High-resolution EBSD and SIMS U–Pb geochronology of zircon, titanite, and apatite: insights from the Lac La Moinerie impact structure, Canada

Abstract

High-resolution electron backscattered diffraction (EBSD) and correlated in situ secondary ion mass spectrometry (SIMS) U–Pb geochronology have been conducted on impact-metamorphosed zircon, titanite, and apatite grains from the 453 Ma Lac La Moinerie impact structure of Québec, Canada. Building on previous work, our new U–Pb results highlight the challenges associated with SIMS dating of low U, common Pb-rich accessory phases (apatite and titanite) within terrestrially shocked impact lithologies. Unlike zircon, microstructurally controlled Pb loss is limited in apatite and titanite, providing evidence that the mechanisms governing impact resetting in these phases are distinct from high U, low common Pb phases (zircon, monazite, baddeleyite). EBSD mapping reveals internal complexity within all three phases, providing new insights into their microstructural evolution during impact-related metamorphism. Correlated EBSD and Raman analysis of Former Reidite In Granular Neoblastic (FRIGN) zircon provides the first documented case of post-recrystallization radiation damage of individual zircon neoblasts. Two modes of recrystallization are recorded in apatite: systematic (Type I), wherein strain-free domains are preferentially oriented along specific crystallographic planes, and non-systematic (Type II), where strain-free domains exhibit no preferred orientation. A new shock twin is observed in titanite that has a disorientation relationship of 74°/ < 103 > , interpreted to be the result of strain-induced misorientation of pre-existing $\left\{ {1\overline{1}1} \right\}$ shock twins. We document the first case of impact-recrystallized titanite, which is intergrown with granular rutile and partially preserves former $\left\{ {1\overline{1}1} \right\}$ shock twins along boundaries of strain-free subdomains. Based on our EBSD observations, we provide a microstructural framework outlining the sequential evolution of zircon, titanite, and apatite during shock and thermal metamorphism.

Introduction

Hypervelocity impact events have strongly influenced the formation of our solar system. Constraining the timing of these events is critical to understanding their role in Earth’s geological and biological evolution. Shock-wave passage and heating of target materials during these events result in the formation of unique textural features (French and Koeberl 2010) and exposure of materials to variable P–T–t (pressure–temperature–-time) conditions, which make dating impact events challenging. Uranium-rich accessory phases such as zircon that have crystallized from impact melt sheets typically yield the most precise impact ages (e.g., Sudbury, Canada, Krogh et al. 1982, Manicouagan, Canada; Hodych and Dunning 1992, McGregor et al. 2021, Morokweng, South Africa, Kenny et al. 2021). However, coherent melt sheets may not be generated (i.e., in smaller structures or structures formed in sedimentary targets; Kieffer and Simonds 1980), and on Earth can be removed by post-impact erosion (Hergarten and Kenkmann 2015). While impact-generated frictional melts and shock veins may be suitable for dating, these materials are typically rapidly cooled and ultrafine, which can present incomplete resetting and crystallization issues. However, success has been achieved, especially using the ⁴⁰Ar/³⁹Ar technique (e.g., Vredefort, Spray et al. 1995). As such, the most commonly available materials for geochronologic studies are impact melt-bearing breccias. These materials are challenging to date as they are petrographically complex, being composed of impact melt (now glass or microcrystalline) particles, rock fragments, and relic (i.e., inherited from target rocks) minerals that have been exposed to variable P–T–t conditions during the cratering process. Understanding the behavior of accessory phase geochronometers (e.g., zircon, monazite, apatite, titanite) within these materials is therefore critical for improving the temporal record of impact events in the solar system.

Inventory of shock metamorphic features in zircon, titanite, and apatite

In attempts to better constrain formation ages of impact events, recent advances have been made in the documentation of shock microstructures within various U–Pb geochronometers. Zircon forms shock deformation twins at > 12 GPa (Moser et al. 2011; Timms et al. 2012), undergoes a pressure-induced polymorphic transformation at > 30 GPa to reidite (Erickson et al. 2017a and references therein), thermally dissociates to ZrO₂ and SiO₂ at >  ~ 1700 °C, and forms polycrystalline granular textures during recrystallization (Wittmann et al. 2006, Timms et al. 2017 and references therein). Pressures  > 30 GPa can also be preserved in granular zircons (i.e., FRIGN zircon; Cavosie et al. 2018), which can be used to infer shock conditions prior to recrystallization during the shock thermal pulse. Unlike zircon, the documentation of impact-induced microstructures in other U-bearing accessory phases such as titanite and apatite remains limited.

Natural observations of impact-induced features in titanite include shock twinning, currently only documented in large (> 150 km rim diameter) complex terrestrial impact structures (Papapavlou et al. 2018; Timms et al. 2020), and micro-vesicles (i.e., ‘MVs’, McGregor et al. 2019, 2020a). In apatite, documented microstructures include planar fractures (i.e., ‘PFs’, Cavosie and Centeno 2014; McGregor et al. 2018; Cox et al. 2020), and micro-vesicles (McGregor et al. 2018, 2020a, b). Evidence for impact-induced recrystallization in apatite has also been documented in grains from variably sized impact structures on Earth (McGregor et al. 2019; Kenny et al. 2020b; Cox et al. 2020), in lunar samples (Černok et al. 2019), and martian (meteoritic) materials (Darling et al. 2020). The formation conditions for shock microstructures in apatite and titanite are poorly constrained due to the lack of experimental studies, and so can only be inferred based on co-existing mineral assemblages within shocked basement lithologies (e.g., Timms et al. 2019).

U–Pb geochronology of shocked accessory phases

Recent advances in dating impact structures has largely been facilitated by the integration of microstructural analysis (e.g., electron backscatter diffraction (EBSD)) and high spatial resolution geochronology (e.g., secondary ion mass spectrometry (SIMS)) of U-rich phases, such as zircon, monazite, and baddeleyite (e.g., Moser et al. 2011; Darling et al. 2016; Kenny et al. 2020a; Erickson et al. 2020). Precise and accurate impact ages have also been obtained from zircon using LA–ICP–MS (laser ablation-–inductively coupled plasma–mass spectrometry, e.g., Schmieder et al. 2019a; Hauser et al. 2019; McGregor et al. 2020b). In these U-rich geochronometers, the extent of isotopic resetting is closely controlled by the level of microstructural deformation, with impact-recrystallized (i.e., neoblastic) domains typically yielding precise impact ages. Recent studies have also highlighted the potential of targeting porous-textured domains in zircon (i.e., pre-impact radiation damaged or ‘metamict’ regions) (McGregor et al. 2019; Schmieder et al. 2019a; Schwarz et al. 2020; Kenny et al. 2020a). However, these domains are susceptible to severe post-impact Pb loss and may yield anomalously young ages that post-date the impact event (McGregor et al. 2019, 2020a). Advances have also been made using (U–Th)/He techniques with various accessory minerals (e.g., van Soest et al. 2011; Biren et al. 2016, 2019).

Despite their widespread abundance and demonstrated suitability as U–Pb geochronometers, the utilization of apatite and titanite for dating terrestrial impact structures has been limited (LA–ICP–MS, McGregor et al. 2018, 2020a; Timms et al. 2020, SIMS Papapavlou et al. 2018). Nevertheless, SIMS U–Pb dating of lunar phosphates (e.g., apatite and merrillite) has proved to be a powerful technique for constraining the timing of impact events on the moon (e.g., Nemchin et al. 2009, Snape et al. 2016, Merle et al. 2017, Theissen et al. 2017). However, there is currently limited understanding of the mechanisms responsible for impact-induced isotopic resetting in these phases. Similar to zircon, recent SIMS dating of shock metamorphosed titanite from Vredefort, South Africa, suggests that impact resetting is microstructurally controlled (Papapavlou et al. 2018). Conversely, recent LA–ICP–MS U–Pb data from shock-twinned titanite within the peak ring of the Chicxulub impact structure, Mexico, reveals a lack of systematic correlation between apparent age and deformation microstructures (Timms et al. 2020). Similarly, multiphase LA–ICP–MS U–Pb dating of apatite and titanite within impact melt-bearing breccias reveals that the degree of isotopic resetting in both geochronometers is largely thermally controlled and influenced by either a given grain’s proximity to impact melt materials (McGregor et al. 2018), or the duration of post-depositional thermal recrystallization of host lithologies (McGregor et al. 2020a, b).

Objectives

This study builds on a previous publication that determined the formation age of the Lac La Moinerie impact structure (McGregor et al. 2019). Here, we deploy an integrated EBSD and SIMS U–Pb dating approach to (1) better elucidate the mechanisms driving Pb loss in low uranium, high common-Pb mineral phases (apatite and titanite); (2) determine if the use of high spatial resolution SIMS U–Pb geochronology can improve the current best-estimate formation age for Lac La Moinerie; and (3) better understand the effects of shock and related thermal metamorphism on zircon, apatite, and titanite. While a precise U–Pb impact age for 453 ± 5 Ma has been obtained using LA–ICP–MS on apatite (McGregor et al. 2019), previous integrated EBSD–SIMS studies on zircon (e.g., Kenny et al. 2020a) and monazite (Erickson et al. 2020) attest to the suitability of this technique to obtain high precision impact ages. Therefore, our aim was to see if similar precisions could be obtained on apatite and titanite using the same approach. Given the limited inventory of documented shock features in apatite and titanite, particular attention is made to describing microstructures in these phases and establishing a microstructural framework for their response to shock and thermal metamorphism.

Sample information and analytical techniques

Sample selection

This study focuses on zircon, titanite, and apatite that occur as relic (i.e., inherited) grains within impact melt-bearing lithologies from the Lac La Moinerie impact structure, Québec, Canada. The structure was formed within the Paleoproterozoic (1.84 – 1.70 Ga) De Pas Suite of the George River Block, comprising granitic lithologies and migmatitic charnockite (Corrigan et al. 2018). Recent studies have revealed that Lac La Moinerie has an apparent rim diameter of  ~ 8 km and a  ~ 4 km central uplift situated below the lake surface, designating it a small complex structure (McGregor et al. 2019). In situ LA–ICP–MS apatite U–Pb geochronology places the impact event at 453 ± 5 Ma, coinciding with the End Ordovician impact cluster (McGregor et al. 2019).

