isotopic fractionation of cu in tektites tektites are terrestrial natural glasses of up to a few centimeters in size that were produced during hypervelocity impacts on the earth s surface it is well established that the chemical and isotopic composition of tektites is generally identical to that of the upper terrestrial continental crust tektites typically have very low water content which has generally been explained by volatilization at high temperature however the exact mechanism is still debated because volatilization can fractionate isotopes comparing the isotopic composition of volatile elements in tektites with those of their source rocks may help to understand the physical conditions during tektite formation interestingly volatile chalcophile elements e g cd and zn seem to be the only elements for which isotopic fractionation is known so far in tektites here we extend this study to cu another volatile chalcophile element we have measured the cu isotopic composition for 20 tektite samples from the four known different strewn fields all of the tektites except the muong nong types are enriched in the heavy isotopes of cu 1 98 delta 61 cu 6 99 in comparison to the terrestrial crust delta 65 cu approximate to 0 with no clear distinction between the different groups the muong nong type tektites and a libyan desert glass sample are not fractionated delta 65 cu approximate to 0 in comparison to the terrestrial crust to refine the cu isotopic composition of the terrestrial crust we also present data for three geological reference materials delta 65 cu approximate to 0 an increase of delta 65 cu with decreasing cu abundance probably reflects that the isotopic fractionation occurred by evaporation during heating a simple rayleigh distillation cannot explain the cu isotopic data and we suggest that the isotopic fractionation is governed by a diffusion limited regime copper is isotopically more fractionated than the more volatile element zn delta 66 64 zn up to 2 49 parts per thousand this difference of behavior between cu and zn is predicted in a diffusion limited regime where the magnitude of the isotopic fractionation is regulated by the competition between the evaporative flux and the diffusive flux at the diffusion boundary layer due to the difference of ionic charge in silicates zn 2 vs cu cu has a diffusion coefficient that is larger than that of zn by at least two orders of magnitude therefore the larger isotopic fractionation in cu than in zn in tektites is due to the significant difference in their respective chemical diffusivity c 2009 elsevier ltd all rights reserved

reply to comments on trace element and isotopic evidence for archean basement in the lonar crater impact breccia deccan volcanic province by ramananda chakrabarti and asish r basu

anatomy of surface rupture zones of two stable continental region earthquakes 1967 koyna and 1993 latur india soil helium surveys in the surface rupture zones of the 1993 latur mw 6 2 and the 1967 koyna mw 6 3 stable continental region scr earthquake sites in peninsular india showed anomalies defining surface traces of the causative faults propagating from the archaean crystalline basement through the deccan basalt cover the seismic fault produced a scarp by uplift along a thrust in the killari area of the latur earthquake whereas the koyna earthquake was associated with a strike slip fault which expressed itself as an en echelon fissure zone core drilling has confirmed that the fault at killari extends downward from the surface rupture zone with an approximate dip of 50 towards ssw the level differences of flow contacts obtained by drilling in the hanging wall and foot wall sides of the fault do not unequivocally establish the amount of displacement but suggest that it might be anything from 1 m to 6 m if the higher figure of 6 m is accepted it would indicate reactivation of an old fault drilling established a wnw dip of the koyna fault resolving a long standing debate

geochemical identification of impactor for lonar crater india the only well known terrestrial analogue of impact craters in basaltic crusts of the rocky planets is the lonar crater india for the first time evidence of the impactor that formed the crater has been identified within the impact spherules which are 0 3 to 1 mm in size and of different aerodynamic shapes including spheres teardrops cylinders dumbbells and spindles they were found in ejecta on the rim of the crater the spherules have high magnetic susceptibility from 0 31 to 0 02 si mass and natural remanent magnetization nrm intensity both nrm and saturation isothermal remanent magnetization sirm intensity are 2 am2 kg demagnetization response by the nrm suggests a complicated history of remanence acquisition the spherules show schlieren structure described by chains of tiny dendritic and octahedral shaped magnetite crystals indicating their quenching from liquid droplets microprobe analyses show that relative to the target basalt compositions the spherules have relatively high average fe2o3 by 1 5 wt mgo 1 wt mn 200 ppm cr 200 ppm co 50 ppm ni 1000 ppm and zn 70 ppm and low na2o 1 wt and p2o5 0 2 wt very high ni contents up to 14 times the average content of lonar basalt require the presence of a meteoritic component in these spherules we interpret the high ni cr and co abundances in these spherules to indicate that the impactor of the lonar crater was a chondrite which is present in abundances of 12 to 20 percent by weight in these impact spherules relatively high zn yet low na2o and p2o5 contents of these spherules indicate exchange of volatiles between the quenching spherule droplets and the impact plume

composition of impact melt particles and the effects of post impact alteration in suevitic rocks at the yaxcopoil 1 drill core chicxulub crater mexico petrographical and chemical analysis of melt particles and alteration minerals of the about 100 m thick suevitic sequence at the chicxulub yax 1 drill core was performed the aim of this study is to determine the composition of the impact melt the variation between different types of melt particles and the effects of post impact hydrothermal alteration we demonstrate that the compositional variation between melt particles of the suevitic rocks is the result of both incomplete homogenization of the target lithologies during impact and subsequent post impact hydrothermal alteration most melt particles are andesitic in composition clinopyroxene rich melt particles possess lower sio2 and higher cao contents these are interpreted by mixing of melts from the silicate basement with overlying carbonate rocks multi stage post impact hydrothermal alteration involved significant mass transfer of most major elements and caused further compositional heterogeneity between melt particles following backwash of seawater into the crater palagonitization of glassy melt particles likely caused depletion of sio2 al2o3 cao na2o and enrichment of k2o and feotot during an early alteration stage since glass is very susceptible to fluid rock interaction the state of primary crystallization of the melt particles had a significant influence on the intensity of the postimpact hydrothermal mass transfer and was more pronounced in glassy melt particles than in wellcrystallized particles in contrast to other occurrences of chicxulub impactites the yax 1 suevitic rocks show strong potassium metasomatism with hydrothermal k feldspar formation and whole rock k20 enrichment especially in the lower unit of the suevitic sequence a late stage of hydrothermal alteration is characterized by precipitation of silica analcime and na bearing mg rich smectite among other minerals this indicates a general evolution from a silica undersaturated fluid at relatively high potassium activities at an early stage toward a silica oversaturated fluid at relatively high sodium activities at later stages in the course of fluid rock interaction

