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

sedimentary signatures and processes during marine bolide impacts a review abstract studies of submarine impact craters resulting from impacts of comets or asteroids demonstrate that the presence of water and the physical properties of target rocks have a major influence on sedimentary processes associated with meteorite impacts this results in difference in sedimentary signature of bolide impacts in marine environments compared to subaerial impact craters in subaerial impacts the targets are commonly hard rocks frequently of igneous and or metamorphic origin whereas in submarine impacts the targets are mostly unconsolidated or poorly lithified sediments or sedimentary rocks with high volumes of pore water such differences result in variability in crater morphology and in sedimentary processes inside and outside the impact area impacts in shallow water marine neritic environments produced craters with low or absent rims and wide and shallow brims as characterize by both the montagnais on the scotian shelf the mjolnir in the barents sea 45 and 40 km in diameter respectively and the chesapeake bay 90 km in diameter lack of elevated rims is thought to be the result of current reworking and resurge of the water back into the excavated cavity as the water in the crater is vaporized during this process resurge gullies can be cut across the rim while mass and debris flows turbidites and other gravity deposits are produced as results of tsunami and crater wall and central high collapse during and after the crater excavation stage such deposits are found both within and outside the crater structure the only difference between gravity deposits triggered by an impact or other rare events such as earthquakes is the admixture of various melt particles and possible enrichments in iridium in the former impacts near the shelf edge may cause partial collapse of the continental margin as shown by the montagnais and chicxulub impacts some of the gravity and debris flows generated by margin collapse may be channelized with final deposits up to several hundred meters thick extending for hundreds of kilometers from the impact site other impact features such as shatter cones tektites spherules and ni spinels shocked quartz isotropication and partial melts are common to both submarine and subaerial impacts theoretical calculations of the destructive forces of mega tsunami waves triggered by meteorite impact in the ocean greatly exceed those based on geologic evidence a dearth of gigantic tsunami evidence for the chicxulub impact outside of the gulf of mexico where from theoretical modelling the maximum near bottom orbital velocity of water flow crossing the deep north atlantic basin should have been 1 m s could be the result of the mitigating effect of the bathymetry of surrounding area causing wave diffraction and interference calculated maximum horizontal orbital velocity near the seafloor at the shelf for the montagnais impact is 22 m s for a 200 m high wave 5 5 m s for a 50 m high wave at 500 km decreasing to 0 5 m s at 1000 km strong enough to scour the deep ocean bottom and produce distinct erosion surfaces and disconformities in marine sedimentary record however lack of cores across impact horizon prevents confirmation of occurrence of such bottom water flows

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

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

a high precision 40ar 39ar age for hydrated impact glass from the dellen impact sweden the dating of terrestrial impact craters and impact glasses that exhibit high degrees of mineralogical complexity can be problematic however if the maximum potential of the terrestrial impact crater record is to be realized accurate and precise ages for crater forming events are critical here we report a high precision 40ar 39ar age for the dellen impact structure sweden previous radio isotopic constraints show a wide variation in age as a result of poor sample characterization and analytical approach a detailed petrographical and mineralogical study provides a solid foundation for interpretation of step heating 40ar 39ar data culminating in a statistically robust age of 140 82 0 51 ma 2 full external precision for the dellen impact event for which data disfavour an inherited argon component primary hydration of the impact melt during cooling quenching and entrapment of molecular water promoted rapid loss of inherited 40ar from the impact melt of rhyolitic composition duplicate analyses of the water content and d of the glass give similar values for the former 1 9 0 1 mol mg 1 but unexpectedly low values for the latter 159 8 with scatter beyond the expected analytical reproducibility due to isotopic heterogeneity this study highlights that the 40ar 39ar technique is unrivalled in its ability to precisely and accurately date the products of hypervelocity collisional events less

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

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

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

the mid ordovician osmussaar breccia in estonia linked to the disruption of the l chondrite parent body in the asteroid belt the middle ordovician 466 ma osmussaar breccia situated along the northwestern coast of estonia is rich in angular chromite grains of extraterrestrial origin 13 grains kg 1 and shocked quartz the angularity of the chromite grains implies that they have not been transported or reworked to any large extent connoting that the brecciation is the result of a contemporary impactor either as a direct consequence of the impact or as a result of an earthquake triggered by the impact and thus is not as previously suggested redeposited material from the nearby similar to 70 m y older neugrund impact structure the chemical composition of the chromite indicates that the impactor was an ordinary chondrite of l type which concurs well with the hypothesis that the influx of large bodies to earth increased during this period due to the breakup of the l chondrite parent body this in turn gives support to the recent suggestion that abundant coeval mega breccias worldwide are impact triggered the presence of extraterrestrial chromite also strengthens the theory that physical pieces of a large celestial body can survive upon impact with earth less

