the rock elm meteorite impact structure wisconsin geology and shock metamorphic effects in quartz the rock elm structure in southwest wisconsin is an anomalous circular area of highly deformed rocks 6 5 km in diameter located in a region of virtually horizontal undeformed sedimentary rocks shock produced planar microstructures pms have been identified in quartz grains in several lithologies associated with the structure sandstones quartzite pebbles and breccia two distinct types of pms are pres ent p1 features which appear identical to planar fractures pfs or cleavage and p2 features which are interpreted as possible incipient planar deformation features pdfs the latter are uniquely produced by the shock waves associated with meteorite impact events both types of pms are oriented parallel to specific crystallographic planes in the quartz most commonly to c 0001 11 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 2 end document 2 and r z 10 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 1 end document 1 the association of unusual structurally deformed strata with distinct shock produced microdeformation features in their quartz bearing rocks establishes rock elm as a meteorite impact structure and supports the view that the presence of multiple parallel cleavages in quartz may be used independently as a criterion for meteorite impact preliminary paleontological studies indicate a minimum age of middle ordovician for the rock elm structure a similar age estimate 450 400 ma is obtained independently by combining the results of studies of the general morphology of complex impact structures with estimated rates of sedimentation for the region such methods may be applicable to dating other old and deeply eroded impact structures formed in sedimentary target rocks

the problem of inherited 40ar in dating impact glass by the 40ar 39ar method evidence from the tswaing impact crater south africa abstract the tswaing meteorite impact crater is a 1 13 km diameter structure located in the 2 05 ga nebo granite of the bushveld complex the impact age had previously been determined by fission track dating to 220 104 ka 40ar 39ar step heating and total fusion experiments performed on single and multi grain impact glass aliquots gave apparent ages ranging from 1 0 0 3 ma to 204 6 ma these ages indicate that the radiogenic ar derived from the target rocks has not been completely degassed as a result of the impact process despite fusion of the target material results of step heating experiments imply that the 40 ar inherited trapped within the glass is located in two distinct reservoirs thought to be the glass matrix and fluid vapor inclusions or un melted residual clasts calculations assuming an age of 0 2 0 1 ma for tswaing fission track data reveal that the amount of inherited 40ar 40 ar inherited relative to the pre impact concentration varies from 0 015 to 4 15 the spread defined by 40 ar inherited likely reflects the various quench rates experienced by the glass most certainly due to the pre impact position of the sample relative to the center of the crater we compare the influence of 40 ar inherited on the apparent 40ar 39ar age determination of five impact structures our calculations show that the main characteristic controlling the age offset for a given proportion of 40 ar inherited is the age difference between the impact and the target rocks i e the 40ar concentration in the target rock the buffer effect for a given crater structure can be predicted knowing the age of the basement and having a rough estimation of the age of the crater structure itself the occurrence of 40 ar inherited is likely influenced by 1 the degree of polymerization i e silicate structure complexity of the target rock and presumably related to the diffusivity of ar in the melt and glass 2 the ar partial pressure at the grain boundary 3 the quantity of energy involved in the impact and 4 the porosity of the target rocks for glass that inevitably suffers inherited and or excess 40ar the use of the inverse isochron technique can be appropriate but should be applied with careful statistical treatment

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

40ar 39ar age of the lake saint martin impact structure canada unchaining the late triassic terrestrial impact craters abstract new 40 ar 39 ar dating of impact melted k feldspars and impact melt rock from the 40 km lake saint martin impact structure in manitoba canada yielded three plateau ages and one mini plateau age in agreement with inverse isochron ages for the k feldspar melt aliquots and a minimum age for a whole rock impact melt sample a combination of two plateau ages and one isochron age with a weighted mean of 227 8 0 9 ma 1 1 ma including all sources of uncertainty 2 mswd 0 52 p 0 59 is considered to represent the best estimate age for the impact the concordant 40 ar 39 ar ages for the melted k feldspars derived from impact melt rocks in the eastern crater moat domain and the partially melted proterozoic central uplift granite suggest that the new dates accurately reflect the lake saint martin impact event in the carnian stage of the late triassic with a relative error of 0 4 on the 40 ar 39 ar age the lake saint martin impact structure counts among the most precisely dated impact structures on earth the new isotopic age for lake saint martin significantly improves upon earlier rb sr and u th he results for this impact structure and contradicts the hypothesis that planet earth experienced the formation of a giant impact crater chain during a major late triassic multiple impact event

