Spatial U-Pb age distribution in shock-recrystallized zircon – A case study from the Rochechouart impact structure, France

Journal Pre-proofs Title: Spatial U–Pb age distribution in shock-recrystallized zircon – a case study from the Rochechouart impact structure, France Cornelia Rasmussen, Daniel F. Stockli, Timmons M. Erickson, Martin Schmieder PII: DOI: Reference:

S0016-7037(20)30029-6 https://doi.org/10.1016/j.gca.2020.01.017 GCA 11597

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Geochimica et Cosmochimica Acta

Received Date: Revised Date: Accepted Date:

1 August 2019 10 January 2020 12 January 2020

Please cite this article as: Rasmussen, C., Stockli, D.F., Erickson, T.M., Schmieder, M., Title: Spatial U–Pb age distribution in shock-recrystallized zircon – a case study from the Rochechouart impact structure, France, Geochimica et Cosmochimica Acta (2020), doi: https://doi.org/10.1016/j.gca.2020.01.017

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Title: Spatial U–Pb age distribution in shock-recrystallized zircon – a case study from the Rochechouart impact structure, France

Cornelia Rasmussen1,2*, Daniel F. Stockli2, Timmons M. Erickson3, and Martin Schmieder4

1

Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, J.J.

Pickle Research Campus, 10100 Burnet Road, Austin, Texas 78758, USA 2

Department of Geological Sciences, Jackson School of Geosciences, University of Texas at

Austin, 2275 Speedway C9000, Austin, Texas 78712, USA 3

Jacobs-JETS, Astromaterials Research and Exploration Science Division, NASA Johnson Space

Center, Houston, Texas, USA. 4

Lunar and Planetary Institute – USRA, 3600 Bay Area Boulevard, Houston TX 77058, USA

*Corresponding author

Abstract

Age determination of impact structures via the zircon U–Pb system remains challenging and often ambiguous due to highly variable effects of shock metamorphism on U-Pb geochronology. It is, therefore, crucial to link the observed zircon microtextures, including their temperature and pressure conditions associated with their formation, directly to the U–Pb ages preserved. Here, we analyzed three recrystallized zircon grains and one plastically deformed zircon crystal from the medium-sized Rochechouart impact structure in the northwestern Massif Central of France. For the Rochechouart impact structure the impact age (206.92 ± 0.32 Ma [40Ar/39Ar]) as well as the 1

tectono-themal history is well established making this study site ideal to test concepts about U-Pb systematics in shocked zircon and to differentiate between shock-driven age resetting and preimpact crystallization and metamorphic overprinting. Zircon microstructures were studied using scanning electron imaging, cathodoluminescence imaging, and electron backscatter diffraction (EBSD) mapping. Further, we conducted U–Pb Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) depth-profiling analysis, allowing us to interpret the resultant age data in discrete steps with increasing ablation time/depth. The U–Pb depth profiling data demonstrate that plastically strained grains are incompletely reset and preserve rim and interior age domains reflecting typical pre-impact (pre-Variscan and Variscan) regional tectonic ages. Our results also reveal that the granular crystals encountered contain microstructural evidence for “former reidite in granular neoblastic” (FRIGN) zircon, reflecting both high-pressure (⩾30GPa) and hightemperature (⩾1200℃) conditions. This signifies that FRIGN zircon is now known from an additional, medium sized, impact structure further supporting the hypothesis that this impact induced microstructure is commonly preserved. In addition, FRIGN zircon has a high potential to preserve completely impact-reset crystal domains (~204 to 207 Ma) that can be identified by our combined analytical approach of U–Pb depth profiling and EBSD mapping, and thus are suitable for determining reliable impact ages.

Keywords: U–Pb depth profiling, Rochechouart, shock metamorphism, EBSD, FRIGN, reidite, zircon

1. INTRODUCTION

2

Improving our understanding of the impact cratering record throughout Earth history is essential to test hypotheses about impact-induced environmental change, such as atmospheric or biotic disturbances (e.g., Alvarez et al. 1980; Hildebrand et al. 1991), the impact flux through time (Grieve and Pesonen, 1996; French, 1998), and the synchronicity of multiple impact events (e.g., Jourdan et al., 2012; Schmieder et al., 2014). Ideally, an impact age would be determined by employing multi-proxy chronometry, consisting of a high-precision 40Ar/39Ar and a high precision U–Pb age (e.g., Ramezani et al., 2005; Kenny et al., 2019) that would not only serve to verify the time of impact, but could also provide an intercalibration point for the geologic, biomagnetostratigraphic, and astronomically tuned Phanerozoic timescale (e.g., Schmieder et al., 2018a,b). However, both techniques have inherent difficulties that stem from the complexities related to chemical alteration of the datable impact material due to the impact and post-impact processes, such as partial (and variable) melting of different phases, variable open system behavior, heterogeneous resetting of the radioisotopic clocks, and post-impact hydrothermal alteration within a cooling impact crater (e.g., Deutsch and Schärer, 1994; Jourdan et al., 2009, 2012). Significant challenges still exist for utilizing the zircon U–Pb system to date impacts, since melt-grown zircon is largely limited to differentiated impact melt sheets and impact glasses, and thus shocked crystals, which tend to be variably reset by impact-heating and associated diffusive loss of Pb, are often the only material available to determine the time of impact (e.g., Deutsch and Schärer, 1994; Jourdan et al., 2009; Kenny et al., 2019). Shock metamorphism is a highly heterogeneous process, in which impactite lithologies, clasts, and mineral fragments experience variable pressures and temperatures, which in turn can lead to variable open system behavior and incomplete resetting of the U–Pb systematics. In addition, because outward impact ejecta flow

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cross-cuts shock pressure isobars during the impact-excavation phase (French, 1998, Kenkmann et al., 2014), allochtonous impactites are commonly composed of materials characterized by different shock levels. Zircon shock features, including irregular fracturing, planar fracturing, crystal-plasticity, planar deformation features, deformation twinning, the transformation to the high-pressure polymorph reidite, and granular recrystallization textures (e.g., Wittmann et al., 2006; Timms et al., 2017a), may all introduce additional fast-diffusion pathways that affect the U– Pb isotopes (Kenny et al., 2019). Zircon crystals, whether formed prior to or during the impact event, can also experience additional Pb-loss when affected by post-impact thermal and/or hydrothermal events (e.g., Krogh et al., 1993a; Jourdan et al., 2009; Schmieder et al., 2015). Therefore, it is possible to encounter ages within the impact melt rock and target lithologies that reflect pre-impact, syn-impact, and post-impact events or a geologically meaningless apparent age from mixing of multiple age components. In addition, metamict (radiation-damaged, U- and Thrich) zircon grains in impact lithologies exhibit diffusion characteristics different from crystalline zircon and, therefore, record impact and other thermal events at lower temperatures (e.g. Schwarz et al., 2016; Stockli et al., 2018; Schmieder et al., 2019; Rasmussen et al., 2019a). Granular neoblastic zircon is a polycrystalline aggregate of variously sized granules formed by the recrystallization of shocked zircon during an impact that has also proven to preferentially preserve younger ages, including U–Pb resetting due to the impact (e.g., Wittmann et al., 2006; Kalleson et al., 2009; Cavosie et al., 2015; Schmieder et al., 2015, 2018; Kenny et al., 2017, 2019). Granular zircon, even if more or less uniform in appearance, can form under vastly different conditions and occur in impact lithologies such as impact melt rock, impact glass, breccia, or ash fall deposits (Kamo et al., 1996; Wittmann et al., 2006; Cavosie et al., 2010; Crow et al., 2017). However, zircon with a granular texture is also found in tectonic shear zones (e.g., Piazolo et al.,

4

2012; Cavosie et al., 2015). Therefore, detailed petrographic analyses, including the microstructural characterization of crystallographic orientations, have to be employed to identify diagnostic evidence for the pressure and temperature conditions under which the granular textures formed and whether they reflect high-temperature and/or high-pressure conditions during the time of impact (e.g., Wittmann et al., 2006; Timms et al., 2017a, b; Cavosie et al., 2018). Zircon shock features, including high-pressure polymorphs, can also be linked to the pressure and temperature conditions (shock stages; e.g., Stöffler, 1984) during impact, and hence can be used as a first-order impact thermo-barometer (e.g., Glass et al., 2002; Wittmann et al., 2006; Timms et al., 2017a). Based on shock experiments and microstructural studies of natural samples, pressures below ~20 GPa may form brittle fractures and crystal-plastic microstructures consistent with dislocation creep, whereas increased shock pressure conditions cause deformation twinning (~20 GPa) (Leroux et al., 1999; Morozova, 2015; Cavosie et al., 2016) and the transformation to reidite (~30 GPa) (Kusaba et al., 1985; Glass et al., 2002; Wittmann et al., 2006; Timms et al., 2017a). Further, low-pressure (< 5-8 GPa) but high-temperature conditions (~1673 °C) lead to the dissociation of zircon to zirconium dioxide (most typically monoclinic baddeleyite; and cubic zirconia at very high temperatures) and silica (Timms et al., 2017a,b). The preservation of systematic crystallographic microstructures, known as ‘phase heritage’, within zircon can be used to infer the former presence of reidite. The parent-daughter transformation results in systematic misorientation relationships within the daughter phase (Timms et al., 2017a). During the reidite - zircon phase transformation alignment of <110>reidite with <001>zircon can result in the inheritance of up to three <001>zircon orientations that are mutually orthogonal (Cavosie et al., 2016, Erickson et al., 2017).

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Despite challenges, zircon crystals (and other accessory phases) are relatively resistant to abrasion and chemical alteration and, thus, able to survive and record a protracted geologic history, such as partial melting and regional or shock metamorphism, while still maintaining at least part of their age information (e.g., time of crystallization and/or pre-impact thermometamorphic events; e.g., Mezger and Krogstad, 1997; Krogh et al., 1993a,b; Kamo and Krogh, 1995). Identifying discrete, meaningful ages from crystals that potentially preserve multiple events can be challenging, however, the obtained ages can give insight into more than just the timing of the impact event. For example Petrus et al., (2016), demonstrated that only a small number (1.5%) record the time of impact, based on a U–Pb study of 3920 zircon crystals from impactite and target rock lithologies of the Sudbury impact structure in Canada. The number of zircon crystals preserving shock features is even lower (< 1%) within detrital samples that were obtained from drainage systems and around (~5-1000 km) the Santa Fee (USA) and the Vredefort (South Africa) impact structures (Erickson et al., 2013; Montalvo et al. 2017, Cavosie et al., 2017; Montalvo et al., 2018). The remaining crystals yielded ages linked to the target rock age pointing to not only deep sourced rocks within the melt sheet, but also to previously unidentified age components of the target lithologies (e.g., Petrus et al., 2016). Therefore, partially reset impact zircon has the potential to reveal fundamental information about the regional and local target rock evolution and crater formation history. The ~18–25 km-wide Rochechouart impact structure (e.g., Lambert et al., 1977; Pohl et al., 1978; Lambert, 2010) in France is an ideal site to test links between zircon microstructures and impact versus target rock age preservation. The impact age has been most recently revised by Cohen et al. (2017), who determined an

40

Ar/39Ar age of 206.92 ± 0.32 Ma supporting a Late

Triassic (Rhaetian) age of the crater, whereas the regional tectono-thermal history of the French

6

Massif Central has been extensively studied as well (e.g., Lambert, 2010; Faure et al., 2009 and references therein). The crystalline target rocks, ranging in age from ~550 to 300 Ma, experienced a complex pre-impact history including pre-Variscan plutonism (550 and 480 Ma; e.g., Lardeaux et al., 2001; Faure et al., 2008) and the Variscan orogeny itself (~420-290 Ma; e.g., Lardeaux et al., 2001; Faure et al., 2009), which resulted in ductile deformation, synorogenic extension, and plutonic intrusions (e.g., Lambert, 2010; Faure et al., 2009). A previous Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) study showed that zircon crystals from Rochechouart impact lithologies can record and preserve the impact age (Horne, 2016). Therefore, zircon grains from the impactites have the potential to yield complex age spectra reflecting both regional tectonics and the impact event. For this study, we utilized shocked zircon crystals from the yellowish-beige vesicular clast free to clast poor impact melt rock found near Babaudus, located close to the estimated center of the Rochechouart impact structure (Fig. 1) (e.g., Lambert, 2010; Sapers et al., 2014) and conducted scanning electron microscopy (SEM), cathodoluminescence imaging (CL-), and electron backscatter diffraction (EBSD) mapping. We undertook U–Pb Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) depth profiling that allowed us to spatially resolve age domains (rim-to-center) in discrete steps within individual zircon crystals. With this analytical approach we seek to directly link impact and target age signals to zircon microstructures to improve our understanding of age preservation within geologic settings with a complex history of (multiphase) thermo-metamorphic activity and alteration.