All studied grains were analyzed in situ within polished thin sections of impact melt-bearing lithologies (i.e., suevites or clast-laden melt rocks) (Fig. S1) previously dated by McGregor et al. (2019). Individual grains occur as mineral clasts within either the matrix of clast-laden melt rocks, or as mineral clasts within impact melt fragments of melt-bearing breccias (Fig. S1). Impact melt-bearing breccias are composed of flow-banded, microcrystalline melt bodies and lithic clasts situated within a fine-clastic matrix (Fig. S1a), while clast-laden impact melt rocks are composed of partially digested lithic and mineral clasts occurring within a glassy impact melt matrix (Fig. S1b, c). The studied grains were selected for EBSD–SIMS analysis based on the presence of unique microstructures observed in BSE imaging and their textural setting. Only those with the most representative microstructures were selected for further investigation. For this study, a total of eight titanite grains were chosen for EBSD analysis, five of which were then selected for SIMS U–Pb geochronology. For apatite, 15 grains were selected for EBSD mapping, while a sub-set of six grains were assessed using SIMS. EBSD mapping was conducted on four zircon grains, with SIMS analysis conducted on two grains.

Backscattered electron imaging (BSE) and electron backscattered diffraction (EBSD)

All zircon, titanite, and apatite grains were characterized using optical microscopy, backscattered electron (BSE) imaging, and electron backscatter diffraction (EBSD). Petrographic thin sections were polished and cleaned with distilled water and ethanol prior to being carbon coated. BSE imaging was conducted using a Hitachi SU-70 field emission scanning electron microscope (FESEM) equipped with a Schottky emitter located at the University of New Brunswick using a 20 kV accelerating voltage and a  ~ 3nA beam current. Orientation analysis of selected grains was conducted via electron backscattered diffraction (EBSD) using an Oxford Instruments Symmetry electron backscatter diffraction (EBSD) detector attached to a JEOL 7600f FESEM housed in the Astromaterials Research and Exploration Science division, NASA Johnson Space Center. Thin sections were polished using a 20 nm colloidal silica dispersion on a Vibromet II for 2–3 h and coated with carbon (< 5 nm) to mitigate charging. Electron backscatter diffraction patterns were collected from rasterized maps with a step size of 100–400 nm. The polished surface was inclined 70° to the incident electron beam, which was operated with 20 kV acceleration and 10 nA current. Indexing of electron backscatter diffraction patterns utilized a match unit based on the zircon unit cell at 1 atmosphere after Hazen and Finger (1979), the hexagonal ternary apatite match unit after Hughes et al. (1990), and the E2312 natural titanite match unit after Hawthorne et al. (1991). The data were processed using the Oxford Instruments Channel 5.11/5.12 software suite to produce crystallographic orientation maps and pole figures (i.e., stereo projections). Noise reduction was applied using the “wild spike” function to remove isolated pixels. All EBSD maps were constructed using the Channel 5 program Tango, while pole figures were processed in Mambo. All pole figures are plotted as lower hemisphere, equal area stereographic projections.

Raman spectroscopy

Raman mapping was conducted using a Renishaw inViaReflex micro-Raman spectrometer at the University of New Brunswick. All analyses were obtained with a 300 mW solid-state 514 nm wavelength edge laser. Raman maps were acquired at 50 × magnification with a 2.8 × 2.8 micron step size and utilizing a 2400 I/mm diffraction grating and 10% laser power. Acquisition exposure times involved 2 accumulations per spot at 10 s exposure time per accumulation. All spectra where obtained within the 100–1200 wave number range. All maps and spectra were processed using the WiRE 3.4 software and Renishaw’s Minerals and Inorganic Materials Database package.

Secondary ion mass spectrometry (SIMS)

A combination of mono-collection spot analyses and multi-collection ion mapping was conducted using a large-geometry CAMECA IMS1280 secondary ion mass spectrometer (SIMS) equipped with a Oregon Physics Hyperion H201 RF-plasma oxygen ion source located at the Swedish Museum of Natural History (NordSIMS facility) in Stockholm, Sweden. To preserve the petrographic context of all grains, thin sections containing individual grains were cut into (~ 8 × 8 mm) squares and mounted in 25 mm round sample holders. Prior to analysis, all sections were cleaned with distilled water and ethanol in an ultrasonic bath to remove surficial contamination and then gold coated. The methodologies used closely follow those described from previous apatite (Snape et al. 2016), titanite (McAteer et al. 2010), and zircon (Whitehouse and Kamber 2005; Jeon and Whitehouse 2015) monocollector spot analysis routines and multicollector ion imaging routines (Whitehouse et al. 2014; Bellucci et al. 2016) at the NordSIMS facility. Minor adjustments were made to each of these methods to accommodate the higher beam density provided by the Hyperion radiofrequency source, which was used in critically focused (Gaussian) rather than aperture illuminated (Köhler) tuning mode. The reference materials NW-1 apatite (1160 Ma, Li et al. 2012), MKED-1 titanite (1518 Ma, Spandler et al. 2016), and Geostandards 91500 zircon (²⁰⁶Pb/²³⁸U age = 1062.4 Ma, Wiedenbeck et al. 1995) were used to calibrate the Pb/U ratios.

Multi-collection scanning ion maps were acquired on individual grains to determine the distribution of U–Pb isotopes (i.e., enrichments/depletions) associated with different microstructural domains identified in EBSD. Raw ion image data were processed using the CAMECA Winimage2 software. Mono-collection spot analyses were selected based on information provided by both ion maps and EBSD. The complete set of the ion images collected for all grains are provided in Supplementary File 1. Individual spot analyses made on each grain are labeled in the corresponding figure as @1, @2 etc., and are provided along with the grain number in Tables S1–S3. All individual spot analyses are presented in either conventional Wetherill (zircon) or Tera–Wasserburg (apatite and titanite) concordia diagrams. Discordancy (%) is calculated following the procedures in Bruce Enlington’s GEODATE program, which yields virtually identical results to the commonly used formula [(disc = 100× (²⁰⁶Pb/²³⁸U_age)/(²⁰⁷Pb/²⁰⁶Pb_age) -1)]. All plots are constructed using Isoplot (Ludwig 2012), with uncertainties given at 1σ (%). All reported zircon ages are ²⁰⁷Pb corrected, while all apatite and titanite data are plotted uncorrected. All ages use an assumed common Pb composition based on the two-stage Pb evolution model of Stacey and Kramers (1975).

Results

Microstructural EBSD and BSE characterization of accessory phases

Zircon

Inverse pole figure (IPF) orientation maps of two granular zircon grains reveal systematic misorientations (Fig. 1b, d) consistent with the former presence of reidite (i.e., FRIGN zircon, Cavosie et al. 2018). Both grains are composed of sub-rounded neoblasts with orientations clustered orthogonally (~ 90°) to each other and coincident with the [001] and  < 110 > directions (Fig. 1, Fig. S3). Several grains also contain thermal dissociation rims of m-ZrO₂ (i.e., monoclinic; Fig. S2a) and mechanical {112} twins (Fig. S2b).

Fig. 1

BSE images and EBSD maps of granular zircons. Overlain on both EBSD maps are 10 µm SIMS spots (Table S1). A Granular and ZrO₂-bearing zircon situated within the melt matrix of a clast-laden impact melt rock (Fig. S1c). (i) Clusters of impinging neoblasts surrounded by minimal impact melt material. (ii) Non-impinging neoblasts entirley surrounded by impact melt and containing inclusions of SiO₂. B Inverse pole figure (IPF) map and pole figures indicating former reidite in granular neoblastic (FRIGN) zircon. White circles are the orientation maximum (orthogonal) indicative of the FRIGN zircon relationship. C Granular and ZrO₂-bearing zircon within same sample as A. (i) Partially preserved dissociation rim containing ZrO₂. Zircon neoblasts within interior containing ZrO₂ inclusions and domains of anatase (c.f., Fig. S4). (ii) High-magnification region of dissociation rim showing the presence of SiO₂, ZrO₂, and anatase (t-TiO₂). D Inverse pole figure (IPF) map and pole figure indicating the presence of a granular-crystalline (i.e., indexed) rim and a granular-amorphous (i.e., non-indexing) core. Pole figures indicate the presence of FRIGN zircon within the granular-crystalline rim. White circles are the orientation maximum (orthogonal) indicative of the FRIGN zircon relationship

In backscattered images, recrystallized grains show pervasive granular textures with individual neoblasts exhibiting variable levels of impingement (i.e., degree of neoblast contact) (Fig. 1a, c). Similar to previous descriptions (McGregor et al. 2020a), neoblasts are defined as either ‘impinging’ or ‘non-impinging’ (Fig. 1a–i, a–ii). Impinging domains comprise neoblasts in partial to complete contact with one another and/or forming clusters (Fig. 1a–i), with small amounts of interstitial glassy material situated between triple junctions that typically appear as pores (e.g., Fig. 1a–i) (c.f., McGregor et al. 2020a, their Fig. 6). In contrast, non-impinging neoblastic domains comprise individual neoblasts with distinct boundaries that are completely surrounded by a significant amount of glassy material (Fig. 1a–ii). Based on BSE imaging, both imaged zircon grains are considered granular (i.e., Fig. 1a, c). EBSD mapping of both grains reveals a notable distinction between each grain. Zircon L5-Zr5 (i.e., Fig. 1b) is entirely composed of crystalline (i.e., indexed) neoblasts that is comparable to previous studies (e.g., Timms et al. 2017), with the whole grain providing evidence for the former presence of reidite (FRIGN zircon, Fig. 1b). In contrast, zircon grain L5-Zr3 contains a distinct granular–amorphous (i.e., non-indexing) core surrounded by a granular–crystalline rim (Fig. 1c, d), which indexes as FRIGN zircon. Moreover, EBSD (Fig. 1d,) and Raman mapping (Fig. S4) of this grain reveal the presence of anatase (t-TiO₂), with high-resolution BSE imaging of the TiO₂ phase indicating a close spatial association with baddeleyite (m-ZrO₂) and SiO₂ within the thermally dissociated rim (Fig. 1c–ii). Anatase also occurs as small patches within the amorphous zircon grain adjacent to the dissociation rim interior where it is closely associated with neoblasts (Fig. 1c–i), or as a discrete phase within the surrounding matrix (Fig. 1c, Fig. S4).