a middle late triassic 40ar 39ar age for the paasselka impact structure se finland abstract 40ar 39ar dating of recrystallized feldspar glass particles separated from clast rich impact melt rocks from the approximately 10 km paasselka impact structure se finland yielded a middle to late triassic ladinian karnian pseudo plateau age of 228 7 3 0 3 4 ma 2 this new age makes paasselka the first known triassic impact structure dated by isotopic methods on the baltic shield the new paasselka impact age is within uncertainty coeval with isotopic ages recently obtained for the lake saint martin impact structure in canada indicating a new middle to late triassic impact crater population on earth the comparatively small crater size however suggests no relationship between the paasselka impact and a postulated extinction event at the middle late triassic boundary

establishing a 14 6 0 2 ma age for the nordlinger ries impact germany a prime example for concordant isotopic ages from various dating materials abstract 40ar 39ar dating of recrystallized k feldspar melt particles separated from partially molten biotite granite in impact melt rocks from the approximately 24 km nordlinger ries crater southern germany yielded a plateau age of 14 37 0 30 0 32 ma 2 this new age for the nordlinger ries is the first age obtained from 1 monomineralic melt 2 separated from an impact metamorphosed target rock clast within 3 ries melt rocks and therewith extends the extensive isotopic age data set for this long time studied impact structure the new age goes very well with the 40ar 39ar step heating and laser probe dating results achieved from mixed glass samples suevite glass and tektites and is slightly younger than the previously obtained fission track and k ar and ages of about 15 ma as well as the k ar and 40ar 39ar age data obtained in the early 1990s taking all the 40ar 39ar age data obtained from ries impact melt lithologies into account data from the literature and this study we suggest an age of 14 59 0 20 ma 2 as best value for the ries impact event

a probabilistic procedure for scenario based seismic hazard maps of greater tehran abstract the seismic hazard assessment of tehran the capital of iran with a population of over 10 million is necessary in the decision making process for risk reduction the main objective of this paper is to present a procedure for obtaining maximum hazard maps in seismically active regions such as tehran with a history of destructive earthquakes but where the number of the instrumentally recorded large magnitude events is small the deterministic and probabilistic seismic hazard methods are two quantitative approaches with some practical limitations to overcome their limitations the bayesian statistics are used to estimate maximum earthquake magnitude by combining prior seismological information with the available limited data the bayesian approach is used to estimate maximum earthquakes corresponding to different individual ground motion scenarios at a specific hazard level the maximum hazard maps are obtained by merging different individual worst case ground motion scenarios at a specific hazard level the hazard analyses under different scenarios are conducted by the finite fault procedure which uses geological and seismological parameters for each seismic source the obtained results are in agreement with the observed intensities of historical earthquakes moreover the conventional approach of probabilistic seismic hazard analysis is presented to show the difference between the two procedures the findings provide information about the seismic design or retrofit of structures and infrastructures

sedimentological analysis of resurge deposits at the lockne and tvaren craters clues to flow dynamics the lockne and tvren craters formed about 455 million years ago in an epicontinental sea where seawater and mainly limestones covered a crystalline basement the target water depth for tvaren apparent basement crater diameter d 2 km was probably not over 150 m and for lockne d 7 5 km recent best fit numerical simulations suggest the target water depth of 500 700 m lockne has crystalline ejecta that partly cover an outer crater 14 km diameter apparent in the target sediments tvren is eroded with only the crater infill preserved we have line logged cores through the resurge deposits within the craters in order to analyze the resurge flow the focus was clast lithology frequencies and size sorting we divide the resurge into resurge proper with water and debris shooting into the crater and ultimately rising into a central water plume anti resurge with flow outward from the collapsing plume and oscillating resurge not covered by the line logging due to methodological reasons with decreasing flow in diverse directions at lockne the deposit of the resurge proper is coarse and moderately sorted whereas the anti resurge deposit is fining upwards and better sorted the tvren crater has a smoothly fining up section deposited by the resurge proper and may lack anti resurge deposits at lockne the content of crystalline relative to limestone clasts generally decreases upwards which is the opposite of tvren this may be a consequence of factors such as crater size i e complex versus simple and the relative target water depth the mean grain size i e the mean phi value per meter and standard deviation i e size sorting for both craters can be expressed by the equation 0 60 1 25

