a tem investigation of shock metamorphism in quartz from the vredefort dome south africa the origin of the vredefort structure in south africa is still debated several causes have been discussed namely asteroid impact internal gas explosion or tectonic processes evidence of dynamic rock deformation is pervasive in the form of planar features in quartz grains shatter cones veins of pseudotachylite and occurrence of coesite and stishovite high pressure quartz polymorphs a number of these characteristics is widely believed to support an impact origin however the planar features in quartz which are generally considered as one of the strongest indicators of impact are in the vredefort case considered as anomalous when compared with those from accepted impact structures we have investigated by optical and transmission electron microscopy tem the defect microstructures in quartz grains from different lithologies sampled at various places at the vredefort structure whatever the locality only thin mechanical brazil twin lamellae in the basal plane are observed by tem so far such defects have only been found in quartz from impact sites but always associated with sets of thin glass lamellae in rhombohedral planes 10 1n with n 1 2 3 and 4 at the scale of the optical microscope brazil twins in 0001 are easily detected in vredefort quartz grains because of the numerous tiny fluid inclusions which decorate them similar alignments of tiny fluid inclusions parallel to other planes are also detected optically but at the tem scale no specific shock defects are detected along their traces if these inclusion alignments initially were shock features they are now so severely weathered that they can no longer be recognized as unambiguous shock lamellae fine grained coesite was detected in the vicinity of narrow pseudotachylite veinlets in a quartzite specimen but stishovite was not found even in areas where its occurrence was previously reported finally definite evidence of high temperature annealing was observed in all the samples these observations lead us to the conclusion that our findings regarding microdeformation in quartz are consistent with an impact origin for the vredefort structure most of the original shock defects are now overprinted by an intense post shock annealing episode only the thin mechanical twin lamellae in the basal plane have survived

40ar 39ar laser probe age determination confirms the ries impact crater as the source of glass particles in graupensand sediments grimmelfingen formation north alpine foreland basin mineralogical and chemical characteristics of reworked suevite material recovered from graupensand deposits grimmelfingen formation north alpine foreland basin middle miocene are identical to those recorded in ries suevite 40ar 39ar laser probe analyses of two glass particles from graupensand components and of a glass fragment from the ries suevite yielded ages ranging between 14 3 and 14 4 ma identical within analytical uncertainties hitherto the ries impact event was dated at 15 ma conventional k ar and 40ar 39ar methods whereas the graupensande have been placed into the ottnangian around 18 5 ma on the basis of mammal biostratigraphy small sample masses involved in laser probe technique allowed rigorous preselection of highly pure glass particles thus reducing possible age determination errors through partly molten or even unmolten mineral fragments from the variscan crystalline basement our results reconfirm previous statements considering the graupensande as reworked products of the ries impact ejecta blanket thus placing the age of the grimmelfingen formation into the badenian of middle miocene rather than into the ottnangian early miocene

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

hf zr and ree partition coefficients between ilmenite and liquid implications for lunar petrogenesis hf zr and ree partition coefficients between ilmenite and coexisting liquid were determined under near lunar conditions ilmenite was synthesized at 1125 c under low oxygen fugacity fo2 10 12 4 from glass of a high ti mare basalt composition doped with 50 100 g g of ree and hf and 500 1000 g g of zr the partition coefficients d of ilmenite for hf and zr obtained by isotope dilution analysis are 0 41 and 0 33 respectively these values are significantly lower than those of ilmenite 3 0 and 1 8 respectively from a kimberlite megacryst fujimaki et al 1984a dree for the synthesized ilmenite are slightly smaller than those of ilmenite from the kimberlite megacryst and the dlu is 0 056 these results suggest that ilmenite may have played an important role in the lu hf evolution of lunar mare basalts using newly determined dlu and dhf for ilmenite the lu hf evolution or lunar cumulates and the coexisting magma was examined for several different crystallization sequences by a model calculation the lu hf variation trend of most high ti mare basalts is well explained by a small degree less than 5 of partial melting of the cumulates however a higher degree of partial melting is required to explain the variation of very low ti basalts green glass and apollo 12 low ti basalts apollo 15 low ti basalts could not be produced by simple partial melting but may require chromite crystallization as well

