impact melt rocks from the ries structure germany an origin as impact melt flows abstract the production of impact melt rocks and glasses is a characteristic feature of hypervelocity impact events on earth and other planetary bodies this investigation represents the first detailed study of an unusual series of coherent impact melt rocks intermittently exposed around the periphery of the 24 km diameter 14 5 ma ries impact structure germany optical and analytical scanning electron microscopy sem reveals that the groundmass comprises sanidine plagioclase quartz and ilmenite decreasing order of abundance with the interstices filled by either fresh or devitrified glassy mesostasis primary crystallites display skeletal dendritic and or spherulitic textures indicating rapid crystallization from a melt the mesostasis is characterized by extreme chemical heterogeneity e g feo and al 2 o 3 contents from 1 to 62 80 wt this is likely due to a combination of crystal liquid fractionation during rapid cooling and crystallization of an originally incompletely homogenized melt vapor phase crystallization of sanidine and cristobalite occurred in miarolitic cavities during late stage cooling of the impact melts the most likely protolith for the impact melt rocks are granitic rocks present in the crystalline basement target the high volatile content of the mesostasis suggests that a large volatile component was retained from this protolith field observations together with analytical data and micro textures indicate that the ries impact melt rocks were molten at the time of and after deposition field relations with other impactites also suggest that these rocks were emplaced subsequent to the excavation stage of crater formation and that they are not therefore ballistic ejecta thus it is proposed that the ries impact melt rocks were emplaced as ground hugging impact melt flows that emanated from different regions of the evolving transient cavity during the modification stage of crater formation this is consistent with and in fact predicted by field relations which indicate that these rocks overlie unequivocal ejecta this represents the first recognition of impact melt flows near the rim of a terrestrial impact structure and is consistent with observations of such features on the moon and venus

penecontemporaneous diagenesis in continental saline sediments bloeditization in quero playa lake la mancha central spain abstract in most continental saline lakes of la mancha central spain bloedite has been identified as a part of recent saline deposits by means of a monitoring of the natural evaporation process of the brines in the quero playa lake has been observed the absence of this mineral as a primary phase in the sequence of saline precipitation however bloedite appears as one of the major components of the saline deposit originated during the aforementioned evaporation period the genesis of this bloedite is caused by a syndepositional diagenetic process we have called bloeditization this process in an early rapid diagenetic genesis of bloedite as a result of the reaction between the previously precipitated epsomite and the concentrating brines the replacement of the epsomite by bloedite leads to the release of magnesium chloride and water into the brine if we use a polythermal thermodynamic simulation of the studied evaporation process we obtain results consistent with observations of the brine precipitation paths and the mineralogical composition of the saline deposit

vredefort bronzite granophyre chemical evidence for origin as a meteorite impact melt abstract computer assisted compositional modelling studies demonstrate that the unusual chemical composition of the bronzite granophyre in the vredefort structure south africa can be produced by the impact melting and mixing of four pre vredefort target rocks which constitute the lower part of the vredefort stratigraphic section ventersdorp lava witwatersrand shale witwatersrand quartzite and old granite similar models show that a simple igneous assimilation mechanism total assimilation of the same wallrock types using a wide range of possible parent magmas cannot duplicate the granophyre composition unless geologically unreasonable amounts of wallrock 100 of the parent magma are assimilated the unusual granophyre composition reflects the incorporation of significant amounts of basalt 30 80 into the impact melt from which the granophyre was derived for various granophyre compositions the granite component varies inversely with basalt from about 45 5 and shale is relatively uniform at 5 20 a significant and nearly constant amount of quartzite about 20 is an unusual feature of the granophyre which given the generally refractory character of quartzite wallrock is difficult to explain by assimilation the granophyre displays a significant range of chemical variation which is similar to the range observed in impact melt bodies from large and small terrestrial impact structures these variations apparently reflect actual differences in the relative amounts of different target rocks incorporated into discrete volumes of impact melt rather than variations in the composition of individual target rock components these results are consistent with the view that the granophyre is an impact melt and that the vredefort structure formed by the impact of an extraterrestrial object this interpretation also explains several long known geological features 1 the close relationship between granophyre and pseudotachylite both of which were formed simultaneously by shock waves of different intensities 2 the absence of shock effects pseudotachylite and shatter cones in the granophyre and 3 the occurrence of quartzite xenoliths in the granophyre at great distances from the nearest quartzite exposures

