vredefort sudbury chicxulub three of a kind vredefort sudbury and chicxulub are the largest known terrestrial impact structures all have been cited as multi ring basins the available data indicate that all have some form of multiple ring attributes most commonly structural features chicxulub however is the only example with morphological ring features there are also commonalities in the structural and lithological features of vredefort and sudbury and it is possible to construct a generalized compilation of the character of 200 300 km diameter impact basins on earth it is not clear however that any of these structures had the original morphological characteristics of large lunar multi ring basins additional data and synthesis are required to fully characterize these structures in order to realize their potential to constrain large scale cratering processes if this is not sufficient incentive for further studies the environmental effects of chicxulub aside the vredefort sudbury and chicxulub impact events are also the reason for

order or chaos origin and mode of emplacement of breccias in floors of large impact structures abstract breccias in the crater floor of large impact structures are pseudotachylites sensu largo authigenic monomict and polymict clastic matrix breccias so called footwall breccias and impact melt breccias pseudotachylite bodies in the center of large impact structures e g vredefort dome south africa appear to have a random distribution and orientation but most dip steeply or vertically large bodies of pseudotachylite in the more distal sectors of the 200 km diameter sudbury structure have been interpreted as ring and terrace collapse features in the vredefort dome networks of randomly distributed pseudotachylite veins accompany large mother lode pseudotachylite dikes in general pseudotachylites in the floors of central parts of impact craters may form through explosive transfer of thermal shock energy in a process that could be termed flash replacement melting whereas pseudotachylites at large distances from the centers of large impact structure are believed to have formed through friction leading to partial or complete melting similar to the formation of tectonic pseudotachylites in smaller structures e g ries and slate islands clastic matrix breccias instead of pseudotachylites occur as the most common breccias in the crater floors they have a chaotic distribution pattern their dips are commonly also steep to vertical melt breccia dikes in the target rocks of the crater floor are associated with melt sheets that fill the lower part of the excavation cavity at vredefort erosion has removed the coherent melt sheet but melt breccia dikes vredefort granophyre in the crater floor are preserved they are characterized by a remarkably homogeneous chemical composition and are believed to represent the initial undifferentiated impact melt near the vredefort collar the granophyre forms more or less concentric dikes in the more central parts of the dome their orientation is more random but in places may be controlled by the archean fabric of the crater basement the offset dikes of the sudbury structure are associated with the sudbury igneous complex that represents in total or in part a differentiated impact melt sheet the dikes in many aspects are similar to vredefort granophyres but their interpretation as undifferentiated bulk melt is problematic on geochemical and structural grounds chemically these dikes are not homogeneous and in places they contain massive sulfide deposits that are unlikely components of an undifferentiated impact melt differentiation and precipitation of massive sulfides are slow processes compared with the presumably high energy and fast emplacement of a supposedly undifferentiated melt pseudotachylites of the central impact crater apparently are mainly produced in the compression phase of the impact process whereas clastic matrix breccias form during the uplift and crater modification phases impact melt breccia dikes contain pseudotachylite bearing inclusions and cut across pseudotachylite bodies such impact melt breccia dikes were probably emplaced during uplift and crater modification

thermal convection in a nonhomogeneous aquifer abstract geothermal activity creates destabilizing temperature gradients in groundwater very often stabilizing salinity gradients also exist in the aquifer the combination of temperature and salinity distributions in the aquifer may induce various kinds of hydrodynamic instability such instabilities are possible mainly in thick aquifers which usually consist of various nonhomogeneous formations in this study we investigate the effect of the aquifer nonhomogeneity on the stability of the flow field as well as on heat and salinity transport through the aquifer by galerkin expansion the criteria of the flow field stability were identified and calculated quantitatively it was found that the point of instability is demonstrated by marginal stability or overstability the nonhomogeneity of the aquifer leads to asymmetry of the convection cells

sudbury breccia canada a product of the 1850 ma sudbury event and host to footwall cu ni pge deposits abstract the sudbury structure formed by meteorite impact at 1850 ma consists of three major components 1 the sudbury basin 2 the sudbury igneous complex which surrounds the basin as an elliptical collar and 3 breccia bodies in the footwall known as sudbury breccia in general the breccia consists of subrounded fragments set in a dark fine grained to aphanitic matrix a comparison of the chemical composition of host rocks clasts and matrices indicates that brecciation was essentially an in situ process sudbury breccia forms irregular shaped bodies or dikes that range in size from mm to km scale contacts with the host rocks are commonly sharp the aspect ratio of most clasts is approximately 2 with the long axes parallel to dike walls the fractal dimension dr 1 55 although there appears to be some concentration of brecciation within concentric zones small sudbury breccia bodies within and outside these zones have more or less random strikes and steep dips sudbury breccia bodies near an embayment structure tend to be subparallel to the base of the sudbury igneous complex sudbury breccia occurs as much as 80 km from the outer margin of the sudbury igneous complex in an inner zone 5 to 15 km wide breccia comprises 5 of exposed bedrock with an increase in brecciation intensity in embayment structures sudbury breccia may be classified into three types based on the nature of the matrix clastic pseudotachylite and microcrystalline clastic sudbury breccia the dominant type in the southern province is characterized by flow surface structures possibly a sudden rise in pore pressure caused explosive dilation and fragmentation followed by fluidization and flowage into extension fractures pseudotachylite sudbury breccia mainly confined to archean rocks apparently formed by comminution and frictional melting microcrystalline sudbury breccia formed as a result of the thermal metamorphism of the north range footwall by the sudbury igneous complex this produced a zone approximately 1 2 km wide wherein the matrix of the breccia either recrystallized or locally melted an overprint of regional metamorphism obliterated contact effects in the south range footwall the ni cu pge magmatic sulphide deposits may be classified into four types based on structural setting sudbury igneous complex footwall contact footwall offset and sheared deposits sudbury breccia is the main host for footwall deposits e g mccreedy east victor lindsley sudbury breccia locally hosts mineralization in radial e g parkin and copper cliff and concentric e g frood stobie offset dikes