Titanite

Titanite exhibits micro-vesicles (MVs), crystal-plastic deformation (CPD), planar deformation bands (PDBs), shock twins, and recrystallization textures. In one example, a single grain contains a planar deformation band coincident with the c-axis of the grain. The planar deformation band crosscuts and offsets partially annealed sub-planar fractures at multiple intervals along its length (Fig. 2). Micro-vesicles occur in two main morphologies: elongate micro-vesicles, which follow the orientation of planar deformation bands (PDBs) (Fig. 2c, d) and as rounded to subrounded micro-vesicles occurring in sub-linear arrays along planar boundaries (Fig. 2a, d, Fig. 3a, a–i).

Fig. 2

BSE images and EBSD maps of titanite displaying crystal plasticity, micro-vesicles (MVs), and containing a planar deformation band (PDB). Grain occurs within melt fragment of an impact melt-bearing breccia (Fig. S1a). Overlain on both EBSD maps are 20 µm SIMS spots (Table S2). A BSE image of entire grain. Note the presence of ilmenite along the grain rim and partially annealed sub-planar fractures. B Textural component (TC) EBSD map indicating cumulative internal misorientation and the presence of a planar deformation band. C Higher-resolution TC map of a planar deformation band (PBD) showing offset across its length along sub-planar microfractures. D High-magnification BSE image of area in C showing the location of sub-linear rounded vesicles and linear arrays of elongate vesicles

Fig. 3

BSE images and EBSD maps from {1–11} shock twin-bearing titanite grains. All grains are situated within the impact melt matrix of clast-laden impact melt rocks (Fig. S1b, c). Overlain on each EBSD map are 20 µm SIMS spots (Table S2). Note that all EBSD maps and corresponding pole figures are textural component (TC) maps of internal misorientation. A BSE image of titanite grain (L5-ttn2) containing a single set of the {1–11} shock twin with a disorientation relationship of 74°/[102]. A–i High-magnification BSE image of boxed region in A. showing the presence of micro-fractures and associated micro-vesicles. A–ii EBSD map of grain showing low levels of crystal-plastic deformation (CPD) and micro-fractures that offset T1. B BSE image of titanite (L4-ttn1) with single set of the {1–11} shock twin. Note the presence of micro-vesicles occurring at the apex of twin lamellae and along the twin/host interface (image insert). B–i Textural component EBSD map and corresponding pole figure of the entire grain showing internal strain within the host, and the relative twin-host disorientation relationship (i.e., 74°/ < 102 > of the {1–11} twin mode). B–ii Higher resolution of boxed area in (B-i) showing the presence of a shear deformation band along which T1 is offset and visibly distorted. C BSE image of titanite grain (L5-ttn3) with multiple {1–11} shock twin sets. C–i Textural component EBSD map and corresponding pole figure highlighting extreme crystal plastic deformation and two sets of {1–11} with twin-host disorientation of 74°/[102] (i.e., T1, T2). C–ii High-resolution TC map of boxed region in (C-i) showing the location of the {1–11} twin with a disorientation relationship of 74°/[103] relationship at the apex of a deformation band. Note the offsetting and distortion of T1 and T2 along this domain

Titanite grains also contain twin lamellae with a twin/host disorientation relationship of  ~ 74°/[102] corresponding to a compositional (K₁) plane of ~$\{1\overline{1}1\}$ and a shear direction (η₁) of  < 110 > (Fig. 3). Twin orientations were identified following the methods of Timms et al. (2019) and confirmed by plotting {221} and {1-11} planes and comparing them with the trace of the twin plane on the polished surface. The disorientation relationship of this twin was first documented in titanite from the Sudbury and Vredefort impact structures (Papapavlou et al. 2018), with compositional planes first resolved in titanite twins from the Chicxulub impact structure (Timms et al. 2019). In this study, twin lamellae occur either as a single set (T1, Fig. 3a, b) or as multiple intersecting sets (T1, T2, Fig. 3c). In BSE images, twins appear as bright, tapered bands of variable thickness (3–5 μm) that are closely associated with linear to sublinear arrays of rounded to subrounded micro-vesicles occuring either at the tapered apex of individual twin lamella (Fig. 3b), or along the twin/host interface (Fig. 3a, b). In all twin-bearing grains, the host exhibits crystal plastic deformation, with up to 25° cumulative misorientation recorded.

Two grains contain discrete deformation bands with high-angle boundaries that record the highest level of crystal plasticity (Fig. 3b–ii, c–ii). The deformation band is characterized by domains of twin offsetting and distortion, high crystal plasticity, sub-grain rotation, and localized dislocation creep (Fig. 3b–i, c–ii). These regions offset shear twins along the outer limits of the deformation band, while twins are notably distorted within their interior (Fig. 3b–ii, c–ii). Associated with these domains we identify a previously undocumented micro-twin with a twin/host disorientation relationship of 74°/ < 103 > (T3, Fig. 3c). This twin coexists within the same grain that hosts intersecting sets (i.e., T1, T2) of the $\{1\overline{1}1\}$ twin mode, and occurs at the apex of discrete deformation bands (Fig. 3c-ii).

In addition to shock-induced microstructures, titanite displays evidence of recrystallization (Fig. 4, Fig. S5). Textural evidence of recrystallization is not readily discernable in BSE images (Fig. 4a, d), although titanite neoblasts  ~ 1 µm in size are visible in one grain (Fig. S5). In EBSD maps, recrystallized titanite grains are composed of clusters of discrete strain-free subdomains that exhibit systematic orientations (Fig. 4b, c, Fig. S5). Within recrystallized domains, the $\{1\overline{1}1\}$ twin mode is preserved along the outer rims of individual subdomains throughout the entire grain (Fig. 4e). Both recrystallized grains documented here are intergrown with granular rutile, which occurs as interlocking strain-free subdomains that exhibit systematic orientations (Fig. 4f, Fig. S5).

Fig. 4

BSE and EBSD results from impact-recrystallized titanite. Overlain on both EBSD maps are 20 µm SIMS spots (Table S2). A BSE image of titanite grain (L3-ttn2) occurring as a mineral clast within a clast-poor impact melt fragment (Fig. S1c). B TC (textural component) map of internal misorientation showing presence of interlocking strain-free domains throughout host grain. C IPF (inverse pole figure) map and corresponding pole figure showing variable orientations of individual strain-free domains. Note the presence of the 74°/[102] micro twin along domain boundaries (white arrow). D BSE image of boxed region in (C) showing intergrowth of titanite with rutile. E High-resolution IPF map of region of interest in (C) showing the presence of the 74°/[102] twin along subdomain boundaries (white arrow). F IPF map and corresponding pole figure of granular rutile showing variable orientations of strain free subdomains with a systematic distribution. See Fig. S5 for EBSD results of additional recrystallized titanite grain

Apatite

The studied apatite grains exhibit planar fractures (PFs) (Fig. 5a, b), micro-vesicles (MVs) (Fig. 5c), crystal-plastic deformation (CPD) (Fig. 5d) and cataclastic textures (Fig. 6a), along with both partial (Fig. 6c) and complete recrystallization (Fig. 7).

Fig. 5

BSE and EBSD maps of planar fracture and micro-vesicle-bearing apatite grains. Overlain on both EBSD maps are 20 µm SIMS spots (Table S3). Ion maps of both grains are provided in Supplementary File 1. A PF-bearing apatite grain occurring as an inclusion in a lithic clast within impact melt-bearing breccia (Fig. S1a). Individual open PFs (planar fractures) intersect orthogonally (see Fig. S9 for acutely intersecting PFs). B Internal misorientation TC map showing variable degrees of crystal-plasticity and corresponding pole figure with minimal dispersion. C MV-bearing apatite occurring as a mineral clast within same sample as A. Both randomly distributed micro-vesicles (MVs) and/or sub-linear MVs are present (see image insert of boxed region). D TC map and pole figure indicating higher degrees of internal misorientation and related dispersion relative to the planar fracture-bearing grain due to higher levels of crystal-plastic deformation

Fig. 6

BSE and EBSD analysis of partially recrystallized apatite grains. A BSE image of in situ cataclastic-recrystallized apatite grain within a clast-laden zone of the impact melt matrix (Fig. S1b). B IPF map and corresponding pole figure of A. Note that individual fragments retain the initial orientation of the host grain; apatite neoblasts are randomly oriented (i.e., non-systematic). Note the distinction between the pole figure here and those in D and Fig. 5. C Partially recrystallized apatite within a clast-poor region of the melt matrix in a clast-laden impact melt rock (Fig. S1c). BSE images showing the presence of an undeformed but strained core rimed by apatite crystallites. D TC map and pole figure indicating the internal misorientation and relative orientation of the core and crystallites. Note the presence of variably strained subdomains that retain the primary orientation of the host (see Fig. S7 for IPF map)

Fig. 7

BSE and EBSD data showing two types of completely recrystallized apatite grains occurring within the impact melt matrix of clast-laden impact melt rocks. A Type I (systematic) apatite grain composed of both (i) elongate and (ii) hexagonal neoblasts domains. Note the presence of a distinct BSE contrast material between individual subdomains (see Fig. S9) and the presence of micro-vesicles within recrystallized domains. B IPF map and pole figure of Type I demonstrating systematic recrystallization; elongate domains oriented about the {00–01} axis (red-yellow) and hexagonal domains about the {11–20} axis (blue-green). C BSE images of Type II recrystallized apatite composed of (i) large impinging subhedral ‘blocks’ with radiating crystallites. (ii) Relationship between discrete block domains, triple junctions and micro-vesicles. D IPF map and pole figure of Type II apatite indicating non-systematic crystallization (i.e., random orientations)

Planar fracture-bearing grains occur either as inclusions within lithic clasts (Fig. 5a) or situated along the rims of lithic/mineral clasts within impact melt zones (i.e., glassy matrices or fragments, Fig. S6). Up to three intersecting sets of planar fractures are observed, which crosscut either orthogonally (Fig. 5a), or at acute angles (Fig. S6). All planar fracture-bearing grains display the lowest degree of crystal plasticity, with 0 – 5° internal misorientation (i.e., little to no dispersion in pole figures; Fig. 5b). All apatite grains contacting impact melt contain micro-vesicles (MVs) (Figs. 5b, 6, 7), with higher degrees of crystal plasticity (up to 20° internal misorientation) recorded in these grains (c.f., Fig. 5b and 6d pole figures). In both planar fracture and micro-vesicle-bearing grains, the original crystallographic orientation of the parent apatite is retained (Fig. 5b, c, pole figures).

