high‐resolution microstructural and compositional analyses
TRANSCRIPT
High-resolution microstructural and compositional analyses of shock deformed
apatite from the peak ring of the Chicxulub Impact Crater
Morgan A. COX *1,2, Timmons M. ERICKSON 1,3, Martin SCHMIEDER 2,Roy CHRISTOFFERSEN3, Daniel K. ROSS3, Aaron J. CAVOSIE 1, Phil A. BLAND 1,
David A. KRING 2, and IODP–ICDP Expedition 364 Scientists
1Space Science and Technology Centre (SSTC), School of Earth and Planetary Science, Curtin University, Perth, Western
Australia 6102, Australia2Lunar and Planetary Institute (LPI)—USRA, 3600 Bay Area Boulevard, Houston, Texas 77058, USA
3Jacobs-JETS, Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, Texas77058, USA
*Correspondence. E-mail: [email protected]
(Received 01 December 2019; revision accepted 09 June 2020)
Abstract–The mineral apatite, Ca5(PO4)3(F,Cl,OH), is a ubiquitous accessory mineral, withits volatile content and isotopic compositions used to interpret the evolution of H2O onplanetary bodies. During hypervelocity impact, extreme pressures shock target rocksresulting in deformation of minerals; however, relatively few microstructural studies ofapatite have been undertaken. Given its widespread distribution in the solar system, it isimportant to understand how apatite responds to progressive shock metamorphism. Here,we present detailed microstructural analyses of shock deformation in ~560 apatite grainsthroughout ~550 m of shocked granitoid rock from the peak ring of the Chicxulub impactstructure, Mexico. A combination of high-resolution backscattered electron (BSE) imaging,electron backscatter diffraction mapping, transmission Kikuchi diffraction mapping, andtransmission electron microscopy is used to characterize deformation within apatite grains.Systematic, crystallographically controlled deformation bands are present within apatite,consistent with tilt boundaries that contain the <c> (axis) and result from slip in <10�10>(direction) on �f1�120g (plane) during shock deformation. Deformation bands containcomplex subgrain domains, isolated dislocations, and low-angle boundaries of ~1° to 2°.Planar fractures within apatite form conjugate sets that are oriented within either {�2110g,{2�1�10g, {�1�120g, or 11�20
� �. Complementary electron microprobe analyses (EPMA) of a
subset of recrystallized and partially recrystallized apatite grains show that there is anapparent change in MgO content in shock-recrystallized apatite compositions. This studyshows that the response of apatite to shock deformation can be highly variable, and thatapplication of a combined microstructural and chemical analysis workflow can revealcomplex deformation histories in apatite grains, some of which result in changes to crystalstructure and composition, which are important for understanding the genesis of apatite inboth terrestrial and extraterrestrial environments.
INTRODUCTION
Impact cratering has played a major role in shapingand reworking planetary bodies within our solar system(e.g., Baldwin 1963; Shoemaker 1983; Melosh 1989). Inorder to confirm an impact structure on Earth, thepresence of shatter cones, meteoritic components, shockdeformation microstructures within minerals, and/or
high-pressure/-temperature polymorphs must beidentified and documented (e.g., French 1998; French andKoeberl 2010). Shock deformation microstructureswithin minerals are, in many cases, a reliable indicator ofpressure conditions experienced by target rocks during animpact event. Shock-produced microstructures incommon crustal minerals such as quartz, feldspar, andzircon have been studied extensively (e.g., Stoffler and
Meteoritics & Planetary Science 1–19 (2020)
doi: 10.1111/maps.13541
1 © The Meteoritical Society, 2020.
Langenhorst 1994; French 1998; Timms et al. 2017),while the response of other accessory minerals such astitanite, apatite, monazite, and xenotime has received lessattention (e.g., Cavosie et al. 2016; Erickson et al. 2016;McGregor et al. 2018; Cernok et al. 2019; Timms et al.2019; Kenny et al. 2020). Here, we present a detailedstudy of shock deformation in apatite from granitoid andimpact melt lithologies in the recently drilled peak ring ofthe Chicxulub impact structure.
Shock Deformation in Apatite
Shock deformation microstructures in apatitedescribed previously include planar fractures (PFs)(Cavosie and Centeno 2014; Sløby et al. 2017;McGregor et al. 2018; see discussion in Montalvo et al.2019), recrystallization (Alwmark et al. 2017; McGregoret al. 2018, 2020; Cernok et al. 2019; Kenny et al.2020), microvesicles (Wittmann et al. 2013; McGregoret al. 2018), crystal–plastic deformation (Cernok et al.2019; Kenny et al. 2020), and cataclastically deformedzones (Birski et al. 2019; Cernok et al. 2019). PFsconsist of multiple sets of parallel, planar features thatcut across the host apatite grain and are typicallyspaced 5–10 µm apart (Cavosie and Centeno 2014; Liand Hsu 2018; McGregor et al. 2018; Montalvo et al.2019). PFs identified in apatite grains throughbackscattered electron imaging appear similar to thoseidentified in shock-deformed zircon and xenotime (e.g.,Kamo et al. 1996; Cavosie et al. 2010, 2016; Ericksonet al. 2013; Cavosie et al. 2016). PFs in apatite havebeen interpreted to form in the { 10�11} orientation(Cavosie and Centeno 2014), which is different fromnatural cleavage directions, {0001} and {hki0}, ofapatite (Deer et al. 2013). Dislocations and PFs inapatite have been produced experimentally withinapatite shock-loaded to ~25 GPa using a plate-wavegenerator (Sclar and Morzenti 1972). In naturalsamples, PFs in apatite have been identified in rocksthat have experienced ~10 to 20 GPa at the Santa Feimpact structure (New Mexico, USA; Cavosie andCenteno 2014; Montalvo et al. 2019), as well as in clastswithin impact breccia from the Nicholson Lake impactstructure (Canada; McGregor et al. 2018) thatexperienced ~10 GPa.