magnetic fabric and microstructural evidence for a tectono thermal overprint of the early proterozoic murray pluton central ontario canada abstract the 2 38 ga old murray granite pluton intruded the southern footwall of the sudbury igneous complex and was subjected to repeated ductile deformation its nw and se domains display differences in strain state ams signature and the nature and distribution of fe oxide minerals the magnetic foliation is parallel to the visible foliation in the nw domain but discordant to it in the se domain microstructural data show that the magnetic fabric in the nw domain coincides with a foliation formed at high temperature probably in response to thermal softening during the emplacement of the adjacent sudbury igneous complex this tectono thermal overprint affected pre existing shape fabrics and was facilitated by incipient melting of the granite near the intrusive contact with the sudbury igneous complex rocks of the se domain are only partially overprinted and two mineral subfabrics are preserved prolate susceptibility ellipsoids and clusters of maximum susceptibility directions at the intersections of the two planar fabrics suggest oblique superposition of strain increments increases in bulk magnetic susceptibility and magnetic anisotropy degree towards the intrusive contact with the sudbury igneous complex can be explained by respective increases in the proportion of magnetite over ilmenite and strain intensity in the murray pluton

structural geology of impact craters abstract the formation of impact craters is a highly dynamic and complex process that subjects the impacted target rocks to numerous types of deformation mechanisms understanding and interpreting these styles of micro meso and macroscale deformation has proved itself challenging for the field of structural geology in this paper we give an overview of the structural inventory found in craters of all size ranges on earth and look into the structures of craters on other planetary bodies structural features are discussed here that are caused by i extremely high pressures and temperatures that occur during the initial passage of the shock wave through the target rock and projectile ii the resulting flow field in the target that excavates and ejects rock materials and iii the gravitationally induced modification of the crater cavity into the final crater form a special focus is put on the effects that low angle impacting bodies have on crater formation we hope that this review will help both planetary scientists and structural geologists understand the deformation processes and resulting structures generated by meteorite impact

structural characteristics of the sudbury impact structure canada impact induced versus orogenic deformation a review orogenic deformation both preceding and following the impact event at sudbury strongly hinders a straightforward assessment of impact induced geological processes that generated the sudbury impact structure central to understanding these processes is the state of strain of the sudbury igneous complex the solidified impact melt sheet its underlying target rocks overlying impact breccias and post impact sedimentary rocks this review addresses 1 major structural metamorphic and magmatic characteristics of the impact melt sheet and associated dikes 2 attempts that have been made to constrain the primary geometry of the igneous complex 3 modes of impact induced deformation as well as 4 mechanisms of pre and post impact orogenic deformation the latter have important consequences for estimating parameters such as magnitude of structural uplift tilting of pre impact huronian strata and displacement on major discontinuities which collectively have not yet been considered in impact models in this regard a mechanism for the emplacement of offset dikes is suggested that accounts for the geometry of the dikes and magmatic characteristics as well as the occurrence of sulfides in the dikes moreover re interpretation of published paleomagnetic data suggests that orogenic folding of the solidified melt sheet commenced shortly after the impact uncertainties still exist as to whether the sudbury impact structure was a peak ring or a multi ring basin and the deformation mechanisms of rock flow during transient cavity formation and crater modification

residence time of osmium in the oceans 1 estimates of osmium residence time in the oceans that are based on oceanic mass balance calculations 35 50 kyr appear irreconcilable with those inferred from the recent evolution of the osmium isotope composition of seawater 3 4 kyr it is argued that the osmium budget of the oceans is currently close to steady state and thus that the estimates made by the two methods should agree as the inventory of osmium in the oceans is relatively well constrained these disparate residence time estimates imply wildly different osmium input fluxes to the oceans an osmium residence time of 8 10 kyr is proposed by evaluating the uncertainties and limitations of both methods and it is argued that osmium inputs to the ocean are currently underestimated by a factor of 3 this reflects in part the underestimation of the river input of osmium to the oceans owing to a bias within the existing data set and in part the probable existence of sources of osmium to the oceans that have not yet been identified the very short residence time of 3 4 kyr inferred from the postglacial change in seawater composition assuming a single step change in input flux is rejected as it implies unreasonably high osmium input fluxes to the oceans it is concluded that a postglacial spike in osmium flux associated with a meltwater event must have driven part of the change in seawater composition however it is also shown that such a spike cannot be the dominant cause of the most recent shift in seawater 187os 188os

microscale haline convection a proposed mechanism for transport and mixing at the water table region field measurements of chloride profiles in the water table region of a deep aquifer are interpreted in terms of haline convection at the microscale level under natural flow conditions the flow field is characterized by a small peclet number between 6 25 10 2 and 6 25 10 3 so that molecular diffusion dominates over mechanical dispersion it is suggested that haline convection is a major transport and mixing mechanism at the water table region of granular aquifers under natural flow conditions wherever the intervention of man changed the chemical composition of natural replenishment in the studied region it is estimated that the critical density difference which overcomes viscous drag forces is 0 23 0 28 kg m3 this value agrees with the magnitude of the critical free convection parameter modified rayleigh number of earlier models

the sudbury structure controversial or misunderstood the origins of the sudbury structure and associated igneous complex have been controversial most models call for a major impact event followed by impact induced igneous activity although totally igneous models are still being proposed much of the controversy is due in our opinion to a misunderstanding of the size of the original sudbury structure by analogy with other terrestrial impact structures the spatial distribution of shock features and huronian cover rocks at the sudbury structure suggest that the transient cavity was 100 km in diameter which places the original final structural rim diameter in the range of 150 200 km theoretical calculations and empirical relationships indicate that the formation of an impact structure of this size will result in 104 km3 of impact melt more than sufficient to produce a melt body the size of the igneous complex present volume 4 8 103 km3 for the igneous complex to be an impact melt sheet it must have a composition similar to that of the target rocks evidence for this has been presented previously for sr and nd isotopic data which suggest a crustal origin here we also present new evidence from least squares mixing models that the average composition of the igneous complex corresponds to a mix of archean granite greenstone terrain with possibly a small component of huronian cover rocks this is a geologically reasonable mix based on the interpreted target rock geology and the geometry of melt formation in an impact event of this size the igneous complex is differentiated which is not a characteristic of previously studied terrestrial impact melt sheets this can be ascribed however to its great thickness and slower cooling that large impact melt sheets can differentiate has important implications for how the lunar samples and the early geologic history of the lunar highlands are interpreted if this working hypothesis is accepted namely that both the sudbury structure and the igneous complex are impact in origin then previous hybrid impact igneous hypotheses can be discarded and the sudbury structure can be studied specifically for the constraints it provides to large scale cratering and the formation of basin sized multiring impact structures

