magnetic investigations of the west hawk deep bay and clearwater impact structures canada due to the effects of erosion tectonism and burial impact structures are often obscured or destroyed geophysical methods are increasingly being used in detecting the signatures of impact structures while gravity lows associated with impact structures are well understood associated magnetic anomaly lows are not in this study drill cores from three canadian impact structures were analyzed for rock magnetic properties and mineralogy in order to explain the magnetic anomaly lows associated with these structures samples from the drill cores were cut and measured for anisotropy of magnetic susceptibility ams and natural remanent magnetization nrm parameters drill cores from the twin impact craters of the clearwater structure exhibited different nrm characteristics and samples from their respective drill cores were subject to demagnetization by alternating field and thermal techniques the difference noted in their nrm characteristics was attributed to the acquisition of a viscous remanent magnetization vrm at depth in clearwater east at all three structures both magnetic susceptibilities and remanent magnetizations are well below regional values in impact generated breccias melt rocks shocked crystalline rocks and in postimpact sedimentary infill the processes of brecciation alteration shock and infill by nonmagnetic sediments contribute to the development of the magnetic lows however a significant contribution to the observed magnetic anomalies was found by first order forward modelling to arise from basement rocks beneath the impact structures this zone of reduced magnetization may be caused by the partial demagnetization of magnetite by the impact induced transient stress wave traveling away from the point of impact

hydrothermally altered impact melt rock and breccia contributions to the soil of mars samples of melt rock and breccia from 12 terrestrial impact craters were examined to identify alteration minerals and their conditions of formation the dominant assemblage in most cases is clay silica k feldspar zeolite indicating hydrothermal alteration at low pressures and temperatures of 100 300 c the clays are predominantly fe chlorites and smectites in most cases depleted in al and enriched in fe and mg relative to their source rocks alteration of impact glass is often complete while alteration of crystalling melt rock is limited to a few percent of the rock volume impact breccia is altered to only a small extent compared to the alteration of glass impact induced hydrothermal alteration is to be expected at martian impact sites if significant quantities of ground ice and or water are present the alteration products are likely to resemble the clays found at terrestrial craters al fe fractionation similar to that observed on earth is strongly suggested by the viking soil analyses the quantitative contribution of hydrothermal clays to the soil of mars is estimated based on a model which predicts the generation of the equivalent of a 61 m thick blanket of impact melt planet wide the inefficiency of melt production in small craters and the inefficiency of alteration in large craters limit the expected production of clay to a few percent of this figure clays produced by impact induced hydrothermal alteration may be expected to contribute to the fines on mars but this contribution is probably not dominant relative to other suggested sources

siderophile rich particles in the melt rocks at the e clearwater impact structure quebec their characteristics and relationship to the impacting body the composition of the sampled melt rocks at the 22 km diameter e clearwater impact structure indicates the presence of 8 c 1 material the meteoritic component is fractionated with refractory siderophiles up to 30 times c 1 abundances concentrated in ten to hundred micron sized magnetic particles these particles consist of the ni sulphide millerite and what is assumed to be a mixture of refractory silicates and magnetite with grain sizes of 1 m the larger particles have a core rim structure with millerite and occasionally very minor galena and possibly pentlandite in the core an origin as a combination of altered meteoritic metal and condensed meteoritic silicate is favored for the origin of the siderophile rich particles if 8 meteoritic material is taken as the average meteoritic contamination in the melt then the e clearwater projectile may have impacted with a velocity of 17 km s 1 peak shock pressures would have been of the order of 300 gpa sufficient to vaporize the silicate component but only melt the metal component of the projectile as the meteoritic material was being driven down into vaporized melted target rocks during the initial stages of impact the melted fe ni metal underwent oxidation fe was removed and meteoritic silicate material recondensed on the cooler essentially ni metal as cavity excavation proceeded these ni metal silicate oxide particles were incorporated in the melt their refractory nature prevented thermal digestion and sulphur in the melt reacted with the metal to produce millerite on final equilibration if this hypothesis is correct it suggests that the e clearwater projectile was a c 2 or c 3 chondrite both of which are compatible with the trace element composition of the melt rocks clearwater lake is a twin impact structure formed by an asteroid pair it is still not clear however what type of projectile formed the 32 km diameter western structure where the surface melt rocks contain no identifiable meteoritic signature

chromium isotopic studies of terrestrial impact craters identification of meteoritic components at bosumtwi clearwater east lappajarvi and rochechouart chromium isotopic compositions and elemental abundances in impact melt rock and impact glass samples from four terrestrial impact craters were measured to verify the presence of an extraterrestrial component and to identify the meteorite type of the impactor all meteorite classes have cr isotopic signatures that are different from those of terrestrial rocks thus precise measurements of cr isotopic abundances can unequivocally distinguish terrestrial from extraterrestrial materials for all four studied craters bosumtwi ghana clearwater east canada lappajarvi finland and rochechouart france we found positive 53cr excesses that eliminate carbonaceous chondrite projectiles because those would show apparent negative excesses and enstatite chondrites because of the magnitude of the excess in all four cases ordinary chondrites have been identified as the best fit for the data in the case of lappajarvi interelement correlations together with the cr isotope data make an h chondrite the most likely projectile whereas in the case of clearwater east both l or h chondrites are possible for bosumtwi and rochechouart the high indigenous contents of the siderophile elements and disturbances of the elemental abundances by weathering and hydrothermal alteration respectively do not allow further constraints to be placed on the type of ordinary chondrite involved in the impact

