cratered cobbles in triassic buntsandstein conglomerates in northeastern spain an indicator of shock deformation in the vicinity of large impacts quartzite cobbles in lower triassic buntsandstein conglomerates from northeastern spain display unusual millimeter to centimeter sized circular craters commonly having central mounds and surrounded by radial fractures the conglomerates are also marked by intense fracturing down to microscopic scale these features have traditionally been attributed to tectonic compression and pressure dissolution at cobble contacts sections through the cratered cobbles reveal pervasive internal fracturing segments detached along concave spall fractures and zones marked by quartz grains with planar deformation features comparison with results of impact experiments on artificial conglomerates suggests that these features were produced by internal accelerations grain collisions and spallation related to shock wave propagation through inhomogeneous deposits the proximity of the outcrops to the azuara and proposed rubielos de la cerida impact structures suggests that shock deformation of conglomerates can provide an easily recognizable regional impact signature

40ar 39ar dating of the manson impact structure a cretaceous tertiary boundary crater candidate the mineralogy of shocked mineral and lithic grains in the cretaceous tertiary k t boundary claystone worldwide is most consistent with a bolide impact on a continent both the concentrations and sizes of these shocked grains are greatest in the western interior of north america these data suggest that the manson impact structure in north central iowa is a viable candidate for the k t boundary impact event argon 40 argon 39 age spectrum dating of shocked microcline from the crystalline central uplift of the manson impact structure indicates that there was severe argon 40 loss at 65 7 1 0 million years ago an age that is indistinguishable from that of the k t boundary within the limits of analytical precision

shock induced microdeformations in quartz and other mineralogical indications of an impact event at the cretaceous tertiary boundary abstract the event terminating the cretaceous period and the mesozoic era caused massive extinctions of flora and fauna worldwide theories of the nature of this event can be classed as endogenic volcanic climatic etc or exogenic extraterrestrial causes mineralogical evidence from the boundary clays and claystones strongly favor the impact of an extraterrestrial body as the cause of this event nonmarine k t boundary claystones are comprised of two separate layers an upper layer composed of high angle ejecta material shocked quartz altered glass and spinel and a basal kaolinitic layer containing spherules clasts and altered glass together with some shocked grains recognition of this dual layered nature of the boundary clay is important for the determination of the timing and processes involved in the impact event and in the assignment and interpretation of geochemical signatures multiple sets of shock induced microdeformations planar features in quartz grains separated from k t boundary clays provide compelling evidence of an impact event this mineralogical manifestation of shock metamorphism is associated worldwide with a large positive anomaly of iridium in these boundary clays which has also been considered indicative of the impact of a large extraterrestrial body global distributions of maximum sizes of shocked quartz grains from the boundary clays and the mineralogy of the ejecta components favor an impact on or near the north american continent spinel crystals magnesioferrite occur in the boundary clays as micrometer sized octahedra or skeletal forms their composition differs from that of spinels found in terrestrial oceanic basalts magnesioferrite crystals are restricted to the high angle ejecta layer of the boundary clays and their small size and skeletal morphology suggest that they are condensation products of a vaporized bolide hollow spherules ranging up to 1 mm in size are ubiquitously associated with the boundary clays in nonmarine sections where a high angle ejecta layer and an underlying kaolinitic layer can be distinguished the spherules are found only in the kaolinitic layer the morphologies and surface features of these spherules suggest that they are original forms and not secondary growths or algal bodies these impact spherules closely resemble microtektites in size and shape all of these features of the boundary clay are uniquely associated with impact and cannot have been formed by volcanic or other terrestrial processes

the stratigraphic distribution of ni rich spinels in cretaceous tertiary boundary rocks at el kef tunisia caravaca spain and hole 761c leg 122 abstract ni rich spinels have been found throughhout the world at the cretaceous tertiary boundary these minerals have no counterparts in terrestrial rocks but have been observed in meteoroid ablation material and impact debris suggesting that the cretaceous tertiary boundary spinels result from an extraterrestrial event their stratigraphic distribution which has been observed at the cretaceous tertiary boundary sites of el kef tunisia caravaca spain and hole 761c odp leg 122 indian ocean are markedly different from site to site and are all much less extended than the ir anomalies regional differences in ni rich spinel distibution are entirely explained by a brief event recorded under different bioturbation conditions the wider stratigraphic dispersion of ir is attributed to postdepositional chemical processes at el kef the site which appears to be the least affected by bioturbation and which has the highest sedimentation rate the 1 2 mm thick ni rich spinel spike indicates that the deposition time did not exceed an upper limit of 100 years this upper limit is consistent only with the collision of an asteroid or a comet at the close of the cretaceous