2. GEOLOGICAL SETTING AND PREVIOUS GEOCHRONOLOGY

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The medium-sized Rochechouart impact structure with a diameter of ~18–25 km is located in France in the Départements Charente and Haute-Vienne, near the north-western edge of the French Massif Central (Limousin region, 45°50’N; 0°56’W) (Lambert et al., 1977; Pohl et al., 1978; Lambert, 2010). As the impact structure is highly eroded it remains challenging to constrain the original crater diameter, while an early estimate of a 15 km minimum crater size was purported by Kraut and French (1971), subsequent studies have proposed larger crater diameters, such as 32 km by Osinski and Ferrière (2016) and up to 40-50 km by Lambert (2010). The target rock consists out of late Neoproterozoic (~550 Ma) to Paleozoic (≥300 Ma) crystalline basement that experienced extensive regional tectonism mainly linked to the Variscan (Hercynian) Orogeny during the Paleozoic (Turpin et al., 1990; Chèvremont et al., 1996; Lambert, 2010; Faure et al., 2009). Recurring tectonic events such as pre-Variscan intrusions, orogenic ductile deformation, synorogenic extension, and plutonic intrusions led to the formation of metamorphic and igneous rocks, such as felsic and metabasic gneiss, amphibolites, and diorite as well as granitic intrusions (Chèvremont et al., 1996; Faure et al., 2007, 2008; Lambert, 2010) (Fig. 1). Post-Variscan overprint of the Massif Central includes early Mesozoic ore mineralization (Marignac and Cuney, 1999), presumably as a result of the opening of the North Atlantic and the emplacement of the Central Atlantic Magmatic Province (CAMP) large igneous province across parts of Europe, North and South America, and Africa around ~200 Ma, during the initial breakup of Pangaea (e.g., Marzoli et al., 1999, 2018; Lindström et al., 2015; Denyszyn et al., 2018). Outside the Rochechouart impact structure, towards the west, the crystalline basement is unconformably overlain by Triassic to Cretaceous limestones of the Aquitaine Basin (Lambert, 1977). The Rochechouart crater experienced post-impact regional tectonics, leading to a ~6° tilt of the crater

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floor to the north, uplifting the southern part of the structure relative to the northern crater domain (Kraut and French, 1971; Lambert, 2010). Even with the original crater being eroded, five complex and heterogeneous impactite units have been described that overlie the shocked basement rocks (Fig. 1). The impact units encompass mono- and polymictic impact breccia and impact melt-bearing lithologies (including clast-poor and clast-rich impact melt rock and suevite) to an ash-like impactoclastic unit (e.g., Kraut, 1969; Lambert, 2010; Schmieder et al., 2010; Sapers et al., 2014). The five impact units are defined as the following: (1) lithic breccia, seemingly devoid of melt or glass clasts (the so-called “Rochechouart breccia”); (2) suevite (lithic impact breccia containing melt and glass particles, the so-called greenish “Chassenon suevite”); (3) basal suevite, a melt-rich impactite containing a large number of lithic and mineral clasts, which is considered a transitional unit between lithic breccia and impact melt rock; (4) particulate, clast rich, impact melt rock with a glassy to crystalline matrix (so-called “Montoume breccia”); and (5) impact melt rock poor in or free of clasts with a finegrained, microcrystalline matrix (locally known as the “Babaudus-type” impact melt rock) (Lambert et al., 2010; Sapers et al., 2014). According to Sapers et al. (2014), units 1 and 2 can be categorized as breccias, whereas unit 3 represents a transitional unit between breccia and impact melt rock and units 4 and 5 are classified as impact melt rocks. A Rb–Sr age of 185.5 ± 2.2 Ma obtained from impact melt rock out-cropping at the Babaudus locality (2σ; Reimold and Oskierski, 1987) was later superseded by an 40Ar/39Ar age of 214 ± 8 Ma obtained from a pseudotachylyte sample from the Champagnac Quarry (6.5 km northeast of the impact structure center [please note the center of the structure is not well defined (Sapers et al., 2014)]) proposed by Kelley and Spray (1997). This age was previously reported as the best-estimate ages of the Rochechouart impact. However, Schmieder et al. (2010) and

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Schmieder et al. (2014) suggested the ~214 Ma age of Kelley and Spray (1997) was presumably too old because insufficiently degassed target rock material may have been incorporated into the analyzed impact melt rock sample. In the study of Schmieder et al. (2010), hydrothermally altered, strongly impact-metamorphosed k-feldspar from gneiss found near Videix from the westerncentral domain of the Rochechouart structure yielded an

40

Ar/39Ar age of 202.7 ± 2.2 Ma

(recalculated in Jourdan et al., 2012). The 40Ar/39Ar age of the Rochechouart impact was more recently revised to 206.92 ± 0.32 Ma (Cohen et al., 2017), calculated using the revised K decay constants and standard ages of Renne et al. (2010, 2011). (Note: the

40

Ar/39Ar age obtained by

Cohen et al. [2017] was from the same Babaudus impact melt rock sample that is used in this study). In addition to the proposed 40Ar/39Ar ages, Horne (2016) presented a U–Th/He age of 191.6 ± 9.1 Ma (from samples obtained from the Montoume breccia and Babaudus impact melt rock) as well as two LA-ICP-MS U–Pb age populations of 202.6 ± 5.8 Ma and 211 ± 13 Ma (from samples obtained from the Montoume breccia) for the Rochechouart impact structure.

3. MATERIAL AND METHODS

3.1. Sample material For this study, zircon crystals were extracted from the K-feldspar and melt-rich, finely crystallized “Babaudus impact melt rock” (Fig. 2; Lambert, 2010; Sapers et al., 2014). The sample was obtained from the Réserve Naturelle del’Astroblème de Rochechouart-Chassenon and the sample was supplied by Scottish Universities Environmental Research Centre. The Babaudus outcrop itself is located at 45°49000.63″N, 0°47031.04″E (see also Cohen et al., 2017; which analyzed the same material as our study). The yellowish-beige rock contains a homogenous matrix

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with only a few preserved (and mostly strongly melt-digested) clasts, evident in hand sample and thin section (Fig. 2). Moreover, elongated vesicles are observed that have been previously interpreted as potential flow features (Sapers et al., 2014). The impact melt rock has an average grain size of 25 μm with grains that characteristically interlock with one another, which previously has been interpreted as an indicator for recrystallization (Sapers et al., 2014; Cohen et al., 2017). The whole rock sample consists to 90% out of K-feldspar, whereas the remaining rock material is composed of smectite clay and iron‐ titanium nickel‐ rich oxides (Sapers et al., 2014; Cohen et al., 2017).

3.2. Analytical approach The impact melt rock was separated by (hand-) crushing, water table, magnetic separation with a hand magnet and Frantz apparatus, and methylene iodide (d = 3.1 g/cc) heavy liquid separation. The zircon crystals were then hand-picked under the binocular microscope and mounted on double-sided tape onto one-inch acrylic discs. We pre-screened 80 zircon crystals via SEM to identity and pick crystals that preserved granular textures. We also chose one additional grain that showed no major surface alteration (R-B-1) to identify the difference in age preservation. Subsequently we conducted coordinated analyses (CL, EBSD, and LA-ICP-MS depth profiling) on those four zircon crystals. Initially all zircon crystals were SEM-imaged to identify the surface morphology using a JEOL 6490LV SEM in low vacuum mode and without carbon coating at the University of Texas at Austin Electron Microbeam Laboratories (Fig. A1). Subsequently, the selected crystals were depth profiled via LA-ICP-MS analyses at the University of Texas at Austin at the UTChron Geo-Thermochronometry Laboratory using a

11

Photon Machine 193 nm Analyte G2 excimer laser-ablation system with large-volume Helex sample cell, coupled to a Thermo Scientific Element2 HR-ICP-MS. We used an ablation spot size of 25 μm and an ablation time of 40 continuous seconds, leading to an ablation depth of 20 μm (ablation rate ~0.5 μm/s). The experimental settings were: (1) laser energy of 4 mJ; (2) laser frequency of 10 Hz; and (3) a gas flow of 0.475 157 L (line 1) and 0.225 L (line 2). The unknowns were analyzed against three in-session interspersed primary and secondary standards: (1) the primary zircon standard GJ-1 (601.7 ± 1.3 Ma; Jackson et al., 2004); (2) the secondary standard Plešovice (337.1 ± 0.37 Ma; Sláma et al., 2008) and (3) and in-house procedural standard Pak1 (43.03 ± 0.01 Ma). (U-Pb depth profile standard data plotted as one-second increment age spectra can be found in Fig. A2). The reduction of raw U–Pb data was performed with the IgorPro based Iolite software (version 3.6) with a VizualAge data reduction scheme (Geochron DRS: Visual age) (Paton et al., 2011; Petrus and Kamber, 2012). The data were corrected for downhole and mass fractionation, using the primary zircon standard GJ-1 which was analyzed using the identical experimental settings as described above. Further, an initial common Pb correction after the Stacey and Kramer model (1975) was applied, using the Pb isotopic composition calculated from the estimated U–Pb ages of the ablation interval (Andersen, 2002) (Fig. 3). The estimated common Pb values (206Pb/204Pbc;

207

Pb/204Pbc), using Stacey and Kramer (1975), are reported in the main data table

(Table A2). The data were exported as one-second increments to identify spatially changing age domains thereby forming “consistent multi-step age domains” (formerly called plateau ages [Rasmussen et al., 2019a] but the name has been changed here to avoid confusion with Ar/Ar geochronology) from rim to center of the individual zircon crystals. The depth profiles were

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visualized with the Isoplot/Ex.4.15 program (Ludwig, 2012) in Wetherill concordia space (Wetherill, 1956) (Fig. 3) and as age spectra, plotting spatial age variations versus ablation time (seconds) and depth (μm) (Figs. 4-7). All ages were reposted on a 95% confidence level (2σ uncertainty). The laser ablation pit depth was estimated based on previous calibration measurements under the same analytical parameters on zircon crystals of variable morphology using a Bruker Optical Profilometer. Numerous intra-laboratory laser interfermometric pit measurements on zircon, including crystals with variable radiation damage, reveal only minor ablation rate variations (< 5%). During the 40 sec. of continuous ablation the goal was to achieve an ablation depth of ~20 μm with an ablation rate of 0.5 μm/sec, which we could verify by polishing the crystal afterward to investigate the ablation depth (R-B-2). (Note that careful polishing is crucial since it can hamper the ability to observe the correct ablation depth). Following the depth profiling analyses, the crystals were removed from the sticky tape and embedded in epoxy and polished to investigate the internal structures of the crystals via CLimaging and EBDS mapping. The CL-imaging was conducted using a Phillips/FEI XL30 Environmental

Scanning

Electron

Microscope

(ESEM)

with

a

Gatan

PanaCL

cathodoluminescence detector at the University of Texas at Austin Electron Microbeam Laboratories. Microstructural EBSD analyses were undertaken on an JEOL 7600F field emission gun scanning electron microscope (FEG-SEM) in the e-beam suite of the Astromaterials Research and Exploration Science division, NASA Johnson Space Center. Electron backscatter diffraction analyses were collected with an Oxford Instruments’ Symmetry EBSD detector and Aztec 4.0 software package. The data was post-processed using Oxford Instruments’ Aztec and Channel5 software packages. The SEM operating conditions included a 17 mm working distance, 20 kV acceleration voltage, and a 9 μA beam current. Individual maps were rastered with a step size of

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0.2 to 0.1 μm. Electron backscatter diffraction patterns were indexed using a structural match unit based on the 1 atm zircon unit-cell parameters of Hazen and Finger (1979) after the methods of Reddy et al. (2008), a reidite match unit with the unit-cell parameters of the 0.69 GPa sample of Farnan et al. (2003), and a baddeleyite match unit based on the unit-cell parameters of Hill and Cranswick (1994). This study shows that spot size analyses prior to the CL imaging and EBSD mapping is feasible. However, it is indisputable that the crystals must be large- (and sturdy) enough (which often cannot be confidently predetermined) so they can be moved after the U-Pb analyses and repositioned for polishing.