Shock-induced recrystallization has been reportedwithin apatite from multiple terrestrial impact structuressuch as Carswell (Canada; Alwmark et al. 2017),Nicholson Lake (McGregor et al. 2018), and Paasselka(Finland; Kenny et al. 2020), as well as in phosphateminerals in lunar samples (Cernok et al. 2019). Apatitegrains in contact with impact melt from the NicholsonLake impact structure were shown to have partiallyreset U-Pb ages, whereas apatite grains with PFs within
clasts largely retain their pre-impact basement ages(McGregor et al. 2018). Cernok et al. (2019) correlatedphosphate microstructures with different shock stagesusing coexisting feldspar and electron backscatterdiffraction (EBSD) mapping of apatite. They showedthat as shock pressure increased, deformation in apatitetransitioned from a brittle deformation regime to thecrystal–plastic regime with low-angle boundaries,followed by subgrain formation and the loss ofcrystallinity (Cernok et al. 2019). Similarly, Kenny et al.(2020) showed that apatite grains from a clast-richimpact melt rock (with lithic and mineral clasts thatexperienced >35 GPa; Schmieder et al. 2008) showevidence for shock recrystallization and intragraincrystal–plastic deformation.
Chicxulub Impact Structure
The Chicxulub impact structure, located on theYucatan Peninsula of Mexico, is ~180 km in diameterand exhibits a well-preserved peak ring (Fig. 1) (e.g.,Hildebrand et al. 1991; Kring 1995, 2005; Morgan et al.1997, 2016; Gulick et al. 2008; Kring et al. 2004; Rilleret al. 2018; Rae et al. 2019). The peak ring of thestructure is ~80 to 90 km in diameter and rises ~400 mabove the structural crater floor (Fig. 1) (Gulick et al.2013). The impact occurred ~66 Ma ago, producedglobal ejecta deposits, and is causatively related to theK-T mass extinction event (e.g., Alvarez et al. 1980;Kring and Boynton 1991; Swisher et al. 1992; Smit1999; Kring et al. 2017; Schulte et al. 2010; Renne et al.2013, 2018; Sprain et al. 2018). Based on the presenceof impact melt rock and shock metamorphic featuresidentified in early drill core materials (samples from theYucatan-6 borehole), Chicxulub was confirmed to haveformed from a hypervelocity impact event (e.g.,Hilderbrand et al. 1991; Kring and Boynton 1991;Sharpton et al. 1992, 1996).
In 2016, the International Ocean DiscoveryProgram (IODP) and International ContinentalScientific Drilling Program (ICDP) drilled 829 m of corefrom the peak ring of the Chicxulub crater duringExpedition 364 (e.g., Morgan et al. 2016; Riller et al.2018). Borehole M0077A (21.45°N, 89.95°W)encountered ~112 m of postimpact deposits, ~130 m ofreworked suevite and underlying impact melt rock, and~587 m of coarse-grained granitoid rocks of thecrystalline crater basement (predominantly ~340 Ma)that also host pre-impact mafic and felsic volcanicdykes, veins and dykes of impact melt, and lithic(cataclastic) breccia dykes (Fig. 1) (e.g., Morgan et al.2016; Riller et al. 2018).
Previously indexed orientations of planardeformation features (PDFs) and PFs in quartz
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constrain shock pressures between ~15 and 18 GPathroughout the basement portion of the core (e.g.,Feignon et al. 2020). Shock deformation features havealso been described within titanite grains (Timms et al.2019), along with the occurrence of the high-pressurepolymorph TiO2–II (Schmieder et al. 2019). Here, wecomplement previous studies of shock deformationwithin IODP-ICDP Expedition 364 core M0077A withnew observations of shock deformation in apatite fromthe peak-ring lithologies and xenocrysts within theimpact melt rock unit that immediately overlies thepeak ring (Morgan et al. 2016; Gulick et al. 2017).
SAMPLES AND METHODS
Seventeen samples from the core were selected fromwithin the interval between 728.5 and 1333.6 m belowsea floor (Fig. 1). The granitoid rock samples (samplenumbers are meters below sea floor (mbsf): 745.7;749.58; 791.64; 828.7; 886.665; 896.56; 932.79; 1050.1;
1132.79; 1240.62; 1250.19; 1278.47; 1320.5; 1333.6; seedata repository for sampling core box numbers) consistof plagioclase, quartz, alkali feldspar, biotite, minormuscovite, apatite, zircon, titanite, epidote, garnet,ilmenite, and magnetite. The green–black impact meltrocks (728.515 mbsf; see also Schulte et al. 2017;Wittmann 2018a, 2018b; Slivicki et al. 2019) and meltveins (917.295 mbsf, 1039.23 mbsf) are aphanitic andare andesitic in composition with quartz and feldsparclasts, and secondary calcite. The melt veins are foundwithin brecciated granitoid lithologies and contain clastsof granite bedrock.
A total of 560 apatite grains were identified andimaged using optical microscopy at the Lunar andPlanetary Institute (LPI), Houston. Backscatteredelectron (BSE) imaging using a JEOL-5910 scanningelectron microscope (SEM), operated at an acceleratingvoltage of 15 kV, was then conducted at the NASAJohnson Space Center (JSC) in order to characterizemicrotextures within select apatite grains.
Fig. 1. Location and geophysical images of the Chicxulub impact structure. A) Bouguer gravity anomaly of the crater modifiedfrom Rae et al. (2019). B) Seismic profile of the peak ring after Gulick et al. (2013). C) Schematic log of the peak-ring drill coreshowing sample locations, modified from Morgan et al. (2016).
Shock deformation in apatite 3
EBSD Mapping, Transmission Kikuchi Diffraction
Mapping, and Transmission Electron Microscopy
Microstructural analyses of a subset of 108 apatitegrains as well as nine zircon grains were conducted usingan Oxford Symmetry EBSD detector on a JEOL 7600ffield emission gun SEM at NASA-JSC. Acquisitionparameters included a 20 kV accelerating voltage, 18 nAbeam current, 20.5 mm working distance, and 70° sampletilt. The Oxford Instruments AZtec 4.1 software was usedto collect the data and Channel5 Tango and Mambomodules were used for post-acquisition processing of thedata to create EBSD maps and pole figures. EBSD mapsof apatite grains were collected at spatial resolutionsbetween 50 and 500 nm.