the genesis of mid ocean ridge basalt the tholeiitic volcanics erupted at mid ocean ridges mid ocean ridge basalts or morb constitute the dominant volcanic lithology on earth analyses of tachylites from atlantic pacific and indian ocean spreading centres range widely in 100 mg mg fe2 ratios m and m varies from 70 to 30 glasses with m 55 65 are the most common variants and only a small percentage of glass analyses has m approaching 70 the latter defines the m value of basaltic melts in equilibrium with residual upper mantle source peridotites with m 88 the frequency histogram of the m values of average compositions of morb glasses at 88 ocean floor localities is similar in analysis distribution to the frequency histograms depicting variation in the m values of glasses from the various spreading centres m values and nickel contents of morb and the nature and compositions of the near liquidus phases crystallized experimentally from morb melts at elevated pressures have been applied to identify primary unfractionated melts erupted in a mid ocean ridge environment however ni abundances and high pressure phase relationships are not necessarily unique or definitive parameters of primary melts the latter are generally linked genetically with mg rich lherzolitic source rocks of pyrolite type m 90 the spectrum of m values displayed by morb glasses with a definite bias towards relatively fe rich compositions average m of approximately 600 morb glasses is 58 6 suggests that the melts may have evolved either via ferromagnesian fractionation of relatively mg rich parental melts m 70 80 or by partial melting of a heterogeneous upper mantle with variable m values or as a result of magma mixing of already fractionated melts and primitive magma batches for a number of reasons fractonation models based on the extraction of olivine or one or more of olivine plagioclase and clinopyroxene either from picritic melts m 75 or primitive basaltic melts with m 70 are questionable as prime controls of morb chemistry these include 1 the extreme rarity of quenched picritic or mg basaltic melts in ocean ridge environments 2 the lack of adequate evidence of the appropriate of necessity voluminous complementary cumulates dunites allivalites troctolites anorthosites demanded by olivine plagioclase or olivine plagioclase fractionation models and 3 the aberrent frequencies of glass m values whereby the assumed derivatives m 55 65 are much more abundant and presumably much more voluminous than the alleged parents or transitional derivatives 65 m 75 the nature of the trends of na2o cao and al2o3 in galapagos spreading centre tachylites of extended composition m 65 30 indicates the gabbroic fractionation is also unlikely to exert important controls on morb chemistry as their m values increase mid ocean ridge basalts increase in al ca ni co cr and decrease in ti mn na k and p except for al and ca these trends are similar to those displayed by upper mantle peridotites increasing in m i e becoming more refractory following one or more partial melting episodes it is suggested that at least a majority of mid ocean ridge basalts is intrinsically primary and generated by variable degrees of partial melting of heterogeneous lherzolitic upper mantle 80 m 90 with variable abundances of elements such as ti al ca and na and also depleted in large ion lithophile lil elements negative europium anomalies in the rare earth patterns of some oceanridge basalts ferrobasalts with low m are ascribed mainly to the persistence of residual plagioclase in relatively fe rich plagioclase lherzolite source rocks following low degrees of partial melting the partial melting events leading to the generation of mid ocean ridge basalts took place over a relatively modes pressure range approximately 8 15 kb which encompassed the transition of plagioclase lherzolite to spinel lherzolite this proposal appears consistent with the nature and occurrence of megacrysts xenocrysts of tschermakitic cr diopside ca43mg52fe5 olivine mg 89 91 plagioclase an92 85 and spinel fe2al60cr38 in some morb the megacryst compositions suggest that these phases represent disaggregated plagioclase peridotite or spinel lherzolite acquired by melts during their passage through the oceanic upper mantle

direct rate measurements of eruption plumes at augustine volcano a problem of scaling and uncontrolled variables the march april 1986 eruption of augustine volcano alaska provided an opportunity to directly measure the flux of gas aerosol and ash particles during explosive eruption most previous direct measurements of volcanic emission rates are on plumes from fuming volcanoes or on very small eruption clouds direct measurements during explosive activity are needed to understand the scale relationships between passive degassing or small eruption plumes and highly explosive events conditions on april 3 1986 were ideal high winds clear visibility moderate activity three measurements were made 1 an airborne correlation spectrometer cospec provided mass flux rates of so2 2 treated filter samples chemically characterized the plume and 3 a quartz crystal microcascade impactor provided particle size distribution atmospheric conditions on april 3 caused the development of a lee wave plume which allowed us to constrain a model of plume dispersion leading to a forecast map of concentrations of so2 at greater distances from the vent on april 3 1986 the emission rate of so2 at augustine was 24 000 t d one of the largest direct volcanic rate measurements yet recorded with a cospec the results coupled with analytical results from samples simultaneously collected on filters allow us to estimate hcl emissions at 10 000 t d and ash eruption rate at 1 5 106 t d based on other data this ash eruption rate is about 1 50 of the maximum rate during the march april activity filter samples show that the gas aerosol proportions for sulfur and chlorine are about 10 1 and 4 1 respectively by contrast measurements of augustine s plume together with ground based gas sampling in july 1986 when the volcano was in a posteruptive fuming state are 380 t d so2 and approximately 8000 t d hcl with no ash emission the observations of large cl releases at augustine support the cl abundance conclusions of johnston 1980 based on study of melt inclusions in the 1976 lavas the results reinforce the need for more measurements during eruptions and for better understanding of scaling of volcanic emissions of various eruptive components

a model of correlated episodicity in magnetic field reversals climate and mass extinctions correlated periodicities of mass extinctions climate magnetic field reversals and other geological phenomena may best be explained by cycles of activity within the mantle and core we propose that these cycles are due to variations in the thickness of the thermal boundary layer at the base of the mantle as the layer alternates between two phases of activity in the quiescent phase little flow occurs and the layer thickens with time by thermal diffusion the active phase begins when the thickening layer becomes dynamically unstable hot material erupts from the layer causing it to become thinner as the layer thins the rate of energy supply to the geodynamo and hence its reversal frequency increases the hot material rises to the surface where it causes widespread basaltic volcanic eruptions these eruptions release large amounts of co 2 and sulfates which have a pronounced effect on the climate and biota

primary and secondary fragmentation of crystal bearing intermediate magma abstract crystal rich intermediate magmas are subjected to both primary and secondary fragmentation processes each of which may produce texturally distinct tephra of particular interest for volcanic hazards is the extent to which each process contributes ash to volcanic plumes one way to address this question is by fragmenting pyroclasts under controlled conditions we fragmented pumice samples from soufriere hills volcano shv montserrat by three methods rapid decompression in a shock tube like apparatus impact by a falling piston and milling in a ball mill grain size distributions of the products reveal that all three mechanisms produce fractal breakage patterns and that the fractal dimension increases from a minimum of 2 1 for decompression fragmentation primary fragmentation to a maximum of 2 7 by repeated impact secondary fragmentation to assess the details of the fragmentation process we quantified the shape texture and components of constituent ash particles ash shape analysis shows that the axial ratio increases during milling and that particle convexity increases with repeated impacts we also quantify the extent to which the matrix is separated from the crystals which shows that secondary processes efficiently remove adhering matrix from crystals particularly during milling abrasion furthermore measurements of crystal size distributions before using x ray computed tomography and after by componentry of individual grain size classes decompression driven fragmentation show not only that crystals influence particular size fractions across the total grain size distribution but also that free crystals are smaller in the fragmented material than in the original pumice clast taken together our results confirm previous work showing both the control of initial texture on the primary fragmentation process and the contributions of secondary processes to ash formation critically however our extension of previous analyses to characterisation of shape texture and componentry provides new analytical tools that can be used to assess contributions of secondary processes to ash deposits of uncertain or mixed origin we illustrate this application with examples from shv deposits