3.3. Age determination from one-second increment U-Pb spectra

Based on consistent multi-step age domains within the depth profiles, weighted mean ages were calculated. Consistent multi-step age domains are defined as: (1) at least three continuous seconds (ideally + n [additional undefined numbers of one-second increments given they fulfill criteria (2) and (3)]) to verify that consistent multi-step age domains are statistically reproducible (Allègre, 2008; Horstwood et al., 2016); (2) All one-second increments have to overlap with one another within 95% confidence (2σ error); And (3) the number of one-second increments are weight against the obtained MSWD of the calculated weighted mean age to verify a chi-squared of 2σ uncertainty (Wendt and Carl 1991; Spencer et al. 2016) (Fig. A3). In context of impact age preservation, we focus on the youngest consistent multi-step age domains within the individual zircon crystals.

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4. RESULTS

Two types of zircon grains were categorized based on their textures: (a) plastically deformed zircon (R-B-1) and (b) recrystallized, granular-textured zircon (R-B-2 to R-B-4) (see also Fig. A1A, B, D, E, G, and H). The zircon U–Pb depth profiles in this study reveal two age distribution patterns: (1) younger rim and older interior age domains (R-B-1, R-B-4) and (2) older rim and younger interior age domains (R-B-2, R-B-3). All reported ages are weighted-mean

206

Pb/238U ages based on consistent multi-step age

domains with the uncertainty being reported at the 95% confidence level. Crystal R-B-1: The euhedral to subhedral zircon crystal has a size of 70 x 100 μm and lacks any discernable shock features on its surface, but exhibits fine irregular fracturing. The interior of the crystal is cross-cut by ~1–2 μm wide lamellae that are more or less planar and parallel to each other (Fig. 4A). The EBSD maps and inverse pole figures reveal that these lamellae are composed of zircon (blue) misoriented up to 16° from the host grain (warm hues) (Fig. 4B). The CL-image shows that the crystal preserves no clear zoning but rather transgressive cross-cutting by localized, curved, zones (Fig. A1C). The U–Pb depth profile indicates a well-developed weighted mean rim age of 368 ± 2.5 Ma (n = 15 steps in a consistent multi-step age domain; MSWD = 1.0; P = 0.45) and an older age plateau for the crystal interior with a weighted mean age of 487 ± 7.0 (2σ; n = 7; MSWD = 1.33; P = 0.27) (Fig. 4C). Crystal R-B-2: The subrounded zircon crystal has a size of 50 x 120 μm and a granular texture, consisting of individual granules (zircon neoblasts) with a size of 0.5–2 μm in diameter. With respect to their crystallographic orientation, the granules form systematic clusters within the

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crystal (Fig. 5A). The pole figures show that the granules are oriented in an orthogonal crystallographic orientation (~90° dispersion), whereby (001) and {110} are subparallel between clusters (Fig. 5B). The CL-image shows a mottled oscillatory zonation (Fig. A1D). The U–Pb depth profile shows that the crystal contains relatively younger age domains in the rim with a weighted mean age of 207 ± 3.6 Ma (2σ; n = 9; MSWD = 1.9; P = 0.062), and apparently older interior age domains with a weighted mean age of 241 ± 2.0 Ma (n = 17; MSWD = 1.7; P = 0.045) (Fig. 5C). Crystal R-B-3: The subhedral to subrounded zircon crystal has a size of 75 x 150 μm, and the surface morphology is described as granular-textured. The SEM image of the crystal’s interior reveals that individual granules have variable sizes, ranging between ~0.25 and 3 μm (Fig. 6A). The pole figures demonstrate that the granules are systematically oriented in an orthogonal crystallographic orientation (~90° dispersion), with (001) and {110} showing coinciding orientation clusters (Fig. 6B). The CL-image reveals that the crystal is more or less homogenous, with weak zoning visible, although locally a rim zone may be preserved (upper left corner in Fig. A1F). The corresponding U–Pb depth profile represents a mixed age spectrum, which precludes the calculation of a meaningful mean age for the rim. However, interior age domains form a relatively coherent multi-step age domain from which a weighted man age of 204 ± 2.2 Ma (n = 23; MSWD = 1.22; P = 0.22) can be calculated (Fig. 6C). Crystal R-B-4: The euhedral zircon crystal has a size of 55 x 135 μm, with a granulartextured surface. The SEM image of the grain’s interior reveals homogeneous granules, with sizes <1.5 μm, and some irregular fracturing (Fig. 7A). The pole figures show that the granules are systematically oriented in an orthogonal crystallographic orientation (~90° dispersion), with (001)

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and {110} showing the same orientation clusters (Fig. 7B). The CL-image shows that the crystal has zones that become progressively darker towards the interior. In addition, the crystal is locally fractured (lower right corner in Fig. A1I). The U–Pb depth profile shows that the crystal becomes apparently younger with increasing ablation depth, with a weighted mean age of 246 ± 7.3 Ma (2σ; n = 6; MSWD = 1.66; P = 0.15) preserved in the outer part of the crystal and a weighted mean age of 194 ± 2.9 Ma (n = 7; MSWD = 0.37; P = 0.90) preserved in the crystal interior (Fig. 7C).

5. DISCUSSION Our results demonstrate that the zircon crystals obtained from the Babaudus impact melt rock, sampled near the center of the Rochechouart impact structure, display a variety of shock metamorphic microstructures and variable calculated ages resulting from partial to total resetting, possibly due to regional tectonics and the impact event itself (the currently best impact age estimate is 206.92 ± 0.32 Ma [Cohen et al., 2017]). However, the following main questions arise: (5.1.) Can U–Pb depth profiling determine discrete ages within shocked zircon crystals? (5.2.) Are specific age results linked to distinct zircon microtextures, as revealed by CL-images and EBSD mapping? And (5.3.) What are the implications of this study for the Rochechouart impact structure and other structures in the terrestrial impact crater record? A discussion on if the Babaudus melt rock preserves a signature of pre-impact tectonism can be found in Appendix 1.

5.1. Can U–Pb depth profiling determine discrete ages within shocked zircon crystals?

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The U–Pb depth profiling and the subsequent data export in one-second increments allows us to visualize the spatial distribution of U–Pb age data in discrete steps and, thus, interpret their geologic significance. We are able to identify how age domains change relative to the zircon microtextures as a function of increasing ablation depth and time. However, individual seconds cannot be interpreted as a robust age and, therefore, identifying consistent multi-step age domains over a number of seconds (n + 3) and assessing their internal statistics is imperative to interpreting the age data in a meaningful way. This was accomplished for all weighted mean age presented in this study (Figs. 3E, 4C–7C). The experimental setup allows us to obtain age information as discrete, one-second steps. However, the laser washout time represents a limitation because it dictates how much time elapses before the signal returns back to pre-ablation levels after the end of the laser pulse (e.g., Horstwood, et al., 2003). For our applied instrumental settings, the determined washout time is <0.5 seconds post-ablation (returning to <5x the background), thus, with an ablation rate of ~0.5 μm/sec, we are able to resolve ablation intervals of 0.3 μm. Hence, one-second increments are still a careful estimate since we allow for two times the nominal washout time. Both downhole and mass fractionation correction represent additional challenges, limiting accuracy and precision of LA-ICP-MS U-Pb ages (e.g., Horn et al., 2000; Paton et al., 2010; Hoeve et al., 2018). During analysis, the measured Pb/U exhibits a time-depended evolution with increasing depth and evacuation from the laser pit due to dissimilar chemical and ionization properties that depend on volatility and mass of the different isotopes (e.g., Horn et al., 2000, Paton et al., 2010, Hoeve et al., 2018). This fractionation behavior depends on a number of factors, such as ablation pit size, ablation rate, and sample gas flows (e.g., Paton et al., 2010). As in our study, the downhole and mass fractionations are simultaneously corrected through co-analysis of a

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matrix-matched zircon reference material (GJ-1 zircon, Jackson et al., 2004), interspersed during the LA-ICP-MS session. This approach is applied under the premise that matrix and ablation characteristics are identical or very similar for the primary age standard and the unknown analysis, although additional factors might play a role, such as crystallinity, radiation damage, U-content, and orientation which might have to be considered especially when analyzing shocked zircon material (e,.g., Nasdala et al., 2005; Kooijman et al., 2012; Marillo-Sialer et al., 2014, 2016; Schaltegger et al., 2015). However, we monitored the robustness of the downhole fractionation correction through the analysis of interspersed secondary reference zircon with different ages and U-concentrations (Plešovice and Pak1 [UT in-house]) as unknowns. Hence, we are confident that the mass and downhole fractionation corrected depth profiles accurately record the isotopic and age information. However, further studies focusing on the effects of highly disturbed material, such as granular zircon, on downhole fraction might be needed. Previous studies pointed out that while LA-ICP-MS depth profiling is fast and effective in establishing rim-core age relationships in zircon, a drawback might be that the technique allows for increased lateral mixing due to the relatively larger spot size used for the analysis in contrast to secondary ion mass spectrometry (SIMS) (Kelley et al., 2014). SIMS depth profiling has an advantage when it comes to resolving U/Pb ages or trace element concentrations within very thin (nm thick) zircon rims, that can form due to metamorphic or hydrothermal fluids along mineral surfaces and fractures (Breeden et al., 2004; Kelley et al., 2014; Skipton et al., 2016). Commonly SIMS depth profiles resolve depth down to ~5 µm (Breeden et al., 2004; Kelley et al., 2014; Skipton et al., 2016), whereas LA-ICP-MS depth profiling not only resolves rim but also interior isotope or trace element concentrations (viable to >40 µm depth) without (re-) polishing the mineral surface. Our U-Pb depth profiling data set demonstrates that interior age domains can also

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hold valuable age information, be it details on the U-Pb signature of the target rock (R-B-1), the proposed impact age (R-B-3), or younger age domains (R-B-4). In summary, we are confident that with aid of LA-ICP-MS depth profiling and data export in one-second increments, we are able to visualize and interpret U–Pb age domains and their spatial variability within impact-modified (and other) zircon grains. In addition, the technique is complementary to other analytical techniques such as SIMS (also employing the depth-profiling technique).

5.2. Are specific age results linked to distinct zircon microtextures, as revealed by CL-images and EBSD mapping?

5.2.1. FRIGN zircon in the Babaudus impact melt rock Three out of the four zircon crystals presented in this study display a granular textures and show systematic crystallographic orientation patterns within the crystals, as indicated by EBSD pole figures (Figs. 5B-7B). The crystallographic orientation of those sub-domains reflects the transformation from zircon to reidite and reversion back to zircon (so-called “former reidite in granular neoblastic” (FRIGN) zircon [Timms et al., 2017a; Cavosie et al., 2016; 2018]) which we report here for the first time from the Rochechouart impact structure. This zircon type is characterized by the alignment of three orthogonal orientation arrays (Figs. 5B-7B) (Erickson et al., 2017; Cavosie et al., 2018). The unique crystallographic “fingerprint” of reidite is initially produced by the transformation of zircon to the high pressure polymorph reidite at shock pressures ⩾32GPa (Kusaba et al., 1985; Leroux et al., 1999; Glass et al., 2002), whereby <001>zircon is aligned parallel

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to <110>reidite (Erickson et al., 2017). The reversion of reidite back to zircon is also controlled by the aforementioned transformation pathways and, therefore, result in the formation of three orthogonal orientation variants (Erickson et al., 2017; Cavosie et al., 2018). The zircon neoblasts (granular texture), which form as a result of nucleation during the back-transition of shockproduced reidite thus inherit their crystallographic orientations from the reidite ‘parent’ phase (Timms et al., 2017a,b). The transformation of reidite back to zircon is linked to high temperatures, as under atmospheric pressures reidite becomes unstable and transforms back to zircon at temperatures ⩾1200℃ (Kusaba et al., 1985; Cavosie et al., 2018). The formation of FRIGN zircon has thus been interpreted by Cavosie et al (2018) as a result of both high-pressure (⩾30GPa) conditions required to produce reidite, as well as high-temperature conditions (at least ⩾1200℃; if ZrO2 is preserved T ⩾1673℃) required to revert reidite back to zircon. Thus, the preservation of FRIGN zircon within the Babaudus impact melt rock formed as a result of the high-pressure and temperature conditions near ground zero of the Rochechouart impact event.