An apatite grain from sample 917.295 mbsfcontaining a representative planar deformation band(PDB) was prepared by focused ion beam (FIB) cross-sectioning using an FEI Quanta dual electron/FIBinstrument at NASA JSC (see data repository for FIBcross section methods).
The FIB section was then analyzed by high-resolution transmission Kikuchi diffraction (TKD) inorder to characterize the degree of misorientationobserved across the PDB. TKD maps were run at12 nm step size, with the apatite section yielding EBSDpatterns of high quality. The mapping used a 25 kVaccelerating voltage, a 10 nA beam current, 20.5 mmworking distance, and −20° sample tilt.
After TKD characterization, transmission electronmicroscopy (TEM) imaging and analysis of the apatiteFIB section utilized a JEOL JEM-2500SE 200 KV field-emission scanning transmission electron microscope(FE-STEM) at NASA-JSC. The 2500SE FE-STEM hasanalytical energy dispersive X-ray spectroscopy (EDS)capabilities for spot analysis and submicrometer scaleelement mapping provided by a JEOL DrySD 60 mm2
silicon drift EDS detector interfaced to a Thermo NSSSystem Seven spectral analyzer system.
The characterization of the apatite FIB sample withFE-STEM utilized the full range of instrumentcapabilities for bright-field (BF)/dark-field (DF)conventional TEM and STEM imaging, with particularemphasis on characterization of dislocations, subgrains,and low-angle/high-angle boundaries by conventionalTEM BF/DF diffraction contrast imaging.
Orientation of PFs in Apatite
Crystallographic orientations of PFs within apatitegrains were measured using the program Image-J fromforescatter images collected during EBSD mapping ofapatite grains. The lineations collected by measuring thePFs in forescatter images were then plotted within the
program Stereonet (Allmendinger et al. 2011; Cardozoand Allmendinger 2013), so that they could be rotatedabout the absolute crystallographic orientation by theaverage Euler angles φ1, Φ, φ2 as determined by EBSD(see data repository). This rotation allowed the c-axis ofthe grains to be centered in stereoplots and, therefore,the fracture sets of all grains could be compared in thesame crystal reference frame to help elucidate anysystematic patterns in order to determine thecrystallographic orientation (hkil) of the PFs.
Electron Microprobe
Apatite grains from impact melt domains insamples 728.515 mbsf and 917.295 mbsf were analyzedusing a JEOL 8530F field emission gun microprobe atNASA-JSC. Working conditions employed anaccelerating voltage of 15 kV, a beam current of 20 nA,and a spot size of 3 µm. Microprobe results from 53spots in 14 apatite grains were collected in order todetermine the local F, Cl, Na, Mg, Al, Si, Ca, S, La,Ce, P, Sr, Y, Fe, Mn, Sm, and Nd abundances.
An additional analytical protocol followed forapatite analyses included the following conditions: F Kα(LD1, 30s), Cl Kα (PETL, 30s), Na Kα (TAP, 30s), MgKα (TAP, 30s), Al Kα (TAP, 30s), Si Kα (TAP, 30s), CaKα (PET, 30s), S Kα (PET, 30s), La Lα (PET, 30s), CeLα (PET, 30s), P Kα (PETL, 30s), Sr Lα (PETL, 30s), YLα (LPET, 30s), Fe Kα (LIFH, 30s), Mn Kα (LIFH, 30s),Sm Lα (LIFH, 30s), and Nd Lα (LIFH, 30s). All elementswere counted for 30 s at their peak positions, and 15 s ateach background position. Standardization of F wasperformed using a Wilberforce (Ontario, Canada)fluorapatite crystal, and Cl using a tugtupite crystal (seedata repository for additional list of standards used).Reanalysis of the Wilberforce fluorapatite standards asunknowns yielded an average K-raw of 105.54 indicatingthat fluorine X-ray yield drifted up compared to theoriginal calibration. An empirical correction of 0.9475was therefore applied to all fluorine data from apatiteunknowns. Analyses on apatite that yielded fluorineconcentrations greater than that which is stoichiometri-cally possible (i.e., >3.76 wt%) were categorized as amisanalysis and removed; only two such analyses werefound. The Cl standards yielded reproducible valuesacross the analyses and, therefore no correction wasapplied to the data.
RESULTS
Optical and BSE Imaging of Apatite
Apatite grains range in length from 30 to 900 µmand exhibit euhedral to subhedral basal and prismatic
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sections. Optical and BSE imaging of 560 apatite grainsrevealed either PFs, sub-PFs, cataclasticallydeformed zones, and granular textures, or a combinationthereof (Fig. 2; see data repository). All samples containgrains with PFs; a total of ~250 apatite grains containPFs in up to three orientations within individual grains(Figs. 2 and 3). Offsets along PFs also occur in heavilyfractured grains, with up to ~5 µm of apparentdisplacement. Sub-PFs are common within apatite.Cataclastic deformation is evident in 60 grains, withsome grains displaced along fractures. Sample 1333.6mbsf contains the best example of cataclasticmicrostructures, with 22 apatite grains containingcomplex brittle deformation microstructures. Sevenapatite grains containing granular microstructures wereidentified from impact melt rock and melt veins insamples 728.515 mbsf and 917.295 mbsf (Fig. 4).Granules are ~10 to 100 µm in size and contain amixture of rounded neoblastic granules and largereuhedral laths (e.g., Fig. 4). Apatite grains hosted bypre-impact biotite exhibit minimal fracturing, with curvi-planar fractures being the predominant microstructureobserved, whereas grains in direct contact with quartz,feldspar, and/or zircon contain multiple sets of PFs thatcrosscut the grains.
