constraints on central uplift structure from the manicouagan impact crater recent drilling operations at the 90 km diameter late triassic manicouagan impact crater of quebec canada have provided new insight into the internal structure of a complex craters central region previous work had indicated that the impact event generated a 55 km diameter sheet of molten rock of relatively consistent originally 400 m thickness floran et al 1978 the drilling data reveals melt sheet thicknesses of up to 1500 m with kilometer scale lateral and substantial vertical variations in the geometry of the crater floor beneath the melt sheet the thickest melt section occurs in a 1500 m deep central trough encircled by a horseshoe shaped uplift of precambrian basement the uplift constitutes a modified central peak structure at least part of which breached the melt sheet mineralogical and compositional segregation differentiation of the thicker melt sheet section coupled with a lack of fractionation in the thinner units shows that the footwall geometry and associated trough structure were in place prior to melt sheet solidification marked lateral changes in sub melt sheet basement relief support the existence of a castellated footwall that was created by high angle impact related offsets of 100s to 1000s of meters this indicates that deformation during the modification stage of the cratering process was primarily facilitated by large displacement fault systems this work suggests that manicouagan is a central peak basin with rings which does not appear to fit with current complex crater classification schemes

thermohydraulic explosions in phreatomagmatic eruptions as evidenced by the comparison between pyroclasts and products from molten fuel coolant interaction experiments 1 thermohydraulic explosions were produced by molten fuel coolant interaction mfci experiments using remelted shoshonitic rocks from vulcano italy the fragmentation history and energy release were recorded the resulting products were recovered and analyzed with the scanning electron microscope fine particles from experiments show shape and surface features that result from melt fragmentation in brittle mode these clasts relate to the thermohydraulic phase of the mfci where most of the mechanical energy is released they are here called active particles the total surface area of such particles is proportional to the energy of the respective explosions other particles from experiments show shape and surface features that result from melt fragmentation in a ductile regime these fragments called passive particles form after the thermohydraulic phase during the expansion phase of the mfci in order to verify thermohydraulic explosions in volcanic eruptions we compared experimental products with samples from phreatomagmatic base surge deposits of vulcano ash particles from the experiments show features similar to those from the deposits suggesting that the experiments reproduced the same fragmentation dynamics to achieve discrimination between active and passive particles we calculated shape parameters from image analysis the mass of active particles in base surge deposits was calculated as the material properties for the natural samples are identical to the experimental ones the energy measurements and calculations of the experiments can be applied for a single phreatomagmatic eruption at vulcano a maximum mechanical energy release of 2 75 1013 j was calculated representing a tnt analogue of 6 5 kt

tookoonooka a large buried early cretaceous impact structure in the eromanga basin of southwestern queensland australia the 66 km wide tookoonooka impact structure 27 degrees 07 s 142 degrees 50 e was first recognised from seismic profiles as a circular structure consisting of a concentric arrangement of anticlines and synclines which surround a complex central dome approximately 22 km wide a gravity low and a central magnetic high characterize the structure now buried by up to 900 m of cretaceous and tertiary clastic sediments the tookoonooka structure was formed approximately 128 ma ago during deposition of the paralic cadna owie formation thin sections from a centrally located exploration well reveal an impact melt breccia composed of local ordovician quartz mica schist bedrock detailed study of planar deformation features pdfs in quartz grains from this breccia show 64 lamellae sets in 25 grains most of the pdf measurements correspond to xi mean 1122 and r z mean 1011 mean 0111 crystallographic indices with five other orientations also measured this distribution of pdfs corresponds to that found in quartz from impact structures in porous sedimentary rock targets thus confirming an impact origin for tookoonooka

sulfur isotope systematics and platinum group element behavior in ree enriched metasomatic fluids a study of mantle xenoliths from dish hill california usa the sulfur isotopic compositions trace and rare earth element ree and platinum group element pge contents were determined for a suite of anhydrous spinel lherzolite xenoliths and their metasomatized equivalents from dish hill california usa to evaluate the mobility of these elements in metasomatic fluids anhydrous xenoliths have ree patterns that vary from being slightly lree depleted to flat as well as variable enrichments of lile ba and sr and hfse th and nb that indicate they were affected by cryptic metasomatism the formation of amphibole in xenoliths modal metasomatism is accompanied by pronounced enrichments in hfses and rees particularly lrees with the exception of a single xenolith with slightly elevated pge contents the metasomatic fluids did not produce any appreciable changes in the platinum group element pge contents suggesting these elements are relatively immobile under these mantle conditions typical concentrations of pges in xenoliths from dish hill are 0 8 ppb os 1 ppb ir 0 6 ppb rh 2 5 ppb pt 1 8 ppb pd and 1 7 ppb au modal metasomatism has resulted in a variable increase in sulfur content of the xenoliths from a value of 10 ppm for anhydrous xenoliths to between 25 and 120 ppm for amphibole rich xenoliths the 34s values of sulfides in anhydrous xenoliths from dish hill are near 7 which are unusually high for the mantle and indicate that the lithospheric mantle under california was most likely affected by subduction of crustal sulfur sulfides in amphibole bearing xenoliths have variable but generally lower 34s values of 3 to 8 7 suggesting that some portion of the metasomatic sulfur was derived from an asthenospheric mantle source with a more primordial sulfur isotopic composition

copper and noble metal enrichments across the lithosphere asthenosphere boundary of mantle diapirs evidence from the lanzo lherzolite massif

geochemical relationships in the sudbury igneous complex origin of the main mass and offset dikes petrological models relating the different rock types constituting the 1 85 ga sudbury igneous complex are constrained with extensive new geochemical data we show that the main mass felsic norite transition zone quartz gabbro and granophyre have similar ratios of the highly incompatible trace elements e g la sm 4 5 7 la nb 2 8 4 2 th zr 0 04 0 05 and that these variations are consistent with the crystallization and differentiation of magma types largely 80 derived from the upper crust with a smaller contribution from a mantle source although there is presently no conclusive proof that magma was generated in situ as a melt sheet produced by meteorite impact we find no principal reason why this model should be rejected however we propose that a small contribution of mantle derived picritic magma is required to explain the abundant ni cu and platinum group elements pge in the sudbury deposits as well as the compositions of the ultramafic inclusions mgo 12 36 wt fo sub 68 87 olivines with 450 3 700 ppm ni and abundant chrome rich spinel and the magnesian composition of the mafic norite 8 14 wt mgo and the sublayer 6 12 wt mgo we believe that the main mass of the sudbury complex achieved its present composition through incorporation of up to 20 percent mantle derived picritic magma emplaced along crustal fractures produced by the impact event these picritic magmas entered the melt sheet as a dense plume vigorously mixing with it and due to the marked compositional shift the mixed magma formed magmatic sulfides which sank through the magma column depleting the melt in ni cu and pge since both the felsic norite and granophyre have indistinguishable ratios of the incompatible trace elements we see no requirement to derive these units of rock by the crystallization of magmas derived from different sources rather the compositional difference between the felsic norite and granophyre is attributed to the in situ differentiation of the magma we show that the main mass has many compositional traits similar to those of most of the offset dike quartz diorites e g the parkin offset dike la sm 6 3 la nb 4 5 th zr 0 05 and of embayment related leucocratic norites from the whistle mine la sm 6 2 la nb 5 0 th zr 0 02 these rocks have compositions intermediate between the felsic norite and the granophyre and therefore crystallized from the same magma type arguably the unmineralized quartz diorites provide the best possible estimate of the original magma from which the sudbury complex crystallized in detail there are subtle variations in composition within and between offset dikes with the largest difference being between the north and south range offsets the north range offset dikes cut archean granitoids and gneisses and have elevated sr la yb la sm and gd yb and low tio 2 whereas the south range dikes cut early proterozoic sediments mafic volcanics and intrusions and have low sr la yb gd yb la sm and high tio 2 these differences may be caused by the assimilation of different country rocks during emplacement of the dike a strongly mineralized offset dike at the creighton mine has geochemical variations that are different when compared to the main mass and in the case of creighton are more similar to the local mineralized sublayer these data suggest that mineralized and barren quartz diorites have different geochemical compositions and that these traits may be of value in mineral exploration

comment on bedout a possible end permian impact crater offshore of northwestern australia nearly a decade ago gorter 1 1 suggested that the bedout basement high offshore western australia might represent an impact structure in view of the site s well defined circular bouguer anomaly and seismic reflection data indicating a possible ring syncline recently becker et al 2

petrology of impact melt rocks at the chicxulub multiring basin yucatan mexico compositions and textures of melt rocks from the upper part of the chicxulub structure are typical of melt rocks at other large terrestrial impact structures apart from variably elevated iridium concentrations less than 1 5 to 13 5 0 9 ppb indicating nonuniform dissemination of a meteoritic component bulk rock and phenocryst compositions imply that these melt rocks were derived exclusively from continental crust and platform sediment target lithologies modest differences in bulk chemistry among samples from wells located approximately 40 km apart suggest minor variations in relative contributions of these target lithologies to the melts subtle variations in the compositions of early formed pyroxene and plagioclase also support minor primary differences in chemistry between the melts evidence for pervasive hydrothermal alteration of the porous mesostasis includes albite k feldspar quartz epidote chlorite and other phyllosilicates as well as siderophile element enriched sulfides suggesting the possibility that chicxulub like sudbury may host important ore deposits

a ferroelectric model for the low emissivity highlands on venus a model to explain the low emissivity venusian highlands is proposed utilizing the temperature dependent dielectric constant of ferroelectric minerals ferroelectric minerals are known to occur in alkaline and carbonatite rocks both of which are plausible for venus ferroelectric minerals possess extremely high dielectric constants 105 over small temperature intervals and are only required in minor 1 abundances to explain the observed emissivities the ferroelectric model can account for 1 the observed reduction in emissivity with increased altitude 2 the abrupt return to normal emissivities at highest elevations and 3 the variations in the critical elevation observed from region to region

impact shocked zircons discovery of shock induced textures reflecting increasing degrees of shock metamorphism abstract textural effects specifically characteristic of shock metamorphism in zircons from impact environments have not been reported previously however planar deformation features pdf due to shock metamorphism are well documented in quartz and other mineral grains from these same environments an etching technique was developed that allows sem visualization of pdf and other probable shock induced textural features such as granular polycrystalline texture in zircons from a variety of impact shock environments these textural features in shocked zircons from k t boundary distal ejecta form a series related to increasing degrees of shock that should correlate with proportionate resetting of the u pb isotopic system

planar lamellar substructures in quartz both single and multiple sets of lamellar features in quartz grains can be produced in shock experiments and are observed at known impact sites they can also be produced in static constant strain rate experiments and are observed at known tectonic and explosive volcanic sites the former are strictly planar and parallel and generally conform to rational crystallographic orientations in the quartz crystal structure the latter can also be planar and parallel but sometimes show slight to moderate curvature with lamellar bifurcations and variable lamellar widths their orientations show a variety of forms with in general a broader angular distribution than that of the impact generated lamellae deductions of physical conditions of deformation from optical morphology spacing multiplicity and orientation information alone remain somewhat conjectural full characterization by transmission electron microscopy tem appears to be necessary

spherule layers records of ancient impacts abstract a large extraterrestrial object striking earth at cosmic velocity melts and vaporizes silicate materials which can condense into highly spheroidal sand size particles that get deposited hundreds to thousands of kilometers from the point of impact these particles known as impact spherules have been detected in great abundance in a relatively small number of thin discrete layers ranging in age from less than a million years to 3 47 billion years unaltered impact spherules consist entirely of glass microtektites or a combination of glass and crystals grown in flight microkrystites impact spherule layers form very rapidly and can be very extensive even global in extent e g the cretaceous tertiary k t boundary layer so they form excellent time stratigraphic markers because they are always found in a stratigraphic context spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts a reco