use of platinum group elements for impactor identification terrestrial impact craters and cretaceous tertiary boundary abstract this study presents the abundances of ru ir pt pd rh and au platinum group elements pge in the ejecta layer at the cretaceous tertiary boundary and melt rocks from east clearwater lake brent new quebec and lake wanapitei impact craters all believed to have been formed by the impact of chondritic projectiles the objective was to evaluate the pure application of pge interelement ratios e g ru ir for impactor identification to compare the utility of both melt rocks and ejecta deposits for this purpose and to expand the pge database for these craters the similarity of pge interelement ratios within the chondrite and iron meteorite groups precludes their use in the fine classification of impactors of this composition introduction of indigenous pge from country rock may contaminate the original meteoritic signature in addition the extent of pge fractionation during impact and melt sheet cooling and of post sedimentary pge remobilization must be evaluated in each case for east clearwater the pge chondrite normalized plot is flat indicating a chondritic impactor based on ru ir and cr ir ratios a cl or l chondrite is most likely however variation of ratios with depth in the core makes any fine classification of the projectile tenuous for the brent and new quebec craters the flat chondrite normalized pge plot indicates a chondritic impactor but finer classification is precluded by possible post syn depositional pge fractionation at lake wanapitei ni cr and cr ir ratios indicate a ll chondrite impactor wolf et al 1980 but sparse pge data for this chondrite type does not allow further classification of the projectile a flat chondrite normalized pge plot indicates a chondritic rather than an iron impactor at lake wanapitei the global ejecta layer at the cretaceous tertiary boundary has a chondritic pge signature but only when the integrated flux of pge on a global scale is considered individual sites may not show chondritic ratios due to differential pge remobilization resulting from local differences in depositional environment continental vs marine and to vaporization and or condensation pge fractionation occurring during impact the results for melt rocks and the k t ejecta deposit indicate that pge can be used for impactor identification but primarily in combination with other siderophile chalcophile meteoritic elements and their ratios

target rocks impact glasses and melt rocks from the lonar crater india highly siderophile element systematics and sr nd os isotopic signatures the lonar crater is a 0 57 myr old impact structure located in the deccan traps of the indian peninsula it probably represents the best preserved impact structure hosted in continental flood basalts providing unique opportunities to study processes of impact cratering in basaltic targets here we present highly siderophile element hse abundances and sr nd and os isotope data for target basalts and impactites impact glasses and impact melt rocks from the lonar area these tools may enable us to better constrain the interplay of a variety of impact related processes such as mixing volatilization and contamination strontium and nd isotopic compositions of impactites confirm and extend earlier suggestions about the incorporation of ancient basement rocks in lonar impactites in the re os isochron plot target basalts exhibit considerable scatter around a 65 6 myr re os reference isochron most likely reflecting weathering and or magma replenishment processes most impactites plot at distinctly lower 187re 188os and 187os 188os ratios compared to the target rocks and exhibit up to two orders of magnitude higher abundances of ir os and ru moreover the impactites show near chondritic interelement ratios of hse we interpret our results in terms of an addition of up to 0 03 of a chondritc component to most impact glasses and impact melt rocks the magnitude of the admixture is significantly lower than the earlier reported 12 20 wt of extraterrestrial component for lonar impact spherules reflecting the typical difference in the distribution of projectile component between impact glass spherules and bulk impactites

temperature measurements and heat transfer in near surface snow at the south pole to study near surface heat flow on the antarctic ice sheet snow temperatures were measured at south pole station to a depth of 3 m at 15 min intervals during most of 1992 solar heating and water vapor transport were negligible during the 6 month winter as was inter grain net thermal radiation leaving conduction as the dominant heat transport mechanism the rate of temperature change at depth over 15 min intervals was smaller than that at the surface by one order of magnitude at 20 cm depth and two orders of magnitude at 1 m depth a finite difference model with conduction as the only heat transfer mechanism and measured temperatures as the upper and lower boundary conditions was applied to foursets of three thermistors each the thermal conductivity was estimated as that which minimized the difference between modeled and measured 15 min changes in temperatures at the center thermistor the thermal conductivity obtained at shallow depths above 40 cm was lower than that given by existing parameterizations based on density probably because the snow grains were freshly deposited cold and poorly bonded a model using only vertical conduction explains on average 87 ofthe observed 15 min temperature changes at less than 60 cm depth and 92 below 60 cm the difference between modeled andmeasured temperature changes decreased with depth the discrepancies between model and observation correlated more strongly with the air snow temperature difference than with the product of that difference with the square of the wind speed suggesting that the residual errors are due more to non vertical conduction and to sub grid scale variabilis of the conductivity than to windpumping the residual heating rate not explained by the model of vertical conduction exceeds 0 2 w m 3 only in the top 60 cm of the near surface snow

ephemeral liquid water at the surface of the martian north polar residual cap results of numerical modelling abstract gypsum a mineral that requires water to form is common on the surface of mars most of it originated before 3 5 gyr when the red planet was more humid than now however occurrences of gypsum dune deposits around the north polar residual cap nprc seem to be surprisingly young late amazonian in age this shows that liquid water was present on mars even at times when surface conditions were as cold and dry as the present day a recently proposed mechanism for gypsum formation involves weathering of dust within ice e g niles p b michalski j 2009 nat geosci 2 215 220 however none of the previous studies have determined if this process is possible under current martian conditions here we use numerical modelling of heat transfer to show that during the warmest days of the summer solar irradiation may be sufficient to melt pure water ice located below a layer of dark dust particles albedo 0 13 lying on the steepest sections of the equator facing slopes of the spiral troughs within martian nprc during the times of high irradiance at the north pole every 51 ka caused by variation of orbital and rotational parameters of mars e g laskar j et al 2002 nature 419 375 377 this process could have taken place over larger parts of the spiral troughs the existence of small amounts of liquid water close to the surface even under current martian conditions fulfils one of the main requirements necessary to explain the formation of the extensive gypsum deposits around the nprc it also changes our understanding of the degree of current geological activity on mars and has important implications for estimating the astrobiological potential of mars