Similar to previous observations (Černok et al. 2019; Cox et al. 2020; McGregor et al. 2020a), apatite displays cataclastic textures. In comparison to these studies, cataclastically deformed apatite observed here exhibits evidence of partial recrystallization. Termed here as ‘cataclastic-recrystallized’ (CR) apatite, these grains are comparable to apatite described from the Brent impact structure (McGregor et al. 2020a). Cataclastically recrystallized apatite occurs in contact with impact melt material and comprises large (5–25 µm) variably rotated, angular to subangular fragments that crudely retain the original shape and crystallographic orientation of the parent grain (Fig. 6a, b). Unlike planar fracture and micro-vesicle-bearing grains, the dispersion of indexed points from individual apatite fragments is distinct, with indexed points occurring as clusters defining a girdle-like band that broadly retains the host orientation (Fig. 6a, b). Intermixed between these fragments are sub-micron size strain-free neoblasts that exhibit no preferred orientation (i.e., non-systematic ‘Type II’ (see below)) (Fig. 6a, b). Several small fragments are completely recrystallized, being composed of clusters of impinging neoblasts that ‘ghost’ (i.e., overprint) pre-existing angular fragments (Fig. 6a, b).

A second type of partially recrystallized apatite is observed where the grain core records no external (i.e., BSE) evidence of recrystallization, but is rimmed by elongate crystallites that nucleate on the host crystal rim (Fig. 6c). Rim crystallites are aligned in a girdle distribution along the $\{10\overline{1}0\}$ crystallographic plane (i.e., similar to systematic ‘Type I’ (see below)) that is distinct from the strained core (Fig. 6c, Fig. S7). EBSD orientation maps reveal that the grain interior is composed of discrete, internally strained ‘subdomains’ that retain the primary crystallographic orientation of the host. Individual subdomains are misoriented relative to neighboring subdomains by  < 10° and record moderate degrees of crystal plasticity (0–10°, Fig. 6d, Fig. S7).

Completely recrystallized apatite grains are here divided into two types based on distinct external (i.e., BSE) and internal (i.e., EBSD) characteristics (Fig. 7, Fig. S8). All completely recrystallized grains occur as individual mineral clasts within the impact melt matrix of clast-laden impact melt rocks (Fig. S1b, c). Externally, Type I apatite grains are composed entirely of interlocking, strain-free elongate ‘crystallites’ (i.e., parallel to c axis) and/or hexagonal neoblast (i.e., basal, perpendicular to c-axis) domains (Fig. 7a, Fig. S8). In contrast, Type II apatite grains are composed of large, interlocking subhedral ‘blocks’ along which occur radiating domains of elongate crystallites and/or neoblasts (Fig. 7c).

Both types of recrystallization are further distinguished based on EBSD orientation maps. Where apatite grains display Type I recrystallization, strain-free domains have crystallographically distinct orientations (i.e., systematic) (Fig. 7b, Fig. S8). In these grains, crystallites (i.e., cut parallel to the c-axis) display a girdle distribution along the < 0001> direction, while sub-rounded neoblasts (i.e., cut perpendicular to the c axis) are oriented along the $<11\overline{2}0>$ direction (Fig. 7b). The degree of intergrowth (i.e., ‘merging’) between these two orientation groups varies, as indicated by the variation in angle between them in pole figures (c.f., Fig. 7 and Fig. S8). In Type II grains, strain-free domains exhibit no preferred orientation (i.e., non-systematic) (Fig. 7d). In these grains, pole figure distributions are comparable to the style of recrystallization observed in cataclastic-recrystallized grains (Fig. 6b).

In both Type I (systematic) and Type II (non-systematic) apatite grains, individual strain-free domains are surrounded by a distinct, dark gray (in BSE) phase of unknown composition that corresponds to regions of low crystallinity (i.e., amorphous) in band contrast (BC) maps (Fig. 8a, Fig. S9). This material has a distinct (lower) atomic mass from the material present within individual micro-vesicles and along triple junctions between interconnected recrystallized domains (dark BSE, Fig. 7a) and is considered as a distinct amorphous material. In both Type I and Type II grains, randomly oriented micro-vesicles occur within the interior of individual recrystallized domains (Fig. 7a–i, Fig. 8c–ii) and are distinct from the linear to sub-linear arrays of micro-vesicles observed in both apatite and titanite.

Fig. 8

BSE, BC (band contrast), ²⁰⁶Pb and ²³⁸U ion maps of selected zircon, titanite and apatite grains. Color scales indicate relative ion counts/pixel intensity. Dashed boxes indicate areas of interest corresponding to ion maps. Dashed ellipses indicate location of individual spot analyses. A Granular crystalline–amorphous zircon L5-Zr3 showing an heterogenous ion distribution with an enrichment of ²⁰⁶Pb and ²³⁸U in the amorphous core relative to the crystalline rim. Corresponding BSE, BC and ion maps of B a highly deformed titanite (Fig. 3c, d) and C a completely recrystallized apatite (Fig. 7a, b) indicating a homogenous, low concentration ion distribution throughout the grains

SIMS U–Pb geochronology: ion mapping and spot analysis

Zircon

Multi-collection scanning ion mapping and mono-collection spot analyses were acquired from two recrystallized zircon grains (Fig. 1, Fig. 8, 10). A total of four spot analyses were obtained based on (1) the nature of individual neoblasts within recrystallized domains (i.e., impinging versus non-impinging), and (2) the level of crystallinity seen in EBSD band contrast maps. For the crystalline granular zircon (grain L5-Zr5, Fig. 1a, b), spots were placed on regions of impinging (@1) and non-impinging neoblasts (@2). For the crystalline–amorphous granular zircon (grain L5-Zr3, Fig. 1c, d), spots where placed on the amorphous–granular core (@1) and the crystalline–granular rim (@2) (Table S1).

Ion mapping conducted on the crystalline–amorphous zircon reveals a heterogenous distribution of ²⁰⁶Pb and ²³⁸U (Fig. 8a). The crystalline–granular rim is strongly depleted in ²⁰⁶Pb and ²³⁸U relative to the amorphous–granular interior, with the latter being strongly enriched in ²⁰⁶Pb and ²³⁸U. Given ion enrichments of ²³⁸U and ²⁰⁶Pb, the lack of an electron diffraction pattern in EBSD (Fig. 1d), and the low intensity of the 985. 15–1015.14 zircon peaks mapped in the core (i.e., amorphous) (Fig. S4), this domain is considered metamict. Individual spot analyses from these domains also confirm this, with the amorphous core containing higher U concentrations (1400 ppm) relative to crystalline domains (840 ppm) and a lower percentage of non-radiogenic Pb (i.e., common Pb, f ²⁰⁶Pb% = 0.46) relative to the crystalline domain (f ²⁰⁶Pb% = 0.63). Calculations of both the pre-impact (t¹) (i.e., time of accumulation between crystallization of the target rocks and the impact event) and post-impact (t²) (i.e., period of time since metamictization started assuming complete annealing during the impact) alpha-decay dosages for these domains indicate that the amorphous core has a significantly higher alpha-decay dosage than the crystalline rim. Following the methods of Meldrum et al. (1998) and McGregor et al. (2019), the crystalline rim has a pre-impact and post-impact alpha dose of 5.41 × 10¹⁶ and 1.24 × 10¹⁴, respectively, while the amorphous core has a pre-impact dose of 9.43 × 10¹⁷ and a post-impact dose of 2.08 × 10¹⁶ (Table S1).

Plotted in conventional concordia space, all spot analyses are variably discordant, recording variable degrees of discordance (Fig. 9a, Table S1). They plot near the lower intercept of the impact regression defined by previous LA–ICP–MS data (McGregor et al. 2019, Fig. 9b). For the amorphous–granular zircon (Fig. 1c, d), the amorphous interior is −29.6% discordant and has a ²⁰⁷Pb/²⁰⁶Pb age of 355 ± 3.0 Ma, while the crystalline rim is −20% discordant and yields a ²⁰⁷Pb/²⁰⁶Pb age of 442 ± 4.0 Ma. In comparison, spot analyses on impinging domains within the crystalline granular zircon (Fig. 1a, b) yield a ²⁰⁷Pb/²⁰⁶Pb age of 442 ± 4.0 Ma, identical to the ²⁰⁷Pb/²⁰⁶Pb age of 442 ± 4.0 Ma obtained from the crystalline rim grain (Fig. 9a, b). Non-impinging domains record the highest level of discordancy (−46%) and yield a ²⁰⁷Pb/²⁰⁶Pb age of 562 ± 12 Ma while impinging neoblast domains (i.e., Fig. 1a–i) contain less common Pb (f ²⁰⁶Pb% = 0.67) relative to non-impinging domains (i.e., Fig. 1a–ii, f ²⁰⁶Pb% = 1.8) (Table S1).

