a high precision 40 ar 39 ar age for the nordlinger ries impact crater germany and implications for the accurate dating of terrestrial impact events abstract 40ar 39ar dating of specimens of moldavite the formation of which is linked to the ries impact in southern germany with a latest generation argus vi multi collector mass spectrometer yielded three fully concordant plateau ages with a weighted mean age of 14 808 0 021 ma 0 038 ma including all external uncertainties 2 mswd 0 40 p 0 67 this new best estimate age for the nordlinger ries is in general agreement with previous 40ar 39ar results for moldavites but constitutes a significantly improved precision with respect to the formation age of the distal ries produced tektites separates of impact glass from proximal ries ejecta suevite glass from three different surface outcrops and partially melted feldspar particles from impact melt rock of the subo 18 enkingen drill core failed to produce meaningful ages these glasses show evidence for excess 40ar introduction which may have been incurred during interaction with hydrothermal fluids only partially reset 40ar 39ar ages could be determined for the feldspathic melt separates from the enkingen core the new 40ar 39ar results for the ries impact structure constrain the duration of crater cooling during the prevailing hydrothermal activity to locally at least 60 kyr with respect to the dating of terrestrial impact events this paper briefly discusses a number of potential issues and effects that may be the cause for seemingly precise but on a kyr scale inaccurate impact ages

ejecta deposits of bakhuysen crater mars abstract bakhuysen crater is a late noachian to early hesperian age 150 km diameter impact structure located in the heavily cratered southern highlands of mars it is one of the largest craters on mars with well preserved ejecta deposits impact ejecta deposits allow an understanding of crustal composition volatile content and evolution of terrestrial planets yet the emplacement of ejecta deposits on bodies throughout the solar system is still debated here we present the first detailed geomorphologic map of bakhuysen crater we distinguish two distinct ejecta units 1 a largely hummocky ejecta unit be1 that comprises the continuous ejecta blanket and 2 an uppermost discontinuous unit be2 with a distinct ponded morphology pitted texture and previously unreported lobate and flow deposits pitted materials are present in the crater interior deposits as well as the ejecta bakhuysen is the largest martian crater thus far identified with crater related pitted material which has been suggested previously to represent volatile rich primary impact melt bearing deposits unit be1 is interpreted as the continuous ejecta blanket emplaced through ballistic sedimentation and radial flow unit be2 shares many attributes with impact melt rich ejecta seen on other planetary bodies e g earth moon and mercury and is interpreted as having been emplaced as ground hugging flows during the later modification stage of crater formation channels are observed as having emanated from bakhuysen impactites and represents a clear example of the role of volatiles in the impact cratering process on mars bakhuysen ejecta deposits and its associated features suggests that if indeed a substantial volatile source is sampled by martian impact events then the formation of ejecta associated channels on mars are likely genetically linked to the formation of pitted materials where present the preservation and exposure of the impact deposits of bakhuysen present a unique opportunity to study these features and their relationship to the impact process on mars

impactoclastic density current emplacement of terrestrial meteorite impact ejecta and the formation of dust pellets and accretionary lapilli evidence from stac fada scotland abstractwe present an impactoclastic density current model for terrestrial meteorite impact ejecta emplacement that is based on a hitherto unreported threefold sequence of lithofacies in the recently identified stac fada impact ejecta outflow blanket in scotland massive suevite division a grades seamlessly up into massive suevite containing whole and broken accretionary lapilli division b and this is overlain by a thin layer of clast supported dust pellets division c the sequence closely resembles sequences in accretionary lapilli bearing ignimbrites emplaced by pyroclastic density currents at volcanoes and we deduce similar emplacement mechanisms we propose that 99 of the thickness of the stac fada ejecta blanket as seen at outcrop was deposited from a single depletive spatially decelerating granular fluid based density current that rapidly waxed and then waned a previously unreported thin layer of pellets division c records direct fallout from a residual atmospheric dust plume we pro

post impact alteration of surficial suevites in ries crater germany hydrothermal modification or weathering processes alteration of surficial suevites at ries crater germany was studied by means of x ray diffraction and scanning electron microscopy here we discuss the origin of hydrous silicate clay phases in these suevites that have been previously interpreted as resulting from post impact hydrothermal processes the results of this study indicate that the dominant alteration phases are dioctahedral al fe montmorillonite and halloysite which are typical low temperature clay minerals we suggest that the surficial suevites are not altered by hydrothermal processes and that alteration occurred by low temperature subsurface weathering processes if the surficial suevites were indeed hydrothermally modified during the early stages of post impact cooling then the alteration was of limited character and is completely masked by later weathering

dust and ice deposition in the martian geologic record abstract the polar layered deposits of mars demonstrate that thick accumulations of dust and ice deposits can develop on the planet if environmental conditions are favorable these deposits appear to be hundreds of millions of years old and other deposits of similar size but of greater age in nonpolar regions may have formed by similar processes possible relict dust deposits include from oldest to youngest noachian intercrater materials including arabia mantle deposits noachian to early hesperian south polar pitted deposits early hesperian hellas and argyre basin deposits late hesperian electris deposits and the amazonian medusae fossae formation these deposits typically are hundreds of meters to a couple kilometers thick and cover upward of a million or more square kilometers the apparent persistence of dust sedimentation at the south pole back to the early hesperian or earlier and the early growth of tharsis during the late noachian and perhaps earlier indicates that extensive polar wandering is unlikely following the middle noachian a scenario for the overall history of dust and perhaps ice deposition on mars includes widespread voluminous accumulations perhaps planetwide during the noachian as impacts volcanism and surface processes generated large amounts of dust the arabia deposits may have formed as ice availability and dust accumulation waned during the early hesperian thick dust sedimentation became restricted to the south pole and the deep hellas and argyre basins the north polar sedimentary record prior to the amazonian is largely obscured deposits at electris and medusae fossae may have resulted from local sources of fine grained material perhaps volcanic eruptions

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

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

lateral transport of suevite inferred from 3d shape fabric analysis evidence from the ries impact crater germany the transport mechanism of suevite parti cles during impact cratering is poorly under stood and was studied at the 15 ma ries crater in southern germany two emplace ment modes of suevite deposits are generally discussed 1 fallback of plume material into the crater and its periphery upon collapse of an ejecta plume and 2 horizontal transport of ejected material akin to emplacement of pyroclastic deposits erupting from volcanic centers in order to differentiate between the two emplacement modes of suevite deposition we analyzed the shape fabrics of suevite components from two localities out side the ries crater by fishape fabric ellipsoids to measured shape fabric ellipses and by applying high resolution x ray computed tomography to analyze the three dimensional shape and orientation of the suevite particles we show that the preferred orientation of long axes of elongate particles is disposed either radially or concentrically with respect to the crater center our obser vations indicate that suevite material was not only derived from an ejecta plume but was transported by lateral fl ow under viscous conditions upon fallback this fl ow regime resembles that known from pyroclastic fl ows

chicxulub testing for post impact hydrothermal input into the tertiary ocean studies of large terrestrial impact craters indicate that post impact hydrothermal activity is a likely consequence of the crustal deformation and heating induced by such events in the case of the chicxulub basin where marine conditions were re established soon after the impact significant fluxing of seawater through the crust and hydrothermal venting into the water column might be anticipated we have carried out geochemical analyses of tertiary carbonate sediments within the yaxcopoil 1 yax 1 drill hole to test for evidence of such post impact hydrothermal circulation hydrothermal activity is most likely to be found close to thick layers of melt rock inside the collapsed transient cavity and it is estimated that yax 1 is located 20 km outside this cavity consequently the most likely signature of hydrothermal venting into the water column would be geochemical anomalies attributable to fallout of suspended particulate matter from a submarine hydrothermal plume samples of tertiary biomicrites from depths of 794 01 to 777 02 m have high concentrations of manganese iron phosphorous titanium and aluminium and low iron manganese ratios relative to samples from higher in the stratigraphic succession this geochemical anomaly decreases fairly systematically between 793 13 m and 777 02 m above which an abrupt change in geochemistry is observed a mass balance calculation suggests that the anomaly is unlikely to be the result of a decreasing detrital input to the carbonate sediments and the nature of the element enrichments is consistent with expectations for fallout from a distal hydrothermal plume we conclude that a postimpact hydrothermal system did develop at chicxulub which led to the expulsion of hydrothermal fluids into the tertiary water column preliminary biostratigraphic and magnetostratigraphic dating on yax 1 core suggest that this hydrothermal activity lasted for at least 300 ka

experimental shock lithification of water bearing rock powders samples of powdered 150 m andesitic basalt to which had been added 0 20 wt water were subjected to shock pressures of 294 490 kb the dry samples are completely lithified by the shock with plagioclase crystals converted to diaplectic glass and partially bonded at the edges the dominant effect of added water is a marked decrease in shock lithification with samples containing 15 20 wt water remaining particulate after shocks of 296 430 kb a secondary effect of water contents below 15 wt is enhancement of the production of vesicular melt glass no glass was observed to have been formed in the totally particulate samples which contained 15 20 wt added water the instantaneous hydration and clay production predicted for impacts into water bearing target materials were not observed in the present experiments

geochemical studies of the subo 18 enkingen drill core and other impact breccias from the ries crater germany suevite and melt breccia compositions in the boreholes enkingen and polsingen are compared with compositions of suevites from other ries boreholes and surface locations and discussed in terms of implications for impact breccia genesis no significant differences in average chemical compositions for the various drill cores or surface samples are noted compositions of suevite and melt breccia from southern and northeastern sectors of the ries crater do not significantly differ this is in stark contrast to the published variations between within crater and out of crater suevites from northern and southern sectors of the bosumtwi impact structure ghana locally occurring alteration overprint on drill cores especially strong on the carbonate impregnated suevite specimens of the enkingen borehole does affect the average compositions overall the composition of the analyzed impact breccias from ries are characterized by very little macroscopically or microscopically recognized sediment clast component the clast populations of suevite and impact melt breccia are dominated consistently by granitic and intermediate granitoid components the polsingen breccia is significantly enriched in a dioritic clast component overall chemical compositions are of intermediate composition as well with dioritic granodioritic silica contents and relatively small contributions from mafic target components selected suevite samples from the enkingen core have elevated ni co cr and ir contents compared with previously analyzed suevites from the ries crater which suggest a small meteoritic component platinum group element pge concentrations for some of the enriched samples indicate somewhat elevated concentrations and near chondritic ratios of the most immobile pge consistent with an extraterrestrial contribution of 0 1 0 2 chondrite equivalent

ries bunte breccia revisited indications for the presence of water in itzing and otting drill cores and implications for the emplacement process we reassessed two drill cores of the bunte breccia deposits of the ries crater germany the objectives of our study were the documentation of evidence for water in the bunte breccia the evaluation of how that water influenced the emplacement processes and from which preimpact water reservoir it was derived the bunte breccia in both cores can be structured into a basal layer composed mainly of local substrate material overlain by texturally and compositionally diverse crater derived breccia units the basal layer is composed of the youngest sediments tertiary clays and upper jurassic limestone and has a razor sharp boundary to the upper breccia units which are composed of older rocks of upper jurassic to upper triassic age sparse material exchange occurred between the basal layer and the rest of the bunte breccia fluids predominantly came from the tertiary and the upper triassic sandstone formation in the basal layer tertiary clays were subjected to intense ductile deformation indicating saturation with water this suggests that water was mixed into the matrix creating a fluidized basal layer with a strong shear localization in the upper units upper triassic sandstones are intensely deformed by granular flow the texture requires that the rocks were disaggregated into granular sand vaporization of pore water probably aided fragmentation of these rocks in the otting core hot suevite t 600 c covered the bunte breccia shortly after its emplacement vertically oriented gas escape pipes in suevite partly emanate directly at the contact to the bunte breccia they indicate that the bunte breccia contained a substantial amount of water in the upper part that was vaporized and escaped through these vents