pioneer vegetation as an indicator of the geochemical parameters in abandoned mine sites using hyperspectral airborne data a general problem that is associated with open pit mining is the total displacement of the former landscape s geocomponents overburden conveyor bridges have an irreversible impact on the environment the consequence of this is a completely new system the so called post mining landscapes these are characterized by extreme geochemical milieus especially acidity which is caused by weathering processes acid mine drainage these extreme environments are often left to natural succession because of the enormous costs and poor prospects of a successful reclamation this study investigated the relationship between pioneer vegetation e g calamagrostis epigejos and the geochemical properties of heap material using hyperspectral airborne hymap data in the west bohemian lignite district near sokolov czech republic thus a fully constrained linear spectral unmixing algorithm was applied to link the geochemical parameters with fractional abundances due to the limitation of pure surfaces in the data set of such fragmented and mixed areas a three stage semiautomated endmember extraction was developed the first stage included the selection of pure surface types using a decision tree with a self developed narrow band and normalized difference indices the second stage extracted the potential endmembers using the sequential maximum angle convex cone algorithm the third stage identified the final endmembers and eliminated the spectral outliers by comparing the extractions to field laboratory spectral libraries the abundance fraction image of the pioneer vegetation which was generated by the linear spectral unmixing showed the spatial distribution and density of the vegetation a linear regression model was used to calibrate the abundance image against the ph value the regression model successfully predicted the ph values of the soil r 2 0 67 rpd 1 42 this method demonstrated that hyperspectral remote sensing data could be used for ecological medium and long term monitoring as well as for spatial assessments in post mining areas in addition this approach works with a small amount of geochemical measurements and the sparse availability of endmembers

18o concentrations in sea ice of the weddell sea antarctica we present data on ice texture salinity and delta o 18 obtained from identical sections of ice cores during the winter weddell sea project 1986 on rv polarstern from july through august 1986 in the longitude range between 5 degrees w and 7 degrees e we find no uniquely definable relationship between delta o 18 values and ice texture in a particular section however most of the snow ice as well as some sections of frazil ice ar found to have negative delta o 18 concentrations this is due to varying degrees of admixtures of meteoric ice snow and sea water during formation of snow ice in contrast to common assumptions our results seem to indicate that a snow cover contributes positively to sea ice growth rather than slowing down the overall growth rate based on a simple model we have estimated the contributions of meteoric ice mean of 3 3 and the combined meteoric ice sea water fraction a minimum of 7 6 to the total ice thickness for the majority of the sampled floes although this is only a moderate contribution to the overall mass balance in the absence of congelation growth it nevertheless enhances ice growth in general this hypothesis is dependently supported by our snow and ice thickness data wadhams and others 1987 which demonstrate that the depression of the snow ice interface below the water line i e a negative freeboard and the formation of snow ice is a common occurrence in the weddell sea therefore we hypothesize that the major part of the observed apparent increase in ice thickness between our inbound and outbound tracks of wwsp 86 may not be derived from regular thermodynamically driven congelation growth but rather from the snow ice component in floes of the weddell sea

natural restoration of soils on mine heaps with similar technogenic parent material a case study of long term soil evolution in silesian krakow upland poland abstract the study focused on the variability in properties and pedogenesis of soils located at post mining sites in the old industrial area in the silesian krakow upland southern poland the investigated sites were comparable in terms of zn pb mining history soil texture and climate but differed in duration of pedogenesis which started after the waste materials were accumulated and covered 30 70 100 and about 400 years of self restoration a wide spectrum of soil parameters including zn pb and cd contents dehydrogenase activity pores space and aggregate formation organic matter decomposition were studied by chemical and micromorphological image analyses with the aim of tracing the evolution of soils and checking if the results of image analysis could be integrated with results obtained by chemical and biological determinations stepwise progressive regression equations were obtained to establish the parameters that varied over time zn pb and cd contents exceeded several times permissible values in all soils studied the evolution of these soils was significantly different from each other during the process of self restoration of the soil the contents of organic carbon and total nitrogen available potassium and phosphorus including dehydrogenase activity increased with increasing age of soils pedogenesis resulted also in the increasing amounts of aggregates of small sizes which were set close to one another together with narrow pores which was able to store water a synchronisation between soil properties obtained by chemical biological and micromorphological analyses indicating their usefulness for the demonstration of temporal changes

petrogenesis of an augite bearing melt rock in the chicxulub structure and its relationship to k t impact spherules in haiti geophysical anomalies on the yucatan peninsula define a buried circular structure with an approximate diameter of 180 km refs 1 3 these anomalies along with stratigraphic and petrological data including evidence for shock metamorphism have been used to interpret the structure as an impact crater4 this structure known as chicxulub is particularly interesting because it formed at or near the end of the cretaceous period in the geographical region where an impact is believed to have occurred in large part because of a thick ejecta deposit found on haiti5 glassy tektite like relics in this deposit6 9 are unusually calcic up to 31 wt cao ref 7 providing a further circumstantial link with the chicxulub structure which penetrates a carbonate and evaporite sequence7 8 here we strengthen this link by showing that a simple chemical relationship exists between the glasses and an augite bearing melt rock found within the chicxulub structure and argue that the composition of this melt rock could not easily have been produced by volcanic processes

an investigation of the thermal conductivity of snow thermal conductivity of snow has been investigated experimentally using the thermal probe method which is a transient method of measurement the measurements have been made over a wide range of snow density for fresh and dense snow for varying temperatures and for different conditions of water content snow grain type etc both in the field and in the laboratory the results are presented along with detailed sample descriptions thermal conductivity of snow increases with density and water content it also increases with temperature and the effect is more pronounced for temperatures between 15 and 0 c