Fig. 9

Individual SIMS U–Pb spot analyses from shocked zircon, titanite, and apatite. Colored ellipses are 1σ and color coded based on the targeted microstructural domains in EBSD maps. Gray ellipses are 2σ and correspond to LA–ICP–MS analyses on shocked zircons from McGregor et al. (2019). A Zircon SIMS data (²⁰⁴Pb corrected, n = 4; Fig. 1; Table S1) plotted on a Wetherill concordia diagram. B SIMS data combined with previously obtained LA–ICP–MS zircon U–Pb analyses (McGregor et al. 2019). C SIMS spot analyses from shocked titanite (uncorrected, n = 13; Table S2) plotted on a Tera–Wasserburg concordia. D SIMS data combined with LA–ICP–MS titanite U–Pb analyses from McGregor et al. (2019) highlighting the relationship between end-member regressions (i.e., basement and impact) defined by LA–ICP–MS ellipses. E SIMS apatite spot analyses from shocked apatite (uncorrected, n = 6; Table S3) plotted on a Tera–Wasserburg concordia. F SIMS data combined with apatite U–Pb analyses obtained by McGregor et al. (2019) showing the relationship between end-member regressions (i.e., basement and impact) defined by LA–ICP–MS ellipses

Titanite

Multicollection ion maps and monocollection spot analyses were obtained on titanite grains displaying discrete microstructural domains in EBSD maps. Ion maps were acquired from the most strongly deformed titanite grain (i.e., L5-ttn3) that contains multiple sets of the $\{1\overline{1}1\}$ shock twin, high degrees of crystal plasticity, and a shear deformation band (Fig. 3c). The ²⁰⁶Pb and ²³⁸U ions counts are low and homogenously distributed throughout the entire grain and, consequently, no ages could be extracted using the ion maps. Unlike zircon, no apparent concentration ‘hot spots’ corresponding to microstructures recorded in EBSD maps were observed, despite the strong level of deformation recorded (Fig. 8b).

A total of 13 spot analyses were obtained on three different titanite grains: (1) low strain host domains (i.e., internal misorientation  < 10°) (n = 3, L5-ttn3, L3-ttn4, L3-ttn2), (2) higher strained host domains (i.e., internal misorientation  > 10°) (n = 3, L5-ttn2, L3-ttn4, L4-ttn1), (3) twin/host overlap domains (n = 4, L5-ttn2. L5-ttn3, L4-ttn1), (4) deformation domains (n = 1, L5-ttn3), and (5) domains of impinging, strain-free subdomains (n = 2, L3-ttn2) (Fig. 9c). Unfortunately, due to bad charging on L5-ttn5, and consequently unreliable U/Pb calibration, the data from this grain are not discussed below (Table S2).

The spot analyses obtained from titanite are strongly discordant, containing high and variable ²⁰⁷Pb/²⁰⁶Pb (~ 0.14–0.75), f ²⁰⁶Pb% (5.6 – 92%), and U concentrations (4.1 – 63 ppm) (Table S2). No linear Pb*/PbC (i.e., radiogenic Pb/common Pb) mixing arrays, nor concordant clusters were obtained from this dataset. As such, no concordia ages, ²⁰⁷Pb/²⁰⁶Pb weighted mean ages, or intercept ages could be obtained (Fig. 9c). There is no clear relationship between microstructure or the level of Pb loss in titanite. Instead, multiple analyses from a given grain, regardless of the apparent level of microstructural deformation observed in EBSD within a single grain, plot in proximity to each other in concordia space and yield similar ²⁰⁷Pb/²⁰⁶Pb and ²³⁸U/²⁰⁶Pb ratios (Fig. 9c, Table S2).

All SIMS data were combined with previous LA–ICP–MS data from McGregor et al. (2019) to determine if any further information could be extracted from the dataset. By doing so, the level of isotopic resetting within the targeted domains could be assessed based on their respective spatial relationship to the triangular array defined by the LA–ICP–MS data (Fig. 10d). In this context, all SIMS analyses can be considered partially reset; plotting between the regressions defining both the basement age and impact age that anchor the two end-members of the triangular array (Fig. 9d).

Fig. 10

A Simplified schematic demonstrating the process of post-recrystallization metamictization (i.e., formation of metamict neoblasts) of a pre-impact metamict zircon through different impact cratering stages (shock compression, decompression, and post-impact time). B Simplified schematic displaying the evolution history of a titanite grain during shock compression and decompression, and the development of the documented 74°/ < 103 > disoriented twin developed from ~$\{1\overline{1}1\}$ shock twins

Apatite

Multi-collection ion mapping and mono-collection spot analysis were conducted on apatite grains exhibiting various microstructures in EBSD. Ion maps were obtained from: (1) planar fracture-bearing grains (Fig. 5a), (2) grains displaying crystal plastic deformation (Fig. 5b), and both (3) partially (i.e., Fig. 6b), and (4) completely recrystallized grains (Fig. 7a) (Fig. 8c, see Supplementary File 1). Similar to titanite, apatite displays low and homogenously distributed ²⁰⁶Pb and ²³⁸U ion emissions throughout all grains, with no concentration ‘hotspots’ associated with discrete microstructures (Fig. 8c).

A total of six SIMS spot analyses were made on five apatite grains based on observed microstructures in EBSD: (1) planar fracture-bearing apatite (n = 1, Fig. 5a), (2) grains recording variable degrees of crystal plasticity (i.e., high and low strain, Fig. 5a, b), (3) micro-vesicular domains (n = 2, L3-Ap4; Fig. 5b), and (4) both partially (n = 1, L5-Ap4 Fig. 6b) and completely recrystallized grains (n = 2, L4-Ap3, Fig. 7a).

Similar to titanite, when plotted in concordia space, no distinct clusters or linear arrays are observed (Fig. 9e). When combined with the LA–ICP–MS dataset (McGregor et al. 2019), analyses of a low strain domain within the center of a PF-bearing apatite (inclusion in a large lithic clast) (Fig. 6a, Fig. 9e, f) are the least isotopically reset and retain the initial Pb composition (i.e., plotting on concordia at the upper intercept of the regressions) (Fig. 9f). Analyses from high-strain domains (grain rim) and low-strain domains (grain center) within a single grain yield similar ²⁰⁷Pb/²⁰⁶Pb and ²³⁸U/²⁰⁶Pb ratios. While both analyses are proximal to the basement regression (i.e., 0% resetting), analyses of the low-strain grain center are less isotopically reset than the high-strain grain rim (Fig. 9f). Analyses acquired from the strained subdomain core of a partially recrystallized grain (Fig. 6b) are partially reset, plotting between the basement and impact regressions (Fig. 9f). Analyses of completely recrystallized grains plot close to the y intercept along the impact regression defined by the LA–ICP–MS data set (Fig. 8a, Fig. 9f). Common Pb is highest within apatite occurring within lithic clasts (i.e., low strain, PF bearing) and completely recrystallized grains (i.e., lowest ²⁰⁶Pb/²⁰⁴Pb, Table S3), while analyses of partially recrystallized grains (i.e., strained subdomains) and those with high crystal plasticity are more radiogenic (highest ²⁰⁶Pb/²⁰⁴Pb, Table S3).

Discussion

Building on previous work on Lac La Moinerie, an integrated EBSD–SIMS U–Pb dating approach has been conducted in an attempt to: (1) improve the best-estimate age for the impact event (i.e., 453 ± 5 Ma, McGregor et al. 2019) by dating impact-recrystallized geochronometers (i.e., zircon, apatite, and titanite) using high-spatial resolution SIMS U–Pb geochronology, (2) target discrete microstructures to assess the mechanisms responsible for impact-related Pb loss in apatite and titanite (i.e., defect enhanced or volume diffusion), and (3) provide insights into the microstructural evolution of zircon, apatite, and titanite during impact metamorphism.

While integrated EBSD–SIMS geochronology of impact-recrystallized zircon (e.g., Moser et al. 2011, Kenny et al. 2020a) and monazite (e.g., Erickson et al. 2020) has been demonstrated to be a highly effective technique for obtaining impact ages, we find that the higher spatial resolution provided by SIMS did not improve the current best-estimate impact age for Lac La Moinerie. In particular, targeting recrystallized domains did not yield concordant data from which impact ages could be calculated. Instead, variable levels of discordance are recorded in all microstructural domains (including recrystallized areas), providing valuable insights into the U–Pb systematics of zircon, titanite, and apatite during impact cratering.

EBSD–SIMS U–Pb geochronology of impact metamorphosed zircon

Spot analyses on FRIGN zircon grains are found to be incompletely reset, recording components of pre-impact radiogenic Pb, common Pb (within interstitial domains between neoblasts), and recent Pb loss (from metamict neoblasts) (Fig. 1; Fig. 9a, b). While individual ²⁰⁷Pb/²⁰⁶Pb ages of  ~ 442 Ma were obtained from domains of impinging crystalline neoblasts, these are younger than the 453 ± 5 Ma best-estimate impact age obtained from apatite using LA–ICP–MS (McGregor et al. 2019). Combined with the anomalously young age of  ~ 350 Ma obtained from the metamict neoblasts within the same grain (Fig. 1c), our zircon results highlight a possible limitation to using single granular zircon grains to obtain impact ages.

While more SIMS spot analyses on granular zircon grains may have provided concordant dates, this is evidently not guaranteed given the observed isotopic heterogeneity, and, at least for Lac La Moinerie, is unlikely given the limited number of concordant analyses obtained from the larger LA–ICP–MS dataset (McGregor et al. 2019). The analysis of multiple grains from different samples is more advantageous when dating such complex lithologies that have experienced variable T–t conditions (i.e., relic grains within impact melt-bearing breccias versus newly grown grains within impact melt rocks). The selection of larger grain populations facilitates a more representative view of the isotopic response of zircon grains to variable T–t conditions within these complex lithologies, and so enables the pooling of an entire dataset to ensure that a reliable impact age is calculated (i.e., allows recent Pb loss and impact-related trends to be discerned).

Record of post-impact Pb loss from metamict zircon neoblasts

While no concordant impact ages were obtained from either of the FRIGN zircon grains assessed here, the correlated EBSD–SIMS approach has provided new insights into the isotope systematics of metamict zircon during impact events. Recent studies have highlighted the susceptibility of pre-impact metamict domains to both ancient (i.e., impact-induced) and recent (i.e., post-impact) Pb loss during impact events (McGregor et al. 2019; Schmieder et al. 2019a; Kenny et al. 2020b; Schwarz et al. 2020). The results presented here are distinct from previous studies in that the targeted metamict domains are not pre-impact (i.e., from relic, minimally shocked grains), but have undergone metamictization after transformation to reidite and granularization. These findings demonstrate that individual neoblasts within impact recrystallized zircon can undergo metamictization following impact events, and can consequently suffer low-temperature, diffusive Pb loss during post-impact time.