Neither zircon grains included within apatite, northose that are in contact with quartz and feldspardisplay evidence of planar microstructures, except fortwo grains that contain one set of planarmicrostructures each (Fig. 7). Zircon grains within theimpact melt rock samples exhibit granularmicrostructures, with evidence of BSE-brightinclusions of ZrO2, presumably baddeleyite (see datarepository).
Orientation of PFs in Apatite
Planar fractures observed within the apatite grainsare crosscutting features that extend across polishedgrain interiors. Fractures are typically spaced ~5 to10 µm apart and occur in up to three orientationswithin individual grains. A total of 24 fracture sets weremeasured from forescatter images of apatite grains inorder to determine their crystallographic orientation.Criteria for measured sets were that fractures mustoccur as multiple sets, they are evenly spaced, planar,and crosscut the grain. Of the 24 fracture sets measured,two distinct conjugate fracture sets are identified whenthe lineations are plotted on stereonets in the crystalreference frame (see data repository). The two conjugatefracture sets coincide with known crystallographicorientations within the hexagonal crystal system, withthe fractures oriented within either the {�2110g, {2�1�10g,{�1�120g, or {11�20g planes.
EBSD Mapping
Microstructural EBSD analyses of 120 apatitegrains in granitoid rocks indicate plastic deformationaffected apatite throughout the core (Figs. 2 and 3).Grains with PFs exhibit a higher degree of intragrainplastic strain than other apatite grains analyzed (Fig. 2).PDBs are observed in five grains, with PDBssystematically misorientated up to 20° from the hostabout (0001) (Fig. 3). Cataclastically deformed grainsexhibit >40° of misorientation between rotatedfragments (see data repository), with adjacent quartzalso showing >10° of crystal–plastic deformation.
High-resolution EBSD mapping of partiallyrecrystallized grains from the impact melt unit showsthat orientation data from host apatite grains are highlydispersed in pole figures, which is attributed to impact-related deformation. In contrast, newly recrystallizeddomains show little to no dispersion in pole figures ofindividual recrystallized granules indicating that theyare virtually strain-free (Fig. 5). Fully recrystallizedapatite grains show differences in crystallinity (Fig. 4),with one grain containing a newly crystallized rim ofapatite, while the interior of the grain does not index byEBSD and is not visible in band contrast.
Apatite included in sheet silicates that exhibit kinkbands within the shocked granitoid rocks shows verylow levels of crystal–plastic deformation, with <5° ofcumulative misorientation observed, while grains incontact with zircon and/or quartz show up to 20°misorientation across the grain, with a particularly highdegree of deformation close to the contact betweenapatite and zircon or quartz (Fig. 6).
A single zircon grain within shocked granitoidsample 1050.1 mbsf contains {112} deformation twins(Fig. 7). The single orientation of twin lamellae is ~50to 100 nm in width and is locally developed withinsubdomains of the zircon grain. The grain contains <5°of crystal–plastic deformation across the crystalstructure and exhibits planar and non-planar fractures.Other zircon grains throughout the shocked granitoidrocks show varying levels of crystal–plastic deformation;one zircon inclusion within apatite contains PDBs thatare misoriented up to 20° from the host zircon grain(see data repository).
Within impact melt rock sample 728.515 mbsf, fourzircon grains have a granular microstructure (e.g.,Fig. 8). EBSD mapping of the granular grains showsthey are composed of individual neoblastic subdomainsthat are misoriented in a systematic way, 90°misorientation relationships about {110} and (001),indicating evidence of the former presence of the high-pressure ZrSiO4 polymorph reidite (Fig. 8) (Cavosieet al. 2018).
Shock deformation in apatite 5
TKD Mapping
An FIB section was prepared from an apatite grain insample 917.295 mbsf (Fig. 9). The grain was selectedbased on EBSD analyses which revealed systematic PDBswith <0001> disorientation axes; the vertical sectionextracted is orthogonal to one of the PDBs (Fig. 9).High-resolution TKD mapping reveals that within thesection, there are two individual deformation bands thatare misoriented up to 10° from the host apatite grain. Thedeformation bands are oriented parallel to the c-axis ofthe grain and are ~2 µm in width. Pole figures of theTKD map show that there is minimal rotation about<0001> of the grain while poles to {11�20} and {10�10}show dispersion up to 20° (Fig. 9).
TEM Analysis
TEM imaging (both conventional TEM bright-fieldand STEM) of an apatite grain from sample 917.295mbsf reveals a complex subgrain microstructure withthe subgrains showing varying degrees of misorientation(but mostly low-angle) across boundaries made up ofcomplex dislocation networks (Fig. 10). Lower densitydislocation substructures are also present inside thesubgrains themselves (Fig. 10). In bright-field STEMimaging, the PDB within the central part of the sampleis prominent and remains in contrast through a range oftilting orientations (Fig. 10A). The strain contrast widthof the dislocation arrays defining the PDB boundaries isat a minimum in imaging normal to the symmetrical c-
Fig. 2. Images of apatite grains from shock-deformed granitoid rock of the Chicxulub peak ring. A) Apatite grain with threeorientations of planar fractures (PFs) from sample 896.56 mbsf. B) Apatite grain with three orientations of PFs from sample932.79 mbsf. C) Apatite grain with three orientations of PFs from sample 749.58 mbsf. D) Apatite grain with up to twoorientations of PFs from sample 1050.1 mbsf. PPL = plane polarized light, BSE = backscattered electron image, BC = bandcontrast, IPF = inverse pole figure
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axis zone orientation, suggesting the boundaries tend tobe oriented parallel to <0001> (e.g., �f1�120g). The c-axiszone orientation also results in a near-extinction
condition for dislocations within the subgrains as wellas the subgrain walls themselves, suggesting thedislocation Burgers vector b is parallel to <0001>, orhas a significant c-axis component.
Although the dislocations and dislocation arraysubstructures associated with the PDB regions in the FIBsample are fundamentally shock-generated in their origin,some amount of thermal strain recovery is also indicatedby the relatively low dislocation density in the regionsbetween the low-angle grain boundaries (i.e., within thesubgrains themselves), and the recovery of intracrystallinestrain by dislocation organization into subgrain walls.Based on the characterization of the apatite foil, therewas no evidence of PFs being infilled with any materialbut to fully address this hypothesis, additional grains withplanar features should be analyzed by TEM.
