multiphase u pb geochronology of sintered breccias from the steen river impact structure canada mixed target considerations for a jurassic cretaceous boundary event abstract in situ u pb geochronology has been conducted using laser ablation inductively coupled mass spectrometry la icp ms on shocked and thermally metamorphosed apatite titanite and zircon grains from the steen river impact structure canada the dated relict mineral phases occur within impact melt bearing breccias that underwent post deposition sintering at 450 c t

petalite postdating spodumene in pegmatite as a consequence of the 2 02 ga meteorite impact in the vredefort structure southern africa abstract rare element granitic pegmatites enriched in lithium are economically important hosts for the lithium aluminosilicates spodumene and petalite most spodumene and petalite crystallize from the pegmatite melt depending on pressure and temperature this paper reports the rare occurrence of petalite postdating the subsolidus stage in spodumene pegmatites within the 2 02 ga vredefort impact structure the largest oldest and most deeply eroded meteorite impact structure currently known on earth the studied spodumene bearing pegmatites postdate the deformation fabric in the surrounding amphibolites of the granitoid greenstone terrane impact related veins of fragment rich pseudotachylites cut across the pegmatites spodumene albite and quartz constitute the primary mineral assemblage of the pegmatites muscovite is late magmatic and contains inclusions of spodumene and albite during the subsolidus stage k feldspar replaced spodumene and albite during pervasive metasomatic alteration samples collected closer to the pseudotachylites show reaction textures between spodumene and quartz without the involvement of muscovite raman spectroscopy and microprobe analyses show that the reaction textures consist of quartz and petalite unlike spodumene petalite does not show a replacement by k feldspar the 40ar 39ar muscovite age of 2027 8 ma coincides with the timing of the meteorite impact and documents its effect on the spodumene pegmatites we suggest that the formation of petalite after spodumene was related to heating which was generated in the target rocks by the post shock thermal event thus the unique conditions associated with the meteorite impact can not only result in the formation of rare minerals in the target rocks but can also account for rare scenarios resulting in phases that would not otherwise form in nature

spatial distribution of zircon with shock microtwins in pseudotachylite bearing granite gneisses vredefort impact structure south africa abstract mechanical microtwins in zircon is a characteristic shock feature and a potential tool for estimating shock pressure and or shear stress however so far zircon with microtwins has rarely been described in situ the present study of pseudotachylite bearing granitic gneisses from the vredefort impact structure focuses on the spatial distribution of zircon with microtwins and other deformation features zircon hosted by granitic gneiss does not reveal extensive shock deformation with decreasing distance to pseudotachylite zircon progressively develops sub planar fractures planar deformation bands low angle boundaries rarely thin 112 microtwin lamellae and closest to the vein it is fragmented in contrast zircon hosted by pseudotachylites contains thick and closely spaced microtwin lamellae which form up to three sets within individual grains microtwin lamellae are associated with planar 112 low angle boundaries and with overprinting crystal plastic and brittle deformation we suggest that planar 112 low angle boundaries in zircon are indicative of shock deformation and that microtwins in zircon within pseudotachylite veins are an indicator of the impact origin for the latter as opposed to endogenic seismically induced friction melts locally elevated shock pressure and shear stress induce the formation of pseudotachylites in the target rock as well as shock microtwins in zircon

seismic properties and mineral crystallographic preferred orientations from ebsd data results from a crustal scale detachment system aegean region abstract the crystallographic preferred orientations cpos were measured on a suite of samples representative of different structural depths along the west cycladic detachment system greece electron backscatter diffraction ebsd analyses were conducted on calcitic and mica schists impure quartzites and a blueschist and the average seismic properties of the rocks were calculated with the voigt reuss hill average of the single minerals elastic stiffness tensor the calcitic and quartzitic rocks have p and s wave velocity anisotropies avp avs averaging 8 1 and 7 1 respectively the anisotropy increases with depth represented by the blueschist with avp averaging 20 3 and avs averaging 14 5 due to the content of aligned glaucophane and mica which strongly control the seismic properties of the rocks localised anisotropies of very high magnitudes are caused by the presence of mica schists as they possess the strongest anisotropies with values of 25 for avp and avs the direction of the fast and slow p wave velocities occurs parallel and perpendicular to the foliation respectively for most samples the fast propagation has the same ne sw orientation as the lithospheric stretching direction experienced in the cyclades since the late oligocene the maximum shear wave anisotropy is subhorizontal similarly concordant with mineral alignment that developed during extension in the aegean radial anisotropy in the aegean mid crust is strongly favoured to azimuthal anisotropy by our results

nanoscale deformation twinning in xenotime a new shocked mineral from the santa fe impact structure new mexico usa shock microstructures in refractory accessory minerals such as zircon and monazite provide crucial evidence for deciphering impact related deformation in a wide variety of planetary materials here we describe the first occurrence of shock deformation in xenotime ypo 4 from a shocked quartz bearing shatter cone in granite at the santa fe impact structure new mexico usa backscattered electron imaging shows that shocked xenotime grains near the surface of a shatter cone contain multiple orientations of closely spaced planar fractures high resolution electron backscatter diffraction mapping reveals that some of the planar microstructures in 112 contain deformation twin lamellae that range from 50 nm to 200 nm in width on the polished surface and occur in up to three crystallographic orientations other features attributed to impact such as planar low angle boundaries and planar deformation bands record crystal plastic deformation shatter cone formation and co existing shocked quartz constrain minimum shock pressure experienced by the xenotime grains to 5 10 gpa an upper limit of 20 gpa is tentatively assigned based on the absence of ypo 4 polymorphs and shock twins in co existing zircon we propose that 112 deformation twins in xenotime constitute a diagnostic record of shock metamorphism similar to 112 twins in zircon they have not previously been reported in nature and occur in a rock with conspicuous evidence of shock deformation documentation of deformation twins in xenotime a widely applied u pb geochronometer can be used to identify hypervelocity deformation in shocked rocks detrital grains and other materials and may be particularly ideal for recording low pressure 20 gpa impact conditions that do not produce diagnostic shock microstructures in zircon

the formation of large neoblasts in shocked zircon and their utility in dating impacts uranium lead u pb geochronology of individual shocked zircon grains has unique potential for dating bolide impact events neoblasts in granular textured zircon have been recognized as the shock related feature most effective at recording the impact age here we report the discovery of large neoblasts 5 100 mu m in dimension in shocked zircon at the sudbury impact structure canada the first report of in situ coarsely granular zircon from a terrestrial impact site other than the vredefort structure south africa the neoblast bearing sample was taken from a heterogeneous lithic clast rich igneous unit associated with the roof rocks of the impact melt sheet making this the first time a crater has been dated using neoblastic zircon from the upper part of its stratigraphy previous in situ discoveries of coarsely granular zircon at vredefort were all in impact generated mafic melt emplaced beneath the impact melt sheet electron backscatter diffraction analysis of the impact aged neoblasts indicates that the high pressure conditions inferred in the formation of many small neoblasts were not necessarily involved in the formation of these large ones their large size internal zonation and occurrence in a slowly cooling environment collectively suggest that large neoblasts at sudbury formed by relatively protracted post impact growth in shocked zircon incorporated into impact related melt based on insight from large neoblast growth in terrestrial settings we suggest that the ca 4 33 ga neoblasts recently reported in lunar zircon may imply a major basin forming event on the moon at that time new knowledge of the cratering environments in which large neoblasts form also raises the prospect of possibly linking ex situ granular zircon in lunar breccias with specific impact structures and thus better calibrating the lunar cratering record with radiometric ages

timing of crystallization of the lunar magma ocean constrained by the oldest zircon the primitive moon was covered with a thick layer of melt known as the lunar magma ocean whose crystallization resulted in the moon s surface as it is observed today dating of the oldest zircon so far in lunar rocks indicates that much of the magma was probably crystallized within 100 million years of the moon s formation

xenon systematics of individual lunar zircons a new window on the history of the lunar surface abstract we demonstrate a new way of investigating the processing of the lunar surface and other planetary regoliths that combines xes xen ages based on uranium fission in individual zircons with their xenon isotopic record of solar wind and cosmic ray exposure we report the first xenon isotopic analyses of individual lunar zircons from apollo 14 soil and breccias samples parallel analyses of a suite of zircons from the vredefort impact structure in south africa revealed xes xen ages that agree well with u pb systematics suggesting that the diffusion kinetics of xenon and lead in zircon are similar in the pressure temperature environment of sub basin floors in contrast all apollo 14 zircons examined exhibit xes xen ages markedly younger than the associated u pb and 207pb 206pb ages and soil zircons with 207pb 206pb ages greater than 3900 ma produced an abundance of xes xen ages 2400 ma suggesting that these samples may be useful for investigating ancient events and regolith processing at an earlier epoch however none of the zircons contain xenon from now extinct 244pu implying that either the samples have completely degassed since 3900 ma or that the initial pu u ratio of the moon is lower than that on earth we also describe a methodology for conducting component deconvolution that can be applied to multi isotopes systems beyond xenon we have also determined new xenon isotopic yields from rare earth element spallation in the lunar environment and high precision yields for neutron induced fission of 235u in geologic samples

new zircon shock phenomena and their use for dating and reconstruction of large impact structures revealed by electron nanobeam ebsd cl eds and isotopic u pb and u th he analysis of the vredefort domethis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh integrated electron nanobeam ebsd cl eds and isotopic measurements u pb u th he of zircon from the collar and centre of the 80 km wide central uplift of the 2020 3 ma vredefort impact structure reveal new shock features in a microstructural progression related to impact basin formation and degree of u pb age resetting 1 planar fractures in 1k0 and 1k2 orientation during initial shock wave compression 2 curviplanar fractures in 1k1 orientation now annealed which host glassy inclusions of partial melt of the host rock 3 microtwin lamellae in an orientation of 65 about 110 attributed to shock wave rarefaction 4 nucleation of impact age crystallites possibly on microtwins during post shock heating by impact melt and 5 crystal plastic deformation linked to crater modification of the core of the central uplift planar fracturing and microtwinning 20 gpa in cold shock zircon in granitoid at a radial distance of 25 km failed to reset zircon age single grain id tims data