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

isotopic composition of osmium in terrestrial samples determined by accelerator mass spectrometry interest has revived in the rhenium osmium system due to significant advances in detection methods1 4 and because of its use for the identification of extraterrestrial material at the cretaceous tertiary boundary5 another perhaps more important application is its use as a tracer for the origin of crustal material this application is however severely restricted by the very low concentrations typical of crustal rocks we present here the first results for the isotopes 187os 188os and 189os measured in sub p p b parts per 109 level samples by accelerator mass spectrometry ams our first exploratory measurements on extraterrestrial and crustal material with os concentrations between 2 p p m parts per 106 and 0 006 p p b show a detection limit of 0 01 p p b and a precision of better than 10 the 187os 186os ratios measured for samples for the canyon diablo meteorite and from the east clearwater crater in quebec canada were close to 1 typical of extraterrestrial material in contrast ratios in samples from the ries crater in germany were around 10 which is evidence of predominantly crustal osmium in these rocks

shock induced kelyphite formation in the core of a complex impact crater we present a compositional and textural analysis of shock induced microtextures in garnet porphyroblasts in migmatitic garnet cordierite biotite paragneisses from the centre of the vredefort impact structure south africa detailed imaging and major element analysis of deformation features in and adjacent to the garnet porphyroblasts record a complex heterogeneous distribution of shock effects at the microscale as the most competent silicate mineral in the assemblage with the highest hugoniot elastic limit and a wide pressure temperature stability field the porphyroblastic garnet preserves a more diverse shock deformation response compared to minerals such as quartz and feldspar which underwent more comprehensive shock metamorphism and subsequent annealing the garnet porphyroblasts display pre impact fractures that are overprinted by later intra granular hertzian and distinctive planar fractures associated with the impact event shock induced strain localization occurred along internal slip planes and defects including pre existing fractures and inclusion boundaries in the garnet symplectitic kelyphitic coronas commonly enclose the garnet porphyroblasts and inhabit intra granular fractures the kelyphite assemblage in fractures with open communication beyond garnet grain boundaries is characterized by orthopyroxene cordierite sapphirine conversely the kelyphite assemblage in closed off intra granular fractures is highly variable comprising spatially restricted combinations of a secondary garnet phase with a majoritic component al rich orthopyroxene sapphirine and cordierite the impedance contrast between garnet porphyroblasts and their inclusions further facilitated the formation of shock induced features al rich orthopyroxene coronas together the textural and mineralogical data suggest that these features provide a record of oscillatory shock perturbations initiated under confining pressure beneath the transient crater floor this occurred as the shocked rock volume underwent post shock expansion forming the core of the central uplift and was followed by variable textural re equilibration this study thus provides a microtextural and mineralogical perspective of the shock regime within confined crust immediately prior to and during central uplift formation

differential melt scaling for oblique impacts on terrestrial planets abstract analytical estimates of melt volumes produced by a given projectile and contained in a given impact crater are derived as a function of impact velocity impact angle planetary gravity target and projectile densities and specific internal energy of melting applications to impact events and impact craters on the earth moon and mars are demonstrated and discussed the most probable oblique impact 45 produces 1 6 times less melt volume than a vertical impact and 1 6 and 3 7 times more melt volume than impacts with 30 and 15 trajectories respectively the melt volume for a particular crater diameter increases with planetary gravity so a crater on earth should have more melt than similar size craters on mars and the moon the melt volume for a particular projectile diameter does not depend on gravity but has a strong dependence on impact velocity so the melt generated by a given projectile on the moon is significantly larger than on mars higher surface temperatures and geothermal gradients increase melt production as do lower energies of melting collectively the results imply thinner central melt sheets and a smaller proportion of melt particles in impact breccias on the moon and mars than on earth these effects are illustrated in a comparison of the chicxulub crater on earth linked to the cretaceous tertiary mass extinction gusev crater on mars where the mars exploration rover spirit landed and tsiolkovsky crater on the moon the results are comparable to those obtained from field and spacecraft observations other analytical expressions and hydrocode simulations

research paperthermohaline convection in a nonhomogeneous aquifer geothermal activity creates destabilizing temperature gradients in groundwater very often stabilizing salinity gradients also exist in the aquifer the combination of temperature and salinity distributions in the aquifer may induce various kinds of hydrodynamic instability such instabilities are possible mainly in thick aquifers which usually consist of various nonhomogeneous formations in this study we investigate the effect of the aquifer nonhomogeneity on the stability of the flow field as well as on heat and salinity transport through the aquifer by galerkin expansion the criteria of the flow field stability were identified and calculated quantitatively it was found that the point of instability is demonstrated by marginal stability or overstability the nonhomogeneity of the aquifer leads to asymmetry of the convection cells