Electron Microprobe
Electron microprobe analysis of 14 apatite grains inimpact melt rock (728.515 mbsf) and impact melt veins(917.295 mbsf) shows that apatite grains in sample728.515 mbsf are chlorine-rich (~0.3 to 0.8 wt%), whilegrains from sample 917.295 mbsf have <0.1 wt% Cl(Table 1, see data repository for microprobe results).Apatite in both samples is fluorine-rich, with Fconcentrations ranging from 3.71 to 2.8 wt%. Bothcalcium and phosphorous concentrations are relativelyconstant throughout apatite grains in both samples, withvalues ranging from ~52 to 55 wt% for CaO and 39 to42 wt% P2O5. Apatite in both samples is also relativelysulfur-rich, with as much as 0.34 wt% SO3 (Table 1).
A single, partially recrystallized grain (sample 163,grain 7) was analyzed by microprobe with 11 individualspots (Fig. 5, Table 1), targeting both recrystallized andpre-existing host grain domains. Recrystallized domainshave higher MgO, with concentrations ranging from~0.2 to 0.01 wt% MgO, while Mg in the pre-existingapatite grain is typically below detection limit (=0.0wt%). The recrystallized domains also have highermeasured wt% SiO2, with the host apatite ranging from0.1 to 0.17 wt% and the recrystallized domains rangingfrom 0.33 to 1.86 wt% SiO2 (Tables 1 and DR1 insupporting information). However, FeO is highlyvariable within both the host apatite and recrystallizeddomains. Phosphorous and chlorine concentrations inboth recrystallized domains and host apatite grain areconstant throughout all spots, but CaO values areslightly higher in the host apatite, reaching up to55.3 wt% while the recrystallized domains are all<54.3 wt% (Table 1). The recrystallized apatite grainswithin the uppermost impact melt rock (728.515 mbsf)also have elevated MgO values ranging from ~0.2 to0.05 wt% MgO.
Fig. 3. Texture component map showing misorientation withinapatite grains in shocked granitoid rock. A) Apatite grain thatcontains PDBs which are up to ~10° misoriented from thehost grain. Texture component map shows up to 18° ofcumulative misorientation across the grain from sample 932.79mbsf. Dotted box = FIB section location. B) Apatite grainthat contains planar fractures and PDBs. Texture componentmap shows up to 17° misorientation across the grain fromsample 932.79 mbsf. Qz = quartz.
Shock deformation in apatite 7
Fig. 4. Electron backscatter diffraction (EBSD) and BSE maps of recrystallized apatite grains. A) BSE image of granular apatitegrain from sample 728.515 mbsf. B) Inverse pole figure showing misorientation of individual granules within the apatite grainfrom (A). C) Pole figure of granular apatite grain from (B), showing random orientations of granules indexed by EBSD. D) BSEimage of granular apatite grain from sample 917.295 mbsf. E) Inverse pole figure showing misorientation of individual granulesand domains within the apatite grain from (D). F) Pole figure of granular apatite grain from B, showing both random andpreferred orientations of granules indexed by EBSD.
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DISCUSSION
Microstructures in Apatite
Apatite grains throughout the samples surveyedshow shock deformation microstructures, ranging fromPFs to crystal–plastic deformation and complete
recrystallization. The orientations of PFs observedwithin apatite from this study are consistent with beingoriented in either the {�2110g, {2�1�10g, {�1�120g, or {11�20gcrystallographic planes. Previous studies havedocumented conjugate sets of PFs in apatite that appearto be oriented parallel to the {10�11g prism plane of thegrain (Cavosie and Centeno 2014). However, it is
Fig. 5. Partially recrystallized apatite grain from sample 917.295 mbsf (sample 163, grain 7). A) BSE image of apatite grain.Dotted line = recrystallized domains, white circles = electron microprobe spots with labels (see Table 1 for results), inlay shows aclose-up of microprobe spots within the recrystallized domain. B) EBSD inverse pole figure (IPF) of misorientation within grain.C) Pole figure for host apatite grain showing dispersed poles. D) Pole figure for undeformed recrystallized domains showing littlemisorientation within individual recrystallized domains. Ap = apatite, Bt = biotite.
Shock deformation in apatite 9
difficult to distinguish between prismatic orientationsusing solely BSE or optical light images. Therefore, ourresults confirm the presence of conjugate sets of PFs inshocked apatite and further build on the suggestedorientations of PFs formed within the mineral resultingfrom shock deformation.
Crystal–plastic deformation observed within apatitefrom the Chicxulub impact structure is similar to thatdescribed in apatite from the Paasselka impact structure(Kenny et al. 2020) as well as in phosphate mineralsfrom the Moon (Cernok et al. 2019). PDBs observedwithin apatite from our samples represent the firstdocumented occurrence of this microstructure in apatite.The deformation bands observed in the apatite grainsare texturally similar to those observed in zircon (e.g.,Erickson et al. 2013), xenotime (Cavosie et al. 2016),
and monazite (Erickson et al. 2016). Deformation bandsin zircon have been suggested to be a product ofimpact-related deformation but are not considereddiagnostic evidence of impact as they have also beendocumented in tectonically deformed rocks (e.g.,Kovaleva et al. 2015). Combining EBSD, TKD, andTEM analyses of the PDB within apatite shows that theband is consistent with tilt boundaries that contain thec-axis and results from slip in <10�10> on �f1�120g asdetermined using high-resolution EBSD and TKD, aswell as the alignment of dislocations when the FIBsection is rotated to the c-axis during TEM imaging.