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

geochemistry of tholeiites from lanai hawaii lanai is the third smallest of the fifteen principal subaerial shield volcanoes of the hawaiian hotspot this volcano apparently became extinct during the shield building stage of volcanism as shown by the absence of both alkalic cap and post erosional lavas major and trace element analyses of 22 new samples collected primarily from 3 stratigraphic sections show that lanai tholeiites span a large range in composition some lanai lavas are unique geochemically among hawaiian tholeiites in having the lowest abundances of incompatible trace elements of any hawaiian lavas and well developed positive eu anomalies the geochemical characteristics of these low abundance lanai tholeiites are not the result of alteration differences in mantle source modal mineralogy the presence of residual accessory mantle phases or fractional crystallization of such phases assimilation of depleted morb wall rock or accumulation resorption of phenocrysts or xenocrysts incompatible trace element ratios e g nb la nb th la th la hf ce pb in lanai tholeiites span considerable ranges and form coherent trends with each other and with absolute abundances of these elements large variations in la sm la yb and absolute ree abundances at constant mgo suggest that lanai tholeiites formed by variable amounts of partial melting however large ranges in incompatible element ratios cannot be explained solely by variations in partial melting of a geochemically homogeneous source but must reflect geochemical heterogeneities in the lanai source partial melting modeling indicates that the mixed lanai source is probably lree enriched i e la yb cn 1 one component in the lanai source exemplified by the low abundance tholeiites has markedly lower ree hfse th hfse alkali hfse and ce pb ratios than other lanai or hawaiian tholeiites and may indicate the presence of recycled residual subduction zone materials in the hawaiian plume source the positive eu anomalies that characterize the low abundance lanai tholeiites are not the result of plagioclase accumulation or assimilation but are a feature of this source component progressive temporal geochemical variations in lanai tholeiites from 2 stratigraphic sections indicate that the source composition of these lavas probably evolved over time this change could have resulted from a progressive decrease in the extent of partial melting of the lanai source the compositional variability of lanai tholeiites suggests that geochemical heterogeneities in their source are larger than the scale of partial melting lanai tholeiites could not have formed by smaller degrees of partial melting of plume material than did the larger volume hawaiian shields therefore volume differences between hawaiian shields must be controlled primarily by differences in the volume of supplied plume material rather than by differences in the degree of partial melting the premature cessation of eruptive activity at lanai may be attributed to relatively large degrees of partial melting of a small plume

experimental constraints on shock induced microstructures in naturally deformed silicates abstract planar deformation features pdfs in various minerals have long been accepted as evidence of impact induced deformation the uniqueness of this association was challenged in the context of the k t boundary extinction debate after mosaicism and microstructures similar to pdfs were reported from the products of explosive volcanism as a result of this debate a significant volume of new experimental and observational data on the development of shock induced microstructures has become available over the last ten years the results reveal that factors such as pre shock temperature pulse duration and crystallographic orientation of target minerals to the shock wave have a primary influence on how these microstructures develop data from diamond anvil cell and high pressure friction experiments reveal that the same solid state amorphization process that produces shock induced pdfs at low temperatures also occurs at much lower strain rates in static experiments the experimental data indicate that the amorphization process is thermally activated and that the character of the resulting pdfs is a function of the applied strain rate shock induced amorphization occurs along those crystallographic planes that are most readily transformed to the high pressure phase during very short pulse durations and produces pdfs that are visible at the optical scale lower strain rate deformation produces tem scale amorphization with orientations that are more homogeneously distributed throughout the target mineral and produces no optically visible pdfs the data confirm the uniqueness of multiple intersecting sets of optically visible pdfs as a diagnostic indicator of hypervelocity impact the data also support the hypothesis that the amorphization process can occur at a wide range of strain rates and that the limiting pressure for the process is controlled by the phase stability of the target mineral under the applied loading conditions not by the hel the data also suggest that the onset pressure the maximum pressure and the pressure range for producing optically visible pdfs decrease with increasing temperature and decreasing strain rate as the pressure range for optically visible pdf formation decreases it is replaced by homogeneous amorphization as observed in the anvil cell experiments thus the uniqueness of multiple intersecting sets of optically visible pdfs to hypervelocity impact is not due to a unique process but rather to a specific set of loading conditions that produce an optically visible microstructure likewise single sets of microdeformations observed in volcanic rocks are produced by the same process but at different loading conditions that preclude the development of multiple intersecting sets of pdfs the data also indicate that shock mosaicism which occurs above the hel represents a plastic response of the target mineral to loading rates that are too large to be accommodated by crystal plastic mechanisms observational data for some naturally deformed samples from the k t boundary the vredefort dome and volcanic rocks along with the experimental observations are used to constrain the range of conditions under which the natural microstructures form and to understand the differences between microstructures produced by impact volcanic and other natural processes finally some possible mechanisms for producing the microstructures observed in volcanic rocks are proposed