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

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

northern hemisphere mid winter vortex displacement and vortex split stratospheric sudden warmings influence of the madden julian oscillation and quasi biennial oscillation we investigate the connection between the equatorial madden julian oscillation mjo and different types of the northern hemisphere mid winter major stratospheric sudden warmings ssws i e vortex displacement and vortex split ssws the mjo ssw relationship for vortex split ssws is stronger than that for vortex displacement ssws as a result of the stronger and more coherent eastward propagating mjos before vortex split ssws than those before vortex displacement ssws composite analysis indicates that both the intensity and propagation features of mjo may influence the mjo related circulation pattern at high latitudes and the type of ssws a pronounced quasi biennial oscillation qbo dependence is found for vortex displacement and vortex split ssws with vortex displacement split ssws occurring preferentially in easterly westerly qbo phases the lagged composites suggest that the mjo related anomalies in the arctic are very likely initiated when the mjo related convection is active over the equatorial indian ocean around the mjo phase 3 further analysis suggests that the qbo may modulate the mjo related wave disturbances via its influence on the upper tropospheric subtropical jet as a result the mjo related circulation pattern in the arctic tends to be wave number one wave number two 25 30 days following phase 3 i e approximately phases 7 8 when the mjo related convection is active over the western pacific during easterly westerly qbo phases which resembles the circulation pattern associated with vortex displacement vortex split ssws

40ar 39ar laser probe dating of the central european tektite producing impact event a new 40ar 39ar data set is presented for tektites from the central european strewn field moldavites this is the only strewn field that is entirely situated in a continental environment and still characterized by scattered ages 14 15 3 myr the main objectives of the study were to define more precisely the moldavite formation age and provide a good calibration for a glass standard proposed for fission track dating the laser total fusion ages obtained on chips from 7 individual specimens from the southern bohemian and moravian subfields are restricted to a narrow interval of time with an average of 14 34 0 08 myr relative to the 27 95 0 09 myr of the fish canyon tuff biotite this result gives a more precise age not only for the tektite field but also for its producing impact if the genetic link between the moldavites and the nordlinger ries impact crater is maintained then this new age has to be considered a reliable estimate for the ries crater also this new value places the formation of central european tektites within the lower serravallian period in the latest geologic timescales evidence of their impact products such as glass spherules or shocked minerals can therefore be sought in sedimentary marine formations in a more precisely defined age interval

impact melting on venus some considerations for the nature of the cratering record abstract modeling the volume of impact melt and its variation with the size of the impact event indicates that for similar sized final craters venusian impacts create about 25 more impact melt than terrestrial impacts more significantly venusian impacts result in approximately a factor of three more impact melt than lunar events producing equivalent sized craters this difference is due to the higher average impact velocity and higher ambient temperatures on venus which enhance impact melt production combined with higher planetary gravity which inhibits crater growth for a given impact event the initial higher intrinsic temperature of incorporated clastic debris also contributes to impact melts with higher initial temperatures lower viscosities and longer cooling times on venus with respect to lunar impact melts the enhanced production of relatively hot low viscosity impact melts under venusian impact conditions may account for the long exterior runout flows and also for the radar smooth interior floors of some venusian craters we also argue that the anomalously deep character of cleopatra may be attributed to drainage of its interior impact melt pool to form the smooth deposits in the adjacent fortuna tessera increasing depth of melting with increasing cavity size resulting in the progressive weakening of transient cavity floor material is offered as a possible explanation for the replacement of uplifted central peaks by rings with increasing crater diameter a consequence of this process is that interior rings will increase in diameter relative to the diameter of the final crater s rim crest with increasing crater size a trend observed on venus and other terrestrial planets this weakening of the target due to relatively enhanced impact melt production in the venusian environment makes it unlikely that orientale style impact basins ever formed on venus

terrestrial chondrules glass spherules and accretionary lapilli from the suevite ries crater germany abstract until recently no terrestrial analogues of meteoritic and lunar chondrules were known only rare glass spherules from the lonar crater india and black magnetic spherules from various localities have been recorded the impact breccia suevite of the no rdlinger ries crater germany contains both chondrules and glass spherules and in addition accretionary lapilli all of which are found imbedded within the fine grained matrix of the suevite the chondrules display many of the textural features characteristic of meteoritic and lunar chondrules lithic chondrules and fluid drop chondrules are present the latter having a composition quite similar to that of glass bombs and glass fragments in the suevite fluid drop chondrules developed from glass spherules by slow devitrification in the hot suevite ejecta masses after deposition on the whole fluid drop chondrules lithic chondrules and glass spherules are rare in the suevite with fluid drop chondrules prevailing detection of chondrules from a terrestrial impact crater supports theories of an impact origin for meteoritic and lunar chondrules accretionary lapilli also represent material formed as a result of impact