boltysh another end cretaceous impact the chixculub impact occurred at the cretaceous tertiary k t boundary and although several other late cretaceous and paleogene impact craters have at times been linked with the k t boundary isotope geochronology has demonstrated that all have significantly different ages the currently accepted age of the 24 km diameter boltysh crater a k ar whole rock age places it in the coniacian at 88 3 ma however comprehensive ar ar dating of a range of melt samples yields a mean age of 65 17 0 64 ma within errors of the k t boundary several of the fresh samples exhibit signs of excess argon but this seems to be concentrated in rapidly crystallized glass rich samples the ar ar age confirms an earlier fission track measurement and thus two dating techniques have yielded an age within errors of the k t boundary for this crater crucially although the ages of boltysh and chixculub are within errors they may not have formed synchronously craters of 24 km diameter occur much more commonly than impacts of chixculub dimensions but their proximity does raise the important question of how many impacts there might have been close to the k t boundary

geochemistry of great salt lake utah ii pleistocene holocene evolution sedimentologic and biostratigraphic evidence is used to develop a geochemical model for great salt lake utah extending back some 30 000 yrs b p hydrologie conditions as defined by the water budget equation are characterized by a lake initially at a low saline stage rising by about 17 000 yrs b p to fresh water basin full conditions bonneville level and then after about 15 000 yrs b p dropping rapidly to a saline stage again as exemplified by the present situation inflow composition has changed through time in response to the hydrologie history during fresh water periods high discharge inflow is dominated by calcium bicarbonate type river waters during saline stages low discharge nacl rich hydrothermal springs are significant solute sources this evolution in lake composition to nacl domination is illustrated by the massive mirabilite deposition free of halite following the rapid drawdown until about 8 000 years ago while historic droughts have yielded principally halite hydrologic history can be combined with inferred inflow composition to derive concentration curves with time for each major solute in the lake calcium concentrations before the drawdown were controlled by calcite solubility and afterwards by aragonite significant amounts of solutes are removed from the lake by diffusion into the sediments na cl and so42 are also involved in salt precipitation by including pore fluid data a surprisingly good fit has been obtained between solute input over the time period considered and the amounts actually found in lake brines pore fluids salt beds and sediments excess amounts are present for calcium carbonate and silica indicating detrital input

brownish inclusions and dark streaks in libyan desert glass evidence for high temperature melting of the target rock dark streaks and different types of inclusions in libyan desert glass ldg collected from the ldg strewn field in egypt were investigated rare transparent spherules enclosed in the glassy matrix are characterized by concentric cracks irregular internal cracks intense twinning and considerable amounts of ti and al raman spectra show that the spherules are a cristobalite their occurrence together with lechatelierite indicates quick heating of the source rock to at least 1550 c followed by rapid quenching leading to crystallization of b cristobalite which upon cooling inverted into a cristobalite brownish inclusions are irregularly shaped elongated objects with smooth contacts to the surrounding glass they contain small roundish to elliptical droplets and a few larger angular grains which compositionally and according to their raman spectra most closely resemble low ca al rich orthopyroxene composition and texture of the orthopyroxene suggest that the brownish inclusions formed by incomplete melting of an al rich orthopyroxene bearing precursor e g mafic phases present in desert surface sands or also of orthopyroxene bearing granulite dykes in the ldg target experimental data on ca poor enstatite also support that the inclusions were heated to about 1550 c analyses of dark streaks in ldg reveal high abundances of al ti mn cr fe and ni and a pronounced correlation between the abundances of cr mn fe and ni as the fe ni mn ni and cr ni ratios are all clearly nonchondritic the source of this material is most likely terrestrial and the dark streaks studied here represent a different type of schlieren compared to those which contain a meteoritic component these findings suggest ldg formation during a short high temperature event melting of al rich orthopyroxene bearing target material seems to suggest an asteroid impact rather than a near surface airburst