undepleted mantle composition beneath hawaii compositions of undepleted mauna loa mantle have been calculated assuming derivative residuum relations between a tholeiitic picrite from the 1852 eruption of mauna loa and mantle residua whose compositions are delineated by experimental data and the compositions of the most refractory picrite phases namely mg rich olivine and cr rich spinel the preferred calculated composition based on 40 picrite 60 residuum composed of olivine orthopyroxene and cr spinel has a lower 100 mg mg fe ratio 84 1 than the ratios of pyrolite like undepleted mantle compositions 88 90 and compared with many such compositions it also has significantly higher abundances of tio2 al2o3 cao na2o k2o and p2o5 the higher abundances of incompatibles have implications for the volumes and character of potential derivative melts a calculated mode which assumes that a titaniferous pargasitic amphibole is a component of the undepleted lherzolite indicates that amphibole could be a significant primary phase at temperatures slightly above the lherzolite solidus amphibole would be eliminated on incongruent melting and its breakdown products mainly clinopyroxene plus minor olivine would become part of a re equilibrated anhydrous peridotitic mineral assemblage the lower 100 mg mg fe ratio of the calculated composition would permit the generation of primary basaltic melts with relatively low 100 mg mg fe2 ratios at lower 40 degrees of melting such lower ratios are usually attributed to olivine fractionation of more mg rich primary melts from pyrolite like mantle sources

the azuara impact structure spain new insights from geophysical and geological investigations a progress report on geophysical and geological investigations in the 35 40 km diameter azuara probable impact structure northeast spain is given the target is a 10 km thick sequence of palaeozoic mesozoic and cenozoic sediments gravity measurements establish a negative residual anomaly of about 100 m s2 and a mass deficiency of 1 24 1014 kg they suggest the existence of a buried inner ring with half the size of the outer ring the result of preliminary model calculations is compatible with the assumption of a flat structure only a few kilometers deep measurements of the total intensity of the earth s magnetic field do not indicate causative bodies related to the structure geological mapping reveals a tectonic style which is characterized by folds and faults with radial and tangential elements and a strong horizontal component ejecta abundant monomict and polymict breccias dislocated megablocks inverted stratigraphy and a megabreccia up to 80 m thick in the outer ring are evidence for intense and violent deformation networks of typical breccia dikes well known from many impact structure cut through nearly all stratigraphical units various types and generations of breccia dikes occur in addition to previously described shock deformation in quartz and mica we present further evidence of high pressure and high temperature signatures in rocks the age of the impact is estimated to be upper eocene or oligocene we conclude that the formation of the azuara structure is difficult to explain by mechanisms other than impact and we discuss our observations within the scope of the contact compression excavation and modification stages of impact cratering with special attention directed to the peculiarities of the azuara sedimentary target

chemostratigraphy across the cretaceous tertiary boundary and a critical assessment of the iridium anomaly the elevated concentration of iridium one of the platinum group elements pge at the cretaceous tertiary boundary is still the most generally accepted evidence that a large bolide struck the earth at the time of the end cretaceous mass extinctions new chemostratigraphic data for cross boundary sections from both hemispheres are not easily explained in terms of such an impact event for example the observation that the pge patterns show marked differences between the hemispheres the new constraints indicate that models of mantle derived pge should be seriously considered and that pge anomalies might not be as useful as previously thought as unambiguous identifiers of large impact events in the earth s history

mean inversion strength of the convective boundary layer over the oceans an equilibrium energy budget for the oceanic convective boundary layer shows that the mean inversion strength in balance with a radiatively driven subsidence is typically 9 k in agreement with the widely observed value

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

estimation of the degree of partial melting by na2o k2o and al2o3sio2 of basic magmas abstract equations for estimating the degree of partial melting f of basic magmas in terms of na2o k2o and al 2 o 3 sio 2 and based on previously published batch equilibrium melting and experimental and petrochemical data are proposed as follows for f 0 10 0 50 f a 1 0 957 na 2 o k 2 o s na 2 o k 2 o m 0 0149 for f f b 1 0 694 na 2 o k 2 o s na 2 o k 2 o m 0 0516 for f 0 15 0 35 f c 0 9348 al 2 o 3 sio 2 s al 2 o sio 2 m 0 1532 0 108 al 2 o 3 sio 2 s al 2 o sio 2 m 0 597 for f f d 1 2437 al 2 o 3 sio 2 s al 2 o sio 2 m 0 2470 3 308 al 2 o 3 sio 2 s al 2 o sio 2 m 0 237 the subscripts s and m denote source and supposedly primary magma or melt respectively a correct estimate or choice of source composition and composition of primary magma for the calculation is important if both compositions can be reasonably assigned or estimated then the degree of partial melting can be calculated from one of these equations with errors generally

dynamic deformation of quartz and feldspar clues to causes of some natural crises abstract indications of natural catastrophes and crises are preserved in some geologic strutures and in the stratigraphic record emphasizing the dynamic nature of the earth s evolution the crises may result either from exogenic or endogenic energy sources distinguishing between these sources is a complex multidisciplinary problem in the earth sciences shock deformation by impact has been well studied both in shock recovery experiments and in known impactites but only recently has the possibility of generation of shock waves by high temperature internal explosions been seriously considered an optically based reconaissance survey of microstructures in quartz and feldspar in some explosive silicic volcanics suggests that they may have been induced by dynamic deformation at high temperature in some respects the character of the microstructures differs greatly from those induced by room temperature shock whereas in others the differences are subtle results from recent shock recovery experiments on quartzite preheated to 440 and shocked to 28 gpa support this contention for two classic cretaceous tertiary sections at gubbio italy it is shown that shock mosaicism and shock lamellae occur over a 4 m interval bisected by the boundary in a pattern approximately correlatable with an independently established iridium enrichment these data are incompatible with any single event and are most plausibly interpreted as reflecting a period 0 5 my of intense probably episodic volcanism with a culmination at the k t transition multiple dynamic events may also account for macroscopic and microscopic structures developed during the evolution of the vredefort structure south africa

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

the atmospheric response to weak sea surface temperature fronts abstractthe response of the atmospheric boundary layer to fronts of sea surface temperature sst is characterized by correlations between wind stress divergence and the downwind component of the sst gradient and between the wind stress curl and the crosswind component of the sst gradient the associated regression or coupling coefficients for the wind stress divergence are consistently larger than those for the wind stress curl to explore the underlying physics the authors introduce a linearized model of the atmospheric boundary layer response to sst induced modulations of boundary layer hydrostatic pressure and vertical mixing in the presence of advection by a background ekman spiral model solutions are a strong function of the sst scale and background advection and recover observed characteristics the coupling coefficients for wind stress divergence and curl are governed by distinct physics wind stress divergence results from either large scale winds crossing the front or from a thermally direct