empirical constraints on shock features in monazite using shocked zircon inclusions shock deformation microstructures in monazite have been systematically characterized for the first time in grains from the vredefort impact structure in south africa electron backscatter diffraction mapping has identified 12 unique orientations of monazite deformation twins including 7 orientations that have not previously been described in experiments or nature other shock features include planar deformation bands and strain free neoblasts which have been shown to date deformation shock twinned zircon inclusions within the deformed monazite require pressures of 20 gpa thus providing critical empirical constraints on formation conditions confirming a hypervelocity impact origin of the monazite microstructures the vredefort monazite grains described here represent the first case of using shocked mineral inclusions to empirically calibrate shock microstructures formed in the host mineral these results conclusively establish monazite as a recorder of shock deformation and highlight its use in identifying and dating impact structures

observations and interpretations at vredefort sudbury and chicxulub towards an empirical model of terrestrial impact basin formation the structural topographic and other characteristics of the vredefort sudbury and chicxulub impact structures are described assuming that the structures originally had the same morphology the observations interpretations for each structure are compared and extended to the other structures this does not result in any major inconsistencies but requires that the observations be scaled spatially in the case of vredefort and sudbury this is accomplished by scaling the outer limit of particular shock metamorphic features in the case of chicxulub scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring the observations interpretations are then used to construct an integrated empirical kinematic model for a terrestrial peak ring basin the major attributes of the model include a set of outward directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor some of which are pre existing structures that have been reactivated during transient cavity formation inward directed motions along the same outermost structures and along a set of structures at intermediate radial distances during transient cavity collapse structural uplift in the center followed by a final set of radially outward directed thrusts at the outer edges of the structural uplift during uplift collapse the rock displacements on the intermediate inward and innermost outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift

zircon radiation damage ages abstract the suggestion that the accumulation of radiation damage in zircons could be used as a dating method was first proposed in the 1950s in the original technique radiation damage was determined by x ray diffraction however this is not suitable for the small sample analysis needed in modern geochronology it is now possible to measure radiation damage and u and th contents on micron sized areas on the polished surface of zircon grains using raman spectroscopy and sims mass spectrometry this opens the way for a reassessment of the potential of radiation damage ages and the purpose of this contribution is to re examine the technique through its application to zircons from two granitoids from the archean yilgarn craton and ancient detrital zircons from the jack hills in western australia the three examples show internally consistent radiation damage ages that are also in accord with independent geochronological evidence the 420 110 ma radiation damage age for the darling range granite is coincident with 500 400 ma biotite rb sr ages in the same region of the yilgarn craton likewise a tonalite from the youanmi terrane in the craton with a 1650 150 ma zircon radiation damage age lies within the domain of a 1600 ma event recorded by biotite rb sr ages the jack hills zircons have a 1120 130 ma radiation damage age that is explained by a complex damage accumulation and annealing history culminating in a mild heating event indicated by biotite ar ar ages of about 1140 ma the positive results for the three case histories suggest that radiation damage ages could play a useful role in dating low temperature thermal events

decline of giant impacts on mars by 4 48 billion years ago and an early opportunity for habitability the timing of the wane in heavy meteorite bombardment of the inner planets is debated its timing determines the onset of crustal conditions consistently below the thermal and shock pressure limits for microbiota survival and so bounds the occurrence of conditions that allow planets to be habitable here we determine this timing for mars by examining the metamorphic histories of the oldest known martian minerals 4 476 4 429 gyr old zircon and baddeleyite grains in meteorites derived from the southern highlands we use electron microscopy and atom probe tomography to show that none of these grains were exposed to the life limiting shock pressure of 78 gpa 97 of the grains exhibit weak to no shock metamorphic features and no thermal overprints from shock induced melting by contrast about 80 of the studied grains from bombarded crust on earth and the moon show such features the giant impact proposed to have created mars hemispheric dichotomy must therefore have taken place more than 4 48 gyr ago with no later cataclysmic bombardments considering thermal habitability models we conclude that portions of mars crust reached habitable pressures and temperatures by 4 2 gyr ago the onset of the martian wet period about 0 5 gyr earlier than the earliest known record of life on earth early abiogenesis by 4 2 gyr ago is now tenable for both planets the oldest known minerals from mars have no strong shock features indicating early cessation of giant impacts there according to microanalysis of zircon and baddeleyite grains in meteorites

solving the martian meteorite age conundrum using micro baddeleyite and launch generated zircon the age of the representative martian meteorite nwa 5298 is determined using spatially correlated electron beam nanostructural and uranium lead isotopic measurements of microminerals resolving a paradox of different age interpretations for the evolution of martian crust a few of the many meteorites that fall to earth are of martian origin the true age of these rare samples of the martian surface has been the subject of a decades long debate with interpreted ages differing by up to 4 billion years desmond moser and colleagues resolve this problem using a new approach to dating meteorite launch events through nanoscale investigation of crystal growth zoning and structures their analysis of the resistant micromineral baddeleyite and host igneous minerals in the highly shocked martian meteorite northwest africa 5298 reveals it as a crystallization product of martian volcanism of the past 400 million years previous estimates of a 4 billion year age of formation have actually dated a remnant signature of the ancient mantle melting event from which the magma was derived these findings confirm the presence of an ancient non convecting mantle beneath a relatively young volcanic martian crust invaluable records of planetary dynamics and evolution can be recovered from the geochemical systematics of single meteorites1 however the interpreted ages of the ejected igneous crust of mars differ by up to four billion years1 2 3 4 5 6 a conundrum7 due in part to the difficulty of using geochemistry alone to distinguish between the ages of formation and the ages of the impact events that launched debris towards earth here we solve the conundrum by combining in situ electron beam nanostructural analyses and u pb uranium lead isotopic measurements of the resistant micromineral baddeleyite zro2 and host igneous minerals in the highly shock metamorphosed shergottite northwest africa 5298 ref 8 which is a basaltic martian meteorite we establish that the micro baddeleyite grains pre date the launch event because they are shocked cogenetic with host igneous minerals and preserve primary igneous growth zoning the grains least affected by shock disturbance and which are rich in radiogenic pb date the basalt crystallization near the martian surface to 187 33 million years before present primitive non radiogenic pb isotope compositions of the host minerals common to most shergottites1 2 3 4 do not help us to date the meteorite instead indicating a magma source region that was fractionated more than four billion years ago9 10 11 12 to form a persistent reservoir so far unique to mars1 9 local impact melting during ejection from mars less than 22 2 million years ago caused the growth of unshocked launch generated zircon and the partial disturbance of baddeleyite dates we can thus confirm the presence of ancient non convecting mantle beneath young volcanic mars place an upper bound on the interplanetary travel time of the ejected martian crust and validate a new approach to the geochronology of the inner solar system

seismic reflection evidence for a dangerous grounds miniplate no extrusion origin for the south china sea the collision of india and asia has caused large strike slip faults to form in east asia resulting in the extrusion of crustal blocks toward the southeast since the eocene as a result of the indentation of rigid india into asia it has been suggested that the south china sea opened as a result of relative motion between a rigid indochina sundaland block and china alternative models propose that rifting and seafloor spreading were driven by trench forces to the south we test these competing models by analysis of seismic reflection profiles across the boundary between sundaland and the southern rifted margin known as the dangerous grounds we show that the southern boundary of the dangerous grounds is a subduction zone that jammed in the middle miocene to the west the dangerous grounds are bounded by a strike slip zone also active until similar to 16 ma that becomes diffuse south of the now inactive south china sea seafloor spreading center we place the western edge of the dangerous grounds just to the east of the natuna arch lupar line the west baram line is confirmed as originating as a major strike slip fault within the dangerous grounds and is continuous with the red river fault zone because the dangerous grounds were independent of sundaland until similar to 6 ma its motion cannot have been constrained by motion of this block making extrusion impossible as a mechanism to rift the south china sea se motion by both the dangerous grounds and sundaland suggests subduction forces were the primary trigger for plate motions our reconstruction places a similar to 280 km upper limit on the motion on the red river fault and a similar to 1400 km width to the paleo south china sea

a pressure temperature phase diagram for zircon at extreme conditions abstract hypervelocity impact processes are uniquely capable of generating shock metamorphism which causes mineralogical transformations and deformation that register pressure p and temperature t conditions far beyond even the most extreme conditions created by terrestrial tectonics the mineral zircon zrsio4 responds to shock deformation in various ways including crystal plasticity twinning polymorphism e g transformation to the isochemical mineral reidite formation of granular texture and dissociation to zro2 sio2 which provide robust thermobarometers that record different extreme conditions the importance of understanding these material processes is two fold first these processes can mobilize and redistribute trace elements and thus be accompanied by variable degrees of resetting of the u pb system which is significant for the use of zircon as a geochronometer second some features described herein form exclusively during shock events and are diagnostic criteria that can be used to confirm the hypervelocity origin of suspected impact structures we present new p t diagrams showing the phase relations of zrsio4 polymorphs and associated dissociation products under extreme conditions using available empirical and theoretical constraints we present case studies to illustrate zircon microstructures formed in extreme environments and present electron backscatter diffraction data for grains from three impact structures mistastin lake of canada ries of germany and acraman of australia that preserve different minerals and microstructures associated with different shock conditions for each locality we demonstrate how systematic crystallographic orientation relationships within and between minerals can be used in conjunction with the new phase diagrams to constrain the p t history we outline a conceptual framework for a zircon based approach to extreme thermobarometry that incorporates both direct observation of high p and high t phases as well as inferences for the former existence of phases from orientation relationships in recrystallised products a concept we refer to here as phase heritage this new approach can be used to unravel the pressure temperature history of zircon bearing samples that have experienced extreme conditions such as rocks that originated in the earth s mantle and those shocked during impact events on earth and other planetary bodies

the age and thermal evolution of the vredefort impact structure a single grain upb zircon study abstract single grain u pb shrimp analyses of a heterogeneous zircon population from a small granite body in the archean basement core of the vredefort dome indicate an age of 2017 5 ma for the granite which is identical within errors to the previously reported age determinations for the vredefort impact event petrographic evidence from the granite indicates that it crystallized after the impact shock event but some doubt remains whether it is a true impact melt a decompression melt generated during rapid uplift of the crater basement following the impact event or a melt generated during an earlier 2 05 2 06 ga granulite facies anatectic event that was still at least partially molten at the time of the vredefort impact event rare zircon xenocrysts displaying planar microdeformation features and giving discordant archean ages are overgrown by polycrystalline zircon rims with u th compositions similar to that of the main 2 02 ga zircon population but with discordant 207pb 206pb ages indicating more recent pb loss