the basal onaping intrusion in the north range roof rocks of the sudbury igneous complex the 1 85 ga sudbury impact structure is one of the largest impact structures on earth igneous bodies the so called basal onaping intrusion occur at the contact between the sudbury igneous complex sic and the overlying onaping formation and occupy 50 of this contact zone the basal onaping intrusion is presently considered part of the onaping formation which is a complex series of breccias here we present petrological and geochemical data from two drill cores and field data from the north range of the sudbury structure which suggests that the basal onaping intrusion is not part of the onaping formation our observations indicate that the basal onaping intrusion crystallized from a melt and has a groundmass comprising a skeletal intergrowth of feldspar and quartz that points to simultaneous cooling of both components increasing grain size and decreasing amounts of clasts with increasing depth are general features of roof rocks of coherent impact melt rocks at other impact structures and the basal onaping intrusion planar deformation features within quartz clasts of the basal onaping intrusion are indicators for shock metamorphism and together with the melt matrix point to the basal onaping intrusion as being an impact melt rock by definition importantly the contact between granophyre of the sic and basal onaping intrusion is transitional and we suggest that the basal onaping intrusion is what remains of the roof rocks of the sic and thus is a unit of the sic and not the onaping formation we suggest henceforth that this unit be referred to as the upper contact unit of the sic

variation of upwelling oceanic conditions during the latest pleistocene through holocene off the central peruvian coast a diatom record diatoms are well preserved in partly laminated holocene and late pleistocene sediments on the upper continental slope of central peru accumulation rates of diatom species previously recognized as tropical oceanic warm water and subtropical neritic cooler water upwelling changed markedly downcore in kasten cores obtained at 11 15 s 12 58 s and 13 37 s latitude subtropical species e g thalassiosira eccentrica group thalassiothrix frauenfeldii chaetoceros spp resting spores delphineis spp predominated in late pleistocene sediments of the two southern cores a mix of tropical and subtropical diatom assemblages occurred in latest pleistocene sediments while tropical species e g coscinodiscus nodulifer radiatus pseudoeunotia doliolus thalassionema nitzschioides var parva predominated in the late holocene sediments in the northern core latest pleistocene sediments also contained a mix of tropical and subtropical species similarly late holocene sediments contained predominately tropical species of diatoms neoglacial periods 200 400 yr b p 2000 2700 yr b p however were sometimes characterized by a greater abundance of subtropical species and punctuated periodically with blooms of skeletonema costatum and rhizosolenia shrubsolei it is proposed that during times of southern hemisphere cooling peruvian coastal waters north of 12 s were subjected to episodic mixing with warmer water masses before being overwhelmed by tropical waters that may have either passed southward across a weakened equatorial front or arose from intensified offshore countercurrents coastal waters south of 12 s latitude were probably unaffected by the tropical influence rather these waters may have undergone intensified upwelling during the last glacial epoch

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

accessory and opaque minerals in impact melt rocks of the boltysh structure ukraine electron microprobe analyses of accessory and opaque minerals from the impact melt rocks of the boltysh structure in the central part of the ukrainian shield are presented in this report our study establishes a variety of minerals represented by native metals alloys oxides sulfides phosphates and silicates formed during several stages of cooling and solidification of the thick impact melt sheet baddeleyite was determined to be the earliest high temperature mineral to occur in the impact melt rocks iron and titanium oxides crystallized earlier or simultaneously with the microliths of orthopyroxene and feldspars high concentrations of tio2 al2o3 and cr2o3 were identified in some hematite varieties cu and ni bearing pyrrhotites occur in impact melt rocks with a glassy matrix native metals copper platinum and silver were likely formed due to the hydrothermal alteration of the upper unit of the impact melt sheet zircon is the only accessory mineral found in impact melt rocks that is preserved from the basement granites of the boltysh structure