While anomalously young ages have been obtained from pre-impact metamict domains in zircon using LA–ICP–MS (McGregor et al. 2019; Schmieder et al. 2019a), concordant impact ages have recently obtained via SIMS (Kenny et al. 2020b; Schwarz et al. 2020). It may be reasoned that these results were enabled by the higher spatial resolution facilitated by SIMS (i.e., 5 µm, Kenny et al. 2020b, 15 µm, Schwarz et al. 2020) compared to LA–ICP–MS (17 µm, McGregor et al. 2019, 25 µm, Schmieder et al. 2019a). However, the SIMS results presented here were obtained using a 10 µm spot size, between that used by Kenny et al. (2020b) and Schwarz et al. (2020). Moreover, calculated alpha dosages for the porous textured domains from these studies show that all porous domains recording a concordant impact age experienced similar post-impact alpha dosages (5.78 × 10¹⁷, Kenny et al. 2020a, b, 2.12 × 10¹⁷, Schwartz et al., 2020) to those obtained here (2.08 × 10¹⁶) (Table S1). This suggests that, rather than being a consequence of differing analytical conditions, these discrepancies a function of an individual zircon grain’s T–t history (i.e., conditions required for annealing metamict domains) and/or its post-impact fluid history. While it is not possible to determine the exact P–T–t conditions experienced by each individual grain given that they are all relics within impact melt-bearing materials, it is evident the thermal evolution of the individual zircon grains (i.e., the T–t conditions of the local grain environment) dated by Kenny et al. (2020a) and Schwarz et al. (2020) was sufficient to anneal pre-impact radiation-damaged domains and inhibit any post-impact Pb loss. While pre-impact metamict domains in zircon may be annealed during clast–melt interactions, this does not preclude the process of post-impact Pb loss from metamict neoblasts. Therefore, our results suggest that while granular (i.e., impact recrystallized) and porous (i.e., radiation damaged) domains in zircon are suitable targets for obtaining impact ages, reliable impact ages are not guaranteed as it is highly dependent on a grain’s T–t history.

EBSD–SIMS U–Pb dating of common Pb-bearing phases: insights into the isotope systematics of impact metamorphosed apatite and titanite

The improved spatial resolution provided by SIMS was unsuccessful in providing concordant (or near-concordant) data points from impact-recrystallized apatite and titanite. The comparison of our SIMS results with previous LA–ICP–MS data from Lac La Moinerie (Fig. 9, McGregor et al. 2019) indicates that both analytical techniques provide comparable precision and isotopic trends. This suggests that the lack of concordance observed in previous U–Pb studies of terrestrially shocked common Pb-bearing phases (e.g., apatite; McGregor et al. 2020a, b) is a function of their chemical makeup (i.e., accommodation of non-radiogenic Pb into the crystal structure) and consequent isotopic composition (i.e., low U, high PbC) rather than the analytical technique deployed or degree of recrystallization. The results of the present study suggest that the higher spatial resolution provided by SIMS is unlikely to yield better results (i.e., concordance) in common Pb-rich phases. The higher data throughput provided by LA–ICP–MS could be considered as a more appropriate tool for dating such complex impactite lithologies due to the rapid data acquisition and higher data throughput of this technique.

Role of variable T–t histories on Pb loss in apatite and titanite

Similar to observations made by Timms et al. (2020) and McGregor et al. (2020b), our results indicate that impact-induced isotopic resetting in apatite and titanite is not microstructurally controlled. Notably, the present data set emphasizes that apatite and titanite do not respond in an isotopic similar manner (i.e., via defect enhanced diffusion) to high-U phases such as zircon (Kenny et al. 2020a), monazite (Erickson et al. 2017b), or baddeleyite (Darling et al. 2016) during impact-related metamorphism. This is evident by (1) the homogenous distribution of ²³⁸U and ²⁰⁶Pb ions within apatite and titanite identified during ion mapping (Fig. 8b, c), (2) similar isotopic ratios obtained in spot analyses from different microstructural domains within an individual grain (Fig. 9d, e), and (3) the lack of concordance from spot analyses on recrystallized domains. Instead, we reason that the degree of thermal insulation and the local temperature of an individual grain’s environment, along with post-impact hydrothermal activity (e.g., Timms et al. 2020) and the cooling trajectory of impactites (e.g., McGregor et al. 2020b) all play a decisive role in the T–t history of individual grains and therefore the resulting degree of Pb loss. If the degree of Pb loss in both apatite and titanite was solely controlled by the level of deformation (i.e., defect enhanced diffusion) as in U-rich geochronometers, it would be expected that: (1) the degree of resetting would correlate with increasing levels of shock deformation (i.e., low crystal plasticity through to recrystallization) regardless of textural setting, and (2) ion ‘hotspots’ would correlate to more strongly deformed domains, as is the case for zircon (Fig. 8).

The effect of protracted heating on the isotope systematics of apatite was recently highlighted in grains within (post-deposition) thermally metamorphosed clast-laden impact melt rocks from the Brent impact structure, Canada (McGregor et al. 2020a). At Brent, the analysis of apatite grains displaying a range of microstructures (e.g., no visible deformation, planar fractures, micro-vesicles, recrystallized) reveals that all grains are 100% impact reset regardless of microstructure, and define a strong Pb*/PbC mixing array from which an impact age was calculated. In the present study, the analysis of low-strain domains within planar fracture-bearing apatite grains are partially reset and plot near the basement regression, while analyses obtained from higher strain domains (including those from strained-subdomain cores of partially recrystallized grains) record higher degrees of Pb loss (Fig. 9f). This suggests a degree of microstructurally controlled resetting. However, grains recording 0% chronometric resetting and exhibiting crystal plastic deformation and planar fractures occur as inclusions within lithic clasts, indicating that the grain was thermally insulated during grain–melt interactions.

Recent studies documenting apatite recrystallization within mylonite zones suggest that complete U–Pb isotopic resetting of apatite is a combined function of volume diffusion, fluid-assisted deformation and recrystallization (Odlum and Stockli 2020). Similar studies have found that reasonable ages are only obtained from grains that do not exhibit evidence of recrystallization, due to the higher common Pb and lower radiogenic Pb compositions of recrystallized grains (Ribeiro et al. 2020). Similar to these studies, analyses obtained from completely recrystallized apatite falling on the impact-regression have the highest common Pb compositions, while the remaining analyses defining the impact-regression record no evidence of recrystallization (McGregor et al. 2019). Evidently, and as observed by Ribeiro et al. (2020), to obtain precise impact ages from relic apatite occurring within impact melt-bearing lithologies, grains displaying a range of microstructures and situated within textural settings that experienced protracted heating (e.g., within impact melt-rich settings) must be targeted to obtain impact ages.

For titanite, spot analyses obtained from different titanite microstructures (i.e., impinging strain-free subdomains, regions of high crystal plasticity, along twin-host boundaries) are all considered partially reset when assessed relative to the LA–ICP–MS data (Fig. 9d, McGregor et al. 2019). Moreover, similar isotopic ratios are obtained from individual titanite grains, regardless of the level of deformation (i.e., microstructural domain). As indicated for apatite, these results suggest that the local grain environment is a primary controlling factor on the level of Pb loss within titanite, which is consequently related to the T–t evolution of the host rock (i.e., impact melt-bearing breccias). This is similar to conclusions made by Timms et al. (2020) and McGregor et al. (2020b), whereby Pb loss was controlled by fluid-mediated Pb loss during post-impact hydrothermal alteration (Timms et al. 2020) and prolonged heating associated with post-depositional recrystallization of impact lithologies (McGregor et al., 2020a).

U–Pb systematics of terrestrial versus lunar apatite: the challenge of common Pb

Based on the SIMS results obtained here for apatite, it is evident that the U–Pb isotope systematics of phosphates are distinct for terrestrial versus lunar impact lithologies. Unlike U–Pb studies of terrestrially shocked apatite, where impact ages are defined by a linear regression through a Pb*/PbC mixing line (e.g., McGregor et al. 2018, 2020a), impact ages from lunar phosphates (apatite and merrillite) are typically produced from concordant, to near-concordant datasets (e.g., Snape et al. 2016, Merle et al. 2017; Theissen et al. 2017). There are several reasons for this.

Firstly, unlike on Earth, the absence of both plate tectonics and extensive water-based weathering/erosion on the moon has favoured the survival of older (pre-4.0 Ga) materials. The lunar surface has undergone minimal reworking since the initial magmatic crystallization (i.e., at  ~ 4.5 Ga) and impact basin formation (i.e., > 3.9 Ga). Consequently, any degree of U–Pb isotopic resetting will result in a shift parallel to concordia, and any clusters on concordia or a linear regression will reflect a resetting event. The resulting phosphate ages are then primarily dependent on ²⁰⁷Pb/²⁰⁶Pb ages, and age calculations can be treated in a similar manner to low PbC phases, such as zircon (i.e., concordia age calculations, intercept ages, or weighted mean ²⁰⁷Pb/²⁰⁶Pb ages; e.g., Nemchin et al. 2009, Merle et al. 2014). In contrast, on Earth there is typically a large time interval between the formation age of the basement (e.g., Mesoproterozoic) and the impact event (e.g., Ordovician, as for Lac La Moinerie). Consequently, instead of moving parallel to concordia, isotopic ratios are distributed in the characteristic fan-like array around an ‘elbow’ in concordia space, with the impact resetting event defined by a linear regression through the youngest apparent ages (i.e., along the impact Pb*/PbC mixing line). These arrays, in turn, may be further complicated by any post-impact tectonic reworking, which theoretically could isotopically reset the impact regression, although this is yet to be observed in terrestrially shocked apatites (or titanite).