identification of the projectile at the brent crater and further considerations of projectile types at terrestrial craters abstract impact melt samples from drill hole b1 59 at the 3 8 km diameter brent crater ontario have been analysed for siderophile trace elements indicative of meteoritic contamination samples from the basal melt zone at 823 857 m depth are enriched in ir os pd ni co cr and se over basement with the abundance pattern suggesting a chondritic projectile for brent from a ni cr correlation of 10 melt samples an l or ll chondrite is inferred the contribution of an ultramafic country rock alnoite in the melt is too small to significantly influence its ni cr ratio glass rich breccias from the allochthonous breccias filling the crater also contain a meteoritic component interelement ratios e g ni cr are however fractionated relative to the melt zone samples this as well as the low au content of all brent samples is probably a product of alteration additional data on impact melts from the 65 km diameter crater manicouagan still did not reveal a meteoritic component as also for the mistastin crater 28 km diameter where cr analyses set an upper limit of 1 of an achondritic projectile component in the melt irghizites tektite like glasses from the zhamanshin impact structure have been found to contain high ni and co concentrations and our data show that ir is also enriched it is however not possible to define the projectile type enrichment of an ivory coast tektite in ir is confirmed there are large differences in siderophile element concentrations among tektites with otherwise similar chemical composition there are now four known craters formed by chondrites clearwater east lapparjarvi wanapitei and brent with brent being the smallest of these for smaller craters the projectiles appear to be limited to iron or stony iron meteorites because of atmospheric destruction of relatively small stony meteorites it appears however that all major classes of meteorites are represented among the projectiles at terrestrial impact craters

nonuniform cratering of the moon and a revised crater chronology of the inner solar system abstract we model the cratering of the moon and terrestrial planets from the present knowledge of the orbital and size distribution of asteroids and comets in the inner solar system in order to refine the crater chronology method impact occurrences locations velocities and incidence angles are calculated semi analytically and scaling laws are used to convert impactor sizes into crater sizes our approach is generalizable to other moons or planets the lunar cratering rate varies with both latitude and longitude with respect to the global average it is about 25 lower at 65 n 90 e and larger by the same amount at the apex of motion 0 n 90 w for the present earth moon separation the measured size frequency distributions of lunar craters are reconciled with the observed population of near earth objects under the assumption that craters smaller than a few kilometers in diameter form in a porous megaregolith varying depths of this megaregolith between the mare and highlands is a plausible partial explanation for differences in previously reported measured size frequency distributions we give a revised analytical relationship between the number of craters and the age of a lunar surface for the inner planets expected size frequency crater distributions are calculated that account for differences in impact conditions and the age of a few key geologic units is given we estimate the orientale and caloris basins to be 3 73 ga old and the surface of venus to be 240 ma old the terrestrial cratering record is consistent with the revised chronology and a constant impact rate over the last 400 ma better knowledge of the orbital dynamics crater scaling laws and megaregolith properties are needed to confidently assess the net uncertainty of the model ages that result from the combination of numerous steps from the observation of asteroids to the formation of craters our model may be inaccurate for periods prior to 3 5 ga because of a different impactor population or for craters smaller than a few kilometers on mars and mercury due to the presence of subsurface ice and to the abundance of large secondaries respectively standard parameter values allow for the first time to naturally reproduce both the size distribution and absolute number of lunar craters up to 3 5 ga ago and give self consistent estimates of the planetary cratering rates relative to the moon