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

mineralogy and petrology of the haiti cretaceous tertiary section abstract the cretaceous tertiary sections in haiti consist of a sequence of graded beds of turbidity current or gravity flow origin each section may consist variously of five major constituents 1 black glass 2 brown globules veinlets and blobs 3 argillaceous limestone clasts 4 white clasts and shards and 5 a fine grained matrix rich in smectite zeolites and calcite in addition there are trace amounts of a high calcium yellow glass the black glass particles are vesicular apparently unaltered with an andesitic to dacitic composition and in sharp contact with the secondary matrix and veinlet alteration products the brown globules veinlets and blobs consist of a palagonite smectite mix of a distinctly different composition than the black glasses white clasts and shards of a still different more hydrated composition are also present all of these materials are sometimes associated with accretionary lapilli the hydration or alteration products brown globules white clasts and shards and matrix amount to something in excess of 95 of the total deposits the sedimentology mineralogy and petrology of the haiti sections has provided a record of a sequence of probable volcanic events

the terrestrial impact cratering record approximately 130 terrestrial hypervelocity impact craters are currently known due to variations in preservation and in geologic knowledge this sample is biased towards young x 20 km craters on the cratons of australia europe including the former u s s r and north america the rate of discovery of new craters is 3 5 craters per year although modified by erosion terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies such as the moon terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters which can be used to constrain interpretations of lunar samples and models of crater formation due to erosion and its effects on form terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects these include shatter cones microscopic planar deformation features solid state and fusion glasses high pressure polymorphs and whole rock melting and vaporization shock recovery experiments indicate that these features occur over shock pressures of 5 gpa to x 100 gpa terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact induced fracturing they include gravity and magnetic lows and reductions in seismic velocity the gravity anomalies are seldom greater than 30 mgal due to the limiting effects of lithostatic pressure on fracturing at large complex craters the gravity signature may include a central relative gravity high due to uplift and short wavelength central magnetic anomalies due to a variety of processes much current work is focused on the effects of impact on earth evolution previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from cretaceous tertiary boundary sites as resulting from a major impact suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record by analogy with the lunar record and modelling of the effects of very large impacts it has been proposed that biological and atmospheric evolution of the earth could not stabilize before the end of the late heavy bombardment 3 8 ga ago the present terrestrial cratering rate is 5 4 2 7 10 15 km 2a 1 for a diameter 20 km this represents a local threat on historic time scales on a global scale a major impact sufficient to cripple human civilization severely will occur on time scales of 106 a

evidence of an impact origin for the azuara structure spain some 50 km south of zaragoza northeast spain there is a tectonically peculiar area which is suspected to be a large impact site it has a morphologically conspicuous ring structure with a diameter of roughly 30 km and consists mainly of mesozoic sediments emerging from the ebro tertiary basin and partly of palaeozoic rocks from the iberian system the strata of the ring in general dip to the center and are highly folded and faulted intense and unusual deformation is indicated by abundant mixed and monomict breccias a single poorly exposed outcrop of a mixed breccia with sedimentary fragments exhibits shock metamorphic effects within quartz grains systems of crystallographically oriented microscopic planar features and planar fractures can be observed as well as kink bands in micas within the breccia matrix the age of the impact is estimated to be between lower cretaceous and miocene

bunte breccia of the ries continuous deposits of large impact craters the ries crater an impact structure of 26 km diameter in south germany is the largest terrestrial crater where substantial amounts of ejecta are preserved on occasion 100 m deep further the target stratigraphy is well known and it is possible to relate specific clasts and breccia lithologies to initial target depth as a consequence the continuous deposits of the ries also known as bunte breccia may be studied with exceptional field control we report field observations and laboratory analyses obtained from 560 m core materials taken at nine different locations that range from 16 to 37 km in radial distance from the impact center the objective is to relate the ries observations to ejection and to emplacement processes of large scale planetary crater deposits the observations regarding the modal stratigraphic characteristics of the bunte breccia may be summarized as follows only 600 m some 0 7 is composed of triassic clasts originating from 300 to 600 m depths lower and middle jurassic horizons approximately 300 150 m constitute some 2 3 and upper jurassic 0 150 m makes up some 31 5 in addition the bunte breccia contains tertiary materials in the form of 1 cm clasts 29 1 and as highly comminuted fine grained matrix 10 gpa constitute 0 1 weight of the entire deposit which indicates that bunte breccia was emplaced at essentially ambient temperatures when possible the above observations are quantified via linear regressions throughout the text all of these observations are consistent with if not predicted by a ballistic emplacement scenario as postulated by oberbeck and co workers primary crater ejecta are expelled ballistically and will form secondary craters in the local substrate a mixture of primary and secondary ejecta results and combines into a highly turbulent ground hugging debris surge as the final phase of ejecta emplacement total emplacement time for the bunte breccia 200 km is estimated to be of the order of 5 min only these findings are compared with cratering theory relating to a number of ejecta thickness decay models and with the so called z model addressing material flow during various stages of crater formation qualitative to fair agreement of observations and predictions results an initial crater radius of 6 5 km an excavation depth of 1650 m an excavation volume of 136 km and an associated transient cavity volume of aproximately 230 km appear to be reasonable estimates approximately 170 km of material was involved in slumping and restoration of the transient cavity for the above radius and z 2 7 the modal composition of ries ejecta with regard to preimpact target stratigraphy indicates that materials contained in the continuous deposits of large complex planetary craters are predominantly derived from depths as small as one hundredth the apparent crater diameter a number of implications are addressed regarding remote sensing of planetary surfaces and investigation of lunar and meteoritic impact breccias