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

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

impact generated hydrothermal systems on earth and mars it has long been suggested that hydrothermal systems might have provided habitats for the origin and evolution of early life on earth and possibly other planets such as mars in this contribution we show that most impact events that result in the formation of complex impact craters i e 2 4 and 5 10 km diameter on earth and mars respectively are potentially capable of generating a hydrothermal system consideration of the impact cratering record on earth suggests that the presence of an impact crater lake is critical for determining the longevity and size of the hydrothermal system we show that there are six main locations within and around impact craters on earth where impact generated hydrothermal deposits can form 1 crater fill impact melt rocks and melt bearing breccias 2 interior of central uplifts 3 outer margin of central uplifts 4 impact ejecta deposits 5 crater rim region and 6 post impact crater lake sediments we suggest that these six locations are applicable to mars as well evidence for impact generated hydrothermal alteration ranges from discrete vugs and veins to pervasive alteration depending on the setting and nature of the system a variety of hydrothermal minerals have been documented in terrestrial impact structures and these can be grouped into three broad categories 1 hydrothermally altered target rock assemblages 2 primary hydrothermal minerals precipitated from solutions and 3 secondary assemblages formed by the alteration of primary hydrothermal minerals target lithology and the origin of the hydrothermal fluids strongly influences the hydrothermal mineral assemblages formed in these post impact hydrothermal systems there is a growing body of evidence for impact generated hydrothermal activity on mars although further detailed studies using high resolution imagery and multispectral information are required such studies have only been done in detail for a handful of martian craters the best example so far is from toro crater marzo g a davila a f tornabene l l dohm j m fairen a g gross c kneissl t bishop j l roush t l mckay c p 2010 icarus 208 667 683 we also present new evidence for impact generated hydrothermal deposits within an unnamed 32 km diameter crater 350 km away from toro and within the larger holden crater synthesizing observations of impact craters on earth and mars we suggest that if there was life on mars early in its history then hydrothermal deposits associated with impact craters may provide the best and most numerous opportunities for finding preserved evidence for life on mars moreover hydrothermally altered and precipitated rocks can provide nutrients and habitats for life long after hydrothermal activity has ceased

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

upper permian magnetic stratigraphy of the lower beaufort group karoo basin abstract we carried out a magnetostratigraphic and geochronological study of late permian sediments in the karoo basin of the western cape province south africa a continuous 700 m thick section of deltaic sediments of the upper waterford formation uppermost ecca group and the fluvial sediments of the abrahamskraal formation lowermost beaufort group were sampled at the meter scale u pb dating of zircons from interbedded volcanic ash beds by ion microprobe shrimp provided absolute age constraints on the age of the sedimentary rocks paleomagnetic analysis reveals a partial overprint of the natural remanent magnetization nrm that is tentatively ascribed to the emplacement of the karoo large igneous province in the western cape region during the middle jurassic a stable component of the nrm was found at temperatures higher than 450 c and was interpreted as a characteristic remanent magnetization chrm acquired during deposition supported by a positive reversals test for this dual polarity chrm the virtual geomagnetic pole position for the waterford and abrahamskraal formations computed from the average chrm direction is in general agreement with the late permian directions for stable gondwana a significantly different average inclination and thus paleomagnetic pole position is obtained by correcting the inclination shallowing error by the elongation inclination method tauxe and kent 2004 the presence of both normal and reversed polarity zones indicate deposition after the end of the kiaman superchron moreover the polarity sequence is in good agreement with the illawarra sequence of steiner 2006 our results indicate a capitanian late guadalupian age for the abrahamskraal fm in agreement with the late permian age based on presence of glossopteris flora and dicynodont fauna traditionally assigned to the fluvial lacustrine sediments of the beaufort group however the u pb zircon ages of ca 264 268 ma suggest an age of 269 ma for the top of the kiaman superchron

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 late triassic age for the rochechouart impact structure france argon 40 argon 39 laser spot fusion dating of pseudotachylyte from the 25 km diameter roche chouart impact structure of western central france yields a matrix age of 214 c 8 ma 20 field evidence indicates that the pseudotachylyte was generated during the modification stage of the impact process prob ably during transient cavity collapse this new age is considerably older than the previously accepted age of 186 8 ma for this structure which was obtained from hydrothermally altered melt sheet samples the new age is in accordance with earlier paleomagnetic and fission track data which indicated that rochechouart was formed during the late triassic moreover the new determination is in agreement with the regional geolog ical setting and field relations of the structure the new age of 214 2 8 ma falls within the norian stage of the triassic system

