multiphase u pb geochronology of sintered breccias from the steen river impact structure canada mixed target considerations for a jurassic cretaceous boundary event abstract in situ u pb geochronology has been conducted using laser ablation inductively coupled mass spectrometry la icp ms on shocked and thermally metamorphosed apatite titanite and zircon grains from the steen river impact structure canada the dated relict mineral phases occur within impact melt bearing breccias that underwent post deposition sintering at 450 c t

recrystallization and chemical changes in apatite in response to hypervelocity impact despite the wide utility of apatite ca5 po4 3 f cl oh in the geosciences including tracing volatile abundances on the moon and mars little is known about how the mineral responds to the extrem

frictional melting processes and the generation of shock veins in terrestrial impact structures evidence from the steen river impact structure alberta canada abstract shock produced melt within crystalline basement rocks of the steen river impact structure sris are observed as thin 1 510 m wide interlocking networks of dark veins which cut across and displace host rock minerals solid state phase transformations such as ferro pargasite to an almandine andradite majorite garnet and amorphization of quartz and feldspar are observed in zones adjacent to comparatively wider 50 500 m sections of the shock veins shock pressure estimates based on the coupled substitution of na ti4 and si4 for divalent cations al3 and cr3 in garnet 14 19 gpa and the pressure required for plagioclase ab62 83 amorphization at elevated temperature 14 20 gpa are not appreciably different from those recorded by deformation effects observed in non veined regions of the bulk rock 14 20 gpa this spatial distribution is the result of an elevated temperature gradient experienced by host rock minerals in contact with larger volumes of impact generated melt and large deviatoric stresses experienced by minerals along vein margins micrometer size equant crystals of almandine pyrope majorite garnet define the shock vein matrix consistent with rapid quench 100 200 ms at 7 5 10 gpa crystallization of the vein occurred during a 0 1 0 15 s shock pressure pulse majoritic garnet formed during shock compression by solid state transformation of pargasite along shock vein margins is observed in tem bright field images as nanometer size gouge particles produced at strain rates in the supersonic field 106 108 these crystals are embedded in vesiculated glass and this texture is interpreted as continued movement and heating along slip planes during pressure release the deformation of high pressure minerals formed during shock compression may be the first evidence of oscillatory slip in natural shock veins which accounts for the production of friction melt via shear when little or no appreciable displacement is observed our observations of the mineralogy chemistry and microtextures of shock veins within crystalline rocks of the sris allow us to propose a model for shock vein formation by shear induced friction melting during shock compression

deep magma body beneath the summit and rift zones of kilauea volcano hawaii a magnitude 7 2 earthquake in 1975 caused the south flank of kilauea volcano hawaii to move seaward in response to slippage along a deep fault since then a large part of the volcano s edifice has been adjusting to this perturbation the summit of kilauea extended at a rate of 0 26 meter per year until 1983 the south flank uplifted more than 0 5 meter and the axes of both the volcano s rift zones extended and subsided the summit continues to subside these ground surface motions have been remarkably steady and much more widespread than those caused by either recurrent inflation and deflation of the summit magma chamber or the episodic propagation of dikes into the rift zones kilauea s magmatic system is therefore probably deeper and more extensive than previously thought the summit and both rift zones may be underlain by a thick near vertical dike like magma system at a depth of 3 to 9 kilometers

composition and structure of the central aleutian island arc from arc parallel wide angle seismic data new results from wide angle seismic data collected parallel to the central aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry the 3 d ray tracing 2 5 d inversion of this sparse wide angle data set which incorporates independent phase interpretations and new constraints on shallow velocity structure produces a faster and smoother result than a previously published velocity model middle crustal velocities of 6 5 7 3 km s over depths of 10 20 km indicate an andesitic to basaltic composition high lower crustal velocities of 7 3 7 7 km s over depths of 20 35 km are interpreted as ultramafic mafic cumulates and or garnet granulites the total crustal thickness is 35 37 km this result indicates that the aleutian island arc has higher velocities and thus more mafic compositions than average continental crust implying that significant modifications would be required for this arc to be a suitable building block for continental crust lateral variations in average crustal velocity below 10 km roughly correspond to trends in major element geochemistry of primitive mg 0 6 lavas the highest lower crustal velocities and presumably most mafic material are detected in the center of an arc segment between unmak and unalaska islands implying that arc segmentation exerts control over crustal composition

isotope systematics and shock wave metamorphism i u pb in zircon titanite and monazite shocked experimentally up to 59 gpa this study reports the first u pb isotope analyses on experimentally shocked zircon titanite and monazite extracted from proterozoic granitoid rocks in all three types of minerals shock waves produce drastic changes in the crystal lattices causing strong lowering of birefringence turbidization and decolorization of the individual grains moreover x ray patterns indicate transition of the crystals into polycrystalline aggregates of 10 5 mm block size precisely dated grains with concordant or nearly concordant ages were embedded in kbr and shocked at 35 47 5 and 59 gpa u pb isotope analyses on these grains show that shock metamorphism does not fractionate pb isotopes within the analytical precision of 0 1 as far as chemical fractionation is concerned there is no difference in degree of concordancy between shocked and unshocked monazite and small degrees 2 of relative upb fractionation in shocked zircon and titanite are due to time integrated pb loss and not to the shock experiment in consequence the data document that shock wave metamorphism alone does not measurably effect the u pb chronometer questioning the view that lower intercept ages of discordant u pb data reflect shock induced re equilibration of the chronometer in moderately to highly shocked rapidly cooling rocks

fission track dating of haughton astrobleme and included biota devon island canada haughton astrobleme is a major extraterrestrial impact structure located on devon island in the canadian arctic archipelago northwest territories apatite grains separated from shocked precambrian gneiss contained in a polymict breccia from the center of the astrobleme yielded a fission track date of 22 4 million 1 4 million years before the present or early miocene aquitanian this provides a date for the impact event and an upper limit on the age of crater filling lake sediments and a flora and vertebrate fauna occurring in them a geologically precise date for these fossils provides an important biostratigraphic reference point for interpreting the biotic evolution of the arctic

evidence of impact melting and post impact decomposition of sedimentary target rocks from the steen river impact structure alberta canada abstract hypervelocity bolide impacts deliver vast amounts of energy to the earth s near surface this crustal process almost universally includes sedimentary target rocks however their response to impact is poorly understood in part because of complexities due to layering pore space and the presence of volatiles that are difficult to model the response of carbonates to bolide impact remains contentious yet whether they melt or decompose and liberate gases by the reaction caco3 s cao s co2 g has significant implications for post impact climatic effects we report on previously unknown carbonate impact melts at the steen river impact structure canada and the first description of naturally shocked barite baso4 carbonate melts are preserved as groundmass supported calcite rich clasts sampled from an up to 164 m thick continuous sequence of crater fill polymict breccias electron microscopy reveals fluidal and ocellar textured calcite and barite intimately associated with silicate melt consistent with these phases being in the liquid state at the same time raman spectroscopy and electron backscatter diffraction ebsd mapping confirm the presence of high pressure phases reidite and coesite within some steen river carbonate melt bearing breccias these minerals attest to the strong shock provenance of the breccia and provide constraints on their shock history preservation of reidite lamellae in zircon indicates a shock pressure 30 gpa

the effects of k metasomatism on the mineralogy and geochemistry of silicic ignimbrites near socorro new mexico abstract k metasomatism of the upper lemitar and hells mesa silicic ignimbrites near socorro new mexico is thought to be the result of downward percolation of alkaline saline brines in a hydrologically closed basin chapin c e lindley j i 1986 potassium metasomatism of igneous and sedimentary rocks in detachment terranes and other sedimentary basins economic implications arizona geological society digest xvi 118 126 during the chemical changes associated with metasomatism na rich phases primarily plagioclase are replaced by secondary mineral phases adularia a low temperature k feldspar is the dominant mineral formed during k metasomatic alteration but mixed layer i s discrete smectite and kaolinite can also be present in the assemblage the formation of discrete smectite within the assemblage during k metasomatism may have occurred during periods of low cation h ratios in the alkaline saline brine upon an increase in the cation h ratio such as could occur during evaporation of the alkaline lake the solution may have become sufficiently concentrated to cause illitization of smectite resulting in the formation of mixed layer i s within the assemblage distribution of phases in the alteration assemblage strongly suggests a dissolution precipitation reaction for k metasomatism in the socorro area as indicated by the presence of dissolution embayments in plagioclase crystals the presence of euhedral adularia and the common occurrence of authigenic clay minerals in the assemblage k metasomatism causes significant chemical modification of the silicic ignimbrite particularly increases in k 2 o and rb and depletion of na 2 o and sr with increasing adularia abundance the correlation between rb and k 2 o suggests that rb is enriched during alteration due to substitution for k in adularia the effect of hydrothermal activity either prior to or following metasomatism is also observed in some samples as shown by high concentrations of elements such as ba as sb pb and cs enrichment of middle and hree in the upper lemitar tuff samples and depletion of middle and hree in hells mesa tuff samples suggests attempted re equilibration between the secondary alteration assemblage and the metasomatizing fluid preliminary data indicate clay minerals within the secondary assemblage may have played an important role in the incorporation of ree during redistribution