the geology and gravity field in the central core of the vredefort structure the centre of the vredefort structure in south africa comprises an uplifted granitic basement with an associated 300 g u gravity high ringed by upturned sedimentary strata detailed geological mapping of the basement core has delineated a three fold concentric zoning of outer granite gneisses ogg steynskraal formation sf and inlandsee leucogranofels ilg an increase in metamorphic grade occurs toward the centre of the structure from greenschist relics on the basement perimeter to upper amphibolite and granulite facies in the sf and granulite facies in the ilg a marked lithological change occurs within the sf in which perimeter greenschists and inner amphibolites give way to paragneisses in the ilg paragneiss and mafic granulite relics predominate these divisions are consistent with the standing on edge model for the basement 1 three regional metamorphic episodes and two thermal events have been recognised m1 and m3 both attained granulite facies at low medium load pressures in the ilg m1 granulite facies assemblages extended into the ogg but the m3 facies was restricted to the ilg m2 was a k metasomatism event limited to the ogg an extensive thermal episode m4a is provisionally correlated with the collar metamorphism pyroxene hornfels facies t 800 c was attained in the centre as opposed to conditions of p 4 5 kbar and t 650 c for alkali granite stocks in the sedimentary collar 2 this event was thus pre or syn uplift but not post uplift a high level high temperature p 3 kbar and t 800 900 c event m4b occurred at a late stage in the overall sequence a suite of amphibole orthopyroxene rocks of basaltic komatiite type cao al2o3 1 are chemically related to younger high mg tholeiites these tholeiites are correlated with similar rocks in the sedimentary collar and were intruded syn to post m4a in several episodes of decreasing basicity shock metamorphic features granitic breccias and pseudotachylite are linked to the late stage history of the structure and a case is presented for a dynamic metamorphic front advancing outward and upward from the centre in response to unroofing of the basement the central gravity high3 is delineated by 1239 new gravity stations at 1 km and 0 5 km intervals a low frequency oval shaped anomaly with a long axis trending nw se has eccentrically positioned on it a plateau extending southwards as a spur a dyke is responsible for the spur but the source for the plateau is from a recent borehole thought to be largely due to sheared altered harzburgite modelling of the low frequency gravity field component was based on the mapped divisions of the ogg sf and ilg and a density contrast of 200 kg m3 was determined between the ogg and the sf and ilg lithologies a purely crustal solution for the excess mass distribution in the basement is conclusively demonstrated

discussion of new zircon shock phenomena and their use for dating and reconstruction of large impact structures revealed by electron nanobeam ebsd cl eds and isotopic u pb and u th he analysis of the vredefort dome 1appears in canadian journal of earth sciences 2011 48 2 117 139 doi 10 1139 e11 011 spodumene pegmatites occur in the southeastern sector of the 2 02 ga vredefort impact structure in south africa moser et al published in 2011 presented an isotope dilution thermal ionization mass spectrometry id tims age of 1136 47 ma for these spodumene pegmatites this discussion questions the reliability of this age field relations indicate that the spodumene pegmatites are not affected by the structural fabric that affected the host amphibolites the crosscutting impact related veins of pseudotachylitic breccia clearly argues a preimpact age for the spodumene pegmatites the high uranium content of one of the two zircons used by moser et al to obtain the age is correlated to the 1 1 ga pb loss event widely observed in different rocks within the vredefort impact structure

preservation of detrital shocked minerals derived from the 1 85 ga sudbury impact structure in modern alluvium and holocene glacial deposits detrital shocked minerals can provide valuable residual records of eroded impact structures recent studies have reported shocked minerals in modern alluvium in a subtropical climate from the deeply eroded 2 02 ga vredefort dome impact basin in south africa to evaluate the detrital shocked mineral record at a large impact structure in a temperate setting with a holocene glacial erosional history we investigated 4000 detrital zircons and 20 000 quartz grains at the less eroded 1 85 ga sudbury basin in ontario canada for the presence of shocked sand grains modern alluvium from rivers within and outside the basin and holocene glaciofluvial sands eskers and outwash deltas across the basin were investigated for shocked minerals shocked zircon and or quartz were found in all modern rivers and most holocene glacial deposits within but not outside the basin petrography and scanning electron microscopy sem backscattered electron bse cathodoluminescence cl imaging and analysis energy dispersive x ray spectroscopy eds electron backscatter diffraction ebsd were used to document shock microstructures of the total detrital zircons surveyed 3 118 3978 were identified as shocked holocene samples contained higher average percentages of shocked zircon 63 1361 or 4 6 with a high of 29 compared to modern alluvium 55 2617 or 2 1 with a high of 6 ebsd analysis revealed a range of shock microstructures including planar fractures deformation microtwins and crystal plastic deformation at sudbury detrital shocked quartz is rare compared to zircon only 15 grains 0 08 were identified all with decorated planar deformation features pdfs these results demonstrate that a detrital shocked mineral record exists at a large impact basin that is in a youthful stage of erosion despite its age in addition to modern alluvium our results also identify glaciofluvial eskers and deltas as reservoirs for detrital shocked minerals glacial episodes thus enhance the dispersal and preservation of shocked detritus in sedimentary systems despite physical differences the observation that the two largest precambrian impact basins continue to contribute detrital shocked minerals 2 b y after impact suggests that a shocked mineral record of impacts on early earth should reside in precambrian siliciclastic rocks

reidite an impact produced high pressure polymorph of zircon found in marine sediments reidite is a high pressure polymorph of zircon with the scheelite structure it has been found in an upper eocene impact ejecta layer in marine sediments on the upper continental slope off new jersey and on barbados reidite occurs epitaxially oriented in shock metamorphosed zircons it is associated with impact glass tektites shocked quartz and feldspar with multiple sets of planar deformation features coesite and trace amounts of stishovite this phase was first produced in high pressure laboratory experiments in 1969 and has also been produced in shock recovery experiments reidite is brittle with an irregular fracture a hardness of 7 5 a calculated density of 5 2 g cm3 a white streak adamantine luster and it does not fluoresce in index oil in transmitted light shocked zircon grains consisting almost entirely of reidite are transparent pleochroism was not observed reidite appears to have parallel extinction and is length slow the maximum birefringence is roughly 0 015 reidite appears to be uniaxial positive it is tetragonal space group i 41 a a 4 738 1 a c 10 506 2 a v 235 84 2 a3 previous shock loading experiments on zircons indicate that the transition to reidite starts at about 30 gpa and is completed around 53 gpa reidite should be a useful indicator of peak pressure in shock metamorphosed rocks reidite is named after alan reid who first produced this phase in the laboratory

zircons from the acraman impact melt rock south australia shock metamorphism u pb and 40ar 39ar systematics and implications for the isotopic dating of impact events abstract this study presents the first optical and scanning electron microscopic characterization and u pb shrimp dating results for zircon grains separated from the most likely autochthonous impact melt rock in the central domain of the large 40 90 km eroded ediacaran acraman impact structure in south australia microtextural characteristics define five zircon subtypes corresponding to different levels of progressive shock metamorphism from virtually unshocked monocrystalline zircon grains that exhibit original magmatic zoning in cathodoluminescence images to fully granular zircons that have completely lost their primary zoning pattern and locally contain neocrystallized submicrometer sized spots of zro 2 probably baddeleyite that pseudomorph pre impact zircon the granular zircons correspond to the highest observed level of shock metamorphism and impact induced recrystallization zro 2 bearing granular zircons indicate shock pressures in excess of 65 70 gpa which are considerably higher than previous shock pressure estimates for the acraman impactites u pb systematics of untreated and chemically abraded melt rock zircons indicate that u pb ratios of the acraman zircons were variably reset during impact weakly shocked crystalline grains yield ages on concordia at 1 59 1 60 ga reflecting the magmatic age of the gawler range volcanics only the entirely granular zircon population was apparently impact reset but based on an ediacaran age from stratigraphic constraints on the ejecta layer experienced significant post impact pb loss the microcrystalline nature of granular zircons could have promoted pb diffusion and recoil in post impact time as suggested by grain size dependent diffusion and recoil modeling a positive correlation of u concentration and shock level suggests that granularization might have preferentially occurred in initially u rich probably metamict zircons 40 ar 39 ar dating of a melted yardea dacite clast from the acraman melt rock as well as k feldspar separated from shocked yardea dacite resulted in post impact alteration plateau ages suggestive of hydrothermal events at 500 ma and 450 ma that selectively affected the impactites exposed in the central domain of the acraman impact structure our study demonstrates that the acraman impact is particularly difficult to date in the absence of accurate and precise isotopic ages for acraman the ediacaran ejecta stratigraphic age of 635 541 ma is considered the most reliable age constraint currently available for the timing of the large acraman impact

microscopic planar deformation features in quartz of the vredefort structure anomalous but still suggestive of an impact origin abstract the orientation and distribution of diagnostic shock produced planar features in quartz has been examined in over 80 samples from both the core and collar rocks of the vredefort structure in south africa these features are widespread occurring in all but four samples the majority of the planar features have a basal 0001 orientation with a few higher pressure so called features and three features occurring near the center of the structure their relative distribution and preservation are however anomalous compared to those at other large terrestrial impact structures and recorded shock pressures calculated on the basis of planar feature orientation do not show a regular decrease radially outward as observed at other known impact structures the apparently anomalous development of planar features is interpreted as a result of post shock recrystallization if the effect of recrystallization of quartz which generally increases inward toward the core of the structure is considered and only the highest pressure type planar feature present in any one sample is taken as a measure of shock pressure then higher pressures are recorded in the core compared to the collar rocks no evidence of a previously reported additional post recrystallization shock event is found in this study although previous researchers report that pseudotachylite which may or may not be associated with the shock event cuts recrystallized quartzites in is concluded that although anomalous the evidence from planar features in quartz is still consistent with the vredefort structure being an erosional remnant of a large complex impact structure