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

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

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

40ar 39ar step heating of impact glasses from the nordlinger ries impact crater implications on excess argon in impact melts and tektites seven impact melts from various places in the nordlinger ries were dated by 40ar 39ar step heating the aim of these measurements was to increase the age data base for ries impact glasses directly from the ries crater because there is only one ar ar step heating spectrum available in the literature almost all samples display saddle shaped age spectra indicating the presence of excess argon in most ries glass samples most probably inherited argon from incompletely degassed melt and possibly also excess argon incorporated during cooling from adjacent phases in contrast moldavites usually contain no inherited argon probably due to their different formation process implying solidification during ballistic transport the plateau age of the only flat spectrum is 14 60 0 16 0 20 ma 2 while the total age of this sample is 14 86 0 20 0 22 ma isochron age 14 72 0 18 0 22 ma 2 proofing the chronological relationship of the ries impact and moldavites the total ages of the other samples range between 15 77 0 52 and 20 4 1 0 ma 2 implying approximately 2 40 excess 40ar compared to the nominal age of the ries crater in respective samples thus the age of 14 60 0 16 0 20 2 14 75 0 16 0 20 ma 2 calculated using the most recent suggestions for the k decay constants can be considered as reliable and is within uncertainties indistinguishable from the most recent compilation for the age of the moldavite tektites

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

the peerless structure daniels county northeastern montana a probable late ordovician impact structure the peerless structure is an 6 km diameter sub surface anomaly located in daniels county northeastern montana the disruption of sedimentary rock in the structure lies between 2624 to 2818 m below the topographic surface seismic mapping shows a typical complex crater composed of a central uplift 2 km across which shows structural uplift of up to 90 m an annular ring 4 km across and an outer rim 6 km in diameter the youngest disrupted rock unit is the upper ordovician red river formation which indicates that the structure was formed about 430450 ma ago

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

the origin of the brunflo fossil meteorite and extraterrestrial chromite in mid ordovician limestone from the garde quarry jamtland central sweden the brunflo fossil meteorite was found in the 1950s in mid ordovician marine limestone in the garde quarry in jamtland it originates from strata that are about 5 million years younger than similar limestone that more recently has yielded 50 fossil meteorites in the thorsberg quarry at kinnekulle 600 km to the south based primarily on the low tio2 content about 1 8 wt of its relict chromite the brunflo meteorite had been tentatively classified as an h chondrite the meteorite hence appears to be an anomaly in relation to the kinnekulle meteorites in which chromite composition chondrule mean diameter and oxygen isotopic composition all indicate an l chondritic origin reflecting an enhanced flux of meteorites to earth following the disruption of the l chondrite parent body 470 ma new chondrule size measurements for the brunflo meteorite indicate that it too is an l chondrite related to the same parent body breakup chromite maximum diameters and well defined chondrule structures further show that brunflo belongs to the l4 or l5 type chromites in recently fallen l4 chondrites commonly have low tio2 contents similar to the brunflo chromites adding support for brunflo being an l4 chondrite the limestone in the circle quarry is relatively rich about 0 45 grain kg 1 in sediment dispersed extraterrestrial chromite grains 63 mu m with chemical composition similar to those in l chondrites and the limestone 1 3 grains kg 1 at kinnekulle suggesting that the enhanced flux of l chondrites prevailed although somewhat diminished at the time when the brunflo meteorite fell less

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

rb sr dating of the impact melt from east clearwater quebec the rb sr composition of eight melt rock and three basement samples from the east clearwater impact structure quebec and two basement samples from the west clearwater structure has been determined the whole rock 87sr 86sr ratios of the melt samples 0 7167 0 7253 are within the range of the basement samples 0 7054 0 7322 and provide further evidence that the melt rocks represent shock melted basement a mineral isochron obtained from a relatively coarse grained melt rock gives an age of 287 26 ma for the crystallization age of the melt this is equivalent to k ar whole rock ages of 285 30 ma and 300 30 ma and a rb sr age of 266 15 ma obtained on melt rocks from west clearwater and confirms the previously generally held assumption that the east and west clearwater structures resulted from the simultaneous impact of two bodies at 285 300 ma ago

comment on the joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by paul r renne et al 2010

