spatial distribution of zircon with shock microtwins in pseudotachylite bearing granite gneisses vredefort impact structure south africa abstract mechanical microtwins in zircon is a characteristic shock feature and a potential tool for estimating shock pressure and or shear stress however so far zircon with microtwins has rarely been described in situ the present study of pseudotachylite bearing granitic gneisses from the vredefort impact structure focuses on the spatial distribution of zircon with microtwins and other deformation features zircon hosted by granitic gneiss does not reveal extensive shock deformation with decreasing distance to pseudotachylite zircon progressively develops sub planar fractures planar deformation bands low angle boundaries rarely thin 112 microtwin lamellae and closest to the vein it is fragmented in contrast zircon hosted by pseudotachylites contains thick and closely spaced microtwin lamellae which form up to three sets within individual grains microtwin lamellae are associated with planar 112 low angle boundaries and with overprinting crystal plastic and brittle deformation we suggest that planar 112 low angle boundaries in zircon are indicative of shock deformation and that microtwins in zircon within pseudotachylite veins are an indicator of the impact origin for the latter as opposed to endogenic seismically induced friction melts locally elevated shock pressure and shear stress induce the formation of pseudotachylites in the target rock as well as shock microtwins in zircon

impact melt and projectile bearing ejecta at barringer crater arizona abstract our understanding of the impact cratering process continues to evolve and even at well known and well studied structures there is still much to be learned here we present the results of a study on impact generated melt phases within ejecta at barringer crater arizona one of the first impact craters on earth to be recognized and arguably the most famous we report on previously unknown impact melt bearing breccias that contain dispersed fragments of the projectile as well as impact glasses that contain a high proportion of projectile material higher than any other glasses previously reported from this site these glasses are distinctly different from so called melt beads that are found as a lag deposit on the present day erosion surface and that we also study it is proposed that the melts in these impact breccias were derived from a more constrained sub region of the melt zone that was very shallow and that also had a larger projectile contribution in addition to low and high fe melt beads documented previously we document ca mg rich glasses and calcite globules within silicate glass that provide definitive evidence that carbonates underwent melting during the formation of barringer crater we propose that the melting of dolomite produces ca mg rich melts from which calcite is the dominant liquidus phase this explains the perhaps surprising finding that despite dolomite being the dominant rock type at many impact sites including barringer crater calcite is the dominant melt product when taken together with our estimate for the amount of impact melt products dispersed on and just below the present day erosional surface it is clear that the amount of melt produced at barringer crater is higher than previously estimated and is more consistent with recent numerical modeling studies this work adds to the growing recognition that sedimentary rocks melt during hypervelocity impact and do not just decompose and or devolatilize as was previously thought this has implications for understanding the processes and products of impacts into sedimentary rocks and for estimating the amount of climatically active gases released by impact events

addendum to stoffler d hamann c and metzler k shock metamorphism of planetary silicate rocks and sediments proposal for an updated classification system meteoritics planetary science 53 5 49 2018

devolatilization or melting of carbonates at meteor crater az we have investigated the carbonates in the impact melts and in a monolithic clast of highly shocked coconino sandstone of meteor crater az to evaluate whether melting or devolatilization is the dominant response of carbonates during high speed meteorite impact both melt and clast carbonates are calcites that have identical crystal habits and that contain anomalously high sio2 and al2o3 also both calcite occurrences lack any meteoritic contamination such as fe or ni which is otherwise abundantly observed in all other impact melts and their crystallization products at meteor crater the carbon and oxygen isotope systematics for both calcite deposits suggest a low temperature environment 100 c for their precipitation from an aqueous solution consistent with caliche we furthermore subjected bulk melt beads to thermogravimetric analysis and monitored the evolving volatiles with a quadrupole mass spectrometer co2 yields were 5 wt with typical values in the 2 wt range also total co2 loss is positively correlated with h2o loss an indication that most of these volatiles derive from the secondary calcite also transparent glasses considered the most pristine impact melts yield 100 wt element totals by empa suggesting complete loss of co2 the target dolomite decomposed into mgo cao and co2 the co2 escaped and the cao and mgo combined with sio2 from coexisting quartz and feo from the impactor to produce the dominant impact melt at meteor crater although confined to meteor crater these findings are in stark contrast to osinski et al 2008 who proposed that melting of carbonates rather than devolatilization is the dominant process during hypervelocity impact into carbonate bearing targets including meteor crater

deep crustal carbonate rocks exposed by meteor impact on mars mars may have once had a co2 rich atmosphere but carbonate rocks that could provide evidence for such conditions are sparse spectral analyses of rocks exposed from deep within an impact crater reveal that carbonate deposits may be extensive on mars but are buried under layers of younger volcanic rocks

impacts into quartz sand crater formation shock metamorphism and ejecta distribution in laboratory experiments and numerical models we investigated the ejection mechanics by a complementary approach of cratering experiments including the microscopic analysis of material sampled from these experiments and 2 d numerical modeling of vertical impacts the study is based on cratering experiments in quartz sand targets performed at the nasa ames vertical gun range in these experiments the preimpact location in the target and the final position of ejecta was determined by using color coded sand and a catcher system for the ejecta the results were compared with numerical simulations of the cratering and ejection process to validate the isale shock physics code in turn the models provide further details on the ejection velocities and angles we quantify the general assumption that ejecta thickness decreases with distance according to a power law and that the relative proportion of shocked material in the ejecta increase with distance we distinguish three types of shock metamorphic particles 1 melt particles 2 shock lithified aggregates and 3 shock comminuted grains the agreement between experiment and model was excellent which provides confidence that the models can predict ejection angles velocities and the degree of shock loading of material expelled from a crater accurately if impact parameters such as impact velocity impactor size and gravity are varied beyond the experimental limitations this study is relevant for a quantitative assessment of impact gardening on planetary surfaces and the evolution of regolith layers on atmosphereless bodies

the l3 6 chondritic regolith breccia northwest africa nwa 869 i petrology chemistry oxygen isotopes and ar ar age determinations northwest africa nwa 869 consists of thousands of individual stones with an estimated total weight of about 7 metric tons it is an l3 6 chondrite and probably represents the largest sample of the rare regolith breccias from the l chondrite asteroid it contains unequilibrated and equilibrated chondrite clasts some of which display shock darkening impact melt rocks imrs both clast free and clast poor are strongly depleted in fe ni metal and sulfides an unequilibrated microbreccia two different light inclusions and two different sio2 bearing objects were found although the matrix of this breccia appears partly clastic it is not a simple mixture of fine grained debris formed from the above lithologies but mainly represents an additional specific lithology of low petrologic type we speculate that this material stems from a region of the parent body that was only weakly consolidated one imr clast and one sio2 bearing object show 17o values similar to bulk nwa 869 suggesting that both are related to the host rock in contrast one light inclusion and one imr clast appear to be unrelated to nwa 869 suggesting that the imr clast is contaminated with impactor material 40ar 39ar analyses of a type 4 chondrite clast yield a plateau age of 4402 7 ma which is interpreted to be the result of impact heating other impact events are recorded by an imr clast at 1790 36 ma and a shock darkened clast at 2216 40 ma demonstrating that nwa 869 escaped major reset in the course of the event at approximately 470 ma that affected many l chondrites

some consequences of a proposed fractal nature of continental faulting a key question in the study of continental deformation concerns the preservation of undeformed regions sometimes quite large in the midst of otherwise penetrative deformation some authors1 have interpreted such regions as being relatively strong but we argue here that the existence of large undeformed regions might instead arise naturally from a fractal style of continental faulting a scaling analysis derived from laboratory simulations of the india asia collision2 5 yields a predicted distribution of undeformed regions which can be tested in the field we also show that if continental faulting is fractal in nature this will limit the validity of homogenization approaches to the modelling of continental deformation in which the rheology of the lithosphere is assumed to be homogeneous at scales larger than a threshold value