variable microstructural response of baddeleyite to shock metamorphism in young basaltic shergottite nwa 5298 and improved u pb dating of solar system events abstract the accurate dating of igneous and impact events is vital for the understanding of solar system evolution but has been hampered by limited knowledge of how shock metamorphism affects mineral and whole rock isotopic systems used for geochronology baddeleyite monoclinic zro2 is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust u pb isotopic systematics but there is little understanding of shock effects on this phase here we present new nano structural measurements of baddeleyite grains in a thin section of the highly shocked basaltic shergottite northwest africa nwa 5298 using high resolution electron backscattered diffraction ebsd and scanning transmission electron microscopy stem techniques to investigate shock effects and their linkage with u pb isotopic disturbance that has previously been documented by in situ u pb isotopic analyses the shock altered state of originally igneous baddeleyite grains is highly variable across the thin section and often within single grains analyzed grains range from those that preserve primary magmatic twinning and trace element zonation baddeleyite shock group 1 to quasi amorphous zro2 group 2 and to recrystallized micro granular domains of baddeleyite group 3 these groups correlate closely with measured u pb isotope compositions primary igneous features in group 1 baddeleyites n 5 are retained in high shock impedance grain environments and an average of these grains yields a revised late amazonian magmatic crystallization age of 175 30 ma for this shergottite the youngest u pb dates occur from group 3 recrystallized nano to micro granular baddeleyite grains indicating that it is post shock heating and new mineral growth that drives much of the isotopic disturbance rather than just shock deformation and phase transitions our data demonstrate that a systematic multi stage microstructural evolution in baddeleyite results from a single cycle of shock loading heating and cooling during transit to space and that this leads to variable disturbance of the u pb isotope system furthermore by linking in situ u pb isotopic measurements with detailed micro to nano structural analyses it is possible to resolve the timing of both endogenic crustal processes and impact events in highly shocked planetary materials using baddeleyite this opens up new opportunities to refine the timing of major events across the solar system

zircon u pb strain chronometry reveals deep impact triggered flow abstract large 100 km meteorite impact cratering events play important roles in surface and biosphere evolution however their potential for widespread ductile modification of the lithosphere has been difficult to assess due partly to our inability to isotopically age correlate deep mineral fabrics with surface records we have integrated benchmark u pb zircon dating methods id tims shrimp with new microstructural techniques ebsd xrd to demonstrate that crystal plastic deformation can cause rapid out diffusion of radiogenic pb and accompanying trace element alteration in crystalline zircon we have used this phenomenon to directly date fabric in archean zircons and xenoliths of the lower crust of south africa at 2023 15 million years coeval with the 2020 3 million year old vredefort cratering event at surface with extent 20 000 km 2 our findings indicate that regional exogenic fabrics similar to high temperature tectonic fabrics exist in ancient crust moreover our results establish that crystal plastic deformation in the lithosphere can now be directly dated and linked to planetary evolution by zircon u pb strain chronometry

radioelement concentrations in the deep profile through precambrian basement of the vredefort structure the vredefort basement granite in south africa forms the core of an undomed and overturned sequence of precambrian strata slawson 1976 proposed that the granite core has also been overturned exposing to view a thick section through the archean crust incrasing in depth toward the center of the dome profiles across the vredefort basement provide an opportunity for studying the distribution of u th and k with depth in the upper and middle crust detailed radioelement concentration profiles across the granitic basement showed that 1 the central part of the basement core is depleted in th and u relative to the perimeter and the concentration of th and u falls off regularly from the basement margin inward 2 the distribution of the radioelements and the distribution of heat production with depth over the outer 8 km are consistent with an exponential depth function 3 the felsic rocks of the central basement are characterized by high k u and th u ratios and the ratios k u k th and th u increase from the margin inward and 4 mafic granulites which occur in the central regions of the dome have normal k u k th and th u ratios high grade metamorphic environment resulted in the depletion of bothmore u and th relative to k in the greater proportion of the felsic rocks from the central regions of the vredefort basement depletion of u was far greater than that of th the normal th u k u and k th ratios in mafic rocks in close proximity to the felsic rocks suggest that mineral assemblages have a strong control on the behavior of the radioelements during regional metamorphism in the vredefort basement less

evolution of the amphibolite granulite facies transition exposed by the vredefort impact structure kaapvaal craton south africa abstract geological mapping is integrated with high precision u pb zircon and monazite geochronology to document in detail the nature and temporal evolution of the amphibolite granulite facies boundary exposed in the vredefort impact structure of the central kaapvaal craton our study has revealed that the preimpact configuration of the amphibolite granulite facies boundary is best preserved in panels between impact related brittle faults and that past concepts of the boundary must be revised a regional lithologic transition exists between amphibolite facies outer granite gneiss 3 084 3 09 ga and a heterogeneous assemblage of granulite facies gneisses ca 3 1 3 3 ga tabular deformed quartz syenite bodies dated at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cy

complicated secondary textures in zircon record evolution of the host granitic rocks studies from western tauern window and otztal stubai crystalline complex eastern alps western austria abstract samples of metamorphosed and deformed granitic rocks were collected from two alpine complexes with well constrained metamorphic history western tauern window and otztal stubai crystalline complex zircon grains from these samples were investigated in situ by a combination of scanning electron microscope techniques cathodoluminescence cl imaging and raman spectroscopy the aims were to describe and interpret complicated secondary textures and microstructures in zircon based on cross cutting relationships between secondary microstructures reconstruct the sequence of processes affecting zircon crystals link the evolution of zircon with the history of the host rocks the results indicate that zircon in the sampled granitic rocks forms growth twins and multi grain aggregates which are unusual for this mineral moreover various secondary textures have been found in the sampled zircon often cross cutting each other in a single crystal these include distorted oscillatory cl zoning with inner zones forming inward penetrating cl bright embayments which are the evidence of dry recrystallization via annealing lattice recovery cl mosaicism with no preservation of growth zoning but abundant nano and micro scale pores and mineral inclusions which are the evidence of recrystallization by coupled dissolution reprecipitation and or leaching embayed zircon boundaries filled with apatite monazite epidote and mylonitic matrix indicating mineral fluid reactions resulting in zircon dissolution and fragmentation overgrowth cl dark rims which contain nano pores and point to transport and precipitation of dissolved zircon matter we conclude that zircon in our meta granites is sensitive to metamorphism deformation events and was reactive with metamorphic fluids additionally we have found evidence of crystal plastic deformation in the form of low angle boundaries and bent grain tips which is a result of shearing and ductile deformation of the host rock we suggest that the observed complicated secondary textures in zircon can be linked to the evolutionary stages of the host rocks such as magmatic crystallization prograde metamorphism peak of amphibolite facies metamorphism post peak cooling and exhumation formation of ductile shear zones and final cooling to 250 c

diffusion in zircon despite its low abundance in most rocks zircon is extraordinarily useful in interpreting crustal histories the importance of u th pb isotopic dating of zircon is well and long established davis et al this volume parrish et al this volume zircon also tends to incorporate trace elements useful as geochemical tracers such as the ree y and hf a number of characteristics of zircon encourage the preservation of internal isotopic and chemical variations often on extremely fine scale which provide valuable insight into thermal histories and past geochemical environments the relative insolubility of zircon in crustal melts and fluids as well as its general resistance to chemical and physical breakdown often result in the existence of several generations of geochemical information in a single zircon grain the fact that this information is so frequently retained as evidenced through backscattered electron or cathodoluminescence imaging that often reveal fine scale zoning down to the sub micron scale has long suggested that diffusion of most elements is quite sluggish in zircon in this chapter we present an overview of the findings to date from laboratory measurements of diffusion of cations and oxygen in zircon because of its importance as a geochronometer attempts have been made to measure diffusion especially of pb for over 30 years but only in the last decade or so have profiling techniques with adequate depth resolution been employed in these studies resulting in a plethora of new diffusion data these findings have important implications for isotopic dating interpretation of stable isotope ratios closure temperatures and formation and preservation of primary chemical composition and zoning in zircon efforts have been made for some time to quantify and characterize diffusion in zircon most notably of pb in deference to its significance in interpreting pb isotopic signatures and refining understanding of thermal histories as is evident from

radiation damage in zircon and monazite monazite and zircon respond differently to ion irradiation and to thermal and irradiation enhanced annealing monazite cannot be amorphized by 800 kev kr ions at temperatures greater than 175 c whereas zircon can be amorphized at temperatures up to 740 c the damage process i e elastic interactions leading to amorphization in radioactive minerals metamictization is basically the same as for the ion beam irradiated samples with the exception of the dose rate which is much lower in the case of natural samples the crystalline to metamict transition in natural samples with different degrees of damage from almost fully crystalline to completely metamict is compared to the sequence of microstructures observed for ion beam irradiated monazite and zircon the damage accumulation process representing the competing effects of radiation induced structural disorder and subsequent annealing mechanisms irradiation enhanced and thermal occurs at much higher temperatures for zircon than for monazite the amorphization dose expressed as displacements per atom is considerably higher in the natural samples and the atomic scale process leading to metamictization appears to develop differently ion beam induced amorphization data were used to calculate the decay event dose required for amorphization in terms of a critical radionuclide concentration i e the concentration above which a sample of a given age will become metamict at a specific temperature this equation was applied to estimate the reliability of u pb ages to provide a qualitative estimate of the thermal history of high u natural zircons and to predict whether actinide bearing zircon or monazite nuclear waste forms will become amorphous metamict over long timescales

evolution of zircon deformation mechanisms in a shear zone lanzo massif western alps abstract magmatic zircons within two sheared gabbroic dykes from the peridotitic massif of lanzo western alps italy revealed evolution of deformation from crystal plasticity to rigid body rotation during shear zone evolution this is the first time that multiple zircon grains have been analysed in a kinematic context in a shear zone zircon grains recorded crystal plastic deformation activating the commonly inferred 001 and 100 glide systems to the newly identified 110 glide system the exact selection of glide system could be dependant of deformation conditions such as pressure temperature and strain rate moreover the activation of one or several glide systems within a single grain could be favoured by the primary orientation of the grains combined with a high strain rate in these sheared gabbros the deformation mechanisms evolve from plastic deformation at low strain rate conditions to increase strain strain softening and localisation of deformation the progressive shear zone development and the softening of the matrix relative to the zircon has lead to a switch from crystal plasticity to rigid body rotation of zircon the zircon grains rigid body rotation involved that their long axes became parallel to the lineation of the shear zone causing reorientation and dispersion of the misorientation axes away from kinematic y