accumulation of carbon nitrogen and phosphorus during soil formation on alder spoil heaps after brown coal mining near sokolov czech republic the accumulation of carbon c nitrogen n and phosphorus p and their vertical distribution in the soil profile in relation to site age were studied in a chronosequence of 19 sites on reclaimed spoil heaps from open cast coal mining near sokolov czech republic and compared to a semi natural alder forest in the vicinity of the mining area the reclaimed sites were located on tertiary clay or quaternary gravel sand spoil material and afforested with alder species alnus glutinosa a incana 4 65 years ago rapid accumulation of c and n in the soil profile was registered within 15 years after reclamation the rate of increase in c and n contents in the whole profile and the thickness of the litter and fermentation layers was slower in 25 year old and older reclaimed sites the soil of the semi natural alder forest was richer in c and n in the 5 10 cm layer compared with 40 year old clays and even with 65 year old gravel sand reclaimed soils n accumulated more slowly in comparison with c especially in deeper 5 10 cm parts of the profile accumulation of organic c resulted in a decrease in substrate ph from alkaline which was characteristic of young sites to slightly acidic on older sites the ph decreased gradually with increasing site age in all soil layers but increased with depth in the soil profile in contrast to c and n p content did not significantly change with site age in any layer of the soil profile the highest amount of p was found in the fermentation layer but there was no difference to the other layers the soil profile of the semi natural sites was richer in p in comparison to 40 year old reclaimed ones

impact related dike breccia lithologies in the icdp drill core yaxcopoil 1 chicxulub impact structure mexico petrographic descriptions of three dike breccia lithologies from drill core yaxcopoil 1 yax 1 are presented they occur within allochthonous units of displaced sedimentary megablocks of the chicxulub impact structure the suevitic dike breccias are the uppermost dike lithology they contain melt rock particles and melt injections into the dike groundmass shock features occur ubiquitously and indicate a strong thermal annealing flow textures suggest a highly energetic emplacement process possibly during the excavation stage as a ground surge related deposit the impact melt rock dikes are present in a strongly brecciated megablock interval as flow textured anastomozing veinlets of impact melt rock that were altered to clay minerals the melt impregnated a dolomitic host rock indicating a low viscosity and thus high initial temperatures brecciation of the impact melt rock dikes occurred while they were still below the glass transition temperature suggesting that dynamic conditions prevailed shortly after the emplacement process major element data indicates that the impact melt rock dikes differ in composition from the homogenized impact melt rock of chicxulub this could point to an emplacement during the late compression or early excavation stages of cratering the clastic polymict dike breccias are coeval with pervasive brittle fracturing of the host rocks they bear clasts including some crystalline basement and possible melt rock particles in a fine grained dolomite matrix with turbulent flow textures fabric and texture indicate a granular flow at ambient pressures such conditions could be envisaged for the excavation phase while the transient cavity grew and fractures opened

technogenic soils developed on mine spoils containing iron sulphides mineral transformations as an indicator of pedogenesis technogenic soils technosols containing iron sulphides developed on mine spoils located in three abandoned industrial sites in poland the siersza hard coal mine in trzebinia staszic pyrite mine in rudki and pyrite mines in wie ciszowice were investigated in order to identify the most important mineral transformations occurring in the soils the x ray diffraction xrd scanning electron microscope energy dispersive spectrometry sem eds techniques and selective extraction methods were applied in the examination of soil mineralogy the soils studied represented weakly developed soil formations in which organic matter accumulation increasing with age was observed along with intense weathering of parent materials containing iron sulphides the degree of sulphide transformations is age dependent in young soils from trzebinia and rudki poorly weathered iron sulphides occurred whereas in old soils from wie ciszowice iron sulphides were almost entirely transformed the main products of sulphide weathering are iron oxides and sulphate minerals gypsum and jarosite group minerals mainly inherited phyllosilicates and pedogenic clay minerals smectite vermiculite kaolinite and mixed layer minerals were present in the soils studied the development of al hydroxy interlayers and the possibility of the formation of poorly crystalline al phases were also documented the processes seem to be ph dependent and occur at ph ranging from 4 3 to 4 4 the development of iron oxides and sulphate minerals in the process of sulphide weathering as well as phyllosilicate transformations e g the development of swelling clay minerals at the expense of inherited mica and chlorite and the formation of hydroxy interlayers in clay minerals are the main mineralogical aspects of pedogenesis of technogenic soils developed on mine spoils containing iron sulphides the wrb and soil taxonomy systems require more precise solutions concerning the classification of technogenic soils investigated the thionic horizon wrb and sulfuric horizon soil taxonomy definitions as well as the sulphidic material wrb and sulfidic material soil taxonomy definitions need to be supplemented the paper suggests the introduction of the sulfic udorthents taxon in the soil taxonomy as well as the introduction of thionic suffix qualifier to the key to the technosols in the wrb

thermal conductivity and heat transfer through the snow on the ice of the beaufort sea 1 eighty nine point measurements of the thermal conductivity ks of the snow on the sea ice of the beaufort sea were made using a heated needle probe average values ranged from 0 078 w m 1 k 1 for new snow to 0 290 w m 1 k 1 for an ubiquitous wind slab ks increased with increasing density consistent with published equations but could also be reliably estimated from the metamorphic state of the snow using measured values of ks and snow stratigraphy the average bulk value for the full snowpack was 0 14 w m 1 k 1 in contrast ks inferred from ice growth and temperature gradients in the snow was 0 33 w m 1 k 1 the mismatch arises in part because the second estimate is based on measurements from an aggregate scale that includes enhanced heat flow due to two and three dimensional snow and ice geometry a finite element model suggests that the complex geometry produces areas of concentrated heat loss at the snow surface these hot spots however increase the apparent conductivity only by a factor of 1 4 not enough to fully explain the mismatch nonconductive heat transfer mechanisms like natural and forced air convection may also be operating in the snowpack though the ubiquitous presence of low permeability wind slabs potentially limits their effectiveness the relative contributions of effects due to snow and ice geometric and nonconductive processes within the snowpack remain uncertain