new zircon shock phenomena and their use for dating and reconstruction of large impact structures revealed by electron nanobeam ebsd cl eds and isotopic u pb and u th he analysis of the vredefort domethis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh integrated electron nanobeam ebsd cl eds and isotopic measurements u pb u th he of zircon from the collar and centre of the 80 km wide central uplift of the 2020 3 ma vredefort impact structure reveal new shock features in a microstructural progression related to impact basin formation and degree of u pb age resetting 1 planar fractures in 1k0 and 1k2 orientation during initial shock wave compression 2 curviplanar fractures in 1k1 orientation now annealed which host glassy inclusions of partial melt of the host rock 3 microtwin lamellae in an orientation of 65 about 110 attributed to shock wave rarefaction 4 nucleation of impact age crystallites possibly on microtwins during post shock heating by impact melt and 5 crystal plastic deformation linked to crater modification of the core of the central uplift planar fracturing and microtwinning 20 gpa in cold shock zircon in granitoid at a radial distance of 25 km failed to reset zircon age single grain id tims data

jurassic cretaceous boundary age and mid ocean ridge type mantle source for shatsky rise basalt sills cored in lowermost berriasian sediments on shatsky rise in the northwest pacific yield a mean 4 0 ar 3 9 ar incremental heating age of 144 6 0 8 ma providing a minimum estimate on the age of the jurassic cretaceous boundary the largest part of the rise was emplaced at rates similar to those of several continental flood basalts the rise formed at an oceanic triple junction and the sills as well as lavas from two dredge sites exhibit an ocean ridge type nd pb sr isotope signature this signature is provisionally consistent with either a perisphere or meteorite impact model but is not predicted by the plume head model of plateau formation however no model can yet be rejected given the very sparse sampling of shatsky rise

deformation associated with the 1997 eruption of okmok volcano alaska 1 okmok volcano located on umnak island in the aleutian chain alaska is the most eruptive caldera system in north america in historic time its most recent eruption occurred in 1997 synthetic aperture radar interferometry shows deflation of the caldera center of up to 140 cm during this time preceded and followed by inflation of smaller magnitude the main part of the observed deformation can be modeled using a pressure point source model the inferred source is located between 2 5 and 5 0 km beneath the approximate center of the caldera and 5 km from the eruptive vent we interpret it as a central magma reservoir the preeruptive period features inflation accompanied by shallow localized subsidence between the caldera center and the vent we hypothesize that this is caused by hydrothermal activity or that magma moved away from the central chamber and toward the later vent since all historic eruptions at okmok have originated from the same cone this feature may be a precursor that indicates an upcoming eruption the erupted magma volume is 9 times the volume that can be accounted for by the observed preeruptive inflation this indicates a much longer inflation interval than we were able to observe the observation that reinflation started shortly after the eruption suggests that inflation spans the whole time interval between eruptions extrapolation of the average subsurface volume change rate is in good agreement with the long term eruption frequency and eruption volumes of okmok

a high precision 40ar 39ar age for hydrated impact glass from the dellen impact sweden the dating of terrestrial impact craters and impact glasses that exhibit high degrees of mineralogical complexity can be problematic however if the maximum potential of the terrestrial impact crater record is to be realized accurate and precise ages for crater forming events are critical here we report a high precision 40ar 39ar age for the dellen impact structure sweden previous radio isotopic constraints show a wide variation in age as a result of poor sample characterization and analytical approach a detailed petrographical and mineralogical study provides a solid foundation for interpretation of step heating 40ar 39ar data culminating in a statistically robust age of 140 82 0 51 ma 2 full external precision for the dellen impact event for which data disfavour an inherited argon component primary hydration of the impact melt during cooling quenching and entrapment of molecular water promoted rapid loss of inherited 40ar from the impact melt of rhyolitic composition duplicate analyses of the water content and d of the glass give similar values for the former 1 9 0 1 mol mg 1 but unexpectedly low values for the latter 159 8 with scatter beyond the expected analytical reproducibility due to isotopic heterogeneity this study highlights that the 40ar 39ar technique is unrivalled in its ability to precisely and accurately date the products of hypervelocity collisional events less

lithification mechanisms for planetary regoliths the glue that binds it is understood how rocks are made on earth however on the moon mercury and to a lesser extent mars and venus there are distinct rock forming processes that we do not fully comprehend the surfaces and crusts of the inner planetary bodies may retain a history of disruption by hypervelocity impact resulting in the generation of disaggregated materials to several kilometers depth the uppermost component of this is called regolith typically 20 m thick on the moon which is part of a more extensive megaregolith that is up to tens of kilometers thick and which in places may pervade the entire crust of a planetary body it is from these pulverized materials that new rocks are reaggregated to form so called breccias this work reviews regolith and megaregolith structure for the inner planetary bodies and investigates how extraterrestrial breccias are produced three principal formation mechanisms are explored thermal sintering shock sintering and the dynamic interaction of impact generated melt w

a pressure temperature phase diagram for zircon at extreme conditions abstract hypervelocity impact processes are uniquely capable of generating shock metamorphism which causes mineralogical transformations and deformation that register pressure p and temperature t conditions far beyond even the most extreme conditions created by terrestrial tectonics the mineral zircon zrsio4 responds to shock deformation in various ways including crystal plasticity twinning polymorphism e g transformation to the isochemical mineral reidite formation of granular texture and dissociation to zro2 sio2 which provide robust thermobarometers that record different extreme conditions the importance of understanding these material processes is two fold first these processes can mobilize and redistribute trace elements and thus be accompanied by variable degrees of resetting of the u pb system which is significant for the use of zircon as a geochronometer second some features described herein form exclusively during shock events and are diagnostic criteria that can be used to confirm the hypervelocity origin of suspected impact structures we present new p t diagrams showing the phase relations of zrsio4 polymorphs and associated dissociation products under extreme conditions using available empirical and theoretical constraints we present case studies to illustrate zircon microstructures formed in extreme environments and present electron backscatter diffraction data for grains from three impact structures mistastin lake of canada ries of germany and acraman of australia that preserve different minerals and microstructures associated with different shock conditions for each locality we demonstrate how systematic crystallographic orientation relationships within and between minerals can be used in conjunction with the new phase diagrams to constrain the p t history we outline a conceptual framework for a zircon based approach to extreme thermobarometry that incorporates both direct observation of high p and high t phases as well as inferences for the former existence of phases from orientation relationships in recrystallised products a concept we refer to here as phase heritage this new approach can be used to unravel the pressure temperature history of zircon bearing samples that have experienced extreme conditions such as rocks that originated in the earth s mantle and those shocked during impact events on earth and other planetary bodies

neodymium isotope disequilibria in subducted sediments and potential consequences for subduction zone recycling neodymium isotope geochemistry is pivotal to understanding magma genesis and crustal recycling processes at convergent plate margins here we present nd isotope compositions of rare earth element ree rich minerals from blueschist to eclogite facies rocks that represent previously subducted continental sediments we find nd isotope heterogeneity in all of our samples apatite preserves diverse detrital isotopic compositions despite metamorphism up to at least 600 c and 3 0 gpa whereas lawsonite appears to be isotopically homogenous epidote and monazite are major ree hosts under eclogite facies conditions in our samples these phases also retain nd isotope heterogeneity even up to ultrahigh pressure conditions of 700 c and 4 0 gpa these findings contest the assumption of nd isotope equilibration during subduction zone metamorphism so caution is needed when using nd isotopes to calculate subducted sediment contributions to arc magmas or for determining rates of crustal recycling through subduction zones

post eruptive deformation associated with the 1986 87 and 1989 lava flows of etna detected by radar interferometry we analysed 92 interferograms produced using ers1 sar images taken on etna between may 1992 and october 1993 nineteen show a local subsidence in the eastern flank of the volcano correlated with the location of the 30 october 1986 1 march 1987 and 27 september 9 october 1989 lava flows using fringe unwrapping and data gridding techniques and assuming that over the sampled time window deformation was a linear function of time we derive a map of along range rate of motion the correlation between the deformation field and the area covered by recent lavas suggests that compaction of the lava flows continues several years after the eruptions the area of maximum subsidence 47 mm yr is localised at the narrowing of the 1989 flow between 1500 and 1700 m a s l we observe that subsidence extends outside the lava flows accounting for at least 12 mm yr assuming a relaxation process of the substrate in response to loading produced by recent lavas a simple 1d maxwell visco elastic model predicts a maximum subsidence rate of 25 mm yr and a relaxation time of about 3 5 years the relaxation time agrees with those derived from post eruptive displacements observed by levelling on etna and piton de la fournaise after several eruptions we conclude that at the time of our measurements 25 to 50 of the deformation was related to relaxation of the substrate and the other part due to compaction of the lava flows