Secondly, and perhaps more importantly, common Pb is largely absent on the moon as a result of volatile depletion during the Giant Impact formation event (Halliday 2004; Day and Moynier 2014). Conseqently, phosphates within lunar materials do not incorporate PbC during initial magmatic crystallization and instead retain the primordial Pb composition of the lunar mantle. Any common Pb recorded in lunar phosphates can largely be assumed to be terrestrial contamination (i.e., ²⁰⁷Pb/²⁰⁶Pb =  ~ 0.85) and can be corrected for [although mixing between terrestrial and lunar initial Pb is possible (Gnos et al. 2004)]. In contrast to the moon, continued differentiation and resulting U and Pb partitioning during crustal reworking on Earth has resulted in a highly evolved initial Pb composition (i.e., ²⁰⁷Pb/²⁰⁶Pb =  < 0.85) (Maltese and Mezger 2020). Consequently, the initial common Pb composition incorporated into phosphates during magmatic crystallization will be more evolved and, in the case of post-crystallization metamorphism (i.e., either regional or impact related), will record mixing between the initial (i.e., target ²⁰⁷Pb/²⁰⁶Pb) and radiogenic Pb composition of the metamorphic event (i.e., the impact event). Thus, while U–Pb age calculations from impact affected lunar apatites can effectively be treated the same way as non-PbC-bearing phases (e.g., zircon, Merle et al. 2014), metamorphosed terrestrial phosphates cannot. Thus, while the U–Pb system in zircon appears to respond similarly during both terrestrial (e.g., Moser et al. 2011) and lunar (Grange et al. 2013, Crow et al. 2017) impact events, it is clear that this is not the case for terrestrial phosphates. While this may seem intuitive, this distinction dictates that direct comparisons between the mechanisms responsible for age resetting in lunar and terrestrially shocked phosphates cannot be made.

Insights into the response of zircon, titanite, and apatite to impact metamorphism.

Post-recrystallization metamictization and preservation of metastable TiO ₂ in zircon

In addition to providing shock pressure constraints for the Lac La Moinerie impact event (i.e., {112} shock twin = 16 GPa, Fig. S2; Timms et al. 2018, and FRIGN zircon =  > 30 GPa, Cavosie et al. 2018), the documentation of amorphous-granular domains within FRIGN zircon (Fig. 1c, d) offers new insight into the microstructural evolution of radiation-damaged domains in zircon during impact metamorphism. This study is the first to document metamict neoblasts within a FRIGN zircon (or granular zircon), making it distinct from previously documented metamict domains in impact-affected zircon (McGregor et al. 2019; Schmieder et al. 2019a; Kenny et al. 2020a). In previous studies, metamict zircon grains exhibit either magmatic-crystalline (McGregor et al. 2019; Schmieder et al. 2019a) or granular–crystalline rims (Kenny et al. 2020a) which surround porous-textured cores (i.e., pre-impact (magmatic) metamict domains). The following section outlines a suggested microstructural formation history of pre-impact metamict zircon and the formation of metamict neoblasts in zircon.

Two morphologically distinct types of reidite can form depending on the crystallinity of the original host zircon (Erickson et al. 2017a; Timms et al. 2017), with lamellar reidite forming within crystalline zircon domains, and granular reidite nucleating within low-crystallinity (i.e., metamict) regions. Given this relationship, we interpret that the fully indexed FRIGN zircon (i.e., completely crystalline, Fig. 1a, b) was entirely crystalline prior to impact and transformed to lamellar reidite during shock wave propagation. In contrast, the parent grain of the crystalline–amorphous zircon (Fig. 1c, d) likely had a crystalline rim surrounding a metamict core (Fig. 10a). Lamellar reidite would have consequently developed in the crystalline rim, while granular reidite would have nucleated within the metamict core (Fig. 10a). The high ²³⁸U ion emission within the amorphous–granular core relative to the crystalline–granular rim (Fig. 8a) indicates that individual neoblasts within the core have retained the U composition of the original pre-impact (i.e., magmatic) metamict domain. Thus, following subsequent reversion of reidite back to zircon via granularization, these U-rich neoblasts will undergo continued radiation damage resulting in the observed amorphous state and, as indicated by the SIMS U–Pb results, will suffer continued low-temperature post-impact Pb loss (Fig. 9a, Fig. 10a).

Within this same zircon grain, Raman mapping reveals the presence of anatase (t-TiO₂) (Fig. S4). Titanium (Ti) is a commonly analyzed trace element within terrestrial (Fu et al. 2008; Page et al. 2007) and lunar zircons (e.g., Crow et al. 2017), with the temperature-dependent substitution of Ti⁴⁺ making Ti-in-zircon a reliable thermometer (Watson et al. 2006; Fu et al. 2008; Page et al. 2007). However, little is known about how impact-induced recrystallization affects the behavior of trace elements such as titanium within zircon. Nanometer-size TiO₂ inclusions within zircon neoblasts have been documented within grains from the Ries impact structure (Schwarz et al. 2020). The TiO₂ phase observed here occurs as both a discrete phase within the matrix (Fig. 1c) and is intergrown with, but separate from (i.e., immiscible), ZrO₂ and SiO₂ within the partially preserved dissociated rim (Fig. 1c-ii, Fig. S4). While crystalline TiO₂ nanoparticles in zircon have been produced in experimental studies (Zhou et al. 2003; Farag et al. 2011), natural observations of TiO₂ within naturally shocked zircon are limited. Although high-pressure TiO₂ polymorphs (e.g., TiO₂-II, brookite) have been documented from several impact structures (El Goresy et al. 2001; Schmieder et al. 2019b), these are pre-impact rutile and/or anatase grains that have undergone high-pressure polymorphic transformation. Here, we consider the presence of anatase to be a consequence of TiO₂ expulsion from the metamict core of the original pre-impact zircon grain during impact-induced recrystallization. This is supported by the close intermingling of TiO₂, ZrO₂, ZrSiO₄, and SiO₂ in the thermally dissociated grain margin (Fig. 1c-ii) and the presence of TiO₂ within porous-metamict zircon cores (McGregor et al. 2019).

Anatase is a metastable TiO₂ polymorph that transforms to stable rutile (Hanaor and Sorrell 2011). This transformation is irreversible and reconstructive (Shannon et. al. 1965), and therefore requires adequate thermal energy for a sufficient duration to proceed to completion (Hanaor and Sorrell 2011). Given the rapid cooling conditions required to partially preserve a ZrO₂-bearing dissociation rim (i.e., quenched), the anatase–rutile transformation was evidently inhibited in this grain, preserving the metastable anatase polymorph. However, given that ZrO₂ and SiO₂ have partially recombined throughout the grain (i.e., to form zircon neoblasts), rutile would be expected to be present, which is not the case. We therefore interpret that the preservation of anatase here is related to the close spatial relationships of TiO₂ with SiO₂ and ZrO₂ in the dissociation rim (Fig. 1c-ii). Both ZrO₂ (Hanaor and Sorrell 2011; Cavalheiro et al. 2017) and SiO₂ (Okada et al. 2001) have been shown to inhibit the anatase-to-rutile transformation, with the increased molar volumes of these oxides increasing the temperature required for the polymorphic transformation back to rutile.

Titanite: shear‑induced twinning, twin preservation, and recrystallization

Unlike zircon, little is known about how titanite responds to impact metamorphism. To date, the EBSD analysis of shocked titanite has only been conducted on grains from larger (>150 km) complex impact structures (e.g., Sudbury, Ontario, and Vredefort, South Africa (Papapavlou et al. 2018), Chicxulub, Mexico (Timms et al. 2019)). This is the first EBSD analysis of titanite from a small (8 km) complex crater.

Similar to previous studies (Timms et al. 2019), we document up to two intersecting sets of the $\{1\overline{1}1\}$ shock twin, which has a disorientation relationship of 74°/ < 102 > (Fig. 3, Fig. 10b). Documentation of lateral displacement of shock twins (Fig. 4) and planar deformation bands (Fig. 2) along microfractures imply that brittle deformation post-dated twin formation (Fig. 10b). We also document a new twin orientation that has a twin-host disorientation relationship of 74°/ < 103 > (Fig. 3c, b). Given that there is no visible offsetting of the 74°/ < 103 > twin along microfractures, formation of the 74°/ < 103 > twin must post date both formation of the $\{1\overline{1}1\}$ twins and microfracturing (Fig. 10b). We interpret the new twin orientation to be the result of distortion of pre-existing $\{1\overline{1}1\}$ shock twins along discrete deformation bands into a more stable configuration about the < 103 > axis (Fig. 10b). This is considered to be the result of increased crystal-plastic deformation, strain localization within the deformation band, and the formation of sub-grain boundaries via dislocation creep during thermal recovery (i.e., grain–melt interaction) (Fig. 10b). While this twin is only observed in a single titanite grain, evidence for twin offsetting and distortion within deformation bands is evident in other grains where the primary TI twin is cross-cut and offset along high-angle boundaries (e.g., Fig. 4b).

We also note the presence of the primary $\{1\overline{1}1\}$ shock twin along the boundaries of individual strain-free domains within recrystallized titanite grains (Fig. 4). This highlights the potential of titanite to preserve evidence of former high-pressure conditions following recrystallization. This is the second observation of recrystallized titanite from a terrestrial impact structure (cf., Papapavlou et al. 2018). However, those documented by Papapavlou et al. (2018) were of tectonic origin and not related to the impact event. Thus, given the preservation of shock twins within recrystallized grains, which strongly supports an impact origin, this study appears to be the first observation of impact-induced recrystallization in titanite. While the presence of shock twins within recrystallized grains confirms that recrystallization occurred during the Lac La Moinerie impact event, titanite is readily recrystallized under tectono-metamorphic conditions and so cannot be considered a diagnostic impact feature (cf., Papapavlou et al. 2017).