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

surface properties and surficial deposits on venus new results from magellan radar altimeter data analysis abstract microwave remote sensing data acquired with magellan venus orbiter are the main source of information about the surface of the planet we analyze variability of the backscattering function dependence of radar cross section on incidence angle for steep incidence angles 0 25 4 75 in the 75 n 55 s latitude zone with data from the magellan radar altimeter at 12 6 cm wavelength we show that all variability of the backscattering function can be described by three parameters describing 1 surface reflectivity 2 relative proportion of horizontal facets and 3 general roughness analysis of maps of these parameters revealed that surficial deposits for example microdune fields are abundant on venus even in places where they are not readily seen in the synthetic aperture radar images properties of surficial deposits rather than original volcanic flow roughness define the shape of the backscattering function on the majority of regional plains a large radar dark flow in bereghinia planitia has anomalously high proportion of horizontal facets which is consistent with it being formed by a relatively recent plain forming volcanic episode some crater associated radar dark diffuse features and splotches are also characterized by increased proportion of horizontal faces which indicate the presence of mantles deposited from fluidized granular material the backscattering functions of the anomalous radar bright material of mountaintops are more consistent with the strong internal scattering hypothesis rather than the exotic surficial material hypothesis obtained maps can be useful for planning future lander missions to sites with access to surface material with known provenance

where have all the craters gone earth s bombardment history and the expected terrestrial cratering record here we make the first quantitative estimates of the maximum total number of large terrestrial impact craters that can survive until the current era on earth our estimates of crater survival are made using the age of earth s crust and the expected flux of large extraterrestrial bodies striking earth this analysis neglects effects such as weathering and burial which tend to preferentially erase smaller craters thus our results represent the maximum number of craters we expect to exist on earth today although the constant recycling of crustal material creates a strong bias toward younger ages our model suggests that the terrestrial cratering record can be used to determine the size frequency distribution of the impactors that bombarded earth over the past 3 5 g y however our model suggests that the cratering record cannot be used to distinguish between scenarios with a constant flux of impactors over time and scenarios where the bombardment rate was considerably higher in earth s ancient history luckily layers of distal impact ejecta may act as records of impacts even when the source craters have been tectonically recycled we suggest that searches for these distal impact ejecta layers will be more fruitful for constraining the bombardment history of earth than searches for large craters

magellan radar performance and data products the magellan venus orbiter carries only one scientific instrument a 12 6 centimeter wavelength radar system shared among three data taking modes the synthetic aperture mode images radar echoes from the venus surface at a resolution of between 120 and 300 meters depending on spacecraft altitude in the altimetric mode relative height measurement accuracies may approach 5 meters depending on the terrain9s roughness although orbital uncertainties place a floor of about 50 meters on the absolute uncertainty in areas of extremely rough topography accuracy is limited by the inherent line of sight radar resolution of about 88 meters the maximum elevation observed to date corresponding to a planetary radius of 6062 kilometers lies within maxwell mons when used as a thermal emission radiometer the system can determine surface emissivities to an absolute accuracy of about 0 02 mosaicked and archival digital data products will be released in compact disk cdrom format

volatile transport on venus and implications for surface geochemistry and geology the high vapor pressure of volatile metal halides and chalcogenides e g of cu zn sn pb as sb bi at typical venus surface temperatures coupled with the altitude dependent temperature gradient of 8 5 k km 1 is calculated to transport volatile metal vapors to the highlands of venus where condensation and accumulation will occur the predicted geochemistry of volatile metals on venus is supported by observations of cu zn sn pb as sb and bi minerals around terrestrial volcanic vents spectroscopic observations of cucl in volcanic gases at kilauea and nyiragongo and large enrichments of these and other volatile elements in terrestrial volcanic aerosols a one dimensional finite difference vapor transport model shows the diffusive migration of a thickness of 0 01 to 10 m yr of moderately to highly volatile phases e g metal halides and chalcogenides from the hot lowlands 740 k to the cold highlands 660 k on venus the diffusive transport of volatile phases on venus may explain the observed low emissivity of the venusian highlands hazes at 6 km altitude observed by two pioneer venus entry probes and the pioneer venus entry probe anomalies at 12 5 km

platinum group elements in mantle xenoliths from eastern china the platinum group element pge composition of a suite of anhydrous spinel peridotite xenoliths from five different localities in eastern china were determined in order to evaluate the geochemical behaviour of these elements during mantle processes the samples are composed of harzburgites of granoblastic textures and lherzolites mainly of protogranular and porphyroclastic textures the total pge contents vary from 16 20 to 50 7 ppb ruthenium ir and pd are correlated with cao and are interpreted to define a partial melting trend the positive correlation between cu and pd supports this interpretation however with the exception of au all peridotitic xenoliths display a downward concave pge pattern indicating pt enrichment this pattern is very different from the flat or negatively sloped pge pattern observed commonly for mantle xenoliths and peridotitic massifs the pt enrichment is ascribed to percolating basaltic magma which is precipitating only small amounts of sulphides as pt is less strongly chalcophilic than pd it may fractionate from the sulphides which host the pd and form pt alloys this process also explains the au enrichment and the pt au correlation as au is also less strongly chalcophilic than pd the strong ir depletion in the protogranular samples is ascribed to ir retention in the deep mantle due to the high metal silicate distribution coefficient of ir at fo2 iw 1

deep drilling into the chesapeake bay impact structure samples from a 1 76 kilometer deep corehole drilled near the center of the late eocene chesapeake bay impact structure virginia usa reveal its geologic hydrologic and biologic history we conducted stratigraphic and petrologic analyses of the cores to elucidate the timing and results of impact melt creation and distribution transient cavity collapse and ocean water resurge comparison of post impact sedimentary sequences inside and outside the structure indicates that compaction of the crater fill influenced long term sedimentation patterns in the mid atlantic region salty connate water of the target remains in the crater fill today where it poses a potential threat to the regional groundwater resource observed depth variations in microbial abundance indicate a complex history of impact related thermal sterilization and habitat modification and subsequent post impact repopulation

cenozoic basalts in se china chalcophile element geochemistry sulfide saturation history and source heterogeneity abstract cenozoic basalts in se china may be derived from a mixture of depleted morb mantle dmm and enriched mantle 2 em2 sources but whether these basalts share a common mantle source or magmatic history remains unknown to investigate these unresolved issues this study sampled basalts from niutoushan and mingxi fujian province xilong zhejiang province and penghu taiwan for geochemical analysis the basalt samples show oib like trace element patterns and have low pge contents with 0 02 0 7 ppb ir and pd 0 05 1 4 ppb ru 0 01 0 2 ppb rh and 0 06 1 1 ppb pt all samples have high cu pd ratios ranging from 69 000 to 3 500 000 and low cu zr ratios ranging from 0 1 to 0 8 suggesting sulfur saturated fractionation model calculations indicate that the basalts are depleted in pge due to the retention of 0 001 to 0 1 sulfide in the mantle and the removal of up to 0 0022 sulfide during magma ascent the crystallization of olivine and spinel and partial melting are insufficient to account for the observed pge variation in these basalts thus the distinct pge patterns in basalts with different ages may reflect the heterogeneity of the mantle source beneath se china the source heterogeneity may be due to compositional heterogeneity particularly variations in oxygen fugacity and pge mineral phases or due to variable fluid melt metasomatic agents in the sub continental lithospheric mantle this heterogeneity is possibly related to the westward subduction of the paleo pacific plate

geology of the giant sudbury polymetallic mining camp ontario canada a new 1 100 000 scale compilation bedrock geology map provided with this issue presents a geologic synthesis of canada s most prolific mining camp the world class sudbury structure with total past production and current reserves of over 1 7 billion tonnes of ni cu co pt pd au ag ore lydon 2007 the polymetallic ore is hosted within one of earth s largest preserved impact craters the new map of the ni cu pge sudbury district after ames et al 2005 is included in a pocket at the back of this issue map 1 and is linked to mineral deposit commodity and geochronology data in tables 1 and 2 of this paper view this table table 1 list of mineral deposits and occurrences of the sudbury map area map 1 view this table table 2 u pb isotope data across the map area since 1891 six compilation maps of the geology of sudbury have been published bell 1891 collins 1937 cooke 1946 card 1969 dressler 1984 ames et al 2005 fig 1 some of the ni cu pge mines have operated for over a century whereas new ore deposits discovered as recently as 2004 are already in production or are in the process of being developed i e advanced prospects in the last 15 years sudbury has been the training ground for 60 postgraduate geoscience students in north america fig 1 from 1903 to 1990 only 23 graduate theses were completed on sudbury related topics eight of them in the 1970s after the discovery of shatter cones dietz 1964 fig 1 number of graduate theses of north american geoscientists trained on sudbury geology per decade showing the minor peak in training after the discovery of shatter cones and the recent education boom with over 50 geoscience graduate theses completed since 1990 the dashed lines represent the regional geologic maps of the sudbury area

the evolution of the onaping formation at the sudbury impact structure the 1 4 1 6 km thick onaping formation consists of a complex series of breccias and melt bodies lying above the sudbury igneous complex sic at the sudbury impact structure based on the presence of shocked lithic clasts and various glassy phases the onaping has been described as a suevitic breccia with an origin at least in part as fallback material recent mapping and a redefined stratigraphy have emphasized similarities and differences in its various vitric phases both as clast types and discrete intrusive bodies the nature of the onaping and that of other suevitic breccias overlying impact melt sheets is reviewed the relative thickness internal stratigraphic and lithological character and the relative chronology of depositional units indicate multiple processes were involved over some time in the formation of the onaping the sudbury structure formed in a foreland basin and water played an essential role in the evolution of the onaping as indicated by a major hydrothermal system generated during its formation taken together observations and interpretations of the onaping suggest a working hypothesis for the origin of the onaping that includes not only impact but also the interaction of sea water with the impact melt resulting in repeated explosive interactions involving proto sic materials and mixing with pre existing lithologies this is complicated by additional brecciation events due to the intrusion of proto sic materials into the evolving and thickening onaping fragmentation mechanisms changed as the system evolved and involved vesiculation in the formation of the upper two thirds of the onaping

origin of the sudbury complex by meteoritic impact neodymium isotopic evidence samarium neodymium isotopic data on whole rocks and minerals of the sudbury complex in canada gave an igneous crystallization age of 1840 21 x 10 6 years the initial epsilon neodymium values for 15 whole rocks are similar to those for average upper continental crust falling on the crustal trend of neodymium isotopic evolution as defined by shales the rare earth element concentration patterns of sudbury rocks are also similar to upper crustal averages these data suggest that the sudbury complex formed from melts generated in the upper crust and are consistent with a meteoritic impact

tectonic evaluation of alpha regio highland and environs venus abstract systematic geologic mapping of an area on venus provides geologic unit and morphologic information about the area that was evaluated to provide a simple geologic history in the study area a large complexly deformed highland alpha regio was found to be among the oldest material units but its relation to other nearby complexly deformed units was indeterminable to better delineate the complex geologic and tectonic history of the area the authors expanded on the initial geologic map assessment by including tectonic unit mapping of the alpha regio highland and geophysical modeling of the highland to assess the possible effect of its formation on surrounding geologic units tectonic unit mapping identified five discrete periods of tectonic activity involved in the formation of the highland additionally the tectonic assessment showed that late stage extensional features within the highland were related to a regional tectonic event that occurred after the formation of the highland geophysical models of highland formation by both upwelling and downwelling were assessed and a downwelling model was applied to the alpha regio highland to calculate stresses related to its formation results of the modeling suggest that tectonic features mapped outside of the highland were not caused by stresses induced by the formation of alpha regio