tectonic influences on the morphometry of the sudbury impact structure implications for terrestrial cratering and modeling impact structures developed on active terrestrial planets earth and venus are susceptible to pre impact tectonic influences on their formation this means that we cannot expect them to conform to ideal cratering models which are commonly based on the response of a homogeneous target devoid of pre existing flaws in the case of the 1 85 ga sudbury impact structure of ontario canada considerable influence has been exerted on modification stage processes by late archean to early proterozoic basement faults two trends are dominant 1 the nnw striking onaping fault system which is parallel to the 2 47 ga matachewan dyke swarm and 2 the ene striking murray fault system which acted as a major paleoproterozoic suture zone that contributed to the development of the huronian sedimentary basin between 2 45 2 2 ga sudbury has also been affected by syn to post impact regional deformation and metamorphism the 1 9 1 8 ga penokean orogeny which involved nnw directed reverse faulting uplift and transpression at mainly greenschist facies grade and the 1 16 0 99 ga grenville orogeny which overprinted the se sector of the impact structure to yield a polydeformed upper amphibolite facies terrain the pre syn and post impact tectonics of the region have rendered the sudbury structure a complicated feature careful reconstruction is required before its original morphometry can be established this is likely to be true for many impact structures developed on active terrestrial planets based on extensive field work combined with remote sensing and geophysical data four ring systems have been identified at sudbury the inner three rings broadly correlate with pseudotachylyte friction melt rich fault systems the first ring has a diameter of 90 km and defines what is interpreted to be the remains of the central uplift the second ring delimits the collapsed transient cavity diameter at 130 km and broadly corresponds to the original melt sheet diameter the third ring has a diameter of 180 km the fourth ring defines the suggested apparent crater diameter at 260 km this approximates the final rim diameter given that erosion in the north range is 6 km and the ring faults are steeply dipping impact damage beyond ring 4 may occur but has not yet been identified in the field one or more rings within the central uplift ring 1 may also exist this form and concentric structure indicates that sudbury is a peak ring or more probably a multi ring basin these parameters provide the foundation for modeling the formation of thisrelatively large terrestrial impact structure

geology of the king crater region new insights into impact melt dynamics on the moon 1 new geologic sketch maps and digital elevation models for king crater and the surrounding lunar farside highlands were created from lunar reconnaissance orbiter camera lroc wide angle camera wac and narrow angle camera nac images nac images reveal that high volume impact melt accumulations exhibit evidence of dynamic processes during and following emplacement that resulted in downwarped zones and other morphologic anomalies visible at the 50 cm pixel scale among the most significant of these forms are negative relief features some of which may represent evidence for near surface caverns offering points of access to subsurface environments and possible shelter from surface hazards other negative relief features may represent regions of extension and separation in response to possible subsurface drainage together with isostatic readjustments contraction and or compaction in the cooling impact melt crater counts on the continuous ejecta blanket suggest a late eratosthenian to copernican age for king crater which is older than the estimate of young 1977 but consistent with those of others

accretion rate of extraterrestrial particles determined from osmium isotope systematics of pacific pelagic clay and manganese nodules abstract pelagic clay and mn nodules from domes sites in the north pacific and a varved glacial lake deposit from connecticut were analysed for os concentration and isotopic composition by isotope dilution secondary ion mass spectrometry after treatment by nis fusion or oxalic acid leaching bulk pelagic clay from domes site c has 187 os 186 os 6 5 and os 0 14 ng g oxalic acid leaches of this same sediment and of mn nodules from domes sites a and c have more radiogenic 187 os 186 os ratios which average 8 3 bulk glacial lake hitchcock sediment has 187 os 186 os 12 5 and os 0 06 ng g the total os flux to north pacific pelagic clay is 7 to 10 ng os cm2 106 y lake hitchcock sediment provides an integrated value for the local crustal 187 os 186 os ratio the oxalic acid leaches are assumed to attack hydrogenous phases selectively a simple model in which the only sources of os to the ocean are continental crust with the isotopic composition of lake hitchcock 187 os 186 os 12 5 and extraterrestrial particles with 187 os 186 os 1 1 results in a cosmic flux of osmium to the sediment of 4 9 ngos cm2 106 y of which 20 is hydrogenous a model in which the sources of os to the ocean are continental crust with an 187os 186os ratio of 30 from the model of palmer and turekian 1986 oceanic mantle or crust with 187 os 186 os 1 04 and extraterrestrial particles with 187 os 186 os 1 1 results in a cosmic flux of os to the sediment of 5 7 ngos cm2 106 y of which none is hydrogenous these extraterrestrial os fluxes correspond to maximum c 1 chondrite accretion rates of between 4 9 104 and 5 6 104 tonnes y

energetics of glass fragmentation experiments on synthetic and natural glasses 1 natural silicate glasses are an essential component of many volcanic rock types including coherent and pyroclastic rocks they span a wide range of compositions occur in diverse environments and form under a variety of pressure temperature conditions in subsurface volcanic environments e g conduits and feeders melts intersect the thermodynamically defined glass transition temperature to form glasses at elevated confining pressures and under differential stresses we present a series of room temperature experiments designed to explore the fundamental mechanical and fragmentation behavior of natural obsidian and synthetic glasses pyrex under confining pressures of 0 1 100 mpa in each experiment glass cores are driven to brittle failure under compressive triaxial stress analysis of the load displacement response curves is used to quantify the storage of energy in samples prior to failure the brittle release of elastic energy at failure and the residual energy stored in the post failure material we then establish a relationship between the energy density within the sample at failure and the grain size distributions d values of the experimental products the relationship between d values and energy density for compressive fragmentation is significantly different from relationships established by previous workers for decompressive fragmentation compressive fragmentation is found to have lower fragmentation efficiency than fragmentation through decompression i e a smaller change in d value with increasing energy density we further show that the stress storage capacity of natural glasses can be enhanced approaching synthetic glasses through heat treatment

cubic zirconia in 2370 c impact melt records earth s hottest crust abstract bolide impacts influence primordial evolution of planetary bodies because they can cause instantaneous melting and vaporization of both crust and impactors temperatures reached by impact generated silicate melts are unknown because meteorite impacts are ephemeral and established mineral and rock thermometers have limited temperature ranges consequently impact melt temperatures in global bombardment models of the early earth and moon are poorly constrained and may not accurately predict the survival stabilization geochemical evolution and cooling of early crustal materials here we show geological evidence for the transformation of zircon to cubic zirconia plus silica in impact melt from the 28 km diameter mistastin lake crater canada which requires super heating in excess of 2370 c this new temperature determination is the highest recorded from any crustal rock our phase heritage approach extends the thermometry range for impact melts by several hundred degrees more closely bridging the gap between nature and theory profusion of 2370 c superheated impact melt during high intensity bombardment of hadean earth likely facilitated consumption of early formed crustal rocks and minerals widespread volatilization of various species including hydrates and formation of dry rigid refractory crust