asteroid breakup linked to the great ordovician biodiversification event the great ordovician biodiversification event coincides 470 million years ago with the break up of a large asteroid and the resultant frequent bombardment of earth with asteroid fragments

petrography geochemistry and argon 40 argon 39 ages of impact melt rocks and breccias from the ames impact structure oklahoma the nicor chestnut 18 4 drill core the 15 km diameter ames structure in northwestern oklahoma is located 2 75 km below surface in cambro ordovician arbuckle dolomite which is overlain by middle ordovician oil creek formation shale the feature is marked by two concentric ring structures with the inner ring of about 5 km diameter probably representing the collapsed remnant of a structural uplift composed of brecciated precambrian granite and arbuckle dolomite wells from both the crater rim and the central uplift are oil and gas producing making ames one of the economically important impact structures petrographic geochemical and age data were obtained on samples from the nicer chestnut 18 4 drill core off the northwest flank of the central uplift these samples represent the largest and best examples of impact melt breccia obtained so far from the ames structure they contain carbonate rocks which are derived from the target sequence the chemical composition of the impact melt breccias is similar to that of target granite with variable carbonate admixture some impact melt rocks are enriched in siderophile elements indicating the possible presence of a meteoritic component based on stratigraphic arguments the age of the crater was estimated at 470 ma previous ar 40 ar 39 dating attempts of impact melt breccias from the dorothy 1 19 core yielded plateau ages of about 285 ma which is in conflict with the stratigraphic age the new ar 40 ar 39 age data obtained on the melt breccias from the nicer chestnut core by ultraviolet uv laser spot analysis resulted in a range of ages with maxima around 300 ma these data could reflect processes related either the regional nemaha uplift or resetting due to hot brines active on a midcontinent wide scale perhaps related to the alleghenian and ouachita orogenies the age data indicate an extended burial phase associated with thermal overprint during late pennsylvanian permian

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

the geology of the malingen structure a probable doublet to the lockne marine target impact crater central sweden the malingen structure is an approximately 700 m wide rimmed sediment filled circular depression in precambrian crystalline basement approximately 16 2 km from the concentric marine target lockne crater inner basement crater diameter approximately 7 5 km total diameter in sedimentary strata approximately 13 5 km we present here results from geologic mapping a 148 8 m deep core drilling from the center of the structure detailed biostratigraphic dating of the structure s formation and its age correlation with lockne chemostratigraphy of the sedimentary infill and indication for shock metamorphism in quartz from breccias below the crater infill the drill core reveals from bottom to the top approximately 33 m of basement rocks with increased fracturing upward approximately 10 m of polymict crystalline breccia with shock features approximately 97 m of slumped cambrian mudstone approximately 4 7 m of a normally graded polymict sedimentary breccia that in its uppermost part grades into sandstone and siltstone cf resurge deposits and approximately 1 6 m of secular sediments the combined data set shows that the malingen structure formed in conjunction with the lockne crater in the same marine setting the shape and depth of the basement crater and the cored sequence of crystalline breccias with shocked quartz slumped sediments and resurge deposits support an impact origin the stratigraphic and geographic relationship with lockne suggests the lockne and malingen craters to be the first described doublet impact structure by a binary asteroid into a marine target setting

the strangways impact structure northern territory australia geological setting and laser probe 40ar 39ar geochronology abstract the strangways impact structure occurs within the mcarthur sedimentary basin northern territory australia the target rocks are primarily quartzites and subordinate shale siltstone sequences of the mesoproterozoic roper group these are primarily underlain by granitoid gneiss which is exposed in a 9 11 km diameter central uplift exposure is relatively limited and the upper parts of the impact structure have been removed by erosion based on the extent of the disturbance of the regional geology the diameter of strangways is estimated to be 24 26 km it is thus a complex crater and probably a central peak basin stratigraphic constraints place the time of impact as post roper group 145 ma in order to better define the time of impact infrared laser spot fusion 40 ar 39 ar data have been obtained from two impact melt lithologies and a highly shocked breccia clast these yield a combined age of 646 42 ma biotite dated from the granitoid gneiss central uplift yields 1686 1237 ma with the younger age reflecting the effects of shock induced resetting and or hydrothermal alteration due to hot melt sheet water interaction following impact the new age of 646 42 ma for strangways places it firmly in the neoproterozoic rather than being ordovician or younger as previously proposed