an u th he age for the shallow marine wetumpka impact structure alabama usa single crystal u th he dating was applied to 24 apatite and 23 zircon grains from the wetumpka impact structure alabama usa this small approximately 5 7 6 km impact crater was formed in a shallow marine environment with no known preserved impact melt thus offering a challenge to common geochronological techniques a mean u th he apatite and zircon age of 84 4 1 4 ma 2 was obtained which is within error of the previously estimated late cretaceous impact age of approximately 83 5 ma in addition helium diffusion modeling of apatite and zircon grains during fireball contact shock metamorphism and hydrothermal events was undertaken to show the influence of these individual thermal processes on resetting u th he ages in the wetumpka samples this study has shown that the u th he geochronological technique has real potential for dating impact structures especially smaller and eroded impact structures that lack impact melt lithologies

in situ u pb dating of impact produced zircons from the vargeao dome southern brazil the vargeao impact structure was formed in the serra gerai basal tic and rhyodacitic to rhyolitic lava flows of southern brazil that belong to the paramf etendeka large igneous province the chapec6 type rhyodacites conta in small baddeleyite crystals recently d a ted at 134 3 0 8 ma which is regarded as the age of this acid volcanism coeva l to the flood basait eruption inside the impact structure a brecciated rhyodacitic sample displays fine veinlets containing numerous lithic fragments in a former melt this impact breccia contains newly formed zircons either in the veins or at the contact between a vein and the volcanic matrix the zircons are 10 50 j lm in length clear and nearly unzoned in situ laser ablation da ting of the zircons provides a concordant early aptian age of 123 0 1 4 ma that is regarded as the age of the impact event as in situ age determination ensures the best possible selection of the analyzed mineral grains the methodology employed in this study also represents a promising method for dating other impact structures

neodymium isotope equilibration during crustal metamorphism revealed by in situ microanalysis of ree rich accessory minerals radiogenic isotopes are widely used to investigate crustal evolutionary processes however recent claims of nd and sr isotope disequilibrium during anatexis question the reliability of such information we have conducted an in situ sm nd isotope study of apatite allanite titanite xenotime and monazite in metasedimentary rocks of different metamorphic grade to test nd isotope equilibrium during metamorphism our results show that apatite retains an original probably detrital highly variable nd isotopic signature until at least 500 c before being isotopically homogenised irrespective of textural context within the rock once equilibrated apatite retains its nd isotope signature throughout anatexis in contrast allanite and titanite are equilibrated at temperatures as low as 350 400 c ree rich accessory minerals in high grade rocks 600 c show very similar initial end values at the time of metamorphism we conclude that under these metamorphic conditions nd isotope disequilibrium between crustal melts and metasedimentary sources is unlikely intra grain nd isotope zoning of monazite indicates that partial melting was open system involving the injection of externally derived melt into migmatites this process likely to be common in anatectic terranes but not always obvious at hand specimen scale or from bulk rock geochemical data can produce isotope variation that could potentially be misinterpreted as disequilibrium between the melt and its protolith

phosphate ages in apollo 14 breccias resolving multiple impact events with high precision u pb sims analyses abstract the u pb systems of apatite and merrillite grains within four separate apollo 14 impact melt breccia samples were analysed by secondary ion mass spectrometry no systematic difference was identified between the 207 pb 206 pb ages of the apatites and merrillites a combined 207 pb 206 pb age of 3927 2 ma 95 conf is determined for three of these samples 14305 103 3926 4 ma 14306 150 3926 6 ma 14314 13 3929 4 ma by combining these data with the ages previously obtained for zircons in apollo 12 impact melt breccia fragments and the lunar meteorite sau 169 a weighted average age of 3926 2 ma 95 conf is obtained which is attributed to the formation of the imbrium basin an age of 3943 5 ma is determined for the fourth breccia 14321 134 which is similar to ages of 3946 15 ma and 3958 19 ma obtained from several older phosphates in 14305 103 and 14314 13 the weighted average of these three older ages is 3944 4 ma 95 conf this is indistinguishable to the age 3938 4 ma 2 obtained for a different apollo 14 impact melt breccia in a previous study after investigating likely sources for this older 3940 ma age we conclude that the humorum or serenitatis basin forming events are likely candidates the potential identification of two large impact events within 15 myrs has important implications for the rate of lunar bombardment around 3 95 3 92 ga this study demonstrates the importance of high precision age determinations for interpreting the impact record of the moon as documented in lunar samples

leaching and differential recrystallization of metamict zircon under experimental hydrothermal conditions we report results of hydrothermal experiments on four alluvial zircons from sri lanka which cover a wide range of radiation damage at 450 c and 1 3 kbar for 744 h with 2 m cacl2 solution as reactive fluid after the hydrothermal treatment the most metamict samples show micrometer thick reaction rims which surround apparently unreacted zircon as revealed by cathodoluminescence cl and nomarski differential interference contrast ndic images these rims have sharp curved and transgressive boundaries with unreacted zircon and are in some cases spread out along cracks the thickness of reaction rims increases with increasing cumulated dosage of the starting materials the reaction rims are strongly enriched in ca up to 7000 ppm and a water species and depleted in radiogenic pb zr and si as revealed by electron microprobe analyses a significant th loss from the reaction rims was detected in the case of the most metamict sample whereas u remained in the structure ft infrared spectrometry and x ray diffraction measurements revealed that the bulk run products were recrystallized using micro raman spectrometry we were able to demonstrate that differential recrystallization took place the reaction rims are strongly recrystallized whereas the unreacted grain interiors underwent only minor recrystallization recrystallization of the rims is accompanied by an enhancement of the integral cl intensity it is suggested that recrystallization in the reaction rims was catalyzed by water infiltration and ion exchange and prevented significant congruent zircon dissolution under the given experimental conditions previous zircon studies have shown that 1 a transgressive morphology 2 a reduced th u ratio and 3 an enhanced cl emission are also characteristics of rims in zircons from high grade metamorphic rocks based on these similarities between natural and experimentally produced rims it is suggested that leaching catalyzed recrystallization is an important alteration process in zircon under wet geological conditions and can account for many complex core rim structures found in natural zircons furthermore the strong enrichment of ca in the reaction rims supports previous assumptions that high ca concentrations in natural zircons are of secondary origin it is suggested that lower u pb concordia intercept ages obtained from single phase zircons with high ca contents date a leaching event

radiation effects in zircon the widespread distribution of zircon in the continental crust its tendency to concentrate trace elements particularly lanthanides and actinides its use in age dating and its resistance to chemical and physical degradation have made zircon the most important accessory mineral in geologic studies because zircon is highly refractory it also has important industrial applications including its use as a lining material in high temperature furnaces however during the past decade zircon has also been proposed for advanced technology applications such as a durable material for the immobilization of plutonium or when modified by ion beam irradiation as an optic waveguide material in all of these applications the change in properties as a function of increasing radiation dose is of critical importance in this chapter we summarize the state of knowledge on the radiation damage accumulation process in zircon

runoff generation from successive simulated rainfalls on a rocky semi arid mediterranean hillslope runoff generation in a mediterranean carbonate rock environment the judaean mountains west bank and israel was studied by a 2 day sprinkling experiment on an 18 x 10 m 2 plot during the first day three spells of 24 1 mm h 1 12 3 mm 27 8 mm h 1 12 3 mm and 14 4 mm h 1 12 5 mm were applied almost saturating the runoff plot on the second day 20 h later two spells of 18 7 mm h 1 34 3 mm and 35 2 mm h 1 25 4 mm followed surface runoff was a combination of infiltration excess runoff from rocky portions of the plot and saturation excess runoff from areas covered by soil soil saturation was accelerated by lateral runoff from adjacent rocky areas once the plot was saturated 80 90 of the applied rainfall became surface runoff significantly different concentrations of chloride sulphate and nitrate ions in the sprinkled water on two successive days served as tracers during the second day a two component hydrograph separation pointed to the importance of shallow reservoirs playing active parts in runoff generation about 14 of the flow collected during the second day originated from water applied during the first day during the second day of sprinkling both water sources obviously mixed in saturated soil reservoirs and contributed in variable percentages to surface flow it is hypothesized that at least during high magnitude rainfall the steep rocky slopes investigated might be regarded as flood generating zones rather than areas of pronounced recharge into the underlying yarkon taninim aquifer

morokweng south africa a large impact structure of jurassic cretaceous boundary age geophysical petrographical and geochemical data indicate the presence of a large impact structure in the area around morokweng northwest province south africa possibly up to 340 km in diameter drill cores from the center of the structure show a thick layer of impact melt rocks with high abundances of cr ni co and the platinum group elements consistent with the presence of up to 5 of a chondritic component ion probe dating of zircons extracted from the impact melt yielded a 206 pb 238 u age of 146 2 1 5 ma and a 208 pb 232 th age of 144 7 1 9 ma these are indistinguishable from the age of the jurassic cretaceous boundary following the identification of the chicxulub impact structure of cretaceous tertiary boundary age the discovery of a second large impact structure at a previously established major chronostratigraphic boundary strengthens suggestions that large impact events have been major factors in the evolution of the earth