strain fabric evolution within and near deformed igneous sheets the sudbury igneous complex canada abstract deformation structures differing in style and orientation within a given terrane are often attributed to distinct tectono metamorphic events however mechanical heterogeneities may locally cause strain perturbations that can have a profound effect on the geometry of such structures here we document highly variable orientations of planar structures metre to kilometre scale folds and shortening directions inferred from brittle fault analysis within the synclinal and layered sudbury igneous complex sic and its huronian host rocks nw se shortening during palaeoproterozoic deformation led to the formation of planar structures in the host rocks that are parallel to the ne sw striking contact of the southern sic during deformation local contact parallel shortening became more important than regional nw se shortening and generated contact orthogonal planar mineral fabrics and folds local contact parallel shortening is attributed to the shape change of the southern sic from a convex outward to a concave inward curvi planar geometry contact parallel shortening accounts for the formation of a previously unidentified kilometre scale buckle fold in the sic and respective axial planar mineral fabrics in huronian host rocks this buckle fold shares similar structural characteristics with known higher order buckle folds of the eastern sic we suggest that non cylindrical buckling and associated mineral fabric development accommodated the shape change of the sic this resulted in mutually perpendicular fabric orientations which are compatible with overall nw se shortening the original shape of the sic seems to have had a profound influence particularly on the geometry of metamorphic foliations our structural analysis supports earlier structural studies advocating that highly discordant planar strain fabrics can be generated by local strain perturbations near igneous sheets under uniform regional shortening

reconstructing the geochemical signature of sudbury breccia ontario canada implications for its formation and trace metal content the sudbury breccia is an impact generated pseudotachylitic breccia formed in the footwall of the 1 85 ga sudbury igneous complex in ontario canada the breccia is commonly developed in areas of contrasting country rock types and it represents a common host to footwall style cu ni platinum group element sulfide mineralization developed in the immediate footwall of large contact type ni cu rich ore deposits at the base of the sudbury igneous complex three models have been proposed for the genesis of the sudbury breccia 1 in situ frictional or shock melting of target rocks 2 cataclasis of the target rocks and or 3 injection of allochthonous impact melt from the sudbury igneous complex into the brecciated crater floor this study reports new mineralogical and geochemical data for samples of sudbury breccia and associated host rocks both proximal and distal to mineralization from six different locations around the sudbury structure a mixing model was used to construct the composition of the sudbury breccia matrix using a combination of country rock and quartz diorite from the offset dikes which are believed to provide an estimate of the composition of the early melt sheet we demonstrate that the breccia matrix can be successfully modeled from footwall lithologies with no requirement for a contribution from the impact melt sheet instead we propose a dynamic parautochthonous in situ melt model where the matrix of the breccia was derived in part by high temperature melting of footwall lithologies these inclusion and crystal laden melts were mixed and transported into tensile fracture zones that developed adjacent to weaknesses in the country rocks principal component analysis of sudbury breccia indicates that the trace metal content within the breccia that developed distal from mineralization is associated with assimilation of mafic units in the footwall in contrast the matrix of sudbury breccia developed closer to mineral zones has a contribution of metal derived from the fractionated sulfide melt and or a contribution of hydrothermally remobilized metal thus by comparing the trace metal content in sudbury breccia with local footwall lithologies baselines can be established that can help to identify anomalous metal contributions that may be derived from the periphery of a footwall mineral system

generation of fragment rich pseudotachylite bodies during central uplift formation in the vredefort impact structure south africa abstract target rocks underlying the central portions of large terrestrial impact structures are characterized by the pervasive presence of fragment rich pseudotachylite bodies debates regarding the formation of these bodies include the origin of pseudotachylitic melts i e friction versus shock induced melting melt mobility causes of target rock fragmentation and timing of fragmentation and melt emplacement with respect to stages of cratering comprehensive structural analysis of pre impact mineral fabrics and properties of fragment rich pseudotachylite in the vredefort dome suggests that melt is allochthonous and was emplaced at an advanced stage of cratering into tensional fracture zones within the crater floor both concentration of bending strains imparted on target rocks during central uplift formation and thermal stresses induced by the emplacement of allochthonous melt led to fragmentation of target rock tensional fracture zones formed in an overall dilational strain field towards the end of cratering likely during collapse of the central uplift and constitute low pressure zones into which melt was forcefully drawn melt may have been drained from the overlying impact melt sheet or from sites within the crater floor and transported into fragment rich dilation zones our field based analysis failed to identify the presence of bona fide shear faults that could potentially have generated in situ frictional melts and fragments rather target rock fragmentation and melt generation that resulted in fragment rich pseudotachylite bodies are processes separated in space and time during cratering

a late mesoproterozoic 40ar 39ar age for a melt breccia from the keurusselka impact structure finland field investigations in the eroded central uplift of the 30 km keurusselka impact structure finland revealed a thin dark melt vein that intersects the autochthonous shatter cone bearing target rocks near the homestead of kirkkoranta close to the center of the impact structure the petrographic analysis of quartz in this melt breccia and the wall rock granite indicate weak shock metamorphic overprint not exceeding 8 10 gpa the mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia 40ar 39ar dating of dark and clast poor whole rock chips yielded five concordant late mesoproterozoic miniplateau ages and one plateau age of 1151 10 ma 11 ma 2 mswd 0 11 p 0 98 considered here as the statistically most robust age for the rock the new 40ar 39ar age is incompatible with 1 88 ga svecofennian tectonism and magmatism in south central finland and probably reflects the keurusselka impact followed by impact induced hydrothermal chloritization of the crater basement in keeping with the crosscutting relationships in the outcrop and the possible influence of postimpact alteration the late mesoproterozoic 40ar 39ar age of 1150 ma should be treated as a minimum age for the impact the new 40ar 39ar results are consistent with paleomagnetic results that suggested a similar age for keurusselka which is shown to be one of the oldest impact structures currently known in europe and worldwide