40ar 39ar laser dating of tektites from the cheb basin czech republic evidence for coevality with moldavites and influence of the dating standard on the age of the ries impact abstract moldavites central european tektites are genetically related to the impact event that produced the 24 km diameter ries crater in germany representing one of the youngest large impact structures on earth although several geochronological studies have been completed there is still no agreement among 40ar 39ar ages on both moldavites and glasses from ries suevites even recently published data yielded within sample mean ages with a nominal spread of more than 0 6 ma 14 24 14 88 ma this age spread which significantly exceeds current internal errors must be in part ascribed to geological and or analytical causes this study reports the results of a detailed geochronological investigation of moldavites from the cheb area czech republic which have never been dated before and for comparison of two samples from type localities one in southern bohemia and the other in western moravia we used 40ar 39ar laser step heating and total fusion techniques in conjunction with microscale petrographic and chemical characterization in addition with the purpose of ascertaining the influence of the dating standards on the age of the ries impact and making data from this study and literature consistent with the now widely used fish canyon sanidine fcs standard we performed a direct calibration of multi grain splits of the fish canyon biotite fct 3 with fcs the intercalibration factors r fcs fct 3 determined for eight stack positions in one of the three performed irradiations were indistinguishable within errors and gave an arithmetic mean and a standard deviation of 1 0086 0 0031 2 in agreement with previous works suggesting that biotite from the fish canyon tuff is somewhat older 0 8 than the coexisting sanidine laser total fusion analysis of milligram to sub milligram splits of five tektite samples from the cheb area yielded mostly concordant intrasample 40ar 39ar ages and within sample weighted mean ages of 14 66 0 08 14 75 0 12 ma 2 internal errors ages relative to fcs that overlap within errors these ages match those obtained for samples from western moravia 14 66 0 08 ma and southern bohemia 14 68 0 11 ma supporting the genetic link between cheb basin tektites and moldavites and consequently between cheb basin tektites and the ries impact in contrast to samples from the cheb area and moravia 40ar 39ar ages from total fusion experiments on the bohemian specimen ranged widely from 14 6 to 17 0 ma older apparent ages however were systematically obtained from fragments characterized by visible surface alteration laser step heating experiments although displaying slightly disturbed age profiles were in line with total fusion analyses and yielded well defined plateau ages of 14 64 0 11 14 71 0 11 ma 2 internal errors ages relative to fcs a thorough comparison of our and previous 40ar 39ar ages on both moldavites and ries suevite glasses recalculated relative to the 40ar 40k ratio recently determined for fcs using intercalibration factors available in or derivable from the literature reveals some inconsistencies which may be ascribed to either geological or analytical causes based on our data decay constants in current use in geochronology and ages calculated relative to fcs we infer that the age of moldavites is 14 68 0 11 ma 2 neglecting uncertainties in the 40k decay constants

40ar 39ar dating of alunite from the pueblo viejo gold silver district dominican republic new 40 ar 39 ar ages for alunite from the moore and monte negro deposits in the pueblo viejo district as well as from a newly discovered alunite bearing zone on loma la cuaba west of the known deposits are reported here the ages range from about 80 to 40 ma with closely adjacent samples exhibiting very different ages interpretation of these results in the context of estimated closure temperatures for alunite and the geologic and tectonic evolution of hispaniola does not lead to a simple conclusion about the age of mineralization the simplest interpretation that mineralization was caused by a buried late cretaceous 80 ma intrusion is complicated by lack of intrusions of this age in the area and absence of alteration in overlying limestone the alternative interpretation that mineralization was formed during early cretaceous 110 ma magmatism and that the 40 ar 39 ar ages were completely reset by late cretaceous thrusting is complicated by a lack of information on the timing and thermal effects of thrusting in central hispaniola alunite studies have yielded similar unclear results in other pre cenozoic ore systems notably those of the lachlan fold belt in australia