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

lake st martin impact structure manitoba canada magnetic anomalies and magnetizations a ground magnetic survey has further delineated the previously known intense 1000 nt negative anomaly at the centre of the lake st martin impact structure manitoba shocked gneiss in the central uplift has a strong reverse remanent magnetization whose direction is similar to that found after alternating field demagnetization of melt rock specimens from within the impact structure shocked gneiss is interpreted as the source of the central negative anomaly the direction of the remanence is consistent with a permo triassic age for the impact event the magnetization in the shocked gneiss is interpreted as a chemical remanent magnetization carried by hematite formed during an extensive alteration of mafic silicates occurring immediately after the impact

calcium aluminum rich inclusions in enstatite chondrites ii oxygen isotopes abstract ln situ ion microprobe analyses of spinel in refractory calcium aluminium rich inclusions cais from type 3 eh chondrites yield 160 rich compositions dl80 and 6170 about 40 0 spinel and feldspar in a cai from an el3 chondrite have significantly heavier isotopic compositions dl80 and 6170 about 5 0 a regression through the data results in a line with slope 1 o on a three isotope plot similar to isotopic results from unaltered minerals in cais from carbonaceous chondrites the existence of cais with 160 rich and 16o poor compositions in carbonaceous as well as enstatite chondrites indicates that cais formed in at least two temporally or spatially distinct oxygen reservoirs general similarities in oxygen isotopic compositions of cais from enstatite carbonaceous and ordinary chondrites indicate a common nebular mechanism or locale for the production of most cais introduction refractory calcium aluminium rich inclusions cais in chondritic meteorites preserve a record of high temperature processing in the early solar nebula including condensation evaporation and melting macpherson

excess argon incorporation in phengite of the mulhacen complex submicroscopic illitization and fluid ingress during late miocene extension in the betic zone south eastern spain abstract 40 ar 39 ar induction furnace and laser step heating of well crystallised post tectonic phengitic mica single grains from gneisses of the mulhacen complex with an early alpine tectonic fabric has resulted in 1 highly scattered integrated ages 2 an abnormally high atmospheric contamination and 3 often anomalously old apparent ages during early 39 ar release that is associated with a high 36 ar air and 37 ar ca contamination this low temperature excess argon 40 ar xs component is probably released from carbonate formed during slight alteration of the mica more than 50 of the samples yielded plateau ages ranging from 15 8 0 4 to 90 1 1 0 ma samples taken only a few metres apart may differ in age by as much as 50 ma a grain that was split over the basal plane yielded plateau ages for each half that differ by 12 the age variation on these different scales is explained by heterogeneous 40 ar xs incorporation during a period with a high transient partial argon pressure in the metamorphic fluid resulting from a late stage reheating event the very swift cooling of 50 100 c ma during exhumation of the mulhacen complex concomitant with late miocene extension may have prevented the equilibration of different 40 ar xs levels in the mica hrtem images of the oldest and youngest phengite specimens show that at least 20 of the lattice is affected by submicroscopic illitisation which is concentrated in several micrometer wide zones and veins that cross cut the basal cleavage these are made up of aggregates of 0 07 0 30 m thick crystallites of three illitic micas types which are chemically and structurally progressively closer to pure illite and occur in different textures the oldest specimen is affected most severely as the veins contain newly formed pseudo illite that does not inherit its crystallographic orientation and chemistry from the host mica in contrast to the youngest sample hrtem aem analyses revealed that phengite and the different illitic micas may be depleted in k the oldest sample is derived from a coarse grained augen gneiss with extensively developed hydraulic cracks which are lacking in the youngest sample a fine grained mylonitic gneiss fluid rock interaction and consequently sub microscopic illitization were therefore more intense in the coarser grained rocks growth of the illitic micas in equilibrium with a high partial 40 ar pressure could account for 40 ar xs incorporation in k vacancies and other lattice imperfections variation in illitisation and associated textural dissimilarities between the oldest and youngest mica permit the different levels of 40 ar xs incorporation that account for the observed age discordance the finding of 40 ar xs plateau ages despite the degassing of intimately intergrown micaceous minerals is interpreted by gas release invoked by in vacuo chemical and structural changes that led to a joint collapse of the lattices of phengite and the illitic micas between 800 c and 1000 c

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

dating impact craters palaeogeographic versus isotopic and stratigraphic methods a brief case study isotopic and stratigraphic ages of the 80 km diameter puchezh katunki russia 220 10 to 167 3 ma and the 20 km diameter obolon ukraine 215 25 to 169 7 ma impact structures are associated with significant age uncertainties as a case study reconstructions of the palaeogeography at the time of crater formation late triassic to middle jurassic based on recent palaeogeographic maps help further to constrain impact ages palaeogeographic studies suggest that puchezh katunki is older than 170 ma and that obolon is younger than 185 ma this also rules out that obolon formed during a 214 ma late triassic multiple impact event as recently discussed