transformation textures mechanisms of formation of high pressure minerals in shock melt veins of l6 chondrites and pressure temperature conditions of the shock events the high pressure polymorphs of olivine pyroxene and plagioclase in or adjacent to shock melt veins smvs in two l6 chondrites sahara 98222 and yamato 74445 were investigated to clarify the related transformation mechanisms and to estimate the pressure temperature conditions of the shock events wadsleyite and jadeite were identified in sahara 98222 wadsleyite ringwoodite majorite akimotoite jadeite and lingunite naalsi3o8 hollandite were identified in yamato 74445 wadsleyite nucleated along the grain boundaries and fractures of original olivine the nucleation and growth of ringwoodite occurred along the grain boundaries of original olivine and as intracrystalline ringwoodite lamellae within original olivine the nucleation and growth of majorite took place along the grain boundaries or fractures in original enstatite jadeite containing assemblages have complicated textures containing particle like stringer like and polycrystalline like phases coexistence of lingunite and jadeite containing assemblages shows a vein like texture we discuss these transformation mechanisms based on our textural observations and chemical composition analyses the shock pressure and temperature conditions in the smvs of these meteorites were also estimated based on the mineral assemblages in the smvs and in comparison with static high pressure experimental results as follows 13 16 gpa 1900 c for sahara 98222 and 17 24 gpa 2100 c for yamato 74445

decline of giant impacts on mars by 4 48 billion years ago and an early opportunity for habitability the timing of the wane in heavy meteorite bombardment of the inner planets is debated its timing determines the onset of crustal conditions consistently below the thermal and shock pressure limits for microbiota survival and so bounds the occurrence of conditions that allow planets to be habitable here we determine this timing for mars by examining the metamorphic histories of the oldest known martian minerals 4 476 4 429 gyr old zircon and baddeleyite grains in meteorites derived from the southern highlands we use electron microscopy and atom probe tomography to show that none of these grains were exposed to the life limiting shock pressure of 78 gpa 97 of the grains exhibit weak to no shock metamorphic features and no thermal overprints from shock induced melting by contrast about 80 of the studied grains from bombarded crust on earth and the moon show such features the giant impact proposed to have created mars hemispheric dichotomy must therefore have taken place more than 4 48 gyr ago with no later cataclysmic bombardments considering thermal habitability models we conclude that portions of mars crust reached habitable pressures and temperatures by 4 2 gyr ago the onset of the martian wet period about 0 5 gyr earlier than the earliest known record of life on earth early abiogenesis by 4 2 gyr ago is now tenable for both planets the oldest known minerals from mars have no strong shock features indicating early cessation of giant impacts there according to microanalysis of zircon and baddeleyite grains in meteorites

ahrensite γ fe 2 sio 4 a new shock metamorphic mineral from the tissint meteorite implications for the tissint shock event on mars ahrensite ima 2013 028 fe 2sio 4 is the natural fe analog of the silicate spinel ringwoodite mg 2sio 4 it occurs in the tissint martian meteorite where it forms through the transformation of the fayalite rich rims of olivine megacrysts or fe rich microphenocrysts in contact with shock melt pockets the typical sequence of phase assemblages traversing across a tissint melt pocket into olivine is quenched melt or fayalite pigeonite intergrowth bridgmanite wustite ahrensite and or ringwoodite highly deformed olivine nanocrystalline ringwoodite deformed olivine we report the first comprehensive set of crystallographic spectroscopic and quantitative chemical analysis of type ahrensite and show that concentrations of ferric iron and inversion in the type material of this newly approved mineral are negligible we also report the occurrence of nanocrystalline ringwoodite in strained olivine and establish correlations between grain size and distance from melt pockets the ahrensite and ringwoodite crystals show no preferred orientation consistent with random nucleation and incoherent growth within a highly strained matrix of olivine grain sizes of ahrensite immediately adjacent to melt pockets are consistent with growth during a shock of moderate duration 1 10 ms

chemical and structural changes induced by thermal annealing of shocked feldspar inclusions in impact melt rocks from lappajarvi crater finland plagioclase and microcline of clastic mineral and rock inclusions in impact melt rocks karnaite were sampled from the coherent melt sheet located in the central crater area of the lappajarvi meteorite crater and analyzed by optical electron optical x ray and microprobe methods both types of feldspars display strong chemical textural and structural alterations compared to the feldspars of their parental crystalline basement rocks as a result of the thermal energy supplied by the surrounding superheated impact melt increase of shock experienced by the clasts prior to their incorporation into the melt results in increasing thermal alteration by the melt in a lower degree of shock both primary plagioclase and microcline transform to checkerboard textured grains that consist of 5 10 m sized subgrains of parallel optical orientation embedded in a mesostasis of eutectically grown sanidine and quartz the subgrains are strongly zoned disordered labradorite to andesine and rimmed by sanidine in a high degree of shock the subgrains are larger and randomly oriented the subgrains are the result of a nonequilibrium fractional crystallization from a liquid state during which a strong chemical exchange between the melt and the feldspar inclusions took place on the basis of model calculations it is concluded that the temperature of the karnaite melt at the time of the incorporation of the clastic debris was in the range of 1800 to 2100 c the equilibration temperature of the clast melt mixture is estimated accordingly to be at least in the range of 1100 to 1140 c or higher depending on the magnitude of the clast s postshock temperature this temperature is above the solidus and in part above the liquidus temperature of a dry granodioritic melt and is also above the solidus temperatures of the feldspars the clast laden melt was at rest when the equilibrium temperature was reached suggesting that the readjustment of the transient crater cavity by central uplifting of the crater took place within a time scale of hours

evidence for shock induced anhydrite recrystallization and decomposition at the unam 7 drill core from the chicxulub impact structure

thermal annealing of experimentally shocked feldspar crystals single crystals of sanidine orthoclase microcline oligoclase and labradorite as well as polycrystalline bytownite were shocked experimentally to peak pressures as high as 45 gpa and subsequently annealed under dry conditions for various times at 800 c 900 c and 1000 c these experiments reveal that alkali feldspars do not change their al si distribution due to shock alone but when subjected to shock pressures 22 gpa and then annealed microcline and orthoclase rapidly transform to sanidine displaying a highly disordered al si distribution high diffusivity of k na and ca was observed in a labradorite sample shocked to 26 gpa subsequent annealing may thus disturb chemical structural and isotopic characteristics of shocked minerals and rocks more easily than of unshocked specimens this is of considerable importance for the interpretation of isotopic age data of impact breccias recrystallization textures of annealed diaplectic plagioclase glasses are very distinctive and may be used as a relative measure of the shock pressure experienced by the samples

ca rich majorite derived from high temperature melt and thermally stressed hornblende in shock veins of crustal rocks from the ries impact crater germany shock veins up to 1 1 mm thick were found within non porous lithic clasts from suevite breccia of the nordlinger ries impact structure these veins were studied by optical microscopy in transmitted and reflected light and by scanning electron microscopy in shocked amphibolites two types of ca rich majorite occur within and adjacent to the veins the first type crystallized from shock induced melts within the veins si contents of these majorites suggest dynamic pressure of 15 17 gpa implying minimum temperatures in the range of 2 150 2 230 c the second type of majorite was formed adjacent to the shock veins within pargasitic hornblende this majorite contains significant amounts of h2o 0 7 0 9 wt based on the textural setting the shrinkage cracks and the chemical compositions of both phases a solid state mechanism is deduced for the hornblende to majorite phase transition both genetic types of ca rich majorite are described for the first time from a terrestrial impact crater along with stishovite majorite constitutes the second silicate mineral displaying sixfold coordination of si at ries using micro raman spectroscopy jadeite coesite and jadeite grossular were identified within local melt glasses of alkali feldspar and plagioclase composition respectively stishovite aggregates produced by solid state reaction along with shock induced high pressure melt glasses of almandine composition were also detected in shock veins of a garnet cordierite sillimanite restite the quenched homogeneous almandine glasses point to melting temperatures of more than 2 500 c for the veins our findings demonstrate that terrestrial shock veins can give valuable information on shock induced mineral transformations and transient high pressures of host rocks during a natural impact event