5.2.2. U–Pb age preservation in FRIGN zircon Our study indicates a link between FRIGN zircon and the preservation of relative younger ages incl. the time of impact. In this study, one out of three granular crystals preserve, within its uncertainty and as its youngest weighted mean age (R-B-2: 207 ± 3.6 Ma; R-B-3; Figs. 7C), the proposed impact age of 206.92 ± 0.32 Ma (Cohen et al., 2017). These findings suggest that Pb is not incorporated into either the reidite and/or zircon neoblasts during the transformation to FRIGN zircon and hence the newly formed neoblasts record the time of impact (Timms et al., 2017a; Kenny et al., 2019). For Pb-loss to have occurred during phase transformation(s), presumably Pb must have been removed from the primary zircon via the phase interface, likely as a phase

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boundary diffusion phenomenon. While crystal R-B-3 preserves a well-developed consistent multi-step age domain (204 ± 2.2 Ma; n = 23; MSWD = 1.22; P = 0.22), crystal R-B-2’s depth profile displays more heterogeneity and hence the weighted mean ages are linked to an increase in data scatter (MSWD) (rim: 207 ± 3.6 Ma; n = 9; MSWD 1.9; P = 0.062; interior: 241 ± 2.0 Ma; n = 17; MSWD 1.7; P = 0.045). The EBSD map of this grain shows that, relative to the other crystals, randomly oriented zircon neoblasts are common and distributed throughout the mineral. In addition, the CL-images show that the crystal is zoned with lighter and darker CL intensities, indicating that the weakly luminescent darker zones likely experienced radiation damage (metamictization). This radiation damage might have facilitated additional Pb loss, in part owing to a reduced crystal annealing temperature (Meldrum et al., 1998; Nasdala et al., 1998; Cherniak and Watson, 2003; Schmieder et al., 2019). It also has to be considered that the standard-based elemental downhole fraction correction (using GJ-1 zircon, analyzed with identical instrumental settings) might have introduced inhomogeneity within the depth profile. However, since we do not observe the same extent of scatter in other depth profiles (e.g. R-B-2 vs R-B-3) that were corrected in the exact same way, we are confident that the downhole-corrected depth profiles reflect U–Pb ages accurately. The fourth crystal (R-B-4), also a FRIGN zircon grain, has consistent multi-step age domains younger (195 ± 2.8 Ma) than the proposed impact age and also too young to be explained by impact-induced hydrothermal activity (e.g., Abramov and Kring, 2007; Schmieder and Jourdan, 2013; Kenny et al., 2019; see also Section 5.3. for further discussion). Since the ~195 Ma zircon age coincides with zircon zones darker in CL, we posit that post-impact Pb annealing (since Pb loss is negligible in crystalline zircon but enhanced in metamict crystals) may have been facilitated due to radiation damage (metamictization) within this crystal due to an unknown post-impact

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thermal event (e.g., Cherniak and Watson, 2003). This young age is similar to the 191.6 ± 9.1 Ma zircon U–Th/He age of Horne (2016) and the ~185 Ma Rb–Sr age of (Reimold and Oskierski, 1987) and might indicate a thermal overprint and alteration of the Rochechouart impactites in the Early Jurassic. To what extent those apparent Jurassic ages are associated with the emplacement of the CAMP volcanism in Europe and concomitant post-Variscan, early Mesozoic mineralization in the French Massif Central (Marignac and Cuney, 1999; Marzoli et al., 1999, 2018; Denyszyn et al., 2018) and whether temperatures sustained by those processes would have been sufficient to partially reset the U–Pb system in (metamict) zircon domains (Cherniak and Watson, 2003) remains, at this point, uncertain. This observation is further supported by the measured U concentration [ppm], which is lower within the older age domains in the rim (315 ppm) and higher (418 ppm) within the interior age domains that yielded the younger age (Fig. 7C). In contrast, the three other crystals show a reversed pattern with higher U concentrations [ppm] in the rim and lower U concentrations [ppm] in the interior zircon zones (Fig. 4C-6C). The decrease of U concentration [ppm] with increasing ablation depth can be observed independent of the crystal type, since we recognize it in two FRIGN zircon crystals (R-B-2 and R-B-3) but also in the grain devoid of neoblasts (R-B-1). Numerous previous studies have focused on the effects of shock metamorphism on the zircon U–Pb system, and the findings show that a high degree of shock pressure is necessary to reset the zircon clock (>59 GPa; Deutsch and Schärer, 1990) in combination with extensive postimpact heating, for example within an impact melt sheet (e.g., Melosh, 1989; Krogh et al., 1993; Kamo et al., 1996; Kalleson et al., 2009; Schmieder et al., 2015, 2017, 2019; Kenny et al., 2019), to sufficiently cause recrystallization (Cavosie et al. 2015). The latter conditions are consistent with the zircon crystals characterized here by EBSD; as the Babaudus impact melt rock contains

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the highest proportion of impact melt (versus unmelted clast material) known at Rochechouart (Lambert, 2010; Sapers et al., 2014). Moreover, previous studies found that granular zircon can record and preserve relatively younger ages, including the proposed impact age (e.g., Krogh et al., 1993; Cavosie et al., 2015; Schmieder et al., 2017, Kenny et al., 2019). This behavior has been explained by (1) the high shock levels, (2) high post-impact temperatures, (3) recrystallization-driven Pb-loss during neoblast formation, and (4) crystals richer in U being more conducive to granule crystallization (Wittmann et al., 2006; Cavosie et al., 2015; Schmieder et al., 2015, Cavosie et al., 2016; Timms et al., 2017a; Kenny et al., 2019). (Note causes (1)-(3) represent a continuous process with point (3) being the most important factor since it is responsible for resetting the zircon U-Pb clock). Our data builds upon those findings and independently demonstrates that FRIGN zircon may provide a suitable targets for the quantification of impact ages by U-Pb LA-ICP-MS or SIMS analyses (see also Hauser et al., 2019). In addition, it has been suggested that, due to small grain size, zircon granules can facilitate preferential volume diffusion and, hence, Pb loss, leading to spurious young ages that post-date the time of impact (Krogh et al., 1993a,b; Kamo et al., 1996; Tohver et al., 2012, Kalleson et al., 2009; Schmieder et al., 2015). Partially metamict crystal R-B-4 may shows signs of such behavior. However, all three FRIGN zircon grains presented in this study also show evidence for partial age resetting due to the Rochechouart impact event within their crystal domains. This partial age resetting might be attributed to zircon domains that did not fully undergo recrystallization due to the impact induced pressure and heating and hence have (partialy) retained their Pb, or, it might be possible that Pb lost from the recrystallized neoblasts became trapped within their interstices. Future work should focus either on LA-ICP-MS crystal surface mapping with smaller, 5 μm–wide

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spots or SIMS analyses for enhanced spatial age resolution in order to distinguish which exact crystal orientations tend to record the time of impact while minimizing the effects of age mixing. Granted, our data set, consisting out of four zircon crystals total, is quite small and one might question how reproducible those data are. All granular zircon crystals preserved evidence of FRIGN zircon and evidence for age resetting. Hence, within this sample of the Babaudus impact melt rock our results are reproducible, in the sense that we observe the same microstructures and that zircon crystals preserving this microstructure have a high potential to preserve younger ages, including the proposed impact age. For the Rochechouart impact structure the impact age and the target rock history, including its thermo-tectonic history, is well constrained (e.g., Faure et al., 2009; Lambert, 2010; Schmieder et al., 2010; Cohen et al., 2017). However, the question remains how well U-Pb depth profiling can serve to obtain the impact age if prior the impact- and/or target rock age is poorly or even unconstrained? In our study, the youngest consistent multistep age domains obtained from the three FRIGN zircon crystals range from ~195 Ma (R-B-4), to ~204 Ma (R-B-3), to ~207 Ma (RB-2), hence the ages alone are not sufficient to determine the time of impact. Therefore the observed zircon microtextures have to be employed as well to evaluate the time of impact (e.g., Krogh et al., 1993; Erickson et al., 2013; Cavosie et al., 2015). In case of the plastically strained crystal R-B-1 this is relatively simple, since the crystal surface (exterior and polished interior) appeared visually undeformed and preserved internal microstructures that are indicative of pressure- and temperature conditions unlikely to result in UPb resetting. Hence, the obtained ages can be relatively confidently linked to the age of the target rock (Fig. 4A-C).

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Evaluating which FRIGN zircon preserves the “true” time of impact is more challenging. Zircon crystal R-B-2 with an obtained rim age of ~207 Ma preserves, in comparison to grains RB-3 and R-B-4, the strongest developed granules (Fig. 5A) and it must be assumed therefore that this crystal likely incorporated less Pb into its zircon neoblasts (e.g., Timms et al., 2017a). Grain R-B-3 has less well developed granules than crystal R-B-2 indicating that less of the host grains U-Pb clock was reset due to the formation of the FRIGN microstructure, as supported by the crystal rim preserving pre-impact age domains. However the crystal interior preserves an age of ~204 Ma, which based on the currently best impact age estimate (206.9 ± 0.32 Ma; Cohen et al., 2017) postdates the impact (Fig. 6C). In the CL image the crystal appears overall homogenous and preserves no growth rims (Fig. A1G). The transition to an amorphous zircon state has been linked to metamictization which facilitates radiogenic Pb diffusion also at lower temperatures (< 200 °C instead of ~900 °C within non-metamict grains [e.g., Cherniak et al., 1991; Meldrum et al., 1998; Salje et al., 1999; Geisler et al. 2002]), however, for grain R-B-3 no higher U (ppm) concentration was measured, in comparison to the two other FRIGN crystals. Still, it might be possible that a post-impact low temperature thermal event (maybe the impact hydrothermal system) produced this post impact age. Similarly, zircon crystals R-B-4 has the least well developed granular texture of the FRIGN zircons presented in this study indicating that grain is composed of less zircon material recrystallized during the transformation from zircon to reidite back to zircon which in turn would result in less resetting of the zircon U-Pb clock which is evident in the rim, despite an unusual young age of ~195 Ma preserved within the crystal interior (Fig. 7A-C). When compared with the CL image of this grain, the age can be linked to CL-dark zones within the crystal interior, indicative

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of metamictization (radiation damage) and therefore an area that potentially facilitated Pb annealing from the crystal long after the impact (Cherniak and Watson, 2003). In conclusion, to determine the time of impact it is critical to link the age information to the observed microstructures. FRIGN crystals that show a high level of granulation have an increased potential to preserve the time of impact due to increased resetting of the U-Pb clock. It appears that FRIGN crystals with less well developed granular texture that might also have experienced radiation damage are also susceptible to resetting but have a tendency to preserve post-impact ages and therefore we suggest that these crystals should be avoided when employing FRIGN zircon to determine the time of an unconstrained impact event.

5.3. What are the implications of this study for the Rochechouart impact structure and other structures in the terrestrial impact crater record?