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

fluid inclusions planar elements and pseudotachylites in the basement rocks of the vredefort structure south africa abstract the basement rocks of the vredefort structure are characterized by high temperature static metamorphism in the course of which a violent shock event occurred quartz in these rocks contains numerous fluid inclusions that consist of almost pure co 2 commonly such fluid inclusions are aligned along planar elements in quartz that were formed during the shock event crystallographic orientations of the planar elements were used to estimate minimum shock pressures of 75 100 30 kbar the fact that fluid inclusions are situated on planar elements indicates that the co 2 gas must have been available immediately after the shock event measurements of the homogenization temperatures of the fluid inclusions in samples from the vredefort basement allow subdivision of the co 2 inclusions into three groups the main group type 1 representing nearly 99 of the co 2 inclusions homogenizes to liquid in the range 26 30 c and therefore has densities of between 0 6 and 0 7 g cm 3 inclusions of this most important group are generally situated on planar elements high density type 2 and low density type 3 inclusions occur isolated in coarse unshocked quartz grains without planar elements in post shock recrystallized quartz grains and in mafic minerals apparently unaffected by the shock event type 2 inclusions with densities of about 0 9 g cm 3 are believed to belong to an early generation which was formed during the granulite facies metamorphism predating the static high temperature vredefort metamorphism low density inclusions of type 3 homogenizing to the gaseous state could be explained as a very late generation of inclusions formed under near surface conditions on the basis of the observation that type 1 inclusions are located on shock induced planar elements the time of entrapment must have been immediately after the shock event and a minimum pressure for co 2 between 2 and 5 kbar can be determined for this time within the entire basement based on independent temperature estimations the observations that a multitude of type 1 inclusions has been found in a minute millimetre sized pseudotachylite vein produced by the shock event is explained by recrystallization of diaplectic glass with this knowledge of microstructures in the vredefort basement rocks it is suggested that the ubiquity of co 2 and the shock event are related to each other

surface processes in the venus highlands results from analysis of magellan and arecibo data the highlands of venus are characterized by an altitude dependent change in radar backscatter and microwave emissivity likely produced by surface atmosphere weathering reactions we analyzed magellan and arecibo data for these regions to study the roughness of the surface lower radar backscatter areas at the highest elevations and possible causes for areas of anomalous behavior in maxwell montes arecibo data show that circular and linear radar polarization ratios rise with decreasing emissivity and increasing fresnel reflectivity supporting the hypothesis that surface scattering dominates the return from the highlands the maximum values of these polarization ratios are consistent with a significant component of multiple bounce scattering we calibrated the arecibo backscatter values using areas of overlap with magellan coverage and found that the echo at high incidence angles up to 70 from the highlands is lower than expected for a predominantly diffuse scattering regime this behavior may be due to geometric effects in multiple scattering from surface rocks but further modeling is required areas of lower radar backscatter above an upper critical elevation are found to be generally consistent across the equatorial highlands with the shift in microwave properties occurring over as little as 500 m of elevation these surfaces are not simply characterized however by the absence of a highly reflective component surface morphology and radar scattering properties suggest that a mantling deposit forms at the highest elevations most likely by in situ erosion of the original rock in ovda regio this process mantles or has removed surface festoon structure at the 1 to 10 m scale implying a significant depth for the weathered layer similar radar dark areas occur in maxwell montes but are apparently unrelated to the current topography of the region possible reasons for these observations include mass wasting from areas of steep slopes compositional or age differences within the montes vertical tectonic shifts of relict contacts local topographic effects on surface temperature or errors in the magellan topography data in the rugged terrain while there is evidence for some of these effects in the existing data no single model at present appears to satisfy all occurrences of high altitude radar dark terrain new measurements of the surface and lower atmosphere chemistry of venus are needed to further refine these conclusions

experimental reproduction of tectonic deformation lamellae in quartz and comparison to shock induced planar deformation features planar features can develop in quartz during comparatively slow tectonic deformation and during very fast dynamic shock metamorphism despite their very different structural nature tectonically induced deformation lamellae have sometimes been mistaken as shock induced planar deformation features pdfs to understand the formation of deformation lamellae and to address the substantial differences between them and pdfs we have conducted deformation experiments on single crystals of quartz in a griggs type apparatus at a temperature of 800 c a confining pressure of 12 kbar and a strain rate of 0 7 1 1 x 10 6 the deformed samples were analyzed with transmission electron microscopy tem and compared to natural pdfs from the ries crater germany tem revealed that tectonic deformation lamellae are associated with numerous sub parallel curved subgrain walls across which the orientation of the crystal changes slightly the formation of deformation lamellae is due to glide and climb controlled deformation in the exponential creep regime in contrast the pdfs in shocked quartz from the ries are perfectly planar crystallographically controlled features that originally represented amorphous lamellae due to postshock annealing and hydrothermal activity they are recrystallized and decorated with fluid inclusions

platinum group minerals in alpine chromitites from southwestern oregon electron microprobe and scanning electron microscopy the platinum group minerals found in the chromitites are laurite porous ru rich alloys os ir and pt rich alloys with highly variable compositions an ir cu rich sulfide a pt arsenide and three relatively rare platinum group sulfides the platinum group minerals are 1 to 30 mu m in diameter and occur as inclusions in chromite grains some of these inclusions also contain silicates and millerite base metal alloys largely ni fe occur in cracks in the chromitites along with magnetite ferritchromite chromian chlorite and serpentine the primary platinum group mineral assemblage is interpreted to have formed before or during crystallization of the chromite grains and not as a result of exsolution from the chromite modified journal abstract

empirical and theoretical comparisons of the chicxulub and sudbury impact structures chicxulub and sudbury are 2 of the largest impact structures on earth research at the buried but well preserved chicxulub crater in mexico has identified 6 concentric structural rings in an analysis of the preserved structural elements in the eroded and tectonically deformed sudbury structure in canada we identified ring like structures corresponding in both radius and nature to 5 out of the 6 rings at chicxulub at sudbury the inner topographic peak ring is missing which if it existed has been eroded reconstructions of the transient cavities for each crater produce the same range of possible diameters 80110 km the close correspondence of structural elements between chicxulub and sudbury suggests that these 2 impact structures are approximately the same size both having a main structural basin diameter of 150 km and outer ring diameters of 200 km and 260 km this similarity in size and structure allows us to combine information from the 2 structures to assess the production of shock melt melt produced directly upon decompression from high pressure impact and impact melt shock melt and melt derived from the digestion of entrained clasts and erosion of the crater wall in large impacts our empirical comparisons suggest that sudbury has 70 more impact melt than does chicxulub 31 000 versus 18 000 km3 and 85 more shock melt 27 000 km3 versus 14 500 km3 to examine possible causes for this difference we develop an empirical method for estimating the amount of shock melt at each crater and then model the formation of shock melt in both comet and asteroid impacts we use an analytical model that gives energy scaling of shock melt production in close agreement with more computationally intense numerical models the results demonstrate that the differences in melt volumes can be readily explained if chicxulub was an asteroid impact and sudbury was a comet impact the estimated 70 difference in melt volumes can be explained by crater size differences only if the extremes in the possible range of melt volumes and crater sizes are invoked preheating of the target rocks at sudbury by the penokean orogeny cannot explain the excess melt at sudbury the majority of which resides in the suevite the greater amount of suevite at sudbury compared to chicxulub may be due to the dispersal of shock melt by cometary volatiles at sudbury

vitric compositions in the onaping formation and their relationship to the sudbury igneous complex sudbury structure the 1 85 ga sudbury structure is the largest well preserved exposed example of a terrestrial impact crater it contains world class ni cu platinum group element pge ores associated with the igneous component a new understanding of the geology and geochemistry of the vitric crater fill sequence the onaping formation provides constraints on the evolution of 1 85 ga igneous rocks in the sudbury structure the onaping formation has a well defined mappable stratigraphy composed of three members seven stratigraphic units and two types of intraformational dikes that are overprinted by an impact generated hydrothermal system stratigraphic and alteration mapping provided a basis for well constrained sampling and allowed the definition of the geochemical characteristics of the less altered rocks from each unit including vitric bombs and blocks and intraformational vitric dikes significantly the identification of a least altered composition of the vitric onaping formation enabled an investigation of the link between the early formed vitric melt phase in the impact crater fill sequence and the sudbury igneous complex and offset dikes the least altered vitric onaping formation represented by the massive xenolith poor cores of vitric bombs blocks and intraformational dikes is andesitic in composition consistent with a crustally derived melt the melt composition 61 6 wt sio 2 4 28 mgo 60 ppm ni is remarkably uniform throughout the onaping formation with a typical enrichment of large ion lithophile and light rare earth elements and pronounced negative nb and ti anomalies incompatible trace element ratios and bulk concentrations of the onaping formation coincide with the main mass norite and offset quartz diorite compositions of the underlying sudbury igneous complex but differ distinctly from the granophyre and quartz gabbro root mean squared differences in composition of the vitric onaping formation with the units of the sudbury igneous complex and quartz diorite offset dikes show that the vitric onaping formation does not directly match any of these igneous units but more closely resembles the composition of the ministic and manchester offset dikes the quartz diorite offset dikes have been proposed as representative of the bulk composition of the original melt of the sudbury igneous complex in detail there are small compositional differences between north and south range offset dikes and between these offset dikes and the concentric manchester offset dike however such differences are lacking in the onaping formation we propose that the least altered vitric composition of the onaping formation represents the best estimate of the bulk composition of an initial quenched melt or shock melt formed early in the evolution of the sudbury structure and is a logical starting liquid to model the evolution of the sudbury main mass and offset dikes the offset dike compositions can be derived from onaping formation vitric shock melt by assimilation of progressively greater amounts of mafic material at deeper levels in the crater this mafic contaminated melt or impact melt developed at the base of the melt sheet and was injected into the offset dikes both prior to and contemporaneously with the earliest stages of sulfide precipitation from the sudbury complex fractionation of the mafic contaminated melt by plagioclase and pyroxene accumulation produced the north range felsic and mafic norite and the south range norites the chalcophile metal content of the onaping formation vitric phase closely reflects the overall metal ratios of the magmatic sulfide ores the vitric onaping formation and unmineralized offset dikes have low liquidus temperatures 1 115 1 125 c and if these units represent the source of metals in the sulfide deposits unusually high mass distribution coefficients for cu and ni and high r factors are necessary to account for the metal endowment