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

fine structures of planar deformation features in shocked olivine a comparison between martian meteorites and experimentally shocked basalts as an indicator for shock pressure

shock effects on the k ar system of plagioclase feldspar and the age of anorthosite inclusions from north eastern minnesota abstract artificial shock pressures up to 52 5 gpa have no influence on the k ar system of plagioclase feldspar the 40ar 39ar analysis of feldspar labradorite an67 from anorthosite of north eastern minnesota shocked up to 45 gpa in vacuo to prevent massive entrapment of atmospheric argon shows that the age spectra and the argon diffusion properties remain unaltered similar feldspar samples labradorite an51 from nain labrador shocked in air up to 52 5 gpa and dated by the conventional k ar method also yield the same age as unshocked samples but with a higher atmospheric argon contribution the minnesota anorthosite has an 40ar 39ar age of 1075 10 m y no information on a possible previous history of the anorthosite became apparent

petrology geochemistry and mineralization of the las aguilas and virorco mafic ultramafic bodies san luis province argentina the layered mafic ultramafic rocks in the sierras de san luis argentina las aguilas las higueras and virorco constitute a 3 5 km wide belt that extends over 100 km from ne to sw they carry a sulphide mineralization consisting of pyrrhotite pentlandite and chalcopyrite in veins and as disseminated to massive ore disseminated spinels are frequently associated with the sulphide minerals as well as platinum group minerals a strong correlation between s ni co cu cr pt and pd indicates the presence of one to three levels of mineralization within the ultramafic units the maximum concentration of these elements coincides with the units containing platinum group minerals pgm and spinel group minerals this clear relationship constitutes a good prospecting guide in the search for layers with high grade ore probably associated with deeper stratigraphic levels where ultramafic rocks are dominant the bulk rock chemistry and concentrations of metals and platinum group elements as well as textural evidence suggest that the parental magma was mafic with tholeiitic affinities and mgo rich the las aguilas layered mafic ultramafic body and the remaining bodies in the area bear similar characteristics to well known stratified complexes developed in extensional tectonic regimes as it is the case of jinchuan china kabanga tanzania and fiambala argentina

akimotoite mg fe sio3 a new silicate mineral of the ilmenite group in the tenham chondrite akimotoite mg fe sio 3 a new silicate mineral of the ilmenite group was found in the shock metamorphosed tenham chondrite it occurs as aggregates adjacent to clinoenstatite in fragments within shock induced melt veins chemical analyses show the simplified formula to be mg sub 0 79 fe sub 0 21 sio 3 the same as for clinoenstatite selected area electron diffraction saed patterns correspond to the synthetic mg fe sio 3 ilmenite phase with space group r3 lattice parameters derived from saed patterns are a 0 478 5 nm c 1 36 1 nm and v 0 269 8 nm 3 in the hexagonal setting the calculated density is 4 0 1 g cm 3 akimotoite in this occurrence is thought to have been transformed from original orthoenstatite in a solid state reaction produced by a shock event peak pressure and temperature generated by shock events in tenham are estimated to be 22 gpa max max 2000 degrees c assuming that equilibrium crystallization of aluminous majorite occurred in the melt veins this new mineral was named after syun iti akimoto

shock metamorphism of planetary silicate rocks and sediments proposal for an updated classification system we reevaluate the systematics and geologic setting of terrestrial lunar martian and asteroidal impactites resulting from single or multiple impacts for impactites derived from silicate rocks and sediments we propose a unified and updated system of progressive shock metamorphism shock metamorphosed rocks occur as lithic clasts or melt particles in proximal impactites at impact craters and rarely in distal impactites they represent a wide range of metamorphism typically ranging from unshocked to shock melted as the degree of shock metamorphism at a given shock pressure depends primarily on the mineralogical composition and the porosity of a rock or sediment sample different shock classification systems are required for different types of planetary rocks and sediments we define shock classification systems for eight rock and sediment classes which are assigned to three major groups of rocks and sediments 1 crystalline rocks with classes f m a and u 2 chondritic rocks class c and 3 sedimentary rocks and sediments with classes sr se and re the abbreviations stand for felsic f mafic m anorthositic a ultramafic u sedimentary rocks sr unconsolidated sediments se and regoliths re in each class the progressive stages of shock metamorphism are denominated s1 to sx these progressive shock stages are introduced as s1 s7 for f s1 s7 for m s1 s6 for a s1 s7 for u s1 s7 for c s1 s7 for sr s1 s5 for se and s1 s6 for re s1 stands for unshocked and sx variable between s5 and s7 stands for whole rock melting we propose a sequence of symbols characterizing the degree of shock metamorphism of a sample i e f s1 to f s7 with the option to add the tabulated pressure ranges in gpa in parentheses

petrology and geochemistry of olivine phyric shergottites lar 12095 and lar 12240 implications for their petrogenetic history on mars larkman nunatak lar 12095 and lar 12240 are recent olivine phyric shergottite lnds we report the results of petrographic and chemical analyses of these two samples to understand their petrogenesis on mars based on our analyses we suggest that these samples are likely paired and are most similar to other depleted olivine phyric shergottites particularly dar al gani dag 476 and sayh al uhaymir sau 005 and samples paired with those the olivine megacryst cores in lar 12095 and lar 12240 are not in equilibrium with the groundmass olivines we infer that these megacrysts are phenocrysts and their major element compositions have been homogenized by diffusion the cores of the olivine megacrysts have mg 70 whereas megacryst rims and groundmass olivines typically have mg 58 60 the rare earth element ree microdistributions in the various phases olivine low and high ca pyroxene maskelynite and merrillite in both samples are similar and support the likelihood that these two shergottites are indeed paired the calculated parent melt i e in equilibrium with the low ca pyroxene which is one of the earliest formed ree bearing minerals has an ree pattern parallel to that of melt in equilibrium with merrillite i e one of the last formed minerals this suggests that the lar 12095 12240 paired shergottites represent the product of closed system fractional crystallization following magma emplacement and crystal accumulation utilizing the europium oxybarometer we estimate that the magmatic oxygen fugacity early in the crystallization sequence was iw finally petrographic evidence indicates that lar 12095 12240 experienced extensive shock prior to being ejected from mars

feather features a low shock pressure indicator in quartz 1 feather features are a recently discovered type of microstructure in shocked quartz they occur as short parallel to subparallel lamellae with spacing similar to that of planar deformation features pdfs these lamellae are always found in combination with a planar fracture pf feather features are crystallographically controlled 0001 101 and 112 orientations are common while 103 and 102 orientations are lacking samples from the nordlinger ries and the matt wilson impact structures are compared with plane wave shock recovery experiments 16 gpa microscopic analysis of quartz grains with feather features show that their formation is linked to shearing along the associated pfs during shock deformation the generation of shear fractures with feather features in plane wave shock recovery experiments suggests a pressure range of 7 10 gpa although further constraints are needed for an upper and lower pressure limit the orientation of feather features in rock samples is suggested to be controlled by the direction of the principal axis of stress with sheared pfs at 45 angles to the axis and most feather feature lamellae aligned parallel to the axis 20 the majority of feather feature lamellae emanate from the pfs in the same direction and in experiments point in the direction from which the shock wave came on the basis of their appearance in shocked quartz grains from 26 impact craters and the current lack of reports of these features in endogenically deformed crustal rocks their uniqueness as a new type of impact induced planar microstructure alongside pdfs and pfs is proposed

co2 release due to impact devolatilization of carbonate results of shock experiments a study of pure single crystal calcite shocked to pressures from 9 0 to 60 8 gpa was conducted to address contradictory data for carbonate shock behavior the recovered materials were analyzed optically and by transmission electron microscopy tem as well as by thermogravimetry tga x ray diffraction xrd and raman spectroscopy in thin section progressive comminution of calcite is observed although grains remain birefringent to at least 60 8 gpa tga analysis reveals a positive correlation between percent of mass loss due to shock and increasing shock pressure r 0 77 and suggests that shock loading leads to the modest removal of structural volatiles in this pressure range xrd patterns of shocked iceland spar samples produce peaks that are qualitatively and quantitatively less intense more diffuse and shift to lower o2 however the regularity observed in these shocked powder patterns suggests that structures with very uniform unit cell separations persist to shock pressures as high as 60 8 gpa raman spectral analyses indicate no band asymmetry and no systematic peak shifting or broadening tem micrographs display progressively diminishing crystallite domain sizes selected area electron diffraction saed patterns reveal no signatures of amorphous material these data show that essentially intact calcite is recovered at shock pressures up to 60 8 gpa with only slight mass loss 7 this work suggests that the amount of co2 gas derived from shock devolatilization of carbonate by large meteorite impacts into carbonate targets has been substantially overestimated