in situ multiphase u pb geochronology and shock analysis of apatite titanite and zircon from the lac la moinerie impact structure canada in situ u pb geochronology has been performed via laser ablation inductively coupled mass spectrometry on shocked apatite titanite and zircon from the 8 km diameter lac la moinerie impact structure canada the three analyzed phases are inherited from target rocks that were transformed to impact melt bearing breccias and clast laden impact melts apatite yields an array of u pb ratios populating a region between a younger regression with a refined age of 453 5 ma and an older regression with a lower intercept age of 1708 10 ma titanite defines a younger regression with an age of 444 15 ma and an older array with a lower intercept at 1844 24 ma zircon yields an upper intercept age of 1810 13 ma and a lower intercept age of 433 21 ma within error of apatite and titanite the oldest ages from all three phases define paleoproterozoic igneous metamorphic events in granitoid target rocks of the de pas suite isotopic resetting in apatite and titanite with relatively high pb diffusion rates was controlled by a combination of proximity to impact generated superheated melt and the formation of fast diffusion pathways facilitated by shock effects and dynamic recrystallization zircon resetting was controlled by shock effects and metamictization which resulted in significant pb loss we outline an approach for discriminating true impact ages from metamict zircons with high common pb that have experienced post impact pb loss lower intercept ages of all phases constrain the age of formation for the lac la moinerie impact structure with apatite yielding the most precise age of 453 5 ma with an age of 453 ma lac la moinerie joins a growing cluster of more than 12 terrestrial impact structures formed in the middle to late ordovician with potential connection to the l chondrite parent body breakup event and a period of enhanced bombardment on earth

nanosims study of seismically deformed zircon evidence of y yb ce and p redistribution and resetting of radiogenic pb lattice defects in zircon can cause trace elements redistribution and disturbance of isotopic systems this study investigates in detail how seismically induced deformation microstructures in zircon correlate with trace element and isotope re equilibration felsic mylonites with pseudotachylyte veins from the ivrea verbano zone in northern italy were studied with special focus on deformed zircon we have revealed the following post growth deformation microstructures planar deformation bands pdbs planar fractures pfs non planar fractures both healed and open and finite strain patterns pdbs are planar portions of crystal lattice that are strictly parallel to 100 crystallographic planes and are rotated to up to 3 with respect to the host grain they are from 0 5 to 1 m wide and have average spacing of 5 m pdbs originate in seismically active environment at elevated differential stress strain rate and temperature several grains in which pdbs are observed were analyzed with ion microprobe ion maps indicate redistribution of radiogenic pb isotopes associated with pdb formation isotopic redistribution preferably occurs in pdbs with larger crystallographic misorientation profiling demonstrated clear spatial correlation of pdbs with variations of ree abundances both gain and loss and possible correlations with increased and decreased hf ti and p abundances trace elements can be depleted or enriched compared to the abundance in surrounding matrix in deformed domains depending on the spacing of pdbs and the proximity of the analyzed volume to grain boundary or to detrital core 207 pb 206 pb ratio demonstrates systematic pb loss in the pdb bearing lattice domains with respect to pdb free domains in some cases pb gain is observed where the pdbs source radiogenic pb from older detrital cores our study has important implications for geochronology and microchemistry of zircon from seismically deformed sections of ivrea verbano zone and from other paleo seismic zones of the world zircon found in seismically deformed rocks near pseudotachylyte veins may demonstrate distorted and even reset isotopic ages and altered trace element abundances enhanced trace element exchange between deformed zircon and host mylonite can influence mass balance calculations for the bulk rock

isotope systematics and shock wave metamorphism ii u pb and rb sr in naturally shocked rocks the haughton impact structure canada minerals and whole rocks from crystalline fragments collected in the allochthonous polymict breccia of the 23 ma old haughton impact crater on devon island arctic canada were analysed for u pb k and rb sr in order to constrain degrees of shock wave induced perturbations in these systems the results show that shock wave metamorphism at 50 gpa with onset of total melting does not significantly affect the u pb chronometers in monazite and zircon monazite preserved its age of crystallization during high grade metamorphism at 1928 2 2 1 2 0 ma 2 errors as dated by the upper intercept of 2 9 discordant monazites the same analyses yield a lower intercept age of 277 5 35 1 35 0 ma in the concordia diagram demonstrating that discordancy is not a direct effect of shock wave metamorphism but a product of time integrated pb diffusion prior to the impact event which occurred 23 ma ago this constrains maximal shock induced pb loss to less than 3 including possible alteration related pb diffusion from the shock damaged grains in 1903 1 1 4 1 1 ma old monazite from an unshocked reference gneiss recent pb loss also reaches 3 corroborating the occurrence of weak alteration related upb fractionation the data substantiate that u pb discordancy patterns are not a reliable chronometer to date impact events small rock fragments and a coherent block of shock stage i to iii vi consist of heterogeneous unequilibrated rb sr sub systems on the 10 m scale with drastically varying isr shock related rbsr fractionation is very limited and in the large block stage iii three biotite fractions yield an rb sr isochron age of 1791 34 ma 2 which is undistinguishable from the 1814 8 ma age of biotite in the unshocked basement gneiss in contrast to this m3 sized fragment rb sr systematics in a fist size sample of shock stage iii do not yield any reliable age information due to small scale 1 cm3 isotopic disequilibrium it appears that such small scale disequilibrium is essentially due to a take up of radiogenic 87sr in feldspar glass from biotite during shock wave passage this prevents dating of the impact events by the rb sr internal isochron method and it can be concluded that total rock melting is the first condition necessary to reset the rb sr chronometer in shocked crystalline rocks

late palaeoproterozoic depositional age for khondalite protoliths in southern india and tectonic implications abstract shrimp dating of detrital zircons from garnet sillimanite cordierite paragneisses khondalites of the trivandrum block southern india revealed nanometer scale pb isotopic inhomogeneity in most grains that we ascribe to annealing processes during ultra high temperature metamorphism at ca 570 ma our age data for zircons from six representative khondalite samples do not document any neo or mesoproterozoic detrital grains and we conclude from the concordant ages and discordant minimum 207pb 206pb ages that the khondalite precursor sediments were deposited more than 2 1 ga ago and were subsequently intruded by granitoid rocks at ca 1765 2100 ma some detrital zircons in the khondalites contain late palaeoproterozoic metamorphic domains suggesting that these grains are derived from an unknown crustal source that most likely experienced late palaeoproterozoic high grade metamorphism both the metasedimentary assemblage and granitoids were severely ductilely deformed metamorphosed and migmatized during the pervasive pan african event at ca 550 580 ma this caused many detrital zircons in the khondalites to become variably recrystallized and to develop metamorphic rims proper interpretation of cathodoluminescence images and zircon morphology is important in interpreting detrital populations the southern indian khondalites were part of an extensive palaeoproterozoic metasedimentary assemblage that may have extended from southern madagascar via the trivandrum block to the highland complex of sri lanka and seems to reflect a continental margin sequence that was deposited on an archaean to early palaeoproterozoic continental terrane possibly the southern margin of the indian dharwar craton the tectonic history of this terrane remains obscure due to the pervasive overprint during the pan african event

the role of midtropospheric winds in the evolution and maintenance of low level mesocyclones abstract using a three dimensional numerical model supercell simulations initialized in environments characterized by hodographs with large curvature in the lowest 3 km and a range of linear midlevel shears are investigated for low values of the midlevel shear 0 005 s 1 the storm develops a mesocyclone at the lowest model level within the first hour of the simulation the gust front starts to move ahead of the main updraft and cuts off the inflow to the storm by approximately 2 h resulting in decay of the initial storm and growth of a new rotating storm on the outflow as the midlevel shear increases to approximately 0 010 s 1 the initial development of the low level mesocyclone is delayed but the mesocyclone that develops is more persistent lasting for over 2 h further increases of the shear to 0 015 s 1 result in the suppression of any low level mesocyclone despite the presence of intense rotation at midlevels of the storm we hypothesize that differences in the distribution of precipitation

impact induced melting of archean granulites in the vredefort dome south africa i anatexis of metapelitic granulites mid crustal archean pelitic granulites in the vredefort dome experienced a static low p granulite facies overprint associated with the formation of the dome by meteorite impact at 2 02 ga heating and exhumation were virtually instantaneous with the main source of heat being provided by energy released from nonadiabatic decay of the impact shock wave maximum temperatures within a radius of a few kilometres of the centre of the structure exceeded 900 c and locally even exceeded 1350 c this led to comprehensive melting of the precursor archean granulite assemblages grt bt qtz pl ksp crd opx sil followed by peritectic crystallization of aluminous alkali feldspar crd spl crn sil parageneses and the segregation of small evolved biotite leucogranite bodies however at a distance of c 6 km from the centre pre impact rock features are largely preserved although partial replacement of garnet by symplectitic coronas of crd opx spl pl and biotite by orthopyroxene indicate that peak temperatures approached 775 50 c thin interstitial moats of k feldspar are closely associated with the orthopyroxene coronas they are interpreted as the remnants of low proportion partial melts generated by biotite breakdown both the textures and mineral compositional data support reduced equilibration volumes in these rocks which reflect rapid isobaric cooling following shock heating and exhumation the high temperatures and strong lateral thermal gradient are consistent with the modelled impact induced isotherm pattern for a 200 300 km diameter impact crater

transformations to granular zircon revealed twinning reidite and zro2 in shocked zircon from meteor crater arizona usa granular zircon in impact environments has long been recognized but remains poorly understood due to lack of experimental data to identify mechanisms involved in its genesis meteor crater in arizona usa contains abundant evidence of shock metamorphism including shocked quartz the high pressure polymorphs coesite and stishovite diaplectic sio2 glass and lechatelierite fused sio2 here we report the presence of granular zircon a new shocked mineral discovery at meteor crater that preserve critical orientation evidence of specific transformations that occurred during formation at extreme impact conditions the zircon grains occur as aggregates of sub micrometer neoblasts in highly shocked coconino sandstone cs comprised of lechatelierite electron backscatter diffraction shows that each grain consists of multiple domains some with boundaries disoriented by 65 around a known 112 shock twin orientation other domains have 001 in alignment with 110 of neighboring domains consistent with the former presence of the high pressure zrsio4 polymorph reidite additionally nearly all zircon preserve zro2 sio2 providing evidence of partial dissociation the genesis of cs granular zircon started with detrital zircon that experienced shock twinning and reidite formation at pressures from 20 to 30 gpa ultimately yielding a phase that retained crystallographic memory this phase subsequently recrystallized to systematically oriented zircon neoblasts and in some areas partially dissociated to zro2 the lechatelierite matrix experimentally constrained to form at 2000 c provided the ultrahigh temperature environment for zircon dissociation 1670 c and neoblast formation the capacity of granular zircon to preserve a cumulative pressure temperature record has not been recognized previously and provides a new method for investigating histories of impact related mineral transformations in the crust at conditions far beyond those at which most rocks melt