zircons from the acraman impact melt rock south australia shock metamorphism u pb and 40ar 39ar systematics and implications for the isotopic dating of impact events abstract this study presents the first optical and scanning electron microscopic characterization and u pb shrimp dating results for zircon grains separated from the most likely autochthonous impact melt rock in the central domain of the large 40 90 km eroded ediacaran acraman impact structure in south australia microtextural characteristics define five zircon subtypes corresponding to different levels of progressive shock metamorphism from virtually unshocked monocrystalline zircon grains that exhibit original magmatic zoning in cathodoluminescence images to fully granular zircons that have completely lost their primary zoning pattern and locally contain neocrystallized submicrometer sized spots of zro 2 probably baddeleyite that pseudomorph pre impact zircon the granular zircons correspond to the highest observed level of shock metamorphism and impact induced recrystallization zro 2 bearing granular zircons indicate shock pressures in excess of 65 70 gpa which are considerably higher than previous shock pressure estimates for the acraman impactites u pb systematics of untreated and chemically abraded melt rock zircons indicate that u pb ratios of the acraman zircons were variably reset during impact weakly shocked crystalline grains yield ages on concordia at 1 59 1 60 ga reflecting the magmatic age of the gawler range volcanics only the entirely granular zircon population was apparently impact reset but based on an ediacaran age from stratigraphic constraints on the ejecta layer experienced significant post impact pb loss the microcrystalline nature of granular zircons could have promoted pb diffusion and recoil in post impact time as suggested by grain size dependent diffusion and recoil modeling a positive correlation of u concentration and shock level suggests that granularization might have preferentially occurred in initially u rich probably metamict zircons 40 ar 39 ar dating of a melted yardea dacite clast from the acraman melt rock as well as k feldspar separated from shocked yardea dacite resulted in post impact alteration plateau ages suggestive of hydrothermal events at 500 ma and 450 ma that selectively affected the impactites exposed in the central domain of the acraman impact structure our study demonstrates that the acraman impact is particularly difficult to date in the absence of accurate and precise isotopic ages for acraman the ediacaran ejecta stratigraphic age of 635 541 ma is considered the most reliable age constraint currently available for the timing of the large acraman impact

the application of u pb geochronology to mafic ultramafic and alkaline rocks an evaluation of three mineral standards abstract a key to proving the accuracy of u pb age determinations by the various techniques available is routine analysis of an internal well characterized matrix matched mineral standard of known age in general there are very few well studied natural mineral standards available to the geological community in this study the nature geochemistry and u pb systematics of baddeleyite titanite and perovskite from three potential mineral standards are evaluated a compilation of concordant to near concordant analyses of a single phalaborwa baddeleyite crystal yields a precise u pb age of 2059 60 0 35 ma a large variation in uranium 51 2124 ppm thorium 0 5 73 ppm and lead 19 782 ppm concentrations and the presence of some u bearing mineral inclusions zircon monazite and baddeleyite zircon intergrowths require some caution when using aliquots of this phalaborwa baddeleyite crystal as a mineral standard the khan titanite crystal investigated here overall has a relatively uniform chemical composition e g u 584 95 ppm th 473 73 ppm pb 57 8 ppm th u 0 81 0 05 but parts of the crystal contain abundant mineral inclusions the model 1 upper intercept age of 522 2 2 2 ma for nine khan titanite analyses is considered the current best estimate for its crystallization age some variability in the u pb systematics with 207pb 206pb ages that vary between 508 and 560 ma was observed and could be related to the presence of tiny u bearing mineral inclusions such as rutile or allanite so careful selection of material from this titanite crystal is also required a single perovskite crystal from the ice river alkaline intrusion has a relatively uniform chemical composition uranium 141 25 ppm thorium 1954 270 ppm lead 41 5 ppm and th u 14 5 1 3 the u pb results for eight perovskite fractions yield a weighted average 206pb 238u age of 356 5 1 0 ma although there are always some limitations to using natural mineral crystals as u pb standards such as the presence of mineral inclusions alteration and geochemical heterogeneity the three crystals investigated here are considered suitable to be used as internal u pb standards for any of the currently used u pb dating techniques

new constraints on the jurassic cretaceous boundary in the high andes using high precision u pb data abstract the jurassic cretaceous j k boundary is poorly constrained and is the only phanerozoic system boundary that lacks an internationally accepted reference stratigraphic section gssp precise radio isotopic u pb data are unavailable for the earliest stage of the cretaceous the berriasian the age of the jurassic cretaceous boundary was based on several assumptions including the relative duration of ammonite zones the constant spreading rates of magnetic anomalies and the extrapolation of rb sr or k ar isotopic data this paper discusses a site in an andean basin of western gondwana showing the j k boundary interval with geographically widespread nannofossil markers which are here uniquely combined with precise radiometric dates the recent finding of a sequence of marine black shales in the high andes of argentina interbedded with ash fall tuffs provides important constraints on this boundary this succession bears calcareous nannofossils and ammonites which allow correlation with well established tethyan floras and faunas in the northern hemisphere the tithonian berriasian transition in the andes was recognized on the basis of ammonite zones and nannofossil bioevents for the first time in the southern hemisphere the new ages obtained are 137 9 0 9 ma by sensitive high resolution ion microprobe shrimp and 139 55 0 09 0 18 ma by chemical abrasion isotope dilution thermal ionization mass spectrometry tims near the base of the berriasian these new ages can be interpreted in two different ways the first alternative would indicate that the present geological time table is correct and the fossil levels should be late berriasian the second alternative is that the j k boundary is 5 ma younger than the recently published geological time scale the authors support the last alternative and propose that the j k boundary should be close to 140 ma

erratum to metamictisation of natural zircon accumulation versus thermal annealing of radioactivity induced damage annealing of the radiation damage generated in decay events is a common phenomenon in natural zircons we have studied relationships between radiation damage age and radionuclide content of natural zircons on a micro scale the degree of metamictisation was estimated by means of confocal laser raman spectroscopic measurements raman band broadening in well ordered to moderately metamict zircons that have not experienced significant healing of the radiation damage since the time of their growth shows a nearly linear dependence on the fluence this dependence is proposed as a calibration line for examining unknown zircons in order to consider complete or incomplete damage storage which may contribute to the understanding of the low t thermal history of their host rocks seven examples for the application are presented radiation damage may anneal heterogeneously with preferred re ordering in more damaged micro areas results suggest that single point defects do not anneal continuously over geologic periods of time but are in contrast a comparably stable type of radiation damage in contrast to recrystallisation processes structural recovery due to simple thermal annealing is not necessarily connected with loss of radiogenic pb the most prominent example for this are concordant zircons from sri lanka although they are excellent standards for u pb dating sri lankan zircons should not be used as standards for structural radiation damage the experimental work is completed by monte carlo simulations of decay events in zircon

dating the cooling of exhumed central uplifts of impact structures by the u th he method a case study at manicouagan abstract forty titanite grain fragments from 9 central uplift samples of metamorphosed anorthosite from the manicouagan impact structure were dated by the u th he technique a u th he central age of 207 1 6 4 ma 2 standard error se n 40 has been determined with 4 outlier ages removed the central age is refined to 208 9 5 1 ma 2 se both of these ages are within error of the previously determined u pb zircon age of 214 1 ma 2 derived from the impact melt manicouagan s central uplift formed due to rapid elevation from 7 10 km depth as part of the modification stage of the impact process which has facilitated the dating of its emplacement due to resulting rapid exhumation and cooling and closure of the u th he system in titanite correlation with the previous u pb zircon 214 1 ma impact melt crystallization age indicates that the u th he titanite dating technique offers a new approach to dating complex impact structures in the absence of viable melt sheets or other melt products the youngest ca 195 ma u th he dates preserved in some titanite fragments are synchronous with early jurassic rift induced lithospheric thinning and associated igneous activity that defines the central atlantic magmatic province camp the u th he titanite data from manicouagan indicate that the influence of this regional event may extend west of the previously proposed limit of camp activity