observations and interpretations at vredefort sudbury and chicxulub towards an empirical model of terrestrial impact basin formation the structural topographic and other characteristics of the vredefort sudbury and chicxulub impact structures are described assuming that the structures originally had the same morphology the observations interpretations for each structure are compared and extended to the other structures this does not result in any major inconsistencies but requires that the observations be scaled spatially in the case of vredefort and sudbury this is accomplished by scaling the outer limit of particular shock metamorphic features in the case of chicxulub scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring the observations interpretations are then used to construct an integrated empirical kinematic model for a terrestrial peak ring basin the major attributes of the model include a set of outward directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor some of which are pre existing structures that have been reactivated during transient cavity formation inward directed motions along the same outermost structures and along a set of structures at intermediate radial distances during transient cavity collapse structural uplift in the center followed by a final set of radially outward directed thrusts at the outer edges of the structural uplift during uplift collapse the rock displacements on the intermediate inward and innermost outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift

radar investigation of mars mercury and titan radar astronomy is the study of the surfaces and near surfaces of solar system objects using active transmission of modulated radio waves and the detection of the reflected energy the scientific goals of such experiments are surprisingly broad and include the study of surface slopes fault lines craters mountain ranges and other morphological structures electrical reflectivities contain information about surface densities and to some extent the chemical composition of the surface layers radar probes the subsurface layers to depths of the order of 10 wavelengths providing geological mapping and determinations of the object s spin state radar also allows one to study an object s atmosphere and ionic layers as well as those of the interplanetary medium precise measurements of the time delay to surface elements provide topographic maps and powerful information on planetary motions and tests of gravitational theories such as general relativity in this paper we limit our discussion to surface and near surface probing of mercury mars and titan and review the work of the past decade which includes fundamentally new techniques for earth based imaging the most primitive experiments involve just the measurement of the total echo power from the object the most sophisticated experiments would produce spatially resolved maps of the reflected power in all four stokes parameters historically the first experiments produced echoes from the moon during the period shortly after world war ii see e g evans 1962 but the subject did not really develop until the early 1960s when the radio equipment was sufficiently sensitive to detect echoes from venus and obtain the first doppler strip maps of that planet the first successful planetary radar systems were the continuous wave cw radar at the goldstone facility of the caltech s jet propulsion laboratory and the pulse radar at the mit lincoln laboratory all of the terrestrial planets were successfully studied during the following decade yielding the spin states of venus and mercury a precise value of the astronomical unit and a host of totally new discoveries concerning the surfaces of the terrestrial planets and the moon this work opened up at least a similar number of new questions although the early work was done at resolution scales on the order of the planetary radii very rapid increases in system sensitivities improved the resolution to the order of 100 km but always with map ambiguities recently unambiguous resolution of 100 m over nearly the entire surface of venus has been achieved from the magellan spacecraft using a side looking synthetic aperture radar reviews of the work up to the magellan era can be found in evans 1962 muhleman et al 1965 evans hagfors 1968 see chapters written by g pettengill t hagfors and j evans and ostro 1993 the radar study of venus from the magellan spacecraft was a tour de force and is well described in special issues of science volume 252 april 12 1991 and in the journal of geophysical research volume 97 august 25 and october 25 1992 venus will not be considered in this paper even though important polarization work on that planet continues at arecibo goldstone and the very large array vla in this paper we review the most recent work in earth based radar astronomy using new techniques of earth rotation super synthesis at the vla in new mexico operated by the national radio astronomy observatory and the recently developed long code techniques at the arecibo observatory in puerto rico operated by cornell university note it was recently brought to our attention that the vla software doubles the flux density of their primary calibrators consequently it is necessary to half the radar power and reflectivity numerical values in all of our published radar results from the vla goldstone radar the symbiotic relationship in these new developments for recent advances in our understanding of mercury and mars is remarkable vla imaging provides for the first time unambiguous images of an entire hemisphere of a planet and the long code technique makes it possible to map mars and mercury using the traditional range gated doppler strip mapping procedure which was apparently developed theoretically at the lincoln laboratory by paul green based on a citation in evans 1962 richard goldstein was the first to obtain range gated planetary maps of venus as reported in carpenter goldstein 1963 such a system was developed earlier for the moon as reported by pettengill 1960 and pettengill henry 1962 we first discuss the synthesis mapping technique

non chondritic platinum group element ratios in the earth s mantle the earth s upper mantle has an overabundance of highly siderophile elements platinum group elements gold and rhenium relative to what would be expected from equilibrium with the metallic core1 3 this excess is now widely believed to have been introduced as a late veneer by meteorite bombardment during early earth history but after separation of the core4 here we report high precision analyses of platinum group elements pges from fertile upper mantle iherzolites which show variations in relative abundances that exceed those in the chondritic meteorites that are thought to have furnished the late veneer5 in particular the pd ir ratio at about 1 76 is significantly greater than that of known chondritic meteorites6 our results combined with previous indications of both non chondritic and nearly chondritic pge ratios in the few other fertile mantle iherzolites that have been analysed2 3 7 11 strongly suggest that the mantle is heterogeneous in its pge content on scales of 100 kilometres if the observed pge ratios can be reconciled with a plausible meteoritic source the observed heterogeneity may reflect an original spatial variation in the late veneer otherwise an additional fractionation mechanism seems to be required

searching for giant ancient impact structures on earth the mesoarchaean maniitsoq structure west greenland a 100km scale circular region in the archaean north atlantic craton centred at 651150n 511500w near maniitsoq town in west greenland comprises a set of highly unusual geological features that were created during a single event involving intense crushing and heating and are incompatible with crustal orogenic processes th presently exposed features of the maniitsoq structure were buried 20 25 km below the surface when this event occurred at c 3ga during waning convergent orogeny these features include a large aeromagnetic anomaly a central 35x50 km2 large area of comminuted quartzo feldspathic material regional scale circular deformation widespread random fractures with feather like textures intense fracture cleavage amphibolite granite matrix breccias unrelated to faulting or intrusions formation and common fluidisation of microbreccias abundant evidence of direct k feldspar and plagioclase melting superimposed on already migmatised rocks deformation of quartz by oc4 slip formation of planar elements in quartz and plagioclase and emplacement of crustally contaminated ultramafic intrusions and regional scale hydrothermal alteration under amphibolite facies conditions the diagnostic tools employed to identify impacting in the upper crust are inadequate for structures preserved deep within the continental crust nevertheless the inferred scale strain rates and temperatures necessary to create the maniitsoq structure rule out a terrestrial origin of the 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 tookoonooka marine impact horizon australia sedimentary and petrologic evidence abstract ejecta from the large subsurface tookoonooka impact structure have been foundin the lower cretaceous strata of the extensive eromanga basin of centralaustralia observations from 31 wells spanning 400 000 km 2 of the basin provide compellingevidence for the presence of a marine impact horizon of regional extent drill core wasexamined to determine the sedimentary context of the tookoonooka impact event thepresence of ejecta and the nature of the impact horizon the base of the wyandra sandstonemember of the cadna owie formation is an unconformity commonly overlain by very poorlysorted sediment with imbricated pebbles exotic clasts and occasional boulders the basalwyandra sandstone member is bimodal a ne sand mode re ects an ambient sedimentcontribution and a coarse mode is interpreted to be impact derived wells thargomindah 1and eromanga 1 within four crater radii of tookoonooka contain distinctive clast supported breccia conglomerate beds at the base of the wyandra sandstone member clastsin these beds include altered accretionary and melt impactoclasts as well as lithic and mineralgrains corresponding to the tookoonooka target rock sequence including basement petrographic evidence includes shock metamorphosed quartz and lithic grains with planardeformation features these breccia conglomerates are in stark contrast to the underlying laterally persistent unimodal cadna owie sediments and overlying shales deposited in anepeiric sea the base of the wyandra sandstone member is therefore interpreted to be thetookoonooka impact horizon the timing of the impact event is con rmed to be thebarremian aptian boundary at 125 1 ma the wyandra sandstone member preservesboth impact ejecta and postimpact marine sediments introductiontookoonooka and talundilly are two subsurfaceanomalies in the eromanga basin of central australiathat were recognized by petroleum exploration in the1980s fig 1 tookoonooka and talundilly are some300 km apart buried under almost 1 km of at lyingsediments near the center of the basin in southwestqueensland the impact origin of subsurface structuresis notoriously dif cult to investigate due to inaccessibilityand paucity of data tookoonooka was con rmed as animpact structure through the identi cation andmeasurement of shock metamorphic features in quartzgrains gorter et al 1989 gostin and therriault 1997 from the structure s buried central uplift however talundilly has not been adequately drilled to date andremains a possible impact structure tookoonooka is the second largest known impactstructure in australia and possibly the tenth largest inthe world earth impact database 2011 it wasestimated to have a nal crater diameter of 66 km gostin and therriault 1997 which is a recalculation ofan earlier estimate of 55 km from seismic data gorteret al 1989 talundilly was estimated to have a 95 kmdiameter from seismic data longley 1989 a talundillycrater diameter cited by bevan 1996 and gorter 1998 of 30 km while it may be more accurate according tomore recent crater scaling techniques cf turtle et al 1

planar deformation features in shocked quartz a transmission electron microscopy investigation abstract we present a detailed investigation by transmission electron microscopy tem of the fine structure of planar deformation features pdf in quartz grains within which shock metamorphism has been detected optically and which originate from various sites slate islands la malbaie and manicouagan lake canada ries crater germany the toba caldera sumatra the vredefort complex in south africa and the k t boundary at raton basin colorado pdfs appear on tem micrographs as straight and very narrow tem reveals many more pdfs than are optically detected probably because only concentrations of such defects in narrow areas can induce an optically detectable contrast caution should therefore be exercised when applying criteria based on the optical observation of single or multiple sets of pdfs pdfs in samples from slate islands and raton basin exhibit a peculiar morphology a high density of very tiny bubbles precipitated on the dislocation lines comparison with the behavior of wet quartz strongly suggests that these materials experienced lengthy annealing at moderate pressure and temperature

sulfur saturation and second stage melts application to the bushveld platinum metal deposits a model is presented for the formation of platinum group element ores by precipitation from sulfur deficient platinum group element rich second stage magmas numerous investigations indicate that basaltic magmas originating from undepleted or mildly depleted source regions are sulfur saturated at the time of eruption mantle derived peridotites residual from midocean ridge basalt magma genesis have pd abundances that are uniformly too high for it to have originated solely from silicate melt retained in the source region at the time of segregation they imply the presence of an accessory immiscible sulfide melt to host the platinum group elements the very high sulfide melt silicate melt partition coefficients of the platinum group elements would result in strong fractionation of these metals into the accessory sulfide component during the melting event the extreme platinum group element enrichment of this accessory sulfide component is evidenced by the discovery ofplatinoid minerals in residual mantle nodules containing pd and pt abundances at the low ppb level keays et al 1981 magmas generated by low to moderate degrees of partial melting of undepleted mantle are therefore sulfur saturated at the time of segregation these first stage magmas become impoverished in platinum group elements during the early stages of fractional crystallization because of coprecipitation of an immiscible sulfide phase e g midocean ridge basalt glasses average