shock effects and argon loss in samples of the leedey l6 chondrite experimentally shocked to 29 70 ga pressures abstract shock recovery experiments between 29 and 70 gpa were made on the leedey l6 chondrite to determine the possible loss of radiogenic ar caused directly by passage of shock waves as opposed to purely thermal loss during prolonged post shock cooling experiments were made in vacuum to prevent shock implantation of terrestrial air and following neutron irradiation the 39 ar 40 ar ratios were measured by stepwise temperature release petrographie examination of experimentally shocked leedey showed predominantly solid state effects in the form of increasing disaggregation of component minerals with increasing shock pressure but very little melting even at the highest shock levels the melting behavior of the minor feldspar phase was difficult to document but likely was complete by the higher shock levels the 39ar 40ar release patterns were complex for both shocked and reference leedey probably because of a natural shock history for the meteorite at ca 3 8 gy the average 39ar 40ar age for shocked leedey decreased with increasing shock levels from essentially no change at 29 gpa to a 20 decrease in age at 60 gpa the feldspar phases degas at lower extraction temperatures and show even greater lowering of 39ar 40ar age with increasing shock pressure the sample shocked at 69 gpa shows less mechanical disruption than samples shocked to lesser pressures and also shows a smaller loss of ar compared to samples shocked at 45 and 60 gpa we conclude that observed ar loss is caused by extensive grain fracturing and heating during shock and that greater ar loss at low extraction temperatures is enhanced by feldspar melting none of the experiments duplicated the magnitude of ar loss commonly seen in naturally shocked materials that lack evidence of pervasive melting we conclude that large scale gas loss in shocked but unmelted samples requires that the materials reside for prolonged periods of time at elevated temperatures in a relatively hot impact formation either inside or outside the crater cavity

shock compaction heating and collisional processes in the production of type 3 ordinary chondrites lessons from the nearly unique l3 chondrite melt breccia northwest africa 8709

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

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

a decade of progress in earth s internal properties and processes a major component of the inter union commission on geodynamics project labeled internal properties and processes included certain experimental and theoretical research in tectonophysics seismology geochemistry petrology volcanology and planetology this review focuses on a few research areas in which there have been surprises and reversals in particular attention is given to the attempts to quantify the thermal profile in the earth s interior and the material properties of the earth s interior

mineral chemistry and in situ upb geochronology of the mare basalt northwest africa 10597 implications for low ti mare volcanism around 3 0 ga abstract unbrecciated mare basalts are rare in the lunar meteorite collection found in 2015 northwest africa nwa 10597 is a medium grained low ti mare basalt with a subophitic texture the meteorite consists mostly of mm sized pyroxene and plagioclase with minor olivine spinel ilmenite phosphates silica and trace zr rich minerals such as baddeleyite zirconolite and tranquillityite a portion of plagioclase and silica has been transformed to their high pressure polymorphs due to shock metamorphism nwa 10597 has a low tio2 content 2 9 wt but is relatively enriched in rare earth elements ree laaverage 65 ci with an overall unfractionated pattern except for a negative eu anomaly calculated ree concentrations of parent melts in equilibrium with mg rich pyroxene and ca rich plagioclase suggest no significant assimilation of ree rich melts after the onset of pyroxene crystallization in situ u pb analyses of baddeleyite and apatite reveal a mutually consistent age of 3 0 ga which is also in excellent agreement with that of low ti mare basalts nwa 4734 and lapaz icefield lap 02205 dated with other independent techniques the concordance suggests no significant thermal disturbance in the u pb isotopic system of nwa 10597 although it was heavily shocked nwa 10597 closely resembles nwa 4734 in terms of petrographic texture mineral chemistry and geochronology indicating a pairing relationship

shock experiments with the h6 chondrite kernouve pressure calibration of microscopic shock effects shock recovery experiments were carried out on samples of the h6 chondrite kernouve at shock pressures of 10 15 20 25 30 35 45 and 60 gpa and preheating temperatures of 293 k low temperature experiments and 920 k high temperature experiments using a calculated equation of state of kernouve pressure pulse durations of 0 3 to 1 2 s were estimated the shocked samples were investigated by optical microscopy to calibrate the various shock effects in olivine orthopyroxene oligoclase and troilite the following pressure calibration is proposed for silicates 1 undulatory extinction of olivine gpa 2 weak mosaicism of olivine from 10 15 gpa to 20 25 gpa 3 onset of strong mosaicism of olivine at 20 25 gpa 4 transformation of oligoclase to diaplectic glass completed at 25 30 gpa low temperature experiments and at 20 25 gpa high temperature experiments 5 onset of weak mosaicism in orthopyroxene at 30 35 gpa low temperature experiments and at 25 30 gpa high temperature experiments and 6 recrystallization or melting of olivine starting at 45 60 gpa low temperature experiments and at 35 45 gpa high temperature experiments and completed above 45 60 gpa in the high temperature experiments troilite displays distinct differences between the samples shocked at low and high temperatures in the low temperature experiments the following effects can be observed in troilite 1 undulatory extinction up to 25 gpa 2 twinning up to 45 gpa 3 partial recrystallization from 30 to 60 gpa and 4 complete recrystallization 35 gpa whereas in the high temperature experiments troilite shows 1 complete recrystallization from 10 up to 45 gpa and 2 melting and crystallization above 45 gpa localized shock induced melting is observed in samples shocked to pressures 15 gpa in the high temperature experiments and 30 gpa for the low temperature experiments in the form of feni metal and troilite melt injections and intergrowths and as pockets and veins of whole rock melt obviously the onset and abundance of shock induced localized melting strongly depends on the initial temperature of the sample

trace element and textural evidence favoring lunar not terrestrial origin of the mini granite in apollo sample 14321 abstract the 14321 lunar granite 14321g has recently been reinterpreted bellucci et al 2019 as a piece of the hadean earth impact transported to the moon in principle samples of such derivation may afford important insights into the nature of earth s hadean crust we have tested the terrestrial provenance hypothesis by comparing trace element data from 14321g versus other lunar evolved rocks and a large data base for terrestrial granites volatile trace metals zn ga and ge are depleted in 14321g below the terrestrial granite ranges and relative to the terrestrial granite averages by factors of 27 2 0 and 18 respectively evidence from shocked chondrites martian meteorites and impact shock studies in general indicates that such major depletions are unlikely to develop without near complete shock melting which clearly did not occur in the mostly still crystalline 14321g moreover other aspects of compositional disparity between 14321g and terrestrial granites involve exclusively refractory trace elements compared to terrestrial granites of similarly high ba content 14321 is enriched in ta by a factor of 10 and the few terrestrial granites that are as ta rich as 14321 have 10 times lower ba the refractory element ratio lu sm is also close to 10 times higher in 14321g than in terrestrial granites of similarly high ba content other highly evolved lunar rocks felsites strongly resemble 14321g in all these respects we conclude that 14321g is probably of wholly lunar derivation this finding stands in contradiction to a recent inference from ti in quartz modeling bellucci et al 2019 that 14321g crystallized at a pressure of 0 69 gpa the geodynamically limited moon was presumably never capable of forming or burying such a highly granitic material 100 km below the base of its crust nor of excavating material from such a depth to the surface

carbonates in cm chondrites complex formational histories and comparison to carbonates in ci chondrites cm chondrites are primitive solar system materials that have undergone high degrees of aqueous alteration resulting in the formation of secondary minerals including carbonates two different carbonate minerals calcite aragonite and dolomite together constitute 1 4 2 8 vol of cm chondrites in contrast ci chondrites contain four different carbonate minerals calcite aragonite dolomite breunnerite and siderite ci chondrites have abundant dolomite a mineral that seems to be absent in the most aqueously altered cm chondrites in this study carbonates in seven cm chondrites y 791198 lapaz icefield 04796 cold bokkeveld nogoya queen alexandra range 93005 allan hills 83100 and meteorite hills 01070 were studied petrographically and by electron microprobe the results indicate that carbonate formation in cm chondrites differs from that in ci chondrites and is more complex than previously recognized our studies of cm chondrites indicate that 1 carbonates formed on the parent asteroid in an aqueous environment that gradually changed in composition 2 at some stage ca and mg activities in the environment were high enough to form metastable dolomite and 3 dolomites disappeared in the most aqueously altered cm chondrites

