u pb isotopic study of relict zircon inclusions recovered from muong nong type tektites abstract isotopic compositions of u and pb were determined for relict zircon inclusions recovered from muong nong type tektites from the australasian and north american strewn fields these inclusions are generally opaque white with granular textures probably representing mechanical disaggregation and or local melting and recrystallization all zircons have strongly discordant u pb isotopic ratios and the isotopic ratios of most of the zircons are dominated by common pb this is interpreted as being the result of complete resetting of the isotopic system during the impact event and exchange with pb from the target sediments ages that may represent the target rocks were only retained by one sample representing a zircon that was extracted from a muong nong type georgiaite a value of 0 6 ga was obtained which is similar to the nd and sr model ages obtained for the north american tektites our results indicate that the u pb isotopic system in almost all zircons from tektites has been totally reset during the impact event this is in contrast to zircons extracted from impact melt rocks at some impact structures and or distal ejecta some of which are only partly reset

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

the formation of large neoblasts in shocked zircon and their utility in dating impacts uranium lead u pb geochronology of individual shocked zircon grains has unique potential for dating bolide impact events neoblasts in granular textured zircon have been recognized as the shock related feature most effective at recording the impact age here we report the discovery of large neoblasts 5 100 mu m in dimension in shocked zircon at the sudbury impact structure canada the first report of in situ coarsely granular zircon from a terrestrial impact site other than the vredefort structure south africa the neoblast bearing sample was taken from a heterogeneous lithic clast rich igneous unit associated with the roof rocks of the impact melt sheet making this the first time a crater has been dated using neoblastic zircon from the upper part of its stratigraphy previous in situ discoveries of coarsely granular zircon at vredefort were all in impact generated mafic melt emplaced beneath the impact melt sheet electron backscatter diffraction analysis of the impact aged neoblasts indicates that the high pressure conditions inferred in the formation of many small neoblasts were not necessarily involved in the formation of these large ones their large size internal zonation and occurrence in a slowly cooling environment collectively suggest that large neoblasts at sudbury formed by relatively protracted post impact growth in shocked zircon incorporated into impact related melt based on insight from large neoblast growth in terrestrial settings we suggest that the ca 4 33 ga neoblasts recently reported in lunar zircon may imply a major basin forming event on the moon at that time new knowledge of the cratering environments in which large neoblasts form also raises the prospect of possibly linking ex situ granular zircon in lunar breccias with specific impact structures and thus better calibrating the lunar cratering record with radiometric ages

timing of crystallization of the lunar magma ocean constrained by the oldest zircon the primitive moon was covered with a thick layer of melt known as the lunar magma ocean whose crystallization resulted in the moon s surface as it is observed today dating of the oldest zircon so far in lunar rocks indicates that much of the magma was probably crystallized within 100 million years of the moon s formation

retention of u and pb in zircons from shocked granite in the siljan impact structure sweden the siljan ring structure central sweden was formed by a devonian impact the u pb zircon isotopic system of the shocked proterozoic granite basement appears to be undisturbed although the zircons were mechanically disrupted by the impact

capturing the acoustic radiation pattern of strombolian eruptions using infrasound sensors aboard a tethered aerostat yasur volcano vanuatu we obtained an unprecedented view of the acoustic radiation from persistent strombolian volcanic explosions at yasur volcano vanuatu from the deployment of infrasound sensors attached to a tethered aerostat while traditional ground based infrasound arrays may sample only a small portion of the eruption pressure wavefield we were able to densely sample angular ranges of 200o in azimuth and 50o in take off angle by placing the aerostat at 38 tethered loiter positions around the active vent the airborne data joined contemporaneously collected ground based infrasound and video recordings over the period 29 july to 1 august 2016 we observe a persistent variation in the acoustic radiation pattern with average eastward directed root mean squared pressures more than 2 times larger than in other directions the observed radiation pattern may be related to both path effects from the crater walls and source directionality