geophysical anomalies and quartz deformation of the warburton west structure central australia abstract this paper reports geophysical anomalies and intra crystalline quartz lamellae in drill cores from the warburton west basin overlapping the border of south australia and the northern territory the pre upper carboniferous 450 300 km large warburton basin north eastern south australia is marked by distinct eastern and western magnetic gravity and low velocity seismic tomography anomalies quartz grains from arenite core samples contain intra crystalline lamellae in carbonate quartz veins and in clastic grains similar to those reported earlier from arenites volcanic rocks and granites from the warburton east basin universal stage measurements of quartz lamellae in both sub basins define miller bravais indices of 10 12 and 10 13 in situ quartz lamellae occur only in pre late carboniferous rocks whereas lamellae bearing clastic quartz grains occur in both pre late carboniferous and post late carboniferous rocks the latter likely redeposited from the pre late carboniferous basement quartz lamellae in clastic quartz grains are mostly curved and bent either due to tectonic deformation or to re deformation of impact generated planar features during crustal rebound or and post impact tectonic deformation seismic tomography low velocity anomalies in both warburton west basin and warburton east basin suggest fracturing of the crust to depths of more than 20 km geophysical modelling of the cooper basin which overlies the eastern warburton east basin suggests existence of a body of high density 2 9 3 0 gr cm 3 and high magnetic susceptibility si 0 012 0 037 at a depth of 6 10 km at the centre of the anomalies in the warburton west basin a large magnetic body of si 0 030 is modelled below 10 km with a large positive gravity anomaly offset to the north of the magnetic anomaly in both the warburton east and warburton west the deep crustal fracturing suggested by the low velocity seismic tomography complicates interpretations of the gravity data universal stage measurements of quartz lamellae suggest presence of both planar deformation features of shock metamorphic derivation and deformed planar lamella the latter may be attributed either to re deformation of impact generated lamella impact rebound deformation or and post impact tectonic deformation the magnetic anomalies in the warburton east and west sub basins are interpreted in terms of 1 presence of deep seated central mafic bodies 2 deep crustal fracturing and 3 removal of devonian and carboniferous strata associated with rebound of a central uplift consequent on large asteroid impact further tests of the warburton structures require deep crustal seismic transects

venus global surface radar reflectivity observations of the surface of venus carried out by the pioneer venus radar mapper at a wavelength of 17 centimeters reveal a global mean reflectivity at normal incidence of 0 13 0 03 over the surface variations from a low of 0 03 0 01 to a high of 0 4 0 1 are found with theia mons previously identified as possibly volcanic showing a value of 0 28 0 07 regions of high reflectivity may consist of rocks with substantial inclusions of highly conductive sulfides

the unique radar properties of silicic lava domes 1 silicic lava domes exhibit distinct morphologic characteristics at scales of centimeters to kilometers multiparameter radar observations capture the unique geometric signatures of silicic domes in a set of radar scattering properties that are unlike any other natural geologic surfaces backscatter cross section values are among the highest observed on terrestrial lava flows and show only a weak decrease with incidence angle cross polarization backscatter hv shows a unique behavior increasing with increasing wavelength circular polarization ratios are relatively high in the 0 3 0 95 range and increase with increasing wavelength field measurements of boulder size frequency distributions and microtopography indicate that silicic dome surfaces are among the roughest ever measured rms heights at a 1 m lateral scale range from 13 cm to 50 cm rms slopes at 1 m spacing range from 12 to 43 degrees modeling of the scattering behavior suggests it results from a combination of rough surface facet scattering and scattering from block edges that act as a random collection of dipoles the unusual wavelength dependence of the radar parameters appears to result from a higher component of edge scattering at large wavelengths producing for example higher cross polarized backscatter at p band 68 cm steep sided volcanic domes on venus superficially resemble terrestrial silicic domes in plan view gross morphology but few similarities remain when the radar scattering and three dimensional shapes of the features are compared the unique radar scattering properties suggest that such volcanic surfaces can be identified with multiparameter radar observations in future planetary radar missions and in active terrestrial volcanoes where dome development can represent serious hazards

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

chemical equilibration of the earth s core and upper mantle the oxygen fugacity fo2 of the earth s upper mantle appears to lie somewhat above that of the iron wustite buffer its fo2 is assumed to have been similar to the present value at the time of core formation in the upper mantle the fe rich liquid protocore that would form under such conditions of fo2 at elevated temperatures would lie predominantly in the system fe s o distribution coefficients for co cu ni ir au ir w re mo ag and ga between such liquids and basalt are known and minimum values are known for ge from these coefficients upper mantle abundances for the above elements can be calculated by assuming cosmic abundances for the whole earth and equilibrium between the fe s o protocore and upper mantle these calculated abundances are surprisingly close to presently known upper mantle abundances agreements are within a factor of 5 except for cu w and mo therefore siderophile element abundances in the upper mantle based on known distribution coefficients do not demand a late stage meteoritic bombardment and a protocore formed from the upper mantle containing s and o seems likely as upper mantle abundances fit a local equilibrium model then either the upper mantle has not been mixed with the rest of the mantle since core formation or else partition coefficients between protocore and mantle were similar for the whole mantle regardless of p t and fo2 the latter possibility seems unlikely over such a p t range

impact crater related surficial deposits on venus multipolarization radar observations with arecibo 1 the distribution of surficial deposits in the vicinity of impact craters on venus was studied using measurements of the polarization properties of the reflected radar wave subsurface scattering of an incident circularly polarized radar signal results in a linearly polarized component in the radar echo due to the differing transmission coefficients at a smooth at wavelength scales atmosphere surface boundary for the horizontal h and vertical v linearly polarized components of the incident wave arecibo 12 6 cm wavelength radar observations in 1999 and 2001 provided images of the surface of venus in the full stokes polarization parameters of the reflected echo from which images in the degree of linear polarization were derived these images show that substantial areas of sedna guinevere and lavinia planitias return a radar echo with a significant degree of linear polarization indicating that mantling deposits may be relatively widespread on the plains of venus the areas showing linear polarization enhancements are strongly correlated with topographic features primarily impact craters dome fields and windblown deposits including dune fields a strong linearly polarized echo component 10 40 linear polarization is found from regions near 45 impact craters including 5 parabolas these linear polarization features typically correspond to diffuse higher backscatter cross section bright regions in magellan images the linearly polarized component in these regions is attributed to subsurface echoes from a mantled substrate or from buried rocks

testing for fullerenes in geologic materials oklo carbonaceous substances karelian shungites sudbury black tuff fullerenes have been reported from diverse geologic environments since their discovery in shungite from karelian russia our investigation is prompted by the presence of onionskin like structures in some carbonaceous substances associated with the fossil nuclear fission reactors of oklo gabon the same series of extractions and the same instrumental techniques laser desorption ionization and high resolution mass spectroscopy electron impact mass spectroscopy were employed to test for fullerenes in samples from three different localities two sites containing putative fullerenes sudbury basin and russian karelia and one new location oklo gabon we confirm the presence of fullerenes c 6 0 and c 7 0 in the black tuff of the onaping formation impact breccia in the sudbury basin but we find no evidence of fullerenes in shungite samples from various locations in russian karelia analysis of carbonaceous substances associated with the natural nuclear fission reactors of oklo yields no definitive signals for fullerenes if fullerenes were produced during sustained nuclear fission at oklo then they are present below the detection limit 100 fmol or they have destabilized since formation contrary to some expectations geologic occurrences of fullerenes are not commonplace

recrystallized impact glasses of the onaping formation and the sudbury igneous complex sudbury structure ontario canada abstract the origin of the sudbury structure and of the associated heterolithic breccias of the onaping formation and the sudbury igneous complex have been controversial while an impact origin of the structure has gained wide acceptance over the last 15 years the origin of the recrystallized onaping formation glasses and of the igneous complex is still being debated recently the interpretation of the breccias of the onaping formation as suevitic fall back impact breccias has been challenged the igneous complex is interpreted either as a differentiated impact melt sheet or as a combination of an upper impact melt represented by the granophyre and a lower impact triggered magmatic body consisting of the norite sublayer formations the onaping formation contains glasses as fluidal and nonfluidal fragments of various shapes and sizes they are recrystallized and our research indicates that they are petrographically heterogeneous and span a wide range of chemical compositions these characteristics are not known from glasses of volcanic deposits this suggests an origin by shock vitrification an interpretation consistent with their association with numerous and varied country rock clasts that exhibit microscopic shock metamorphic features the recrystallized glass fragments represent individual solid state and liquid state vitrified rocks or relatively small melt pods the basal member lies beneath the gray and black members of the onaping formation and where not metamorphic has an igneous matrix igneous textured melt bodies occur in the upper two members and above the basal member a comparison of the chemical compositions of recrystallized glasses and of the matrices of the basal member and the melt bodies with the components and the bulk composition of the igneous complex is inconclusive as to the origin of the igneous complex basal member matrix and melt bodies on average are chemically similar to the granophyre of the sudbury igneous complex suggesting that they are genetically related our chemical results allow interpretation of the entire igneous complex as a differentiated impact melt however they are also consistent with the granophyre alone being the impact melt and the norite and quartz gabbro beneath it representing an impact triggered magmatic body this interpretation is preferred as it is consistent with a number of field observations a re evaluation and extension of structural field studies and of geochemical data as well as a systematic study of the contact relationships of the various igneous phases of the igneous complex are needed to establish a sudbury impact model consistent with all data and observations

partitioning of palladium iridium platinum and gold between sulfide liquid and basalt melt at 1200 c abstract iron nickel monosulfide and basalt glass containing trace amounts of pge equilibrated at 1200 c and fo 2 10 9 2 close to the wustite magnetite buffer and fs 2 10 0 9 have been analyzed for noble metals by radiochemical and instrumental neutron activation analysis the average contents of pge in coexisting fe ni sulfide and basalt glass respectively are pd 50 ppm and 0 5 ppb ir 50 ppm and 0 5 ppb pt 100 ppm and 10 ppb and au 0 7 ppm and 0 8 ppb the sulfide liquid silicate melt partition coefficients d values for the noble metals are 9 7 10 4 for pd 1 0 7 10 5 for ir 9 6 10 3 for pt and 1 0 9 10 3 for au the noble metals are strongly partitioned into sulfide liquid but the affinity of pd and ir for sulfide liquid is about 50 times greater than that of pt and about 500 times greater than that of au the d values indicate that equilibrium partitioning between immiscible sulfide liquid and basalt magma would result in fractionation of the noble metals which differs significantly from that generally observed in nature

the sudbury igneous complex a differentiated impact melt sheet the sudbury structure ontario is the remnant of a 1 85 ga old impact crater which originally had a diameter of 200 to 250 km the sudbury igneous complex occurs within the sudbury structure the sudbury igneous complex is a 2 5 to 3 0 km thick 60 27 km elliptical igneous rock body which consists of four major lithologies from top to bottom traditionally termed granophyre quartz gabbro norite and contact sublayer sulfide and inclusion bearing noritic rock with the exception of the latter all these lithologies are continuous across the structure modal analyses reveal that following the iugs system of nomenclature quartz gabbro samples are in fact quartz monzogabbros a few of the norite samples are quartz gabbros and most norite samples are quartz monzogabbros in view of these observations and in order to clarify the nomenclature an updated terminology is proposed from top to bottom upper unit middle unit lower unit and contact sublayer the bulk composition of the sudbury igneous complex from north range data is granodioritic continuous and gradational mineralogical and geochemical variations between the lithological units are evidence that the complex behaved as a single melt system all the sudbury igneous complex lithologies have the same light to heavy rare earth element ree ratio and an overall pattern of increased light ree and depleted heavy ree the occurrence of primary hydrous minerals hornblende and biotite deuteric alteration and abundant micrographic and granophyric intergrowths demonstrate that the melt was rich in h2o moreover the granophyric and other far from equilibrium textures are most likely due to rapid crystallization triggered by exsolution of a volatile phase the sudbury igneous complex differs from traditional layered mafic complexes in the following aspects it has an overall intermediate composition a hydrous nature a crustal isotopic signature normative corundum and an unusually large volume of granophyre the sudbury complex differs from known terrestrial impact melt sheets only by its great thickness and the presence of chemical and therefore mineralogical layering reported here for the first time and similar to those found in impact melt rocks elsewhere are the occurrences of plagioclase xenocrysts with complex twinning and zoning patterns and planar deformation features in quartz xenocrysts the well known ore deposits of the sudbury region are directly related to the genesis of the sudbury igneous complex some ores precipitated from the sudbury melt whereas others were concentrated by hydrothermal fluids that percolated through the crystallized complex it is concluded that the sudbury igneous complex is the best exposed and only well documented to date terrestrial impact melt sheet to have differentiated