shocked quartz grains from the malingen structure sweden evidence for a twin crater of the lockne impact structure the malingen structure in sweden has for a long time been suspected to be the result of an impact however no hard evidence i e shock metamorphic features or traces of the impactor has so far been presented here we show that quartz grains displaying planar deformation features pdfs oriented along crystallographic planes typical for shock metamorphism are present in drill core samples from the structure the shocked material was recovered from basement breccias below the sediment infill and the distribution of the orientation of the shock produced pdfs indicates that the studied material experienced low shock pressures based on our findings we can exclude that the material is transported from the nearby lockne impact structure which means that the malingen structure is a separate impact structure the seventh confirmed impact structure in sweden furthermore sedimentological and biostratigraphic aspects of the deposits that fill the depression at malingen are very similar to features at the lockne impact structure this implies a coeval formation age and thus also the confirmation of the first known marine target doublet impact craters on earth i e the lockne malingen pair

modeling the ries steinheim impact event and the formation of the moldavite strewn field using detailed geological petrographic geochemical and geographical constraints we have performed numerical modeling studies that relate the steinheim crater apparent diameter da 3 8 km the ries crater da 24 km in southern germany and the moldavite tektite strewn field in bohemia and moravia czech republic lusatia east germany and lower austria the moldavite strewn field extends from 200 to 450 km from the center of the ries to the east northeast forming a fan with an angle of 57 an oblique impact of a binary asteroid from a west southwest direction appears to explain the locations of the craters and the formation and distribution of the moldavites the impactor must have been a binary asteroid with two widely separated components some 1 5 and 0 15 km in diameter respectively we carried out a series of three dimensional hydrocode simulations of a ries type impact the results confirm previous results suggesting that impacts around 30 50 from the horizontal are the most favorable angles for near surface melting and consequently for the formation of tektites finally modeling of the motion of impact produced tektite particles through the atmosphere produces in the downrange direction a narrow angle distribution of the moldavites tektites in a fan like field with an angle of 75 an additional result of modeling the motion of melt inside and outside the crater is the preferred flow of melt from the main melt zone of the crystalline basement downrange towards the east northeast rim this explains perfectly the occurrence of coherent impact melt bodies some tens of meters in size in a restricted zone of the downrange rim of the ries crater the origin of these melt bodies which represent chemically a mixture of crystalline basement rocks similar to the main melt mass contained as melt particles 0 5 m in size in the suevite do not occur at any other portion of the ries crater rim and remained enigmatic until now although the calculated distribution of moldavites still deviates to some degree from the known distribution our results represent an important step toward a better understanding of the origin and distribution of the high velocity surface melts and the low velocity deep seated melt resulting from an oblique impact on a stratified target

intercalibration of 40ar 39ar age standards nl 25 hb3gr hornblende ga1550 sb 3 hd b1 biotite and bmus 2 muscovite abstract 40 ar 39 ar dating requires precise and accurate age data of standard fluence monitors besides primary calibration of k and 40 ar concentrations i e the ages are calculated with precise k ar dating intercalibration of the different monitors in use provides important cross checks of the validity of standard ages and implicitly reliability and uncertainty of chronological studies that rely on these standards here we report intercalibration of 5 primary international 40 ar 39 ar standards and one internal laboratory monitor a major goal of this study was to check the ages of the nl 25 hornblende standard and of the muscovite laboratory standard bmus 2 against the international standards hd b1 ga1550 sb 3 and hb3gr in replicate measurements of three irradiation series 40 ar 39 ar ages for the fluence monitors were found which agree within 1 error limits with the reported age values of 2660 9 ma for nl 25 hb3gr 1073 6 4 6 ma bmus 2 328 5 1 1 ma sb 3 162 9 0 8 ga1550 98 79 0 54 and 24 21 0 32 for hd b1 for the hornblende nl 25 fluence monitor a new slightly different 40 ar 39 ar age of 2657 4 ma is suggested for the age of the muscovite bmus 2 laboratory standard we recommend 328 5 1 1 ma and for biotite hd b1 a more precise value of 24 18 0 09 ma