With our new results from Chicxulub, impact-induced recrystallization of apatite has now beendescribed from five terrestrial impact structures withsimilar microstructures of lath-like grains and rounded
Fig. 6. BSE and EBSD maps of apatite grains exhibiting impedance mismatching with neighboring phases from sample 1050.1mbsf. A) BSE image of virtually undeformed apatite grain found within sheet silicates (chloritized biotite). B) Texturecomponent map showing that apatite grain from A contains <3° misorientation. C) BSE image of deformed apatite grain foundwithin sheet silicates and in contact with zircon. Parts of the grain in contact with zircon contain planar fractures while otherregions of the grain that are only in contact with sheet silicates contain few planar fractures. D) Texture component mapshowing that apatite grain from B contains up to 12° misorientation, with a higher degree of strain toward the contact withadjacent apatite and zircon grains. Ap = apatite, Bt = biotite, Chl = chlorite, Zrn = zircon.
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neoblasts observed in recrystallized grains from theCarswell (Canada; Alwmark et al. 2017), Lac LaMoinerie (Canada; McGregor et al. 2019), Paasselka(Finland; Kenny et al. 2020), and Steen River (Canada;McGregor et al. 2020) impact structures.
Shock Barometry of Peak-Ring Samples
To evaluate shock pressures in peak-ring granitoidrocks and impact melt within them, we begin by utilizing
PDFs within quartz grains. These features have beentraditionally used to determine (peak) shock pressuresbecause they have been extensively studied in nature andreproduced in controlled experiments (e.g., Stoffler andLangenhorst 1994). Based on experimental replication,different specific orientations of PDFs in quartz indicatedifferent pressure ranges, with {10 �1 3} and {10 �1 4}suggesting >10 to <20 GPa, while the presence of {10 �12} PDFs indicates slightly higher pressures around~20 GPa (according to the shock barometry calibrated
Fig. 7. EBSD and BSE maps of shock-deformed zircon grain from granitoid rock sample 1050.1 mbsf. A) BSE image of zircongrain showing subplanar microstructures. B) Inverse pole figure (IPF) showing misorientation of the grain and {112}deformation twins identified within the grain. C) Pole figure from inset for shock-deformed zircon grain showing the relationshipof the {112} deformation twins with the host zircon (Zrn).
Shock deformation in apatite 11
for non-porous quartzofeldspathic rocks; Stoffler andLangenhorst 1994; Stoffler et al. 2018). In the case of thePDFs in shocked quartz grains within granitoid rocks,the relatively high abundance of {10 �1 3} and {10 �1 4}along with subordinate {10 �1 2}, {11 �2 1}, and {51 �6 1}orientations suggests the granitoid target rocks of theChicxulub peak ring experienced shock pressures of ~15to 18 GPa (Feignon et al. 2020).
The presence of {112} twins within a zircon grainfrom granitoid rock sample 1050.1 mbsf supports ourinterpretation of the pressure estimates derived fromquartz PDF indexing, as empirical studies indicate thatzircon requires shock pressures of ~20 GPa to form{112} deformation twins (e.g., Moser et al. 2011; Timmset al. 2017; Cox et al. 2018). Due to the low abundanceof shock-twinned zircon (one grain) identified within the
Fig. 8. EBSD maps of granular zircon grain that contains misorientation relationships indicating former reidite in granularneoblastic zircon (FRIGN zircon) from sample 728.515 mbsf. A) BSE image of granular zircon grain. B) Band contrast (BC)image of zircon grain showing subgranular microstructures as well as intact host grain domains. C) Inverse pole figure (IPF)showing misorientation of the grain. D) Stereonet of shock-deformed zircon grain showing the 90° relationships of the granulesand host indicating the former presence of the high-pressure polymorph reidite (Cavosie et al. 2018).
12 M. A. Cox et al.
samples, as well as only one orientation of extremelyfine twins (~50 nm in width) therein, it appears that thenon-melted granitoid rocks of the peak ring did notexperience shock pressures much higher than ~20 GPa,and/or that the shock twins may have resulted fromlocalized amplification of peak shock pressure (e.g.,along grain boundaries that may have served as zonesof enhanced shock impedance [e.g., Stoffler 1971; ElGoresy et al. 2001]. The estimated pressure range(≤20 GPa) is consistent with the absence within thegranitoid rocks of reidite, the high-pressure polymorphof ZrSiO4, which requires shock pressures of ~30 GPato form in granitoid rocks (e.g., Kusaba et al. 1985;Leroux et al. 1999; Glass et al. 2002; Erickson et al.2017; Cox et al. 2018).
More variable and potentially higher shockpressures are reasonably expected in the impact meltrock within the peak ring. The identification of theformer presence of reidite in granular zircon (FRIGNzircon) within the impact melt rock samples (Fig. 8)suggests that individual zircon grains experiencedpressures >30 GPa (e.g., Cavosie et al. 2018). Previouswork on target rock components of the impact melt inthe Chicxulub structure suggested pressures of ~60 GPa(e.g., Morgan et al. 2016), while other rock and mineralclasts underwent complete melting. However, becauseshock metamorphism is an inherently heterogeneousprocess and because shock pressure isobars crosscutexcavation flow lines during the impact crateringprocess (e.g., French 1998), impact melt rocks andbreccias typically contain a mix of variably (weakly toseverely) shocked target rock and mineral fragments(e.g., Stoffler 1971; Schmieder et al. 2015). The zirconmicrostructures alone cannot constrain the pressures
required to form granular microstructures in apatitegrains within the impact melt, but are suggestive of aminimum shock pressure of ~30 GPa that affectedtarget rock clasts and mineral grains incorporated intothe Chicxulub impact melt.