Two weighted mean ages, 207 ± 3.6 Ma (R-B-2) and 204 ± 2.2 Ma (R-B-3) obtained via U–Pb depth profiling from FRIGN zircon support previous findings, and the ages are, within their uncertainty, equivalent to the currently best impact age estimate of 206.92 ± 0.32 Ma (Cohen et al., 2017) and the age of 202.7 ± 2.2 Ma proposed by Schmieder et al. (2010) (recalculated by Jourdan et al., 2012). Hence, those LA-ICP-MS U–Pb zircon results reproduce previous findings with a different geochronometer, albeit with lower analytical uncertainty. However, both previously proposed

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Ar/39Ar impact ages (Schmieder et al., 2010; Cohen et al., 2017) are

reproduced in our study indicating that younger, post-impact, possibly hydrothermally altered zircon age domains are preserved within the FRIGN zircon crystals.

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The thermal history of Rochechouart is highly complex (see also discussion on regional tectonics in A1), which comprises the thermal-tectono history that pre-dates the impact event (e.g., summary in Faure et al., 2009) but also the thermal evolution of the impact structure including an impact induced hydrothermal system (e.g., Schmieder et al., 2010; Simpson et al., 2017). At a preerosional crater diameter of ~18–25 km (Lambert, 2010), the Rochechouart impact structure likely hosted an extensive and relatively long-lived hydrothermal system (> 1 Myr) (e.g., Naumov, 2005; Abramov and Kring, 2007; Schmieder et al., 2010; Osinski et al., 2013; Cohen et al., 2017). The offset of >1 Myr between the most recent 40Ar/39Ar age of 206.92 ± 0.32 Ma for the Rochechouart impact (Cohen et al., 2017) and the age of an adularia-overprinted shocked gneiss (recalculated age 202.7 ± 2.2 Ma for the “K-metasomatite” of Schmieder et al., 2010; see Jourdan et al., 2012) may, in part, reflect the lifetime of that impact-induced hydrothermal system, however, even if taking the minimum uncertainty between the ages proposed by Schmieder et al. (2010) and Cohen et al. (2017) the duration would be, at least, 1.7 Myr. This duration appears unlikely since this length of time has been proposed as a maximum duration for the much larger (180 km) Chicxulub impact structure (Abranov and Kring, 2007). Hence, rock alteration of the samples employed for 40

Ar/39Ar analyses cannot be ruled out as a contributing factor for the difference of the two

previously proposed ages. In addition, the predicted temperatures (typically ~300–100°C; e.g., Zürcher and Kring, 2004) of the impact-induced hydrothermal system may have caused a prolonged period of Ar loss and, therefore, resulted in apparent ages younger than the time of impact (Schmieder et al., 2010, 2014, 2018a,b; Cohen et al., 2017; Biren et al., 2019; Kenny et al., 2019). The 40Ar/39Ar age of Cohen et al. (2017) for impact melt rock samples that were presumably derived from the near-surface, therefore, seem to indicate (slightly) faster cooling of the Rochechouart impact melt than the adularia age of Schmieder et al. (2010), which likely reflects

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hydrothermal alteration within or underneath the melt sheet. However, the Rochechouart impact structure is today eroded to approximately the structural crater floor level (i.e., the base of craterfilling impact breccias overlying the crystalline crater basement; e.g., Lambert, 2010). Therefore, it is difficult to reconstruct how much (now removed) material previously overlaid the impact melt sheet, and in turn the original sample depth and how crater cooling time varied between different structural crater domains and sample localities. A compounding effect may be argon redistribution via recoil (induced during neutron irradiation of the sample aliquot before

40

Ar/39Ar analysis)

known to cause undulating argon-argon age spectra and, commonly, slightly older apparent ages (e.g., Turner and Cadogan 1974; McDougall and Harrison, 1999; Jourdan et al., 2009), especially in fine-grained impact melt rock or impact glass samples, such as the Babaudus impact melt rock (Cohen et al., 2017). However, in case of the Cohen et al. (2017) study, Ar recoil seems unlikely since the provided plateau ages do not show the typical recoil shape (tilde-shaped) reflecting an exchange between K and Ca rich reservoirs, whereas the age spectra provided by these authors form well developed plateaus that agree with one another rather indicating that possibly rock alterations are minor (Jourdan and Renne, 2013). In medium-sized impact structures in continental settings, post-impact hydrothermal hotfluid flow can persist for a few million years (>1 Myr; Schmieder and Jourdan, 2013; Kenny et al., 2019). Within a marine setting, or as in case or the Rochechouart impact structure in a proposed marginal marine setting (e.g., Lambert, 1977; Kelley and Spray, 1997; Lambert et al., 2010), where an increased water source is available the hydrothermal activity might be prolonged as well (Osinski et al., 2013; Simpson et al., 2017). Numerical modeling suggests that larger impact structures, such as the ~180 km-diameter, end-Cretaceous Chicxulub crater in Mexico, can sustain impact-induced hydrothermal activity for ~1.5 to 2.3 Myr (e.g., Ames et al., 1998; Abramov and

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Kring, 2004, 2007). In addition, low temperature geochronology (40Ar/39Ar [feldspar] and (UTh)/He analyses [zircon, apatite]) conducted on the peak ring rocks from Chicxulub impact crater point towards a prolonged, low-temperature, hydrothermal activity of possibly up to 6 Myr (Pickersgill et al., 2019; Rasmussen et al., 2019b). Based on those previous assumption, our U–Pb age of ~195 Ma cannot be linked to the impact-induced hydrothermal system (but rather to a post-impact thermal event unrelated to the impact event) since it would reflect a lifetime of at least ~11 Myr (as a conservative estimate based on the impact age proposed by Schmieder et al., 2010) and hence would have lasted longer than hydrothermal activity in impact structures much larger than Rochechouart. Finally, although reidite has been found in bedrocks outcropping at central uplifts (e.g., Cox et al., 2018), clasts within suevite impact breccias (e.g., Erickson et al., 2017), and impact ejecta deposits (Glass et al., 2002), FRIGN zircon has, thus far, only be described from impact melt rocks and glasses formed at high shock pressures and temperatures, including the Rochechouart sample described in this study (e.g., Cavosie et al., 2018; Kenny et al., 2019). FRIGN zircon is currently known from a number of impact structures, such as Meteor Crater, Arizona, USA (Cavosie et al., 2016), Acraman, Australia (Timms et al., 2017a), Nördlinger-Ries, Germany (Erickson et al. 2017), Luizi, Democratic Republic of the Congo (Cavosie et al., 2018), Pantasma, Nicaragua (Cavosie et al., 2018), Lappajärvi, Finland (Kenny et al., 2019), Mien, Sweden (Martell et al., 2019), and in the Libyan Desert Glass, western Egypt (Cavosie and Koeberl, 2019). With the Rochechouart crater, one more medium-sized (~18 to 25 km in diameter) impact structure is now known to host this high-pressure and high-temperature form of zircon. This further strengthens the earlier suggestion (Cavosie et al., 2018; Kenny et al.,

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2019) that FRIGN zircon may be of particular use in geochronology when assessing the age of terrestrial impact structures and their target lithologies.

6. CONCLUSION

We report, for the first time, the occurrence of “former reidite in granular neoblastic” (FRIGN) zircon from the medium-sized Rochechouart impact structure, France, demonstrating that zircon crystals within impact melt rock from the central uplift (Babaudus) experienced highshock pressure (⩾30 GPa) and high-temperature conditions (⩾1200 ℃). By applying U–Pb depth profiling, we are able to spatially resolve age domains within those variably altered zircon crystals, demonstrating that FRIGN zircon is favorable to preserve the time of impact (~204–207 Ma). Moreover, a plastically deformed zircon crystal indicative of lower shock conditions preserves age domains reflecting regional tectonics, including pre-Variscan magmatic intrusions (~480 Ma) and metamorphic overgrowth possibly linked to the Variscan orogeny (~360 Ma). Aided by detailed petrographic analyses (SEM, CL, and EBSD) in combination with U–Pb depth profiling, allowing age interpretations in discrete steps, we are able to identify and visualize much of the pre-impact tectono-thermal history together with a reasonably precise impact age of the Rochechouart structure from just a few zircon crystals, demonstrating that combined electron beam characterization and LA-ICP-MS depth profiling are a powerful tool in impact crater geochronology.

Acknowledgments

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We would like to thank to Sarah Simpson, Annemarie Pickersgill, and Darren Mark for supplying the sample of the Babaudus impact melt rock. DFS would like to acknowledge financial support for this study from the Chevron (Gulf) Centennial Professorship Endowement and the UTChron Laboratory Fund. We also would like to especially thank Lisa Stockli UTChron GeoThermochronometry Laboratory at UT Austin) for her exceptional support during the LA-ICP-MS analyses. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. We would like to very much thank the editor Fred Jourdan and the three reviewers Ludovic Ferrière, Aaron Cavosie, and Nick Timms for their comments and suggestions greatly helping us to improve our manuscript. This is a UTIG contribution no. 3607 and LPI Contribution no. XXXX.

Figure captions

Fig. 1: Geological setting of the Rochechouart impact structure, modified after Lambert et al. (2010), Simpson et al. (2017), and Cohen et al. (2017). The target rocks consist of gneiss and granitoid intrusions that have been dated at ~400-300 Ma (Faure et al., 2009). Preserved impact lithologies encompass monomict lithic breccia and polymict lithic breccia, impact melt-bearing breccia, and impact melt rock.

Fig. 2: Hand sample: (A) Yellowish-beige hand sample of the homogenous Babaudus impact melt rock from the Rochechouart impact structure. (B) Close up of (A), showing rare lithic clasts within the matrix (stippled black lines) and the commonly occurring vesicles. (C) Thin section of hand sample. Arrows point towards open vesicles.

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Fig. 3: Wetherill (1956) Concordia plots for the individual, one-second increments for each zircon crystal analyses. The seconds representing consistent multi-step age domains and calculated weighted mean ages are color coded based on the depth profiles (Figs. 4C-7C). (A) Zircon R-B-1; (B) Zircon R-B-2; (C) Zircon R-B-3; and (D) Zircon R-B-4. The criteria for grouping one-second age increments can be found in section 3.3.

Fig. 4: Plastically deformed zircon from the Babaudus impact melt rock from the Rochechouart impact structure (A) (1) BSE image of zircon containing subparallel lamellae and fine-scale tension cracks, (2) close up of (1), (3) EBSD texture component map revealing multiple lamellae orientations, and (4) close up of (3); (B) Pole figures of (001) and {110} crystallographic orientations, showing that 1-2 μm wide lamella preserved within the zircon host grain are misoriented relative the host crystal up to 16°; (C) U–Pb depth profile including the calculated weighted mean ages and associated U concentration [ppm] (2σ) for the respective zircon crystal. Individual boxes represent one-second increments within a single analysis. The criteria for grouping one-second age increments can be found in section 3.3. *Current best impact age estimate by Cohen et al. (2017).

Fig. 5: Former reidite in granular neoblastic (FRIGN) zircon from the Babaudus impact melt rock from the Rochechouart impact structure. (A) (1) BSE image of granular zircon (please note: Zircon crystal shows laser ablation depth profile pit in its center), (2) close up of (1), (3) close up of (1), (4) EBSD map of granular zircon, (5) close up of (4), and (6) close up of (4); (B) Pole figures showing crystallographic orientations of neoblasts. FRIGN zircon is indicated by three orthogonal

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orientation clusters ~ 90° misoriented about (001) and {110}; (C) U–Pb depth profile including the calculated weighted mean ages and associated U concentration [ppm] (2-sigma) for the respective zircon crystal. Individual boxes represent one-second increments within a single analysis. The criteria for grouping one-second age increments can be found in section 3.3. *Current best impact age estimate by Cohen et al. (2017).

Fig. 6: Former reidite in granular neoblastic (FRIGN) zircon from the Babaudus impact melt rock from the Rochechouart impact structure. (A) (1) BSE image of granular zircon (2) close up of (1), (3) EBSD map of granular zircon, and (4) close up of (3); (B) Inverse pole figures showing crystallographic orientations of neoblasts. FRIGN zircon is indicated by three orientation clusters with a ~90° misorientation angle apparent in (001) and {110}; (C) U–Pb depth profile including the calculated weighted mean ages and associated U concentration [ppm] (2σ) for the respective zircon crystal. Individual boxes represent one-second increments within a single analysis. The criteria for grouping one-second age increments can be found in section 3.3. *Current best impact age estimate by Cohen et al. (2017).