highly siderophile elements re os ir ru rh pd au in impact melts from three european impact craters saaksjarvi mien and dellen clues to the nature of the impacting bodies twenty two large 10 g impact melt samples from three scandinavian craters i e saaksjarvi finland and mien and dellen sweden were analyzed for highly siderophile elements hse platinum group elements rh and au by the nickel sulfide technique in combination with neutron activation the ten impact melt samples from saaksjarvi are enriched in ir and other highly siderophile elements ir 2 48 0 73 ng g relative to average upper crust concentrations 0 03 0 02 ng g ir the twelve dellen and mien samples are marginally enriched in ir 0 48 0 23 ng g for dellen and 0 37 0 23 ng g for mien the amount of meteoritic component corresponds to 0 5 of a nominal ci component for saaksjarvi and about 0 1 for mien and dellen the saaksjarvi pattern is fractionated relative to ci chondrites normalized abundances increase from the refractory to the more volatile siderophile elements ir ni the trend is qualitatively similar to magmatic iron meteorites and corresponds to 0 4 of a nominal magmatic iron meteorite component tamarugal iiiab for mien and dellen no projectile assignment can be made all samples from the three impact craters have low osir ratios compared to chondritic ratios either the projectile had low osir ratios or os was lost during the impact as volatile oso4 based on the results of this study and on a compilation of literature data average upper crustal abundances of the highly siderophile elements in the target area baltic shield are estimated as 0 03 ng g for ir and os 1 1 ng g for ru 0 38 ng g for rh 2 0 ng g for pd 8 g g for co and 37 g g for cr these data are representative of the upper crust they allow for the first time reliable estimates of crustal abundances for rh and ru upper crustal abundances also show that ir and os are the most favourable elements for the identification of meteoritic signatures fractionated osir ratios in magmatic iron meteorites or potential losses of os during the impact leaves ir as the best indicator element for meteoritic contamination with ir extraterrestrial components as low as 2 104 ci may be identified

final amalgamation of the central asian orogenic belt in ne china paleo asian ocean closure versus paleo pacific plate subduction a review of the evidence abstract the central asian orogenic belt caob evolved through complex closure of the paleo asian ocean from the neoproterozoic to the late phanerozoic this caused the chinese cratons to collide with eurasia and led to the formation of the world s largest phanerozoic orogenic belt ocean closure commenced in the west and was completed in the east near changchun closure of the paleo asian ocean in ne china was along the solonker xar moron changchun yanji suture and this was likely completed in the late permian although associated activity continued into the triassic there was an overlap in the latest permian early triassic between terminal activity associated with paleo asian ocean closure and the onset of tectonism associated with subduction of the paleo pacific plate this switch in geodynamic setting occurred at 260 250 ma and is reflected by a relaxing of north south directed compression and the onset of east west directed processes related to paleo pacific subduction by the early jurassic events associated with the westward advance of the paleo pacific plate dominated leading to extensive development of i type granites as far inland as the great xing an range from 140 ma the paleo pacific plate retreated eastward resulting in an extensional setting in the early cretaceous the effects of which were enhanced by regional thinning of the lithosphere commonly attributed to delamination throughout this period the eastern asian margin was tectonically complex the north south oriented jiamusi khanka bureya block was rifted away from the eastern margin of the caob in the late triassic but was then re united in the jurassic by westward advancing subduction that affected both the western and eastern margins of the block accretionary complexes continued to evolve in the cretaceous along the whole eastern margin of asia with final accretion of the nadanhada terrane part of the sikhote alin accretionary terrane with the caob at 130 ma followed by the emplacement of s type granites

subsurface damage from oblique impacts into low impedance layers 1 layered planetary surfaces occur ubiquitously in the solar system where sedimentary sequences or icy layers overlay crystalline bedrock previous experimental studies investigated how the presence of weak layer overlying a strong basement affects crater morphology subsurface damage and soft sediment compression numerical studies generally focus on the final morphology as a function of thicknesses and burial depths of weak layers in field studies of impact craters the shock state of minerals is a key metric here we evaluate the effect of a surficial low impedance layer on peak pressure magnitudes and consequent damage extent in the competent substrate laboratory experiments coupled with 3d cth models of oblique 30 from horizontal hypervelocity impacts at laboratory and planetary scales show that surface layers with a thickness on the order of the projectile diameter shield the underlying surface and absorb scatter 70 of the impact energy numerical simulations reveal that surficial layers reduce peak pressure magnitudes within the subsurface by 60 70 while damage in the substrate is due to shear failure sedimentary layers are more efficient shields than icy layers but both reduce the extent of subsurface damage and the resulting shock levels recorded by minerals these results indicate that a thin surficial low impedance layer mitigates the expression of shocked minerals in the substrate even when a structural response is still observed

erratum to glacial weathering and the hafnium isotope composition of seawater earth planet sci lett 198 2002 167 175

morokweng south africa a large impact structure of jurassic cretaceous boundary age geophysical petrographical and geochemical data indicate the presence of a large impact structure in the area around morokweng northwest province south africa possibly up to 340 km in diameter drill cores from the center of the structure show a thick layer of impact melt rocks with high abundances of cr ni co and the platinum group elements consistent with the presence of up to 5 of a chondritic component ion probe dating of zircons extracted from the impact melt yielded a 206 pb 238 u age of 146 2 1 5 ma and a 208 pb 232 th age of 144 7 1 9 ma these are indistinguishable from the age of the jurassic cretaceous boundary following the identification of the chicxulub impact structure of cretaceous tertiary boundary age the discovery of a second large impact structure at a previously established major chronostratigraphic boundary strengthens suggestions that large impact events have been major factors in the evolution of the earth