absence of shock metamorphic effects in the bushveld complex south africa results of an intensive search abstract an extensive search was made for distinctive shock metamorphic effects in order to test the theory that the bushveld complex bc was formed by the impact of large extraterrestrial objects field and petrographie studies concentrated on two geological environments in which current impact cratering theories predict the formation and preservation of such shock metamorphic effects as shatter cones unusual fragmental or melt breccias and shock lamellae in quartz these are 1 pre bc quartzite xenoliths in the rooiberg group a thick sequence of acid lavas rhyolites and rhyodacites and related rocks proposed by earlier workers as a possible crater filling impact melt and 2 the locally observable contact between the rooiberg group and the underlying pre bc basement pretoria group a contact possibly corresponding to the original crater floor in established impact structures more than 320 samples of pre bc quartzite were collected and examined petrographically for shock metamorphic effects no megascopic shock metamorphic effects shatter cones or unusual breccias were observed in any outcrop in thin section no petrographic shock effects e g shock lamellae hi quartz and kink bands in biotite were observed in any sample the quartzites from both the basement 80 samples and the rooiberg xenoliths 245 samples are virtually undeformed cataclastic textures are rare and individual quartz grains show only normal metamorphic deformation lamellae bohm lamellae and single or multiple sets of fluid inclusions arranged along irregular planes which appear to be healed fractures although some of these parallel sets superficially resemble so called decorated shock lamellae universal stage measurements of their orientation demonstrate that they are totally different the absence of both megascopic and microscopic shock metamorphic effects in a large suite of carefully selected samples strongly indicates that impact models for the bc are not correct and that the bc is not an impact structure this conclusion also agrees with 1 the complex stratigraphy now recognized in the lower rooiberg group which is not consistent with an impact melt origin 2 the absence in the rooiberg group of any of the distinctive melt rich fragmentai breccias which dominate the crater filling deposits in established large impact structures and 3 the undeformed character of the basement rocks below the rooiberg group and the apparently concordant contact between them

the water balance equations in saline playa lakes comparison between experimental and recent data from quero playa lake central spain abstract the quero playa lake is an ephemeral saline playa lake located in the la mancha region of central spain in this study a daily monitoring of the brine physical properties water activity brine depth and main climatic parameters was simultaneously carried out together with determining the precipitation sequence of minerals field data were compared with the results of simulating the water evaporation in an environmental chamber in this simulation a similar hydrochemical composition for the saline lake was used and the main climatic parameters temperature and humidity were controlled the water balance equation for saline lakes has usually been described using the wood and sanford equation econ geol 85 1990 1226 1235 our experimental results required us to revise the water balance equation for the brine depth variations dh dt that may be expressed as follows d h d t p 1 k a b a l s i s o h d where p mm is the precipitation k is the drainage coefficient of the lake al is the lake surface ab is the drainage basin surface si and so are the contribution of influent and effluent seepage to the depth of brine in the lake the term is the evaporation condensation defined as kpw aw rh where k is the mass transfer coefficient dalton s equation pw is the water pressure in equilibrium with the air aw is the water activity of the brine rh is the relative humidity the other terms h and d correct the brine depth loss or and gain a consequence of hydrated saline mineral precipitation and early diagenetic hydration dehydration reactions as a consequence of the above we suggest that the water balance equation for saline lakes can be an important consideration in the interpretation of their evolution the precipitation of hydrated saline minerals and the early diagenetic dehydration hydration reactions imply changes in the dh dt curves as a result the interpretation of the sequence of primary saline minerals in older evaporitic deposits may be used as a paleoenvironmental marker

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

the geochronology of large igneous provinces terrestrial impact craters and their relationship to mass extinctions on earth the relationship between sudden catastrophic events and sudden mass extinctions on earth has attracted a great deal of research effort including measurements of the absolute and relative ages of these events the result has been a rapid improvement in the precision and accuracy of radiometric dates for large igneous provinces lips for which there is now a consensus on the peak eruption times over the last 250 ma in parallel there has been a more gradual improvement in the radiometric dates for craters formed by hypervelocity impacts hvis of asteroids and comets with earth the absolute ages of the largest craters are known to within a few million years but the ages of many smaller craters are poorly constrained in addition whereas relatively few lips have formed in the last 250 ma at least one hvi crater larger than 20 km diameter has formed on earth every million years only 25 such craters have been discovered so coincidence between the ages of hvi crater formation and ages of mass extinctions is related as much to crater preservation as to coincidence of the events the difference in quality of geochronology between hvis and lips is the result of differing availability of samples lips cover large areas and present an array of basalt samples suitable for dating whereas hvi craters are often poorly preserved and present few datable samples the radiometric data for preserved hvis and lips are now sufficiently precise and accurate for us to be confident that there is no one to one correlation between either lips or hvis and global mass extinctions of life on earth however there is a close association between a significant number of lips and sudden environmental change and a very close association between at least one massive hvi crater formation and a mass extinction further there is no apparent correlation between hvis and lips and thus no reason to suggest a causative relationship finally the radiometric dating of hvi craters indicates several clusters in the geological record but the correlation with sudden environmental change is no better than for large craters