lahar infrasound associated with volcan villarrica s 3 march 2015 eruption the paroxysmal 2015 eruption of volcan villarrica chile produced a 2 5 h long lahar which descended more than 20 km within the rio correntoso turbio drainage and destroyed two small bridges a three element infrasound array 10 km from the summit and 4 km from the lahar s closest approach was used to study the flow s progression array processing using cross correlation lag times and semblance places constraints on the lahar s dynamics including detection of an initial flow pulse that traveled from 2 to 12 km at an average speed of 38 m s subsequently the lahar signal evolved to a relatively stationary infrasonic tremor located 10 to 12 km from the vent and adjacent to a topographic notch through which sound may have preferentially diffracted toward the recording site this study demonstrates the powerful capabilities of infrasound arrays for lahar study and suggests their potential application for future hazard monitoring

discovery of high pressure zrsio4 polymorph in naturally occurring shock metamorphosed zircons shock metamorphosed zircons were recovered from an upper eocene impact ejecta layer probably from the chesapeake bay impact structure found at three deep sea drilling project ocean drilling program sites 612 903 and 904 on the upper continental slope off new jersey the shocked zircons exhibit granular textures and some have planar features x ray diffraction patterns show that some of the shocked zircons contain a high pressure zrsio4 polymorph with a scheelite type structure that was first produced in laboratory experiments in 1969 the presence of the high pressure zrsio4 polymorph in some of the zircons indicates that they have been shock metamorphosed at pressures between 20 and 90 gpa the discovery of this phase in naturally occurring shocked zircons confirms previous predictions that this phase would be found in shock metamorphosed rocks from impact structures and it provides a new tool for calibrating peak shock pressures in impact structures and ejecta in addition because the high pressure zrsio4 polymorph has now been found in naturally occurring samples it can be given a mineral name

analysis of dynamics of vulcanian activity of ubinas volcano using multicomponent seismic antennas abstract a series of 16 vulcanian explosions occurred at ubinas volcano between may 24 and june 14 2009 the intervals between explosions were from 2 1 h to more than 6 days mean interval 33 h considering only the first nine explosions the average time interval was 7 8 h most of the explosions occurred after a short time interval

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

a terrestrial perspective on using ex situ shocked zircons to date lunar impacts deformed lunar zircons yielding u pb ages from 4333 ma to 1407 ma have been interpreted as dating discrete impacts on the moon however the cause of age resetting in lunar zircons is equivocal as ex situ grains in breccias they lack lithologic context and most do not contain microstructures diagnostic of shock that are found in terrestrial zircons detrital shocked zircons provide a terrestrial analog to ex situ lunar grains for both identifying diagnostic shock evidence and also evaluating the feasibility of dating impacts with ex situ zircons electron backscatter diffraction and sensitive high resolution ion microprobe u pb analysis of zircons eroded from the ca 2020 ma vredefort impact structure south africa show that complete impact age resetting did not occur in microstructural domains characterized by microtwins planar fractures and low angle boundaries which record ages from 2890 ma to 2645 ma an impact age of 1975 39 ma was detected in neoblasts within a granular zircon that also contains shock microtwins which link neoblast formation to the impact however we show that granular texture can form during regional metamorphism and thus is not unique to impact environments these results demonstrate that dating an impact with ex situ shocked zircon requires identifying diagnostic shock evidence to establish impact provenance and then targeting specific age reset microstructures with the recognition that zircon can deform plastically in both impact and magmatic environments age resetting in lunar zircons that lack diagnostic shock deformation may record magmatic processes rather than discrete impacts identifying shock microstructures that record complete age resetting for geochronological analysis is thus crucial for constructing accurate zircon based impact chronologies for the moon earth or other planetary bodies