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

comments on trace element and isotopic evidence for archean basement in the lonar crater impact breccia deccan volcanic province by ramananda chakrabarti and asish r basu in earth and planetary science letters 247 2006 197 211

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

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

40ar 39ar age of the lonar crater and consequence for the geochronology of planetary impacts asteroid impacts play an important role in the evolution of planetary surfaces in the inner solar system the large majority of impacts occur on bodies e g asteroids the moon mars covered by primitive igneous rocks however most of the impacts recorded on earth occur on different rock types and are poor proxies for planetary impacts the lonar crater is a 1 88 km diameter quaternary age crater fig 1 located on the ca 66 ma deccan basaltic traps in maharashtra india and is one of the very few craters on earth emplaced directly on basaltic lava flows we carried out 12 40 ar 39 ar step heating experiments on 4 melt rock samples in order to 1 obtain a precise age for the lonar crater 2 study the response of isotopic chronometers during impacts on mafic target rocks and 3 better understand the dating of extraterrestrial impact craters we obtained 10 plateau and 9 inverse isochron ages on various aliquots combination of selected data into a global inverse isochron yielded an age of 570 47 ka mswd 1 1 p 0 24 in comparison previous nonisotopic investigations on rocks thought to be affected by secondary processes yielded a range of much younger ages ca 12 62 ka the measured 40 ar 36 ar trapped values offer a direct comparison with the atmospheric benchmark value and allow us to test the inherited 40 ar degassing capacity of basaltic impact melt rocks the 40 ar 36 ar ratio of 296 5 1 7 is indistinguishable from the atmospheric composition and suggests that inherited 40 ar is absent from the melt rock this result substantiates diffusion models that predict a near complete degassing of low viscosity melt e g basalts during impact and demonstrates for the first time that inherited 40 ar is less problematic for 40 ar 39 ar dating of impact events in basaltic igneous rocks compared to si rich rocks these results provide direct evidence that basaltic melt rocks are excellent candidates for recording the timing of planetary impact events and as far as dating is concerned should be the preferred targets of sample recovery by future missions

geology of lonar crater india lonar crater india is one of the youngest and best preserved impact structures on earth the 1 88 km diameter simple crater formed entirely within the deccan traps making it a useful analogue for small craters on the basaltic surfaces of the other terrestrial planets and the moon in this study we present a meter scale resolution digital elevation model geological map of lonar crater and the surrounding area and radiocarbon ages for histosols beneath the distal ejecta impact related deformation of the target rock consists of upturned basalt fl ows in the upper crater walls and recumbent folding around rim concentric subhorizontal noncylindrical fold axes at the crater rim the rim fold hinge is preserved around 10 15 of the crater although tearing in the rim fold is inferred from fi eld and paleomagnetic observations no tear faults are identifi ed indicating that large displacements in the crater walls are not characteristic of small craters in basalt one signifi cant normal fault structure is observed in the crater wall that offsets slightly older layer parallel slip faults there is little fl uvial erosion of the continuous ejecta blanket portions of the ejecta blanket are overlain by aerodynamically and rotationally sculpted glassy impact spherules in particular in the eastern and western rim as well as in the depression north of the crater known as little lonar the emplacement of the continuous ejecta blanket can be likened to a radial groundhugging debris fl ow based on the preserved thickness distribution of the ejecta the effi cient exchange of clasts between the ejecta fl ow and the underlying histosol and the lack of sorting and stratifi cation in the bulk of the ejecta the ejecta profi le is thickened at the distal edge and similar to fl ejecta structures observed on mars

evidence for a chondritic impactor evaporation condensation effects and melting of the precambrian basement beneath the target deccan basalts at lonar crater india the similar to 1 88 km diameter lonar impact crater formed similar to 570 ka ago and is an almost circular depression hosted entirely in the poladpur suite of the similar to 65 ma old basalts of the deccan traps to understand the effects of impact cratering on basaltic targets commonly found on the surfaces of inner solar system planetary bodies major and trace element concentrations as well as nd and sr isotopic compositions were determined on a suite of selected samples composed of basalts a red bole sample which is a product of basalt alteration impact breccia and impact glasses either in the form of spherules 1 mm and 43 0 the group 1 spherules are further subdivided into groups 1a and 1b with group 1a spherules showing higher ni and mostly higher cr compared to the group 1b spherules iridium and cr concentrations of the spherules are consistent with the admixture of 1 8 wt of a chondritic impactor to the basaltic target rocks the impactor contribution is most prominent in the group 1a and group 2 spherules which show higher ni co ni cr and cr co ratios compared to the target basalts in contrast the group 1b spherules show major and trace element compositions that overlap with those of the impact breccia and are characterized by high efth enrichment factor for th defined as the nb normalized concentration of th relative to that of the average basalt as well as fractionated la sm n and higher large ion lithophile element lile concentrations compared to the basalts the relatively more radiogenic sr and less radiogenic nd isotopic composition of the impact breccia and non spherical impact glasses compared to the target basalts are consistent with melting and mixing of the precambrian basement beneath the deccan basalt with up to 15 wt contribution of the basement to these samples variations in the moderately siderophile element mse concentration ratios of the impact breccia as well as all the spherules are best explained by contributions from three components a chondritic impactor the basaltic target rocks at lonar and the basement underlying the deccan basalts the large variations in concentrations of volatile elements like zn and cu and correlated variations of efcu efzn efpb efzn efk efzn and efna efzn particularly in the group 1a spherules are best explained by evaporation condensation effects during impact while most spherules irrespective of their general major and trace element composition show a loss in volatile elements e g zn and cu relative to the target basalts some spherules mainly of group 1 display enrichments in these elements that are interpreted to reflect the unique preservation of volatile rich vapour condensates resulting from geochemical fractionation in a vertical direction within the vapour cloud c 2017 elsevier ltd all rights reserved

size and morphology of the chicxulub impact crater the chicxulub impact in mexico has been linked to the mass extinction of species at the end of the cretaceous period from seismic data collected across the offshore portion of the impact crater the diameter of the transient cavity is determined to be about 100 km this parameter is critical for constraining impact related effects on the cretaceous environment with previous estimates of the cavity diameter spanning an order of magnitude in impact energy the offshore seismic data indicate that the chicxulub crater has a multi ring basin morphology similar to large impact structures observed on other planets such as venus

