in the Kaibab limestone in Meteor Crater this technique using smaller grain sizes. A problem with and its ejecta sheet show shock deformation. The calcite was analyzing shocked carbonate minerals with X-ray powder petrographically categorized as being not, little, moderately, diffraction is making sure that the grain size of the powder is and highly shock deformed. This classification is based solely small enough without affecting peak shapes during the sample on a relative comparison among the samples. Petrographic preparation. Differentiation of relative shock pressures using data as well as XRD and ESR analysis indicate that XRD may also be better if the target material is subjected to a deformation features in the calcite crystals experienced shock wide range of shocked pressures. The samples from this study pressures below 3.0 GPa. However, the type of deformation were apparently subjected to low and rather similar shock features cannot be directly correlated to distinct levels of pressures, making discrimination between shock levels shock because a correlation between the results of the difficult. The Rietveld method also seems to be a better different analytical techniques was not possible. It is curious approach in the analysis of peak broadening than single peak that seemingly so few high shock deformation features occur profile fitting in determining the pressure history of the in the Kaibab limestone samples throughout the crater and the samples from this and other studies (Sk·la and Jakeö 1999). ejecta sheet as high pressure polymorphs of quartz occur in Analysis of the Kaibab limestone samples using ESR the underlying Coconino sandstone. 306 J. B. Burt et al.

ACKNOWLEDGMENTS Numerical modeling of laboratory experiments. In Catastrophic events and mass extinctions: Impacts and beyond, edited by This paper is dedicated to the late David Roddy, whose Koeberl C. and MacLeod K. G. GSA Special Paper #356. pp. 587–594. expertise at Meteor Crater was invaluable. We thank him for Kieffer S. W. 1971. of the Coconino sandstone the time he spent at the crater with us, and he will be missed. at Meteor Crater. Journal of Geophysical Research 76:5449– Special thanks go to Nick Foit for his assistance with the 5473. XRD analysis and Mickey Gunter for advice on Rietveld Langenhorst F., Boustie M., Deutsch A., Hornemann U., refinement. We would also like thank Roger Willet, Jim Matignon C., Migault A., and Romain J. P. 2002. Experimental techniques for the simulation of shock metamorphism: A case Woods, and Thomas Ahrens for their help with the electron study on calcite. 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