recrystallization and chemical changes in apatite in response to hypervelocity impact despite the wide utility of apatite ca5 po4 3 f cl oh in the geosciences including tracing volatile abundances on the moon and mars little is known about how the mineral responds to the extrem

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

chronology and tectonic significance of cenozoic faults in the liupanshan arcuate tectonic belt at the northeastern margin of the qinghai tibet plateau abstract the liupanshan arcuate tectonic belt latb is located at the northeastern margin of the qinghai tibet plateau major strike slip and thrust faults in the liupanshan area are prominent cenozoic structures which are critical in understanding and reconstructing the tectonic deformation history this paper not only provides detailed investigations on geometric and kinematic characteristics of these faults in the latb but also dates the faults movements by electron spin resonance esr the latb underwent a succession of compression extension and again compression tectonic deformation processes since the cenozoic the liupanshan curved faults first experienced sinistral strike slip shear during 57 61 ma the liupanshan curved faults responded to the deformation caused by the eastward escape of the qinghai tibet plateau and acted as the northeastern boundary of the deformation timing for the formation of the liupanshan curved faults shows that the collision of the indian and eurasian plates must have occurred earlier than these faults activity because the latter is reflected the far field effect of the collision

performance of low fold scalar migration for downhole seismic imaging of massive sulfide ore deposits at norman west sudbury canada abstractthe downhole seismic imaging consortium conducted two consecutive vertical seismic profiling surveys in the norman west mining camp sudbury canada in 1998 and 1999 these were aimed toward imaging a massive sulfide ore deposit situated within the footwall of the sudbury igneous complex sic three component seismic data were acquired in four boreholes with variable signal to noise ratio and poor polarization quality consequently the images suffered from strong azimuthal ambiguity a strike filter passing only reflections originating from within the sic was applied during migration to enhance interpretability of the images obtained migrated images showed structures correlating with the known position of an ore deposit located 1800 m away from one borehole n40 diffraction coherency migration enhanced the image of the deposit and suggested strong seismic scattering from within the footwall of the sic

new evidence for persistent impact generated hydrothermal activity in the miocene ries impact structure germany the extent of impact generated hydrothermal activity in the 24 km sized ries impact structure has been controversially discussed to date mineralogical and isotopic investigations point to a restriction of hydrothermal activity to the impact melt bearing breccias specifically the crater fill suevite here we present new petrographic geochemical and isotopic data of postimpact carbonate deposits which indicate a hydrothermal activity more extended than previously assumed specifically carbonates of the erbisberg a spring mound located upon the inner crystalline ring of the crater show travertine facies types not seen in any of the previously investigated sublacustrine soda lake spring mounds of the ries basin in particular the streamer carbonates which result from the encrustation of microbial filaments in subaerial spring effluents between 60 and 70 c are characteristic of a hydrothermal origin while much of the primary geochemical and isotopic signatures in the mound carbonates have been obliterated by diagenesis a postimpact calcite vein from brecciated gneiss of the subsurface crater floor revealed a flat rare earth element pattern with a clear positive eu anomaly indicating a hydrothermal fluid convection in the crater basement finally the strontium isotope stratigraphic correlation of the travertine mound with the crater basin succession suggests a hydrothermal activity for about 250 000 yr after the impact which would be much longer than previously assumed