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

interferometric synthetic aperture radar study of okmok volcano alaska 1992 2003 magma supply dynamics and postemplacement lava flow deformation 1 okmok volcano located in the central aleutian arc alaska is a dominantly basaltic complex topped with a 10 km wide caldera that formed circa 2 05 ka okmok erupted several times during the 20th century most recently in 1997 eruptions in 1945 1958 and 1997 produced lava flows within the caldera we used 80 interferometric synthetic aperture radar insar images interferograms to study transient deformation of the volcano before during and after the 1997 eruption point source models suggest that a magma reservoir at a depth of 3 2 km below sea level located beneath the center of the caldera and about 5 km northeast of the 1997 vent is responsible for observed volcano wide deformation the preeruption uplift rate decreased from about 10 cm yr 1 during 1992 1993 to 2 3 cm yr 1 during 1993 1995 and then to about 1 2 cm yr 1 during 1995 1996 the posteruption inflation rate generally decreased with time during 1997 2001 but increased significantly during 2001 2003 by the summer of 2003 30 60 of the magma volume lost from the reservoir in the 1997 eruption had been replenished interferograms for periods before the 1997 eruption indicate consistent subsidence of the surface of the 1958 lava flows most likely due to thermal contraction interferograms for periods after the eruption suggest at least four distinct deformation processes 1 volcano wide inflation due to replenishment of the shallow magma reservoir 2 subsidence of the 1997 lava flows most likely due to thermal contraction 3 deformation of the 1958 lava flows due to loading by the 1997 flows and 4 continuing subsidence of 1958 lava flows buried beneath 1997 flows our results provide insights into the postemplacement behavior of lava flows and have cautionary implications for the interpretation of inflation patterns at active volcanoes

mineralogy and petrology of melt rocks from the popigai impact structure siberia the late eocene popigai impact structure of siberia comprises a 0 5 1 5 km thick 100 km diameter sequence of clast rich and clast poor andesitic to rhyolitic impact melt rocks and impact breccias underlain by archean to proterozoic crystalline basement and proterozoic to phanerozoic sedimentary rocks the fine grained to cryptocrystalline texture of the more melt rich rocks despite their occurrence in bodies locally in excess of 800 m thick and 28 km long suggests that the melt crystallized in response to 1 cooling by the clast load and or 2 rapid nucleation on finely brecciated clasts which have since been assimilated and or 3 crystallization enhanced by the relatively low water contents of the melts rapid crystallisation of the melt is indicated by the lack of zoning in minerals the presence of glass the lack of strain recovery features in clasts and the lack of evidence for fractionation in the major and trace elements including the rare earth elements optical and analytical electron microscopy reveal that the previously reported division of the melt rocks into high and low temperature variants based on hand sample appearance or glass content is not warranted clasts within the melt rich rocks exhibit a wide range of shock metamorphic features though they are not distributed in the impact melts in a systematic manner this indicates that the melt rich rocks were well mixed during their formation thus juxtaposing unshocked with shocked material injection of mesostasis melt into partially melted checkerboard plagioclase and orthopyroxene clasts also occurred during this mixing stage

sub million year age resolution of precambrian igneous events by thermal extraction thermal ionization mass spectrometer pb dating of zircon application to crystallization of the sudbury impact melt sheet a new approach to zircon dating is described that potentially offers a considerable improvement in the accuracy of 207 pb 206 pb ages on a wider variety of samples and at less cost than that generally available from conventional techniques zircon is first preheated in a vacuum to evaporate pb from altered domains leaving predominantly pb with isotopes that preserve the primary age of the sample refractory pb is removed from the zircon by thermal extraction at higher temperature directly into a silica melt ionization activator within a thermal ionization mass spectrometer this produces strong ion beams that allow 207 pb 206 pb isotope ratios to be measured to high precision 10 4 with almost negligible contamination from common pb isotope fractionation appears to be more reproducible than with conventional solution analysis allowing routine age determinations with precision of 0 2 m y for the precambrian as a test application zircon from a noritic boundary phase of the sudbury impact melt gives 1849 53 0 21 ma while a phase from several hundred meters higher in the noritic layer is resolvably younger at 1849 11 0 19 ma 95 confidence errors the enhanced precision and ease of application of this method should greatly increase the scientific power and availability of zircon dating

late jurassic age for the morokweng impact structure southern africa abstract a roughly 70 km diameter circular feature buried beneath the kalahari sands in south africa is revealed on regional aeromagnetic maps boreholes drilled into the centre of the structure intercept a 250 m thick sheet of quartz norite interpreted as an impact melt which overlies brecciated and shock metamorphosed basement granite zircons recovered from the quartz norite yield u pb ages of 145 0 8 ma and biotites provide ar ar ages of 144 4 ma these data provide strong evidence for the occurrence of a late jurassic impact crater the morokweng impact structure 100 m beneath the surface

radiation damage in zircon and monazite monazite and zircon respond differently to ion irradiation and to thermal and irradiation enhanced annealing monazite cannot be amorphized by 800 kev kr ions at temperatures greater than 175 c whereas zircon can be amorphized at temperatures up to 740 c the damage process i e elastic interactions leading to amorphization in radioactive minerals metamictization is basically the same as for the ion beam irradiated samples with the exception of the dose rate which is much lower in the case of natural samples the crystalline to metamict transition in natural samples with different degrees of damage from almost fully crystalline to completely metamict is compared to the sequence of microstructures observed for ion beam irradiated monazite and zircon the damage accumulation process representing the competing effects of radiation induced structural disorder and subsequent annealing mechanisms irradiation enhanced and thermal occurs at much higher temperatures for zircon than for monazite the amorphization dose expressed as displacements per atom is considerably higher in the natural samples and the atomic scale process leading to metamictization appears to develop differently ion beam induced amorphization data were used to calculate the decay event dose required for amorphization in terms of a critical radionuclide concentration i e the concentration above which a sample of a given age will become metamict at a specific temperature this equation was applied to estimate the reliability of u pb ages to provide a qualitative estimate of the thermal history of high u natural zircons and to predict whether actinide bearing zircon or monazite nuclear waste forms will become amorphous metamict over long timescales

u th he dating of terrestrial impact structures the manicouagan example the accurate dating of meteorite impact structures on earth has proven to be challenging melt sheets are amenable to high precision dating by the u pb and 40ar 39ar methods but many impact events do not produce them or they are not preserved in cases where high temperature shock metamorphism of the target materials has occurred without widespread melting these isotopic chronometers may be partially reset and yield dates that are difficult to interpret unambiguously as the age of impact however the u th he chronometer is sensitive to thermal resetting and can provide a powerful new tool for dating impactites we report u th he dates for accessory minerals from the manicouagan impact structure in quebec canada nine zircons from a melt sheet sample yield a weighted mean age of 213 2 5 4 ma 2se indistinguishable from the published 214 1 ma 2 u pb zircon age for the impact in contrast five apatites from this sample yield dates between 205 9 6 5 and 162 0 5 3 ma 2 indicating variable postimpact helium loss due to low temperature thermal disturbance preimpact titanite crystals from a shocked meta anorthosite sample yield two dates consistent with the impact age at 212 27 and 214 13 ma 2 and two younger dates of 189 6 6 9 and 192 2 9 8 ma 2 suggestive of postimpact helium loss these results indicate that u th he chronometry is a suitable method for dating impact events although interpretation of the results requires recognition of possible 4he loss related to reheating subsequent to impact

fluid driven resetting of titanite following ultrahigh temperature metamorphism in southern madagascar abstract la icp ms u pb dates and epma trace element maps of titanite were collected from calc silicate gneisses of the ikalamavony peak t 750 800 c and anosyen peak t 900 950 c domains in southern madagascar to evaluate how titanite responds to high temperature metamorphism cooling and retrogression fluid mediated replacement of precursor titanite by titanite domains of different composition interface coupled dissolution precipitation icdr was the primary mechanism by which titanite was reset following high grade metamorphism comparison of titanite u pb dates 530 490 ma with independent petrology and thermochronology indicates that the alteration occurred at temperatures as low as 300 500 c apparent zr temperatures temperatures calculated assuming titanite quartz zircon equilibrium in altered titanite are less than or equal to the metamorphic peak but higher than the inferred alteration temperature implying that zr was removed but that titanite quartz zircon equilibrium was not achieved during alteration although evidence for icdr was observed in titanite samples separated by 10 s to 100 km differences in u pb dates among samples and even among titanite grains in the same thin section suggest that alteration at any given time was localized

in situ multiphase u pb geochronology and shock analysis of apatite titanite and zircon from the lac la moinerie impact structure canada in situ u pb geochronology has been performed via laser ablation inductively coupled mass spectrometry on shocked apatite titanite and zircon from the 8 km diameter lac la moinerie impact structure canada the three analyzed phases are inherited from target rocks that were transformed to impact melt bearing breccias and clast laden impact melts apatite yields an array of u pb ratios populating a region between a younger regression with a refined age of 453 5 ma and an older regression with a lower intercept age of 1708 10 ma titanite defines a younger regression with an age of 444 15 ma and an older array with a lower intercept at 1844 24 ma zircon yields an upper intercept age of 1810 13 ma and a lower intercept age of 433 21 ma within error of apatite and titanite the oldest ages from all three phases define paleoproterozoic igneous metamorphic events in granitoid target rocks of the de pas suite isotopic resetting in apatite and titanite with relatively high pb diffusion rates was controlled by a combination of proximity to impact generated superheated melt and the formation of fast diffusion pathways facilitated by shock effects and dynamic recrystallization zircon resetting was controlled by shock effects and metamictization which resulted in significant pb loss we outline an approach for discriminating true impact ages from metamict zircons with high common pb that have experienced post impact pb loss lower intercept ages of all phases constrain the age of formation for the lac la moinerie impact structure with apatite yielding the most precise age of 453 5 ma with an age of 453 ma lac la moinerie joins a growing cluster of more than 12 terrestrial impact structures formed in the middle to late ordovician with potential connection to the l chondrite parent body breakup event and a period of enhanced bombardment on earth