Fig. 7: Former reidite in granular neoblastic (FRIGN) zircon from the Babaudus impact melt rock from the Rochechouart impact structure. (A) (1) BSE image of granular zircon, (2) close up of (1), (3) EBSD map of granular zircon, and (4) close up of (3); (B) Pole figures showing crystallographic orientations of neoblasts. FRIGN zircon is indicated by three orientation cluster with ~ 90° misorientation angles, apparent in (001) and {110}; (C) U–Pb depth profile including the calculated weighted mean ages and associated U concentration [ppm] (2σ) for the respective zircon crystal. Individual boxes represent one-second increments within a single analysis. The

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criteria for grouping one-second age increments can be found in section 3.3. *Current best impact age estimate by Cohen et al. (2017).

References cited Abramov O. and Kring D. A. (2004) Numerical modeling of an impact‐ induced hydrothermal system at the Sudbury crater. J. Geophys. Res. Planets. 109, 1–16. Abramov O. and Kring D. A. (2007) Numerical modeling of impact‐ induced hydrothermal activity at the Chicxulub crater. Meteoritics Planet. Sci. 42, 93–112. Allègre C. J. (2008) Isotope geology. Cambridge University Press. Alexandrov P., Cheilletz A., Deloule É. and Cuney M. (2000) 319±7 Ma crystallization age for the Blond granite (northwest Limousin, French Massif Central) obtained by U/Pb ionprobe dating of zircons. Comptes Rendus de l'Académie des Sciences-Series IIA-Earth and Planetary Science, 330, 617–622. Alvarez L. W., Alvarez W., Asaro F. and Michel H. V. (1980) Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208, 1095–1108. Ames D. E., Watkinson D. H. and Parrish R. R. (1998) Dating of a regional hydrothermal system induced by the 1850 Ma Sudbury impact event. Geology 26, 447–450. Andersen, T. (2002) Correction of common lead in U–Pb analyses that do not report 204Pb. Chem. Geol. 192, 59–79. Breeding C. M., Ague J. J., Grove M. and Rupke A. L. (2004) Isotopic and chemical alteration of zircon by metamorphic fluids: U–Pb age depth-profiling of zircon crystals from Barrow’s garnet zone, northeast Scotland. Am. Mineral. 89, 1067–1077.

35

Biren M. B., Wartho J. A., van Soest M. C., Hodges K. V., Cathey H., Glass B. P., Koeberl C., Horton Jr J. W. and Hale W. (2019) (U‐ Th)/He zircon dating of Chesapeake Bay distal impact ejecta from ODP site 1073. Meteoritics Planet. Sci. (in press), pp. 13, doi: 10.1111/maps.13316. Burg J. P. and Matte P. J. (1978) A cross section through the French Massif Central and the scope of its Variscan geodynamic evolution. Zeit. d. Deutschen Geol. Ges. 129, 429–460. Cavosie A. J. and Koeberl C. (2019) Overestimation of threat from 100 Mt–class airbursts? Highpressure evidence from zircon in Libyan Desert Glass. Geology 47, 609–612. Cavosie A. J., Quintero R. R., Radovan H. A. and Moser D. (2010) A record of ancient cataclysm in modern sand: Shock microstructures in detrital minerals from the Vaal River, Vredefort Dome, South Africa. GSA Bull. 122, 1968–1980. Cavosie A. J., Erickson T. M., Timms N. E., Reddy S. M., Talavera C., Montalvo S. D., Pincus M. R., Gibbon R. J. and Moser D. (2015) A terrestrial perspective on using ex situ shocked zircons to date lunar impacts. Geology, 43, 999–1002. Cavosie A. J., Timms N. E., Erickson T. M., Hagerty J. J. and Hörz F. (2016) Transformations to granular zircon revealed: Twinning, reidite, and ZrO2 in shocked zircon from Meteor Crater (Arizona, USA). Geology 44, 703-–706. Cavosie, A.J., Erickson, T.M., Montalvo, P.E., Prado, D.C., Cintron, N.O. and Gibbon, R.J. (2017) The Rietputs Formation in South Africa. In Microstructural Geochronology (eds D.E. Moser, F. Corfu, J.R. Darling, S.M. Reddy and K. Tait). Cavosie A. J., Timms N. E., Ferrière L. and Rochette P. (2018) FRIGN zircon - The only terrestrial mineral diagnostic of high-pressure and high-temperature shock deformation. Geology, 46,

36

891–894.Cherniak D. J. and Watson E. B. (2003) Diffusion in zircon: Rev. Mineral. Geochem. 53, 113–143. Cherniak D. J. and Watson E. B. (2003) Diffusion in zircon: Rev. Mineral. Geochem. 53, 113– 143. Cherniak D. J., Lanford W. A. and Ryerson F. J. (1991) Lead diffusion in apatite and zircon using ion implantation and Rutherford backscattering techniques. Geochim. Cosmochim. Acta 55, 1663-1673. Chevremont P., Floc’h J-P M. F., Stussi J., Delbos R., Sauret B., Bles J., Courbe C., Vuaillat D. and Gravelat C. (1996) Notice explicative de la Carte géologique de la France à 1/50 000. Rochechouart, Orléans, BRGM. Cohen B. E., Mark D. F., Lee M. R. and Simpson S. L. (2017) A new high‐ precision 40Ar/39Ar age for the Rochechouart impact structure: At least 5 Ma older than the Triassic–Jurassic boundary. Meteoritics Planet. Sci. 52, 1600–1611. Cox M. A., Cavosie A. J., Bland P. A., Miljković K. and Wingate M. T. (2018) Microstructural dynamics of central uplifts: Reidite offset by zircon twins at the Woodleigh impact structure, Australia. Geology 46, 983–986. Denyszyn S. W., Fiorentini M. L., Maas R. and Dering G. (2018) A bigger tent for CAMP. Geology 46, 823–826. Deutsch A. and Schärer U. (1990) Isotope systematics and shock-wave metamorphism: I. U–Pb in zircon, titanite and monazite, shocked experimentally up to 59 GPa. Geochim. Cosmochim. Acta 54, 3427–3434. Deutsch A. and Schärer U. (1994) Dating terrestrial impact events. Meteoritics 29, 301–322.

37

Erickson T. M., Pearce M. A., Reddy S. M., Timms N. E., Cavosie A. J., Bourdet J., Rickard W. D. A. and Nemchin A. A. (2017) Microstructural constraints on the mechanisms of the transformation to reidite in naturally shocked zircon. Contrib. Mineral. Petrol. 172, 1–26. Farnan I., Balan E., Pickard C. J. and Mauri F. (2003) The effect of radiation damage on local structure in the crystalline fraction of ZrSiO4: Investigating the29Si NMR response to pressure in zircon and reidite. Am. Mineral. 88, 1663–1667. Faure, M., Ledru, P. and Lardeaux, J. M. (2007) Palaeozoic orogenies in the French Massif Central; A cross section from Béziers to Lyon. in Guide book of field excursion. Géol. France 2, 7–54. Faure M., Bé Mézème E., Cocherie A., Rossi P., Chemenda A. and Boutelier D. (2008) Devonian geodynamic evolution of the Variscan Belt, insights from the French Massif Central and Massif Armoricain. Tectonics 27, 1–19. Faure M., Lardeaux J. M. and Ledru P. (2009) A review of the pre-Permian geology of the Variscan French Massif Central. Comptes Rendus Geoscience 341, 202–213. French B. M. (1998) Traces of catastrophe. A handbook of shock-metamorphic effects in terrestrial meteorite impact structures. LPI Contrib. 954. Lunar and Planetary Institute, Houston, Texas, USA, 120 pp. Hauser N., Reimold W. U., Cavosie A. J., Crósta A. P., Schwarz W. H., Trieloff M., Da Silva Maia de Souza C., Pereira L. A., Rodrigues E. N. and Brown M. (2019) Linking shock textures revealed by BSE, CL, and EBSD with U‐ Pb data (LA‐ ICP‐ MS and SIMS) from zircon from the Araguainha impact structure, Brazil. Meteoritics Planet. Sci. 54, 22862311.

38

Horn, I., Rudnick, R. L., & McDonough, W. F. (2000). Precise elemental and isotope ratio determination by simultaneous solution nebulization and laser ablation-ICP-MS: application to U–Pb geochronology. Chem. Geol. 164, 281-301. Gébelin A., Brunel M., Monié P., Faure M. and Arnaud N. (2007) Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: Structural and 40

Ar/39Ar investigations. Tectonics 26, 1–27.

Geisler T., Pidgeon R. T., Van Bronswijk W. And Kurtz R. (2002) Transport of uranium, thorium, and lead in metamict zircon under low-temperature hydrothermal conditions. Chem. Geol. 191, 141-154. Glass B. P., Liu S. and Leavens P. B. (2002) Reidite: An impact-produced high-pressure polymorph of zircon found in marine sediments. Am. Mineral. 87, 562–565. Grieve R. A. and Pesonen L. J. (1996) Terrestrial impact craters: their spatial and temporal distribution and impacting bodies. In Worlds in Interaction: Small Bodies and Planets of the Solar System (pp. 357–376). Springer, Dordrecht. Hazen R. M. and Finger L. W. (1979) Crystal structure and compressibility of zircon at high pressure. Am. Mineral. 64, 196–201. Hildebrand A. R., Penfield G. T., Kring D. A., Pilkington M., Camargo Z. A., Jacobsen S. B. and Boynton W.V. (1991) Chicxulub crater: a possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico. Geology 19, 867–871. Hill R. J. and Cranswick L. M. D. (1994) International union of crystallography. Commission on powder diffraction. Rietveld refinement round Robin. II. Analysis of monoclinic ZrO2. J. Appl. Crystal. 27, 802–844.

39

Horne A. (2016) (U–Th)/He, U/Pb, and Radiation Damage Dating of the Rochechouart-Chassenon Impact Structure, France. Arizona State University 90 pp. Horstwood M. S., Foster G. L., Parrish R. R., Noble S. R. and Nowell G. M. (2003) Common-Pb corrected in situ U–Pb accessory mineral geochronology by LA-MC-ICP-MS. J. Anal. Atomic Spectr. 18, 837–846. Horstwood M. S., Košler J., Gehrels G., Jackson S. E., McLean N. M., Paton C., Pearson N. J., Sircombe K., Sylvester P., Vermeesch P., Bowring, J. F., Condon D. J. and Schoene B. (2016) Community‐ derived standards for LA‐ ICP‐ MS U‐ (Th‐ ) Pb geochronology– Uncertainty propagation, age interpretation and data reporting. Geostand. and Geoanal. Res. 40, 311-332. Jackson S. E., Pearson N. J., Griffin W. L. and Belousova E. A. (2004) The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U–Pb zircon geochronology. Chem. Geol. 211, 47–69. Jourdan F. and Renne P. R. (2014) Neutron-induced 37Ar recoil ejection in Ca-rich minerals and implications for 40Ar/39Ar dating. Geol. Soc. Spec. Publ. 378, 33-52. Jourdan F., Renne P. R. and Reimold W. U. (2009) An appraisal of the ages of terrestrial impact structures. Earth Planet. Sci. Lett. 286, 1–13. Jourdan F., Reimold W. U. and Deutsch A. (2012) Dating terrestrial impact structures. Elements 8, 49–53. Kamo S. L. and Krogh T. E. (1995) Chicxulub crater source for shocked zircon crystals from the Cretaceous-Tertiary boundary layer, Saskatchewan: Evidence from new U–Pb data. Geology 23, 281–284.