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

a late mesoproterozoic 40ar 39ar age for a melt breccia from the keurusselka impact structure finland field investigations in the eroded central uplift of the 30 km keurusselka impact structure finland revealed a thin dark melt vein that intersects the autochthonous shatter cone bearing target rocks near the homestead of kirkkoranta close to the center of the impact structure the petrographic analysis of quartz in this melt breccia and the wall rock granite indicate weak shock metamorphic overprint not exceeding 8 10 gpa the mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia 40ar 39ar dating of dark and clast poor whole rock chips yielded five concordant late mesoproterozoic miniplateau ages and one plateau age of 1151 10 ma 11 ma 2 mswd 0 11 p 0 98 considered here as the statistically most robust age for the rock the new 40ar 39ar age is incompatible with 1 88 ga svecofennian tectonism and magmatism in south central finland and probably reflects the keurusselka impact followed by impact induced hydrothermal chloritization of the crater basement in keeping with the crosscutting relationships in the outcrop and the possible influence of postimpact alteration the late mesoproterozoic 40ar 39ar age of 1150 ma should be treated as a minimum age for the impact the new 40ar 39ar results are consistent with paleomagnetic results that suggested a similar age for keurusselka which is shown to be one of the oldest impact structures currently known in europe and worldwide

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

base of the kiaman its definition and global stratigraphic significance defining and dating the base of the kiaman the long period of reverse polarity that spans the time from the late carboniferous to the middle permian have proven difficult in an attempt to do this an integrated stratigraphic paleomagnetic and geochronologic study was undertaken in carboniferous rocks in the northern tamworth belt of the new england orogen eastern australia new paleomagnetic measurements at 145 horizons sites in the new england orogen of eastern australia provide an accurate record of the late namurian to westphalian geomagnetic field they have high unblocking temperatures satisfy tilt conglomerate and slump tests and show excellent agreement between a wide variety of igneous and sedimentary rock types indicating that magnetism was acquired at or soon after deposition magnetite and to a lesser degree hematite are the carriers of magnetization the record is contained in sequences of glaciogene sediments and arc derived volcano sedimentary units folded into broad structures in the late permian to triassic new isotopic dates for the volcanic units indicate ages from 321 to 306 ma late namurian to westphalian magnetizations are steeply inclined downward reversed polarity in the southern hemisphere except within the clifden formation of the rocky creek syncline where a change from normal to reverse polarity referred to as the wanganui reversal records the onset of the kiaman superchron integrating complex stratigraphic isotopic and paleomagnetic data we correlate the wanganui reversal to the top of n6 in the joggins section nova scotia canada and estimate that its age is between 318 and 316 ma further work may substantiate a younger normal zone in australia that would support a slightly younger age but within these limits these considerations indicate that the wanganui reversal occurs in the late namurian marsdenian and after the time of the mississippian pennsylvania boundary the mean direction of magnetization of the upper carboniferous australian sequences is declination d 195 3 inclination i 76 9 25 95 3 1 with a paleopole at 51 9 s 141 2 e 95 5 7 and a paleolatitude of 65 0 5 6 s p 0 05 this paleolatitude is consistent with the glacial origin of some of the sedimentary units

impact ejecta emplacement on terrestrial planets abstract impact cratering is one of the most fundamental processes responsible for shaping the surfaces of solid planetary bodies one of the principal characteristics of impact events is the formation and emplacement of ejecta deposits an understanding of which is critical for planetary exploration current models of ejecta emplacement however do not account for several important observations of ejecta deposits on the terrestrial planets in particular the presence of more than one layer of ejecta furthermore there is also no universal model for the origin and emplacement of ejecta on different planetary bodies we present a unifying working hypothesis for the origin and emplacement of ejecta on the terrestrial planets in which the ejecta are emplaced in a multi stage process the generation of the continuous ejecta blanket occurs during the excavation stage of cratering via the conventional ballistic sedimentation and radial flow model this is followed by the emplacement of more melt rich ground hugging flows the surface melt flow phase during the terminal stages of crater excavation and the modification stage of crater formation minor fallback occurs during the final stages of crater formation several factors will affect the final morphology and character of ejecta deposits the volatile content and cohesiveness of the uppermost target rocks will significantly affect the runout distance of the ballistically emplaced continuous ejecta blanket with impact angle also influencing the overall geometry of the deposits e g the production of the characteristic butterfly pattern seen in very oblique impacts ejecta deposited during the surface melt flow stage is influenced by several factors most importantly planetary gravity surface temperature and the physical properties of the target rocks topography and angle of impact play important roles in determining the final distribution of surface melt flow ejecta deposits with respect to the source crater this working hypothesis of ballistic sedimentation and surface melt flow provides a framework in which observations of ejecta at impact craters can be compared and placed in the context of the respective terrestrial planets