shock metamorphism and petrography of the shergotty achondrite the shergotty subsamples 1 and 12 consist of augite and pigeonite 67 5 maskelynite 24 ilmenite and titanomagnetite 2 pyrrhotite 0 4 whitlockite 1 8 apatite 0 1 quartz 0 5 baddeleyite trace fayalite 0 4 mesostasis 3 and shock induced local polymineralic melt products 0 6 the overall modal composition is similar to other shergotty samples except for the rather high whitlockite content the shock effects observed in the mineral constituents include mosaicism deformation bands planar fractures and mechanical twin lamellae in clinopyroxene isotropization of plagioclase with very rare remnants of birefringence planar deformation structures mosaicism and strongly reduced birefringence in quartz mechanical twinning of ilmenite localized in situ melting of neighbouring minerals at the contact of low and high density phases based on the refractive index of maskelynite average 1 5467 with average an content of 49 and the degree of isotropization of the plagioclase an equilibrium shock pressure of 29 1 gpa is derived the inferred post shock temperature is 200 20 c no heating event could have exceeded 400 c duke 1968 local stress and temperature concentrations reach 60 80 gpa and 1600 2000 c the observed shock effects can be explained by a single shock event a second weaker shock event as found by others appears to be highly improbable equilibrium shock pressures and post shock temperatures for the other known shergottites are 31 2 gpa and 220 50 c zagami 43 2 gpa and 400 800 c alha 77005 the pressure estimate for eeta 79001 by lambert 1985 is confirmed 34 1 gpa the post shock temperature is 250 50 c the abundance and textural setting of localized melt products in these meteorites confirm increasing shock pressures in the sequence shergotty zagami eeta 79001 alha 77005 undoubtedly the melts could have been formed by the same single shock which produced the equilibrium shock effects e g maskelynitization in these meteorites the shock induced particle velocities inferred from hugoniot data of basalts are in the 1 5 2 0 km s range for the parental rocks of the shergottites ejection velocities are therefore in the order of 3 4 km s special ejection mechanisms are required in order to exceed the escape velocity of a planet like mars without producing higher degrees of shock e g melting than those observed in the shergottites

spinel assemblages in lunar meteorites graves nunataks 06157 and dhofar 1528 implications for impact melting and equilibration in the moon s upper mantle

microcomputed tomography and shock microdeformation studies on shatter cones one of the aspects of impact cratering that are still not fully understood is the formation of shatter cones and related fracturing phenomena yet shatter cones have been applied as an impact diagnostic criterion for decades without the role of shock waves and target rock defects in their formation having been elucidated ever we have tested the application of the nondestructive microcomputed tomography ct method to visualize the interior of shatter cones in order to possibly resolve links between fracture patterns and shatter cone surface features striations and intervening valleys shatter coned samples from different impact sites and in different lithologies were investigated for their ct suitability with a shatter cone in sandstone from the serra da cangalha impact structure brazil remaining as the most promising candidate because of the fracture resolution achieved to validate the obtained ct data the scanned specimen was cut into three orthogonal sets of thin sections scans with 13 m resolution were obtained ct scans and microscopic analysis unraveled an orientation of subplanar fractures and related fluid inclusion trails and planar fracture pf orientations in the interior of shatter cones planar deformation features pdf were observed predominantly near the shatter cone surface previously undescribed varieties of feather features ff in the form of lamellae emanating from curviplanar and curved fractures as well as an arrowhead like ff development with microlamellae originating from both sides of a pf were observed the timing of shatter cone formation was investigated by establishing temporal relations to the generation of various shock microscopic effects shatter cones are thus generated post or syn formation of pf ff subplanar fractures and pdf the earliest possible time for shatter cone formation is during the late stage of the compressional phase that is shock wave passage of an impact event

discovery of bridgmanite the most abundant mineral in earth in a shocked meteorite meteorites exposed to high pressures and temperatures during impact induced shock often contain minerals whose occurrence and stability normally confine them to the deeper portions of earth s mantle one exception has been mgsio 3 in the perovskite structure which is the most abundant solid phase in earth here we report the discovery of this important phase as a mineral in the tenham l6 chondrite and approved by the international mineralogical association specimen ima 2014 017 mgsio 3 perovskite is now called bridgmanite the associated phase assemblage constrains peak shock conditions to 24 gigapascals and 2300 kelvin the discovery concludes a half century of efforts to find identify and characterize a natural specimen of this important mineral

shock metamorphism of enstatite chondrites abstract we have extended the shock metamorphism classification scheme of stoffler et al 1991 to account for shock effects in orthopyroxene and applied it to sixty enstatite chondrites orthopyroxene exhibits the following sequence of progressive shock effects shock stage s1 unshocked sharp optical extinction s2 very weakly shocked undulose extinction s3 weakly shocked development of clinoenstatite lamellae parallel to 100 s4 moderately shocked weak mosaicism s5 strongly shocked strong mosaicism as in the stoffler et al 1991 scheme stage s5 is characterized principally by the solid state transformation of crystalline plagioclase into maskelynite most el3 chondrites exhibit foliations caused by impact deformation of chondrules and metal particles el5 and el6 chondrites are all shock stage s2 because opaque veins are rare in s2 ordinary chondrites it is plausible that the centimeter size kamacite veins in atlanta and blithfield and the 1 6 cm long oldhamite rich vein in jajh deh kot lalu all el6 formed when their hosts were shocked to s3 s5 levels because removal of shock stage s3 s5 features including repair of shock damaged orthopyroxene requires levels of metamorphism comparable to those experienced by petrologic type 5 to 6 chondrites we infer that proto el6 material was shocked to s3 s5 levels prior to peak metamorphism and shocked again to stage s2 after metamorphism overall enstatite chondrites appear to have suffered greater shock damage than ordinary and carbonaceous chondrites

helium loss from martian meteorites mainly induced by shock metamorphism evidence from new data and a literature compilation noble gas data from martian meteorites have provided key constraints about their origin and evolution and their parent body these meteorites have witnessed varying shock metamorphic overprinting at least 5 to 14 gpa for the nakhlites and up to 45 55 gpa e g the lherzolitic shergottite allan hills alh a77005 solar heating cosmic ray exposure and weathering both on mars and earth influences on the helium budgets of martian meteorites were evaluated by using a new data set and literature data concentrations of 3he 4he u and th are measured and shock pressures for same sample aliquots of 13 martian meteorites were determined to asses a possible relationship between shock pressure and helium concentration partitioning of 4he into cosmogenic and radiogenic components was performed using the lowest 4he 3he ratio we measured on mineral separates 4he 3he 4 1 pyroxene of alha77005 our study revealed significant losses of radiogenic 4he systematics of cosmogenic 3he and neon led to the conclusion that solar radiation heating during transfer from mars to earth and terrestrial weathering can be ruled out as major causes of the observed losses of radiogenic helium in bulk meteorites for bulk rock we observed a correlation of shock pressure and radiogenic 4he loss ranging between 20 for chassigny and other moderately shocked martian meteorites up to total loss for meteorites shocked above 40 gpa a steep increase of loss occurs around 30 gpa the pressure at which plagioclase transforms to maskelynite this correlation suggests significant 4he loss induced by shock metamorphism noble gas loss in rocks is seen as diffusion due to 1 the temperature increase during shock loading shock temperature and 2 the remaining waste heat after adiabatic unloading post shock temperature modeling of 4he diffusion in the main u th carrier phase apatite showed that post shock temperatures of 300 c are necessary to explain observed losses this temperature corresponds to the post shock temperature calculated for bulk rocks shocked at about 40 gpa from our investigation data survey and modeling we conclude that the shock event during launch of the meteorites is the principal cause for 4he loss