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 lappajarvi impact structure finland age duration of crater cooling and implications for early life abstract an in depth approach of 40 ar 39 ar dating of the 23 km lappajarvi impact structure finland was performed using carefully selected single grain aliquots of optically fresh clast poor impact melt rock and recrystallized k feldspar melt particles separated from impact metamorphosed granite pegmatite step heating analysis yielded a set of 13 statistically robust plateau ages obtained on six melt rock and seven k feldspar samples the melt rocks yielded fully concordant ages with a weighted mean of 76 37 0 46 ma dating of the k feldspar melt particles resulted in a series of younger plateau ages ranging from 75 11 0 36 to 76 11 0 35 ma the melt rock results combined with the oldest syn melt rock k feldspar age and including all sources of uncertainties yielded a weighted mean age of 76 20 0 29 ma 2 mswd 1 02 p 0 41 which is interpreted to represent the best estimate age of the lappajarvi impact the age spread of at least 1 1 0 5 ma between the crystallization of the impact melt rocks and the k feldspar melt particles warns against the use of single 40 ar 39 ar analysis to derive a meaningful impact age in turn the age offset monitors both rapid cooling of the lappajarvi melt sheet versus slow cooling and impact induced hydrothermal circulation within the crater basement most likely driven by long lasting heat flow from the hot central uplift of the impact structure this interpretation is in line with the grain size dependent argon diffusion parameters and apparent closure temperatures of 230 410 c determined for various domain sizes of k feldspars observed in this study our study shows that even in comparatively small impact craters post impact hydrothermal activity can be estimated to last between 600 ka and 1 6 ma and is technically resolved by means of the 40 ar 39 ar dating technique therefore the longevity of hydrothermal systems in medium sized impact craters might be an order of magnitude longer than previously estimated prolonged post impact heat flow in medium sized craters in analogy to lappajarvi might have played an important role in the emergence of life on early earth and possibly mars

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

a rhaetian 40 ar 39 ar age for the rochechouart impact structure france and implications for the latest triassic sedimentary record ar 39 ar dating of potassium feldspar primary spherulitic blocky and secondary idiomorphic k feldspar separated from impact metamorphosed gneiss found near videix in the western central part of the rochechouart impact structure nw massif central france yielded a rhaetian combined age of 201 2 ma 2r indistinguishable within uncertainty from the age of the triassic jurassic boundary ballen quartz intergrown with the primary k feldspar indicates post shock temperatures exceeding approximately 1000 c that affected the precursor gneiss geochemically both feldspar types represent essentially pure potassium end members apart from the approximately 15 km diameter impact deposit area the youngest crystallization age known for basement rocks in this part of the massif central is approximately 300 ma no endogenic magmatic thermal events are known to have occurred later in this region the k feldspar recrystallized from local feldspar melts and superimposed post shock hydrothermal crystallization probably within some thousands of years after the impact it is therefore suggested that the 40 ar 39 ar age for the videix gneiss as a potassic impact metasomatite dates the rochechouart impact in consistence with evidence for k metasomatism in the rochechouart impactites the new age value is distinctly younger than the previously obtained karnian norian age for rochechouart and thus contradicts the late triassic multiple impact theory postulated some years ago in agreement with the paleogeographic conditions in the western tethys domain around the triassic jurassic boundary the near coastal to shallow marine rochechouart impact is compatible with the formation of seismites and tsunami deposits in the latest triassic of the british isles and possible related deposits in other parts of europe

l chondrite asteroid breakup tied to ordovician meteorite shower by multiple isochron 40ar 39ar dating radiochronometry of l chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 myr ago fossil meteorites from sweden indicate a highly increased influx of extraterrestrial matter in the middle ordovician 480 myr ago an association with the l chondrite parent body event was suggested but a definite link is precluded by the lack of more precise radiometric ages suggested ages range between 450 30 myr and 520 60 myr and can neither convincingly prove a single breakup event nor constrain the delivery times of meteorites from the asteroid belt to earth here we report the discovery of multiple 40ar 39ar isochrons in shocked l chondrites particularly the regolith breccia ghubara that allow the separation of radiogenic argon from multiple excess argon components this approach applied to several l chondrites yields an improved age value that indicates a single asteroid breakup event at 470 6 myr fully consistent with a refined age estimate of the middle ordovician meteorite shower at 467 3 1 6 myr according to a geologic time scale 2004 our results link these fossil meteorites directly to the l chondrite asteroid destruction rapidly transferred from the asteroid belt the increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate which implies severe environmental stress