multiple zircon growth within early archaean tonalitic gneiss from the ancient gneiss complex swaziland abstract u pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the ancient gneiss complex the oldest episode at3644 4ma 2 produced the dominant type of zircon characteristically purplish brown and massive in texture this probably constitutes unaltered zircon precipitated from the original magma recrystallization accompanied and obscured by early pb loss took place within the oldest grains at3504 6ma and3433 8ma whole new grains grew at these times also we interpret the post 3644 ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss in addition many grains are visibly overgrown by two layers of younger zircon of different colour and texture dated at2986 20ma and2867 30ma euhedral finely zoned whole grains having the 2986 ma age are present also evidently contributed by very thin felsic veins associated with the nearby lochiel granite the age of3644 4ma combined with precise zircon u pb dating of volcanics from the onverwacht group reported elsewhere demonstrates that at least part of the ancient gneiss complex is older than the barberton greenstone belt

ries crater and suevite revisited observations and modeling part ii modeling we present the results of numerical modeling of the formation of the ries crater utilizing the two hydrocodes sova and isale these standard models allow us to reproduce crater shape size and morphology and composition and extension of the continuous ejecta blanket some of these results cannot however be readily reconciled with observations the impact plume above the crater consists mainly of molten and vaporized sedimentary rocks containing very little material in comparison with the ejecta curtain at the end of the modification stage the crater floor is covered by a thick layer of impact melt with a total volume of 6 11 km3 the thickness of true fallback material from the plume inside the crater does not exceed a couple of meters ejecta from all stratigraphic units of the target are transported ballistically no separation of sedimentary and crystalline rocks as observed between suevites and bunte breccia at ries is noted we also present numerical results quantifying the existing geological hypotheses of ries ejecta emplacement from an impact plume by melt flow or by a pyroclastic density current the results show that none of these mechanisms is consistent with physical constraints and or observations finally we suggest a new hypothesis of suevite formation and emplacement by postimpact interaction of hot impact melt with water or volatile rich sedimentary rocks

distribution petrography and shock metamorphism of the ejecta of the ries crater in germany a review abstract the lithologies and distribution of the various ejecta formations from the ries crater germany are described they may be subdivided into 1 low shock level 10 gpa represented by polymict crystalline breccias fall out suevites crater suevite and tektites moldavites the ejecta from the mesozoic sedimentary column do not show effects of shock pressures exceeding 10 gpa and only basement gneisses were shock melted to yield suevite glasses it is therefore concluded that the maximum impact energy was transferred from the impactor to the target rocks at a depth of at least 1000 m below ground level where a complex of gneisses underlies shallower granitic intrusions the estimated depth of the transient crater was about 2700 m and its diameter was 12 13 km bunte breccia megablocks of sedimentary rocks and crystalline breccias and megablocks were ejected in ballistic trajectories and form the earliest ejecta deposits extending up to a radius of 22 km fall out suevites are understood to have been lofted from the transient crater by an expanding plume of vaporized rocks and deposited outside the crater as fluidized turbulent masses on top of the earlier ejecta in contrast the crater suevite never left the crater cavity it represents the main mass of suevite moldavites are understood to have been formed during the first encounter of the impactor with the ground by condensation from vaporized and ionizised tertiary sands covering the surface of the target area

hematite pyroxene and phyllosilicates on mars implications from oxidized impact melt rocks from manicouagan crater quebec canada visible and near ir reflectivity mossbauer and x ray diffraction data were obtained on powders of impact melt rock from the manicouagan impact crater located in quebec canada the iron mineralogy is dominated by pyroxene for the least oxidized samples and by hematite for the most oxidized samples phyllosilicate smectite contents up to 15 wt were found in some heavily oxidized samples nanophase hematite and or paramagnetic ferric iron is observed in all samples no hydrous ferric oxides e g goethite lepidocrocite and ferrihydrite were detected which implies the alteration occurred above 250 c oxidative alteration is thought to have occurred predominantly during late stage crystallization and subsolidus cooling of the impact melt by invasion of oxidizing vapors and or solutions while the impact melt rocks were still hot the near ir band minimum correlated with the extent of aleration fe3 fetot and ranged from 1000 nm high ca pyroxene to 850 nm bulk well crystalline hematite for least and most oxidized samples respectively intermediate band positions 900 920 nm are attributed to low ca pyroxene and or a composite band from hematite pyroxene assemblages manicouagan data are consistent with previous assignments of hematite and pyroxene to the 850 and 1000 nm bands observed in martian reflectivity spectra manicouagan data also show that possible assignments for intermediate band positions 900 920 nm in martian spectra are pyroxene and or hematite pyroxene assemblages by analogy with impact melt sheets and in agreement with observables for mars oxidative alteration of martian impact melt sheets above 250 c and subsequent erosion could produce rocks and soils with variable proportions of hematite both bulk and nanophase pyroxene and phyllosilicates as iron bearing mineralogies if this process is dominant these phases on mars were formed rapidly at relatively high temperatures on a sporadic basis throughout the history of the planet the manicouagan samples also show that this mineralogical diversity can be accomplished at constant chemical composition which is also indicated for mars from analyses of soil at the two viking landing sites

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

impact induced hydrothermal activity within the haughton impact structure arctic canada generation of a transient warm wet oasis field studies and analytical scanning electron microscopy indicate that a hydrothermal system was created by the interaction of water with hot impact generated rocks following formation of the 24 km diameter 23 ma haughton impact structure hydrothermal alteration is recognized in two settings within polymict impact breccias overlying the central portion of the structure and within localized pipes in impact generated concentric fault systems the intra breccia alteration comprises three varieties of cavity and fracture filling a sulfide with carbonate b sulfate and c carbonate these are accompanied by subordinate celestite barite fluorite quartz and marcasite selenite is also developed particularly in the lower levels of the impact breccia sheet the fault related hydrothermal alteration occurs in 1 7 m diameter subvertical pipes that are exposed for lengths of up 20 m the pipes are defined by a monomict quartz carbonate breccia showing pronounced fe hydroxide alteration associated sulfides include marcasite pyrite and chalcopyrite we propose three distinct stages in the evolution of the hydrothermal system 1 early stage 200 c with the precipitation of quartz vapor phase dominated 2 main stage 200 100 c with the development of a two phase vapor plus liquid zone leading to calcite celestite barite marcasite and fluorite precipitation and 3 late stage 100 c with selenite and fibroferrite development through liquid phase dominated precipitation we estimate that it took several tens of thousands of years to cool below 50 c following impact during this time haughton supported a 14 km diameter crater lake and subsurface water system providing a warmer wetter niche relative to the surrounding terrain the results reveal how understanding the internal structure of impact craters is necessary in order to determine their plumbing and cooling systems

evidence for a spatially extensive hydrothermal system at the ries impact structure germany the 15 ma 26 km diameter ries impact structure in south central germany was one of the first terrestrial impact structures where evidence of impact associated hydrothermal alteration was recognized previous studies suggested that pervasive high temperature hydrothermal activity was restricted to the area within the inner ring i e the crater fill impactite units here we present mineralogical evidence for localized hydrothermal activity in the ejecta beyond the crater rim in two previously unstudied settings a pervasively altered lens of suevite ejecta directly overlying the bunte breccia at the aumuhle quarry and suevite ejecta at depth overlain by 20 m of lacustrine sediments sampled by the wornitzostheim 1965 drill core a comprehensive set of x ray diffraction analyses indicates five distinct alteration regimes 1 surficial ambient weathering characterized by smectite and a minor illitic component 2 locally restricted hydrothermal activity characterized by an illitic component and minor smectite 3 hydrothermal activity at depth characterized by smectite a minor illitic component and calcite 4 hydrothermal activity at depth characterized by smectite a minor illitic component calcite zeolites and clinochlore and 5 pervasive hydrothermal activity at depth characterized by smectite a minor illitic component and minor clinochlore these data spatially extend the ries postimpact hydrothermal system suggesting a much more extensive complex and dynamic system than previously thought constraining the mineralogical alteration regimes at the ries impact structure may also further our understanding of impact associated phyllosilicate formation on mars with implications for climate models and habitability

paleomagnetism of impact breccias from the chicxulub crater implications for ejecta emplacement and hydrothermal processes abstract results of a paleomagnetic study of 89 samples from impact breccias from the chicxulub crater are presented and used to investigate on ejecta emplacement hydrothermal and alteration processes the impactite sequence sampled in cores from yaxcopoil 1 borehole is 100 m thick and formed by six subunits characterized by petrographic scan images geochemical and textural analyses which point to different emplacement modes impact occurred within reverse polarity chron c29r therefore magnetization acquired at crater formation and shortly thereafter will present reverse polarity which is supported by paleomagnetic measurements on melt samples from yucatan 6 borehole results after step wise thermal and alternating field demagnetization document a pattern of 23 upward and 29 downward inclinations at yaxcopoil 1 around 33 and 38 respectively measurements of magnetic hysteresis and variation of low field susceptibility with temperature suggest low ti titanomagnetites and magnetite as main magnetic carriers petrographic observations indicate the presence of fine grained magnetite hematite and fe oxyhydroxides related to hydrothermal alteration processes curie points are in the range of 520 580 c the wide range of unblocking temperature spectra points to variable grain sizes of a magnetic phase that unblocks around 580 c hysteresis ratio plots indicate most samples fall in the pseudo single domain field in some samples wasp waist constrained hysteresis loops suggest magnetite and hematite analyses of vector plots and coercivity and unblocking temperature spectra do not show apparent differences within and between subunits subunits are characterized by distinct textural and compositional differences in size type and relative abundance of clasts and melt rich or carbonate rich matrix types bulk properties vary with composition and clast contents particularly within the middle suevite and brecciated melt rock subunits that show higher values while low values characterize the lower suevite carbonate rich with rare basement clasts subunit magnetization acquisition mechanisms are discussed with reference to impactite characteristics and may relate to remagnetization from hydrothermal and post impact alteration processes