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

terrestrial impact structures impact structures are the dominant landform on planets that have retained portions of their earliest crust the present surface of the earth however has comparatively few recognized impact structures this is due to its relative youthfulness and the dynamic nature of the terrestrial geosphere both of which serve to obscure and remove the impact record although not generally viewed as an important terrestrial as opposed to planetary geologic process the role of impact in earth evolution is now receiving mounting consideration for example large scale impact events may have been responsible for such phenomena as the formation of the earth s moon and certain mass extinctions in the biologic record the importance of the terrestrial impact record is greater than the relatively small number of known structures would indicate impact is a highly transient high energy event it is inherently difficult to study through experimentation because of the problem of scale in addition sophisticated finite element code calculations of impact cratering are gen erally limited to relatively early time phenomena as a result of high com putational costs terrestrial impact structures provide the only ground truth against which computational and experimental results can be com pared these structures provide information on aspects of the third dimen sion the preand postimpact distribution of target lithologies and the nature of the lithologic and mineralogic changes produced by the passage of a shock wave they also provide data for cratering rate estimates and in some cases on the nature of the impacting body in this review emphasis is placed on the nature of terrestrial impact structures the criteria for their identification and their contribution to

high precision 40ar 39ar age of the janisjarvi impact structure russia abstract the 14 km diameter janisjarvi impact structure is located in svecofennian proterozoic terrain in the southeastern part of the baltic shield karelia russia previous radioisotopic dating attempts gave k ar and 40 ar 39 ar ages of 700 5 ma and 698 22 ma respectively with both results being difficult to interpret recent paleomagnetic results have challenged these ages and proposed instead ages of either 500 ma or 850 900 ma in order to better constrain the age of the janisjarvi impact structure we present new 40 ar 39 ar data for the janisjarvi impact melt rock we obtained five concordant isochron ages that yield a combined isochron age of 682 4 ma 2 with a mswd of 1 2 p 0 14 and 40 ar 36 ar intercept of 475 3 we suggest that this date indicates the age of the impact and therefore can be used in conjunction with existing paleomagnetic results to define the position of the baltica paleocontinent at that time argon isotopic results imply that melt homogenization was achieved at the hundred micrometer scale certainly because of the low silica content of the molten target rock that allows fast 40 ar diffusion in the melt however the large range of f 40 ar inherited 4 1 to 11 0 observed for seven grains shows that complete isotopic homogenization was not reached at the centimeter and perhaps the millimeter scale the f 40 ar inherited results are also in good agreement with previous rb and sr isotopic data

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

uppermost impact fallback layer in the bosumtwi crater ghana mineralogy geochemistry and comparison with ivory coast tektites in 2004 an international continental scientific drilling program icdp drilling project at the bosumtwi impact crater ghana 10 5 km in diameter 1 07 myr old was performed to study the sediments that fill the lake as well as the underlying impactites in one lb 05 of 16 cores drilled into the lake sediments the zone between the impact breccias and the post impact sediments was penetrated preserving the final fine grained impact fallback layer this 30 cm thick layer contains in the top 10 cm accretionary lapilli microtektite like glass spherules and shocked quartz grains glass particlesmostly of splash form less than 1 mm size make up the bulk of the grains 70 78 by number in the coarser size fraction 125 m of the top of the fallback layer about one third of all quartz grains in the uppermost part of the layer are shocked with planar deformation features pdfs almost half of these grains are highly shocked with 3 or more sets of pdfs k feldspar grains also occur and some show shock deformation the abundance of shocked quartz grains and the average shock level as indicated by the number of sets of pdfs for both quartz and k feldspar decrease with depth into the layer the well preserved glass spherules and fragments are chemically rather homogeneous within each particle and also show relatively small variations between the various particles on average the composition of the fallback spherules from core lb 5b is very similar to the composition of ivory coast tektites and microtektites with the exception of cao contents which are about 1 5 to 2 times higher in the fallback spherules this is a rare case in which the uppermost fallback layer and the transition to the post impact sediments has been preserved in an impact structure its presence indicates that the impactite sequence at bosumtwi is complete and that bosumtwi is a very well preserved impact crater