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

an ordinary chondrite impactor for the popigai crater siberia with a diameter of 100 km popigai in northern siberia is the largest crater known in the cenozoic the concentrations in platinum group elements pge were analyzed in twenty samples of homogeneous impact melt collected in the northwestern flank of the crater to identify the composition of the projectile the method selected was preconcentration by nis fire assay followed by inductively coupled plasma mass spectrometry icp ms this technique measures all the pge except os and by using aliquots 10g the results are highly reproducible the major and trace element composition of the impact melt resembles that of gneissic lithologies of the anabar shield which are representative of the target rock the pge are enriched in the melt by factors of 3 to 14 compared to the main target lithology but the meteoritic contamination is only around 0 2 wt using plots of elemental ratios such as ru rh vs pt pd or ru rh vs pd ir the popigai impactor is clearly identified as an ordinary chondrite and most likely l chondrite this study indicates that pge elemental ratios allow discrimination of the type of impactor even in the case of low meteoritic contamination this study confirms that a significant fraction of the crater forming projectiles presently documented could have an ordinary chondrite composition their probable source the s type asteroids appears to form the majority of the bodies in the main asteroid belt and among near earth objects neos the ordinary chondrite origin of the popigai projectile supports an asteroidal origin for the late eocene impacts as a plausible alternative to the comet shower scenario proposed by farley et al 1998 copyright 2005 elsevier ltd

new 40ar 39ar dating of the clearwater lake impact structures quebec canada not the binary asteroid impact it seems abstract the two clearwater lake impact structures quebec canada are generally interpreted as a crater doublet formed by the impact of a binary asteroid here arguments are presented that raise important questions about the proposed double impact scenario new 40 ar 39 ar dating of two virtually fresh impact melt rock samples from the 36 km west clearwater lake impact structure yielded two statistically robust early permian plateau ages with a weighted mean of 286 2 2 2 2 6 ma 2 mswd 0 33 p 0 57 in contrast 40 ar 39 ar results for two chloritized melt rocks from the 26 km east clearwater lake impact structure produced disturbed age spectra suggestive of a distinct extraneous argon component although individually weakly robust age spectra corrected for the trapped argon component and inverse isochron plots for the east clearwater melt rocks consistently yielded apparent ages around 460 470 ma no permian signal was found in either of these melt aliquots our new 40 ar 39 ar results reproduce earlier 40 ar 39 ar plateau ages 283 ma and 465 ma respectively for the two impact structures by bottomley et al 1990 and are in conflict with a previous statistically non robust rb sr age of 287 293 26 ma for east clearwater the combined cluster of apparent ages of 460 470 ma derived from four different samples across the impact melt sheet is very unlikely to represent a false age effect due to the incorporation of extraneous argon into the melt instead it strongly favors a middle ordovician age for the east clearwater impact and impact induced hydrothermal chloritization moreover the clearwater impact structures are characterized by different natural remanent magnetizations testifying to separate geologic histories an effect unexpected in the case of a permian double impact whereas the west clearwater impact affected ordovician carbonates incorporated into the impact breccia drill core reports from the 1960s concluded that clasts of ordovician sedimentary rocks are seemingly absent in the impact breccia lens of the east clearwater lake impact structure which is overlain by 100 m of post impact sandstones shales and carbonates no resolvable impactor contamination has so far been detected in the west clearwater impact melt rocks whereas east clearwater carries a distinct ordinary possibly l chondritic impactor signature in its melt rocks east clearwater lake might thus represent one among a long list of ordovician impact structures in north america and northern europe that were presumably formed in response to the l chondrite asteroid breakup event 470 ma ago paleogeographic reconstructions show that the ordovician east clearwater impact probably occurred in a near coastal to shallow marine setting while the permian west clearwater impact hit continental pangaea along with the new 40 ar 39 ar data the paleomagnetic sedimentologic and geochemical findings suggest that the close spatial arrangement of the two clearwater lakes is most likely pure coincidence the two impact structures seem to represent a false doublet struck by impacts separated by 180 million years in time the new results for the clearwater lake impact structures have major implications for the reliable identification of doublet impact craters and the rate of binary asteroid impacts on earth and on other planetary bodies in the inner solar system

characterization and morphological reconstruction of the terny impact structure central ukraine the terny impact structure located in central ukraine displays a variety of diagnostic indicators of shock metamorphism including shatter cones planar deformation features in quartz diaplectic glass selective melting of minerals and whole rock melting the structure has been modified by erosion and subsequently buried by recent sediments although there are no natural outcrops of the deformed basement rocks within the area mining exploration has provided surface and subsurface access to the structure exposing impact melt rocks shocked parautochthonous target rocks and allochthonous impact breccias including impact melt bearing breccias similar to suevites observed at the ries structure we have collected and studied samples from surface and subsurface exposures to a depth of approximately 750 m below the surface this analysis indicates the terny crater is centered on geographic coordinates 48 13 n 33 52 e the center location and the distribution of shock pressures constrain the transient crater diameter to be no less than approximately 8 4 km using widely accepted morphometric scaling relations we estimate the pre erosional rim diameter of terny crater to be approximately 16 19 km making it close in original size to the well preserved el gygytgyn crater in siberia comparison with el gygytgyn yields useful insights into the original morphology of the terny crater and indicates that the amount of erosion terny experienced prior to burial probably does not exceed 320 m