mixed hydrothermal fluids and the origin of the martian soil the martian soil consists mainly of weathered basaltic rock and a salt component enriched in volatile and mobile elements primarily sulfur chlorine sodium and potassium since the discovery of this mobile element component by the viking x ray fluorescence spectrometer volcanic aerosols have been considered a likely source for these enrichments alternative sources of the mobile element component are fluids supplied to the surface by hydrothermal processes related to impact craters and volcanism we show that a mixed hydrothermal fluid model can successfully explain the composition of the mobile element component of the soil this model is constructed using terrestrial analogs constrained by the viking and pathfinder measurements of the martian soil chemistry the potential hydrothermal fluid contributions to the soil are tightly constrained because the relative abundances of the mobile elements in hydrothermal fluids are nearly constant the two end member types of hydrothermal fluids considered in the model are the neutral chloride and the acid sulfate type the neutral chloride fluids have high cl and na contents and a significant amount of k but their s abundance relative to cl s cl ratio is too low to match the martian soil however acid sulfate hydrothermal systems characterized by substantial vapor transport and high s cl ratios are likely on mars due to the low abundance of water indeed as the availability of water declines in a hydrothermal system the neutral chloride system can evolve to a vapor dominated acid sulfate system a combination of the two types of fluids can quantitatively explain the s and cl abundance in the soil and provide reasonable abundances of other mobile elements including na k and br without predicting an excess of any element measured in the soil however the hydrothermal fluids are unlikely to be involved in the formation of the magnetic minerals in the soil because the fe contents of the fluids are too low although the acid sulfate fluids have fe concentrations a factor of 100 larger than the neutral chloride fluids the acid sulfate fluids would contribute less than 1 of the total fe in the soil the volcanic aerosol models are poorly constrained in contrast to the hydrothermal fluid models because in volcanic aerosols the relative abundances of the mobile elements are highly variable therefore while volcanic aerosols can explain the mobile element component of the soil significant contributions from other sources cannot be excluded determining the relative contributions of hydrothermal components compared to volcanic aerosols will require measurements in the martian soil of trace elements that provide signatures of the hydrothermal fluids such as high lithium contents that are associated with neutral chloride fluids

hyperspectral imaging of drill core from the steen river impact structure canada implications for hydrothermal activity and formation of suevite like breccias

the nature of the groundmass of surficial suevite from the ries impact structure germany and constraints on its origin surficial suevites from the ries impact structure have been investigated in the field and using optical and analytical scanning electron microscopy the groundmass of these suevites comprises calcite clay minerals impact melt glass crystallites plagioclase garnet and pyroxene francolite and ba phillipsite the latter zeolite is a secondary phase abundant textures have been observed intricate flow textures between the various groundmass phases globules of each phase in the other phases spheroids of pyrrhotite in calcite the budding off of clay globules into silicate glass and or calcite euhedral overgrowths of francolite on apatite clasts and quench textured crystallites in the groundmass groundmass forming calcite displays higher feo mno and sio2 contents than limestone target material the composition of suevite clay minerals is highly variable and not always consistent with montmorillonite three types of glasses are distinguished in the groundmass type 1 glasses are sio2 rich and are clearly derived from sandstones in the sedimentary cover while the protoliths of the other two glass types remains unclear analytical data and micro textures indicate that the calcite silicate glass francolite and clay minerals of the groundmass of the ries suevites represent a series of impact generated melts that were molten at the time of and after deposition on cooling plagioclase pyroxene and garnet crystallized from the groundmass these results are at variance with the current traditional descriptive definition of suevite given that ries is the original type occurrence of suevite some modification to the traditional definition may be in order as the results of this study are most consistent with the groundmass of ries surficial suevites representing a mix of several types of impact generated melts we suggest that a possible origin for these suevites is as some form of impact melt flow s that emanated from different regions of the evolving crater

a case study of impact induced hydrothermal activity the haughton impact structure devon island canadian high arctic the well preserved state and excellent exposure at the 39 ma haughton impact structure 23 km in diameter allows a clearer picture to be made of the nature and distribution of hydrothermal deposits within mid size complex impact craters a moderate to low temperature hydrothermal system was generated at haughton by the interaction of groundwaters with the hot impact melt breccias that filled the interior of the crater four distinct settings and styles of hydrothermal mineralization are recognized at haughton a vugs and veins within the impact melt breccias with an increase in intensity of alteration towards the base b cementation of brecciated lithologies in the interior of the central uplift c intense veining around the heavily faulted and fractured outer margin of the central uplift and d hydrothermal pipe structures or gossans and mineralization along fault surfaces around the faulted crater rim each setting is associated with a different suite of hydrothermal minerals that were deposited at different stages in the development of the hydrothermal system minor early quartz precipitation in the impact melt breccias was followed by the deposition of calcite and marcasite within cavities and fractures plus minor celestite barite and fluorite this occurred at temperatures of at least 200 c and down to 100 120 c hydrothermal circulation through the faulted crater rim with the deposition of calcite quartz marcasite and pyrite occurred at similar temperatures quartz mineralization within breccias of the interior of the central uplift occurred in two distinct episodes 250 down to 90 c and 80 c appreciation of the structural features of impact craters has proven to be key to understanding the distribution of hydrothermal deposits at haughton

ries crater and suevite revisited observations and modeling part i observations we report results of an interdisciplinary project devoted to the 26 km diameter ries crater and to the genesis of suevite recent laboratory analyses of crater suevite occurring within the central crater basin and of outer suevite on top of the continuous ejecta blanket as well as data accumulated during the past 50 years are interpreted within the boundary conditions imposed by a comprehensive new effort to model the crater formation and its ejecta deposits by computer code calculations artemieva et al 2013 the properties of suevite are considered on all scales from megascopic to submicroscopic in the context of its geological setting in a new approach we reconstruct the minimum maximum volumes of all allochthonous impact formations 108 116 km3 of suevite 14 22 km3 and the total volume of impact melt 4 9 8 0 km3 produced by the ries impact event prior to erosion these volumes are reasonably compatible with corresponding values obtained by numerical modeling taking all data on modal composition texture chemistry and shock metamorphism of suevite and the results of modeling into account we arrive at a new empirical model implying five main consecutive phases of crater formation and ejecta emplacement numerical modeling indicates that only a very small fraction of suevite can be derived from the primary ejecta plume which is possibly represented by the fine grained basal layer of outer suevite the main mass of suevite was deposited from a secondary plume induced by an explosive reaction fuel coolant interaction of impact melt with water and volatile rich sedimentary rocks within a clast laden temporary melt pool both melt pool and plume appear to be heterogeneous in space and time outer suevite appears to be derived from an early formed melt rich and clast poor plume region rich in strongly shocked components melt clasts and originating from an upper more marginal zone of the melt pool crater suevite is obviously deposited from later formed clast rich and melt poor plumes dominated by unshocked and weakly shocked clasts and derived from a deeper central zone of the melt pool genetically we distinguish between primary suevite which includes dike suevite the lower sublayer of crater suevite and possibly a basal layer of outer suevite and secondary suevite represented by the massive upper sublayer of crater suevite and the main mass of outer suevite

fluidization and hydrothermal alteration of the suevite deposit at the ries crater west germany and implications for mars the formation cooling and alteration of the impact ejecta deposits at the ries crater are an analog for impact processes and the formation of regoliths and soils on mars we have studied the suevite deposit an impact melt bearing breccia found outside of the crater rim this suevite contains large numbers of chimney like degassing pipes apparently formed by high gas pressures in the hot 500 c fluidized suevite immediately after deposition of the suevite the likely sources of the gas are volatiles from the melted and shocked basement inclusions the suevite outside of the crater rim contains about 15 wt clay indicating a large amount of alteration of the suevite the clay is an fe rich montmorillonite with less than 5 interstratified illite indicating that the temperature at which most of the alteration occurred was less than 100 130 c the duration of this low temperature period may have been extended by additional heating from the exothermal alteration reactions the surfaces of the degassing pipes are not altered to clays however but have a thin fe rich oxide or hydroxide coating the absence of alteration in the pipes suggests that the main alteration event outside the crater occurred under unsaturated conditions such that the walls of the pipes were dry suevite deposits are likely to exist on mars because of the presence of volatile rich target rock martian suevites may have also been fluidized and hydrothermally altered contributing significantly to the formation of the clay like phases inferred to exist in the martian soil

ancient impact and aqueous processes at endeavour crater mars the rover opportunity has investigated the rim of endeavour crater a large ancient impact crater on mars basaltic breccias produced by the impact form the rim deposits with stratigraphy similar to that observed at similar sized craters on earth highly localized zinc enrichments in some breccia materials suggest hydrothermal alteration of rim deposits gypsum rich veins cut sedimentary rocks adjacent to the crater rim the gypsum was precipitated from low temperature aqueous fluids flowing upward from the ancient materials of the rim leading temporarily to potentially habitable conditions and providing some of the waters involved in formation of the ubiquitous sulfate rich sandstones of the meridiani region

geology of quartz and hydrated silica bearing deposits near antoniadi crater mars 1 the only area on mars where crystalline quartz has been identified from orbit is near antoniadi crater on the northern edge of the syrtis major shield volcano however the method of quartz formation has remained unknown in this study we use high resolution satellite imagery as well as thermal and near infrared spectroscopy to construct a geologic history of these deposits and their local context we find that the quartz bearing deposits are consistently co located with hydrated silica this spatial coherence suggests that the quartz formed as a diagenetic product of amorphous silica rather than as a primary igneous mineral diagenetic quartz is a mature alteration product of hydrated amorphous silica and indicates more persistent water and or higher temperatures at this site beneath the silica bearing rocks we also find spectral evidence for smectites in the lowermost exposed noachian aged breccia a similar stratigraphic sequence smectite bearing breccias beneath deposits containing minerals suggesting a greater degree of alteration has also been found at nearby exposures at nili fossae and toro crater suggesting a widespread sequence of alteration by merging the mineral detections of thermal infrared quartz feldspar and near infrared spectroscopy hydrated silica smectite clays we are able to construct a more complete geologic history from orbit

geomorphological evidence for transient water flow on vesta abstract vesta the second most massive asteroid has long been perceived as anhydrous recent studies suggesting the presence of hydrated minerals and past subsurface water have challenged this long standing perception yet direct geologic indications of water activity on vesta s surface were unexpected herein we show evidence that transient water flowed on the surface in a debris flow like process and left distinctive geomorphologic features based on detailed analysis of highest resolution 20 m pixel images obtained by the dawn spacecraft we identify a class of locally occurring interconnected and curvilinear gully networks on the walls of young

igneous rocks formed by hypervelocity impact abstract igneous rocks are the primary building blocks of planetary crusts most igneous rocks originate via decompression melting and or wet melting of protolith lithologies within planetary interiors and their classification and compositional petrographic and textural characteristics are well studied as our exploration of the solar system continues so too does the inventory of intrusive and extrusive igneous rocks settings and processes the results of planetary exploration have also clearly demonstrated that impact cratering is a ubiquitous geological process that has affected and will continue to affect all planetary objects with a solid surface whether that be rock or ice it is now recognized that the production of igneous rocks is a fundamental outcome of hypervelocity impact the goal of this review is to provide an up to date synthesis of our knowledge and understanding of igneous rocks formed by hypervelocity impact following a brief overview of the basics of the impact process we describe how and why melts are generated during impact events and how impact melting differs from endogenic igneous processes while the process may differ we show that the products of hypervelocity impact can share close similarities with volcanic and shallow intrusive igneous rocks of endogenic origin such impact melt rocks as they are termed can display lobate margins and cooling cracks columnar joints and at the hand specimen and microscopic scale such rocks can display mineral textures that are typical of volcanic rocks such as quench crystallites ophitic porphyritic as well as features such as vesicles flow textures and so on historically these similarities led to the misidentification of some igneous rocks now known to be impact melt rocks as being of endogenic origin this raises the question as to how to distinguish between an impact versus an endogenic origin for igneous like rocks on other planetary bodies where fieldwork and sample analysis may not be possible and all that may be available is remote sensing data while the interpretation of some impact melt rocks may be relatively straightforward e g for clast rich varieties and those with clear projectile contamination we conclude that distinguishing between impact and endogenic igneous rocks is a non trivial task that ultimately may require sample investigation and analysis to be conducted caution is therefore urged in the interpretation of igneous rocks on planetary surfaces