mineral equilibria and the high radar reflectivity of venus mountaintops we studied the relationship between altitude and microwave emissivity the complement of power reflectivity in 10 highland regions of venus using the magellan data set above a critical altitude that ranges from 4 75 km on maxwell montes to 2 49 km on sapas mons emissivity undergoes an abrupt decrease to values so low 0 6 that the surface mineralogy on these mountaintops must differ from that at lower altitudes in such a way as to enhance the bulk dielectric constant of the highland surface material this emissivity effect probably is produced not by basalt minerals but by a secondary weathered mineral assemblage we developed the phase diagrams descriptive of equilibrium secondary mineral assemblages on venus as a function of altitude and atmospheric redox state the mineral responsible for low emissivity on mountaintops appears to be the electrical semiconductor pyrite fes2 variation from one highland area to another in the altitude at which pyrite becomes the stable fe mineral as opposed to magnetite at lower altitudes may be attributable to local modulation by topography of the atmospheric thermal profile the altitudes at which the magnetite pyrite phase boundary is encountered vary with the redox slate of the atmosphere and observed values of this altitude can be used to estimate an oxygen fugacity at plains level of 10 21 bars maat mons a volcanic peak almost as lofty as maxwell montes is the only high mountaintop on venus that has not weathered to a low emissivity mineral assemblage presumably this means flows at the highest altitudes are negligibly weathered and the volcano is relatively young

reflection and emission properties on venus alpha regio the retrieval of geophysical surface properties from radar measurements is relatively straightforward when dealing with near normal incidence angles and physical surfaces which are relatively smooth procedures developed from hagfors 1964 scattering model have been used with success to extract various properties reflectivity emissivity slope etc from both pioneer venus pettengill et al 1988 and magellan tyler et al 1991 pettengill et al 1991 data however under certain circumstances such as physically rough areas or areas of nonhomogeneous composition hagfors model may lead to serious errors in the interpretation of radar and radiometry data we examine the magellan derived surface properties for some areas of below average emissivity in alpha regio and in an area to the east of alpha crater stuart in order to determine if hagfors model is applicable we also apply methods of classical multiple scattering theory to show enhanced radar backscattering and depressed thermal emission can be expected with a matrix of silicic or mafic rocks embedded in low loss soils

structural outer rim of chesapeake bay impact crater seismic and bore hole evidence nine seismic reflection profiles and four continuous core holes define the gross structural and stratigraphic framework of the outer rim of the chesapeake bay impact crater the rim is manifested as a 90 km diameter ring of terraced normal fault blocks which forms a 320 m 1200 m high rim escarpment the top of the rim escarpment is covered by a 20 m 30 m thick ejecta blanket the escarpment encircles a flat floored annular trough which is partly filled with an 250 m thick breccia lens exmore breccia the exmore breccia overlies a 200 m 800 m thick interval of slumped sedimentary megablocks which in turn rests on crystalline basement rocks all postimpact strata upper eocene to quaternary sag structurally into the annular trough and most units also thicken as they cross the rim into the crater postimpact compaction and subsidence of the exmore breccia have created extensive normal faulting in overlying strata

siderophile element abundances in the upper mantle evidence for a sulfide signature and equilibrium with the core abstract measurements of the intrinsic oxygen fugacities of samples from the earth s upper mantle have shown that the characteristic oxidation state ranges from values close to fe ni metal saturation to more oxidized values tectonic processes can account for the progressive and variable oxidation of a mantle that was originally in equilibrium with metal so that the oxygen fugacity data give strong support to the possibility of protocore protomantle equilibrium the data for apparently overabundant siderophile elements in the mantle are reviewed and 2 groups group a ge au pd pt ru rh ir re os group b sn sb ag ga cu as co ni w mo are recognized group a abundances result from an overprinting by a late accreting chondritic component of a mantle that had suffered major extraction of the indigenous complement by core forming metal chou 1978 group b however are 10 more abundant than group a and may be distinguished as a group by their greater chalcophile tendencies this is highlighted for example by the large cl normalized sn ge ratio of the mantle which is unexpected as a result of metal silicate fractionation alone but can be accounted for if some sn together with sulfur was retained in the mantle during core separation industrial experience in the equilibrium of molten fe and silicate slags shows that sulfur is not overwhelmingly partitioned into metal so that the concept of partial 20 of original accreting component retention of sulfur and group b elements in the mantle has experimental and theoretical support following core separation progressive loss of sulfide from the mantle is indicated by the anomalously high ni at a given mg number in archaean basalts and siliceous mg poor sediments it is also a feature of oceanic and continental basalts that some u pb fractionation must have occurred in the mantle source regions following core separation and this is most satisfactorily achieved if pb is partially removed from u in a sulfide phase

planar deformation features and impact glass in inclusions from the vredefort granophyre south africa the vredefort granophyre represents impact melt that was injected downward into fractures in the floor of the vredefort impact structure south africa this unit contains inclusions of country rock that were derived from different locations within the impact structure and are predominantly composed of quartzite feldspathic quartzite arkose and granitic material with minor proportions of shale and epidiorite two of the least recrystallized inclusions contain quartz with single or multiple sets of planar deformation features quartz grains in other inclusions display a vermicular texture which is reminiscent of checkerboard feldspar feldspars range from large twinned crystals in some inclusions to fine grained aggregates that apparently are the product of decomposition of larger primary crystals in rare inclusions a mafic mineral probably biotite or amphibole has been transformed to very fine grained aggregates of secondary phases that include small euhedral crystals of fe rich spinel these data indicate that inclusions within the vredefort granophyre were exposed to shock pressures ranging from 5 to 8 30 gpa many of these inclusions contain small rounded melt pockets composed of a groundmass of devitrified or metamorphosed glass containing microlites of a variety of minerals including k feldspar quartz augite low ca pyroxene and magnetite the composition of this devitrified glass varies from inclusion to inclusion but is generally consistent with a mixture of quartz and feldspar with minor proportions of mafic minerals in the case of granitoid inclusions melt pockets commonly occur at the boundaries between feldspar and quartz grains in metasedimentary inclusions some of these melt pockets contain remnants of partially melted feldspar grains these melt pockets may have formed by eutectic melting caused by inclusion of these fragments in the hot 650 to 1610 oc impact melt that crystallized to form the vredefort granophyre

noble metals in the marginal rocks of the bushveld complex two major subdivisions of marginal rocks occur in the eastern bushveld complex a pyroxenitic group borders the lower and lower critical zones and a gabbroic group borders the upper critical and main zones three main facies of the gabbroic group can be recognized on the bases of field relations petrography and chemical composition five sampling sites were chosen that would best represent the varieties of marginal rocks by study of the field relations and bulk rock compositions of 184 previously analyzed samples these sites were resampled and the rocks analyzed for major and trace elements and the noble metals ir rh pt pd and au the field relations petrography and mineral and bulk rock composition of the samples are presented the range in noble metal abundances is ir 0 2 to 0 6 rh 3 to 12 pt 30 to 50 pd 10 to 20 and au 10 to 40 ppb the values are considerably higher than corresponding abundances in most basalts and chilled margins of other layered complexes especially for au all marginal rocks have broadly similar chondrite normalized noble metal distribution patterns with enrichment in rh and au relative to pt and pd abundances of noble metals in a liquid from which the ug 2 chromitite layer and the merensky reef may have crystallized is calculated from marginal rock compositions assuming a conical magma chamber the liquid is composed of a approximately 52 48 percent pyroxenitic gabbroic mixture and has a composition of ir 0 41 rh 7 7 pt 40 pd 16 and au 21 ppb maximum distribution ratio values d super sulfide liquid super silicate liquid computed for the mixture assuming no fractional crystallization are between 1 8 and 26 times higher in the ug 2 chromite environment than in the merensky reef silicate environment distribution ratio values of ir 6 600 rh 1 100 pt 3 900 pt 4 000 au 480 and cu 480 were obtained for the merensky reef with 50 percent fractional crystallization and incompatible behavior of the metals the distribution ratios attain more realistic values of ir 3 300 rh 550 pt 1 950 pd 2 000 au 240 and cu 240

backscatter model for the unusual radar properties of the greenland ice sheet a number of planetary objects exhibit large radar reflectivity and polarization ratios and more recently a similar behavior has been observed over a vast portion of the earth s surface the percolation facies of the greenland ice sheet surface based ranging radar data and snow stratigraphy studies demonstrated that the radar properties of that portion of greenland are caused by enhanced scattering from massive large solid ice bodies buried in the top few meters of the dry cold clean snowy surface of the ice sheet and created by seasonal melting and refreezing events from author

isotopic evidence for distinct crustal sources of north and south range ores sudbury igneous complex lead isotope ratios were measured on fe cu ni sulphide ores and associated silicate phases from nine mines on the north and south ranges of the sudbury igneous complex sic these data are compared with the lead isotope signatures of country rocks and shock melted sudbury breccias around the complex both ore and feldspar data from the south range fall along a 1 85 ga reference line the age of the igneous complex and indicate a magmatic origin for lead in the ore data from the north range are also consistent with a magmatic origin but are displaced to lower 206pb 204pb and 207pb 204pb ratios these two suites fall close to the lead isotope composition of country rocks adjacent to the south and north ranges respectively furthermore there is a close correspondence between lead signatures of the sic and sudbury breccia on each side of the complex the evidence suggests an origin for the sic by shock melting of different crustal units on the two sides of the complex this is consistent with the impact of a meteorite on the southward thickening apron of huronian strata which overlay the margin of the superior craton at 1 85 ga