original size of the vredefort structure implications for the geological evolution of the witwatersrand basin abstract historically there have been a range of diameter estimates for the large deeply eroded vredefort impact structure within the witwatersrand basin south africa here we estimate the diameter of the transient cavity at the present level of erosion as 124 140 km based on the spatial distribution of shock metamorphic features in the floor of the structure and downfaulted transvaal outliers taking erosion into account 6 km and scaling to original final rim diameter an estimate of close to 300 km for the rim diameter is obtained independent estimates of the final rim diameter based on an empirical relation of central uplift diameter to rim diameter spatial distribution of pseudotachylites and concentric large scale structural patterns give a similar estimate of close to 300 km for the original final rim diameter an impact structure of this size is expected to have had an original multi ring form at this size the vredefort impact structure encompasses the bulk of the witwatersrand basin which appears to owe its preservation to the vredefort impact in addition the vredefort impact event may have been the thermal driver for some of the widespread hydrothermal activity in the area which in recent interpretations is believed to be a component in the creation of the world class gold deposits of the witwatersrand basin

improved accuracy of u pb zircon dating by selection of more concordant fractions using a high gradient magnetic separation technique abstract a loop of soft iron wire or a paper clip or a ferromagnetic grid mounted between the poles of an electromagnet picks up and allows further magnetic separation of zircons previously found to be non magnetic on a frantz isodynamic separator tests on previously analysed samples indicate that most such zircons that are fairly discordant say 10 can be picked up and isolated from associated grains that are more concordant tests on new samples indicate that even when most grains can be picked up the last few percent of the sample contain less uranium and are more concordant than the bulk sample the degree of discordance is the dominant factor affecting the uncertainty of u pb zircon ages both because of the error amplification in projections and because the assumption of a simple two stage system may not be valid only by eliminating or reducing discordance can errors approaching the uncertainty in a single analysis say 2 m y for 2700 m y rocks be achieved rutile normally concentrated with zircon as non magnetic has been successfully removed from a small amount of low uranium zircon using the high intensity separation technique

evidence for a caledonian amphibolite to eclogite facies pressure gradient in the middle allochthon lindas nappe sw norway the proterozoic anorthosite mangerite charnockite complex dominating the lindas nappe in the scandinavian caledonides was locally eclogitized in the southwestern part of the nappe during the caledonian orogeny whereas only amphibolite facies assemblages are recorded in the rest of the nappe sveconorwegian granulites of anorthositic to jotunitic composition in the northernmost eclogite free exposures of the nappe exhibit large garnet phenoblasts ca 900 c that are fractured and partly replaced by a caledonian symplectitic amphibolite facies assemblage ca 515 c metamorphic zircon attributed to this garnet breakdown is dated by id tims u pb at 430 3 ma suggesting that the amphibolite facies overprint was coeval with the formation of eclogite 30 km further south probably implying that the section across the nappe represents a caledonian pressure gradient the rocks also preserve a complex sveconorwegian history including an age of 969 6 ma which we interpret as dating magmatic emplacement of jotunitic anorthositic portions of the complex 936 12 ma reflecting the granulite facies metamorphism and 908 16 ma representing a late generation of zircon best explained as having formed by metasomatic processes caledonian shearing severely deformed zircon grains in an amphibolite facies shear zone resetting their u pb systems and forming new ones hereby also demonstrating a case of resetting and recrystallization of low u zircon our data gained from diverse lithologies illustrate several processes involved in making and resetting zircon as well as indicate the contemporaneous evolution and similar origin of the lindas nappe and the jotun nappe complex

u pb geochronology of apatite and zircon from the brent impact structure canada a late ordovician sandbian katian boundary event associated with l chondrite parent body disruption in situ la icp ms u pb geochronology has been performed on apatite and zircon within thermally recrystallized clast laden and clast poor impact melt rocks from the brent impact structure a total of 377 laser analyses on 120 impact melt grown n 9 and impact recrystallized zircon grains n 111 were obtained from which a concordia age of 452 8 2 7 ma mswd 0 57 n 11 and a weighted average mean 206pb 238u age of 453 2 2 9 ma mswd 0 60 n 11 are calculated a total of 300 laser analyses from 100 relict apatite grains were obtained with an unanchored regression through all data yielding a lower intercept age of 453 2 6 0 ma mswd 5 8 n 300 that overlaps within error of zircon 207pb 206pb ratios obtained from feldspar clasts within clast laden impact melt retain the same initial pb composition as the target rocks from which they are derived while feldspars that crystallized from impact melt have 207pb 206pb ratios indicative of isotopic re equilibration between basement lithologies of two different ages a similar variability in 207pb 206pb is recorded by apatite this provides evidence for the involvement of neoproterozoic lake nipissing alkaline suite as well as mesoproterozoic grenville gneisses in the production of impact melt at brent recrystallized apatite grains exhibit enrichments in light rare earth elements lrees along neoblast grain boundaries indicative of trace element substitution and phase precipitation during impact induced recrystallization an age of 452 8 2 7 ma from zircon and 453 2 6 0 ma from apatite places the impact event in the late ordovician at or near the sandbian katian boundary confirming brent s involvement in the middle to late ordovician crater cluster event a period of enhanced impactor flux to earth related to the l chondrite parent body disruption

a 2d and 3d nanostructural study of naturally deformed pyrite assessing the links between trace element mobility and defect structures the links between deformation induced micro and nanostructures and trace element mobility in sulphide minerals have recently become a popular subject of research in the earth sciences due to its connections with metallic ore paragenesis it has been shown that plastic deformation in pyrite creates diffusion pathways in the form of low angle grain boundaries that act as traps for base and precious metals however the plastic behavior of pyrite and the physiochemical processes that concentrate these trace elements in deformation induced micro and nanostructures remain poorly understood in this study we develop strategies for 2d and 3d analysis of naturally deformed sulphides by combining electron backscatter diffraction electron channeling contrast imaging and atom probe tomography on pyrite in an attempt to better understand the underlying diffusion processes that mobilize trace elements the combined results reveal structures associated with crystal plastic deformation in the form of dislocations stacking faults and low angle grain boundaries that are decorated by as and co although our data support a dislocation impurity pair diffusion model we have evidence that multiple diffusion mechanisms may have acted simultaneously in this study we applied new data processing techniques that allow for orientation measurement of nanostructural crystal defects from atom probe tomography data dislocations within our studied sample occur along the 110 planes suggesting glide on 110

a record of ancient cataclysm in modern sand shock microstructures in detrital minerals from the vaal river vredefort dome south africa the record of terrestrial meteorite impacts is fragmentary because most impact structures and ejecta are removed by erosion or buried discovery of the missing impact record from hadean to present may be advanced through identifi cation of residual shocked detritus to evaluate which shocked minerals survive erosion and sedimentary transport we investigated modern sands from the vaal river in south africa where it crosses the 2 02 ga vredefort dome the largest terrestrial impact structure known to date shocked minerals were identifi ed in all sediment samples including from the vaal channel and tributaries within the structure in transmitted light detrital quartz preserves discontinuous decorated planar features previously identifi ed as brazil twins which are readily visible as bright continuous features in cathodoluminescence images detrital zircons preserve fi ve orientations of planar fractures pfs which can produce dramatically offset growth zoning and apparent rotation of subgrains other zircons contain fi lled fractures that may represent a new shock microstructure detrital monazite preserves four orientations of pfs and many grains contain oscillatoryzoned shocked zircon inclusions which thus represent shocked inclusions within shocked accessory grains zircon and monazite with granular texture were also identifi ed this study is proof of the concept that shocked minerals can be identifi ed in sediments up to 2 billion years after an impact event and it demonstrates their potential for preserving evidence of ancient impacts the recognition of a new geological repository for impact evi dence provides a means for identifying distal shocked detritus from eroded structures of any age and may be particularly relevant to early earth studies

diachroneity of the clearwater west and clearwater east impact structures indicated by the u th he dating method the u th he method has been applied to constrain the formation ages of the clearwater west and east impact structures of quebec canada zircons were separated from impact melt samples derived from a surface exposure at clearwater west 32 km diameter and from a drill core at clearwater east 26 km diameter the u th he results indicate ages of 280 27 ma 2 n 7 for clearwater west and 450 56 ma 2 n 8 for clearwater east our u th he date for clearwater west supports the findings of previous rb sr 266 15 ma 2 and 40ar 39ar 280 4 ma and 283 8 2 2 ma 2 impact melt studies our u th he date for clearwater east also overlaps with previously published 40ar 39ar dating results which yielded u shaped spectra with maximum and best estimate dates of 460 470 ma our results support the contention previously based solely on 40ar 39ar data that the clearwater west and east impact structures do not comprise an impact doublet that formed coevally from a binary asteroid pair

nanoscale records of ancient shock deformation reidite zrsio4 in sandstone at the ordovician rock elm impact crater the terrestrial record of meteorite impacts is difficult to decipher because unequivocal evidence of impact is increasingly destroyed with time by erosion burial and tectonics zircon survives these processes as a shocked mineral and above 20 gpa transforms to reidite a high pressure zrsio 4 polymorph diagnostic of impact however the utility of reidite has been limited by its occurrence it has only been reported from three relatively young 36 ma impact craters globally here we report a new occurrence of reidite in brecciated sandstone from the ordovician rock elm impact crater in wisconsin united states electron backscatter diffraction mapping was used to identify reidite and microtwins within shocked zircons smaller than 50 m in diameter reidite occurs both as 200 500 nm wide lamellar intergrowths and as nanoparticulate grains and not only provides the first diagnostic evidence for ultrahigh pressure shock metamorphism at rock elm but is also the oldest reported occurrence of reidite considering its small size and the ubiquitous presence of detrital zircon in siliciclastic rocks reidite may be more common in the rock record than has been reported but has potentially gone undetected the recognition that nanoscale reidite can be preserved over deep time within zircon in shock metamorphosed sandstone presents new opportunities for investigating earth9s impact record as it could potentially preserve nanoscopic evidence of impact events much older than the one that formed rock elm given that shocked zircons have been shown to survive sedimentary cycling the identification of reidite within zircons in siliciclastic rocks could facilitate investigating the impact chronology over much of the geological time scale as the oldest terrestrial minerals known are detrital zircons