late alteration in titanite catisio5 redistribution and remobilization of rare earth elements and implications for u pb and th pb geochronology and nuclear waste disposal abstract translucent dark coloured titanites from a wide variety of geological environments are commonly partly replaced by very fine grained aggregates of ree rich minerals including allanite monazite and bastnasite rutile quartz calcite apatite and chlorite along grain fractures and boundaries the aggregates of ree rich minerals are bordered by an irregular zone of altered titanite which by secondary ion mass spectrometry sims is considerably lower in rees u and th than the adjacent unaltered matrix light rees and th are differentially depleted in the altered zone and correspondingly enriched in secondary minerals additionally hrees and u have been lost to the external system the loss of hrees being in proportion to the increase in atomic number natural radiation damage was not an important factor in promoting the late alteration of titanite titanite which has been subjected to late hydrothermal activity would not remain a closed system in respect to either u or th decay series and therefore cannot be used for u pb and th pb geochronology the observed leaching and partial dissolution of titanite under hydrothermal conditions are generally consistent with laboratory experimentation indicating that proper isolation from any communication with hydrothermal fluids is essential in the application of titanite based ceramics for the immobilization of radioactive waste

context matters ar ar results from in and around the manicouagan impact structure canada implications for martian meteorite chronology abstract as an analog for interpretations of the ages of martian shergottite meteorites we have conducted an argon isotopic study of plagioclase feldspars exhibiting varying levels of shock from in and around the manicouagan impact structure canada plagioclase from the impact melt sheet at manicouagan yields an age of 215 40 0 16 ma which indicates the time of impact plagioclase from a clast within melt bearing breccias of the melt sheet and a hornfels adjacent to the melt sheet yield ages of 216 3 ma and 218 7 ma respectively which are interpreted to have been reset by contact metamorphism from the impact melt country rocks that were unaffected by the impact gives 849 ma ages consistent with the known grenvillian target rock history maskelynite amorphous plagioclase which has been transformed in the solid state yields an age of 567 6 ma this age is geologically meaningless because it is not consistent with the target age the impact age or regional metamorphic ages at manicouagan our results show that maskelynite argon ages are not meaningful and that context is critical for proper interpretation of impact affected argon ages

shocked titanite records chicxulub hydrothermal alteration and impact age abstract hydrothermal activity is a common phenomenon in the wake of impact events yet identifying and dating impact hydrothermal systems can be challenging this study provides the first detailed assessment of the effects of shock microstructures and impact related alteration on the u pb systematics and trace elements of titanite catisio5 focusing on shocked granite target rocks from the peak ring of the chicxulub impact structure mexico a 1 mm long shock twinned titanite grain preserves a dense network of irregular microcracks some of which exploit shock twin interfaces secondary microcrystalline anatase and pyrite are heterogeneously distributed along some microcracks in situ laser ablation multi collector inductively coupled plasma mass spectrometry la mc icpms analysis reveals a mixture of three end member pb components the pb components are 1 common pb consistent with the pb isotopic signature of adjacent alkali feldspar 2 radiogenic pb accumulated since magmatic crystallization and 3 a secondary younger pb signature due to impact related complete radiogenic pb loss the youngest derived ages define a regression from common pb that intersects concordia at 67 4 ma in agreement with the established age of 66 04 0 05 ma for the chicxulub impact event contour maps of la mc icpms data reveal that the young ages are spatially restricted to microstructurally complex domains that correlate with significant depletion in trace elements ree y zr nb mo sn th and reduction in magnitude of the eu eu anomaly mapping by time of flight secondary ion mass spectrometry tof sims show that patterns of localised element depletion in titanite are spatially related to microcracks which are enriched in al the spatial correlation of ages and trace element abundance is consistent with localised removal of pb and other trace elements from a pervasive network of fast fluid pathways in fractured domains via a fluid mediated element transport process associated with the impact event here we interpret the 67 4 ma u pb age to represent hydrothermal pb loss in the chicxulub peak ring in the wake of the impact event these results highlight the potential of our analytical approach using titanite geochronology and geochemistry for dating post impact hydrothermal activity in impact structures elsewhere

routine low damage apatite u pb dating using laser ablation multicollector icpms 1 apatite is a common u bearing accessory mineral with a u pb closure temperature of 500 c making u pb dating of apatite a potentially valuable thermochronometer however its low u concentration and tendency to incorporate common lead has limited widespread application to destructive isotope dilution methods we overcome previous limitations by using a nu plasma multicollector icpms with an attached short pulse excimer laser and by identifying two new matrix matched reference apatites to correct for elemental fractionation gem quality 485 myr old apatite from madagascar which we independently characterized by id tims analysis and 523 5 ma apatite from the mcclure mountain syenite source of the40ar 39ar reference mmhb common pb is corrected using measured 204pb isobarically corrected for hg interference and a five step iterative process using stacey and kramers common pb model we accurately reproduce ages of numerous independently characterized apatites regularly achieving precision of 2 2 by pooling as few as five 30 m spot analyses data quality in apatite with low u concentrations low 206pb 204pb values 30 and young ages 75 ma is compromised by the goal of avoiding significant grain damage such limitations can be overcome by using spot sizes 65 m or greater but at the expense of substantial grain damage for single detrital apatite grains with ages of 500 ma precision of 4 2 was achieved by pooling 2 to 3 spots per grain the accuracy of our detrital results is supported by a good age match with similar closure temperature 40ar 39ar detrital hornblende ages from the same sediment

ground surface deformation patterns magma supply and magma storage at okmok volcano alaska from insar analysis 1 intereruption deformation 1997 2008 eruption magma storage had increased by 3 7 5 2 1 0 7 m 3 or 85 100 of the 1997 eruption volume we propose that the supply rate decreased in response to the diminishing pressure gradient between the shallow storage zone and a deeper magma source region eventuallytheeffectsofcontinuingmagmasupplyandvesiculationofstoredmagmacaused acriticalpressurethresholdtobeexceeded triggeringthe2008eruption asimilarpatternof initially rapid inflation followed by oscillatory but generally slowing inflation was observed prior to the 1997 eruption in both cases withdrawal of magma during the eruptions depressurized the shallow storage zone causing significant volcano wide subsidence and initiating a new intereruption deformation cycle

a previously unrecognized high temperature impactite from the steen river impact structure alberta canada here we report a previously unrecognized impactite from the steen river impact structure in alberta canada which was intersected by continuous diamond drill core into the allochthonous proximal deposits of this buried 25 km diameter complex crater a suite of high temperature minerals defines the matrix formed by grain growth in a solid state by static recrystallization of an originally clastic matrix deposited at temperatures 800 c this rock type is predominantly a result of the recrystallization of target material driven by the acceleration of hot gasses from volatilized sedimentary cover mixed with variably shocked crystalline basement approximately one third of terrestrial impact structures occur in mixed target rocks therefore this type of impactite may be more common than previously realized contact metamorphism between entrained sedimentary target rocks and the juxtaposed hot matrix resulted in carbonate decomposition to form a rare spinel group mineral magnesioferrite in crater environments magnesioferrite has been found in the distal chicxulub mexico ejecta and may prove a novel indicator mineral for impact into carbonate bearing target rocks

the influence of geomorphological position and vegetation cover on the erosional and hydrological processes on a mediterranean hillslope soil erosion and runoff rates are assumed to be highly dependent on slope position however little knowledge exists about the hydrogeomorphological processes at the pedon scale that support this idea in order to assess the hydrological and erosional behaviour of soils at different slope positions simulated rainfall experiments 55 mm was applied during one hour were carried out on a south facing slope with underlying limestone in south east spain in the mean terms the erosion rates 9 g m2 hr 1 and the runoff coefficients 12 were very low at the scale of measurement 0 25 m2 the slope position does not affect erosion rates when the measurements are carried out under extreme dry conditions during summer the low runoff rates found in summer under thunderstorms of high intensity 5 year return period and the runon into surfaces with higher infiltration rates resulted in no detectable direct surface runoff hortonian at the slope scale this implies that erosion as a consequence of surface overland flow will only take place during events of high magnitude 55 mm hr 1 and low frequency 5 years vegetation is the most important factor determining the soil erosion and runoff rates within the slope 1998 john wiley sons ltd

dynamic deformation of etna volcano observed by satellite radar interferometry satellite radar interferometry of mt etna volcano sicily italy reveals a sequence of deformation characterized by deflation during the end of the 1993 eruption inflation from 1993 1995 with an increase in the inflation rate immediately before its resumed eruptive activity in late 1995 this was followed by very low deformation levels during the following year the source of the deformation changed from a depth of 9 km during deflation to more than 11 14 km during the subsequent inflation consistent with a model in which deflation at shallower levels is followed by inflation at greater depth as the volcano system recharges from below before its next eruption this study demonstrates that radar interferometry provides an important contribution towards understanding the dynamic deformation of volcanoes by revealing large scale changes in their pre eruption deformation rates radar interferometry could play an important role in volcano eruption monitoring