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

shock experiments on pre heated α and β quartz i optical and density data abstract discs of single crystal quartz unheated and pre heated to 275 c and 540 c i e quartz and 630 c i e quartz were experimentally shocked to pressures ranging from 20 to 40 gpa with the shock front propagating parallel to either 10 1 0 or 0001 refractive indices density and the orientation of planar deformation features pdfs were determined on the recovered quartz samples refractive indices of pre heated quartz are unaffected up to 25 gpa but density starts to decrease slightly up to this pressure above 25 gpa pre heating causes drastic variations refractive indices and birefringence of quartz shocked at ambient temperature decrease continuously until complete isotropization is reached at 35 gpa in quartz shocked at 630 c refractivity drops discontinuously in the interval from 25 to 26 gpa and complete transformation to diaplectic glass is reached at 26 gpa density follows the trends demonstrated by the optical parameters with higher pre shock temperatures yielding lower density at a given shock pressure these results indicate that the threshold pressure for the onset of transformation to diaplectic quartz glass is largely temperature invariant lying at 25 gpa whereas the pressure limit for complete transformation decreases with increasing pre shock temperature from 35 to 26 gpa quartz shocked parallel to 0001 always has a higher density and refractivity than that shocked parallel to 10 1 0 indicating a significant influence of the structural anisotropy this is also evident from the distribution of pdf orientations pressures 25 gpa cause in quartz shocked parallel to 10 1 0 pdfs that are predominantly oriented parallel to 10 1 2 while quartz shocked to the same pressures but parallel to 0001 contains almost exclusively pdfs parallel to 10 1 3 pdf orientations in quartz shocked at ambient temperature parallel to 10 1 0 show the following characteristics i the frequency of 10 1 3 is high at 20 gpa declines with increasing pressure and finally is totally absent at 32 gpa ii 10 1 3 begins to occur at 25 gpa increases and finally is the only remaining orientation at 32 gpa and iii 10 1 2 and 11 2 2 are only present at 20 and 25 gpa in contrast quartz shocked at 630 c parallel to 10 1 0 displays a broad pdf distribution with indistinct maxima and 10 1 3 is totally absent the small structural difference between and quartz which is reflected in the pdf distribution should allow in principle evaluation of the pre shock temperature in naturally impacted metamorphic rocks the results substantiate that formation of shock effects in quartz is dependent on i shock pressure ii pre shock temperature and iii shock wave direction so far the latter two parameters have not been considered in the application of experimental results to impact sites

mesoscale and microscale shock effects in the ll6 s4 chondrites saint severin and elbert a tale of two breccias

shocked quartz in the cretaceous tertiary boundary clays evidence for a global distribution shocked quartz grains displaying planar features were isolated from cretaceous tertiary boundary clays at five sites in europe a core from the north central pacific ocean and a site in new zealand at all of these sites the planar features in the shocked quartz can be indexed to rational crystallographic planes of the quartz lattice the grains display streaking indicative of shock in x ray diffraction photographs and also show reduced refractive indices these characteristic features of shocked quartz at several sites worldwide confirm that an impact event at the cretaceous tertiary boundary distributed ejecta products in an earth girdling dust cloud as postulated by the alvarez impact hypothesis

the barringer award address presented 1996 july 25 berlin germany impact experiments related to the evolution of planetary regoliths impact induced comminution of planetary surfaces is pervasive throughout the solar system and occurs on submillimeter to global scales resulting in comminution products that range from fine grained surface soils to massive polymict ejecta deposits to collisionally fragmented objects within this wide range of comminution products we define regoliths in a narrow sense as materials that were processed by repetitive impacts to dimensional scales comparable to or smaller than that of component minerals of the progenitor rock s in this paper we summarize a wide variety of impact experiments and other observations that were primarily intended to understand the evolution of regoliths on lunar basalt flows and we discuss some of their implications for asteroidal surfaces cratering experiments in both rock and noncohesive materials combined with photogeologic observations of the lunar surface demonstrate that craters 500 m in diameter contribute most to the excavation of local bedrock for subsequent processing by micrometeorites the overall excavation rate and thus growth rate of the debris layer decreases with time because the increasingly thicker fragmental layer will prevent progressively larger projectiles from reaching bedrock typical growth rates for a 5 m thick lunar soil layer are initially 3 ga ago a few mm ma and slowed to 1 mm ma at present the coarse grained crater ejecta are efficiently comminuted by collisional fragmentation processes and the mean residence time of a 1 kg rock is typically 10 ma the actual comminution of either lithic or monomineralic detritus is highly mineral specific with feldspar and mesostasis comminuting preferentially over pyroxene and olivine thus resulting in mechanically fractionated fines especially at grain sizes 20 m such fractionated fines also participate preferentially in the shock melting of lunar soils thus giving rise to agglutinate melts as a consequence agglutinate melts are systematically enriched in feldspar components relative to the bulk composition of their respective host soil s compositionally homogeneous impact derived glass beads in lunar soils seem to result from micrometeorite impacts on rock surfaces reflecting lithic regolith components and associated mineral mixtures cumulatively experimental and observational evidence from lunar mare soils suggests that regoliths derive substantially from the comminution of local bedrock the addition of foreign exotic components is not necessary to explain the modal and chemical compositions of diverse grain size fractions from typical lunar soils regoliths on asteroids are qualitatively different from those of the moon the modest impact velocities in the asteroid belt some 5 km s 1 are barely sufficient to produce impact melts also substantially more crater mass is being displaced on low gravity asteroids compared to the moon collisional processing of surface boulders should therefore be more prominent in producing comminuted asteroid surfaces these processes combine into asteroidal surface deposits that have suffered modest levels of shock metamorphism compared to the moon impact melting does not seem to be a significant process under these conditions however the role of cometary particles encountering asteroid surfaces at presumably higher velocities has not been addressed in the past unfortunately the asteroidal surface processes that seemingly modify the spectral properties of ordinary chondrites to match telescopically obtained spectra of s type asteroids remain poorly understood at present despite the extensive experimental and theoretical insights summarized in this report and our fairly mature understanding of lunar surface processes and regolith evolution

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

the nature of the l chondrite parent body s disruption as deduced from high pressure phases in the sixiangkou l6 chondrite the disruption of the l chondrite parent body lcpb at similar to 470ma is currently the best documented catastrophic celestial impact event based on the large number of l chondritic materials associated with this event uranium lead u pb dating of apatite and its high pressure decomposition product tuite in the sixiangkou l6 chondrite provides a temporal link to this event the u pb system of phosphates adjacent to shock melt veins was altered to varying degrees and the discordance of the u pb system correlates closely with the extent of apatite decomposition this suggests that the u pb system of apatite could be substantially disturbed by high temperature pulse during shock compression from natural impacts at least on the scale of mineral grains although many l chondrites can be temporally related to the catastrophic lcpb impact event the shock conditions experienced by each individual meteorite vary this could be due to the different geologic settings of these meteorites on their parent body the shock pressure and duration derived from most meteorites may only reflect local shock features rather than the impact conditions although they could provide lower limits to the impact conditions the sixiangkou shock duration similar to 4s estimated from high pressure transformation kinetics provides a lower limit to the high pressure pulse of the lcpb disruption impact combined with available literature data of l chondrites associated with this impact event our results suggest that the lcpb suffered a catastrophic collision with a large projectile with a diameter of at least 18 22km at a low impact velocity 5 6kms 1 this is consistent with astronomical estimates based on the dynamical evolution of l chondritic asteroids

estimating average shock pressures recorded by impactite samples based on universal stage investigations of planar deformation features in quartz sources of error and recommendations planar deformation features pdfs in quartz are the most widely used indicator of shock metamorphism in terrestrial rocks they can also be used for estimating average shock pressures that quartz bearing rocks have been subjected to here we report on a number of observations and problems that we have encountered when performing universal stage measurements and crystallographically indexing of pdf orientations in quartz these include a comparison between manual and automated methods of indexing pdfs an evaluation of the new stereographic projection template and observations regarding the pdf statistics related to the c axis position and rhombohedral plane symmetry we further discuss the implications that our findings have for shock barometry studies our study shows that the currently used stereographic projection template for indexing pdfs in quartz might induce an overestimation of rhombohedral planes with low miller bravais indices we suggest based on a comparison of different shock barometry methods that a unified method of assigning shock pressures to samples based on pdfs in quartz is necessary to allow comparison of data sets this method needs to take into account not only the average number of pdf sets grain but also the number of high miller bravais index planes both of which are important factors according to our study finally we present a suggestion for such a method which is valid for nonporous quartz bearing rock types which consists of assigning quartz grains into types a e based on the pdf orientation pattern and then calculation of a mean shock pressure for each sample less