trace element and isotopic evidence for archean basement in the lonar crater impact breccia deccan volcanic province abstract the lonar impact crater in the deccan traps of the indian peninsula provides unique opportunities to study physical and chemical processes of impact cratering on basaltic targets because terrestrial impact craters on basalts are extremely rare such studies are needed for determining provenance and other parameters of the excavated rocks and the cratering phenomenon that may have implications for similar crater formations in lunar martian and other basaltic targets in the solar system considering some of these objectives we analyzed trace elements and nd sr pb isotopes of impact breccia rocks and target basalts collected from the lonar crater chondrite normalized rare earth element ree patterns in the target basalts and breccia rocks show similar light ree enriched patterns although in detail the impact breccia are more fractionated in la sm compared to the target basalts the target basalts also show much lower concentrations of rb ba th u and pb compared to the breccia and are characterized by rb ba and pb depletions with respect to the primitive mantle normalized th u nb ta and the ree contents the breccia rocks are significantly enriched in rb ba and pb and to a lesser extent in th and u compared to the target rocks the nd sr and pb isotopic compositions of the lonar target basalts can be correlated with those of the poladpur suite one of the mid section volcano stratigraphic units of the deccan traps in contrast to the host basalts the impact breccia rocks show more radiogenic sr less radiogenic nd and higher rb sr and lower sm nd ratios indicating an additional component other than the target basalt that must have been derived from beneath the basaltic target rocks at the impact site the deccan traps in western india are underlain by archean to mid proterozoic cratonic rocks the overall geochemical signatures of the impact breccia rocks specifically the trace element concentrations negative end values radiogenic sr isotopic composition as well as the high 207pb 204pb at low 206pb 204pb indicate that a major component of the lonar impact breccia was derived from melting of archean basement rocks we argue that the archean component in the breccia cannot be from the incorporation of paleosols that are weathering products of the target basalts or from the inter trappean sediments that are most commonly cherts and limestones of mesozoic age similarly the possible role of eolian sediments in causing the archean pb isotopic signature identical to those of the deccan basement in the breccia rocks can be excluded the basement beneath the lonar region is believed to be similar to the dharwar craton of peninsular india based on their similar pb isotopic compositions with the breccia rocks we suggest the archean chitradurga group of rocks of this craton to be present in the basement beneath the deccan lavas of the lonar region the thickness of the basaltic target rocks at the crater site 400 m and the inferred crater depth 350 m 610 m based on depth to diameter ratios in simple planetary craters 0 2 0 33 are consistent with our conclusion regarding melting and incorporation of these ancient basement rocks in the impact breccia of the lonar crater using pi group scaling relations the observed crater diameter and density of the basaltic target rocks we have estimated with reasonable approximation the diameter of the lonar impactor to be either 70 m 86 m or 120 m assuming the bolide to be an iron meteorite stony iron meteorite or an ordinary chondrite respectively

the thermal conductivity of seasonal snow twenty seven studies on the thermal conductivity of snow k eff have been published since 1886 combined they comprise 354 values of k eff and have been used to derive over 13 regression equation and predicting k eff vs density due to large and largely undocumented differences in measurement methods and accuracy sample temperature and snow type it is not possible to know what part of the variability in this data set is the result of snow microstructure we present a new data set containing 488 measurements for which the temperature type and measurement accuracy are known a quadratic equation where is in g cm 3 and k eff is in w m 1 k 1 can be fit to the new data r 2 0 79 a logarithmic expression can also be used the first regression is better when estimating values beyond the limits of the data the second when estimating values for low density snow within the data set snow types resulting from kinetic growth show density independent behavior rounded grain and wind blown snow show strong density dependence the new data set has a higher mean value of density but a lower mean value of thermal conductivity than the old set this shift is attributed to differences in snow types and sample temperatures in the sets using both data sets we show that there are well defined limits to the geometric configurations that natural seasonal snow can take

microstructural change around a needle probe to measure thermal conductivity of snow the thermal conductivity of snow determines to a large extent ground heat flux and snow metamorphism one common method of measuring the thermal conductivity of snow is with a needle probe we measured the microstructural changes in the snow around a typical needle using micro computed tomography the insertion of the needle probe caused structural changes up to a radial distance of 0 5 1 mm using a commercial needle probe with short 30 s heating time we measured thermal conductivity values in snow that were 50 lower than the conductivity measured using a calibrated guarded hot plate apparatus numerically simulated time temperature curves with an introduced air gap that simulated the needle damage reproduced the observed needle probe results well confirming that poor contact resistance between the needle and the snow will lead to measurement discrepancies and a large bias while most measurements in snow have been done using a long heating time and are not subject to this error the thermal conductivity of snow measured with needle probes with short measurement times should be avoided this same effect is probably observed in other brittle and highly porous materials a condition that may not be widely appreciated

target rocks impact glasses and melt rocks from the lonar impact crater india petrography and geochemistry the lonar crater india is the only well preserved simple crater on earth in continental flood basalts it is excavated in the deccan trap basalts of cretaceous tertiary age a representative set of target basalts including the basalt flows excavated by the crater and a variety of impact breccias and impact glasses were analyzed for their major and trace element compositions impact glasses and breccias were found inside and outside the crater rim in a variety of morphological forms and shapes comparable geochemical patterns of immobile elements e g rees for glass melt rock and basalt indicates minimal fractionation between the target rocks and the impactites we found only little indication of post impact hydrothermal alteration in terms of volatile trace element changes no clear indication of an extraterrestrial component was found in any of our breccias and impact glasses indicating either a low level of contamination or a non chondritic or otherwise iridium poor impactor