mobile element analysis by secondary ion mass spectrometry sims of impactite matrix samples from the yaxcopoil 1 drill core in the chicxulub impact structure the concentrations of the fluid mobile trace elements lithium beryllium boron and barium were measured in samples of the altered matrix of several impactite breccias of the yaxcopoil 1 drill core using secondary ion mass spectrometry sims to determine the extent of transport due to aqueous or hydrothermal processes three of the elements li be and b have higher concentrations in the upper suevite impact breccias than in the lower impact melt deposits by factors of 3 5 2 2 and 1 5 respectively lithium and b are the most enriched elements up section and appear to have had the greatest mobility the similar fractionation of li and b is consistent with fluid transport and alteration under low temperature conditions of less than 150 c based on published experimental studies in contrast to li be and b the concentration of ba in the altered matrix materials decreases upward in the section and the concentration of ba in the matrix is an order of magnitude less than the bulk concentrations likely due to the presence of barite the origin of the elemental variations with depth may be related to different protolith compositions in the upper versus the lower impactite units a different protolith in the altered matrix is suggested by the mg rich composition of the lower units versus the al rich composition of the upper units which largely correlates with the mobile element variations the possibility that vertical transport of mobile elements is due to a postimpact hydrothermal system is supported by published data showing that the sediments immediately overlying the impactites are enriched in mobile elements derived from a hydrothermal system however the mobile elements in the sediments do not have to originate from the underlying impactites in conclusion our data suggests that the impactites at this location did not experience extensive high temperature hydrothermal processing and that only limited transport of some elements including li be and b occurred

the role of fluidization in the emplacement of pyroclastic flows 2 experimental results and their interpretation abstract although often quoted as being important in the emplacement of pyroclastic flows little relevant experimental fluidization work has been done fluidization processes are first reviewed for a simple system against which ignimbrite samples can be compared results from fluidization experiments on ignimbrite samples show that their behaviour differs radically from any simple system principally because of their extremely poor sorting at a certain gas velocity u ie whose value cannot be predicted some ignimbrite samples begin to expand whilst at a higher gas velocity u mp the samples begin to show segregation structures the minimum fluidization velocity u mf in simple systems is replaced by u mp the value of the latter in pyroclastic materials cannot be easily or reliably predicted from published u mf correlations and a new empirical method for determining u mp is presented from the experimental data during fluidization ignimbrite samples expand much less than more conventional materials due to the bypassing of gas through segregation channels the total amount of expansion is also limited e g a 100 m thick pyroclastic flow will from fairly high gas velocities deflate to form an ignimbrite which is not less than 70 85 m thick during fluidization the poor sorting causes only part of the weight of the ignimbrite samples to be supported by the gas flow the degree of support increasing with an improvement in sorting segregation structures and grain size and compositional variations within fluidized ignimbrite samples show varying behaviour depending on the sorting of the sample after subjecting a sample to high gas velocities the net result is a grain size and compositionally zoned bed which becomes richer upwards in pumice and fine material once formed these features cannot be destroyed under laboratory conditions implying that some segregation structures should be present in any ignimbrite where u mp was exceeded

hydrothermal alteration at the lonar lake impact structure india implications for impact cratering on mars the 50 000 year old 1 8 km diameter lonar crater is one of only two known terrestrial craters to be emplaced in basaltic target rock the 65 million year old deccan traps the composition of the lonar basalts is similar to martian basaltic meteorites which establishes lonar as an excellent analogue for similarly sized craters on the surface of mars samples from cores drilled into the lonar crater floor show that there are basaltic impact breccias that have been altered by post impact hydrothermal processes to produce an assemblage of secondary alteration minerals microprobe data and x ray diffraction analyses show that the alteration mineral assemblage consists primarily of saponite with minor celadonite and carbonate thermodynamic modeling and terrestrial volcanic analogues were used to demonstrate that these clay minerals formed at temperatures between 130 c and 200 c by comparing the lonar alteration assemblage with alteration at other terrestrial craters we conclude that the lonar crater represents a lower size limit for impact induced hydrothermal activity based on these results we suggest that similarly sized craters on mars have the potential to form hydrothermal systems as long as liquid water was present on or near the martian surface furthermore the fe rich alteration minerals produced by post impact hydrothermal processes could contribute to the minor iron enrichment associated with the formation of the martian soil

3820 ma zircons from a tonalitic armisoq gneiss in the godthab district of southern west greenland abstract zircons from a sample of the tonalitic amitsoq gneisses near godthhb west greenland exhibit several stages of crystal growth measurements of the u th pb isotopic composition of 30 m areas within individual grains using the ion microprobe shrimp reveal that components of magmatic zircon from the original igneous protolith are at least 3822 5 ma l old slightly but significantly older than the age of zircons from a fclsic volcanic unit in the isua supracrustal belt as measured at high precision by the same technique and older than all ages previously determined by mineral and whole rock isotopic techniques for amitsoq gneisses at godthab and at isukasia overgrowths of younger zircon formed at ca 3630 ma coinciding with an episode of major pb loss from the older grains these results indicate that previous whole rock isochron ages refer to later isotopic redistribution and or that other phases of the amitsoq gneisses intruded during this later period and dominate the isotopic systematics comparison of the ion probe results with previous conventional u th pb data for the same zircon sample demonstrates the strong influence of discordant high uranium parts of zircon on the mean isotopic composition of multi grain samples

hydrothermal alteration in the core of the yaxcopoil 1 borehole chicxulub impact structure mexico petrographic electron microprobe and raman spectrometric analyses of yaxcopoil 1 core samples from the chicxulub crater indicate that the impact generated a hydrothermal system relative textural and vein crosscutting relations and systematic distribution of alteration products reveal a progression of the hydrothermal event in space and time and provide constraints on the nature of the fluids the earliest calcite halite and gaylussite suggest that the impactite sequence was initially permeated by a low temperature saline brine subsequent development of a higher temperature hydrothermal regime is indicated by thermal metamorphic diopside hedenbergite aeg3fs 18 33 en32 11wo 47 53 after primary augite and widespread na k for ca metasomatic alkali exchange in plagioclase hydrothermal sphene apatite magnetite bornite as well as early calcite combined 3 to 8 vol were introduced with metasomatic feldspar a lower temperature regime characterized by smectite after probable primary glass secondary chlorite and other pre existing mafic minerals as well as very abundant calcite veins and open space fillings extensively overprinted the early hydrothermal stage the composition of early and late hydrothermal minerals show that the solution was chlorine rich cl f 10 and that its fe mg ratio and oxidation state increased substantially 4 to 5 logo2 units as temperature decreased through time the most altered zone in the impactite sequence occurs 30 m above the impact melt the lack of mineralogical zoning about the impact melt and convective modeling constraints suggest that this unit was too thin at yaxcopoil 1 to provide the necessary heat to drive fluids and implies that the hydrothermal system resulted from the combined effects of a pre existing saline brine and heat that traveled to the yaxcopoil 1 site from adjacent areas where the melt sheet was thicker limonite after iron oxides is more common toward the top of the sequence and suggests that the impactite section was subjected to weathering before deposition of the tertiary marine cover in addition scarce latest anatase stringers chalcopyrite and barite in vugs francolite after apatite and recrystallized halite are the likely products of limited post ydrothermal ambient temperature diagenesis or ocean and or meteoric water circulation

stable isotope composition of smectite in suevites at the ries crater germany implications for hydrous alteration of impactites abstract the 24 km diameter ries crater germany exhibits well preserved crater filling and surficial melt rich breccia deposits that are believed to have been altered by post impact hydrothermal fluids the alteration mineralogy of the crater filling breccias is characterized by clay smectite chlorite and a zeolite assemblage and secondary clay phases smectite minor halloysite in surficial melt bearing breccia deposits using stable isotope analysis of secondary smectitic clay fractions evidence of significant hydrous alteration of impactites at large water rock ratios was found the estimated fluid temperatures using data derived by 18 o and d fractionation suggest smectite precipitation in surficial breccias in equilibrium with meteoric fluids at temperatures 16 5 c in agreement with the long term variation of modern precipitation in the area the stable isotope composition of smectite in crater fill breccia however suggests a trend of monotonously increasing temperatures from 43 to 112 c with increasing depth through the breccia sequence this demonstrates a different origin of alteration and temperature distribution for the surficial and crater filling melt bearing impact breccias in the ries crater our results suggest that the inverted structure of hydrothermal systems observed in some terrestrial impact craters including the ries crater could indicate the initial configuration of a thermal anomaly in the crater filling sequence but which is replaced with a normal hydrothermal convection in crater proper during the course of post impact cooling

petrography of the impact breccias of the enkingen subo 18 drill core southern ries crater germany new estimate of impact melt volume a new drill core subo 18 that intersects a continuous sequence of impact breccias has been obtained at the town of enkingen at the southern edge of the ries inner crater detailed petrographic analysis of this core provides important new information regarding the nature and origin of within crater suevite which is not only relevant for the understanding of the formation of the ries impact structure but has strong implications for impact breccia deposition in general in the enkingen core below 21 m of crater sediment 90 m of impact breccia have been sampled most of this material is what in the past has been referred to as suevite defined as groundmass of clastic material entraining cogenetic melt fragments and target rock clasts only the bottommost 10 m of core are melt dominated impact breccia any relatively thick melt rock intersection along this core is revealed to be composed of individual centimeter to decimeter sized melt bodies best described as agglomerate due to its close resemblance to volcaniclastics of that type detailed macro meso and microscopic petrographic analysis has established that the subo 18 impact breccias cannot be divided into distinct units the groundmass is extensively altered to phyllosilicate smectite or chlorite or carbonate minerals results of a detailed scanning electron microscopic investigation of the groundmasses of samples from all parts of this sequence emphatically support the traditional definition of suevite irrespective of scale modal analysis at macroscopic to submicroscopic scales has shown that the overall average melt content of enkingen suevite is on the order of 48 vol this exceeds by far previous estimates and brings the ries crater melt volume in line with values traditionally suggested from crater and melt volume scaling for similar sized impact structures in crystalline and mixed targets

origin of the valley networks on mars a hydrological perspective abstract the geomorphology of the martian valley networks is examined from a hydrological perspective for the compatibility with an origin by rainfall globally higher heat flow and localized hydrothermal systems comparison of morphology and spatial distribution of valleys on geologic surfaces with terrestrial fluvial valleys suggests that most martian valleys are probably not indicative of a rainfall origin nor are they indicative of formation by an early global uniformly higher heat flow in general valleys are not uniformly distributed within geologic surface materials as are terrestrial fluvial valleys valleys tend to form either as isolated systems or in clusters on a geologic surface unit leaving large expanses of the unit virtually untouched by erosion with the exception of fluvial valleys on some volcanoes most martian valleys exhibit a sapping morphology and do not appear to have formed along with those that exhibit runoff morphology in contrast terrestrial sapping valleys form from and along with runoff valleys the isolated or clustered distribution of valleys suggests localized water sources were important in drainage development persistent groundwater outflow driven by localized but vigorous hydrothermal circulation associated with magmatism volcanism impacts or tectonism is however consistent with valley morphology and distribution snowfall from sublimating ice covered lakes or seas may have provided an atmospheric source of water for the formation of some valleys in regions where the surface is easily eroded and where localized geothermal hydrothermal activity is sufficient to melt accumulated snowpacks