Impedance Mismatching
Impedance mismatching refers to variations in thevelocity of the shock wave through different, coexistingminerals due to differences in physical properties andcrystal structure, such as density, hardness, andorientation of the phase relative to the shock wave(e.g., Stoffler 1972; Kusaba et al. 1988). Impedancemismatching is inferred in all granitoid samples, withapatite grains that are enclosed within (chloritized)biotite showing <5° of crystal plastic deformation. Incontrast, apatite grains fully surrounded by quartz orin contact with zircon and coexisting apatite contain>10° crystal–plastic deformation, PDBs, and PFs(Fig. 6). Biotite and chloritized biotite within thegranitoid samples appear to have compressed duringshock wave propagation, forming typical kink bandsand, therefore, may have accommodated most of thestrain, allowing it to largely bypass enclosed apatitegrains (e.g., HOrz and Ahrens 1969). Shock-inducedcompression of sheet silicates has been documentedwithin grains from multiple impact structures (e.g.,Santa Fe; Montalvo et al. 2019), as well as inexperimentally shocked mica (e.g., Lambert andMackinnon 1984). The inferred impedancemismatching suggests that apatite responds to the sameshock pressures (from the same shock wave front) indifferent ways, depending on the mineralogy of the
Fig. 9. High-resolution transmission Kikuchi diffraction (TKD) mapping of focused ion beam (FIB) section removed fromapatite grain in sample 917.295 mbsf (shown in Fig. 3A). A) Texture component map showing up to 10° misorientation acrosssection. One distinct orientation of planar deformation bands is observed. B) Pole figure showing misorientation within theapatite grain, as well as little dispersion of poles about the c-axis, suggesting PDBs are parallel to the c-axis.
Shock deformation in apatite 13
surrounding host lithology. While local shock pressureamplification can occur at grain boundaries causinglocalized pressure excursions (e.g., El Goresy et al.2001), shock pressures can also be notably reduced instrain-accommodating “shock-buffer” zones such asmica flakes and other sheet silicates.
Shock-Induced Recrystallization
The partially recrystallized apatite grains from thisstudy (Fig. 5) show that recrystallized domainsidentified in both EBSD and BSE imaging have higherMgO compared to the host grain, while the latter has
Fig. 10. TEM images of planar deformation bands and other shock-related defects in an apatite grain from sample 917.295mbsf, prepared by FIB sectioning (Figs. 3A and 9). A) Bright-field STEM image of entire FIB section showing locations of mostprominent PDBs. B) Conventional TEM symmetrical zone axis bright-field image showing detail of subgrains and low-anglesubgrain walls associated with a PDB. The PDB contains internal subgrain domains as well as isolated dislocations outside ofthe PDB. C) Conventional TEM bright-field image of a PDB in a slightly off-axis orientation relative to the [0001] (c-axis).Narrow width of the strain contrast across the PDBs in this diffraction orientation are consistent with minimal inclination of theboundary planes relative to the c-axis (i.e., boundaries are likely along �f1�120g). D) Conventional bright-field TEM image ofPDB in non-systematic diffraction orientation highlighting the dislocation array along the PDB boundary.
14 M. A. Cox et al.
slightly higher CaO. This indicates that during thepartial recrystallization of apatite within the impactmelt, Mg2+ may have been substituted for Ca2+ and,therefore, the composition of apatite was locally alteredin response to shock-induced or post-shocktemperature-induced recrystallization. There is alsoelevated SiO2 within the recrystallized domains of theapatite grain suggesting that the granitic bedrock maypotentially be a source of fluids during impact-inducedmelting. However, variability in FeO concentrationsthat is uncorrelated with recrystallized apatite texturesdomains is evidence for pre-impact heterogeneity withinthe grains. Partial recrystallization created an opennetwork within the grain and, therefore, these areaswere able to thermally interact with the hot impactmelt, as supported by elevated MgO and SiO2 valueswithin fully recrystallized grains within the upper mostimpact melt unit (e.g., Fig. 4, Table 1).
The differences in both the Cl concentrations inrecrystallized apatite grains from the upper impactmelt unit and deeper melt veins could all be due topostimpact hydrothermal alteration or pre-impactvariations in magmatic fluids. The differences in MgOconcentrations within the partially recrystallized graincould potentially be due to a long-lived hydrothermalsystem triggered by the Chicxulub impact (Zurcherand Kring 2004; Abramov and Kring 2007; Kringet al. 2017). Magnesium was originally present withinthe target rocks at the Chicxulub structure in the form
of dolomite in the sedimentary portion of the targetrock and mafic (dolerite) veins and amphibolite unitswithin the deeper, crystalline portion of the targetrock (e.g., Sharpton et al. 1996; Kring 2005;Schmieder et al. 2017, 2018). Therefore, elevated Mgwithin the apatite could be due to impact-inducedalkali and Mg-/Ca-metasomatism (e.g., Rowe et al.2004; Tuchscherer et al. 2004; Zurcher and Kring2004; Trepmann et al. 2005), but further chemicalcharacterization of variably shocked apatite in bothfresh and hydrothermally altered target rock andimpactite samples from the Chicxulub crater isrequired to test this hypothesis.
Observations within our sample suite indicate thatimpact-induced recrystallization of apatite can changethe chemical composition of the mineral. A recent studyby Kenny et al. (2020) showed that partiallyrecrystallized apatite grains from the Paasselka impactstructure in Finland also have observed changes inchemistry, with recrystallized domains being depleted inMg and Fe relative to the host grain. Therefore, acombination of microstructural and geochemicalcharacterization of any potential apatite targets,especially from meteorites and lunar samples where thepetrographic context is often poorly characterized orunknown, is necessary before determining meaningfulgeochronologic ages or potentially measuring apatiteOH, Cl, and F concentrations to assess planetaryvolatile abundances.
Table 1. Representative microprobe analyses of recrystallized apatite grains (728.515 mbsf—sample 89) and apartially recrystallized grain (917.295 mbsf—sample 163) from impact melt domains, core M0077A.