isotopic composition of osmium in terrestrial samples determined by accelerator mass spectrometry interest has revived in the rhenium osmium system due to significant advances in detection methods1 4 and because of its use for the identification of extraterrestrial material at the cretaceous tertiary boundary5 another perhaps more important application is its use as a tracer for the origin of crustal material this application is however severely restricted by the very low concentrations typical of crustal rocks we present here the first results for the isotopes 187os 188os and 189os measured in sub p p b parts per 109 level samples by accelerator mass spectrometry ams our first exploratory measurements on extraterrestrial and crustal material with os concentrations between 2 p p m parts per 106 and 0 006 p p b show a detection limit of 0 01 p p b and a precision of better than 10 the 187os 186os ratios measured for samples for the canyon diablo meteorite and from the east clearwater crater in quebec canada were close to 1 typical of extraterrestrial material in contrast ratios in samples from the ries crater in germany were around 10 which is evidence of predominantly crustal osmium in these rocks

constraints on the timing of uplift of the yanshan fold and thrust belt north china this paper reports in situ u pb ages and hf isotopic data on detrital zircons from paleozoic to late mesozoic strata in the xishan area near beijing that provide important constraints on the evolution and paleogeography of the northern part of the north china craton ncc zircons from carboniferous to late mesozoic sandstones are characterized by three major groups of u pb ages 2 5 2 3 ga 1 9 1 7 ga and late phanerozoic ages the neoarchean and paleoproterozoic zircons and some phanerozoic zircons have negative ehf t values similar to igneous zircons from intrusive rocks known from the ncc indicating that the source of the sandstones was locally derived however late triassic to late jurassic 205 158 ma sandstones contain a group of phanerozoic zircons with positive ehf t values distinct from known igneous zircons in the ncc suggesting an exotic source the nearest potential source is the xing meng orogenic belt xmob to the north of the ncc where considerable amounts of juvenile crustal materials exist it is concluded that there are two sudden shifts in source provenances of the xishan sediments from the ncc before the late triassic to a mixed source from the ncc and xmob during the late triassic to late jurassic and then from the ncc again during the late jurassic 158 ma the first shift in source resulted from the exhumation and denudation of rocks due to uplift of the xmob which coincided with post collisional extension in the late triassic and intraplate extension of the ncc in the middle to late jurassic the second source shift resulted from the uplift of the yanshan fold and thrust belt thus constraining the timing of this to the late jurassic

the manicouagan impact melt sheet evidence for isotopic homogenization with limited assimilation abstract the rb sr pb pb and sm nd isotope chemistry of impact generated melt and target rocks has been determined from the 214 ma 90 km rim diameter manicouagan impact structure of canada fifty seven samples were obtained from 8 field sites and 11 drill core sites across the impact melt sheet located on the 55 km diameter central island ile rene levasseur the results reveal that the impact melt though locally differentiated via fractional crystallization was isotopically homogenized during its formation following its derivation primarily from charnockites and mesocratic gneisses the isotopic signatures of the target rocks indicate that the protolith for the melt was the proterozoic manicouagan imbricate zone and that the underlying archean gagnon terrane was not involved in melt production or its subsequent modification via assimilation despite impact melt resting on archean lithologies in the southwest sector of the melt sheet limited assimilation of the footwall in this sector is attributed to the relatively rapid cooling of a thinner peripheral melt body

mid sized complex crater formation in mixed crystalline sedimentary targets insight from modeling and observation large impact crater formation is an important geologic process that is not fully understood the current paradigm for impact crater formation is based on models and observations of impacts in homogeneous targets real targets are rarely uniform for example the majority of earth s surface is covered by sedimentary rocks and or a water layer the ubiquity of layering across solar system bodies makes it important to understand the effect target properties have on the cratering process to advance understanding of the mechanics of crater collapse and the effect of variations in target properties on crater formation the first bridging the gap workshop recommended that geological observation and numerical modeling focussed on mid sized 15 30 km diameter craters on earth these are large enough to be complex small enough to be mapped surveyed and modelled at high resolution and numerous enough for the effects of target properties to be potentially disentangled from the effects of other variables in this paper we compare observations and numerical models of three 18 26 km diameter craters formed in different target lithology ries germany haughton canada and el gygytgyn russia based on the first order assumption that the impact energy was the same in all three impacts we performed numerical simulations of each crater to construct a simple quantitative model for mid sized complex crater formation in a subaerial mixed crystalline sedimentary target we compared our results with interpreted geological profiles of ries and haughton based on detailed new and published geological mapping and published geophysical surveys our combined observational and numerical modeling work suggests that the major structural differences between each crater can be explained by the difference in thickness of the pre impact sedimentary cover in each case we conclude that the presence of an inner ring at ries and not at haughton is because basement rocks that are stronger than the overlying sediments are sufficiently close to the surface that they are uplifted and overturned during excavation and remain as an uplifted ring after modification and post impact erosion for constant impact energy transient and final crater diameters increase with increasing sediment thickness