roter kamm impact crater namibia geochemistry of basement rocks and breccias abstract the roter kamm crater in the southern namib desert has previously been identified as an impact structure on the basis of crater morphology and the presence of impact melt breccias which contain shock metamorphosed quartz and lithic clasts to better define the variety of target rocks and breccias we studied the petrography and chemical composition of a new suite of twenty eight basement and breccia samples from the roter kamm crater based on chemical data for target lithologies and breccias we suggest that the crater was formed in a two layer target region an upper layer of gariep metasediments schist marble quartzite and sandstone overlying the crystalline basement of the namaqualand metamorphic complex the basement was also heavily intruded by coarse grained quartz veins and quartz and quartz feldspar pegmatites the clast population in the melt breccias indicates that impact induced melting involved mainly metasedimentary target rocks with rarely detected contributions from pegmatite and granite granodiorite three varieties of melt breccias can be defined 1 schistose 2 quartzitic melt breccias 3 true impact melt breccias these melt breccia types are chemically heterogeneous and even the impact melt breccias may have been produced in situ and not from a coherent melt body the shapes of the schistose melt breccias previously thought to be ejected impact breccias are most likely caused by erosion and these breccias are now interpreted to be locally derived the crater basement as exposed at the rim was structurally severely affected and at least locally considerable thermal energy was generated during formation of large volumes of cataclastic mylonitic and pseudotachylitic breccias analyses of mylonite and pseudotachylites from the crater rim as well as their respective host rocks show that these breccias were mainly formed from local material analyses of pseudotachylite like breccias indicate that these possible friction melts are generated by preferential melting of hydrous ferromagnesian minerals and feldspar similar to their tectonically produced counterparts although no significant fluid effects resulting from formation of mylonites or pseudotachylites are indicated several breccias compared to their host rocks do show evidence of severe chemical alteration chloritisation and sericitisation the presence of large vesicles filled with hydrothermal mineral assemblages in some schistose breccias and other petrographic and chemical data support the hypothesis of an impact induced hydrothermal event in the crater area

widespread crater related pitted materials on mars further evidence for the role of target volatiles during the impact process abstract recently acquired high resolution images of martian impact craters provide further evidence for the interaction between subsurface volatiles and the impact cratering process a densely pitted crater related unit has been identified in images of 204 craters from the mars reconnaissance orbiter this sample of craters are nearly equally distributed between the two hemispheres spanning from 53 s to 62 n latitude they range in diameter from 1 to 150 km and are found at elevations between 5 5 to 5 2 km relative to the martian datum the pits are polygonal to quasi circular depressions that often occur in dense clusters and range in size from 10 m to as large as 3 km pit sizes scale with both the host crater s diameter and the host deposit size these pits have subtle raised rims and unlike primary and secondary impact craters they lack well defined ejecta deposits and overlapping stratigraphic relationships they also lack any sign of any preferential alignment expected of volcanic or tectonic collapse features morphologic and stratigraphic evidence in support of an impact origin includes the observation that pitted materials primarily occur as ponded and flow like deposits on crater floors behind terraces and infilling the lowest local topographic depressions atop the ejecta blanket similar to the distribution of impact melt bearing bodies on the moon based on the observations and comparisons to terrestrial and lunar analogs we conclude that the pit bearing materials are impactite deposits the presence of these deposits in older craters where preserved suggests that they have formed on mars throughout most of its geologic history thus understanding their origin may help to constrain the hydrological and climate history of mars

petrographic investigation of melt and matrix relationships in chicxulub crater yaxcopoil 1 brecciated melt rock and melt rock bearing suevite 846 885 m units 4 and 5 abstract the drill core from the international continental drill program s chicxulub scientific drilling project yaxcopoil 1 yax 1 borehole in the annular trough of the chicxulub crater exhibits from 794 to 895 m a continuous sequence of impactites consisting of reworked fallout fallout suevite and brecciated impact melt rock these impact breccias exhibit a complex history of deposition fracturing matrix emplacement and hydrothermal alteration detailed investigation of the mineralogy and chemistry of these breccias has led to a better understanding of the complex events involved in their formation we find that the paragenesis of the brecciated impact melt rock unit 5 861 885 m involved fracturing of melt rock and early k metasomatism during a hydrothermal alteration episode as suggested by earlier work however the present work has identified the role of multiple episodes of precipitation of mg rich phyllosilicates and formation and dissolution of accessory minerals in a relatively high temperature 300 c hydrothermal event the earliest matrix formation event involved precipitation of mg rich phyllosilicate accessory quartz calcite apatite and andradite garnet from a hydrothermal fluid with a brine or seawater component the fluid could have partly incorporated elements and shock metamorphosed mineral phases derived from sedimentary lithologies including calcite and dolomite that underwent complex phase transformations such as melting decomposition and possible back reactions the discovery of andradite garnet in the matrix confirms the presence of an early high temperature hydrothermal event previously identified by mineralogical stable isotope and fluid inclusion studies the mineral assemblage including mg rich saponite suggests the involvement of seawater by comparison with similar alteration assemblages in hydrothermal systems involving seawater around the world the presence of a later low temperature phase of the hydrothermal system with different fluid chemistry is indicated by the partial dissolution of andradite garnet and continued precipitation of matrix phyllosilicate minerals but without the accessory quartz abundant calcite or andradite and without an accompanying k metasomatism event

compositional heterogeneity of impact melt rocks at the haughton impact structure canada implications for planetary processes and remote sensing connecting the surface expression of impact crater related lithologies to planetary or regional subsurface compositions requires an understanding of material transport during crater formation here we use imaging spectroscopy of 6 clast rich impact melt rock outcrops within the well preserved 23 5 ma 23 km diameter haughton impact structure canada to determine melt rock composition and spatial heterogeneity we compare results from outcrop to outcrop using clasts groundmass and integrated clast groundmass compositions as tracers of transport during crater fill melt rock formation and cooling supporting laboratory imaging spectroscopy analyses of 91 melt bearing breccia and clast samples and microscopic x ray fluorescence elemental mapping of cut samples paired with spectroscopy of identical surfaces validate outcrop scale lithological determinations results show different clast rich impact melt rock compositions at three sites kilometers apart and an inverse correlation between silica rich sandstone gneiss and phyllosilicate rich shales and gypsum rich rocks that suggests differences in source depth with location in the target stratigraphy gypsum is primarily sourced from 1 km depth while gneiss is from 1 8 km depth sandstone from 1 3 km and shales from 1 6 1 7 km observed heterogeneities likely result from different excavation depths coupled with rapid quenching of the melt due to high content of cool clasts results provide quantitative constraints for numerical models of impact structure formation and give new details on melt rock heterogeneity important in interpreting mission data and planning sample return of impactites particularly for bodies with impacts into sedimentary and volatile bearing targets e g mars and ceres

hornblende alteration and fluid inclusions in kardla impact crater estonia evidence for impact induced hydrothermal activity the well preserved krdla impact crater on hiiumaa island estonia is a 4 km diameter structure formed in a shallow ordovician sea about 455 ma ago into a target composed of thin 150 m unconsolidated sedimentary layer above a crystalline basement composed of migmatite granites amphibolites and gneisses the fractured and crushed amphibolites in the crater area are strongly altered and replaced with secondary chloritic minerals the most intensive chloritization is found in permeable breccias and heavily shattered basement around and above the central uplift alteration is believed to have resulted from convective flow of hydrothermal fluids through the central areas of the crater chloritic mineral associations suggest formation temperatures of 100 300 c in agreement with the most frequent quartz fluid inclusion homogenization temperatures of 150 300 c in allochthonous breccia the rather low salinity of fluids in krdla crater 13 wt nacleq suggests that the hydrothermal system was recharged either by infiltration of meteoric waters from the cater rim walls raised above sea level after the impact or by invasion of sea water through the disturbed sedimentary cover and fractured crystalline basement the well developed hydrothermal system in krdla crater shows that the thermal history of the shock heated and uplifted rocks in the central crater area rather than cooling of impact melt or suevite sheets controlled the distribution and intensity of the impact induced hydrothermal processes

origin of two verwey transitions in different generations of magnetite from the chesapeake bay impact structure usa 1 we observed two different verwey transition temperatures in fragments of crystalline basement rocks and impact sediments from the chesapeake bay impact structure usa our study aims to the question if this feature can be used as shock indicator in impact craters we distinguished three generations of magnetite 1 primary magnetite in crystalline basement rocks has average grain sizes up to several hundreds of micrometers and shows a regular tv at 121 k 2 shocked magnetite occurs in fragments of crystalline basement rocks and also in the suevite and impact breccia these magnetites show two verwey transitions a regular one and a low temperature transition ltv at around 89 k ltv is related to a small grain size fraction whereas a larger grain size fraction some hundreds of micrometers causes the regular tv the small grain size fraction contains a distinctly higher amount of superficially oxidized material due to the high surface volume ratio which causes a decrease of the verwey transition temperature ltv 3 a secondary magnetite generation shows also two verwey transition temperatures one at 121 k and a ltv range between 91 and 105 k the ltv in this generation is also linked to thin oxidized surface layers this study shows that especially the verwey transition temperature of small magnetite grains reacts very sensitively to surface oxidation and can therefore not be used as a reliable pressure indicator for impact structures on earth

airgrav results a comparison of airborne gravity data with gsc test site data sander geophysics sgl has developed an airborne gravimeter system that uses three orthogonal accelerometers mounted on a three axis gyroscopically stabilized platform the system called airgrav airborne inertially referenced gravimeter was designed specifically for airborne use this has resulted in an instrument with significant advantages over the modified sea gravimeters commonly used for airborne surveying the main benefit of the new instrument is that it is more stable in attitude and less subject to noise from horizontal accelerations