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

isotopic fractionation of zinc in tektites abstract tektites are terrestrial natural glasses produced during a hypervelocity impact of an extraterrestrial projectile onto the earth s surface the similarity between the chemical and isotopic compositions of tektites and terrestrial upper continental crust implies that the tektites formed by fusion of such target rock tektites are among the driest rocks on earth although volatilization at high temperature may have caused this extreme dryness the exact mechanism of the water loss and the behavior of other volatile species during tektite formation are still debated volatilization can fractionate isotopes therefore comparing the isotope composition of volatile elements in tektites with that of their source rocks may help to understand the physical conditions during tektite formation for this study we have measured the zn isotopic composition of 20 tektites from four different strewn fields almost all samples are enriched in heavy isotopes of zn compared to the upper continental crust on average the different groups of tektites are isotopically distinct listed from the isotopically lightest to the heaviest muong nong type indochinites 66 64 zn 0 61 0 30 north american bediasites 66 64 zn 1 61 0 49 ivory coast tektites 66 64 zn 1 66 0 18 the australasian tektites others than the muong nong type indochinites 66 64 zn 1 84 0 42 and central european moldavites 66 64 zn 2 04 0 19 these results are contrasted with a narrow range of 66 64 zn 0 0 7 for a diverse spectrum of upper continental crust materials the elemental abundance of zn is negatively correlated with 66 64 zn which may reflect that isotopic fractionation occurred by evaporation during the heating event upon tektite formation simple rayleigh distillation predicts isotopic fractionations much larger than what is actually observed therefore such a model cannot account for the observed zn isotope fractionation in tektites we have developed a more realistic model of evaporation of zn from a molten sphere during its hypervelocity trajectory the molten surface of the tektite will be entrained by viscous coupling with air that will then induce a velocity field inside the molten sphere this velocity field induces significant radial chemical mixing within the tektite that accelerates the evaporation process our model albeit parameter dependent shows that both the isotopic composition and the chemical abundances measured in tektites can be produced by evaporation in a diffusion limited regime

highly heterogeneous precambrian basement under the central deccan traps india direct evidence from xenoliths in dykes abstract crustal or mantle xenoliths are not common in evolved tholeiitic flood basalts that cover huge areas of the precambrian shields yet the occasional occurrences provide the most direct and unequivocal evidence on basement composition few xenolith occurrences are known from the deccan traps india and inferences about the deccan basement have necessarily depended on geophysical studies and geochemistry of deccan lavas and intrusions here we report two basalt dykes rajmane and talwade dykes from the central deccan traps that are extremely rich in crustal xenoliths of great lithological variety gneisses quartzites granite mylonite felsic granulite carbonate rock tuff because the dykes are parallel and only 4 km apart and only a few kilometres long the xenoliths provide clear evidence for high small scale lithological heterogeneity and strong tectonic deformation in the precambrian indian crust beneath measured 87sr 86sr ratios in the xenoliths range from 0 70935 carbonate to 0 78479 granite mylonite the rajmane dyke sampled away from any of the xenoliths shows a present day 87sr 86sr ratio of 0 70465 and initial at 66 ma ratio of 0 70445 the dyke is subalkalic and fairly evolved mg no 44 1 and broadly similar in its sr isotopic and elemental composition to some of the lavas of the mahabaleshwar formation the xenoliths are comparable lithologically and geochemically to basement rocks from the archaean dharwar craton forming much of southern india as several lines of evidence suggest the dharwar craton may extend at least 350 400 km north under the deccan lava cover this is significant for precambrian crustal evolution of india besides continental reconstructions

geochemical anatomy of a spheroidally weathered diabase major trace element concentrations and nd sr isotope ratios were measured in micro drilled samples of a 237 ga old hand specimen sized spheroidally weathered diabase from southern india a sample of the un weathered diabase dike was also analyzed x ray micro ct imaging of the weathered sample shows three dominant mineral phases which are plagioclase pyroxene and a fe bearing phase possibly hematite and or ilmenite this imaging documents the pervasive nature of two generations of ribbon like cross cutting fractures the older fracture is sealed while the more recent fracture is open without any in filling the values of the chemical index of alteration cia of the samples show a wide range but are less than 50 despite being a relatively less weathered rock we observe that concentrations of major minor and trace elements vary significantly with the percentage relative standard deviation rsd for the elements ranging from 10 2 41 8 the cia of the samples do not show any trend with the position of the sample in the hand specimen barring ca and li whose concentrations decrease from the core to the rim of the sample there is no significant spatial trend in the concentrations of the elements concentrations of na al and sr increase with increasing cia values while concentrations of mg fe and sc decrease with increasing cia the strong positive correlations of na and al as well as na and sr indicates preferential weathering of plagioclase in the diabase na ca increases while mg al mg na mg ca fe al and sc sr decrease with increasing cia values and the un weathered rock plots in the middle of these trends such variations are explained in terms of differential weathering of plagioclase in samples with lower cia than the un weathered rock w1 type and pyroxene in samples with higher cia than the un weathered rock w2 type which have varying resistance to weathering at the hand specimen scale the variability in the weathering indices like cia are controlled by differential weathering of minerals and might not accurately reflect the intensity of weathering chondrite normalized la sm and gd lu co vary with cia values indicating mobility of the rees during spheroidal weathering even at the hand specimen scale the eu anomaly also increases with increasing cia values which is explained by differential weathering of pyroxene and plagioclase we observe large percentage deviations of the nb normalized concentrations of elements from the un weathered rock in specific samples but no spatial trend is observed overall the variations in element concentrations can be explained by varying fluid mobility of the elements selective weathering of the minerals in the diabase and ambient environmental conditions considerable nd and sr isotopic variability is observed at the hand specimen scale and is explained in terms of weathering related fractionation of parent daughter ratios this elemental fractionation must have happened long time ago to allow for radiogenic decay of the long lived isotopes of rb 87 and sm 147 the spread rsd in the initial sr and nd isotope compositions of the weathered samples reach a minimum value around 12 13 ga which we interpret as the timing of the peak weathering event which led to fractionation of the parent daughter ratios for nd isotopes the average epsilon nd 1 2 ga of the weathered samples coincides with the epsilon nd 1 2 ga of the unweathered rock the timing of the weathering event coincides with the timing of the breakup of the columbia supercontinent and follows wide spread alkaline volcanism in the indian subcontinent this is the first such attempt to determine the timing of a weathering event in rocks using long lived radioactive isotopes c 2016 elsevier b v all rights reserved