u pb geochronology of apatite and zircon from the brent impact structure canada a late ordovician sandbian katian boundary event associated with l chondrite parent body disruption in situ la icp ms u pb geochronology has been performed on apatite and zircon within thermally recrystallized clast laden and clast poor impact melt rocks from the brent impact structure a total of 377 laser analyses on 120 impact melt grown n 9 and impact recrystallized zircon grains n 111 were obtained from which a concordia age of 452 8 2 7 ma mswd 0 57 n 11 and a weighted average mean 206pb 238u age of 453 2 2 9 ma mswd 0 60 n 11 are calculated a total of 300 laser analyses from 100 relict apatite grains were obtained with an unanchored regression through all data yielding a lower intercept age of 453 2 6 0 ma mswd 5 8 n 300 that overlaps within error of zircon 207pb 206pb ratios obtained from feldspar clasts within clast laden impact melt retain the same initial pb composition as the target rocks from which they are derived while feldspars that crystallized from impact melt have 207pb 206pb ratios indicative of isotopic re equilibration between basement lithologies of two different ages a similar variability in 207pb 206pb is recorded by apatite this provides evidence for the involvement of neoproterozoic lake nipissing alkaline suite as well as mesoproterozoic grenville gneisses in the production of impact melt at brent recrystallized apatite grains exhibit enrichments in light rare earth elements lrees along neoblast grain boundaries indicative of trace element substitution and phase precipitation during impact induced recrystallization an age of 452 8 2 7 ma from zircon and 453 2 6 0 ma from apatite places the impact event in the late ordovician at or near the sandbian katian boundary confirming brent s involvement in the middle to late ordovician crater cluster event a period of enhanced impactor flux to earth related to the l chondrite parent body disruption

estimating lava volume by precision combination of multiple baseline spaceborne and airborne interferometric synthetic aperture radar the 1997 eruption of okmok volcano alaska interferometric synthetic aperture radar insar techniques are used to calculate the volume of extrusion at okmok volcano alaska by constructing precise digital elevation models dems that represent volcano topography before and after the 1997 eruption the posteruption dem is generated using airborne topographic synthetic aperture radar topsar data where a three dimensional affine transformation is used to account for the misalignments between different dem patches the preeruption dem is produced using repeat pass european remote sensing satellite data multiple interferograms are combined to reduce errors due to atmospheric variations and deformation rates are estimated independently and removed from the interferograms used for dem generation the extrusive flow volume associated with the 1997 eruption of okmok volcano is 0 154 spl plusmn 0 025 km sup 3 the thickest portion is approximately 50 m although field measurements of the flow margin s height do not exceed 20 m the in situ measurements at lava edges are not representative of the total thickness and precise dem data are absolutely essential to calculate eruption volume based on lava thickness estimations this study is an example that demonstrates how insar will play a significant role in studying volcanoes in remote areas

quantifying the release of climate active gases by large meteorite impacts with a case study of chicxulub potentially hazardous asteroids and comets have hit earth throughout its history with catastrophic consequences in the case of the chicxulub impact here we reexamine one of the mechanisms that allow an impact to have a global effect the release of climate active gases from sedimentary rocks we use the sova hydrocode and model ejected materials for a sufficient time after impact to quantify the volume of gases that reach high enough altitudes 25 km to have global consequences we vary impact angle sediment thickness and porosity water depth and shock pressure for devolatilization and present the results in a dimensionless form so that the released gases can be estimated for any impact into a sedimentary target using new constraints on the chicxulub impact angle and target composition we estimate that 325 130 gt of sulfur and 425 160 gt co2 were ejected and produced severe changes to the global climate

u pb isotopic dating of titanite microstructures potential implications for the chronology and identification of large impact structures identifying and dating large impact structures is challenging as many of the traditional shock indicator phases can be modified by post impact processes refractory accessory phases such as zircon while faithful recorders of shock wave passage commonly respond with partial u pb age resetting during impact events titanite is an accessory phase with lower pb closure temperature than many other robust chronometers but its potential as indicator and chronometer of impact related processes remains poorly constrained in this study we examined titanite grains from the sudbury ontario canada and vredefort south africa impact structures combining quantitative microstructural and u pb dating techniques titanite grains from both craters host planar microstructures and microtwins that show a common twin host disorientation relationship of 74 about in the vredefort impact structure the microtwins deformed internally and developed high and low angle grain boundaries that resulted in the growth of neoblastic crystallites u pb isotopic dating of magmatic titanite grains with deformation microtwins from the sudbury impact structure yielded a 207pb 206pb age of 1851 12 ma that records either the shock heating or the crater modification stage of the impact event the titanite grains from the vredefort impact structure yielded primarily pre impact ages recording the cooling of the ultra high temperature ventersdorp event but domains with microtwins or planar microstructures show evidence of u pb isotopic disturbance despite that the identified microtwins are not diagnostic of shock metamorphic processes our contribution demonstrates that titanite has great potential to inform studies of the terrestrial impact crater record

in situ la icp ms apatite and zircon u pb geochronology of the nicholson lake impact structure canada shock and related thermal effects abstract in situ laser ablation inductively coupled plasma mass spectrometry la icp ms has been deployed to determine the u pb ages of impact metamorphosed apatite and zircon associated with the 12 5 km diameter nicholson lake impact structure northwest territories canada the dated phases occur within both impact melt bearing breccias and clast laden impact melts a total of 84 laser ablation analyses from 57 apatite grains within seven rock samples yield a minimum refined lower intercept at 387 5 ma mswd 0 87 2 n 26 and maximum age with lower intercept of 1740 ma a total of 90 laser ablation analyses on 52 zircon grains from two rock samples yield a minimum intercept age of 384 8 ma mswd 1 3 2 n 22 with a maximum upper intercept age of 2679 14 ma and a second discordia with a 1740 ma upper intercept age the results are consistent with the target rocks comprising archean snow island suite 2 7 ga and paleoproterozoic nueltin plutonic suite 1 74 ga with the impact event occurring at approximately 385 ma the degree of resetting of inherited apatite can be related to its proximity to impact melt now partly devitrified glass within the host impact melt bearing breccias and clast laden impact melt rocks those grains in direct contact with melt bodies are reset while those occurring as inclusions in lithic or mineral clasts partly or wholly retain their original isotopic compositions the youngest impact reset apatite ages are most closely associated with the highest shock levels i e granular textured and thermally dissociated zircons which we relate to juxtaposition of apatite with superheated melt due to accumulated radiation damage prior to impact many of the relic zircons are metamict which facilitated enhanced pb diffusion and their resetting to the impact age granular zircons record impact ages while those exhibiting planar fractures or experiencing negligible shock record basement ages we link apatite and zircon geochronology to the textural and structural states of zrsio4 and its associated shock levels via field emission scanning electron microscopy and micro raman spectrometry

on the absence of insar detected volcano deformation spanning the 1995 1996 and 1999 eruptions of shishaldin volcano alaska shishaldin volcano a large frequently active basaltic andesite volcano located on unimak island in the aleutian arc of alaska had a minor eruption in 1995 1996 and a vei 3 sub plinian basaltic eruption in 1999 we used 21 synthetic aperture radar images acquired by ers 1 ers 2 jers 1 and radarsat 1 satellites to construct 12 coherent interferograms that span most of the 1993 2003 time interval all interferograms lack coherence within 5 km of the summit primarily due to persistent snow and ice cover on the edifice remarkably in the 5 15 km distance range where interferograms are coherent the insar images show no intrusion or withdrawal related deformation at shishaldin during this entire time period however several insar images do show deformation associated with a shallow ml 5 2 earthquake located 14 km west of shishaldin that occurred 6 weeks before the 1999 eruption we use a theoretical model to predict deformation magnitudes due to a volumetric expansion source having a volume equivalent to the 1999 erupted volume and find that deformation magnitudes for sources shallower than 10 km are within the expected detection capabilities for interferograms generated from c band ers 1 2 and radarsat 1 synthetic aperture radar images we also find that insar images cannot resolve relatively shallow deformation sources 1 2 km below sea level due to spatial gaps in the insar images caused by lost coherence the lack of any deformation particularly for the 1999 eruption leads us to speculate that magma feeding eruptions at the summit moves rapidly at least 80m day from n10 km depth and that the intrusion eruption cycle at shishaldin does not produce significant permanent deformation at the surface published by elsevier b v

effect of natural restoration time of abandoned farmland on soil detachment by overland flow in the loess plateau of china vegetation restoration has significant effects on soil properties and vegetation cover and thus affects soil detachment by overland flow few studies have been conducted to evaluate this effect in the loess plateau where a great green project was implemented in the past decade this study was carried out to quantify the effects of age of abandoned farmland under natural vegetation restoration on soil detachment by overland flow and soil resistance to erosion as reflected by soil erodibility and critical shear stress the undisturbed soil samples were collected from five abandoned farmlands with natural restoration age varying from 3 to 37 years the samples were subjected to flow scouring in a 4 0 m long by 0 35 m wide hydraulic flume under six different shear stresses ranging from 5 60 to 18 15 pa the results showed that the measured soil detachment capacities in currently cultivated farmland were 24 1 to 35 4 times greater than those of the abandoned farmlands for the abandoned farmlands soil detachment capacities fluctuated greatly due to the complex effects of root density and biological crust thickness and could be simulated well by flow shear stress and biological crust thickness with a power function nse 0 851 soil erodibility of abandoned farmlands decreased gradually with restoration age and reached a steady stage when restoration age was greater than 28 years the critical shear stress of the natural abandoned farmlands declined when restoration age was less than 18 years and then increased due to the episodic influences of vegetation recovery and biological crust development more studies in the loess plateau are necessary to quantify the relationship between soil detachment capacity and biological crust thickness for better understanding the mechanism of soil detachment under natural vegetation restoration copyright 2013 john wiley sons ltd