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

postshock annealing and postannealing shock in equilibrated ordinary chondrites implications for the thermal and shock histories of chondritic asteroids abstract in addition to shock effects in olivine plagioclase orthopyroxene and ca pyroxene petrographic shock indicators in equilibrated ordinary chondrites oc include chromite veinlets chromite plagioclase assemblages polycrystalline troilite metallic cu irregularly shaped troilite grains within metallic fe ni rapidly solidified metal sulfide intergrowths martensite and various types of plessite metal sulfide veins large metal and or sulfide nodules silicate melt veins silicate darkening low ca clinopyroxene silicate melt pockets and large regions of silicate melt the presence of some of these indicators in every petrologic type 4 to 6 ordinary chondrite demonstrates that collisional events caused all equilibrated oc to reach shock stages s3 s6 those type 4 to 6 oc that are classified as shock stage s1 on the basis of sharp optical extinction in olivine underwent postshock annealing due to burial beneath materials heated by the impact event those type 4 to 6 oc that are classified s2 on the basis of undulose extinction and lack of planar fractures in olivine were shocked to stage s3 s6 annealed to stage s1 and then shocked again to stage s2 some oc were probably shocked to stage s3 after annealing it seems likely that many oc experienced multiple episodes of shock and annealing because 40ar 39ar chronological data indicate that mil 99301 ll6 s1 was annealed 4 26 ga ago presumably as a consequence of a major impact it seems reasonable to suggest that other equilibrated s1 and s2 oc which contain relict shock features were also annealed by impacts because some type 6 s1 oc e g guarena kernouve portales valley all of which contain relict shock features were annealed 4 44 4 45 ga ago during a period when impacts were prevalent and most oc were thermally metamorphosed it follows that impact induced annealing could have contributed significantly to oc thermal metamorphism

groundwater seepage controls salinity in a hydrologically terminal basin of semi arid northwest australia abstract very small groundwater outflows have the potential to significantly impact the hydrochemistry and salt accumulation processes of notionally terminal basins in arid environments however this limited groundwater outflow can be very difficult to quantify using classical water budget calculations due to large uncertainties in estimates of evaporation and evapotranspiration rates from the surface of dry lake beds in this study we used a dimensionless time evaporation model to estimate the range of groundwater outflow required to maintain salinity levels observed at the fortescue marsh fm one of the largest wetlands of semi arid northwest australia 1100 km2 the groundwater outflow from aquifers underlying the fm to the lower fortescue catchment is constrained by an extremely low hydraulic gradient of 300 g l after 45 ka we calculated that only a very small seepage of 2g l yr 0 03 of the fm water volume is sufficient to maintain current salinity conditions the minimum time required to develop the current hydrochemical groundwater composition under the fm ranges from 60 to 165 ka we conclude that a dimensionless time evaporation model versus inflow over outflow ratio model is likely more suitable than classical water budget calculations for determining outflow from large saline lakes and to estimate groundwater seepage from hydrologically terminal basins

impact melt sheet formation on mars and its implication for hydrothermal systems and exobiology abstract hydrothermal deposits are a prime exobiological research target on mars and impact craters are potentially a significant source for these deposits key factors in the longevity of terrestrial impact crater hydrothermal systems are the volume of shock melt produced and the degree to which this melt is quenched by mixing with un melted clasts mixing occurs when the shock melt is smeared along the crater floor and the degree of mixing is inversely proportional to the thickness of this smeared layer to investigate the potential development of thick melt sheets on mars we apply an analytical impact model to asteroid and long period lp comet impacts that produces martian craters with final diameters of 30 640 km we combine our model shock melt volumes with estimates of the surface area of the transient cavity to estimate initial shock melt thickness which we use as a proxy measure of the amount of mixing between shock melt and un melted clasts we calculate this shock melt thickness parameter for a range terrestrial craters with and without thick melt sheets from which we infer the melt sheet characteristics for the model mars craters the results demonstrate that most asteroid impacts on mars do not produce thick clast free melt sheets only asteroid impact craters 580 km in diameter on mars produce shock melt thicknesses comparable to earth s 200 km diameter sudbury crater and martian craters 65 km in diameter produce shock melt thicknesses comparable to terrestrial craters that lack melts sheets entirely lp comet impacts on mars are more likely to produce thick melt sheets lp comet impact craters 280 km in diameter on mars produce shock melt thicknesses comparable to that of the sudbury crater given that lp comet impacts on mars are rare compared to asteroid impacts we conclude that large long lived impact hydrothermal systems are likewise rare on mars comparisons of the mars asteroid impact rate with the potential longevity of impact hydrothermal systems indicate that such systems were active 1 of the time in the last 3 5 ga which raises questions about the exobiological importance of impact crater hydrothermal systems on mars after the noachian