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

compositional variation and mixing of impact melts on microscopic scales we investigated the compositional characteristics of schlieren rich holohyaline impact glasses from ries wabar and meteor crater using a cameca sx 100 scanning electron microprobe this instrument is capable of producing detailed maps of major elements at spatial resolutions of 10 m the objective was to characterize the composition of an unusually large number of individual schlieren and to evaluate details of the process that causes melts of lithologically diverse target rocks to mix on scales of micrometers the ries and meteor crater impacts involved lithologically heterogeneous targets whereas wabar crater formed in relatively uniform dune sand texturally heterogeneous schlieren rich glasses from the ries crater illustrate that schlieren of highly variable color can be surprisingly similar in composition as first detailed by stahle 1972 consistent with these earlier findings most schlieren represent mixtures of diverse rock melts their compositions deviate only subtly from the average melt and do not resemble monomineralic melts nor binary mixtures of major rock forming minerals a specific population of schlieren is enriched in mafic elements mg fe and ca which suggests incomplete homogenization of an amphibolite progenitor in the case of wabar crater a compositionally simple melt of dune sand mixed with projectile iiia iron meteorite materials and specific schlieren are variable mixtures of these two progenitors the optically homogeneous glass from meteor crater is compositionally homogeneous as well which suggests ideal mixing of such diverse lithologies as platform carbonates sandstone and a class iiia iron meteorite the mixing of projectile and target melts at wabar and meteor crater unambiguously demonstrates that melts initially produced in distinctly different stratigraphic structural locations will undergo wholesale mixing if not homogenization also the projectile melts unquestionably formed relatively early in the cratering process and their dissemination throughout the prospective melt volume albeit at variable concentration levels suggests that the entire mixing process may be an early cratering feature this also follows from the fact that we investigated ballistic melt ejecta which thereby eliminates all of those mixing processes that may additionally operate during the pooling and generation of massive melt ponds following gravitational collapse of large structurally complex craters substantial turbulence ranging from field dimensions to microscopic scales seems inescapable to accomplish the observed degree of mixing yet this is not readily inferred from current models of macroscopic material motions during hypervelocity impact

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

formation of the axial relief at the very slow spreading southwest indian ridge 49 to 69 e the comparison of segment lengths relief and gravity signature along the very slow spreading southwest indian ridge swir between 49oe and 69oe suggests that the marked change in segmentation style that occurs across the melville transform 60o45 e reflects a change in the modes of formation of the axial topography we propose that the axial relief east of melville is largely due to volcanic constructions that load the axial lithosphere from above by contrast the axial relief in segments west of the melville fracture zone appears to be primarily due as proposed for segments of the faster spreading mid atlantic ridge to along axis changes in the depth of the axial valley and to partial compensation of negative loads thicker lower crust and or lighter upper mantle acting within the plate or at the bottom of the plate in terms of geology this means that the contribution of the uppermost effusive part of the crust to along axis crustal thickness variations may be greater east of melville than in other regions of the study area regional axial depths suggest that the ridge east of melville is also characterized by a low melt supply and is underlain by cold mantle a simple model of mantle melting and regional isostatic compensation suggests that differences in mantle temperature and in melt thickness between this deep eastern ridge region and the shallower region west of the gallieni transform 52o20 e are of the order of 80oc and 4 km respectively