mass extinctions the view of a sceptic the concept of periodic mass extinctions by extraterrestrial impacts has contributed very strongly to the current revival of interest in the problem of mass extinctions in this context several radical claims have been made asserting that the newly emerging picture of mass extinctions as a separate class of biological phenomena caused by a separate class of processes demands a far reaching revision of the the neodarwinian paradigm of evolution and implies a vindication of the cuvierian perspective on earth9s history the validity of these claims however is questionable at all levels from the interpretation of particular events to the concept of mass extinctions as a special class of phenomena to their proposed causation by impacts and to their alleged historical pedigree and implications for current evolutionary theory mass extinctions may actually be clusters of more or less accidentally aggregated in time but causally distinct extinction episodes

terrestrial impact structures impact structures are the dominant landform on planets that have retained portions of their earliest crust the present surface of the earth however has comparatively few recognized impact structures this is due to its relative youthfulness and the dynamic nature of the terrestrial geosphere both of which serve to obscure and remove the impact record although not generally viewed as an important terrestrial as opposed to planetary geologic process the role of impact in earth evolution is now receiving mounting consideration for example large scale impact events may have been responsible for such phenomena as the formation of the earth s moon and certain mass extinctions in the biologic record the importance of the terrestrial impact record is greater than the relatively small number of known structures would indicate impact is a highly transient high energy event it is inherently difficult to study through experimentation because of the problem of scale in addition sophisticated finite element code calculations of impact cratering are gen erally limited to relatively early time phenomena as a result of high com putational costs terrestrial impact structures provide the only ground truth against which computational and experimental results can be com pared these structures provide information on aspects of the third dimen sion the preand postimpact distribution of target lithologies and the nature of the lithologic and mineralogic changes produced by the passage of a shock wave they also provide data for cratering rate estimates and in some cases on the nature of the impacting body in this review emphasis is placed on the nature of terrestrial impact structures the criteria for their identification and their contribution to

crater taphonomy and bombardment rates in the phanerozoic impact structures can be catalogued according to the age of the rocks in which they are now found country rocks the observed frequency distribution of craters by age of country rock is shown to be statistically indistinguishable from the predictions of a simple model in which it is assumed that the survival time for craters is the same as that for their target rocks other models are considered but do not match the data a lower limit of the rate of bombardment through the phanerozoic based only on documented craters is 0 13 pm 0 09 events ma 10 8 km 2 for craters with diameters geq 10 km and 0 09 pm 0 08 events ma 10 8 km 2 for craters with diameters geq 20 km the data allow but do not demand an increase in meteorite flux over the phanerozoic but do not allow any significant decrease we estimate that only about 6 of the existing terrestrial impact structures of diameter greater than 10 km have been discovered to date and only 16 of those with diameter greater th

cratered cobbles in triassic buntsandstein conglomerates in northeastern spain an indicator of shock deformation in the vicinity of large impacts comment and reply comment ernstson et al 2001 1 interpreted pitted and fractured quartzite cobbles of lower triassic conglomerates as new evidence in support of the existence of two large impact structures in the iberian chain azuara and rubielos de la cerida fig 1 2 in this discussion we disagree with their

shocked minerals at the k t boundary explosive volcanism as a source abstract azimuthal symmetry of p waves first motion up and diminished s wave amplitude indicates an explosion to be the source of the seismicity of the 18 may 1980 eruption at mt st helens explosion coupling efficiencies and scaling from underground mining excavation and nuclear explosions imply that peak pressures could have been as high as 1000 kbar which is more than sufficient for the formation of shocked minerals in addition high load rates as well as contamination may lower phase transition pressures a region of formation of shocked minerals surrounding the magma chamber is indicated to have a thickness on the order of 100 m the outer portions of this region should be sufficiently cool so that shocked features are not annealed hence the source of shocked minerals should be the country rock surrounding the magma chamber it is unlikely any of the magmatic material would be able to retain shocked features due its high temperature the explosive mechanism may be quench supersaturation which theoretically can yield shock producing pressures but chemical processes should not be dismissed as an explosive source tektite distributions show no iridium or mass extinction association industrial and present volcanic atmospheric discharge are known to have worldwide distribution the clays present at the k t boundary may have had a volcanic origin and it is now known that the deccan traps which have been suggested by others as a source of the iridium lie athwart the k t boundary as there is common intimacy between silicic and mafic volcanism both should be anticipated at the k t boundary and their global appearance there may be a manifestation of a surge in whole mantle convection in short there is a definite possibility shocked minerals at the k t boundary have a volcanic source

the microstructural response of quartz and feldspar under shock loading at variable temperatures shock recovery experiments were carried out on westerly granite and hospital hill quartzite targets in the peak pressure range 8 to 25 gpa preshock temperatures of 25 450 and 750 c and pulse durations of 2 to 7 s using internally heated momentum traps and explosive plane wave generators optical and transmission electron microscopy analyses of quartz and feldspar shocked at 25 c revealed the previously documented progression with increasing pressure 1 fracturing 2 planar fractures and shock mosaicism 3 shock mosaicism and planar deformation features pdfs and 4 isotropization this same sequence is observed for experiments at elevated preshock temperature but with specific microstructures occurring at lower pressures than those in previous experiments at room temperature at 750 c strong shock mosaicism partially thermally recovered is characteristic of feldspar shocked to 8 gpa whereas 15 gpa is required for its development in quartz and for the generation of pdfs in both minerals the results suggest that threshold pressures for formation of the various microstructures and phases are expected to vary systematically as a function of the preshock temperature of the target material we suggest that pdfs are generated in the shock transition by progressive heterogeneous phase transformation of the crystal structure to form dense glass or high pressure polymorphs the onset pressures for pdfs in specific crystallographic orientations is not influenced strongly by temperature but the character of the pdfs does change as preshock temperature is increased at the same peak shock stress the change from multiple sets of thin pdfs at low temperature to thicker single sets of pdfs at moderate temperature and finally to complete isotropization at high temperatures reflects a change in the phase transformation mechanism as a function of temperature in contrast development of shock mosaicism in quartz and feldspar occurs throughout the duration of shock loading and is better developed at elevated temperatures where the kinetics are enhanced by the additional thermal energy in the target