40

Kamo S. L., Reimold W. U., Krogh T. E. and Colliston W. P. (1996) A 2.023 Ga age for the Vredefort impact event and a first report of shock metamorphosed zircons in pseudotachylitic breccias and granophyre. Earth Planet. Sci. Lett. 144, 369–387. Kalleson E., Corfu F. and Dypvik H. (2009) U–Pb systematics of zircon and titanite from the Gardnos impact structure, Norway: Evidence for impact at 546 Ma? Geochim. Cosmochim. Acta 73, 3077–3092. Kenkmann T., Poelchau M. H. and Wulf G. (2014) Structural geology of impact craters. J. Struct. Geol. 62, 156–182. Kelley S. P. and Spray J. G. (1997) A late Triassic age for the Rochechouart impact structure, France. Meteoritics Planet. Sci. 32, 629–636. Kelly C. J., McFarlane C. R., Schneider D. A. and Jackson S. E. (2014) Dating Micrometre‐ Thin Rims Using a LA‐ ICP‐ MS Depth Profiling Technique on Zircon from an Archaean Metasediment: Comparison with the SIMS Depth Profiling Method. Geostandards and Geoanalytical Res.earch 38, 389–407. Kenny G. G., Morales L. F., Whitehouse M. J., Petrus J. A. and Kamber B. S. (2017) The formation of large neoblasts in shocked zircon and their utility in dating impacts. Geology 45, 1003– 1006. Kenny G. G., Schmieder M., Whitehouse M. J., Nemchin A. A., Morales L. F., Buchner E., Bellucci J. J. and Snape J. F. (2019) A new U–Pb age for shock-recrystallised zircon from the Lappajärvi impact crater, Finland, and implications for the accurate dating of impact events. Geochim. Cosmochim. Acta 245, 479–494. Kooijman E., Berndt J. and Mezger K. (2012) U-Pb dating of zircon by laser ablation ICP-MS: recent improvements and new insights. European Journal of Mineralogy 24, 5-21.

41

Kraut F. (1969) Über ein neues Impaktit-Vorkommen im Gebiete von Rochechouart-Chassenon (Départements Haute-Vienne und Charente, Frankreich). Geologica Bavarica 61, 428– 450. Kraut F. and French B. M. (1971) The Rochechouart meteorite impact structure, France: Preliminary geological results. J. Geophys. Res. 76, 5407–5413. Krogh T. E., Kamo S. L., Sharpton V. L., Marin L. E. and Hildebrands A. R. (1993a) U–Pb ages of single shocked zircons linking distal K/T ejecta to the Chicxulub crater. Nature 366, 731–734. Krogh T. E., Kamo S. L. and Bohor B. A. (1993b) Fingerprinting the K/T impact site and determining the time of impact by U-Pb dating of single shocked zircons from distal ejecta. Earth Planet. Sci. Lett. 119, 425–429. Kusaba K., Yasuhiko Syono M. K. and Fukuoka K. (1985) Shock behavior of zircon: Phase transition to scheelite structure and decomposition. Earth Planet. Sci. Lett. 72, 433–439. Lambert P. (1977) Les effets des ondes de choc naturelles et artificielles, et le cratère d'impact de Rochechouart (Limousin, France). Dissertation, 515 pp. Lambert P. (2010) Target and impact deposits at Rochechouart impact structure, France. GSA Spec. Pap. 465, 509–541. Lardeaux J. M., Schulmann K., Faure M., Janoušek V., Lexa O., Skrzypek E., Edel J. B. and Štípská

P.

(2014)

The

moldanubian

zone

in

the

French

Massif

Central,

Vosges/Schwarzwald and Bohemian Massif revisited: differences and similarities. Geol. Soc. London Spec Publ. 405, 7–44.

42

Ledru P., Lardeaux J. M., Santallier D., Autran A., Quenardel J-M., Floch J-P., Lerouge G., Maillet N., Marchand J. and Ploquin A. (1989) Où sont les nappes dans le Massif central français? Bull. Soc. géol. France 3, 605–618. Leroux H., Reimold W. U., Koeberl C., Hornemann U. and Doukhan J.-C. (1999) Experimental shock deformation in zircon: A transmission electron microscopic study. Earth Planet. Sci. Lett. 169, 291–301. Lindström S., Pedersen G. K., Van De Schootbrugge B., Hansen K. H., Kuhlmann N., Thein J., Johansson L., Petersen H. I., Alwmark C., Dybkjær K. and Weibel R. (2015) Intense and widespread seismicity during the end-Triassic mass extinction due to emplacement of a large igneous province. Geology 43, 387–390. Lotout C., Pitra P., Poujol M. and Van Den Driessche J. (2017) Ordovician magmatism in the Lévézou massif (French Massif Central): tectonic and geodynamic implications. Int. J. Earth Sci. 106, 501–515. Ludwig K. (2012) Isoplot/Ex version 3.7. A geochronological toolkit for Microsoft Excel. Berkeley Geochronological Centre, Special Publication, 70 pp. Marignac C. and Cuney M. (1999) Ore deposits of the French Massif Central: insight into the metallogenesis of the Variscan collision belt. Mineralium Deposita 34, 472-504. Marsh J. H. and Stockli D. F. (2015) Zircon U–Pb and trace element zoning characteristics in an anatectic granulite domain: Insights from LA-ICP-MS depth profiling. Lithos 239, 170– 185. Marzoli A., Renne P. R., Piccirillo E. M., Ernesto M., Bellieni G. and De Min A. (1999) Extensive 200-million-year-old continental flood basalts of the Central Atlantic Magmatic Province. Science 284, 616–618.

43

Marzoli A., Callegaro S., Dal Corso J., Davies J.H., Chiaradia M., Youbi N., Bertrand H., Reisberg L., Merle R. and Jourdan F. (2018) The Central Atlantic magmatic province (CAMP): a review: In L. H. Tanner (Ed.) The Late Triassic World (pp. 91-125). Springer, Cham, Switzerland. Matte P. (1986) Tectonics and plate tectonics model for the Variscan belt of Europe. Tectonophysics 126, 329–374. Marignac C. and Cuney M. (1999) Ore deposits of the French Massif Central: insight into the metallogenesis of the Variscan collision belt. Mineralium Deposita 34, 472–504. Marillo-Sialer E., Woodhead J., Hergt J., Greig A., Guillong M., Gleadow A., Evans N. and Paton C. (2014) The zircon ‘matrix effect’: evidence for an ablation rate control on the accuracy of U–Pb age determinations by LA-ICP-MS. J. Anal. At. Spectrom. 29, 981-989. Marillo-Sialer E., Woodhead J., Hanchar J. M., Reddy S. M., Greig A., Hergt J. and Kohn B. (2016) An investigation of the laser-induced zircon ‘matrix effect’. Chem. Geol. 438, 1124. McDougall I. and Harrison T. M. (1999) Geochronology and Thermochronology by the 40Ar/39Ar Method. Oxford University Press on Demand, 199 pp. Meldrum A., Boatner L. A., Weber W. J. and Ewing, R. C. (1998) Radiation damage in zircon and monazite. Geochim. Cosmochim. Acta 62, 2509-2520. Melosh H. J. (1989) Impact Cratering - A Geologic Process. Oxford University Press, New York, USA, 245 pp. Mezger K. and Krogstad E. (1997) Interpretation of discordant U‐ Pb zircon ages: an evaluation. J. Metam. Geol. 15, 127–140.

44

Montalvo S. D., Cavosie A. J., Erickson T. M. and Talavera C. (2017) Fluvial transport of impact evidence from cratonic interior to passive margin: Vredefort-derived shocked zircon on the Atlantic coast of South Africak. Am. Mineral. 102, 813-823. Montalvo P. E., Cavosie A. J., Kirkland C. L., Evans N. J., McDonald B. J., Talavera C., Erickson T. M., and Lugo-Centeno C. (2018) Detrital shocked zircon provides first radiometric age constraint (< 1472 Ma) for the Santa Fe impact structure, New Mexico, USA. GSA Bull. 131, 845-863. Morozova I. (2015) Strength study of zircon under high pressure. University of Western Ontario 111 pp. Nasdala L., Pidgeon R. T., Wolf D. and Irmer G. (1998) Metamictization and U–Pb isotopic discordance in single zircons: a combined Raman microprobe and SHRIMP ion probe study. Min. Petrol. 62, 1–27. Nasdala L., Hanchar J. M., Kronz A. and Whitehouse M. J. (2005) Long-term stability of alpha particle damage in natural zircon. Chem. Geol. 220, 83-103. Naumov M. V. (2005) Principal features of impact‐ generated hydrothermal circulation systems: Mineralogical and geochemical evidence. Geofluids 5, 165–184. Osinski G. R., Tornabene L. L., Banerjee N. R., Cockell C. S., Flemming R., Izawa M. R., McCutcheon J., Parnell J., Preston L. J., Pickersgill A. E. and Pontefract A. (2013) Impactgenerated hydrothermal systems on Earth and Mars. Icarus 224, 347–363. Osinski G. R. and Ferrière L. (2016) Shatter cones: (Mis) understood?. Science Advances 2, e1600616.

45

Paton C., Woodhead J. D., Hellstrom J. C., Hergt J. M., Greig A. and Maas R. (2010) Improved laser ablation U‐ Pb zircon geochronology through robust downhole fractionation correction. Geochem. Geophys. Geosyst. 11, 1-36. Paton C., Hellstrom J., Paul B., Woodhead J. and Hergt J. (2011) Iolite: Freeware for the visualisation and processing of mass spectrometric data. J. Anal. Atomic Spectr. 26, 2508– 2518. Petrus J. A. and Kamber B. S. (2012) VizualAge: A novel approach to laser ablation ICP‐ MS U‐ Pb geochronology data reduction. Geostandards and Geoanalytical Research 36, 247–270. Petrus J. A., Kenny G. G., Ayer J. A., Lightfoot P. C. and Kamber B. S. (2016) Uranium - lead zircon systematics in the Sudbury impact crater-fill: implications for target lithologies and crater evolution. J. Geol. Soc. 173, 59–75. Piazolo S., Austrheim H., and Whitehouse M. (2012) Brittle-ductile microfabrics in naturally deformed zircon: Deformation mechanisms and consequences for U-Pb dating. Am. Mineral. 97, 1544–1563. Pickersgill A., Christou E., Mark D. F., Lee M. R., Tremblay M.M., Rasmussen C., Morgan J. V., Gulick S. P. S., Schmieder M., Bach W., Osinski G. R., Simpson S., Kring D. A., Cockell C., Collins G. S., Christeson G., Tikoo S., Stockli D., Ross C., Wittmann A., Swindle T. and the Expedition 364 Scientists. (2019) 6 million years of hydrothermal activity at Chicxulub?. Large Meteorite Impacts and Planetary Evolution VI conference, 5082. Pohl J., Ernstson K., and Lambert P. (1978) Gravity measurements in the Rochechouart impact structure (France). Meteoritics 13, 601– 604.

46

Ramezani J., Bowring S. A., Pringle M. S., Winslow III F. D. and Rasbury E. T. (2005) The Manicouagan impact melt rock: A proposed standard for the intercalibration of U–Pb and 40

Ar/39Ar isotopic systems. Geochim. Cosmochim. Acta 69, 321.

Rasmussen C., Stockli D. F., Ross C. H., Pickersgill A., Gulick S. P., Schmieder M., Christeson G. L., Wittmann A., Kring D. A., Morgan J. V., and IODP-ICDP Expedition 364 Science Party. (2019a) U-Pb memory behavior in Chicxulub's peak ring - Applying U-Pb depth profiling to shocked zircon. Chem. Geol. 525, 356-367. Rasmussen C., Stockli D., Chatterjee R., Pickersgill A. E., Gulick S. P. S., Schmieder M., Kring D. A., Wittmann A., Ross C., Christeson G., Tikoo S., Xiao L., Morgan J. V., and the IODP Exp. 364 Science Party. (2019b) Thermal history of Chicxulub’s peak ring – constraints from zircon U-Pb and (U-Th)/He double dating. Large Meteorite Impacts and Planetary Evolution VI conference, 5081. Reddy S. M., Timms N. E. and Eglington B. M. (2008) Electron backscatter diffraction analysis of zircon: A systematic assessment of match unit characteristics and pattern indexing optimization. Am. Mineral. 93, 187–197. Reimold W. U. and Oskierski W. (1987) The Rb-Sr-age of the Rochechouart impact structure, France, and geochemical constraints on impact melt-target rock-meteorite compositions: In Pohl, J. (Ed.) Research in terrestrial impact structures, Vieweg + Teubner Verlag, Wiesbaden, pp. 94–114. Salje E. K. H., Chrosch J. and Ewing R. C. (1999) Is “metamictization” of zircon a phase transition?. Am. Mineral. 84, 1107-1116.