chronostratigraphy of the hottah terrane and great bear magmatic zone of wopmay orogen canada and exploration of a terrane translation model the paleoproterozoic hottah terrane is the westernmost exposed bedrock of the canadian shield and a critical component for understanding the evolution of the wopmay orogen thirteen new high precision u pb zircon crystallization ages are presented and support field observations of a volcano plutonic continuum from hottah terrane through to the end of the great bear magmatism from 1950 to 1850 ma the new crystallization ages new geochemical data and newly published detrital zircon u pb data are used to challenge hitherto accepted models for the evolution of the hottah terrane as an exotic arc and microcontinent that arrived over a west dipping subduction zone and collided with the slave craton at ca 1 88 ga although the hottah terrane does have a tectonic history that is distinct from that of the neighbouring slave craton it shares a temporal history with a number of domains to the south and east domains that were tied to the slave craton by ca 1 97 ga it is interpreted herein that hottah terra

u pb ages of shocked zircon grains link distal k pg boundary sites in spain and italy with the chicxulub impact abstract the u pb ages of shocked zircon crystals from the chicxulub impact crater and cretaceous paleogene k pg boundary sites in haiti the usa and canada and the pattern of decreasing particle size with paleodistance from the crater have been used as evidence of a genetic link between chicxulub and the k pg boundary despite this the inference that the k pg boundary layer formed as a direct consequence of the chicxulub impact has been repeatedly questioned here we present u pb id tims ages and textural evidence of shock metamorphosed zircon grains from the k pg boundary at caravaca spain and petriccio italy that establish a causal connection between the impact and formation of the k pg boundary layer the shocked zircon grains give data that produce a characteristic age pattern which indicates a primary source age of 549 5 5 7 ma and a secondary event at the approximate time of impact at 66 ma the intensity of the shock features is proportional to the degree of isotopic resetting and all textural features and ages are analytically identical to those of previously analyzed zircon from chicxulub and k pg boundary sites in north america caravaca and petriccio were 8000 km from chicxulub at the time of impact and are therefore the farthest k pg sites identified that can be linked to chicxulub through the dating of individual shocked zircon grains we conclude that the combined age data and textural observations provide unambiguous evidence that ejecta from the chicxulub impact formed the global k pg boundary layer these data cannot be explained by the alternative scenario that the chicxulub impact occurred 300 ka prior to the k pg boundary

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

shock metamorphosed zircon in terrestrial impact craters to ascertain the progressive stages of shock metamorphism of zircon samples from three well studied impact craters were analyzed by optical microscopy scanning electron microscopy sem and raman spectroscopy in thin section and grain separates these samples are comprised of well preserved rapidly quenched impactites from the ries crater germany strongly annealed impactites from the popigai crater siberia and altered variably quenched impactites from the chicxulub crater mexico the natural samples were compared with samples of experimentally shock metamorphosed zircon below 20 gpa zircon exhibits no distinct shock features above 20 gpa optically resolvable planar microstructures occur together with the high pressure polymorph reidite which was only retained in the ries samples decomposition of zircon to zro2 only occurs in shock stage iv melt fragments that were rapidly quenched this is not only a result of post shock temperatures in excess of 1700 c but could also be shock pressure induced which is indicated by possible relics of a high pressure polymorph of zro2 however zro2 was found to revert to zircon with a granular texture during devitrification of impact melts other granular textures represent recrystallized amorphous zrsio4 and reidite that reverted to zircon this requires annealing temperatures 1100 c a systematic study of zircons from a continuous impactite sequence of the chicxulub impact structure yields implications for the post shock temperature history of suevite like rocks until cooling below 600 c

mjolnir structure an impact crater in the barents sea a systematic search for impact indicators was conducted on a core of late jurassic early cretaceous sedimentary strata from the vicinity of the proposed mjolnir impact structure barents sea a 0 8 m thick section of the core was found to contain unequivocal indicators of meteoritic impact shocked quartz grains and a strong enrichment in iridium the ejecta bearing strata were discovered only 30 km north northeast of the structure within a stratigraphic interval corresponding to the seismically defined deformation event at mjolnir further study of this unusually well presented impact crater ejecta layer pair may help constrain poorly understood aspects of large magnitude meteorite impacts into the oceans 14 refs 4 figs

topographic zonation of infiltration in the hilly loess region north china field infiltration tests using portable rainfall infiltrometers were conducted in the wangjiagou experimental basin in the hilly loess region of north china based on data collected at 27 sites a topographic zonation of infiltration characteristics was observed the average steady infiltration rate and the average ponding time decreased from the hilltop to the hillslope and further decreased to the gully wall such a zonation is closely related to the variations of topography soil and land use conditions in the study area a general infiltration model is proposed collected field data are used to establish the applicability of the proposed model in the study area

new shock microstructures in titanite catisio5 from the peak ring of the chicxulub impact structure mexico accessory mineral geochronometers such as apatite baddeleyite monazite xenotime and zircon are increasingly being recognized for their ability to preserve diagnostic microstructural evidence of hypervelocity impact processes to date little is known about the response of titanite to shock metamorphism even though it is a widespread accessory phase and a u pb geochronometer here we report two new mechanical twin modes in titanite within shocked granitoid from the chicxulub impact structure mexico titanite grains in the newly acquired core from the international ocean discovery program hole m0077a preserve multiple sets of polysynthetic twins most commonly with composition planes k1 1 11 and shear direction 1 and less commonly with the mode k1 130 1 in some grains 130 deformation bands have formed concurrently with the deformation twins indicating dislocation slip with burgers vector b can be active during impact metamorphism titanite twins in the modes described here have not been reported from endogenically deformed rocks we therefore propose this newly identified twin form as a result of shock deformation formation conditions of the twins have not been experimentally calibrated and are here empirically constrained by the presence of planar deformation features in quartz 12 5 and 17 5 gpa and the absence of shock twins in zircon 20 gpa while the lower threshold of titanite twin formation remains poorly constrained identification of these twins highlight the utility of titanite as a shock indicator over the pressure range between 12 and 17 gpa given the challenges to find diagnostic indicators of shock metamorphism to identify both ancient and recent impact evidence on earth microstructural analysis of titanite is here demonstrated to provide a new tool for recognizing impact deformation in rocks where other impact evidence may be erased altered or did not manifest due to generally low 20 gpa shock pressure

the suevite of drill hole yucatan 6 in the chicxulub impact crater the suevite breccia of the chicxulub impact crater yucatan mexico is more variable and complex in terms of composition and stratigraphy than suevites observed at other craters detailed studies microscope electron microprobe sem xrf have been carried out on a noncontinuous set of samples from the drill hole yucatn 6 y6 located 50 km sw from the center of the impact structure three subunits can be distinguished in the suevite the upper unit is a fine grained carbonate rich suevite breccia with few shocked basement clasts mostly altered melt fragments and formerly melted carbonate material the middle suevite is a coarse grained suevite with shocked basement clasts and altered silicate melt fragments the lower suevite unit is composed of shocked basement and melt fragments and large evaporite clasts the matrix of the suevite is not clastic but recrystallized and composed mainly of feldspar and pyroxene the composition of the upper members of the suevite is dominated by the sedimentary cover of the yucatan target rock with depth in well y6 the amount of carbonate decreases and the proportion of evaporite and silicate basement rocks increases significantly even at the thin section scale melt phases of different chemistry can be identified showing that no widespread homogenization of the melt took place the melt compositions also reflect the heterogeneity of the deep yucatan basement calcite with characteristic feathery texture indicates the existence of formerly pure carbonate melt the proportion of carbonate to evaporite clasts is less than 5 1 except in the lower suevite where large evaporite clasts are present this proportion constrains the amount of co2 and sox released by the impact event

the chicxulub impact event and its environmental consequences at the cretaceous tertiary boundary abstract an impact mass extinction hypothesis for the cretaceous tertiary k t boundary transition has been confirmed with multiple lines of evidence beginning with the discovery of impact derived ir in k t boundary sediments and culminating in the discovery of the chicxulub impact crater likewise a link between the chicxulub impact crater and k t boundary sediments has been confirmed with multiple lines of evidence including stratigraphic petrological geochemical and isotopic data the environmental effects of the chicxulub impact event were global in their extent largely because of the interaction of ejected impact debris with the atmosphere the environmental consequences of the chicxulub impact event and their association with the k t boundary mass extinction event indicate that impact cratering processes can affect both the geologic and biologic evolution of our planet