Coexisting within recrystallized titanite grains is microcrystalline–granular rutile (TiO₂) (Fig. 4d, Fig. S5). Rutile is a common exsolution phase of titanite (Zack and Kooijman 2017), and is therefore considered a pre-impact feature of the grain. Evidence for rutile recrystallization is limited in tectono-metamorphic settings (Zack and Kooijman 2017; Plavsa et al. 2018), and has only been documented from the Chicxulub impact structure (Schmieder et al. 2019b). In the latter study, microcrystalline rutile partially replaces the pre-existing high pressure TiO-II polymorph (Schmieder et al. 2019b). Similar to that study, rutile neoblasts observed here are systematically oriented relative to one another in pole figures (Fig. 4f). Given the high pressures estimated for Lac La Moinerie based on the presence of FRIGN zircon (i.e., > 30 GPa) and high post-shock temperatures  > 1673 °C indicated by occurrence of dissociation, the polycrystalline rutile observed may be the final product of back-reversion (i.e., overprinting) from the initial high-pressure TiO-II polymorph (El Goresy et al. 2001; Schmieder et al. 2019b). If correct, this indicates that, similar to FRIGN zircon (Cavosie et al. 2018), polycrystalline rutile may preserve evidence for the former presence of a high-pressure polymorph.

Apatite: micro-vesiculation, cataclasis, and styles of recrystallization

Based on the EBSD data obtained from apatite, and comparisons with previous studies (McGregor et al. 2018, 2020a; Černok et al. 2019; Kenny et al. 2020a; Cox et al. 2020), we propose a microstructural evolution model for apatite during shock compression and post-shock decompression during impact cratering events (Figs. 11, 12).

Fig. 11

Graphic representations of the microstructural evolution of apatite grains during shock compression and decompression, and the relationship of these microstructures to a grain’s textural setting

Fig. 12

Simplified diagrams outlining the formation of Type I and Type II styles of recrystallization observed in apatite. Note that the color is representative of crystallographic variations as revealed by EBSD mapping

Micro-vesiculation

Apatite occurring as inclusions within lithic clasts contain planar fractures (PFs), while micro-vesicles (MVs) are absent (Fig. 5a, b, Fig. 11). In contrast, apatite grains in contact with impact melt contain linear to sublinear arrays of micro-vesicles (Fig. 5c, d, Fig. S9), similar to apatite grains within impact melt-bearing lithologies from other impact structures (McGregor et al. 2018,2019,2020a, 2020b). While comparable micro-vesicles are seen along twin/host interfaces in titanite (this study, Fig. 3a, c), zircon (Moser et al. 2011; Timms et al. 2012), and monazite (Erickson et al. 2017b), micro-twins have yet to be documented in shocked apatite. We therefore interpret the observed spatial relationship between a given grain’s textural setting (e.g., clast versus melt) and the presence/absence of linear and sub-linear micro-vesicles to be a consequence of partial annealing of planar fractures (PFs) and non-planar fractures (NPFs) during grain–melt interactions (Fig. 11). During these interactions, planar fractures and non-planar fractures would serve as percolation pathways for impact melt, the heat of which would facilitate annealing of apatite along pre-existing fractures, and consequently result in entrapment of (glassy) impact melt (i.e., the formation of micro-vesicles). The final grain microstructure is then thermally induced, with micro-vesicles preserving the orientation of the initial non-planar fractures (i.e., sub-linear arrays of micro-vesicles) and planar fractures (i.e., linear arrays of micro-vesicles) (Fig. 11).

Cataclasis

We document partially recrystallized cataclastically deformed apatite grains, which we here term “cataclastic-recrystallized” (Fig. 6, 11). Cataclasis is considered a low-pressure process whereby grains undergo brittle deformation, with individual fragments undergoing cataclastic flow via grain sliding and block rotation (Passchier and Trouw 1996, Exner and Tschegg 2012). Recent high-pressure experimental studies demonstrate that brittle deformation and cataclasis can also occur during shock wave loading and unloading (Takagi et al. 2020). Cataclastically deformed apatite has recently been observed in grains from the Chicxulub impact structure (Cox et al. 2020) and in lunar phosphates (Černok et al. 2019). In these studies, cataclastically deformed apatite grains are composed of interlocking subdomains with low angle boundaries (Černok et al. 2019), or individual, variably rotated angular fragments (Cox et al. 2020). In both cases, grains retain the primary orientation of the host apatite. In these studies, grains occur as either inclusions in lithic materials (i.e., clasts within lunar lithic clasts; Černok et al. 2019), or within basement rocks (i.e., inclusions in granitoids from a central uplift; Cox et al. 2020). In contrast, and similar to apatite within impact melt-bearing breccias from the Brent impact structure (McGregor et al. 2020a), the cataclastic-recrystallized apatite grains observed here occur in direct contact with impact melt and display notably higher levels of cataclasis (Fig. 6). We suggest that the progressive development of cataclasis in apatite is controlled by its textural setting, with grains liberated from lithic clasts and contacting impact melt (i.e., upon shock unloading and post-shock demcompression) being subjected to higher levels of cataclasis and subsequent recrystallization during grain–melt contact (Fig. 11). The level of cataclasis in apatite grains may be due to either the initial location of apatite within the basement during shock wave propagation (e.g., proximity to isobaric core) and/or the result of impedance mismatch between co-existing phases prior to being liberated from the host materials (e.g., quartz/feldspar (Černok et al. 2019) versus biotite (Cox et al. 2020)) (Fig. 11).

Types of recrystallization: non-systematic versus systematic

Microstructural EBSD analysis of completely recrystallized apatite here reveals two modes of (complete) recrystallization in apatite: systematic (Type I, Fig. 6d, Fig. 7b, Fig. 12) and non-systematic (Type II, Fig. 7d, Fig. 12). Previous EBSD analysis of completely recrystallized apatite indicates that strain-free subdomains are all preferentially oriented along a single crystallographic plane (Černok et al. 2019; Kenny et al. 2020b; Cox et al. 2020). The orientation of recrystallized grains identified in those studies are similar to the partially recrystallized grains observed here (Fig. 6a), where strained subdomain cores are rimmed by crystallites that follow the orientation of the primary apatite (Fig. 6c, d, Fig. 12). Given these similarities, we consider this to be the result of recrystallization of a defect-free parent grain, with strain-free domains aligning along the same orientation of the host (‘Type I a’, Fig. 12). In contrast, where newly crystallized neoblasts and crystallites are preferentially oriented along two crystallographic planes (Fig. 7a, b), we interpret this to be the result of a pre-existing defect within the original parent grain (e.g., growth twin), or two interpenetrating grains followed by brittle block rotation (‘Type Ib’, Fig. 12). While it is not possible to state exactly what this defect is, it is unlikely to be an impact-related defect (e.g., shock twin or planar fracture), given the observed girdle orientation, which is inconsistent with nucleation from a specific crystallographic orientation. In completley recrystalllized grains where recrystallization is non-systematic (i.e., ‘Type II’, Fig. 7c, d, Fig. 12), the crystallographic orientation of neoblasts is comparable to that seen in cataclastic-recrystallized grains (i.e., the partially recrystallized equivalent of Type II, c.f., Fig. 6b, 8c). These newly grown apatite “blocks” are strain free and are considered a higher temperature equivalent of the cataclastic-recrystallized grains (Fig. 12). Under conditions of prolonged melt interaction, strain-free apatite domains would nucleate within pre-existing high-strain fragments that are variably oriented and rotated (e.g., neoblastic overprinting of fragments; Fig. 11). Continued growth would then result in the impingement of individual domains and further nucleation of neoblasts/crystallites along domain rims that have non-systematic orientations reflective of the original fragment orientation (Fig. 12).

In addition to the observed styles of apatite recrystallization, two interesting textural features are documented in completely recrystallized grains that provide insight into the mechanisms responsible for apatite recrystallization. Firstly, randomly oriented micro-vesicles are observed within the interior of apatite neoblasts and crystallites within completely recrystallized apatite (both Type I and Type II, Fig. 7). Comparable micro-vesicles (or glass inclusions) have been observed in the center of newly-grown apatite during experimental nucleation and crystallization studies (e.g., Höland et al. 1999; Hill et al. 2007; Wisniewski et al. 2015). In these studies, apatite nucleates on glassy droplets, with crystal growth continuing until it encompasses the pre-existing glass phase (Höland et al. 1999; Hill et al. 2007). Secondly, individual strain-free domains (i.e., both neoblasts and crystallites) are surrounded by a low-crystallinity phase that is readily observable in BSE images (Fig. 7, Fig. S9). This material is distinct from material within micro-vesicles and triple junctions (Fig. 7a, b), and its location is strikingly similar to REE enrichments surrounding neoblasts within apatite from the Brent impact structure, Canada (McGregor et al. 2020a, their Fig. 7D). Fine-scale amorphous phase separation during apatite crystallization has also been observed in experimental studies (Hill et al. 2007). Based on these similarities, and the absence of any spatial relationship with microstructures, it is possible that apatite recrystallization during impact events occurs via similar mechanisms. However, given the natural abundance of volatiles in apatite, and the presence of multiple micro-vesicles within a given neoblast interior (e.g., Fig. 7), it cannot be dismissed that these features may form after apatite recrystallization during volatile release from the crystal lattice under disequilibrium conditions.

Conclusions

This study utilized an integrated EBSD––SIMS approach to investigate zircon, apatite, and titanite from the Lac La Moinerie impact structure to provide insights in the microstructural and isotopic behavior of accessory phases during impact metamorphism.

• Our results demonstrate that impact-induced isotopic resetting in common Pb-rich geochronometers (apatite and titanite) is distinct from that of high-U phases (zircon), emphasizing the underlying challenges involved with dating common Pb-rich accessory phases within terrestrially shocked impact melt-bearing lithologies.

• We demonstrate that unlike high-U geochronometers, impact resetting in apatite and titanite is minimally controlled by microstructural deformation and is largely related to an individual grain’s textural setting (i.e., T–t history).

• We document the first known case of metamict neoblasts within FRIGN zircon. Our findings indicate that zircon can retain the high-U compositions of pre-impact metamict domains during impact-induced polymorphic transformations and recrystallization

• Raman mapping has revealed the presence of metastable anatase (t-TiO₂) within thermal dissociation rims, with its preservation interpreted to be the result of ZrO₂–SiO₂ controlled stabilization of anatase.

• Two modes of recrystallization are observed in apatite: systematic (Type I), in which strain-free domains nucleate in one or more preferred orientations that are controlled by pre-existing structural defects (if present) in the parent grain; and non-systematic (Type II), whereby strain-free domains nucleate on angular fragments within cataclastically deformed parent grains.