47

Sapers H. M., Osinski G. R., Banerjee N. R., Ferrière L., Lambert P. and Izawa M. R. (2014) Revisiting the Rochechouart impact structure, France. Meteoritics Planet. Sci. 49, 2152– 2168. Schaltegger U., Schmitt A. K. and Horstwood M. S. A. (2015) U–Th–Pb zircon geochronology by ID-TIMS, SIMS, and laser ablation ICP-MS: recipes, interpretations, and opportunities. Chem. Geol. 402, 89-110. Simpson S. L., Boyce A. J., Lambert P., Lindgren P., and Lee, M. R. (2017) Evidence for an impact-induced biosphere from the δ34S signature of sulphides in the Rochechouart impact structure, France. Earth Planet. Sci. Lett. 460, 192-200. Schmieder M. and Jourdan F. (2013) The Lappajärvi impact structure (Finland): Age, duration of crater cooling, and implications for early life. Geochim. Cosmochim. Acta 112, 321–339. Schmieder M., Buchner E., Schwarz W. H., Trieloff M. and Lambert P. (2010) A Rhaetian 40

Ar/39Ar age for the Rochechouart impact structure (France) and implications for the latest

Triassic sedimentary record. Meteoritics Planet. Sci. 45, 1225–1242. Schmieder M., Jourdan F., Tohver E. and Cloutis E. A. (2014)

40

Ar/39Ar age of the Lake Saint

Martin impact structure (Canada) - Unchaining the Late Triassic terrestrial impact craters. Earth Planet. Sci. Lett. 406, 37–48. Schmieder M., Tohver E., Jourdan F., Denyszyn S. W. and Haines P. W. (2015) Zircons from the Acraman impact melt rock (South Australia): Shock metamorphism, U–Pb and 40Ar/39Ar systematics, and implications for the isotopic dating of impact events. Geochim. Cosmochim. Acta 161, 71–100.

48

Schmieder M., Shaulis B. J., Lapen T. J. and Kring D. A. (2017) U–Th–Pb systematics in zircon and apatite from the Chicxulub impact crater, Yucatán, Mexico. Geol. Mag. 155, 1330– 1350. Schmieder M., Kennedy T., Jourdan F., Buchner E. and Reimold W.U. (2018a) A high-precision 40

Ar/39Ar age for the Nördlinger Ries impact crater, Germany, and implications for the

accurate dating of terrestrial impact events. Geochim. Cosmochim. Acta 220, 146–157. Schmieder M., Kennedy T., Jourdan F., Buchner E. and Reimold W.U. (2018b) Response to comment on “A high-precision

40

Ar/39Ar age for the Nördlinger Ries impact crater,

Germany, and implications for the accurate dating of terrestrial impact events” by Schmieder et al. (Geochim. Cosmochim. Acta 220 (2018) 146–157): Geochim. Cosmochim. Acta 238, 602–605. Schmieder M., Shaulis B. J., Lapen T. J., Buchner E. and Kring D. A. (2019) In situ U–Pb analysis of shocked zircon from the Charlevoix impact structure, Québec, Canada. Meteoritics Planet. Sci. (in press), 20 pp., doi: 10.1111/maps.13315. Schwarz W. H., Breutmann G., Schmitt A. K., Trieloff M., Ludwig T., Hanel M., Buchner E., Schmieder M., Pesonen L. J. and Moilanen J. (2016) U/Pb dating of zircon from the Suvasvesi impact structures, Finland. LPI Contribution 1921, abstract no. 6297. Simpson S. L., Boyce A. J., Lambert P., Lindgren P. and Lee M. R. (2017) Evidence for an impactinduced biosphere from the δ34S signature of sulphides in the Rochechouart impact structure, France. Earth Planet. Sci. Lett. 460, 192–200. Sláma J., Košler J., Condon D. J., Crowley J. L., Gerdes A., Hanchar J. M., Horstwood M. S. A., Morris G. A., Nasdala L., Norberg N., Schaltegger U., Schoene B., Tubrett M. N. and

49

Whitehouse M. J. (2008) Plešovice zircon - a new natural reference material for U-Pb and Hf isotopic microanalysis. Chem. Geol. 249, 1-35. Skipton D. R., Schneider D. A., McFarlane C. R. M., St-Onge M. R. and Jackson S. E. (2016) Multi-stage zircon and monazite growth revealed by depth profiling and in situ U-Pb geochronology: resolving the Paleoproterozoic tectonics of the Trans-Hudson Orogen on southeastern Baffin Island, Canada. Precambr. Res. 285, 272-298. Spencer C. J., Kirkland C. L. and Taylor R. J. (2016) Strategies towards statistically robust interpretations of in situ U–Pb zircon geochronology. Geoscience Frontiers 7, 581–589. Stacey J. S. and Kramers J. D. (1975) Approximation of terrestrial lead isotope evolution by a twostage model. Earth Planet. Sci. Lett. 26, 207–221. Stockli D. F., Rasmussen C., Ross C., Stockli L. D., Gulick S. P. S., Chatterjee R., Christeson G. L., Wittmann A., Schmieder M., Kring D. A., Xiao L., Morgan J. V. and Expedition 364 Science Party. (2018) Zircon and apatite U–Pb and (U–Th)/He geo- and thermochronometry of the crystalline basement in the Chicxulub’s peak ring structure: Indianapolis, Indiana. GSA Abstracts with Programs 50, 6. https://doi.org/10. 1130/abs/2018am-320351. Stöffler D. (1984) Glasses formed by hypervelocity impact. J. Non-Cryst. Solids 67, 465–502. Timms N. E., Erickson T. M., Pearce M. A., Cavosie A. J., Schmieder M., Tohver E., Reddy S. M., Zanetti M. R. and Nemchin A. A. (2017a) A pressure-temperature phase diagram for zircon at extreme conditions. Earth Sci. Rev. 165, 185–202. Timms N. E., Erickson T. M., Zanetti M. R., Pearce M. A., Cayron C., Cavosie A. J., Reddy S. M. Wittmann A. and Carpenter P. K. (2017b) Cubic zirconia in >2370 °C impact melt records Earth’s hottest crust. Earth Planet. Sci. Lett. 477, 52–58.

50

Tohver E., Lana C., Cawood P. A., Fletcher I. R., Jourdan F., Sherlock S., Rasmussen B., Trindade R. I. F., Yokoyama E., Souza Filho C. R. and Marangoni Y. (2012). Geochronological constraints on the age of a Permo–Triassic impact event: U–Pb and 40Ar/39Ar results for the 40 km Araguainha structure of central Brazil. Geochim. Cosmochim. Acta 86, 214227. Turner G. and Cadogan P. H. (1974) Possible effects of 39Ar recoil in 40Ar-39Ar dating. In Lunar and Planetary Science Conference Proceedings 5, 1601–1615. Turpin L., Cuney M., Friedrich M., Bouchez J. L. and Aubertin M. (1990) Meta-igneous origin of Hercynian peraluminous granites in NW French Massif Central: implications for crustal history reconstructions. Contrib. Mineral. Petrol. 104, 163–172. Wendt I. and Carl C. (1991) The statistical distribution of the mean squared weighted deviation. Chem. Geol.: Isotope Geoscience Section 86, 275–285. Wetherill G. W. (1956) Discordant uranium‐ lead ages: I. EOS. Transactions American Geophysical Union 37, 320–326. Ver Hoeve T. J., Scoates J. S., Wall C. J., Weis D. and Amini M. (2018) Evaluating downhole fractionation corrections in LA-ICP-MS U-Pb zircon geochronology. Chem. Geol. 483, 201-217. Wittmann A., Kenkmann T., Schmitt R. T. and Stöffler D. (2006) Shock‐ metamorphosed zircon in terrestrial impact craters. Meteoritics Planet. Sci. 41, 433–454. Zürcher L. and Kring D. A. (2004) Hydrothermal alteration in the core of the Yaxcopoil‐ 1 borehole, Chicxulub impact structure, Mexico. Meteoritics Planet. Sci. 39, 1199–1221.

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0° 47’ 31’’E

France N

Ri

N

ve

Rochechoaurt impact structure

rV ien

ne

300 km

Crater center Impactite units Impact meltbearing breccia Polymict lithic breccia Monomict lithic breccis

45° 49’N

Impact melt rock

Target rock units Granitoids Layered gneiss Gneiss

5 km

Babaudus sample locality

B 1 cm B

A

1 cm

C

1 mm

B

A R-B-1

520

0.065

360

0.4

206Pb/238U

0.044

300

0.024 0.0

190

220 180

0.4

207Pb/235U

One-second increment ellipses

0.1

0.2

0.3

207Pb/235U

R-B-4

0.4

0.5

300 260

0.040 0.036

0.6

Rim age domains = RAD*

0.8

IAD

220

0.032 180

0.028

IAD 0.2

RAD

170

0.044

0.032 0.028

IAD

210

0.032

D

260

0.040

230

0.036

0.048

R-B-3

250

0.024 0.0

0.8

207Pb/235U

C

0.036

0.6

270

R-B-2

0.028

RAD

320

0.045 0.2

0.048

IAD

400

0.055

0.052

206Pb/238U

440

0.044 0.040

480

0.075

206Pb/238U

206Pb/238U

0.085

0.024 0.0

0.1

Interior age domains = IAD*

RAD 0.2

0.3

207Pb/235U

0.4

0.5

data-point error ellipses are 2σ *Used to calculate weighted mean ages

A

10μm

B

10μm

x

(001)

2

C 1

550

2 10μm

{110} 0

500

Age (Ma)

10μm

16°

450

Depth (µm)

5

R-B-1

10

3

4

15

18

⁴⁰Ar/³⁹Ar impact age: 206.92 +/- 0.32 Ma*

368 +/- 2.5 Ma MSWD = 1.0 P = 0.45 311 +/- 16 U [ppm]

400

487 +/- 7.0 Ma MSWD = 1.33 P = 0.27 165 +/- 2 U [ppm]

350

4 0°

y

300 0

box heights are 2σ

5

10

15

20

Time (sec.)

25

30

35

38

A

10μm

2

10μm

B

y x

(001)

C 10μm

5

2 5

3 6

10μm (001)

6

250

5

R-B-2

10

10μm

10μm

150 0

15

18

⁴⁰Ar/³⁹Ar impact age: 206.92 +/- 0.32 Ma*

207 +/- 3.6 Ma MSWD = 1.9 P = 0.062 355 +/- 7 ppm [U]

241 +/- 2.0 Ma MSWD = 1.7 P = 0.045 145 +/- 1 ppm [U]

200

{100} {110}

0 350

300

Age (Ma)

1 4

3

{110}

Depth (µm)

box heights are 2σ

5

10

15

20

25

Time (sec.)

30

35

38

B

A

2

x

(001)

Melt

C

1 (001)

y

10μm

2

350

10μm

{110} 0

5

R-B-3

Depth (µm) 10

18

⁴⁰Ar/³⁹Ar impact age: 206.92 +/- 0.32 Ma*

{100}

300

Age (Ma)

{110}

4

3

15

204 +/- 2.2 Ma MSWD = 1.2 P = 0.90 229 +/- 3 ppm [U]

250

200 419 +/- 16 ppm [U]

10μm

4

step size = 100 nm

10μm

150

box heights are 2σ

0

5

10

15

20

25

Time (sec.)

30

35

38

A

B

y x

2 (001)

C 1 (001)

10μm

2

350

10μm

{100}

{110} 0

5

R-B-4

10

Age (Ma)

4

250

246 +/- 7.3 Ma MSWD = 1.6 P = 0.15 315 +/- 5 ppm [U]

150

4

10μm

18

194 +/- 2.9 Ma MSWD = 0.37 P = 0.90 418 +/- 8 ppm [U]

200

10μm

15

⁴⁰Ar/³⁹Ar impact age: 206.92 +/- 0.32 Ma*

300

{110}

3

Depth (µm)

0

5

10

15

20

box heights are 2σ

25

Time (sec.)

30

35

38

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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