causes of extreme ridges that induce california droughts abstractcalifornia droughts are often caused by high amplitude and persistent ridges near and off the west coast of north america without apparent connections with enso here with a hierarchy of climate models it is demonstrated that extreme ridges in this region are associated with a continuum of zonal wavenumber 5 circumglobal teleconnection patterns that originate from midlatitude atmospheric internal dynamics although tropical diabatic heating anomalies are not essential to the formation and maintenance of these wave patterns certain persistent heating anomalies may double the probability of ridges with amplitudes in the 90th percentile occurring on interannual time scales those heating anomalies can be caused by either natural variability or possibly by climate change and they do not necessarily depend on enso the extreme ridges that occurred during the 2013 14 and 2014 15 winters could be examples of ridges produced by heating anomalies that are not associated with enso this mechanism could p

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

the lawn hill annulus an ordovician meteorite impact into water saturated dolomite the lawn hill impact structure lhis is located 250 km n of mt isa in nw queensland australia and is marked by a highly deformed dolomite annulus with an outer diameter of similar to 18 km overlying low metamorphic grade siltstone sandstone and shale along the ne margin of the georgina basin this study provides detailed field observations from sections of the lawn hill annulus and adjacent areas that demonstrate a clear link between the deformation of the dolomite and the lawn hill impact ar 40 ar 39 dating of impact related melt particles provides a time of impact in the ordovician 472 8 ma when the georgina basin was an active depocenter the timing and stratigraphic thickness of the dolomite sequence in the annulus suggest that there was possibly up to 300 m of additional sedimentary rocks on top of the currently exposed thorntonia limestone at the time of impact the exposed annulus is remarkably well preserved with preservation attributed to postimpact sedimentation the lhis has an atypical crater morphology with no central uplift the heterogeneous target materials at lawn hill were probably low strength porous and water saturated with all three properties affecting the crater morphology the water saturated nature of the carbonate unit at the time of impact is thought to have influenced the highly brecciated nature of the annulus and restricted melt production the impact timing raises the possibility that the lawn hill structure may be a member of a group of impacts resulting from an asteroid breakup that occurred in the mid ordovician 470 6 ma

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

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

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

on the significance of crater ages new ages for dellen sweden and araguainha brazil samples of a few grams in weight from the impact melt at the dellen crater sweden and the araguainha dome brazil were analyzed by rb sr and sm nd techniques for dellen pure labradorite orthopyroxene and glass separates from a clast free impact melt rock with holohyaline ground mass yield an internal rb sr isochron with 89 0 2 7 ma isr 0 83571 0 00007 2 compared to ar ar plateau ages of 102 3 2 ma and 109 6 2 0 ma published for total melt rocks various whole rock values not plotting on this rb sr isochron indicate primary variations in 87sr86sr over the melt sheet the age of the araguainha dome brazil is bracketed by the rb sr model age of 243 19 ma isr 0 7226 0 0005 for altered cordierite in the impact melt and by the internal rb sr isochron age of 449 9 ma isr 0 7140 0 0007 for an uplifted granite shock stage ib in melt rocks from both impact structures 143nd144nd was not homogenized during total rock melting sm nd data either scatter around the primary isochron araguainha or point to a partial resetting dellen this data array may be caused by tiny refractory ree rich inclusions in the main mineral phases which dominate the sm nd system of the impact melt rocks the significant difference between the new isochron age for dellen and ar ar ages show that the data base for analysis of periodicity in cratering record most probably lack precision and accuracy

measuring pore water oxygen of a high energy beach using buried probes abstract we conducted six field campaigns to investigate the spatial and temporal evolution of pore water oxygen content on a high energy sandy beach aquifer during several tidal cycles at different seasons we buried autonomous probes in intertidal sands to record dissolved oxygen saturation salinity temperature and water head at a 2 10 min frequency oxygen concentrations display significant changes both with time tide and seasons and space cross shore and vertical variations seawater circulation in tidal sands forms a saline pore water circulation cell with oxygen saturated pore water in the zone of seawater recharge oxygen depleted pore water in the lower beach is the result of in situ respiration processes that occur during seawater circulation oxygen depletion varies throughout the year and anoxic conditions are reached at the end of spring as planktonic organic matter becomes abundant in seawater and more organic matter is therefore supplied to pore water on a shorter time scale weeks to minutes oxygen variations are driven directly by physical forcing tidal amplitude affects the extent of the saltwater circulation cell and the associated location of the recharging and discharging zones of the beach the evolution of the water table level during the tidal cycle influences the circulation of pore water in the sand and ultimately the timing of oxygen variations during flood and ebb this first in situ study in a high energy sandy beach shows that the dynamics of pore water oxygen are governed by biogeochemical processes at the seasonal scale and by physical forcing at the time scale of minutes to a few days