geochemistry of the melt and country rocks of the lake st martin impact structure manitoba canada impact melt and country rocks from the 23 km diameter lake st martin structure in manitoba were analysed for major and trace element abundances and for rb sr isotopic compositions our aim was to better understand the formation of the melt rocks in this probable impact structure to search for traces of the meteoritic projectile to date the melt sheet in order to improve the chronological record of impact events on the earth s surface and to constrain the target from which the melt rocks could have formed major element analyses are consistent with the observation by simonds and mcgee 1979 that many of the melt rocks from lake st martin are chemically homogenized to a degree similar to that of melt bodies from other impact structures however some samples are of slightly different composition this can be explained by post impact hydrothermal alteration particularly near the present erosion surface chemical exchange between melt bearing breccia and basement rock near the bottom of the melt sheet or limited mixing of different target lithologies so called pseudotachylites from the central uplift of the structure are chemically diverse and do not always represent the composition of their host rock we conclude that such samples are not pseudotachylite but probably represent injected impact breccia rees and other lithophile elements are well homogenized in the melt in comparison with the country rock variation there is a slight enrichment of the siderophile elements co and au and of cr in the melt rocks but further analyses are required to verify whether this is caused by a small contribution from the meteoritic projectile the melt rocks are also homogenized isotopically when compared with the spread of isotopic ratios determined for the country rocks the melt rock isotopic composition however can only be achieved by mixing of various granitoid country rock types with another component of high 87rb86sr ratio furthermore the melt rocks are isotopically reequilibrated whole rock rb sr isotope data yield an age of 212 4 43 ma isr 7132 3 at 2 level whole rock and mineral separate data yield an age of t 219 32 ma isr 7131 3 this age consistent with previously reported k ar ages of 200 25 and 250 25 ma is thought to be the best approximation to the age of the melt rocks and thus of the impact event however the 2 error of 32 ma is too high to allow this age to become a significant addition to the data base of well constrained impact ages for analysis of periodicity in the cratering record an age of t 2785 3 157 ma 2 for isr 6997 11 was obtained for the basement rocks of the lake st martin structure but the true meaning of this age is still unclear while the age obtained is comparable to other superior province data it appears that the lake st martin country rock data define a mixing line the new chemical and isotopic results are consistent with an impact origin for the lake st martin structure harmonic least squares mixing calculations suggest that the melt rocks were formed from ca 80 monzonitic and 15 granitic basement plus a small addition of ca 6 carbonate rock this is in good agreement with the results by simonds and mcgee 1979 and favors their suggestion that carbonaterich target rocks even when they form the upper portion of the target stratigraphy do not significantly take part in impact melt formation but are largely removed in a co2 rich vapor phase

the age of the kara impact structure russia for the 65 km sized kara impact structure close to the polar ural we report an age of 70 3 2 2 ma 2s defined by the mean of 40ar 39ar plateau ages for three glassy or crystalline impact melt rocks cleaned from mineral and rock clasts the fine structure of the age spectra of these samples can quantitatively be simulated by modeling taking into account 39ar recoil effects without assuming the presence of excess ar the calculations corroborate our age results by showing that 39ar recoil does not affect the plateau fractions previously kara has been proposed as a probable k t impact site or was related to the campanian maastrichtian boundary at 73 ma at the 2s level both suggestions are ruled out by the well constrained age for the kara impact structure