impacts of an accumulation hiatus on the physical properties of firn at a low accumulation polar site 1 recent field investigations of a megadune region of east antarctica provide evidence that differences in grain size thermal conductivity and permeability across a megadune profile are due to spatial accumulation variability in the absence of significant microclimate variations the megadunes are low amplitude 2 8 m long wavelength 2 5 km bands with perceptible but low accumulation less than 40 mm water equivalent weq yr 1 and accumulation hiatus within several kilometers proximity as determined by remote sensing surface feature classification and ground penetrating radar profiling our hypothesis that accumulation rate impacts the extent of temperature gradient driven metamorphic growth in low accumulation rate sites is supported by measurements of various firn physical properties relatively small differences in accumulation rate less than 40 mm weq yr 1 result in large differences in physical properties including grain size thermal conductivity and permeability which are apparent in satellite based microwave data from both passive and active sensors the differences in physical snow structure between low accumulation areas and accumulation hiatus areas in the near surface are sufficiently distinct that evidence of past accumulation hiatus should be observable in the physical and chemical properties of an ice core record

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

origin of moldavites abstract moldavites are supposed to be products of the ries impact but neither has the source material been identified in the ries area nor is the mode of moldavite glass formation sufficiently clear it is the purpose of this investigation to elucidate both problems by means of new chemical data fifty nine moldavites from 20 localities in bohemia and moravia and 30 samples of middle miocene sediments from the ries were analyzed for major oxides x ray fluorescence 30 trace elements neutron activation water content infrared spectroscopy and 18 o 16 o ratios microprobe analysis shows that moldavites are composed of thin laminae of optically and chemically somewhat different glasses bent and curled in three dimensions indicating the confluence of primary melt units under conditions of laminar flow the moldavites cannot be derived from crystalline basement rocks and mesozoic sediments involved in the ries impact the only lithology from which moldavites could have been formed is middle miocene sands which formed a thin veneer in the ries area at the time of the impact moldavites and these middle miocene sands are compositionally similar but not identical which indicates that moldavite formation from sands involved chemical fractionation enrichments and depletions of individual elements in moldavites can not be explained by fractional vaporization this was confirmed also by melting experiments with sand samples moreover the negligible water content in moldavites and the scarcity of vesicles excludes the formation of moldavite melt by simple fusion of sands for individual elements the distribution ratios q concentration in moldavites concentration in sands and ionic radii r are significantly correlated q 2 21r 1 18 it is inferred that at the encounter of the impactor with the ground surficial sands were transformed by high pressures and temperatures into a plasma like vapor in the expanding and ascending vapor droplets of a liquid phase condensed consisting of a framework of interconnected si al o4 tetrahedra which selectively incorporated cations to compensate its negative charge the uptake of cations was controlled by the ionic radius because ions fit better into the interstices of the liquid framework the larger they are the primary melt droplets separated from residual vapor and coalesced to moldavite bodies the lower o content of moldavites 11 5 as compared to dry sands 16 is explained by an admixture meteoric water o 10 which filled the pore space

petrography and geochemistry of target rocks and impactites from the ilyinets crater ukraine the 400 ma old ilyinets impact structure was formed in the precambrian basement of the ukrainian shield and is now mostly covered by quaternary sediments various impact breccias and melts are exposed in its southern section the crater is a complex structure with a central uplift that is surrounded by an annular deposit of breccias and melt rocks in the annulus brecciated basement rocks are overlain by up to 80 m of glass poor suevitic breccia which is overlain and partly intercalated by glass rich suevite with a thickness of up to 130 m impact melt rocks occur within and on top of the suevites in some cases in the form of devitrified bomb shaped impact glass fragments we have studied the petrographic and geochemical characteristics of 31 mostly shocked target rock samples granites gneisses and one amphibolite obtained from drill cores within the structure and impact breccias and melt rock samples from drill cores and surface exposures multiple sets of planar deformation features pdfs are common in quartz potassium feldspar and plagioclase of the shocked target rocks the breccias comprise more or less devitrified impact melt with shocked clasts the impact melt rocks bombs show abundant vesicles and in some cases glass is still present as brownish patches and schlieren all impact breccias including the melt rocks are strongly altered and have significantly elevated k contents and lower na contents than the target rocks the alteration could have occurred in an impact induced hydrothermal system the bomb shaped melt rocks have lower mg and ca contents than other rock types at the crater compared to target rocks only minor enrichments of siderophile element contents e g ni co ir in impact melt rocks were found

petrology of impactites from el gygytgyn crater breccias in icdp drill core 1c glassy impact melt rocks and spherules el gygytgyn is a 18 km diameter 3 6 ma old impact crater in ne siberia international continental scientific drilling program el gygytgyn hole 1c was drilled on the frozen crater lake 2 3 km from the crater center to a final depth of 517 m below the lake floor petrographic and geochemical analyses of 26 drill core samples three impact melt rocks from the surface and seven glass spherules from surface deposits outside the crater are used to characterize the impactite inventory at el gygytgyn the bottom 98 m of hole 1c intersected monomict brecciated unshocked rhyolitic ignimbrite with minor intercalations of polymict breccia and mafic inclusions these lithologies are overlain by 89 m of polymict breccia whose components occasionally exhibit scarce low degree shock metamorphic features this unit is succeeded by 10 m of suevite that contains about 1 vol glassy impact melt shards 1 cm in size and a low amount of shock metamorphosed lithic clasts the suevite is capped by a reworked fallout deposit that constitutes a transition over 4 m into lacustrine sedimentation a higher abundance of shock metamorphosed lithic clasts and glass spherules some with ni rich spinel and admixture of an ultramafic component characterize this unit we tentatively interpret this impactite section as allochthonous breccia in the vicinity of el gygytgyn s central ring uplift the geochemical compositions of seven glass spherules from terrace deposits 2 km outside the crater and eight spherules from the reworked fallout deposit in hole 1c show far greater variability than the composition of impact melt shards and impact melt rocks some of these spherules also show strong enrichments in siderophile elements