exploding xenoliths and the absence of elephants graveyards in granite batholiths abstract biotite monzogranite near the eastern margin of the late devonian south mountain batholith at portuguese cove nova scotia surrounds a large 20 m 25 m anisotropic decimetre scale interbedded psammite and semipelite xenolith of the metamorphic regional greenschist thermal cordierite hornfels country rocks of the cambro ordovician meguma group the xenolith has two distinct types of surfaces against the monzogranite i one rounded diffuse and apparently old surface that had prolonged contact with the granite magma and ii two straight sharp and apparently new surfaces that had more recently come into contact with the granite magma more than 375 thin sheets width n 224 0 1 0 5 cm n 66 0 5 1 0 cm n 51 1 0 5 0 cm n 30 5 0 10 0 cm n 3 10 cm n 2 ranging in composition from monzogranite to pegmatite to coarse quartz sericite cut the xenolith parallel to bedding producing a cumulative volume expansion of 14 in the xenolith aspect ratios in some of the thin sheets exceed 1000 1 several sheets cut the xenolith perpendicular to bedding producing a volume expansion of

application of the aquifer s average characteristics for determining the onset of thermohaline convection in a heterogeneous aquifer abstract due to layered deposition of sedimentary rocks the aquifer s characteristics usually vary with the depth such variations are mainly significant in thick aquifers the variable characteristics cause the aquifer to be heterogeneous the basic equations used for the simulation of transport phenomena in the heterogeneous aquifer are nonlinear therefore simulation of thermohaline convection in such a stratum requires complicated mathematical models the present study considers the application of the aquifer s average characteristic method for such a simulation according to this method the model considers constant average characteristics of the aquifer instead of considering variable local characteristics of the saturated porous medium such an approach considerably simplifies the calculations the study verifies the applicability of the method for the predictions concerning diffusive thermohaline convection in a heterogeneous aquifer on this basis the study presents an analysis of important features of dispersive thermohaline convection in a heterogeneous aquifer by applying the method of the aquifer s average characteristics

an osmium based method for assessing the source of dissolved rhenium and molybdenum to archean seawater abstract authigenic enrichments of re and mo in 2 7 2 5 ga organic rich mudrocks orm indicate that dissolved re and mo were present in late archean seawater however it can be difficult to determine if the dissolved re and mo were sourced from oxidative crustal weathering or magmatic hydrothermal extraterrestrial mhe inputs comparison of re mo and 192 os non radiogenic os isotope concentrations and initial 187 os 188 os isotope compositions from re os isochron regressions of orm can shed insight on the source of re and mo to archean seawater seawater 187 os 188 os reflects the relative contributions of radiogenic crustal versus unradiogenic mhe os inputs to seawater during the phanerozoic transient increases in magmatic hydrothermal activity marked by unradiogenic 187 os 188 os excursions and higher 192 os concentrations in orm are accompanied in some cases by higher re concentrations these unradiogenic os sources could thus have played a greater role in the archean marine re and possibly mo budget given that the riverine re and mo flux at that time was likely smaller than the phanerozoic flux hence a positive correlation between re mo and 192 os concentrations in archean orm that yields unradiogenic initial 187 os 188 os may reflect a mhe origin for dissolved re and mo in the overlying paleo seawater local oxidative weathering in the presence of photosynthetic o 2 of mafic ultramafic rocks and crustal sulfide minerals with low re os ratios is an alternative explanation but the two scenarios cannot be readily distinguished on the basis of initial 187 os 188 os by contrast an initial 187 os 188 os from archean orm that is statistically higher than the mhe baseline of 0 10 0 11 points to oxidative mobilization of re os and mo from crustal sulfide minerals rhenium and mo concentrations that are negatively correlated with 192 os concentrations in archean orm yielding unradiogenic initial 187 os 188 os suggests that some re and mo were derived from oxidative weathering such negative correlations may reflect an increased marine os sink adsorption to oxide minerals during a mild increase in local environmental o 2 levels these concepts may help trace the onset of photosynthetic o 2 production and accumulation along archean ocean margins

the osmium isotopic composition of the continental crust osmium isotopic compositions and concentrations have been determined for deltaic and continental shelf sediments from three major rivers the amazon changjiang and mississippi and for two loesses from the upper mississippi river valley the loesses have heavy mineral and clay compositions characteristic of mississippi river and delta sediment concentrations range from 15 to 120 pg os g and 187os 186os ratios range from 8 2 to 10 9 sediments which are polluted surface sediment from the changjiang estuary or which have scavenged metals from seawater gulf of mexico slope surface sediment and distal amazon shelf sediment have high osmium concentrations and less radiogenic isotopic compositions purely terrigenous sediments have 15 90 pg os g and 187os 186os 10 0 10 9 north american loess and mississippi delta sediment osmium isotopic compositions are indistinguishable we believe that 187os 186os 10 11 is a good average for currently eroding upper continental crust these sediments have neodymium isotopic compositions close to the global mean for river sediment terrigenous 187os 186os is significantly more radiogenic than seawater 187os 186os 8 6 requiring a nonriverine source of osmium to seawater terrigenous osmium is also significantly more radiogenic than bulk pelagic clay osmium we use the improved estimate of terrigenous 187os 186os to recalculate the flux of extraterrestrial osmium to pacific marine sediment if the residence age of osmium in an upper crustal region is similar to the neodymium model age of a sediment derived from that region a model dependent upper crustal 187re 186os ratio can be calculated for the model ages of the sediments analyzed in this study tdm 1 5 1 7 ga model 187re 186os ratios are 340 360 and can be used to correct for bias resulting from the preferential erosion of young crust the 187os 186os ratio of 2 2 ga crust with 187re 186os 400 is 14 2 we have also used the data from this study to estimate upper crustal osmium and rhenium concentrations of 50 pg g and 390 pg g respectively