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

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

chemical and ecological evolution of the miocene ries impact crater lake germany a reinterpretation based on the enkingen subo 18 drill core scientific drilling conducted at the inner slope of the miocene central ries impact crater recovered a partial section of crater lake sediments four sequences were recovered composed of suevite derived sandstones thin lignite seams bituminous shales and marlstones to claystones these flooding evaporation sequences reflect the impact of short term climatic fluctuations on a hydrologically closed basin the superimposed trend from sequences rich in bituminous shales in the lower parts of the section to sequences dominated by organic poor claystones and intercalated lignites in the upper parts of the section resembles that of the 300 m thick central crater basin succession which has previously been considered to reflect a climate controlled development from an alkaline saline lake to a freshwater lake with temporary coal swamps in the sediment core of enkingen however the change from bituminous shales to organic poor claystones with intercalated lignites is associated with a general increase in salinity as indicated by 1 palynomorphs 2 increase in 13c of the lipid biomarker archaeol bis o phytanylglycerol and 3 the occurrence of 13c enriched c20 c25 archaeol o phytanyl o sesterterpanylglycerol specific to halophilic archaea in addition the unidirectional trend in 87sr 86sr of carbonates declining from ratios of variscan basement rocks toward marine ratios indicates a change from 1 weathering of crystalline rocks and suevite to 2 ejected jurassic sediments bunte breccia in the catchment area as the major source of ion influx to the lake from that trend a change in lake water composition and a general increase in ion concentrations are inferred these new results can be applied to a reassessment of major parts of the lacustrine succession of the ries crater we use these data to propose a new hypothetical model for the chemical and ecological evolution of the ries crater lake 1 after the establishment of a stratified brackish eutrophic soda lake due to silicate weathering and evaporation the increasing influx of waters from the bunte breccia carbonate and authigenic silicate precipitation led to a mesotrophic halite lake with marine like ion ratios and concentrations 2 further increase in ions among them mg2 and sr2 resulted in hypersaline conditions with gypsum precipitation low primary production and phreatic sr rich dolomitization in marginal carbonates 3 the final sudden change to oligotrophic freshwater conditions is explained by the formation of an outlet late in the lake history we conclude that the chemical and ecological evolution of the ries lake therefore appears to have been mainly controlled by the weathering history of the catchment area with climate fluctuations causing superimposed cycles similarly changes in terrestrial palynomorph associations may at least partly reflect a change in soil types in the catchment area from fertile moist soils on suevite to dry karst soils and soils on bunte breccia these interpretations imply that the initial suevite blanket of the ries crater was much more continuous and widespread than previously assumed

evaluation of the 87rb decay constant by age comparison against the u pb system abstract the rb sr decay system is one of the most widely used geochronometers for obtaining ages and cooling rates of terrestrial magmatic metamorphic and hydrothermal events it has also been extensively applied to date extraterrestrial early solar system events the accuracy of rb sr ages however strongly depends on the accuracy of the 87 rb decay constant 87 rb we determined 87 rb relative to the decay constants of 235 u and 238 u by comparing rb sr ages of minerals with u pb ages obtained from the same intrusion comparison of u pb emplacement ages with high precision rb sr mineral ages from three rapidly cooled igneous rocks covering an age range of ca 2 5 ga yields an unweighted mean 87 rb of 1 393 0 004 10 11 yr 1 i e 0 3 corresponding to a half life of 49 76 10 9 years because this decay constant is 2 lower than the presently recommended one many previously published ages are 2 too young and the resulting geologic interpretations may need revision

the winneshiek biota exceptionally well preserved fossils in a middle ordovician impact crater the winneshiek shale middle ordovician darriwilian was deposited in a meteorite crater the decorah impact structure in ne iowa this crater is 5 6 km in diameter and penetrates cambrian and ordovician cratonic strata it was probably situated close to land in an embayment connected to the epicontinental sea typical shelly marine taxa are absent the konservat lagerstatte within the winneshiek shale is important because it represents an interval when exceptional preservation is rare the biota includes the earliest eurypterid a giant form as well as a new basal chelicerate and the earliest ceratiocarid phyllocarid conodonts some of giant size occur as bedding plane assemblages bromalites and rarer elements including a linguloid brachiopod and a probable jawless fish are also present similar fossils occur in the coeval ames impact structure in oklahoma demonstrating that meteorite craters represent a novel and under recognized setting for konservat lagerstatten

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