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

bunte breccia of the ries continuous deposits of large impact craters the ries crater an impact structure of 26 km diameter in south germany is the largest terrestrial crater where substantial amounts of ejecta are preserved on occasion 100 m deep further the target stratigraphy is well known and it is possible to relate specific clasts and breccia lithologies to initial target depth as a consequence the continuous deposits of the ries also known as bunte breccia may be studied with exceptional field control we report field observations and laboratory analyses obtained from 560 m core materials taken at nine different locations that range from 16 to 37 km in radial distance from the impact center the objective is to relate the ries observations to ejection and to emplacement processes of large scale planetary crater deposits the observations regarding the modal stratigraphic characteristics of the bunte breccia may be summarized as follows only 600 m some 0 7 is composed of triassic clasts originating from 300 to 600 m depths lower and middle jurassic horizons approximately 300 150 m constitute some 2 3 and upper jurassic 0 150 m makes up some 31 5 in addition the bunte breccia contains tertiary materials in the form of 1 cm clasts 29 1 and as highly comminuted fine grained matrix 10 gpa constitute 0 1 weight of the entire deposit which indicates that bunte breccia was emplaced at essentially ambient temperatures when possible the above observations are quantified via linear regressions throughout the text all of these observations are consistent with if not predicted by a ballistic emplacement scenario as postulated by oberbeck and co workers primary crater ejecta are expelled ballistically and will form secondary craters in the local substrate a mixture of primary and secondary ejecta results and combines into a highly turbulent ground hugging debris surge as the final phase of ejecta emplacement total emplacement time for the bunte breccia 200 km is estimated to be of the order of 5 min only these findings are compared with cratering theory relating to a number of ejecta thickness decay models and with the so called z model addressing material flow during various stages of crater formation qualitative to fair agreement of observations and predictions results an initial crater radius of 6 5 km an excavation depth of 1650 m an excavation volume of 136 km and an associated transient cavity volume of aproximately 230 km appear to be reasonable estimates approximately 170 km of material was involved in slumping and restoration of the transient cavity for the above radius and z 2 7 the modal composition of ries ejecta with regard to preimpact target stratigraphy indicates that materials contained in the continuous deposits of large complex planetary craters are predominantly derived from depths as small as one hundredth the apparent crater diameter a number of implications are addressed regarding remote sensing of planetary surfaces and investigation of lunar and meteoritic impact breccias

loss of radiogenic argon from shocked granitic clasts in suevite deposits from the ries crater five granitic clasts from the otting and aumuhle quarries in the suevite ejecta deposits beyond the rim of the ries germany impact crater have been characterized as to modal composition degree of shock and loss of radiogenic argon the shock pressures experienced by this selected suite of samples range from 60 gpa and the samples have been modestly shocked to heavily shocked or even melted ages for these samples were determined by the 39ar40ar technique the least shocked specimen 10 15 gpa shows very little loss of radiogenic ar relative to the approximately 320 my age of the variscan basement at the ries all other samples 28 gpa 42 gpa 52 gpa and an impact melt suggest complete to nearly complete loss of radiogenic ar at the time of the 15 my old impact event most of the radiogenic ar in these granitic samples is contained in feldspar with a lesser amount in biotite diffusive loss of ar occurs relatively easily from these highly disordered feldspars the essentially complete loss of radiogenic ar from 4 of the 5 samples is in large part ascribed to thermal activation during the cooling history of the suevite rather than during initial passage of the shock wave these data are consistent with the conclusion from previous paleomagnetic studies that the suevite was deposited at temperatures above 500 c the fact that staudacheret al 1982 found almost no loss of radiogenic ar in a wide variety of ries samples is ascribed to two causes a most of their samples were shocked to 15 gpa and were collected in drill cores from the modestly shocked crater bottom which was not as hot as the suevite and b their observations were primarily made on hornblende and biotite separates that are more resistant to shock effects and to diffusive loss of ar compared to shocked feldspars loss of argon from our suevite samples likely occurred in a hot post impact ejecta layer from data presented here we estimate that 50 of the material composing the hot ries ejecta should show at least partially reset k ar ages such materials compose no more than 5 of the total displaced crater volume

context of ancient aqueous environments on mars from in situ geologic mapping at endeavour crater using the mars exploration rover opportunity we have compiled one of the first field geologic maps on mars while traversing the noachian terrain along the rim of the 22 km diameter endeavour crater latitude 2 16 33 longitude 5 10 51 in situ mapping of the petrographic elemental structural and stratigraphic characteristics of outcrops and rocks distinguishes four mappable bedrock lithologic units three of these rock units predate the surrounding burns formation sulfate rich sandstones and one the matijevic formation represents conditions on early mars predating the formation of endeavour crater the stratigraphy assembled from these observations includes several geologic unconformities the differences in lithologic units across these unconformities record changes in the character and intensity of the martian aqueous environment over geologic time water circulated through fractures in the oldest rocks over periods long enough that texturally and elementally significant alteration occurred in fracture walls these oldest pre endeavour rocks and their network of mineralized and altered fractures were preserved by burial beneath impact ejecta and were subsequently exhumed and exposed the alteration along joints in the oldest rocks and the mineralized veins and concentrations of trace metals in overlying lithologic units is direct evidence that copious volumes of mineralized and or hydrothermal fluids circulated through the early martian crust the wide range in intensity of structural and chemical modification from outcrop to outcrop along the crater rim shows that the ejecta of large 8 km in diameter impact craters is complex these results imply that geologic complexity is to be anticipated in other areas of mars where cratering has been a fundamental process in the local and regional geology and mineralogy

the history of allan hills 84001 revised multiple shock events the geologic history of martian meteorite allan hills alh 84001 is more complex than previously recognized with evidence for four or five crater forming impacts onto mars this history of repeated deformation and shock metamorphism appears to weaken some arguments that have been offered for and against the hypothesis of ancient martian life in alh 84001 allan hills 84001 formed originally from basaltic magma its first impact event i1 is inferred from the deformation d1 that produced the granular textured bands crush zones that transect the original igneous fabric deformation d1 is characterized by intense shear and may represent excavation or rebound flow of rock beneath a large impact crater an intense thermal metamorphism followed d1 and may be related to it the next impact i2 produced fractures fr2 in which carbonate pancakes were deposited and produced feldspathic glass from some of the igneous feldspars and silica after i2 carbonate pancakes and globules were deposited in fr2 fractures and replaced feldspathic glass and possibly crystalline silicates next feldspars feldspathic glass and possibly some carbonates were mobilized and melted in the third impact i3 microfaulting intense fracturing and shear are also associated with i3 in the fourth impact i4 the rock was fractured and deformed without significant heating which permitted remnant magnetization directions to vary across fracture surfaces finally alh 84001 was ejected from mars in event i5 which could be identical to i4 this history of multiple impacts is consistent with the photogeology of the martian highlands and may help resolve some apparent contradictions among recent results on alh 84001 for example the submicron rounded magnetite grains in the carbonate globules could be contemporaneous with carbonate deposition whereas the elongate magnetite grains epitaxial on carbonates could be ascribed to vapor phase deposition during i3

a geomorphic analysis of hale crater mars the effects of impact into ice rich crust abstract hale crater a 125 150 km impact crater located near the intersection of uzboi vallis and the northern rim of argyre basin at 35 7 s 323 6 e is surrounded by channels that radiate from incise and transport material within hale s ejecta the spatial and temporal relationship between the channels and hale s ejecta strongly suggests the impact event created or modified the channels and emplaced fluidized debris flow lobes over an extensive area 200 000 km2 we estimate 1010 m3 of liquid water was required to form some of hale s smaller channels a volume we propose was supplied by subsurface ice melted and mobilized by the hale forming impact if 10 of the subsurface volume was ice based on a conservative porosity estimate for the upper martian crust 1012 m3 of liquid water could have been present in the ejecta we determine a crater retention age of 1 ga inside the primary cavity providing a minimum age for hale and a time at which we propose the subsurface was volatile rich hale crater demonstrates the important role impacts may play in supplying liquid water to the martian surface they are capable of producing fluvially modified terrains that may be analogous to some landforms of noachian mars

age and tectonic setting of late archean greenstone gneiss terrain in henan province china as revealed by single grain zircon dating the authors report precise u pb zircon ages for single grains of a metarhyodacite from the late archean dengfeng greenstone belt in henan province china near the southern margin of the north china craton most grains belong to an igneous population whose u pb isotopic systematics define a straight line intersecting concordia at 2512 12 ma and this is interpreted as the time of crystallization of the original greenstone volcanics several grains are distinctly older between 2576 9 and 2945 44 ma and the authors interpret the older grains as xenocrysts of pre greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava the xenocrysts suggest evolution of the dengfeng greenstone belt in a continental environment that may be represented by the taihua high grade gneisses bordering the dengfeng greenstones and for which the authors obtained ages of 2806 7 and 2841 6 ma the data add evidence to the now widely held concept that most archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination in such cases conventional zircon dating may not always record the precise age of rock formation

enigmatic tubular features in impact glass we describe the fi rst putative microbial trace fossils hosted in meteorite impact glass we conducted optical and scanning electron microscopy energy dispersive x ray spectroscopy and fourier transform infrared spectroscopy on postimpact tubular features hosted in impact glasses from the ries impact structure germany the morphologies of the tubules are inconsistent with known mineralogical crystallization mechanisms and combined with evidence of organic molecules suggest that these tubules cannot be formed through purely abiotic processes the simplest and most consistent explanation of the data is that biological activity played a role in the formation of the tubular textures in the ries glasses likely during postimpact hydrothermal activity as impact glass is a ubiquitous substrate on rocky bodies throughout the solar system and likely common on the early earth the preservation of biological activity in impact glass has signifi cant astrobiological implications for life on early earth as well as for the search for life on other planets

impact crater lakes on mars the robotic search for life on mars centers on identifying accessible environments where the biological catalyst water has existed the formation of large impact craters on mars 65 km diameter may have resulted in the creation of ice covered impact crater lakes which would not freeze for thousands of years even under present climatic conditions water could be supplied from deep confined aquifers penetrated by the impact craters without the need for surface melt water freezing of the lakes is postponed owing to heat from impact generated melt bearing deposits from impact related uplift of hotter rocks from depth and from the latent heat of freezing of a deep crater lake abundant morphologic evidence for ancient crater lakes has not been found in viking images except for craters associated with outflow channels however ice covered crater lakes could have formed and further searches for evidence of these lakes are warranted the lake deposits from dissected impact craters may represent one of the best targets for future surface exobiology investigations or sample return missions from mars

weathering features in shocked quartz from the ries impact crater germany shocked quartz from the ejecta of the ries impact structure has been investigated by analytical transmission electron microscopy atem quartz grains display numerous planar fractures pfs and planar deformation features pdfs both are partly or fully replaced by a mineral of the kaolinite group likely halloysite its formation involves fluid circulation into the dense fracture networks dissolution and removal of the amorphous phase initially present in pdfs and finally precipitation and crystallization of the kaolinite group mineral from solutions resulting from the chemical alteration of adjacent minerals feldspars and biotite kaolinite group minerals are typical of hydrothermal alteration at low temperature in humid climate and under moderately acid conditions and thus this alteration may not be directly related to the impact event itself however the weathering features were strongly enhanced by the shock generated microstructure in particular by fractures that provided pathways for fluid circulation

origin of small pits in martian impact craters we propose a numerical model for the formation of the closely spaced pits found in the thin ejecta related deposits superposed on the floors interior terrace blocks and near rim ejecta blankets of well preserved martian impact craters our model predicts the explosive degassing of water from this pitted material which is assumed to originally be water bearing impact melt rich breccia at the time of deposition this process is analogous to what occurred in the fall out suevite deposits at the ries impact structure in germany at ries impact heating of water bearing target material resulted in the rapid degassing of its water and other volatiles the martian environment plays an important role in enhancing the effects of this degassing by increasing the flow speed of the escaping gas the high flow rate of gas through particulate materials such as suevite tends to quickly form segregation channels or vent pipes similar to those found in the ries deposits these pipes act as conduits for the efficient high speed escape of the gas and small clasts that it entrains escaping gas and entrained clasts abraded and eroded the conduit walls flaring them to form pits above a network of pipes