predictions for impactor contamination on ceres based on hypervelocity impact experiments new experiments predict that ceres should be extensively contaminated with meteoritic debris derived from the asteroid belt all types of impactors likely contribute to the contamination ceres may accrete debris more efficiently if it is ice rich because of enhanced projectile survival and retention in porous ice targets experiments indicate that if a silicate regolith lag protects subsurface ice then some of the projectile should be injected into the regolith during high angle impacts thereby hiding part of the projectile component from view if impacts excavate ice sublimation will gradually concentrate projectile relics into a surficial lag in contrast if the near surface lacks ice then accreted meteoritic debris should be distributed throughout a vertically mixed regolith high resolution images may reveal pristine projectile relics lining some crater floors moreover we predict that the surface of ceres is not exclusively endogenic and may be dominated by delivered exogenic debris

origin of toasted quartz in terrestrial impact structures quartz present in the target rocks at impact sites and as clasts within impact melt rocks and some pseudotachylytes commonly exhibits a brown nonpleochroic grainy appearance in plane and cross polarized light known as toasted quartz this variety of quartz also possesses a higher albedo in hand sample than colorless examples despite the intense coloration no compositional origin for the browning is evident a high proportion of exceptionally small typically submicron scale fluid inclusions which are typically too small to be observed with the petrographic microscope is considered to be the cause these fluid inclusions are predominantly located along decorated planar deformation features and they enhance scattering of transmitted light and reflect larger quantities of incident light statistically lower sio 2 and total oxide contents present in toasted quartz relative to untoasted quartz are consistent with increased surface electron scattering during microprobe analysis by fluid inclusions intersecting the surface

did the paleo asian ocean between north china block and mongolia block exist during the late paleozoic first paleomagnetic evidence from central eastern inner mongolia china 1 the tectonic evolution of the paleo asian ocean between the north china block ncb and the mongolia block mob is a contentious issue and geodynamic models remain speculative in an effort to puzzle out this controversy a paleomagnetic study was carried out on the silurian to permian formations in central eastern inner mongolia china more than 680 sedimentary and volcanic samples were collected from 86 sites we have established titanium poor magnetite and hematite as the principal magnetic carriers anisotropy of the magnetic susceptibility measurements demonstrate negligible deformation of the majority of study rocks with sedimentary fabrics from primary magnetizations a late devonian and a permian pole are calculated for inner mongolia block imb at 46 8 n 349 1 e dp 14 6 dm 27 3 with n 3 and 48 7 n 3 7 e dp 5 2 dm 9 1 with n 6 respectively two stages of secondary magnetization are also identified probably due to early permian and early cretaceous magmatic events as preliminary results the comparison of our new paleomagnetic poles with available data from ncb mob and siberia indicates that 1 the paleolatitudes of imb ncb and mob are consistent between late devonian and permian suggesting pre late devonian closure of the paleo asian ocean and further evaluation of these three blocks as a single entity and 2 post permian intracontinental deformation was significant and characterized by block rotations which are due to strike slip faulting within the welded ncb imb mob block

platinum group elements in the morokweng impact structure south africa evidence for the impact of a large ordinary chondrite projectile at the jurassic cretaceous boundary radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to overlap in time with major boundaries in earth history e g the cretaceous tertiary k t and jurassic cretaceous j k boundaries the morokweng impact crater in south africa is coincident in age with the j k boundary however the types of objects that generate large craters are poorly known because it is difficult to unambiguously identify the projectile from the signature it imparts into the impact rocks meteorites are highly enriched in the platinum group elements pge which have been widely used as a tool for identifying the presence of a meteorite signature here we present new pge analyses from the morokweng impact melt sheet our data reveal high pge concentrations and high degree of pge correlation through the melt sheet regression analysis was used to determine the projectile pge signature and constrain input from the terrestrial target rocks the closest match to morokweng is the pge signature of ordinary l or ll chondrite meteorites which is broadly in agreement with the results of an earlier cr isotope study the results of these independent studies provide strong evidence that a large ordinary chondrite projectile struck the area of morokweng in the late jurassic

paleo pacific subduction accretion evidence from geochemical and u pb zircon dating of the nadanhada accretionary complex ne china the nadanhada terrane located along the eastern margin of eurasia contains a typical accretionary complex related to paleo pacific plate subduction accretion the yuejinshan complex is the first stage accretion complex that consists of meta clastic rocks and metamafic ultramafic rocks whereas the raohe complex forms the main parts of the terrane and consists of limestone bedded chert and mafic ultramafic rocks embedded as olistolith blocks in a weakly sheared matrix of clastic meta sedimentary rocks geochemical data indicate that the yuejinshan metabasalts have normal mid ocean ridge basalt n morb affinity whereas the raohe basaltic pillow lavas have an affinity to ocean island basalts oib sensitive high resolution ion microprobe shrimp u pb zircon analyses of gabbro in the raohe complex yield a weighted mean 206pb 238u zircon age of 216 5 ma whereas two samples of granite intruded into the complex yield weighted mean 206pb 238u zircon ages of 128 2 and 129 2 ma laser ablation inductively coupled plasma mass spectrometry la icpms u pb zircon analyses of basaltic pillow lava in the raohe complex define a weighted mean age of 167 1 ma two sandstone samples in the raohe complex record younger concordant zircon weighted mean ages of 167 17 and 137 3 ma these new data support the view that accretion of the raohe complex was between 170 and 137 ma and that final emplacement of the raohe complex took place at 137 130 ma the accretion of the yuejinshan complex probably occurred between the 210 and 180 ma suggesting that paleo pacific plate subduction was initiated in the late triassic to early jurassic