a late ordovician high energy temperate water carbonate ramp southern quebec canada implications for late ordovician oceanography the trenton group late ordovician the youngest carbonate unit in the taconic foreland basin of southern quebec is a tripartite unit with a distinctive coarse grained middle part the deschambault formation lithofacies of the deschambault formation are dominated by coarse grained bioclastic intraclastic limestones finer grained lithofacies are ubiquitous but subordinate the complete spectrum of lithofacies indicates sedimentation ranging from above fairweather to below storm wave base skeletal components are indicative of the modern temperate water bryomol association non skeletal elements are represented by peloids and intraclasts accretion rates from areas of continuous sedimentation were low 14 cm 103 years from sedimentological and faunal evidence it is proposed that the late ordovician deschambault ramp was bathed by temperate waters the model compares favourably with modern cool water shelves rimming the southern edge of the australian continent palaeomagnetic data locate southern quebec in a low latitudinal setting during the late ordovician upper ordovician facies distribution in eastern canada and progressive disappearance of some faunal provinces through late ordovician time are used to conclude that the initiation of the late ordovician glaciation that covered most of gondwana was instrumental in easing northward movement of cold oceanic currents this resulted in the rapid contraction of the southern hemisphere warm water tropical belt from a 30 latitudinal wide zone in the early caradoc to a 15 zone in the late caradoc

a russian record of a middle ordovician meteorite shower extraterrestrial chromite at lynna river st petersburg region numerous fossil meteorites and high concentrations of sediment dispersed extraterrestrial chromite ec grains with ordinary chondritic composition have previously been documented from 467 1 6 ma middle ordovician darriwilian strata these finds probably reflect a temporarily enhanced influx of l chondritic matter following the disruption of the l chondrite parent body in the asteroid belt 470 6 ma in this study a volkhovian kundan limestone marl succession at lynna river northwestern russia has been searched for ec grains 63 mu m eight samples forming two separate sample sets were collected five samples from strata around the asaphus expansusa raniceps trilobite zone boundary in the lower middle kundan yielded a total of 496 ec grains in 65 5 kg of rock average 7 6 ec grains kg 1 but up to 10 2 grains kg 1 these are extremely high concentrations three orders of magnitude higher than background levels in similar condensed sediment from other periods ec grains are typically about 50 times more abundant than terrestrial chrome spinel in the samples and about as common as terrestrial ilmenite three stratigraphically lower lying samples close to the a lepidurusa expansus trilobite zone boundary at the volkhov kunda boundary yielded only two ec grains in 38 2 kg of rock 0 05 grains kg 1 the lack of commonly occurring ec grains in the lower interval probably reflects that these strata formed before the disruption of the l chondrite parent body the great similarity in ec chemical composition between this and other comparable studies indicates that all or most ec grains in these russian mid ordovician strata share a common sourcethe l chondrite parent body less

extraterrestrial chromite distribution across the mid ordovician puxi river section central china evidence for a global major spike in flux of l chondritic matter previous studies of mid ordovician limestone in sweden have shown that over a stratigraphic interval representing a few million years there is a two orders of magnitude enrichment in fossil l chondritic meteorites empty set 1 21 cm and sediment dispersed extraterrestrial chromite ec grains 63 mu m this has been interpreted as a dramatic increase in the flux of l chondritic matter to earth following the breakup of the l chondrite parent body which based on ar ar gas retention ages 470 6 ma of recently fallen meteorites occurred at about this time here we show that the general trend in the distribution of sediment dispersed ec grains can be reproduced in the puxi river section in central china a total of 288 kg of limestone was searched for chrome spinels in samples spanning the lower 8 m of the section representing the paroistodus originalis and lenodus antivariabilis conodont zones a total of 110 kg of limestone yielded only one ec grain similarly to the swedish sections ec grains begin to be common in the overlying l variabilis zone and remain common throughout the upper 9 m of the section representing the l variabilis yangtzeplacognathus crassus and l pseudoplanus zones in this part of the section 178 kg of limestone yielded 290 ec grains with an average chemical composition very similar to chromite from recent l chondrites in most of the beds over this interval one finds 1 4 ec grains per kilogram rock a clear two orders of magnitude enrichment relative to the lower part of the section small bed by bed variations in the ec content over the upper interval most likely reflect small variations in sedimentation rates the puxi river section contains only very rare terrestrial chrome spinel grains which can be distinguished already by their rounded abraded appearance compared to the angular pristine extraterrestrial spinels in the mid ordovician based on paleoplate reconstructions the puxi river site was positioned at mid latitudes on the southern hemisphere a couple of thousand kilometers east of the swedish sites the prominent enrichment of ec grains over the same stratigraphic interval in china and sweden is supporting evidence for a dramatic increase in the flux of l chondritic matter to earth shortly after the disruption of the l chondrite parent body in the asteroid belt c 2010 elsevier inc all rights reserved less