climate surface pore water interactions on a salt crusted playa implications for crust pattern and surface roughness development measured using terrestrial laser scanning sodium accumulating playas also termed sodic or natric playas are typically covered by polygonal crusts with different pattern characteristics but little is known about the short term hours dynamics of these patterns or how pore water may respond to or drive changing salt crust patterning and surface roughness it is important to understand these interactions because playa crust surface pore water and roughness both influence wind erosion and dust emission through controlling erodibility and erosivity here we present the first high resolution 10 3m hours co located measurements of changing moisture and salt crust topography using terrestrial laser scanning tls and infra red imagery for sua pan botswana maximum nocturnal moisture pattern change was found on the crests of ridged surfaces during periods of low temperature and high relative humidity these peaks experienced non elastic expansion overnight of up to 30 mm and up to an average of 1 5 mm night during the 39 day measurement period continuous crusts on the other hand showed little nocturnal change in moisture or elevation the dynamic nature of salt crusts and the complex feedback patterns identified emphasise how processes both above and below the surface may govern the response of playa surfaces to microclimate diurnal cycles

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

ground water control of evaporite deposition topographically closed basins may be hydrologically open as a result of seepage losses to underlying or surrounding ground water systems in such cases these losses can have a substantial control over the suite and the thicknesses of evaporite minerals formed in the basin the ratio of ground water outflow to inflow flux ratio in hydrologically open basins is as important in determining the mineralogy and thicknesses of evaporite deposits as the solute composition of the inflow water attainment of steady state flux ratios permits large thicknesses of two or three minerals to form rather than thin veneers of many minerals the presence or absence of glauberite mirabilite halite bloedite polyhalite and hexahydrite caused by subtle changes in the ground water seepage is illustrated using an example from the southern high plains of texas and new mexico however the model is general and is applicable with any solute composition including that of seawater and the use of surface rather than ground water an analytical lumped parameter solute mass balance model is developed to define the concept of a ground water flux ratio as it applies to topographically closed basins in which evaporation exceeds precipitation diffusion advection and density driven flow are proposed as mechanisms by which solutes can escape to the ground water from these closed basins the geochemical reaction computer program phrqpitz is used to document the effects of various flux ratios on the mineralogy and thickness of deposits solute analyses used in conjunction with the model can be used to screen prospective basins as well as to provide insights for exploratory drilling program

modeling the interaction between evaporation and chemical composition in a natural saline system summary evaporation controls the salinity of many natural and anthropic brine systems but the reverse is also true by controlling water activity salinity affects evaporation rates we present a method to compute the evolution of water activity in high salinity systems so as to evaluate evaporation rates we place special emphasis on the assessment of invariant points where activity is controlled by the set of precipitated minerals the algorithm is tested on a natural mg so 4 rich brine evaporation experiment in accordance with the experiments the model predicts two intervals invariant points in which water activity the concentration of all species and the amount of liquid water remain constant because evaporating water comes from the dissolution of hydrated minerals this suggests that mineral paragenesis might have a considerable influence on shallow brine lake evolution by fixing chemical composition for a significant portion of time this conjecture was tested with a simplified model of a perennial saline playa lake an analytical solution was developed to illustrate the evolution of the proposed system according to the calculations the system tends to a cyclical steady state for both lake level and chemical composition the latter remains fixed at invariant points during long time intervals where hydrated minerals act as the water source for evaporation this situation is to be expected in epsomite lakes where two invariant points can be found in equilibrium with relative humidities of 57 and 50