kardla hiiumaa island estonia the buried and well preserved ordovician marine impact structure abstract the kardla marine impact structure estonia 58 58 n 22 46 e was formed at 455 ma upper ordovician in a shallow epicontinental sea some tens of kilometres from the land and erosion area the iron rich projectile about 200 m in diameter approached from the west at an angle of 30 45 the impactor penetrated about 50 m thick water layer and the sedimentary cover and exploded in the uppermost part of the crystalline basement a complex crater 4 km wide and about 500 m deep with a central uplift rising 130 m from the crater floor was formed the highest point of the rimwall is 110 m above the target level the rimwall is cut by at least two resurge excavated gullies the variable height of the rimwall obviously results from the obliqueness of the impact outside the crater an elliptical area was revealed 12 15 km in diameter with deformed sedimentary rocks below the target level the elliptical shape of this area may also be due to the oblique impact because the crater and its surroundings were buried directly after the impact the whole complex of impact related sediments is preserved there they are recovered by 160 wells six of which penetrate the entire complex of impact breccias inside the crater

argon isotopic analysis of breccia veins from the roter kamm crater namibia and implications for their thermal history the rocks exposed in the rim of the 2 5 km wide and 3 7 ma old roter kamm crater in southwest namibia are cut by breccia veins that macroscopically resemble and were originally described as pseudotachylytes the veins were later shown to be cataclasites with no evidence for melting 40ar 39ar data for vein and host rock samples indicate a low grade metamorphic event at around 300 ma but provide no evidence for an impact age the samples have suffered 5 7 ar loss which we associate with the impact event all the samples record similar ranges of possible timetemperature conditions and there are no resolvable differences between the results for the vein and the host rock samples as would be expected if frictional heating played an important role in breccia formation modeling the 40ar 39ar data assuming instantaneous impact heating followed by extended cooling and coupling these results to published data on fluid inclusions quartz precipitation shock effects and crater degradation suggest that the veins reached maximum temperatures of 230 290 c during impact and never approached melting temperatures of the precursor rocks

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

geochemical evolution and metamorphic development of the early precambrian in eastern hebei china abstract within the archaean qianxi group in eastern hebei china two roughly coeval tectonic units can be recognized the qian an granulite complex and the zunhua high grade greenstone belt both belong to the high grade terranes and have recorded almost the same history of metamorphism the granulite facies metamorphism was generally followed by a retrograde metamorphism of amphibolite facies and multistage anatexis despite the similar metamorphic histories the geochemical features of the two units are different 1 1 the protoliths of the qian an granulite complex consist of a calc alkaline volcanic series dominated by andesitic composition and a shallow water sedimentary series whereas that of the zunhua greenstone belt clearly show the character of a bimodal magmatic series 2 2 most metabasalts of the qian an complex belong to the continental basalt type and grade into andesite whereas in the zunhua belt they are characterized by a suite of high magnesian series low k2o tholeiites and a large amount of ultramafic mafic layered complexes 3 3 in terms of regional geochemistry the qian an complex is rich in potassium and potassium charnockites of autochthonous anatectic and intrusive origin are widespread in contrast the zunhua belt is rich in sodium and tonalitic gneisses of deep seated mixed origin are widespread finally a model for archaean crustal evolution of high grade terranes is suggested

boron isotope composition of secondary smectite in suevites at the ries crater germany boron fractionation in weathering and hydrothermal processes abstract the 24 km diameter ries crater in germany is one of the best preserved terrestrial complex impact structure and clay minerals in the groundmass of the ries suevites have been recognized for several decades it is generally accepted that these clays were formed by post impact aqueous alteration of impact generated glasses and or finely comminuted crystalline basement material at temperatures above ambient conditions the ries impact structure provides a good opportunity to study the evolution of post impact development of impact craters here we present a study of boron isotopic compositions 11 b and b concentrations of the secondary smectite clay fraction in fall out and crater suevites from the ries crater germany these data were used to study the boron fractionation in clay mineral precipitation processes and to model using b isotope fractionation the characteristics ph temperature and salinity of fluids during fluid rock interaction at different positions within the crater the results of this study show that the boron isotopic composition of smectite in fall out suevites sampled from 4 different locations average 24 5 1 8 was significantly different from that of the smectite in crater suevites sampled from the nordlingen 1973 drill core at different depth intervals from 370 m to 525 m average 4 07 similarly the 11 b composition of the fluids responsible for the alteration calculated from smectite isotopic composition in the fall out suevites 7 5 1 6 differed from the alteration fluid composition in crater suevites 17 6 10 8 indicating a different origin of the fluids responsible for the alteration of the fall out and crater suevites in the ries crater our results suggest that the alteration in fall out suevites occurred at lower temperatures than the crater suevites and at a lower ph which is consistent with the smectite precipitation in equilibrium with meteoritic fluids the boron isotopic composition of smectite in the crater suevites suggests secondary clay formation at higher temperatures and or at elevated ph 8 9 low 11 b values of fixed boron in secondary smectite in fall out suevites suggests that b was incorporated into the mineral structure during the precipitation of authigenic smectite whereas the smectite precipitated in equilibrium with the boron and hydrogen isotopic composition in the meteoric water that percolated through the system our study provides new constraints on the origin and evolution of the geothermal fluids that were involved in the formation of altered minerals at the ries crater and suggests that boron isotope composition studies of the secondary clay phases associated with post impact cooling may have important implications for the changes in fluid flow and changes in geochemical signatures of the fluids and provide new constraints on fluid rock interaction and mass transport during the cooling of the impact crater

geochemistry of impact glasses from the k t boundary in haiti relation to smectites and a new type of glass we have individually analyzed twelve black and seven yellow glasses from the k t boundary section at beloc haiti plus three smectite mantles around black glasses the main chemical differences between black and yellow glasses are higher s ca mg zn as br sb and au but lower k na and si abundances in the yellow glasses we have found high cao yellow glasses with low s contents 0 08 0 17 wt which may be explained by fusion of carbonate rich sediments microprobe studies of individual glasses show that the black glasses are very homogeneous in their composition at the 10 100 m level while yellow glasses show much more variability and different intra sample elemental correlations one of the black glasses has higher sio2 and k2o abundances but lower concentrations of all other major elements this suggests the existence of a third glass type which we have named the high si k variety hsi k glass the glass shows areas consisting of pure sio2 maybe lechatelierite and some schlieren with lower sio2 content the chemical composition indicates that the hsi k glass may have originated from melting of a sedimentary source rock such as shale or argillite severe changes in major and trace element composition occur during alteration of the glass as documented by analyses of adhering smectites the ree patterns in the glass are similar to average upper crust but the smectites exhibit much lower abundances and flat patterns not unlike the patterns observed in claystones at numerous k t boundary sites the suggestion that the chicxulub structure in yucatan may be the source crater for the k t impact glasses is not fully supported by the currently available data impact glasses and tektites are produced from the surface layers of their targets at chicxulub the target stratigraphy comprised mainly carbonates and evaporites although we cannot exclude chicxulub as a source the present data do not provide any firm chemical evidence for a connection we conclude however that the haiti glasses have formed by impact and have later undergone alteration by low temperature hydrothermal processes to produce clays the boundary claystones at k t boundary sites may very well be alteration products of these impact glasses

anomalous quartz from the roter kamm impact crater namibia evidence for post impact hydrothermal activity abstract centimeter sized quartz pebbles have been found on the rim of the roter kamm impact crater the roter kamm crater has a diameter of about 2 5 km and is situated in the namib desert swa namibia because of the sand coverage impact products breccias impact melt shocked rocks are exposed exclusively in the form of ejecta on the crater rim the quartz pebbles were found close to the main deposits of the impact breccias and show signs of wind abrasion thin sections revealed that the pebbles consist of individual quartz domains that are up to 1 mm in size under crossed nicols polarized light all individual domains show extinction almost simultaneously within 2 which is a rare phenomenon microprobe studies neutron activation analyses and x ray diffractometry confirmed that the material consists of pure quartz the quartz contains three different types of fluid inclusions primary inclusions size about 5 10 m that record the formation conditions of the quartz very small m secondary inclusions associated with the grain boundaries and late inclusions of irregular size freezing point depression measurements of the primary inclusions indicate fluid salinities between 18 3 and 19 6 wt nacl homogenization temperatures t h for the primary inclusions range from 165 to 250 c the quartz and the primary inclusions may provide evidence for a post impact phase of extensive hydrothermal activity generated by the residual heat from the kinetic energy of the impact

the importance of being cratered the new role of meteorite impact as a normal geological process this paper is a personal and in many ways incomplete view of the past development of impact geology and of the newly recognized importance of impact events in terrestrial geological history it also identifies some exciting scientific challenges for future investigators to determine the full range of impact effects preserved on the earth to apply the knowledge obtained from impact phenomena to more general geological problems and to continue the merger of the once exotic field of impact geology with mainstream geosciences since the recognition of an impact event at the cretaceous tertiary k t boundary much current activity in impact geology has been promoted by traditionally trained geoscientists who have unexpectedly encountered impact effects in the course of their work their studies have involved 1 the recognition of additional major impact effects in the geological record the chesapeake bay crater the alamo breccia and multiple layers of impact spherules in precambrian rocks and 2 the use of impact structures as laboratories to study general geological processes e g igneous petrogenesis at sudbury canada and archean crustal evolution at vredefort south africa other research areas in which impact studies could contribute to major geoscience problems in the future include 1 comparative studies between low level 7 gpa shock deformation of quartz and the production of quartz cleavage in both impact and tectonic environments and 2 the nature origin and significance of bulk organic carbon kerogen and other carbon species in some impact structures gardnos norway and sudbury canada

chemical components of the martian soil melt degassing hydrothermal alteration and chondritic debris on the basis of measurements by the viking and soviet phobos missions the fine grained surficial drift material on mars is enriched in moderately volatile and mobile elements such as s cl k and br the compounds of these elements may amount to as much as 16 wt of the soil while the bulk of the soil appears to consist primarily of crustal material whose composition is similar to that of the basaltic mars meteorites clark 1993 three possible geochemical sources melt degassing hydrothermal alteration and accretion of chondritic material may have contributed to the enrichments we have evaluated possible chemical signatures for these sources by examining the relative abundances of chemical elements in the different components to the measured k abundance in the mars soil and have identified characteristic elements for each component the relative abundances of mobile elements in the different components are much less variable than their absolute abundances which strongly depend on conditions such as temperature and water composition the volatility controlled elemental signature of the melt degassing process includes enrichments of zn mo cd ba and w relative to the elemental abundances in the other components and martian meteorites hydrothermal alteration of minerals and glass controlled by low temperature solubility makes up a second component with a characteristic lithium enrichment signature a chondritic component is enriched in siderophile and chalcophile elements including ni compared to material derived from the martian mantle which was depleted in siderophile elements due to core formation unfortunately the existing data for the mars soil do not include any of these key signature elements and the uncertainties for the viking analyses could allow large amounts of each of these elements the mars pathfinder inorganic analysis experiment and the 2001 orbital gamma ray experiment have the potential to determine some of these characteristic elements and to reveal the possible contributions to the soil the enriched components of the soil especially cd and pb could also be a potential hazard for future martian explorers who will be exposed to the dust carried into their habitats on their space suits