extraterrestrial chromite in the resurge deposits of the early late ordovician lockne crater central sweden the distribution of extraterrestrial chromite grains 63 mu m has been studied in the resurge deposit of the early late ordovician 458 ma lockne impact structure in central sweden a 1 kg sample of resurge sediment from the rim of the crater is extremely rich in extraterrestrial chromite in total the sample contains 125 chromium spinel grains kg 1 most grains being chromite due to post depositional alterations the origin of several of the grains is dubious although it is clear that the major part of the chromites is of extraterrestrial origin 75 grains kg 1 most likely derived from the impactor the alterations are primarily due to the hydrothermal system induced by the impactor with zinc enrichment in the chromites as the most common characteristic the element chemistry of the least altered chromite grains indicates that the impactor was an ordinary l chondrite this concurs with a suggested increase in the flux of l chondritic asteroids to earth following the disruption of the l chondrite parent body at ca 470 ma more than 170 impact craters are known from around the earth but only few of the impactors have been identified mainly due to the low survival rate of projectile material further studies of impact craters will reveal if survival of extraterrestrial chromite is unique for the lockne structure c 2006 elsevier b v all rights reserved less

an early ordovician 40ar 39ar age for the 50 km carswell impact structure canada the formation age of the large 50 km carswell impact structure canada has been a matter of debate ever since its discovery five decades ago with proposed ages ranging from mesoproterozoic to early cretaceous here we present new 40ar 39ar data for aliquots of euhedral adularia separated from vesicles in an impact melt rock from the central uplift of the structure the analyses of the adularia yielded a statistically robust early ordovician crystallization age of 481 5 0 8 ma 2 mean square of weighted deviates 1 06 p 0 30 the most plausible explanation for the formation of vesicle filling adularia is through low temperature mineral precipitation during residual hydrothermal circulation that followed the impact as no other known major intrusive extrusive or thermal events have occurred in the carswell region in the phanerozoic the new age of the carswell impact structure overlaps within uncertainty with the most precise ar ar ages proposed for the l chondrite parent body breakup event but not with the age of the stratigraphic sequence from which the meteorites and micrometeorites from this event were recovered thus either the carswell impact represents a separate unrelated impact event or the dynamic evolution of the l chondrite parent body breakup is more complicated than presently understood and carswell represents one of the earliest and largest known impacts of this event on earth less

multiple fluvial reworking of impact ejecta a case study from the ries crater southern germany impact ejecta eroded and transported by gravity flows tsunamis or glaciers have been reported from a number of impact structures on earth impact ejecta reworked by fluvial processes however are sparsely mentioned in the literature this suggests that shocked mineral grains and impact glasses are unstable when eroded and transported in a fluvial system as a case study we here present a report of impact ejecta affected by multiple fluvial reworking including rounded quartz grains with planar deformation features and diaplectic quartz and feldspar glass in pebbles of fluvial sandstones from the monheimer hohensande 10 km east of the ries crater in southern germany

structural control of uranium mineralization at the dominique peter deposit saskatchewan canada the dominique peter deposit is part of a cluster of unconformity related uranium deposits that have been exploited since 1980 at the cluff lake mine in northern saskatchewan it is located on the south side of the 18 km basement core of the 39 km diameter circular carswell structure the metamorphic basement cylinder has risen at least 1 200 m through the mesoproterozoic sandstones underground structural study at dominique peter resulted in a revision of the understanding of the succession of tectonic events controlling mineralization emplacement and deformation the main premineralization ductile deformation d 1 and d 2 produced a flat lying gneissosity and subsequent development of foliation conformable mylonites and exhibits some contrasts with the eastern athabasca trans hudsonian tectonics east west to northwest southeast general extension ca 1 3 ga resulted in the formation of a set of 015 degrees to 050 degrees listric normal faults the latter controlled the opening of quartz then uraninite veins and induced a general tilting of fault blocks to the east mechanisms of mineralization emplacement are described in detail foliation reactivation in mylonite and aluminous gneiss is favorable to vein opening the postmineralization formation of the carswell circular structure ca 480 ma displays a centrifugal deformation with a symmetry axis located in the center of the basement dome a suite of pseudotachylyte veins breccias and shatter cones accompanies this deformation overall geometry of the carswell dome and surrounding rings as well as the nature of associated breccias presents strong similarities with those of other impact related structures