terrestrial impact melt rocks and glasses abstract the effects of meteorite and comet impact on earth are rock brecciation the formation of shock metamorphic features rock melting and the formation of impact structures i e simple craters complex craters and multi ring basins large events such as the 65 ma chicxulub impact are believed to have had catastrophic environmental effects that profoundly influenced the development of life on earth in this review an attempt is made to summarize some of the voluminous literature on impact melting one important aspect of planetary impact provide some comments on this process and to make suggestions for future research the products of impact melting are glasses impact melt rocks and pseudotachylites our treatise deals mainly with the geological setting petrography and major element chemistry of melt rocks and glasses impact glasses in several petrographic aspects are similar to volcanic glasses but they are associated with shock metamorphosed mineral and rock fragments and in places with siderophile element anomalies suggestive of meteoritic contamination they are found in allogenic breccia deposits within fall back suevite and outside fall out suevite impact craters and as spherules in distal ejecta large events such as the k t boundary chicxulub impact are responsible for the formation of worldwide ejecta horizons which are associated with siderophile element anomalies and shock metamorphosed mineral and rock debris impact glasses have a bulk chemical composition that is homogeneous but exemptions to this rule are common on a microscopic scale however impact glasses are commonly strikingly heterogeneous tektites are glasses ejected from craters over large distances they are characterized by very low water and volatile contents and element abundances and ratios that are evidence that tektites formed by melting of upper crustal sedimentary rocks four tektite strewn fields are known three of which can be tied to specific impact craters impact melt rocks form sheets lenses and dike like bodies within or beneath allogenic fallback breccia deposits in the impact crater and possibly on crater terraces and flanks dikes of impact melt rocks also intrude the rocks of the crater floor they commonly contain shock metamorphosed target rock and mineral fragments in various stages of assimilation and are glassy or fine to coarse grained chemically they are strikingly homogeneous but as with impact glasses exemptions to this rule do exist large and thick melt bodies such as the sudbury igneous complex sic are differentiated or may represent a combination of impact melt rocks sensu strictu and impact triggered deep crustal melts a concerted multidisciplinary approach to future research on impact melting and on other aspects of meteorite and comet impact is advocated impact models are models only and uncritical reliance on their validity will not lead to a better understanding of impact processes especially of melting excavation and deposition of allogenic breccias and the spatial position of breccias in relation to sheets and lenses of melt rocks within the crater impact triggered pressure release melting of target rocks beneath the excavation cavity may be responsible for the existence of melt rocks beneath the impact melt rocks sensu strictu this controversial idea needs to be tested by a re evaluation of existing data and models be they based on field or laboratory research only a relatively small number of terrestrial impact structures has been investigated in sufficient detail as it relates to geological and geophysical mapping in this review we summarize observations made on impact melt rocks and impact glasses in a number of north american brent haughton manicouagan new quebec sudbury wanapitei all in canada asian popigai russia zhamanshin kazakhstan two south african structures morokweng and vredefort the henbury crater field of australia and one european crater ries germany our tables listing major element chemical compositions of impact glasses and melt rocks however include also data from structures not dealt with in further detail

tem study of meteorite impact glass at new zealand cretaceous tertiary sites evidence for multiple impacts or differentiation during global circulation abstract study by transmission electron microscopy of samples from the cretaceous tertiary k t boundary clay at flaxbourne river and woodside creek new zealand has revealed the occurrence of nanometer sized meteorite impact derived glass the average glass composition is exceptionally ca rich and is distinct from other glass found on earth apart from glass inferred to be of impact origin at mexican and haitian k t sites the glass shards are partially altered to montmorillonite like smectite with the dominant interlayer cation ca reflecting the composition of the parent glass the data imply a heterogeneous global distribution in composition of k t boundary impact glass si rich and ca rich in mexico and haiti si rich in denmark and ca rich in new zealand this heterogeneous distribution may relate to dispersal processes similar to those used to account for the asymmetric distribution of clastic debris from the chicxulub impact site however recent discovery of an impact crater of k t boundary age in ukraine raises the possibility of impact clusters which produce material of heterogeneous composition

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