clues to the nature of the impacting bodies from platinum group elements rhenium and gold in borehole samples from the clearwater east crater canada and the boltysh impact crater ukraine seven large 10 g impact melt rock samples from boreholes from the boltysh impact crater ukraine and six samples from the east clearwater crater canada were analyzed for os ir ru rh pd re and au by the nickel sulfide technique in combination with neutron activation earlier analyses of clearwater east impact melt rocks have shown that they are strongly enriched in ir os pd and re in this work i confirm earlier findings and demonstrate similarly high enrichments of rh and ru the average os ir ru ir pd ir rh ir and ru rh ratios of the melt rock samples from clearwater east are ci chondritic and yield an average ir content of 25 2 6 5 ng g relative to an average upper crust concentration of 0 03 0 02 ng g ir the amount of meteoritic component corresponds to 4 to 7 of a nominal ci component for clearwater east the impact melt rock samples from a bore hole from boltysh are low in ir with an average of 0 2 0 1 ng g the ci normalized abundances increase from the refractory to the more volatile siderophile elements os ir ru rh pd au ni co because of the low ir anomaly and uncertainties in making corrections correlations are weak for indigenous siderophile elements no clear projectile assignment can be made

partial melt topology in statically and dynamically recrystallized granite microstructures of the murray granite pluton central ontario canada show evidence of both static and dynamic crystallization subsequent to partial melting backscattered electron analyses reveal interstitial k feldspar and plagioclase at triple junctions of strain free isometric quartz grains the geometry of the quartz feldspar boundaries mimics the original topology of the quartz melt contacts during crystallization this conclusion is suggested by the occurrence of both rounded and planar faces of quartz grains and by low 27 dihedral angles of quartz quartz feldspar boundaries similar to dihedral angles in experimentally crystallized quartz quartz silicic melt systems in contrast feldspar seams in deformed granites have high axial ratios are usually elongated perpendicular to the foliation plane and are located preferentially along individual grain boundaries quartz grains are dynamically recrystallized and occasionally transected by feldspar seams indicating that fracturing occurred in the presence of melt during crystal plastic deformation of quartz the subparallel orientation in quartz grains of intragranular feldspar bearing fractures and interstitial feldspar seams suggests that these features originated as intragranular and intergranular fractures respectively partial melt topology was therefore controlled by intergranular and occasionally by intragranular fracturing

thermal metamorphic signature of an impact event in the vredefort dome south africa pseudotachylitic breccias and shock deformation features related to the 2 02 ga formation of the vredefort dome by meteorite impact were overprinted by a static metamorphic event the intensity of which decreased from granulite facies t 700 c in the center of the dome to greenschist facies t 400 c around its margins geobarometric estimates of 0 2 0 3 gpa for the metamorphic parageneses indicate some 8 11 km of erosion since the impact event the strong lateral thermal gradient implied by these p t results is attributed to the combined effects of differential uplift of mid crustal rocks heated along a pre impact geotherm and increased shock heating of the target crust toward the center of the impact structure we suggest that the exceptionally high grade of metamorphism in the center of the dome may in part reflect an elevated regional geothermal gradient of 25 c km in the target crust due to lingering thermal effects related to the 2 05 2 06 ga bushveld magmatic event

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

metamorphism on the moon a terrestrial analogue in the vredefort dome south africa a new model is proposed to explain the origin of enigmatic fine grained granulite facies rocks sampled from the moon based on observations from the vredefort dome south africa the dome is the deeply eroded central uplift of the 300 km diameter vre defort impact structure in the dome fine grained granulites dis playing poikilitic or granoblastic microstructures were formed by relatively slow cooling of shock 6 friction melts derived at t 1350 8c slow cooling was achieved owing to the 7 km depth of burial of the rocks following the impact at least some of the lunar granulitic impactites may also have formed by shock heating and slow cooling at deep levels within the central uplifts of large impact structures without the need for additional heating by younger in trusive or impact melt bodies

petrographic and geochemical evidence for an allochthonous possibly impact melt origin of pseudotachylite from the vredefort dome south africa abstract hypotheses proposed to explain the origin of pseudotachylite bodies formed during impact cratering include 1 frictional heating 2 shock loading 3 decompression or 4 drainage of impact melt into target rocks in order to differentiate among these processes we conducted detailed geochemical and petrographic analysis of the matrices in pseudotachylitic veins and dikes and of their respective wall rocks our analyses indicate that the chemical compositions of matrices locally deviate significantly from their immediate wall rocks and that assimilation of wall rock has substantially modified the pseudotachylite matrix compositions in places variable magnitudes of assimilation can be explained by the surface area of wall rock or its fragments in contact with melt as well as the initial temperature and cooling rate of the pseudotachylitic melt chemical trends observed can be explained either by admixture of an exotic melt component with immediate wall rock or by mixing of melts derived from local lithologies trends in the compositional deviation of centimetre to metre wide pseudotachylite dikes from their immediate wall rocks are consistent with the presence of a primary melt component having granitoid composition akin to the average composition of vredefort granophyre dikes within veins melt transport can be geochemically and petrographically traced for distances of centimetres to metres with the direction of melt transport from larger pseudotachylite veins toward smaller ones and into apophyses sulphide and silicate mineralogy indicates that the initial temperature of pseudotachylitic melt must have been at least 1200 1700 c collectively these characteristics point to an allochthonous origin of pseudotachylitic melt we advocate the possibility that impact melt from the initially superheated impact melt sheet contributed to the formation of pseudotachylite bodies at vredefort