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

geophysical anomalies and quartz microstructures eastern warburton basin north east south australia tectonic or impact shock metamorphic origin abstract the eastern warburton basin northeast south australia features major geophysical anomalies including a magnetic high of near 200 nt centred on a 25 km wide magnetic low 30 000 km large eastern warburton basin the qz pe include multiple intersecting planar to curved sub planar elements with relic lamellae less than 2 m wide with spacing of 4 5 m qz pe are commonly re deformed displaying bent and wavy patterns accompanied with fluid inclusions u stage measurements of a total of 243 planar sets in 157 quartz grains indicate dominance of 10 12 10 13 and subsidiary 11 22 22 41 m 10 11 and x 51 61 planes transmission electron microscopy tem analysis displays relic narrow 1 m wide lamellae and relic non sub grain boundaries where crystal segments maintain optical continuity extensive sericite alteration of feldspar suggests hydrothermal alteration to a depth of 500 m below the unconformity which overlies the qz pe bearing warburton basin terrain the data are discussed in terms of a tectonic metamorphic deformation and b impact shock metamorphism producing planar deformation features qz pdf deformed qz pe are compared to re deformed qz pdfs in the sudbury vredefort manicouagan and charlevoix impact structures a 4 5 km uplift of the big lake granite suite during 298 295 ma is consistent with missing of upper ordovician to devonian strata and possible impact rebound the occurrence of circular seismic tomography anomalies below the east warburton basin the poolowana basin and the woodleigh impact structure signifies a potential diagnostic nature of circular tomographic anomalies

geochemistry of the j m howland reef of the stillwater complex minneapolis adit area i sulfide chemistry and sulfide olivine equilibrium platinum mineralization in the stillwater complex occurs within a complex sequence of olivine bearing plagioclase rich cumulates about 400 m above the base of the banded zone the reef consists of a 2 m thick layer of disseminated sulfides extending over the entire strike length of the complex in the area of this study the host rock to the reef sulfides shows a lateral transition over a strike length of 700 m from a heterogeneous olivine plagioclase cumulate mixed rock to a plagioclase olivine cumulate containing a high proportion of postcumulus bronzite with olivine preserved only as rare resorbed cores on bronzite grains platinum group elements within the reef correlate strongly with abundances of sulfides which show convincing textural evidence for having originated as immiscible sulfide liquid strong correlations are also observed between all six platinum group elements ni and cu despite the gradation in host rock mineralogy the sulfide fraction of the reef displays a constant bulk composition over 700 m of strike length containing 9 3 percent ni 6 9 percent cu and approximately 0 7 percent total platinum group elements principally pd these platinum group element concentrations are two to three orders of magnitude higher than those found in typical magmatic sulfide deposits and are attributed to equilibration of sulfide liquid with a very large volume of silicate melt olivines from the reef show a range of compositions from fo 71 to fo 85 and from 2 400 to 4 500 ppm ni a positive correlation is observed between nio and feo in olivines from sulfide bearing samples indicating ni fe exchange between olivine and sulfides on a local scale such that ni contents of olivines were buffered by that of the sulfide phase a value of 9 70 or 1 49 is obtained for the ni fe distribution coefficient between sulfide and olivine in agreement with values obtained at magmatic temperatures by boctor 1982 these observations may be interpreted in the framework of the plume model of campbell et al 1983 a pulse of replenishing magma enters the stillwater chamber as a buoyant plume and subsequently spreads to form a layer of hybrid melt sulfide liquation and olivine crystallization occur within the plume and the hybrid layer in response to mixing and subsequent cooling sulfide liquid attains high platinum group element concentrations during turbulent ascent and spreading of the plume crystallization of olivine from the hybrid layer gives rise to olivine sulfide boulders which settle to the floor of the chamber reaction between the boulders and the evolved liquid at the bottom of the chamber causes olivine to become more iron rich subsequent reequilibration between olivine and sulfide grains gives rise to the observed ni fe distribution

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

geophysical and structural criteria for the identification of buried impact structures with reference to australia the discovery of large asteroid impact structures likely and possible impact structures onshore and offshore the australian continent woodleigh 120 km 360 ma gnargoo 75 km lower permian upper cretaceous tookoonooka 55 65 km 125 ma talundilly 84 km 125 ma mount ashmore 100 km end eocene and warburton twin structures 400 km pre end carboniferous requires re examination of the diagnostic criteria used for their identification bouguer anomalies of established impact structures chicxulub 170 km 64 98 0 05 ma woodleigh impact structure and gnargoo probable impact structure display a unique structural architecture where pre impact structural ridges are intersected and truncated by the outer ring of the circular structure seismic reflection data outline circular central uplift domes basement plugs and rim synclines sharp circular seismic tomography anomalies indicate low velocity columns under both the woodleigh impact structure and warburton probable impact hinting at deep crustal fracturing deformed curved and clouded intra crystalline planar deformation features in quartz qz pdfs displaying miller indices 10 11 10 12 10 13 diagnostic of shock metamorphism abound around exposed established impact structures vredefort 298 km 2023 4 ma sudbury 250 km 1850 3 ma charlevoix 54 km 342 15 ma manicouagan 100 km 214 1 ma tookoonooka and talundilly deformed qz pdfs allow recognition of shock metamorphism in buried impact structures where original qz pdfs were bent recrystallized and or clouded during formation of the central uplift and hydrothermal activity triggered by the impact planar deformation in quartz can also occur in explosive pyroclastic units but are limited to boehm lamella brazil twins with single lamella sets 0001 it has been suggested that a class of microstructures in quartz referred to as metamorphic deformation lamella qz mdl occur in endogenic tectonic metamorphic terrains however no type locality has been established for qz mdl of non impact origin

venus global surface radio emissivity observations of thermal radio emission from the surface of venus made by the pioneer venus radar mapper at a wavelength of 17 centimeters show variations that are dominated by changes in surface emissivity the regions of lowest emissivity 0 54 0 05 for the highland areas of aphrodite terra and theia mons correspond closely to regions of high radar reflectivity reported earlier these results support the inference of inclusions of material with high electrical conductivity in the surface rock of these areas

osmium isotope evidence for a crustal origin of platinum group elements in the sudbury nickel ore ontario canada abstract sulphide ores from the international nickel company s inco creighton mine sudbury were analysed for osmium isotope ratios by icp ms rhenium and osmium abundances were determined by isotope dilution using a mixed spike in solid solution in a nickel sulphide matrix calculated initial 187 os 188 os ratios at 1 85 ga the emplacement age of the sudbury complex cluster around 0 60 the occurrence of less radiogenic compositions is attributed to post emplacement open system behaviour of the re os system the creighton results strongly overlap the initial ratios of published osmium data from two other sudbury mines suggesting a narrow range of isotope ratio in the original ore within the range of estimated osmium isotope compositions in the country rock at 1 85 ga using published and new data therefore the sulphide ores of the sudbury complex can have an entirely crustal source without a mantle derived contribution this conclusion is consistent with published neodymium isotope data for the complex and supports the meteorite impact hypothesis since this is the most effective means of fusing the large quantity of crustal rocks necessary to generate the complex the extraction of platinum group elements pge from this silicate melt by a nickel sulphide liquid was probably analogous to the laboratory fire assay procedure

origin of pge depleted ni cu sulfide mineralization in the triassic hongqiling no 7 orthopyroxenite intrusion central asian orogenic belt northeastern china the 216 ma hongqiling no 7 intrusion is located in the southern margin of the central asian orogenic belt northeastern china it was the second largest ni producer after jinchuan in china the intrusion crops out in an area of 0 013 km2 on the surface it intruded gneisses and marbles of the paleozoic hulan formation the intrusion is mainly composed of orthopyroxenite 95 vol with minor harzburgite and norite over 90 vol of the intrusion contains disseminated net textured or massive sulfides the bulk sulfide ores recalculated to 100 sulfide have low average pge concentrations 5 6 ppb os 3 1 ppb ir 4 7 ppb ru 2 2 ppb rh 31 ppb pt and 11 ppb pd and high cu pd ratios 3 105 3 107 consistent with the compositions of sulfide liquids segregated from pge depleted magma olivine composition fo 86 mol indicates a fractionated parental magma back calculation shows that the parental magma was possibly derived by olivine fractionation from a primary magma containing 16 wt mgo numerical modeling indicates that pge depletion in the parental magma of the hongqiling no 7 intrusion was due to previous sulfide segregation in the staging chamber whole rock data show light rare earth element ree enrichments negative nb ta anomalies and positive nd t 216 ma values from 3 to 5 which can be explained by mixing of a mantle derived magma with a granitic melt formed by magma underplating in the crust the os t 216 ma values of sulfide ores vary from 50 to 187 the highly variable os t values coupled with similar nd t values indicate preferential assimilation of crustal sulfides during magma rock interaction based on the observations we propose that the parental magma of the hongqiling no 7 intrusion formed by contamination of mantle derived s undersaturated magma by crustally derived granitic melt together with olivine crystallization in a staging chamber the contaminated magma became pge depleted due to sulfide segregation in response to crustal contamination the pge depleted magma became s undersaturated again during ascent due to the negative effect of pressure on the maximum solubility of sulfur in magma upon arrival near the present level of the hongqiling no 7 intrusion a second event of sulfide segregation took place due to addition of crustal sulfides from immediate country rocks the immiscible sulfide liquids and orthopyroxene crystallizing from the magma were retained in a dynamic conduit to form the ore bearing intrusion while much of the magma left the conduit to form some of the nearby sulfide poor intrusions we suggest future exploration in the region to focus on olivine bearing and orthopyroxene dominant intrusions with ages similar to that of the hongqiling no 7 intrusion

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

chemical record of the projectile in the graded fall back sedimentary unit from the ries crater germany abstract the so called graded unit comprises 17 2 m of the core from the research borehole nordlingen 1973 and is thought to represent fall back material which has been sorted similar to tephra samples from six levels of these air borne impact debris sediments in the ries crater have been analyzed for ir ni co cr and fe the concentrations of these elements are negatively correlated with average grain size and accordingly the depth of the samples indicating the presence of a surface correlated component and are hence consistent with vapour deposition of meteoritic material on small dust grains the largest ir concentration found is 230 pg g a factor of 28 higher than the indigenous concentration enrichment factors in the uppermost layer of the graded unit are about 28 for ir approximately 2 for fe co ni and 1 5 for cr if the enrichment of all elements determined were of meteoritic origin then the net meteoritic composition is most closely matched by aubrites as suggested previously however it seems more likely that the observed concentration trends of cr fe co and ni are due to mineral sorting therefore the only as yet certain indication of meteoritic material is the presence of ir which is not sufficient to further constrain the nature of the projectile

microscopic planar deformation features in quartz of the vredefort structure anomalous but still suggestive of an impact origin abstract the orientation and distribution of diagnostic shock produced planar features in quartz has been examined in over 80 samples from both the core and collar rocks of the vredefort structure in south africa these features are widespread occurring in all but four samples the majority of the planar features have a basal 0001 orientation with a few higher pressure so called features and three features occurring near the center of the structure their relative distribution and preservation are however anomalous compared to those at other large terrestrial impact structures and recorded shock pressures calculated on the basis of planar feature orientation do not show a regular decrease radially outward as observed at other known impact structures the apparently anomalous development of planar features is interpreted as a result of post shock recrystallization if the effect of recrystallization of quartz which generally increases inward toward the core of the structure is considered and only the highest pressure type planar feature present in any one sample is taken as a measure of shock pressure then higher pressures are recorded in the core compared to the collar rocks no evidence of a previously reported additional post recrystallization shock event is found in this study although previous researchers report that pseudotachylite which may or may not be associated with the shock event cuts recrystallized quartzites in is concluded that although anomalous the evidence from planar features in quartz is still consistent with the vredefort structure being an erosional remnant of a large complex impact structure