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

complex crater formation insights from combining observations of shock pressure distribution with numerical models at the west clearwater lake impact structure large impact structures have complex morphologies with zones of structural uplift that can be expressed topographically as central peaks and or peak rings internal to the crater rim the formation of these structures requires transient strength reduction in the target material and one of the proposed mechanisms to explain this behavior is acoustic fluidization here samples of shock metamorphosed quartz bearing lithologies at the west clearwater lake impact structure canada are used to estimate the maximum recorded shock pressures in three dimensions across the crater these measurements demonstrate that the currently observed distribution of shock metamorphism is strongly controlled by the formation of the structural uplift the distribution of peak shock pressures together with apparent crater morphology and geological observations is compared with numerical impact simulations to constrain parameters used in the block model implementation of acoustic fluidization the numerical simulations produce craters that are consistent with morphological and geological observations the results show that the regeneration of acoustic energy must be an important feature of acoustic fluidization in crater collapse and should be included in future implementations based on the comparison between observational data and impact simulations we conclude that the west clearwater lake structure had an original rim final crater diameter of 35 40 km and has since experienced up to 2 km of differential erosion

delivering a projectile component to the vestan regolith abstract dark material on vesta may consist of carbonaceous chondrite like material delivered by impact events this study uses hypervelocity impact experiments to assess the viability of the impact delivery hypothesis experiments reveal that impact events deliver significant fractions of the projectile to the target during impacts at average vestan speeds hence dark material can plausibly be delivered to vesta by impacts with the projectile component accumulating in the regolith with time projectile retention is sensitive to impact angle ranging from 7 for 30 impacts measured from horizontal to 72 for vertical impacts averaged over the probability distribution of impact angles 17 of the projectile s mass remains in or near the crater projectile contaminated breccias largely remain inside the crater for vertical impacts in oblique impacts projectile contaminated pieces concentrate downrange beyond the crater rim based on experiments projectile delivery is expected for typical vestan impact conditions not only for extraordinary events such as low probability and very low speed e g

clues to the nature of the impacting bodies from platinum group elements rhenium and gold in borehole samples from the clearwater east crater canada and the boltysh impact crater ukraine seven large 10 g impact melt rock samples from boreholes from the boltysh impact crater ukraine and six samples from the east clearwater crater canada were analyzed for os ir ru rh pd re and au by the nickel sulfide technique in combination with neutron activation earlier analyses of clearwater east impact melt rocks have shown that they are strongly enriched in ir os pd and re in this work i confirm earlier findings and demonstrate similarly high enrichments of rh and ru the average os ir ru ir pd ir rh ir and ru rh ratios of the melt rock samples from clearwater east are ci chondritic and yield an average ir content of 25 2 6 5 ng g relative to an average upper crust concentration of 0 03 0 02 ng g ir the amount of meteoritic component corresponds to 4 to 7 of a nominal ci component for clearwater east the impact melt rock samples from a bore hole from boltysh are low in ir with an average of 0 2 0 1 ng g the ci normalized abundances increase from the refractory to the more volatile siderophile elements os ir ru rh pd au ni co because of the low ir anomaly and uncertainties in making corrections correlations are weak for indigenous siderophile elements no clear projectile assignment can be made

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

geochronology and geochemistry of middle permian middle triassic intrusive rocks from central eastern jilin province ne china constraints on the tectonic evolution of the eastern segment of the paleo asian ocean abstract to constrain the permian early mesozoic tectonic evolution of the eastern segment of the paleo asian ocean we conducted zircon u pb dating and whole rock geochemical analyses on six middle permian middle triassic intrusive plutons in central eastern jilin province ne china zircons from the six plutons display distinct oscillatory zoning and striped absorption in cathodoluminescence cl images and th u ratios of 0 11 1 41 indicating a magmatic origin zircon u pb dating indicates that the intrusive rocks formed in the middle permian ca 260 ma to middle triassic ca 245 ma in central eastern jilin province the middle permian and middle triassic plutons are composed mainly of strongly deformed monzogranites with affinities to adakitic rocks which are formed from partial melting of thickened mafic lower crust in contrast the late permian the early triassic plutons in central eastern jilin province consist of a bimodal association including gabbros and granitoids and deformed monzonites which typically form in extensional settings these observations along with the results of previous studies on early middle permian granitoids in western jilin province indicate that the eastern segment of the paleo asian ocean underwent the initial closure in central western jilin province during the middle permian and the final closure in eastern jilin province in the middle triassic