recurrent magmatic activity on a lithosphere scale structure crystallization and deformation in kimberlitic zircons kimberlites are not only the most economically important source of diamonds they also carry unique information encapsulated in rock fragments entrained as the magma traverses the whole thickness of the lithosphere the nurbinskaya pipe in the siberian kimberlite province russia is one of several intruded along the vilyui rift a major terrane boundary the pipe contains three populations of mantle derived zircon xenocrysts archean mean age 2709 9 ma devonian mean age 371 2 3 ma and a subset of grains with evidence of brittle deformation and rehealing and a range of ages between 370 and 450 ma the hf isotope o isotope and trace element signatures of the last group provide a link between the archean and devonian events indicating at least three episodes of magmatic activity and zircon crystallization in the lithosphere beneath the pipe the emplacement of the nurbinskaya pipe ca 370 ma ago was only the youngest activity in a magma plumbing system that has been periodically reactivated over at least 2 7 billion years controlled by the lithosphere scale structure of the vilyui rift

in situ la icp ms apatite and zircon u pb geochronology of the nicholson lake impact structure canada shock and related thermal effects abstract in situ laser ablation inductively coupled plasma mass spectrometry la icp ms has been deployed to determine the u pb ages of impact metamorphosed apatite and zircon associated with the 12 5 km diameter nicholson lake impact structure northwest territories canada the dated phases occur within both impact melt bearing breccias and clast laden impact melts a total of 84 laser ablation analyses from 57 apatite grains within seven rock samples yield a minimum refined lower intercept at 387 5 ma mswd 0 87 2 n 26 and maximum age with lower intercept of 1740 ma a total of 90 laser ablation analyses on 52 zircon grains from two rock samples yield a minimum intercept age of 384 8 ma mswd 1 3 2 n 22 with a maximum upper intercept age of 2679 14 ma and a second discordia with a 1740 ma upper intercept age the results are consistent with the target rocks comprising archean snow island suite 2 7 ga and paleoproterozoic nueltin plutonic suite 1 74 ga with the impact event occurring at approximately 385 ma the degree of resetting of inherited apatite can be related to its proximity to impact melt now partly devitrified glass within the host impact melt bearing breccias and clast laden impact melt rocks those grains in direct contact with melt bodies are reset while those occurring as inclusions in lithic or mineral clasts partly or wholly retain their original isotopic compositions the youngest impact reset apatite ages are most closely associated with the highest shock levels i e granular textured and thermally dissociated zircons which we relate to juxtaposition of apatite with superheated melt due to accumulated radiation damage prior to impact many of the relic zircons are metamict which facilitated enhanced pb diffusion and their resetting to the impact age granular zircons record impact ages while those exhibiting planar fractures or experiencing negligible shock record basement ages we link apatite and zircon geochronology to the textural and structural states of zrsio4 and its associated shock levels via field emission scanning electron microscopy and micro raman spectrometry

planar microstructures in zircon from paleo seismic zones pseudotachylytes resulted from frictional melts associated with ultramylonites in high grade metapelitic rocks from the ivrea verbano zone in the southern alps northern italy were studied with focus on the deformation microstructures in zircon the aims were to investigate the characteristics of zircon deformation in seismic zones and to recognize specific microstructures generated in zircon during earthquakes which could be useful for mineral dating of paleo seismic events helps to understand how seismic energy is released at depth and interacts with metamorphic processes the interior of polished zircon grains ranging from 30 to 150 m in length were investigated with optical microscope and scanning electron microscope sem techniques including secondary electron se backscattered electron bse forward scattered electron fse cathodoluminescence cl imaging and crystallographic orientation mapping by electron backscatter diffraction analysis ebsd grains were studied in situ and as separated fractions embedded in epoxy disks among different cataclastic and crystal plastic deformation microstructures in zircon we identified characteristic planar deformation bands pdbs planar fractures pfs and curviplanar fractures cfs planar deformation bands in zircon are crystallographically controlled planar lattice volumes with misorientation from the host grain which varies from 0 4 to 2 7 pdbs are usually parallel to 100 crystallographic planes have width from 0 3 to 1 m and average spacing of 5 m in 2d sections planar deformation bands appear as contrast lamellae in orientation contrast images and in ebsd maps and in rare cases can be observed with the optical microscope pdbs form in specifically oriented grains due to high differential stresses high temperatures and high strain rates generated in seismically active environment and or due to shearing in the vicinity of frictional melts discovered structures represent a result of crystal plastic deformation of zircon grains with operating dislocations having 010 glide system and misorientation axis therefore they can be classified as a new type 4 lattice distortion pattern according to the existing classification for zircon piazolo et al 2012 kovaleva et al 2014 we have demonstrated that formation of planar fractures in zircon takes place not only during impacts but also in seismically active zones we observe at least two cases of formation of pfs with 100 orientation 1 as a result of evolution of pdbs and 2 as micro cleavage this study demonstrates that planar microstructures in terrestrial zircon do not exclusively form during impact events but also as a result of seismic events at depth due to unusually high differential stress strain rate and temperature according to the new findings pdbs in zircon from the deep crust are supposed to represent newly recognized evidence of seismicity

correlating planar microstructures in shocked zircon from the vredefort dome at multiple scales crystallographic modeling external and internal imaging and ebsd structural analysis microstructural and geochronological analysis of shocked zircon has greatly advanced understanding the formation and evolution of impact structures however fundamental aspects of shock produced planar microstructures in zircon remain poorly known such as their deformation mechanisms crystallographic orientations and how planar microstructures visible at the grain scale by scanning electron microscopy correlate to microstructures visible at sub micrometer scales by transmission electron microscopy and electron backscatter diffraction ebsd to unify observations of planar microstructures in zircon made at different scales into a consistent framework we integrate the results of 1 three dimensional crystallographic modeling of planar microstructure orientations with 2 360 external prism backscattered electron imaging at the grain scale and 3 polished section cathodoluminescence and ebsd analysis at the sub micrometer scale for a suite of detrital shocked zircons eroded from the vredefort dome in south africa our combined approach resulted in the documentation of seven planar microstructure orientations that can be correlated from grain to sub micrometer scales of observation 010 100 112 112 112 1 12 and 011 all orientations of planar microstructures exhibit minor variations in style however all are considered to be fractures no amorphous zrsio4 lamellae were identified we therefore favor the usage of planar fracture pf over planar deformation feature pdf for describing the observed planar microstructures in zircon based broadly on the nomenclature developed for shocked quartz some 112 pfs visible at the grain scale contain impact microtwins detectable by ebsd and are the first report of polysynthetic twinning in zircon the microtwins consist of parallel sets of thin lamellae of zircon oriented 65 about and occur in multiple crosscutting 112 orientations within single grains curviplanar fractures and injected melt are additional impact related microstructures associated with pf formation crosscutting relations of shock microstructures reveal the following chronology 1 early development of c axis parallel pfs in 010 and 100 orientations 2 the development of up to four 112 pfs including some with microtwins 3 the development of curviplanar fractures and the injection of impact derived melt 4 the development of 011 pfs associated with compressional deformation and 5 grain scale non discrete crystal plastic deformation experimental constraints for the onset of pfs together with the absence of reidite suggest formation conditions from 20 to 40 gpa for all of the planar microstructures described here

mechanisms of strain accommodation in plastically deformed zircon under simple shear deformation conditions during amphibolite facies metamorphism abstract this study documents the strain accommodation mechanisms in zircon under amphibolite facies metamorphic conditions in simple shear microstructural data from undeformed fractured and crystal plastically deformed zircon crystals are described in the context of the host shear zone and evaluated in the light of zircon elastic anisotropy our work challenges the existing model of zircon evolution and shows previously undescribed rheological characteristics for this important accessory mineral crystal plastically deformed zircon grains have axis oriented parallel to the foliation plane with the majority of deformed grains having axis parallel to the lineation zircon accommodates strain by a network of stepped low angle boundaries formed by switching between tilt dislocations with the slip systems 010 and 1 10 110 and rotation axis 001 twist dislocations with the rotation axis 001 and tilt dislocations with the slip system 001 and rotation axis 010 the slip system 1 10 110 is newly described for zircon most misorientation axes in plastically deformed zircon grains are parallel to the xy plane of the sample and have 001 crystallographic direction such behaviour of strained zircon lattice is caused by elastic anisotropy that has a direct geometric control on the rheology deformation mechanisms and dominant slip systems in zircon young s modulus and p wave velocity have highest values parallel to zircon 001 axis indicating that zircon is elastically strong along this direction poisson ratio and shear modulus demonstrate that zircon is also most resistant to shearing along 001 thus 001 axis is the most common rotation axis in zircon the described zircon behaviour is important to take into account during structural and geochronological investigations of poly metamorphic terrains geometry of dislocations in zircon may help reconstructing the geometry of the host shear zone s large scale stresses in the crust and possibly the timing of deformation if the isotopic systems of deformed zircon were reset

impact triggered nanoscale pb clustering and pb loss domains in archean zircon zircon microcrystals are found in many planetary crustal rocks and nanoscale research on grains from well characterized impact environments on earth provide a baseline for reconstructing extreme shock and thermal histories elsewhere however using zircon to date large impact events can be challenging given that shock related isotopic re setting of u pb ratios when measured at micrometre scale is often incomplete and difficult to interpret as the underlying pb migration mechanisms are unclear to better understand shocked zircon u pb systematics we performed atom probe tomography and electron microscopy with the previous sims analyses of two shock metamorphosed mesoarchean zircon grains from deep 15 km beneath the centre of the 2 020 ga giant vredefort impact structure we find evidence of two types of impact related nanoscale pb mobility in one grain clustering has produced 10 nm diameter bodies of radiogenic pb unsupported by u and co located with al with an average 207pb 206pb ratio of 0 50 n 4 the value extant in the grain at the time of impact conversely nearby nanodomains exhibit randomly distributed radiogenic pb u and al and yield 206pb 238u dates consistent with 100 loss of pre impact radiogenic pb atoms during shock metamorphic processes notably domains with multiple pb clusters occur within micrometres of domains that experienced 100 pb loss precluding a uniform radial pattern of thermally driven pb diffusion at the grain scale these cases of broadly coeval clustering and outward pb mobility during geologically instantaneous shock metamorphism point to unusually rapid multi path diffusion processes within sub micrometre volumes which when averaged yield normally discordant u pb dates the isolation of spatially variable styles of pb retention and loss at nanoscale amidst classical grain scale shock microstructures shows promise for recognizing and resolving bombardment histories in planetary crusts using zircon