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

role of kilometer scale weak circular heterogeneities on upper crustal deformation patterns evidence from scaled analogue modeling and the sudbury basin canada the sudbury basin is a non cylindrical fold basin occupying the central portion of the sudbury impact structure the impact structure lends itself excellently to explore the structural evolution of continental crust containing a circular region of long term weakness in a series of scaled analogue experiments various model crustal configurations were shortened horizontally at a constant rate in mechanically weakened crust model basins formed that mimic several first order structural characteristics of the sudbury basin 1 asymmetric non cylindrical folding of the basin 2 structures indicating concentric shortening around lateral basin termini and 3 the presence of a zone of strain concentration near the hinge zones of model basins geometrically and kinematically this zone corresponds to the south range shear zone of the sudbury basin according to our experiments this shear zone is a direct mechanical consequence of basin formation rather than the result of thrusting following folding overall the models highlight the structurally anomalous character of the sudbury basin within the paleoproterozoic eastern penokean orogen in particular our models suggest that the basin formed by pure shear thickening of crust whereas transpressive deformation prevailed elsewhere in the orogen the model basin is deformed by thickening and non cylindrical synformal buckling while conjugate transpressive shear zones propagated away from its lateral tips this is consistent with pure shear deformation of a weak circular inclusion in a strong matrix the models suggest that the sudbury basin formed as a consequence of long term weakening of the upper crust by meteorite impact

estimation of continental 187os 186os values by using 187os 186os and 143nd 144nd ratios in marine manganese nodules abstract the relationship between 187os 186os and 143nd 144nd in different manganese nodule fields is used to determine the 187os 186os ratio of the continental terrains bounding the major ocean basins the atlantic ocean drainages yield 187os 186os of about 11 the pacific ocean between 25 and 36 and the western indian ocean 20 by assuming a two component continental crust composed of ultramafic rocks high os concentration low 187os 186os and granite with only radiogenic 187os produced in accessory re bearing molybdenite the ultramafic contribution to weathering is about 0 2 some or most of this may come from the alteration of oceanic ultramafics

crustal modelling of the ivrea verbano zone in northern italy re examined coseismic cataclasis versus extensional shear zones and sideways rotation abstract new field and microstructural observations in the ivrea verbano zone ivz northern italy reveal extensive post hercynian cataclasis indentation structures microtectonic escape structures microcataclastic injectites and occasional true pseudotachylytes the deformation occurred in situ and was governed by pure shear these observations are incompatible with prevailing crustal models of the ivz most of these models assume the presence of lower crustal originally flat lying extensional shear zones sideways block rotation and exposure of the petrological moho although the original structural observations from the central ivz suggested that the post hercynian deformation was cataclastic rather than shear dominated other already published information also contradicts the sideways rotation of a crustal block comprising the ivz and the adjacent serie dei laghi sdl after the early permian as assumed in the models the ivz and sdl have different detrital zircon populations and are separated by a steep shear zone and a transcurrent fault not rotated listric faults as assumed in the models rotation is also prevented by a flat lying zone of miarolitic cavities in the early permian baveno granite and by contemporaneous vertical basic dykes the widespread in situ cataclasis is interpreted as due to coseismic deformation by high frequency seismic waves generated by earthquakes along the insubric fault and exerting oscillatory compression dilation and torsion in a 10 km wide zone along the footwall of the insubric line the in situ post hercynian cataclasis absence of lower crustal extensional shear zones and lack of sideways rotation of the ivz and sdl call for a new and simpler crustal model of the region

the theoretical plausibility of central pit crater formation via melt drainage central pit craters are seen in large craters on some icy satellites and on mars we investigate the hypothesis that central pits form when impact melt drains into fractures beneath the impact crater for this process to occur the volume of melt generated during the impact the volume of void space in fractures beneath the impact crater and the volume of melt able to drain before the fractures freeze shut all must exceed the volume of the observed central pits we estimate the volume of melt generated using results from previous numerical modeling studies the fracture volume is estimated using gravity anomalies over terrestrial craters to estimate the amount of melt able to drain before freezing we consider flow through plane parallel fractures these calculations all suggest that enough liquid water could drain into fractured ice beneath a crater on ganymede to form a central pit on earth and the moon silicate impact melt will freeze before a large volume is able to drain so we do not expect to see central pits in impact craters in targets with no ice in summary we find our calculations are consistent with observed central pits in craters on ganymede and the lack of central pits in craters on earth and the moon

emplacement history of granophyre dikes in the vredefort impact structure south africa inferred from geochemical evidence abstract the central vredefort impact structure is characterised by impact melt rocks known as the vredefort granophyre dikes the mode of emplacement of which is not well known whole rock and petrographic analyses of two dikes were conducted and compared to published geochemical data to elucidate the mode and timing of dike formation the dikes are characterised by compositional and textural heterogeneity between and within individual dikes specifically central dike portions are felsic and rich in wall rock fragments whereas marginal dike phases are more mafic and fragment poor collectively this suggests that melt was derived from compositionally different parental melts and emplaced in at least two pulses in addition the chemical heterogeneity between fragment rich and fragment poor dike zones can be explained by variable assimilation of a mafic component notably ventersdorp basalt at the base of the impact melt sheet from which melt of the granophyre dikes is derived this scenario accounts for the mafic and fragment poor character of melt emplaced first in the dikes and the more felsic and fragment rich nature of melts of the following emplacement pulse i e when the impact melt was less hot and thus less capable of digesting large quantities of mafic wall rock fragments differences in geometrical textural chemical and fragment characteristics between the granophyre dikes and pseudotachylite bodies can be explained by the same process i e impact melt drainage but operating at different times after impact

187os 186os in marine manganese nodules and the constraints on the crustal geochemistries of rhenium and osmium during their formation marine manganese or more properly ferromanganese oxide nodules include and commonly concentrate elements dissolved in seawater this attribute makes them especially valuable recorders of the isotopic composition of elements supplied to the oceans having radiogenic isotopes for these can be used in exploring crustal composition and evolution1 5 from measurements of 187os 186os ratios in marine manganese nodules and comparison with the 143nd 144nd ratios of the same nodules we have calculated the os isotopic composition and the osmium and rhenium concentrations of the weatherable portion of the continental crust together with the proportions of ultra mafic and granitic rock that constitute this crustal mass