paleomagnetism and rock magnetism of east and west clearwater lake impact structures the east and west cleawater lake impact structures wiy sh kimi lake quebec 26 and 32 km in diameter respectively have been proposed to represent an impact doublet we investigated their pale

a tectonic transition from closure of the paleo asian οcean to subduction of the paleo pacific plate insights from early mesozoic igneous rocks in eastern jilin province ne china abstract the timing of tectonic transition from the paleo asian oceanic regime to the paleo pacific regime in the easternmost central asian orogenic belt remains controversial to constrain the event we present new zircon u pb ages hf isotopic compositions and whole rock major and trace element data for early mesozoic igneous rocks from eastern jilin province ne china these new ages together with existing age data indicate that early mesozoic magmatism in the kaiyuan yanji area can be subdivided into four stages early triassic ca 250 ma early middle triassic 249 235 ma late triassic 229 208 ma and latest triassic to early jurassic 207 175 ma the early triassic igneous rocks constitute a bimodal igneous rock association that formed in an extensional environment caused by break off of the subducting slab during final closure of the paleo asian ocean the early middle triassic magmatism is dominated by adakitic granitoids whose primary magmas were derived from partial melting of thickened mafic lower crust suggesting that the north china craton and songliao xilinhot block were still undergoing collisional orogenesis at 249 235 ma the late triassic igneous rocks comprise a typical bimodal association of rocks in an e w trending belt formed in a post orogenic extensional environment related to final closure of the paleo asian ocean the latest triassic to early jurassic igneous rocks have calc alkaline characteristics were emplaced as a ne sw trending belt parallel to the eastern margin of eurasia and formed in an active continental margin setting associated with initial subduction of the paleo pacific plate we conclude that the tectonic transition between the paleo asian and the paleo pacific regimes took place during the latest triassic

contrasting meteoritic signatures within the clearwater east and clearwater west impact structures the view from osmium isotopes abstract osmium isotopes provide a powerful tool for identifying meteoritic signatures in impactites we apply the osmium isotope method to impact melt and country rocks from the clearwater east and clearwater west craters located in quebec canada impact melts from clearwater east have 187 os 188 os ratios of 0 1281 0 1285 these values indicate a significant meteoritic component which exceeds that in impact melts at all terrestrial craters studied to date except morokweng such findings align with earlier results from chromium isotopes and platinum group elements in contrast impact melts from clearwater west have 187 os 188 os ratios between 6 604 and 59 12 these highly radiogenic ratios are indistinguishable from the 187 os 188 os ratios in country rocks hence osmium isotopes provide no evidence for a meteoritic component in impact melts at clearwater west the clearwater craters formed in almost identical targets therefore target effects cannot readily explain the stark difference between the two clearwater craters if melt sheet heterogeneity is similar at the two craters the probability that melts at clearwater west host an undetected chondritic component is 1 may best explain the unusually strong meteoritic signature although the signature or its nondetection at each crater may be related to asymmetric preservation of the impactor component the results presented here provide further evidence that clearwater east and clearwater west were temporally separate impact events

the deep impact crater on 9p tempel 1 from stardust next abstract the stardust next sdn mission returned to comet 9p tempel 1 and viewed the site of the deep impact di collision just over one comet year later comparisons between pre impact images from the its camera on the di probe and sdn images reveal a 50 m diameter crater surrounded by a low rim about 180 m in diameter the removal of a small mound uprange but offset from the trajectory from the impact site can be related to changes in the evolution of ejecta a narrow 6 gap in the ejecta curtain downrange indicates that a ridge extending from the impact facing scarp downrange interrupted the final stages of cratering in one small region together these observations indicate that the di excavation crater diameter was about 200 m 20 m a value consistent with the ejected mass derived from earth and space based observations with the assumption that this mass represents only 10 20 of the total ejected mass as a result the di crater visible today is consistent with either a larger transient crater which collapsed or a central crater of a nested crater resembling an inverted sombrero the latter alternative would be expected from a layered target a loose particulate surface about 1 2 m deep over a slightly more competent substrate

clearwater east impact structure a re interpretation of the projectile type using new platinum group element data from meteorites platinum group element pge concentrations and ratios obtained from samples of the clearwater east impact melt have been used along with other siderophile element ratios to classify the impacting projectile as a carbonaceous chondrite this is at odds with recent chromium isotope analyses that suggest ordinary chondrite type material is present the present study reviews and reinterprets the available pge data in the light of new pge data from meteorites and concludes that the pge ratios in the impact melt are most consistent with ordinary possibly type l chondrite source material not carbonaceous chondrites therefore the structure was most probably formed by the impact of an asteroid composed of material similar to ordinary chondrites