geologic history of martian regolith breccia northwest africa 7034 evidence for hydrothermal activity and lithologic diversity in the martian crust the timing and mode of deposition for martian regolith breccia northwest africa nwa 7034 were determined by combining petrography shape analysis and thermochronology nwa 7034 is composed of igneous impact and brecciated clasts within a thermally annealed submicron matrix of pulverized crustal rocks and devitrified impact volcanic glass the brecciated clasts are likely lithified portions of martian regolith with some evidence of past hydrothermal activity represented lithologies are primarily ancient crustal materials with crystallization ages as old as 4 4 ga one ancient zircon was hosted by an alkali rich basalt clast confirming that alkalic volcanism occurred on mars very early nwa 7034 is composed of fragmented particles that do not exhibit evidence of having undergone bed load transport by wind or water the clast size distribution is similar to terrestrial pyroclastic deposits we infer that the clasts were deposited by atmospheric rainout subsequent to a pyroclastic eruption s and or impact event s although the ancient ages of igneous components favor mobilization by impact s despite ancient components the breccia has undergone a single pervasive thermal event at 500 800 c evident by groundmass texture and concordance of 1 5 ga dates for bulk rock k ar u pb in apatite and u pb in metamict zircons the 1 5 ga age is likely a thermal event that coincides with rainout breccia lithification we infer that the episodic process of regolith lithification dominated sedimentary processes during the amazonian epoch the absence of pre amazonian high temperature metamorphic events recorded in ancient zircons indicates source domains of static southern highland crust punctuated by episodic impact modification

the nature distribution and genesis of the coesite and stishovite associated with the pseudotachylite of the vredefort dome south africa abstract the vredefort dome represents the uplift centre of a well known 2 00 ga old impact structure of unusually large magnitude shock features like shatter cones planar features and high pressure silica polymorphs are common the present study deals with the description mode of occurrence field distribution and post shock metamorphic alteration of coesite and stishovite which were poorly documented up to now these minerals occur as unusually large crystals in the quartzite of the wall rock in contact with very thin pseudotachylite veins in the pseudotachylite itself which can be interpreted as a friction and or a shock recovery melt fine needles of kyanite are ubiquitous it is proposed that these thin pseudotachylites a type formed during the transit of the shock wave they preceded the development of thick pseudotachylite and microbreccia veins b type which formed during the tensional period which immediately followed from comparison with the model of formation of high pressure polymorphs in porous sandstone it is suggested that higher pressure and stress was concentrated along the a type veins at the arrival of the shock front at this time the quartz was transformed into a high pressure phase which was probably poorly crystalline behind the shock front that is during the rarefaction the pressure was progressively released and the polymorphs crystallized from this initial high pressure phase stishovite crystallized first followed by coesite the high pressure conditions may have lasted an unusually long time due to the magnitude of the vredefort impact this long time probably about one second may account for the large size of the coesite and stishovite crystals most of the high pressure silica polymorphs are corroded to a variable degree by secondary quartz and preserved only in a restricted area of the impact structure this alteration is attributed to post shock metamorphism due to the temperature of the rock before impact plus the heat left after shock recovery some of the evidence put forward by authors against the impact model is discussed according to the facts arising from this investigation it is concluded that an origin by impact remains the simplest and most realistic model to explain the origin of the vredefort dome

shock metamorphism of carbonaceous chondrites abstract we have studied shock effects in carbonaceous chondrites using optical microscopy of thin sections and find that our petrographic classification of progressive shock metamorphism in ordinary chondrites can also be applied to carbonaceous chondrites we find that sixty nine carbonaceous chondrites can be assigned to four shock stages si to s4 largely on the basis of shock effects in olivine the least shocked chondrite groups are cm2 and co3 thirty six out of thirty eight members are classified as shock stage s1 differences between the mean shock levels of certain groups of chondrites e g co3 and cv3 are probably due to stochastic differences in the sampling of their parent bodies however there is an overall tendency for the mean shock level of carbonaceous chondrites and ordinary chondrites to increase with increasing petrologic type that may reflect real differences in the shock level of near surface materials on their parent bodies caused by intrinsic variations in the chemical and physical properties of these materials petrologic type 2 and 3 chondrites are more porous and richer in volatiles than types 4 to 6 the greater porosity of types 2 3 causes higher post shock temperatures and melting at lower pressures and the higher volatile contents ensure that strongly shocked material is more readily dispersed on release from high pressure we suggest that type 2 3 material shocked above 20 30 gpa normally escapes from the parent asteroids and forms particles that are too small to survive as meteorites in the cv3 group there is a correlation between the degree of chondrule flattening and the intensity of shock metamorphism analogous to that discovered in ordinary chondrites by sneyd et al 1988 suggesting that shock rather than static overburden pressure is responsible for chondrule flattening we infer that the correlations are due to collapse of pores under shock pressures of at least 5 10 gpa and that shock is an important process affecting many physical properties of type 2 4 chondrites

orbital identification of carbonate bearing rocks on mars geochemical models for mars predict carbonate formation during aqueous alteration carbonate bearing rocks had not previously been detected on mars surface but mars reconnaissance orbiter mapping reveals a regional rock layer with near infrared spectral characteristics that are consistent with the presence of magnesium carbonate in the nili fossae region the carbonate is closely associated with both phyllosilicate bearing and olivine rich rock units and probably formed during the noachian or early hesperian era from the alteration of olivine by either hydrothermal fluids or near surface water the presence of carbonate as well as accompanying clays suggests that waters were neutral to alkaline at the time of its formation and that acidic weathering proposed to be characteristic of hesperian mars did not destroy these carbonates and thus did not dominate all aqueous environments

experimental generation of shock induced pseudotachylites along lithological interfaces to understand the mechanism of formation of shock induced pseudotachylites and particularly the role that rock heterogeneities and interfaces play in their formation shock recovery experiments were carried out on samples consisting of two distinct lithologies dunite and quartzite it was possible to generate melt veins of 1 6 m width along lithological interfaces at moderate shock pressures 6 to 34 gpa the magnitudes of displacement along the interface strain rate and the kinetic heat production indicate that friction is an important heat source that largely contributes to the energy budget of the melt veins the experimentally produced veins resemble natural s type pseudotachylites the geometry of the veins depends on the orientation of the interface with respect to the shock front and includes strong variations in thickness formation of melt pockets and injection veins sudden changes in vein orientation and sharp vein margins two types of melt were observed vesicle free and vesicular melts dense vesicle free melt rock is likely to represent high pressure melts vesicular melts were also generated during shock compression but they remained in a molten state during pressure release and continued shearing intermingling of comminuted olivine and melt suggests that ultracataclasis of olivine induced by a dynamic tensile failure is a precursor stage to frictional melting shock wave interferences at the lithological interface provide the necessary stress conditions to start dynamic failure of olivine the composition of the frictional melts ranges from olivine normative to enstatite normative and is thus largely determined by olivine melting the validity of the sequence of friction melting susceptibilities of rock forming minerals inferred from tectonically produced pseudotachylites is confirmed and can now be applied to ultra high strain rates during shock compression

diogenites as polymict breccias composed of orthopyroxenite and harzburgite abstract a few relatively unbrecciated olivine rich diogenites consist of an equilibrium assemblage of olivine and magnesian orthopyroxene harzburgite more common diogenites with smaller amounts of olivine are breccias containing two distinct orthopyroxenes one magnesian and one ferroan these diogenites are mixtures of a harzburgite lithology that is more magnesian with the normal orthopyroxenite lithology that is ferroan and may contain small amounts of plagioclase both lithologies likely formed by fractional crystallization in multiple plutons emplaced within the crust of asteroid 4 vesta minor element trends in orthopyroxenes indicate that these plutons exhibited a range of compositions we propose a revised taxonomy for the hed howardites eucrites and diogenites suite where all ultramafic samples are referred to as diogenites within this group the prefixes dunitic harzburgitic and orthopyroxenitic are used to distinguish diogenites consisting of more than or equal to 90 olivine olivine orthopyroxene and more than or equal to 90 orthopyroxene respectively the prefix polymict is used to describe brecciated mixtures of any of these rock types the recognition that olivine is a significant phase in some diogenites is consistent with spectral interpretations of olivine in a deeply excavated crater on vesta and has important implications for the bulk composition and petrogenesis of that body