Grain spot 89 23-1 89 6-5 89 7-2 89 7-3 163 7-2 163 7-3 163 7-4 163 7-5 163 7-8 163 7-9 163 7-11
CaO 54.9 53.9 53.6 53.8 55.1 55.0 55.1 54.3 54.2 53.6 55.3P2O5 42.7 41.9 41.7 41.7 42.0 41.8 42.0 41.4 41.1 40.2 42.4
F 3.36 3.26 3.51 3.49 3.42 3.57 3.52 3.46 3.33 3.54 3.26Cl 0.31 0.63 0.62 0.62 0.01 0.01 b.d.l. 0.02 0.01 0.02 0.01SiO2 0.18 0.22 0.31 0.27 0.15 0.17 0.12 0.61 0.68 0.66 0.10Al2O3 b.d.l. b.d.l. b.d.l. 0.02 0.01 0.01 0.01 0.18 0.25 0.10 0.01
Y2O3 b.d.l. b.d.l. 0.10 0.14 0.04 0.03 0.04 0.04 b.d.l. 0.02 0.01La2O3 b.d.l. 0.02 0.02 b.d.l. b.d.l. 0.02 b.d.l. 0.05 b.d.l. 0.02 0.03Ce2O3 0.14 0.08 0.04 0.10 0.08 0.07 0.05 0.16 0.03 0.15 0.07
Nd2O3 0.07 0.01 0.08 0.08 0.01 0.03 b.d.l. 0.04 0.03 b.d.l. 0.06Sm2O3 0.03 0.02 b.d.l. 0.04 b.d.l. b.d.l. b.d.l. 0.02 0.02 b.d.l. b.d.l.FeO 0.09 0.38 0.39 0.33 0.36 0.45 0.34 0.59 0.63 0.37 0.33
MnO 0.07 0.14 0.14 0.16 0.10 0.10 0.12 0.09 0.10 0.10 0.07MgO 0.01 0.20 0.19 0.16 b.d.l. b.d.l. b.d.l. 0.13 0.20 0.04 b.d.l.SrO b.d.l. 0.01 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.
Na2O 0.05 0.04 0.07 0.07 0.05 0.08 0.06 0.07 0.05 0.10 0.04SO3 0.17 0.08 0.11 0.15 0.10 0.22 0.17 0.19 0.13 0.20 0.06O = F + Cl −1.56 −1.59 −1.70 −1.69 −1.52 −1.59 −1.57 −1.54 −1.58 −1.45 −1.40Total 100.64 99.44 99.42 99.62 100.09 100.19 100.24 100.03 99.66 97.53 100.41
Units in table are in wt%. XX XX-X = sample, grain spot no.
b.d.l. = below detection limit.
Shock deformation in apatite 15
CONCLUSION
Developing a better understanding of how apatiteresponds to shock deformation is crucial in order tounderstand more about changes in the crystal structureand composition of the mineral on other planetarymaterials, in particular, older materials that haveexperienced complex histories of impact bombardmentover billions of years. Using multiple, correlatedtechniques to characterize and identify deformationmicrostructures and recrystallization textures withinapatite is important, as no single technique alone canidentify all changes in the mineral resulting from shockdeformation. Recrystallized and partially recrystallizedapatite grains analyzed in this study show that shockrecrystallization can cause apparent changes in apatitecomposition.
The crystallographically controlled PDBs identifiedwithin apatite in this study are the first to be reportedin this mineral and result from active slip in <10�10> on�f1�120g. The PDBs contain complex subgrain domains
and low-angle boundaries with the bands defined bydislocation arrays, confirming that apatite responds toshock deformation in a complex way even at thenanometer to micrometer scale. We show that the PFswithin apatite form conjugate sets and are orientedwithin either the {�2110g, {2�1�10g, {�1�120g, or 11�20
� �
planes, which increase the known orientations of PFs innaturally shocked apatite. Additionally, the suspectedimpedance mismatching observed within the samplesshows that apatite deforms differently based on itspetrographic context with coexisting adjacent mineralsand, therefore, the surrounding mineralogy of the hostrock may significantly influence the behavior of apatiteduring shock deformation processes.
Acknowledgments—The IODP-ICDP Expedition 364Science Party is composed of S. Gulick (US), J. V.Morgan (UK), G. Carter (UK), E. Chenot (France), G.Christeson (US), Ph. Claeys (Belgium), C. Cockell(UK), M. J. L. Coolen (Australia), L. Ferriere(Austria), C. Gebhardt (Germany), K. Goto (Japan), H.Jones (US), D. A. Kring (US), J. Lofi (France), C.Lowery (US), R. Ocampo-Torres (France), L. Perez-Cruz (Mexico), A. Pickersgill (UK), M. Poelchau(Germany), A. Rae (UK), C. Rasmussen (US), M.Rebolledo-Vieyra (Mexico), U. Riller (Germany), H.Sato (Japan), J. Smit (Netherlands), S. Tikoo (US), N.Tomioka (Japan), M. Whalen (US), A. Wittmann (US),J. Urrutia-Fucugauchi (Mexico), L. Xiao (China), andK. E. Yamaguchi (Japan). Work by MAC, MS, andDAK at the LPI was partially supported by NationalScience Foundation (NSF) award 1736826. Support wasalso provided by the LPI Summer Intern Program in
Planetary Sciences and the Space Science andTechnology Center at Curtin University. LPIContribution no. 2370. LPI is operated by USRA undera cooperative agreement with the Science MissionDirectorate of the National Aeronautics and SpaceAdministration. We thank the editor Christian Koeberland reviewers A. Cernok and an anonymous reviewerfor constructive comments.
Editorial Handling—Dr.Christian Koeberl
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SUPPORTING INFORMATION
Additional supporting information may be found inthe online version of this article.
Item DR1. Sample location in meters below seafloor, preparation, and extended methods.
Item DR2. Plane polarized light image of planarfractures within apatite grains.
Item DR3. Backscattered electron images of planarfractures in apatite grains
Item DR4. Stereonet of the orientation of planarfractures in apatite.
Item DR5. Electron backscatter diffraction maps ofcataclastically deformed apatite grains.
Item DR6. Electron backscatter diffraction map ofapatite with deformed zircon inclusion.
Item DR7. BSE images of apatite (sample 89) withspot numbers for microprobe analysis of grains.
Item DR8. BSE images of apatite (sample 163) withspot numbers for microprobe analysis of grains.
Table Item DR1. Electron backscatter diffractionmap of apatite with deformed zircon inclusion.
Shock deformation in apatite 19