mineralogy composition and alteration of mars pathfinder rocks and soils evidence from multispectral elemental and magnetic data on terrestrial analogue snc meteorite and pathfinder samples major element multispectral and magnetic properties data were obtained at ares vallis during the mars pathfinder mission to understand the compositional mineralogical and process implications of these data we obtained major element mineralogical and magnetic data for well crystalline and nanophase ferric minerals terrestrial analogue samples with known geologic context and snc meteorites analogue samples include unaltered palagonitic and sulfatetic tephra from mauna kea volcano hydrolytic and acid sulfate alteration steam vent material from kilauea volcano hydrolytic alteration and impactites from meteor crater relithification salient results for mars pathfinder include 1 band depths bd530b and bd600 and the reflectivity ratio r800 r750 are consistent with the dominant ferric mineral being nanophase ferric oxide associated with an unknown amount of h2o and occurring in composite particles along with subordinate amounts of other ferric minerals hematite and hematite plus nanophase goethite are most consistent with the data but maghemite akaganeite schwertmannite and nanophase lepidocrocite are also possible interpretations ferric oxides that are consistently not favored by the data as sole alteration products are jarosites and well crystalline goethite and lepidocrocite 2 the strength of the ferric adsorption edge r750 r445 implies the fe3 fe2 values for pathfinder rock and soil are within the ranges 0 7 3 and 3 20 respectively 3 ferrous silicates are indicated for subsets of pathfinder rocks and soils one subset has a band minimum near 930 nm that can attributed to low ca pyroxene alternatively the band could be a second manifestation of certain ferric oxides including nanophase goethite maghemite akaganeite and schwertmannite another subset has a negative spectral slope from 800 to 1005 nm which could result from the high energy wing of a high ca pyroxene and or olivine band a mixture of bright and dark materials and for rocks thin coatings of bright dust on dark rocks 4 chemical data on pathfinder rocks and soils are consistent with two component mixtures between an andesitic rock with low mgo and so3 concentrations soil free rock and a global basaltic soil with high mgo and so3 concentrations rock free soil pathfinder rock free soil can be modeled as a chemical mixture of snc meteorites and the pathfinder soil free rock 5 pathfinder soil cannot be obtained by chemical alteration of pathfinder rocks by any of the hydrolytic and acid sulfate alteration processes we studied presumably global mixing has obscured and possibly erased the elemental signatures of chemical alteration 6 the strongly magnetic phase in palagonitic and sulfatetic tephra is titanomagnetite and possibly its oxidation product titanomaghemite fe ti spinels the saturation magnetization of the tephra samples 0 5 2 0 am2 kg is at or below the low end of the range inferred for martian dust 4 2 am2 kg implying that lithogenic fe ti spinels are a possible candidate for the martian strongly magnetic phase 7 the predominantly palagonitic spectral signature and magnetic nature of martian soil and dust are consistent with glassy precursors with imbedded fe ti spinel particles comparison with lunar glass production rates suggests that production of sufficient quantities of glassy materials on mars by volcanic and impact processes is sufficient to account for these observations

potential for impact glass to preserve microbial metabolism abstract here we provide the first high resolution geochemical evidence for microbial metabolism to be preserved in impact generated materials this study is unique as not only do we merge complimentary analytical techniques such as high resolution spectromicroscopy to assess the biogenicity of tubules in impact glasses but we compare these results to those from co occurring abiotic quench crystallites as an intrinsic negative control scanning transmission x ray microscopy stxm near edge x ray absorption fine structure spectroscopy nexafs at the fe l 3 and c k edges revealed iron speciation patterns and organic c associated with tubular features in the impact glass the high spatial resolution of stxm combined with nexafs allowed organic carbon to be localized to the tubule features the fine energy resolution of nexafs allowed for unique populations of organic carbon to be spectrally differentiated between the tubule features and the matrix the distinct and systematic variation in iron redox states observed is consistent with microbially mediated dissimilatory iron reduction the ries tubules comprise the first trace fossil preserved in a substrate unique to the impact process thus illustrating the potential for microbial metabolism to be preserved in impact materials

suevite of the ries crater germany source rocks and implications for cratering mechanics suevites are impact breccias with a montmorillonitic matrix that contains shocked and unshocked mineral and rock fragments from the crystalline basement glass inclusions and a small amount of sedimentary clasts data are given of the modal composition of fall out suevites deposited at isolated points around the crater and crater suevite forming a layer below post impact lake sediments in the crater cavity fall out suevites contain aerodynamically shaped bombs which are absent in crater suevite taking into account not only large glass fragments and bombs but also the finer fractions the glass content of fall out and crater suevites amounts to 47 and 29 vol respectively crystalline clasts in suevites consist of all igneous and metamorphic rock types that constitute the local basement which consists of an upper layer of igneous rocks mainly granites and a lower series of gneisses and amphibolite based on a collection of 1 200 clasts from 13 suevite occurrences the average crystalline clast population of suevites was determined suevites contain on the average 46 igneous and 54 metamorphic clasts in constrast weakly shocked and unshocked crystalline ejecta of the ries structure consist of 82 igneous and 18 metamorphic rocks

impact craters in the northern hemisphere of mars layered ejecta and central pit characteristics mars global surveyor mgs and mars odyssey data are being used to revise the catalog of large martian impact craters analysis of data in the revised catalog provides new details on the distribution and morphologic details of 6795 impact craters in the northern hemisphere of mars this report focuses on the ejecta morphologies and central pit characteristics of these craters the results indicate that single layer ejecta sle morphology is most consistent with impact into an ice rich target double layer ejecta dle and multiple layer ejecta mle craters also likely form in volatile rich materials but the interaction of the ejecta curtain and target produced vapor with the thin martian atmosphere may be responsible for the large runout distances of these ejecta pancake craters appear to be a modified form of double layer craters where the thin outer layer has been destroyed or is unobservable at present resolutions pedestal craters are proposed to form in an ice rich mantle deposited during high obliquity periods from which the ice has subsequently sublimated central pits likely form by the release of vapor produced by impact into ice soil mixed targets therefore results from the present study are consistent with target volatiles playing a dominant role in the formation of crater morphologies found in the martian northern hemisphere

hydrothermally enhanced magnetization at the center of the haughton impact structure haughton is a 24 myr old mid size apparent diameter 23 km complex impact structure located on devon island in nunavut canada the center of the structure shows a negative gravity anomaly of 12 mgal coupled to a localized positive magnetic field anomaly of 900 nt a field expedition in 2013 led to the acquisition of new ground magnetic field mapping and electrical resistivity datasets as well as the first subsurface drill cores down to 13 m depth at the top of the magnetic field anomaly petrography rock magnetic and petrophysical measurements were performed on the cores and revealed two different types of clast rich polymict impactites 1 a white hydrothermally altered impact breccia not previously observed at haughton and 2 a grey impact breccia with no macroscopic sign of alteration in the altered core gypsum is present in macroscopic veins and in the form of intergranular selenite associated with colored and zoned carbonate clasts this altered core has a natural remanent magnetization nrm four to five times higher than materials from the other core but the same magnetic susceptibility their magnetization is still higher than the surrounding crater fill impact melt rocks x ray fluorescence data indicate a similar proportion of iron rich phases in both cores and an enrichment in silicates within the altered core in addition alternating field demagnetization results show that one main process remagnetized the rocks these results support the hypothesis that intense and possibly localized post impact hydrothermal alteration enhanced the magnetization of the clast rich impact melt rocks by crystallization of magnetite within the center of the haughton impact structure subsequent erosion was followed by in situ concentration in the subsurface leading to large magnetic gradient on surface

martian cratering revisited implications for early geologic evolution improved crater statistics from varied martian terrains are compared to lunar crater populations the distribution functions for the average martian cratered terrain and the average lunar highlands over the diameter range 8 2000 km are quite similar the martian population is less dense by approximately 0 70 from 8 to 256 km diameter and diverges to proportionally lower densities at greater diameters crater densities on martian pure terra give a lower limit to the mars moon integrated crater flux of 0 75 since the last stabilization of the respective planetary crusts the crater population 8 km diameter postdating the martian northern plains is statistically indistinguishable from that population postdating the lunar maria monte carlo simulations were performed to constrain plausible mechanisms of crater obliteration the models demonstrate that if the crater density difference between the lunar and martian terra has been due to resurfacing processes random intercrater plains formation cannot be the sole process if plains preferentially form in and obliterate larger craters then the observed martian distribution retains its shape as the crater density decreases this result is consistent with the morphology of martian intercrater plains

the effect of target lithology on the products of impact melting impact cratering is an important geological process on the terrestrial planets and rocky and icy moons of the outer solar system impact events generate pressures and temperatures that can melt a substantial volume of the target however there remains considerable discussion as to the effect of target lithology on the generation of impact melts early studies showed that for impacts into crystalline targets coherent impact melt rocks or sheets are formed with these rocks often displaying classic igneous structures e g columnar jointing and textures for impact structures containing some amount of sedimentary rocks in the target sequence a wide range of impactgenerated lithologies have been described although it has generally been suggested that impact melt is either lacking or is volumetrically minor this is surprising given theoretical constraints which show that as much melt should be produced during impacts into sedimentary targets the question then arises where has all the melt gone the goal of this synthesis is to explore the effect of target lithology on the products of impact melting a comparative study of the similarly sized haughton mistastin and ries impact structures suggests that the fundamental processes of impact melting are basically the same in sedimentary and crystalline targets regardless of target properties furthermore using advanced microbeam analytical techniques it is apparent that for the structures under consideration here a large proportion of the melt is retained within the crater as crater fill impactites for impacts into sedimentary bearing target rocks thus it is suggested that the basic products are genetically equivalent but they just appear different that is it is the textural chemical and physical properties of the products that vary

impact glasses in fallout suevites from the ries impact structure germany an analytical sem study t impact generated glasses from fallout suevite deposits at the ries impact structure have been investigated using analytical scanning electron microscopy approximately 320 analyses of glass clasts were obtained four glass types are distinguished on the basis of composition and microtextures type 1 glasses correspond to the aerodynamically shaped glass bombs studied previously by many workers major oxide concentrations indicate the involvement of granitic rocks amphibolites and minor al rich gneisses during melting type 2 glasses are chemically heterogeneous even within individual clasts with variations of several wt in most of the major oxides e g 57 70 wt sio2 this suggests incomplete mixing of 1 mineral derived melts or 2 whole rock melts from a wide range of lithologies aluminium rich clinopyroxene and fe mg rich plagioclase quench crystals are present in type 1 and 2 glasses respectively type 3 glasses contain substantial amounts of h2o 12 17 wt low sio2 50 53 wt high al2o3 17 21 wt and high cao 5 7 wt contents this suggests an origin due to shock melting of part of the sedimentary cover type 4 glasses form a ubiquitous component of the suevites based on their high sio2 content 85 100 wt the only possible protolith are sandstones in the lowermost part of the sedimentary succession calcite forms globules within type 1 glasses with which it develops microtextures indicative of liquid immiscibility unequivocal evidence also exists for liquid immiscibility between what are now montmorillonite globules and type 1 2 and 4 glasses indicating that montmorillonite was originally an impact melt glass clearly the melt zone at the ries must have incorporated a substantial fraction of the sedimentary cover as well as the underlying crystalline basement rocks impact melts were derived from different target lithologies and these separate disaggregated melts did not substantially mix in most cases type 2 3 and 4 glasses and carbonate melts