geochemistry and petrography of impact breccias and target rocks from the 145 ma morokweng impact structure south africa morokweng is a large 145 ma impact structure in the northwest province of south africa the impact origin of this structure and its melt rock has been confirmed by ample evidence of shock metamorphism in clasts within the melt rock and samples from granitoid basement below the melt body the age of this structure is indistinguishable from the biostratigraphic age of the jurassic cretaceous j k boundary the size of morokweng for which diameters ranging from 70 to 165 kilometers have been quoted before and which has important implications regarding its relation to the j k boundary remains an open question here we present new results of a detailed petrographic and chemical investigation of impact melt rock and country rock samples the granophyric melt rock is mostly unaltered and contains a large number of gabbroic and felsic clasts the occurrence of baddeleyite formed from high temperature dissociation of primary zircon indicates a high temperature origin the impact melt rock body which in the cores investigated here has a thickness of at least 120 m shows no statistically significant variation or trend in chemical composition with depth or geographic location chemical data for impact melt rock breccia dike vein breccia samples granite quartzite and basic to mafic clasts were used in harmonic least squares mixing calculations to determine the source rock types and their proportions involved in the formation of the impact melt rock granite is the dominant target rock component 50 to 63 by weight depending on target composition input to the mixing models with significant 35 to 50 mafic contributions and a possible minor contribution of quartzite new platinum group element ru rh pd os ir and pt re and au data as well as data for other siderophile elements cr co ni and ir confirm the presence of up to 5 of a chondritic component in the melt rock the indigenous contribution of the pges from the target rocks is negligible normalized pge abundance patterns and interelement ratios of morokweng impact melt rock indicate that the projectile was likely of ordinary chondritic possibly l chondrite composition but the choice of the meteoritic compositional data influences this interpretation

rhenium behavior in molybdenite in hypogene and near surface environments implications for re os geochronometry abstract rhenium is concentrated mainly in molybdenite mos2 and occurs as a major cation in only a few rare minerals this affinity makes molybdenite an ideal mineral for re os geochronometry but research on the behavior of re in molybdenite is limited infrared microscope xrd back scattered electron bse and microprobe techniques have been used in this study to document re behavior in molybdenite affected by hypogene and near surface processes in the hypogene environment both 3r and 2h molybdenite can experience re loss during hydrothermal alteration which also causes increased infrared transparency ir alteration at temperatures as low as 150 c can cause re loss in the presence of advecting fluids and will affect re os dating if it occurs long after primary mineralization re loss in 3r and 2h molybdenite under supergene conditions does not increase ir transparency rhenium is not incorporated into supergene ferrimolybdite but is enriched in k al silicate intergrowths which may be illite these minute intergrowths are present in several samples and would not be detected in a simple optical examination in the supergene environment elemental os is stable whereas re is not rhenium in molybdenite may be removed by supergene fluids after some has decayed to 187os causing erroneously old ages or it may be adsorbed into the illite intergrowths creating ages which are too young in the weathering environment proterozoic molybdenites have altered to re enriched powellite which can be detected using back scattered electron imagery combined microprobe xrd bse and infrared microscopy can be successfully used to detect alteration in molybdenite prior to dating these techniques are non destructive and should be performed before any molybdenite is dated by the re os system

use of platinum group elements for impactor identification terrestrial impact craters and cretaceous tertiary boundary abstract this study presents the abundances of ru ir pt pd rh and au platinum group elements pge in the ejecta layer at the cretaceous tertiary boundary and melt rocks from east clearwater lake brent new quebec and lake wanapitei impact craters all believed to have been formed by the impact of chondritic projectiles the objective was to evaluate the pure application of pge interelement ratios e g ru ir for impactor identification to compare the utility of both melt rocks and ejecta deposits for this purpose and to expand the pge database for these craters the similarity of pge interelement ratios within the chondrite and iron meteorite groups precludes their use in the fine classification of impactors of this composition introduction of indigenous pge from country rock may contaminate the original meteoritic signature in addition the extent of pge fractionation during impact and melt sheet cooling and of post sedimentary pge remobilization must be evaluated in each case for east clearwater the pge chondrite normalized plot is flat indicating a chondritic impactor based on ru ir and cr ir ratios a cl or l chondrite is most likely however variation of ratios with depth in the core makes any fine classification of the projectile tenuous for the brent and new quebec craters the flat chondrite normalized pge plot indicates a chondritic impactor but finer classification is precluded by possible post syn depositional pge fractionation at lake wanapitei ni cr and cr ir ratios indicate a ll chondrite impactor wolf et al 1980 but sparse pge data for this chondrite type does not allow further classification of the projectile a flat chondrite normalized pge plot indicates a chondritic rather than an iron impactor at lake wanapitei the global ejecta layer at the cretaceous tertiary boundary has a chondritic pge signature but only when the integrated flux of pge on a global scale is considered individual sites may not show chondritic ratios due to differential pge remobilization resulting from local differences in depositional environment continental vs marine and to vaporization and or condensation pge fractionation occurring during impact the results for melt rocks and the k t ejecta deposit indicate that pge can be used for impactor identification but primarily in combination with other siderophile chalcophile meteoritic elements and their ratios