sediment dispersed extraterrestrial chromite traces a major asteroid disruption event abundant extraterrestrial chromite grains from decomposed meteorites occur in middle ordovician 480 million years ago marine limestone over an area of 250 000 square kilometers in southern sweden the chromite anomaly gives support for an increase of two orders of magnitude in the influx of meteorites to earth during the mid ordovician as previously indicated by fossil meteorites extraterrestrial chromite grains in mid ordovician limestone can be used to constrain in detail the temporal variations in flux of extraterrestrial matter after one of the largest asteroid disruption events in the asteroid belt in late solar system history

noble gases in fossil micrometeorites and meteorites from 470 myr old sediments from southern sweden and new evidence for the l chondrite parent body breakup event we present noble gas analyses of sediment dispersed extraterrestrial chromite grains recovered from similar to 470 myr old sediments from two quarries hallekis and thorsberg and of relict chromites in a coeval fossil meteorite from the gullhogen quarry all located ill southern sweden both the sediment dispersed grains and the meteorite gullhogen 001 were generated in the l chondrite parent body breakup about 470 myr ago which was also the event responsible for the abundant fossil meteorites previously found in the thorsberg quarry trapped solar noble gases in the sediment dispersed chromite grains have partly been retained during similar to 470 myr of terrestrial residence and despite harsh chemical treatment in the laboratory this shows that chromite is highly retentive for solar noble gases the solar noble gases imply that a sizeable fraction of the sediment dispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites the grains in the oldest sediment beds were rapidly delivered to earth likely by direct injection into all orbital resonance in the inner asteroid belt whereas grains in younger sediments arrived by orbital decay due to poynting robertson p r drag the fossil meteorite gullhogen 001 has a low cosmic ray exposure age of similar to 0 9 myr based on new he and ne production rates in chromite determined experimentally this age is comparable to the ages of the fossil meteorites from thorsberg providing additional evidence for very rapid transfer times of material after the l chondrite parent body breakup

a rain of ordinary chondritic meteorites in the early ordovician forty fossil meteorites with a total original mass of v7 7 kg have been recovered in the first systematic search for fossil meteorites pursued in an active quarry in lower ordovician 480 ma marine limestone in southern sweden the meteorites represent at least 12 different falls over a seafloor area of v6000 m 2 during 91 75 myr making the quarry one of the most meteorite dense areas known in the world geochemical analyses of relict chromite grains indicate that all or most of the meteorites are ordinary chondrites and probably l chondrites mechanisms for meteorite delivery from the asteroid belt to earth were the same 480 ma as today however the flux was one to two orders of magnitude higher most likely reflecting the disruption of the l chondrite parent body at about that time this is a major event in late solar system history which may also have led to an enhanced flux of asteroids to earth during v30 myr fl 2001 elsevier science b v all rights reserved

40ar 39ar age of northwest africa 091 more evidence for a link between l chondrites and fossil meteorites abstract most 40ar 39ar ages of l chondrites record an event at approximately 500 ma indicating a large collisional impact at that time however there is a spread in ages from 400 to 600 ma in these meteorites that is greater than the analytical uncertainty identification of and correction for trapped ar in a few l chondrites has given an age of 470 6 ma this age coincides with ordivician fossil meteorites that fell to earth at 467 2 ma as these fossil meteorites were originally l chondrites the apparent conclusion is that a large impact sent a flood of l chondrite material to earth while material that remained on the l chondrite parent body was strongly heated and reset we have reduced 40ar 39ar data for northwest africa 091 using various techniques that appear in the literature including identification and subtraction of trapped ar these techniques give a range of ages from 455 to 520 ma and show the importance of making accurate corrections by using the most straightforward technique to identify and remove a trapped ar component which is neither terrestrial nor primordial an 40ar 39ar age of 475 6 ma is found for northwest africa 091 showing a temporal link to fossil meteorites in addition high temperature releases of northwest africa 091 contain evidence for a second trapped component and subtraction of this component indicates a possible second collisional impact at approximately 800 ma this earlier age coincides with 40ar 39ar ages of some h and l chondrites and lunar samples