origin of large volume pseudotachylite in terrestrial impact structures large volume pseudotachylite bodies in impact structures are dike like and consist of angular and rounded wall rock fragments enveloped by a microcrystalline and sporadically glassy matrix that crystallized from a melt knowledge of the formation of pseudotachylite bodies is important for understanding mechanics of complex crater formation most current hypotheses of pseudotachylite formation inherently assume that fragmentation and melt generation occur during a single process based on the structure of pseudotachylite bodies at sudbury canada and vredefort south africa we show that these processes differ in time and space we demonstrate that the centimeter to kilometer scale bodies are effectively fragment and melt fi lled tension fractures that formed by differential rotation of target rock during cratering highly variable pseudotachylite characteristics can be accounted for by a single process i e drainage of initially superheated impact melt into tension fractures of the crater fl oor

revision and recalibration of existing shock classifications for quartzose rocks using low shock pressure 2 5 20 gpa recovery experiments and mesoscale numerical modeling a combination of shock recovery experiments and numerical modeling of shock deformation in the low shock pressure range from 2 5 to 20 gpa for two dry sandstone types of different porosity a completely water saturated sandstone and a well indurated quartzite provides new insights into strongly heterogeneous distribution of different shock features 1 for nonporous quartzo feldspathic rocks the traditional classification scheme stoffler 1984 is suitable with slight changes in pressure calibration 2 for water saturated quartzose rocks a cataclastic texture microbreccia seems to be typical for the shock pressure range up to 20 gpa this microbreccia does not show formation of pdfs but diaplectic quartz glass sio2 melt is formed at 20 gpa 1 vol 3 for porous quartzose rocks the following sequence of shock features is observed with progressive increase in shock pressure 1 crushing of pores 2 intense fracturing of quartz grains and 3 increasing formation of diaplectic quartz glass sio2 melt replacing fracturing the formation of diaplectic quartz glass sio2 melt together with sio2 high pressure phases is a continuous process that strongly depends on porosity this experimental observation is confirmed by our concomitant numerical modeling recalibration of the shock classification scheme results in a porosity versus shock pressure diagram illustrating distinct boundaries for the different shock stages

fe3 in shocked olivine crystals of the alha 77005 meteorite abstract alha 77005 a shocked achondrite of the shergottite group is unique in containing cumulus olivine crystals which display a brownish color in thin section mossbauer epr and optical spectroscopic studies show the presence of fe 3 in the olivine approximately 4 5 wt of the total iron in the olivine 24 wt as feo is in the trivalent state the fe 3 ions preferentially occupy m2 lattice positions charge transfer between fe 3 and fe 2 as well as between fe 3 and oxygen ligands causes a broad absorption band in the optical spectrum at high energies which is the reason for the olivine color it is concluded that the color of the alha 77005 olivine is preterrestrial and may be the result of shock induced oxidation of the olivine crystals on the meteorite parent body

regolith mixing by impacts lateral diffusion of basin melt abstract impact cratering has been the primary process to alter the distribution of lunar highland material since the formation of a crust this impact history is recorded in the radiogenic clocks of impact melts which are accessible for study on lunar samples and meteorites however primary impact melt is exposed to a long time gardening process i e re melting excavation burial and re excavation by subsequent impacts resulting in a complex spatial distribution of materials representing specific impact events to investigate the diffusion behavior of impact melt a model tracing the evolving distribution of melt laterally and with depth was built using a monte carlo approach given scaling laws concerning melt production and ejecta distribution the size frequency distribution of impact craters and the rate function for crater formation we examine the evolution of melt component occurrence of different ages three mid to late forming basins serenitatis crisium and imbrium are chosen as a case study for the diffusion of melt from major basin forming events the survival probability of basin melt occurrence at the apollo and luna sampling spots is derived it is expected to find abundant imbrium and crisium melt at the apollo and luna sampling sites consistent with the k ar radiometric dates of highland samples whereas the older serenitatis melt was subjected to the later long term gardening strongly influenced by later local impacts and thus is less abundant understanding the diffusion of impact melt is helpful for interpretation of radiometric ages of lunar samples and can be used to predict the distribution of differently aged melts at future landing sampling sites such as the chinese chang e 4 ce 4

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

diaplectic quartz glass and sio2 melt experimentally generated at only 5 gpa shock pressure in porous sandstone laboratory observations and meso scale numerical modeling abstract a combination of shock recovery experiments and numerical modeling of shock deformation in the low pressure range from 2 5 to 17 5 gpa in dry porous seeberger sandstone provides new significant insights with respect to the heterogeneous nature of shock distribution in such important upper crustal material for which to date no pressure calibrated scheme for shock metamorphism exists we found that pores are already completely closed at 2 5 gpa shock pressure whole quartz grains or parts of them are transformed to diaplectic quartz glass and or sio 2 melt starting already at 5 gpa whereas these effects are not observed below shock pressures of 30 35 and 45 gpa respectively in shock experiments with quartz single crystals the appearance of diaplectic glass or melt is not restricted to the zone directly below the impacted surface but is related to the occurrence of pores in a much broader zone the combined amount of these phases increases distinctly with increasing shock pressure from 0 03 vol at 5 gpa to 80 vol at 17 5 gpa in accordance with a previous shock classification for silica phases in naturally shocked coconino sandstone from meteor crater that was based on varied slopes of the coconino sandstone hugoniot curve our observations allow us to construct a shock pressure classification for porous sandstone consistent with shock stages 1b 4 of the progressive shock metamorphism classification of kieffer 1971 numerical modeling at the meso scale provides the explanation for the discrepancy of shock deformation in porous material and single crystal quartz in keeping with our experimental results it confirms that pore space is completely collapsed at low nominal pressure and demonstrates that pore space collapse results in localized pressure amplification that can exceed 4 times the initial pressure this provides an explanation for the formation of diaplectic quartz glass and lechatelierite as observed in the low shock pressure experiments the numerical models predict an amount of sio 2 melt similar to that observed in the shock experiments this also shows that numerical models are essential to provide information beyond experimental capabilities

regolith breccia northwest africa 7533 mineralogy and petrology with implications for early mars northwest africa 7533 a polymict martian breccia consists of fine grained clast laden melt particles and microcrystalline matrix while both melt and matrix contain medium grained noritic monzonitic material and crystal clasts the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars microbasaltic clasts vitrophyric and microcrystalline spherules and shards the clast laden melt rocks contain clump like aggregates of orthopyroxene surrounded by aureoles of plagioclase some shards of vesicular melt rocks resemble the pyroxene plagioclase clump aureole structures submicron size matrix grains show some triple junctions but most are irregular with high intergranular porosity the noritic monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths and cooled more slowly than rocks with zoned pyroxene or fine grain size noritic material contains orthopyroxene or inverted pigeonite augite calcic to intermediate plagioclase and chromite to cr bearing magnetite monzonitic clasts contain augite sodic plagioclase k feldspar ti bearing magnetite ilmenite chlorapatite and zircon these feldspathic rocks show similarities to some rocks at gale crater like black trout mara and jake m the most magnesian orthopyroxene clasts are close to alh 84001 orthopyroxene in composition all these materials are enriched in siderophile elements indicating impact melting and incorporation of a projectile component except for ni poor pyroxene clasts which are from pristine rocks clast laden melt rocks spherules shards and siderophile element contents indicate formation of nwa 7533 as a regolith breccia the zircons mainly derived from monzonitic melt rocks crystallized at 4 43 0 03 ga humayun et al 2013 and a 147sm 143nd isochron for nwa 7034 yielding 4 42 0 07 ga nyquist et al 2016 defines the crystallization age of all its igneous portions the zircon from the monzonitic rocks has a higher 17o than other martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration nemchin et al 2014 this record of protolith interaction with atmosphere hydrosphere during regolith formation before melting demonstrates a thin atmosphere a wet early surface environment on mars and an evolved crust likely to have contaminated younger extrusive rocks the latest events recorded when the breccia was on mars are resetting of apatite much feldspar and some zircons at 1 35 1 4 ga bellucci et al 2015 and formation of ni bearing pyrite veins during or shortly after this disturbance lorand et al 2015

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