distinguishing shocked from tectonically deformed quartz by the use of the sem and chemical etching abstract multiple sets of crystallographically oriented planar deformation features pdfs are generated by high strain rate shock waves at pressures of 12 gpa in naturally shocked quartz samples on surfaces pdfs appear as narrow 50 500 nm lamellae filled with amorphosed quartz diaplectic glass which can be etched with hydrofluoric acid or with hydrothermal alkaline solutions in contrast slow strain rate tectonic deformation pressure produces wider semi linear and widely spaced arrays of dislocation loops that are not glass filled etching samples with hf before examination in a scanning electron microscope sem allows for unambiguous visual distinction between glass filled pdfs and glass free tectonic deformation arrays in quartz this etching also reveals the internal pillaring often characteristic of shock induced pdfs this technique is useful for easily distinguishing between shock and tectonic deformation in quartz but does not replace optical techniques for characterizing the shock features

end cretaceous mass extinction event argument for terrestrial causation the end cretaceous mass extinctions were not a geologically instantaneous event and were selective in character these features are incompatible with the original alvarez hypothesis of their being caused by a single asteroid impact that produced a world embracing dust cloud with devastating environmental consequences by analysis of physical and chemical evidence from the stratigraphic record it is shown that a modified extraterrestrial model in which stepwise extinctions resulted from encounter with a comet shower is less plausible than one intrinsic to the earth involving significant disturbance in the mantle

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

the role of mantle co2 in volcanism abstract carbon dioxide is the propellant gas in volcanic eruptions and is also found in mantle xenoliths it is speculated that co 2 occurs as a free gas phase in the mantle because there is no reason to expect co 2 to be so universally associated with volcanic rocks unless the co 2 comes from the same source as the volcanic rocks and their xenoliths if correct the presence of a free gas in the mantle would lead to physical instability with excess gas pressure providing the cause of both buoyancy of volcanic melts and seismicity in volcanic regions convection in the mantle and episodic volcanic eruptions are likely necessary consequences this suggestion has considerable implications for those responsible for providing warnings of impending disasters resulting from volcanic eruptions and earthquakes in volcanic regions

origin of archean ferropicrites geochemical constraints from the boston creek flow abitibi greenstone belt ontario canada abstract new major element trace element and nd isotope data for non cumulate samples from the fe enriched boston creek flow bcf have permitted the development of a petrogenetic model for the origin of this geochemically unusual flow the data reveal high contents of nb ta lree ti ni co and ir as well as in fe and low contents of al al 2 o 3 tio 2 6 hree y v sc pd and pt compared to komatiitic rocks and typical tholeiitic rocks of similar magnesian content primitive mantlenormalized incompatible trace element profiles for the bcf peak in the ta nb la region la sm n 2 0 la yb n 5 4 th ta n 0 7 and bottom in the al sc region la sc n 11 0 la al n 9 7 and reveal slight depletions in zr and hf relative to mree sm zr n 1 3 and ti nd t 2720 ma values for the flow are 2 5 indicating derivation of the parental melt from a long term depleted mantle source these geochemical characteristics are somewhat similar to those of modern oceanic within plate basalts these unusual geochemical characteristics and comparisons to proterozoic ferropicrite flows at pechenga russia indicate that the bcf is ferropicritic rather than komatiitic in affinity alteration assimilation and magma mixing and crystal fractionation processes cannot explain the bcf therefore the distinctive geochemistry of the bcf is concluded to have been a feature of its mantle source and is attributed to mantle processes models involving melting of core material in the mantle source may explain the enrichment of fe and high ni in the bcf but they are contradicted by the relatively low pt and pd models involving olivine equilibration in the mantle are consistent with the high ni and co similar composition of assumed bcf parental melt equilibrium olivine at 25 kbar and that of modern fe rich lherzolite olivine fo85 and comparison of the mgo and feo content of the melt with those of experimentally derived melts that are saturated in olivine at high pressure models involving garnet equilibration in the mantle could explain the low al hree y v and sc but are contradicted by the very low al2o3 content of the assumed parental melt compared with those of experimentally derived melts of similar feo and mgo content that are saturated in garnet at high pressure alternatively the unusual geochemistry of the bcf can be explained by a two source component mixing model somewhat like those commonly accepted for oceanic within plate basalts petrogenesis the first source component is peridotite depleted by extraction of melt s prior to generation of the bcf parental melt which explains the relatively high compatible element contents and low al hree sc pd and pt contents the second source component is highly enriched small degree melt fractions formed at mantle depths sufficient to stabilize garnet majorite which explains the relative enrichments in fe ti and the highly incompatible elements mixing of the two source components must have occurred immediately prior to melting to maintain the depleted nd isotope composition and may have been facilitated by metasomatic amphibole stabilization at relatively shallow 100 km depths in the mantle

a petrologic model for the constitution of the upper mantle and crust of the koolau shield oahu hawaii and hawaiian magmatism abstract a petrological model for the uppermost upper mantle and crust under the koolau shield to a depth of about 60 km has been derived on the basis of petrology of the upper mantle and crustal xenoliths in nephelinites of the honolulu volcanic series three main xenolith suites exist in the koolau shield dunites spinel lherzolites and garnet bearing pyroxenites on the basis of mineralogy it is inferred that the dunites represent cumulates in shallow crustal tholeiitic magma chambers the spinel lherzolites form a thick 40 km layer in the upper mantle and the garnet pyroxenite suite occurs as veins and stringers in the spinel lherzolites at about 60 km depth the eruption sequence in a hawaiian volcano i e tholeiite transitional basalt alkali basalt is generated by partial melting of a volatile bearing garnet lherzolite part of a lithospheric plate as it rides over a hot spot if the tholeiite transitional and alkali basalts of hawaiian volcanoes are generated at the same depth then the observed sequence of lavas requires replenishment of the source area with volatiles and gradual decrease of the degree of partial melting with time post erosional olivine nephelinites are produced from isotopically distinct deeper source area which may be the asthenosphere