the melt rocks of the boltysh impact crater ukraine ussr the 100 12 m y 25 km diameter boltysh impact crater was formed in precambrian granites and granite gneisses of the ukrainian shield the crater deposits have undergone minimal post impact erosion and it is possible to study a complete vertical section of the underlying 200 m thick melt sheet the melt rocks as sampled in two drill holes can be subdivided into two major textural classes microcrystalline and glassy the microcrystalline melt rocks form an uppermost and two lowermost units with the glassy variety occupying the middle of the melt sheet the microcrystalline units contain 25 zoned plagioclase phenocrysts set in a microcrystalline matrix of intergrown alkali feldspar and quartz pyroxene has been replaced by sheet silicates mineral and lithic clasts make up 5 15 and show varying degrees of shock and resorption the glassy melt rocks are characterized by 10 30 zoned plagioclase and 5 10 orthopyroxene set in a fresh to partially devitrified glassy matrix clast content is 5 chemically the melt rocks are relatively homogeneous and correspond to a mixture of kirovograd granites and gneisses in the ratio of 5 to 1 with ni ir and cr showing slight enrichments over the target rocks there are minor differences in the fe2o3 feo ratio and the alkalis between the microcrystalline and glassy varieties the increase in matrix crystallinity at the upper and lower contacts is contrary to observations at other impact melt sheets where greater matrix crystallinity occurs in the interiors of the melt sheets one possible explanation is that the melt matrix was originally glassy throughout due to its high sio2 content and the microcrystalline matrix is the result of extensive devitrification involving minor alkali exchange with circulating ground waters

stable isotope composition of impact glasses from the nordlinger ries impact crater germany abstract the ries impact which today is represented by a 24 km diameter complex crater occurred at 15 ma it is estimated that a projectile of about 1 km diameter formed a transient crater of 6 to 7 km radius and 2 8 km depth impact melt glasses investigated are found within the suevite the breccia that forms the uppermost layers of the ejecta blanket around the crater the glasses are considered to represent quenched melts produced as a result of the impact the oxygen isotope compositions of several glasses sampled from widely spaced localities are very homogeneous with 18 o values in the range of 6 7 to 7 4 and 17 o of 3 3 to 3 7 with increasing devitrification and alteration the 18 o values increase up to 15 8 17 o up to 8 1 water content also increases with devitrification from fresh glasses with about 1 3 wt to 3 8 wt in devitrified glass d values decrease with increasing water content from about 87 to 127 whole rock oxygen isotope compositions of the sedimentary cover sequence range from 17 to 27 and crystalline basement rocks range from 8 8 to 13 5 for granites and gneisses whereas amphibolites have 18 o values of 5 2 and 6 1 models suggesting that the glass in the suevite represents a mixed melt derived from all the rocks present in the suevite in proportion of their occurrence engelhardt 1997 do not agree with the oxygen isotope composition of the glass the simplest explanation for the homogeneous chemical and oxygen isotope composition is that the glass represents quenched melts from a few closely spaced lithologies only high fe and ni contents in the glass cr ni and cr co ratios and relatively low 18 o values indicate involvement of amphibolites during melting the balance of the melt was made up of spatially associated granites or gneisses as indicated by the major element and rare earth element geochemistry the best agreement between major element and oxygen isotope composition of the glass and model melts is obtained by mixtures of amphibolite and granite in proportions similar to their average occurrence in the fallout suevite

suevite breccia of the ries impact crater germany petrography chemistry and shock metamorphism of crystalline rock clasts clasts of deep seated crystalline basement rocks in suevites of the ries crater germany were catalogued lithologically and classified with regard to their degree of shock metamorphism the sample suite consisted of 806 clasts from 10 outcrops in fallout suevites and 447 clasts from drill cores encountering crater suevite in the crater interior these clasts can be grouped into seven types of metamorphic and nine types of igneous rocks one hundred forty three clasts representing these lithologies were analyzed for major element bulk composition the fallout suevite contains on average 4 vol of crystalline basement clasts 0 4 vol of sedimentary rocks 16 vol of glass bodies some of them aerodynamically shaped and 79 vol of groundmass on average 52 of all crystalline clasts are from metamorphic sources and 42 are of igneous origin using the shock classification of stoffler 1974 8 of all crystalline clasts appear unshocked 10 gpa and 34 30 and 27 of clasts are shocked to stages i 10 35 gpa ii 35 45 gpa and iii 45 60 gpa respectively the bulk composition of suevite glasses is consistent with the modal proportions of crystalline rock types observed in the clast populations this indicates that the glasses originate by shock fusion of a similarly composed basement the crater suevite contains the same crystalline rock types that occur in the fallout suevites the bore hole nordlingen 1973 yields an average of 62 vol metamorphic and 38 vol igneous rocks the crater suevite differs from fallout suevites by a higher clast glass ratio by preponderance 65 95 of clasts shocked to stage i only and by the absence of aerodynamically shaped glass bodies the source of crystalline clasts and melt particles of suevites is a volume of rocks located deep in the crystalline basement to which the projectile transmittted most of its energy so that only rocks of the basement were shocked by pressures exceeding 10 gpa deep burst impact model fallout suevites were ejected propelled by an expanding plume of vaporized rock and withdrew preferentially from this volume melt and highly shocked clasts leaving in the transient cavity the crater suevite with more clasts of modest shock levels and less melt