geochronological constraints on the age of a permo triassic impact event u pb and 40ar 39ar results for the 40 km araguainha structure of central brazil abstract impact cratering has been a fundamental geological process in earth history with major ramifications for the biosphere the complexity of shocked and melted rocks within impact structures presents difficulties for accurate and precise radiogenic isotope age determination hampering the assessment of the effects of an individual event in the geological record we demonstrate the utility of a multi chronometer approach in our study of samples from the 40 km diameter araguainha impact structure of central brazil samples of uplifted basement granite display abundant evidence of shock deformation but u pb ages of shocked zircons and the 40ar 39ar ages of feldspar from the granite largely preserve the igneous crystallization and cooling history mixed results are obtained from in situ 40ar 39ar spot analyses of shocked igneous biotites in the granite with deformation along kink bands resulting in highly localized partial resetting in these grains likewise spot analyses of perlitic glass from pseudotachylitic breccia samples reflect a combination of argon inheritance from wall rock material the age of the glass itself and post impact devitrification the timing of crater formation is better assessed using samples of impact generated melt rock where isotopic resetting is associated with textural evidence of melting and in situ crystallization granular aggregates of neocrystallized zircon form a cluster of ten u pb ages that yield a concordia age of 247 8 3 8 ma the possibility of pb loss from this population suggests that this is a minimum age for the impact event the best evidence for the age of the impact comes from the u th pb dating of neocrystallized monazite and 40ar 39ar step heating of three separate populations of post impact inclusion rich quartz grains that are derived from the infill of miarolitic cavities the 206pb 238u age of 254 5 3 2 ma 2 error and 208pb 232th age of 255 2 4 8 ma 2 error of monazite together with the inverse 18 point isochron age of 254 10 ma mswd 0 52 for the inclusion rich quartz grains yield a weighted mean age of 254 7 2 5 ma 0 99 2 error for the impact event the age of the araguainha crater overlaps with the timing of the permo triassic boundary within error but the calculated energy released by the araguainha impact is insufficient to be a direct cause of the global mass extinction however the regional effects of the araguainha impact event in the parana karoo basin may have been substantial

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

sims u pb study of zircon from apollo 14 and 17 breccias implications for the evolution of lunar kreep abstract we report the results of a sims u pb study of 112 zircons from breccia samples from the apollo 14 and 17 landing sites zircon occurs in the breccia matrices as rounded irregular shaped broken and rarely euhedral grains and as constituent minerals in a variety of lithic clasts ranging in composition from ultra mafic and mafic rocks to highly evolved granophyres crystallisation of zircon in magmatic rocks is governed by the zirconium saturation in the melt as a consequence the presence of zircon in mafic rocks on the moon implies enrichment of their parent melts in the kreep component our sims results show that the ages of zircons from mafic to ultramafic clasts range from ca 4 35 ga to ca 4 00 ga demonstrating multiple generations of kreepy mafic and ultramafic magmas over this time period individual zircon clasts in breccia matrices have a similar age range to zircons in igneous clasts and all represent zircons that have been incorporated into the breccia from older parents the age distributions of zircons from breccias from both the apollo 14 and apollo 17 landing sites are essentially identical in the range 4 35 4 20 ga however whereas apollo 14 zircons additionally show ages from 4 20 to 3 90 ga no zircons from apollo 17 samples have primary ages less than ca 4 20 ga also in contrast to previous suggestions that the magmatism in the lunar crust is continuous our results show that the zircon age distribution is uneven with distinct peaks of magmatic activity at ca 4 35 ga ca 4 20 ga in apollo 14 and 17 and a possible third peak in zircons from apollo 14 at ca 4 00 ga to explain the differences in the zircon age distributions between the apollo 14 and 17 landing sites we propose that episodes of kreep magmatism were generated from a primary reservoir and that this reservoir contracted over time towards the centre of procellarum kreep terrane we attribute the peaks in kreep magmatism to impact induced emplacement of kreep magma from a primary mantle source or to a progressive thermal build up in the mantle source until the temperature exceeds the threshold for generation of kreep magma which is transported into the crust by an unspecified possibly plume like process