u pb ages of shocked zircon grains link distal k pg boundary sites in spain and italy with the chicxulub impact abstract the u pb ages of shocked zircon crystals from the chicxulub impact crater and cretaceous paleogene k pg boundary sites in haiti the usa and canada and the pattern of decreasing particle size with paleodistance from the crater have been used as evidence of a genetic link between chicxulub and the k pg boundary despite this the inference that the k pg boundary layer formed as a direct consequence of the chicxulub impact has been repeatedly questioned here we present u pb id tims ages and textural evidence of shock metamorphosed zircon grains from the k pg boundary at caravaca spain and petriccio italy that establish a causal connection between the impact and formation of the k pg boundary layer the shocked zircon grains give data that produce a characteristic age pattern which indicates a primary source age of 549 5 5 7 ma and a secondary event at the approximate time of impact at 66 ma the intensity of the shock features is proportional to the degree of isotopic resetting and all textural features and ages are analytically identical to those of previously analyzed zircon from chicxulub and k pg boundary sites in north america caravaca and petriccio were 8000 km from chicxulub at the time of impact and are therefore the farthest k pg sites identified that can be linked to chicxulub through the dating of individual shocked zircon grains we conclude that the combined age data and textural observations provide unambiguous evidence that ejecta from the chicxulub impact formed the global k pg boundary layer these data cannot be explained by the alternative scenario that the chicxulub impact occurred 300 ka prior to the k pg boundary

resolution of impact related microstructures in lunar zircon a shock deformation mechanism map abstract the microstructures of lunar zircon grains from breccia samples 72215 73215 73235 and 76295 collected during the apollo 17 mission have been characterized via optical microscopy cathodoluminescence imaging and electron backscatter diffraction mapping these zircon grains preserve deformation microstructures that show a wide range in style and complexity planar deformation features pdfs are documented in lunar zircon for the first time and occur along 001 110 and 112 typically with 0 1 25 m spacing the widest pdfs associated with 112 contain microtwin lamellae with 65 misorientation relationships deformation bands parallel to 100 planes and irregular low angle misorientation axes this geometry is consistent with a dislocation glide system with 010 during dislocation creep nonplanar fractures recrystallized domains with sharp irregular interfaces and localized annealing textures along fractures are also observed no occurrences of reidite were detected shock deformation microstructures in zircon are explained in terms of elastic anisotropy of zircon pdfs form along a limited number of specific hkl planes that are perpendicular to directions of high young s modulus suggesting that pdfs are likely to be planes of longitudinal lattice damage twinned 112 pdfs also contain directions of high shear modulus a conceptual model is proposed for the development of different deformation microstructures during an impact event this shock deformation mechanism map is used to explain the relative timing conditions and complexity relationships between impact related deformation microstructures in zircon

archean age for the granulite facies metamorphism near the center of the vredefort structure south africa granulite facies metamorphic assemblages in rocks exposed near the center of the 2 02 ga vredefort impact structure previously have been interpreted either as early proterozoic genetically related to the 2060 ma bushveld complex or as archean and representative of lower crust that rebounded to upper crustal levels following an impact event zircon and monazite recovered from the granulite facies rocks record high grade metamorphism at 3107 9 ma and a primary age of 3425 ma for detrital zircon a shock deformed but otherwise pristine dolerite dike that intrudes the granulite terrane yields a u pb zircon age of 2560 ma providing a minimum age for the metamorphism these isotopic age data are difficult to reconcile with a regional high grade metamorphic event in the crust beneath vredefort at 2060 ma instead the preimpact high temperature metamorphic history observed in the vredefort lower crustal rocks indicates an enigmatic high temperature event during the stabilization of diamondiferous archean tectosphere

shock metamorphosed zircon in terrestrial impact craters to ascertain the progressive stages of shock metamorphism of zircon samples from three well studied impact craters were analyzed by optical microscopy scanning electron microscopy sem and raman spectroscopy in thin section and grain separates these samples are comprised of well preserved rapidly quenched impactites from the ries crater germany strongly annealed impactites from the popigai crater siberia and altered variably quenched impactites from the chicxulub crater mexico the natural samples were compared with samples of experimentally shock metamorphosed zircon below 20 gpa zircon exhibits no distinct shock features above 20 gpa optically resolvable planar microstructures occur together with the high pressure polymorph reidite which was only retained in the ries samples decomposition of zircon to zro2 only occurs in shock stage iv melt fragments that were rapidly quenched this is not only a result of post shock temperatures in excess of 1700 c but could also be shock pressure induced which is indicated by possible relics of a high pressure polymorph of zro2 however zro2 was found to revert to zircon with a granular texture during devitrification of impact melts other granular textures represent recrystallized amorphous zrsio4 and reidite that reverted to zircon this requires annealing temperatures 1100 c a systematic study of zircons from a continuous impactite sequence of the chicxulub impact structure yields implications for the post shock temperature history of suevite like rocks until cooling below 600 c

experimental shock deformation in zircon a transmission electron microscopic study abstract in recent years apparently shock induced and thus impact characteristic microdeformations in the form of planar microdeformation features and so called strawberry granular texture have been observed in zircons in rocks from confirmed impact structures and from the k t boundary the nature of the planar microdeformations in this mineral is however still unknown and critical information is needed regarding the shock pressure range in which these deformation effects are produced we experimentally shock deformed two series of thin zircon zrsio 4 target plates cut perpendicular to the c axis at shock pressures of 20 40 and 60 gpa the recovered samples were characterized by optical and scanning electron microscopy in addition one sample series was studied by transmission electron microscopy tem microdeformation effects observed at 20 gpa include pervasive micro cleavage and dislocation patterns plastic deformation is indicated by a high density of straight dislocations in glide configuration the dominant glide systems are 010 micro cleavages induced by shear stresses during the compression stage occur mostly in the 100 planes the large density of dislocations at crack tips shows that plastic deformation was initiated by the micro cracking processs at 40 gpa the sample was partly transformed from the zircon z to a scheelite cawo 4 type s structure planar deformation features pdfs containing an amorphous phase of zircon composition are present in the not yet transformed zircon relics the phase with scheelite structure initiated in the 100 planes of zircon consists of thin 0 1 to several m bands that crosscut the zircon matrix the phase transformation is displacive martensitic and can be related by 100 z 112 s and 001 z s the scheelite structure phase is densely twinned with twins in the 112 plane the 60 gpa sample consists completely of the scheelite structure phase crosscutting and displacing relationships between twins and pdfs demonstrate that pdfs are formed in the zircon structure i e before the phase transformation to the scheelite structure occurred most likely at the shock front crystallographic orientations of optically visible planar features in zircon in comparison with orientations of planar defects at the tem scale suggest that the optically visible features are more likely planar microfractures than pdfs

dating the shock wave and thermal imprint of the giant vredefort impact south africa u pb geochronology of single grains of zircon and monazite has been used to date an episode of intense postimpact metamorphism in the core of the deeply eroded vredefort impact structure of south africa results from two basement units exposed in the uplifted central region indicate that the impact and a later pyroxene hornfels metamorphic event were penecontemporaneous at 2020 3 ma discovery of a synimpact to postimpact dike of norite that intruded at 2019 2 ma is the first recognition of mafic igneous activity related to impact the dike is either derived from a sudbury type impact melt layer since eroded or is the product of decompression melting of kaapvaal mantle in response to the ablation of 15 km of crust at the center of the crater the combined heating effects of the shock wave and impact triggered magmas are thought to have created the 300 km 2 thermal imprint of the asteroid collision with kaapvaal craton and account for the nearly coeval timing relationship between core metamorphism and shock revealed by this study

dating terrestrial impact structures u pb depth profiles and u th he ages of zircon we investigate the presence of epitaxial overgrowth rims and reset zircon complete loss of radiogenic lead pb from terrestrial impactites to constrain the occurrence of such phenomenon in impact environments and their possible use in dating impact events we also explore u th he dating of zircon to evaluate this geochronometer in accurately identifying impact ages particularly when no dateable melt sheet exists our results show that u th he ages of zircon from the brecciated and presumably shocked target can accurately date an impact event and provides another tool to determine impact ages when no melt sheet exists an alternative to problematic interpretations of commonly used apparent 40ar 39ar plateau ages no evidence of epitaxial overgrowth rims and or reset zircon was observed suggesting that zircons within shocked impactites have remarkably slow pb diffusion and possibly explaining the relatively few reset grains reported in terrestrial impactites

on the geometric relationship between deformation microstructures in zircon and the kinematic framework of the shear zone abstract we present novel microstructural analyses of zircon from a variety of strained rocks for the first time multiple plastically deformed zircon crystals were analyzed in a kinematic context of the respective host shear zones our aim was to derive how the orientation of zircon grains in a shear zone affects their deformation based on careful in situ observations for sampling we selected zircon bearing rocks that were deformed by simple shear samples covered a range of p t conditions and lithologies including various meta igneous and meta sedimentary gneisses microstructural analyses of zircon crystals in situ with scanning electron backscatter diffraction mapping show strong geometrical relationships between orientations of i the long axes of plastically deformed zircon crystals ii the crystallographic orientation of misorientation axes in plastically deformed zircon crystals and iii the foliation and lineation directions of the respective samples we assume that zircon crystals did not experience post deformation rigid body rotation and thus the true geometric link can be observed the relationships are the following a plastically deformed zircon crystals usually have long axes parallel to the mylonitic foliation plane b crystals with axes oriented at an angle 15 to the foliation plane are undeformed or fractured zircon crystals that have axes aligned parallel or normal to the stretching lineation within the foliation plane develop misorientation and rotation axes parallel to the 001 crystallographic direction zircon grains with the axis aligned at 30 60 to the lineation within the foliation plane often develop either two low miller indices misorientation axes or one high miller indices misorientation axis host phases have a significant influence on deformation mechanisms in a relatively soft matrix zircon is more likely to develop low miller indices misorientation axes than in a relatively strong matrix these relationships are independent of zircon s grain size and shape and reflect the strong geometric control of the macroscopic kinematic rotation axis on the crystal plastic deformation behavior of zircon and on the geometry of its slip systems we describe previously unknown rheological and crystallographic properties of zircon which suggest a tool for deriving an orientation of the plastically deformed zircon crystals with respect to the associated foliation and stretching lineation additionally relationships between zircon deformation microstructures and the macroscopic kinematic framework have implications for zircon geochronology if deformation events result in zircon distortion and as a consequence partial or complete resetting of the zircon isotopic system the age of deformation can be accurately dated