cosmic impact versus terrestrial origin of the azuara structure spain a review the azuara structure is the largest one proposed so far in spain as possibly related to a cosmic impact event a review of the evidence set forward in favor of and against its cosmic origin indicates that the discussion is not yet finished some megascopic features inverted stratigraphy megabreccia negative gravity anomalies and shock metamorphic effects planar deformational features have been described in relation with the structure although their real significance has been questioned and is still being debated comparison with other similar sized verified impacts suggests that unequivocal impactogenic features are yet to be found before the azuara structure can be related to a cosmic impact until then the azuara structure should be considered as an unverified impact structure and should not be included in global comprehensive maps of terrestrial impact structures

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

evidence from cathodoluminescence for non volcanic origin of shocked quartz at the cretaceous tertiary boundary bohor et al 1 have described shocked quartz displaying one or more sets of planar elements indexed to rational crystallographic planes associated with anomalous siderophile abundances at the cretaceous tertiary k t stratigraphic boundary in eastern montana shocked quartz has subsequently been shown at several k t boundary sections around the world and has been attributed to the effects of impact on crustal rocks2 7 this impact may have occurred within the scenario of comet or meteor collision8 9 or alternatively shocked quartz at the k t boundary could be the product of volcanic eruption rather than bolide impact10 14 here we describe a cathodoluminescence study of shocked quartz at the k t boundary in southeastern colorado that reveals a diversity of luminescence colours not present in quartz from known volcanic ejecta but typical of quartz in crustal supracrustal lithologies cathodoluminescence colour diversity indicates that the grains were originally derived from a variety of igneous metamorphic and sedimentary rocks subsequently overprinted with planar shock features these observations lend support to the hypothesis of bolide impact at the end of the cretaceous period

the kilmichael structure mississippi evidence for a meteorite impact origin structural evidence from the 9 6 km roughly circular kilmichael disturbance montgomery co mississippi 33 30 n 89 33 w indicates a possible origin by meteorite impact probably in late tertiary time interpretation of its origin is based on structural similarities with terrestrial impact structures of comparable diameter in sedimentary rocks at the surface steep normal faults outline a structural ring graben at a diameter of 6 5 km in turn partly surrounded by a peripheral faulted zone in which kilometer sized fault blocks weakly define a horst ring and an outer peripheral graben in the center largely unconsolidated tertiary and cretaceous sediments have been disturbed to 1 km depth and have been raised 440 470 m forming a central uplift 2 km across ringed by a peripheral trough 3 2 km in diameter depressed 180 m below normal stratigraphic level the uplift and peripheral trough have no topographic expression unsorted gravels and sands filling a shallow basin surmounting the central uplift m

a test of the longevity of impact induced faults as preferred sites for later tectonic activity the hypothesis that impact induced faults have been preferred sites for later deformation in response to lithospheric stresses has been suggested for several planets and satellites this hypothesis is investigated on earth by examining whether terrestrial impact structures show higher rates of nearby earthquake activity than do surrounding intraplate regions for 28 of 30 probable impact structures having an original crater 20 km or more in diameter the rates of nearby seismicity have been no higher than the regional background rates for two large probable impact structures vredefort and charlevoix with higher than normal rates of nearby seismicity factors other than slip on impact induced faults appear to control the occurrence of earthquakes it is concluded that impact induced faults at least on earth do not persist as lithospheric weak zones for periods in excess of several million years after the impact event

coupling of the radiative convective and surface fluxes over the equatorial pacific abstract an idealized energy balance model for a closed circulation is first presented to illustrate the coupling between the net tropospheric radiative cooling the surface fluxes and the mean subsidence away from the precipitation zones then a one dimensional diagnostic model and a radiation model with boundary layer clouds are combined to explore this coupling for a specific region using mean sounding data over the tropical pacific the radiatively driven subsidence rate at the top of the convective boundary layer is approximately 35 mb day 1 0 04 pa s 1 and is largely independent of boundary layer cloud fraction the sensitivity of the corresponding convective heat flux profiles to the mass divergence profile and cloud fraction within the boundary layer is explored reasonable assumptions give realistic surface sensible and latent heat fluxes for this region of approximately 10 and 130 w m 1 the paper illustrates the important background climatic control of the radiation field on the tropical surfa

the petrology of some picrites from mauna loa and kilauea volcanoes hawaii the mineralogy and chemistry of picrites from mauna loa and kilauea have been investigated to evaluate for hawaiian tholeiitic picrites the contrasting genetic models which have been proposed for these mg rich volcanics namely products of direct crystallization of high mg melts 20 25 mgo or the result of accumulation of olivine phenocrysts into less mg rich melts genetic interpretations rely heavily on mg fe partitioning relations between olivines and their picrite hosts although the 100 mg mg fe2 ratios m of picrites are wide ranging m 73 6 82 9 for fe2o3 feo 0 15 with mgo as high as 27 8 the average 100 mg mg fe ratios mg of the cores of olivine phenocrysts megacrysts show only restricted compositional variation mg 87 2 89 0 successive olivine generations are progressively more fe rich olivine liquid mg fe partitioning data and the mn and ni abundances in olivine phenocrysts collectively indicate that they were precipitated by mg rich basaltic melts with 12 14 mgo spinel compositions range from liquidus magnesiochromites occurring mainly as inclusions in olivine phenocrysts to groundmass titanomagnetites which crystallized at nearsolidus temperatures the cr2o3 contents and m values of liquidus magnesiochromites suggest that their parent melts were neither mg rich picritic mgo 20 nor relatively mg poor basaltic types