progressive deformation of feldspar recording low barometry impact processes tenoumer impact structure mauritania the tenoumer impact structure is a small well preserved crater within archean to paleoproterozoic amphibolite gneiss and granite of the reguibat shield north central mauritania the structure is surrounded by a thin ejecta blanket of crystalline blocks granitic gneiss granite and amphibolite and impact melt rocks evidence of shock metamorphism of quartz most notably planar deformation features pdfs occurs exclusively in granitic clasts entrained within small bodies of polymict glass rich breccia impact related deformation features in oligoclase and microcline grains on the other hand occur both within clasts in melt breccia deposits where they co occur with quartz pdfs and also within melt free crystalline ejecta in the absence of co occurring quartz pdfs feldspar deformation features include multiple orientations of pdfs enhanced optical relief of grain components selective disordering of alternate twins inclined lamellae within alternate twins and combinations of these individual textures the distribution of shock features in quartz and feldspar suggests that deformation textures within feldspar can record a wide range of average pressures starting below that required for shock deformation of quartz we suggest that experimental analysis of feldspar behavior combined with detailed mapping of shock metamorphism of feldspar in natural systems may provide critical data to constrain energy dissipation within impact regimes that experienced low average shock pressures

correlated δ 18 o and ti in lunar zircons a terrestrial perspective for magma temperatures and water content on the moon zircon grains were separated from lunar regolith and rocks returned from four apollo landing sites and analyzed in situ by secondary ion mass spectrometry many regolith zircons preserve magmatic 18o and trace element compositions and although out of petrologic context represent a relatively unexplored resource for study of the moon and possibly other bodies in the solar system the combination of oxygen isotope ratios and ti provides a unique geochemical signature that identifies zircons from the moon the oxygen isotope ratios of lunar zircons are remarkably constant and unexpectedly higher in 18o 5 61 0 07 vsmow than zircons from earth s oceanic crust 5 20 0 03 even though mare basalt whole rock samples are nearly the same in 18o as oceanic basalts on earth 5 6 thus the average fractionation of oxygen isotopes between primitive basalt and zircon is smaller on the moon 18o wr zrc 0 08 0 09 than on earth 0 37 0 04 the smaller fractionations on the moon suggest higher temperatures of zircon crystallization in lunar magmas and are consistent with higher ti in lunar zircons phase equilibria estimates also indicate high temperatures for lunar magmas but not specifically for evolved zircon forming melts if the solidus temperature of a given magma is a function of its water content then so is the crystallization temperature of any zircon forming in that melt the systematic nature of o and ti data for lunar zircons suggests a model based on the following observations many of the analyzed lunar zircons are likely from k rare earth elements p kreep zr rich magmas zircon does not saturate in normal mafic magmas igneous zircons in mafic rocks are typically late and formed in the last most evolved portion of melts even if initial bulk water content is moderately low the late zircon forming melt can concentrate water locally in general water lowers crystallization temperatures in which case late igneous zircon can form at significantly lower temperatures than the solidus inferred for a bulk rock composition although lunar basalts could readily lose h2 to space during eruption lowering water fugacity the morphology large size and presence in plutonic rocks suggest that many zircons crystallized at depths that retarded degassing in this case the crystallization temperatures of zircons are a sensitive monitor of the water content of the parental magma as well as the evolved zircon forming melt if the smaller 18o zircon mare basalt values reported here are characteristic of the moon then that would suggest that even highly evolved zircon forming magmas on the moon crystallized at higher temperature than similar magmas on earth and that magmas though not necessarily water free were generally drier on the moon

observations and modeling of wind driven currents in the northeast pacific abstract velocity measurements from the interior of the alaskan gyre are presented from a current meter mooring deployed in 4000 m of water at 49 33 n 138 38 w in the vicinity of ocean weather station p the mooring held five current meters which spanned the depth of the water column the data reveal surface intensified motions with flow fluctuations in the upper layers of the water column characterized by long period o 100 d time scales of variability at abyssal depths the flow displays shorter o 20 d time scales of variability the data are compared with observations from one of the nepac moorings in the northeast pacific 42 n 152 w similar characteristics in kinetic energy levels in the vertical structure of the flow and in the vertical variation of eddy time scales are found at this location the current measurements are considered in terms of the linear theory of directly wind driven variability of muller and frankignoul a comparison with simulated currents from a quasigeostrophic num