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

impact origin for the hummeln structure sweden and its link to the ordovician disruption of the l chondrite parent body several studies of meteorites show that a large disruption of an asteroid occurred ca 470 ma in our solar system s asteroid belt as a consequence a large number of meteorite impacts occurred on earth during the following few million years the finding and characterization for the first time of planar deformation features in quartz grains from rocks collected at the middle ordovician hummeln structure sweden prove the hypervelocity impact origin of the structure the unambiguous shock features allow us to close an similar to 200 yr old discussion about its origin and further the hypothesis of enhanced asteroid bombardment during the middle ordovician adding an impact crater to the increasing number confirmed and properly dated from this period despite its relatively small size similar to 1 2 km in diameter similar to the young meteor crater arizona usa and its old age the hummeln structure is remarkably well preserved contradicting the general assumption that small craters are not preserved on earth for more than a few tens of thousands to a couple of million years less

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

determining the impactor of the ordovician lockne crater oxygen and neon isotopes in chromite versus sedimentary pge signatures abundant chromite grains with l chondritic composition in the resurge deposits of the lockne impact crater 458 myr old dia similar to 10 km in sweden have been inferred to represent relict fragments of an impactor from the break up of the l chondrite parent body at 470 ma this view has been challenged based on it cr and platinum group element pge patterns of the same resurge deposits and a reinterpretation of the origin of the chromite grains an impactor of the non magmatic iron meteorite type was proposed instead here we show that single grain oxygen and noble gas isotope analyses of the chromite grains from the resurge deposits further support an origin from an l chondritic asteroid we also present pge analyses and ir cr ratios for fossil l chondritic meteorites found in mid ordovician marine limestone in sweden the l chondritic origin has been confirmed by several independent methods including major element and oxygen isotopic analyses of chromite although the meteorites show the same order of magnitude pge and cr concentrations as recent l chondrites the elements have been redistributed to the extent that it is problematic to establish the original meteorite type from these proxies different pge data processing approaches can lead to highly variable results as also shown here for the lockne resurge deposits we conclude that the lockne crater was formed by an l chondritic impactor and that considerable care must be taken when inferring projectile type from pges in sedimentary ejecta deposits c 2011 elsevier b v all rights reserved less

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

shocked monazite chronometry integrating microstructural and in situ isotopic age data for determining precise impact ages monazite is a robust geochronometer and occurs in a wide range of rock types monazite also records shock deformation from meteorite impact but the effects of impact related microstructures on the u th pb systematics remain poorly constrained we have therefore analyzed shock deformed monazite grains from the central uplift of the vredefort impact structure south africa and impact melt from the araguainha impact structure brazil using electron backscatter diffraction electron microprobe elemental mapping and secondary ion mass spectrometry sims crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal plastic deformation features including shock twins planar deformation bands and neoblasts shock twins were documented in up to four different orientations within individual monazite grains occurring as compound and or type one twins in 001 100 left 10 bar 1 right 110 left 212 right and type two irrational twin planes with rational shear directions in 0 bar 1 bar 1 and bar 1 bar 1 0 sims u th pb analyses of the plastically deformed parent domains reveal discordant age arrays where discordance scales with increasing plastic strain the correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event neoblasts in granular monazite domains are strain free having grown during the impact events via consumption of strained parent grains neoblastic monazite from the inlandsee leucogranofels at vredefort records a 207pb 206pb age of 2010 15 ma 2 n 9 consistent with previous impact age estimates of 2020 ma neoblastic monazite from araguainha impact melt yield a concordia age of 259 5 ma 2 n 7 which is consistent with previous impact age estimates of 255 3 ma our results demonstrate that targeting discrete microstructural domains in shocked monazite as identified through orientation mapping for in situ u th pb analysis can date impact related deformation monazite is therefore one of the few high temperature geochronometers that can be used for accurate and precise dating of meteorite impacts