coeval argon 40 argon 39 ages of moldavites from the bohemian and lusatian strewn fields 40ar 39ar ages of four tektites moldavites from southern bohemia near cesk budejovice czech republic and a tektite from lusatia near dresden germany have been determined by 11 step degassing experiments the purpose of the study was to enlarge the 40ar 39ar data base of moldavites and to check the age relations of the bohemian and lusatian samples the mean plateau age of the bohemian samples which range from 14 42 to 14 70 ma is 14 50 0 16 0 42 2 ma errors in parentheses include age error and uncertainty of standard monitor age the plateau age of the lusatian sample of 14 38 0 26 0 44 2 ma confirms the previously published 40ar 39ar age of 14 52 0 08 0 40 2 ma and demonstrates that the fall of lusatian and bohemian tektites were contemporaneous because of their geochemistry and their ages there is no doubt that the lusatian tektites are moldavites accepting that moldavites are ejecta from the nrdlinger ries impact the new ages also date the impact event this age is slightly younger about 0 2 0 3 ma than the age suggested by earlier k ar determinations

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

carbonate silicate liquid immiscibility upon impact melting ries crater germany the 24 km diameter ries impact crater in southern germany is one of the most studied impact structures on earth the ries impactor struck a triassic to upper jurassic sedimentary sequence overlying hercynian crystalline basement at the time of impact 14 87 0 36 ma storzer et al 1995 the 350 m thick malm limestone was present only to the south and east of the impact site to the north and west the malm had been eroded away exposing the underlying dogger and lias the largest proportion of shocked target material is in the impact melt bearing breccia suevite the suevite had been believed to be derived entirely from the crystalline basement calcite in the suevite has been interpreted as a postimpact hydrothermal deposit from optical inspection of 540 thin sections of suevite from 32 sites i find that calcite in the suevite shows textural evidence of liquid immiscibility with the silicate impact melt textural evidence of liquid immiscibility between silicate and carbonate melt in the ries suevite includes carbonate globules within silicate glass silicate globules embedded in carbonate deformable and coalescing carbonate spheres within silicate glass sharp menisci or cusps and budding between silicate and carbonate melt fluidal textures and gas vesicles in carbonate schlieren a quench crystallization sequence of the carbonate spinifex textured quenched carbonate separate carbonate spherules in the suevite mineral fragment matrix and inclusions of mineral fragments suspended in carbonate blebs given this evidence of liquid immiscibility the carbonate in the suevite therefore has like the silicate melt a primary origin by impact shock melting evidence of carbonate silicate liquid immiscibility is abundant in the suevites from the southwest to east of the ries crater the rarer suevites to the west to northeast of the crater are nearly devoid of carbonate melts this correspondence between the occurrence of outcropping limestones at the target surface and the formation of carbonate melt indicates that the malm limestones are the source rocks of the carbonate impact melt this correspondence shows that the suevites preserve a compositional memory of their source rocks from the regional distribution of suevites with or without immiscible carbonate melts it is inferred that the ries impactor hit the steep albtrauf escarpment at its toe in an oblique impact from the north

impacts do not initiate volcanic eruptions eruptions close to the crater many papers on meteorite impact suggest that large impacts can induce volcanic eruptions through decompression melting of the underlying rocks we perform numerical simulations of the impact of an asteroid with a diameter of 20 km striking at 15 km s 1 into a target with a near surface temperature gradient of 13 k km 1 cold case or 30 k km 1 hot case the impact creates a 250 300 km diameter crater with 10 000 km3 of impact melt however the crater collapses almost flat and the pressure field returns almost to the initial lithostat even an impact this large cannot raise mantle material above the peridotite solidus by decompression statistical considerations also suggest that impacts cannot be the common initiator of large igneous provinces any time in post heavy bombardment earth history

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