re os isotope systematics of ni cu sulfide ores sudbury igneous complex ontario evidence for a major crustal component sudbury igneous complex sublayer ores from the levack west falconbridge and strathcona mines were analyzed for their re and os concentrations and os isotopic compositions the re os isotope systematics of three ores from the different mines give isochron ages of1 840 60 ma 1770 60 ma and 1780 110 ma suggesting that the re os system became closed at the time of or soon after the 1850 1 ma crystallization age of the complex the os isotopic compositions of different portions of the complex at the time of crystallization varied considerably with initial187os 186os1850 ranging from 4 64 at levack west to 7 55 at strathcona these heterogeneities require that the os and probably also the other platinum group elements contained in the ores were derived from at least two sources in addition the high initial187os 186os ratios indicate that the os was derived predominantly from ancient crust previous studies have suggested that the complex either crystallized from a mixture of mantle derived basaltic melt and ancient continental crust or was derived exclusively from the fusion of ancient continental crust resulting from a meteorite impact results of modelling suggest that if a contemporaneous mantle derived basaltic melt was involved in the origin of the sic it likely contributed 50 of the os to all three ores the large percentage of ancient crust involved in the production of the ores is most consistent with an interpretation of substantial crustal fusion resulting from meteorite impact

dynamic deformation of volcanic ejecta from the toba caldera possible relevance to cretaceous tertiary boundary phenomena plagioclase and biotite phenocrysts in ignimbrites erupted from the toba caldera sumatra show microstructures and textures indicative of shock stress levels higher than 10 gpa strong dynamic deformation has resulted in intense kinking in biotite and with increasing shock intensity the development of plagioclase of planar features shock mosaicism incipient recrystallization and possible partial melting microstructures in quartz indicative of strong shock deformation are rare however and many shock lamellae if formed may have healed during post shock residence in the hot ignimbrite they might be preserved in ash falls peak shock stresses from explosive silicic volcanism and other endogenous processes may be high and if so would obviate the need for extraterrestrial impacts to produce all dynamically deformed structures possibly including shock features observed near the cretaceous tertiary boundary 38 references 3 figures

laufey regio a newly discovered topographic rise on venus 1 laufey regio is a 0 5 km high elongate topographic rise measuring 1000 by 2000 km located within northern navka planitia venus it is dominated by two large volcanoes var and atanua montes and numerous coronae with associated flow deposits the area has been geologically mapped showing that large scale edifice formation focused at var and atanua montes and overlapped in time with the formation of smaller coronae several coronae and volcanoes have had complex and protracted histories with multiple episodes of volcanism and tectonism supporting a nondirectional history for this region the broad topography of the rise is surrounded by a concentric set of wrinkle ridges interpreted to have formed after the onset of centralized volcanism admittance studies indicate both bottom and top loading but lithospheric parameters are not well constrained top loading models give a range of 0 48 km for elastic thickness te and 35 115 km for crustal thickness we interpret the te range as most likely indicating isostatic compensation bottom loading models yield lower bounds of 30 75 km for te and 32 74 km for the apparent depth of compensation adc the combination of the large te and low topography indicates that the plume is no longer active bottom loading and a small adc may indicate that a layer of low density residuum produced by pressure release melting in the now extinct plume is partially supporting the topography on the basis of its broad regional topography abundant volcanism and gravity signature we interpret laufey to be a volcano dominated rise in a late stage of evolution

structure and origin of australian ring and dome features with reference to the search for asteroid impact events abstract ring dome and crater features on the australian continent and shelf include a 38 structures of confirmed or probable asteroid and meteorite impact origin and b numerous buried and exposed ring dome and crater features of undefined origin a large number of the latter include structural and geophysical elements consistent with impact structures pending test by field investigations and or drilling this paper documents and briefly describes 43 ring and dome features with the aim of appraising their similarities and differences from those of impact structures discrimination between impact structures and igneous plugs volcanic caldera and salt domes require field work and or drilling where crater like morphological patterns intersect pre existing linear structural features and contain central morphological highs and unique thrust and fault patterns an impact connection needs to tested in the field hints of potential buried impact structures may be furnished by single or multi ring tmi patterns circular tmi quiet zones corresponding gravity patterns low velocity and non reflective seismic zones a examples of crater form and dome form features containing elements consistent with an impact origin though unproven include auvergne delamere fiery creek monte christo mount moffatt tanami east youngerina and tingha b examples of buried multi ring features of possible to probable impact origin include augathella balfour downs calvert hills camooweal green swamp well herbert ikybon river ilkurka lennis mclarty hills mount davies mulkara neale sheridan creek oodjuongari and renehan c examples of igneous plugs unrelated to impacts include the monto gabbro and numerous circular granitoid plugs such as windinie hills granite and yataga granodiorite d large circular structures such as mount ashmore and gnargoo are considered to have convincing structural deformation features warranting classification as probable impact structures the origin of very large circular tmi and gravity patterns such as of the diamantina river drainage feature coonamona anomaly and the multiple tmi ring pattern of the deniliquin booligal remain unresolved the advent of 40 m tmi grid coverage promises to further uncover ring and dome features such as the mclarty hills multi ring feature potentially increasing the inventory of ring structures on the australian continent compared with frequency distribution patterns of extra terrestrial impact structures worldwide the australian record displays a relatively common occurrence of large impact structures and relative depletion in small impact structures and craters this is explained by the better preservation of large structures at deep crustal zones as compared to the erosion of small craters

deep submarine pyroclastic eruptions theory and predicted landforms and deposits submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows however that significant pyroclastic activity can occur even at depths in excess of 3000 m in order to document and illustrate the full range of submarine eruption styles we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions 1 no gas exsolution 2 gas exsolution but no magma disruption 3 gas exsolution magma disruption and hawaiian style fountaining 4 volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse 5 magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity and 6 volatile content builds up in the top of a dike leading to vulcanian eruptions we also examine the role of bulk interaction steam explosivity and contact surface steam explosivity as processes contributing to volcaniclastic formation in these environments we concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities the most likely circumstance is the quiet effusion of magma with minor gas exsolution and the production of somewhat vesicular pillow lavas or sheet flows depending on effusion rate the amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption the most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity due to the localized concentration of volatiles in magma that has a low rise rate magmatic gas collects by bubble coalescence and ascends in large isolated bubbles which disrupt the magma surface in the vent producing localized blocks bombs and pyroclastic deposits another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions these deposits being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater should be easily distinguishable from strombolian and hawaiian eruptions however we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles in this case a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian style activity the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile induced hawaiian eruption specifically typical pyroclast sizes might be smaller fountain heights may exceed those expected for the purely magmatic hawaiian case cooling of descending pyroclasts would be more efficient leading to different types of proximal deposits and runout distances for density flows would be greater potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer in addition flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles and thus extremely poor in vesicles relative to typical flows associated with hawaiian style eruptions in the primary magmatic gas case we examine several cases of reported submarine volcaniclastic deposits found at depths as great as 3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid ocean ridges furthermore the latter stages of development of volcanic edifices seamounts formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth but also to 1 the presence of a summit magma reservoir which favors the buildup of magmatic foams enhancing hawaiian style activity and episodic dike emplacement which favors strombolian style eruptions and 2 the common occurrence of alkalic basalts the co2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts these models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles

gravity and magnetic field modeling and structure of the chicxulub crater mexico the chicxulub crater is an 180 km diameter peak ring crater based on drill hole logs and samples potential fields seismic reflection profiles and surface fracture patterns a structural cross section produced based on these constraints has the features expected for a large complex impact crater the bouguer gravity anomaly consists of a broad 90 km radius 30 mgal low with a central 20 km radius 20 mgal high and two 5 mgal concentric lows at 35 and 60 km radius the gravity anomaly is disrupted by large scale basement anomalies and possibly by large scale slumping and backwash erosion effects the magnetic field anomaly over the crater consists of three zones an outer zone from 45 to 90 km radius of low amplitude short wavelength anomalies with an irregular perimeter a middle zone from 20 to 45 km radius of high amplitude short wavelength anomalies slightly elongated nnw sse and an inner 20 km radius single large amplitude anomaly magnetic field modeling indicates that the melt pool averages 90 km in diameter and the melt volume in the crater is estimated at 20 000 km3 the melt pool size constrains the collapsed transient cavity diameter to 90 km gravity and magnetic field modeling indicate that the structural uplift is irregular in shape but 40 km in diameter and underlies or protrudes into the melt pool the preliminary structural cross section indicates that the inferred peak ring is decoupled from the structural uplift the geometry and bouguer gravity signature of the crater indicate that no significant uplift of the moho or relaxation of the crater has occurred

experimental study of the solubility of platinum and iridium in basic silicate melts implications for the differentiation of platinum group elements during magmatic processes this study presents evidence for the dissolution of two platinum group elements pge s pt and ir in silicate melts of basic composition in the absence of any associated fluid phase the dissolution of these pge s is attributed to the formation of a complex ion in which the noble metal is uncharged an increase in oxygen fugacity in equilibrium with the melt brings about a considerable decrease in ir solubility in this way we can explain the early enrichment in ir group pge s ipge s ir ru os observed during the crystallization of ultrabasic cumulates by contrast however pt remains soluble in the melt an increase in the fs2 results in a pronounced increase in the solubility of pt in the silicate melt the sulfur saturation which leads to the precipitation of the base metal sulfides bms s also brings about the fixation of pt group pge s ppge s pt pd rh which are dissolved in bms s this accounts for the presence of ppge s in the nickel sulfide horizons of stratiform deposits the occurrence of platiniferous fe crystallizing with chromite in alaskan type complexes at oxygen fugacities of 10 6 bar can be explained by the low activity of fe in fe pt alloys such alloys have been formed in our laboratory at such fo2

volcanic deposits in shield fields and highland regions on venus surface properties from radar polarimetry 1 we compare arecibo dual polarization radar image data for venus to magellan images and emissivity data to investigate the physical properties of volcanic deposits radar waves can easily penetrate smooth mantling layers such as ash aeolian and crater derived deposits if a circularly polarized radar wave refracts into a surface that is smooth at wavelength scales the vertical component of the wave will be preferentially transmitted resulting in a net linear polarized echo component arecibo polarimetry data were used to create maps of the degree of linear polarization in the radar echo we find that some volcanic fields in plains regions on venus are associated with enhanced linear polarization these fields sometimes have nearby windstreaks which suggest fine grained surface material and we infer that the radar wave is penetrating into mantling deposits that are a few centimeters to 1 m thick enhanced linear polarization values are also correlated with specific lava flows these lava flows have emissivity values of 0 80 to 0 84 similar to many other flows on venus the enhanced linear polarization may be produced by penetration of the radar wave into very smooth lava flows with internal air gaps high reflectivity low emissivity areas near the summits of theia and tepev montes also have a linearly polarized echo component consistent with surface penetration by the radar wave the cause of the high linear polarization in summit regions remains uncertain but perhaps the radar wave is able to penetrate into high dielectric material in limited cases of very smooth surface texture

microscopic lamellar deformation features in quartz discriminative characteristics of shock generated varieties the appearance and orientation of microscopic lamellar deformation features in quartz have been examined in samples from different geologic environments lamellar deformation features from known shock and or impact environments are sharp parallel closely spaced and tend to extend in a continuous manner across entire grains they have specific well documented orientations and occur in multiple sets per grain lamellar deformation features from other geologic environments including tectonic and explosive volcanic only superficially resemble shock features they are generally less well defined slightly curved wider spaced and tend to extend in a discontinuous manner across parts of grains they have relatively random orientations only some of which correspond to known shock orientations and generally occur as single sets per grain on the basis of these observations we conclude that there is a characteristic set of microscopic deformation features produced in quartz by shock these so called planar features differ from microscopic lamellar deformation features produced by other dynamic processes therefore the case for the formation of characteristic planar features in quartz by natural processes other than impact is still not proven