u pb systematics of zircon and titanite from the gardnos impact structure norway evidence for impact at 546 ma abstract zircon and titanite were investigated in impactites of the gardnos structure a crater formed in sveconorwegian ca 1 ga crust which was then overridden in the devonian by caledonian nappes observed deformation features in zircons are granular texture planar microstructures and likely the incorporation of organic carbon during impact causing black staining of the zircon grains the grains were studied by scanning electron microscopy sem and cathode luminescence cl and dated by u pb isotope dilution thermo ionization mass spectrometry id tims zircon grains without impact related features have u pb data showing moderate discordance 5 13 and indicating formation ages mostly in the range of 1600 1000 ma except detrital zircon ages as old as 2481 ma reflecting the diversity of target rocks in the area titanite with concordant ages of 995 999 ma dates metamorphism during final juxtaposition of the telemarkia on the idefjorden terrane to the east zircon grains with demonstrated or presumed shock features yield highly discordant 14 40 u pb data with a majority of them plotting along an array with a lower intercept of about 340 ma reflecting the influence of the caledonian orogeny and recent pb loss one zircon grain was totally reset at 379 ma during late caledonian metamorphism which also caused local growth of new titanite a specific group of zircon grains yields data with relatively high discordance for moderate u contents and five of these analyses including that of a grain with proven granular or aggregate texture fit a discordia line with an upper intercept of 546 5 ma these features are interpreted as indicating zircon break down to an amorphous state during impact with subsequent recrystallization into microcrystalline aggregates causing extensive to complete pb loss we further suggest that their crystallinity prevented pb loss during the caledonian orogeny while the small subgrain size and increasing metamictisation allowed more recent disturbances we thus interpret the 546 ma age as the approximate time of impact

preservation of detrital shocked minerals derived from the 1 85 ga sudbury impact structure in modern alluvium and holocene glacial deposits detrital shocked minerals can provide valuable residual records of eroded impact structures recent studies have reported shocked minerals in modern alluvium in a subtropical climate from the deeply eroded 2 02 ga vredefort dome impact basin in south africa to evaluate the detrital shocked mineral record at a large impact structure in a temperate setting with a holocene glacial erosional history we investigated 4000 detrital zircons and 20 000 quartz grains at the less eroded 1 85 ga sudbury basin in ontario canada for the presence of shocked sand grains modern alluvium from rivers within and outside the basin and holocene glaciofluvial sands eskers and outwash deltas across the basin were investigated for shocked minerals shocked zircon and or quartz were found in all modern rivers and most holocene glacial deposits within but not outside the basin petrography and scanning electron microscopy sem backscattered electron bse cathodoluminescence cl imaging and analysis energy dispersive x ray spectroscopy eds electron backscatter diffraction ebsd were used to document shock microstructures of the total detrital zircons surveyed 3 118 3978 were identified as shocked holocene samples contained higher average percentages of shocked zircon 63 1361 or 4 6 with a high of 29 compared to modern alluvium 55 2617 or 2 1 with a high of 6 ebsd analysis revealed a range of shock microstructures including planar fractures deformation microtwins and crystal plastic deformation at sudbury detrital shocked quartz is rare compared to zircon only 15 grains 0 08 were identified all with decorated planar deformation features pdfs these results demonstrate that a detrital shocked mineral record exists at a large impact basin that is in a youthful stage of erosion despite its age in addition to modern alluvium our results also identify glaciofluvial eskers and deltas as reservoirs for detrital shocked minerals glacial episodes thus enhance the dispersal and preservation of shocked detritus in sedimentary systems despite physical differences the observation that the two largest precambrian impact basins continue to contribute detrital shocked minerals 2 b y after impact suggests that a shocked mineral record of impacts on early earth should reside in precambrian siliciclastic rocks

reidite an impact produced high pressure polymorph of zircon found in marine sediments reidite is a high pressure polymorph of zircon with the scheelite structure it has been found in an upper eocene impact ejecta layer in marine sediments on the upper continental slope off new jersey and on barbados reidite occurs epitaxially oriented in shock metamorphosed zircons it is associated with impact glass tektites shocked quartz and feldspar with multiple sets of planar deformation features coesite and trace amounts of stishovite this phase was first produced in high pressure laboratory experiments in 1969 and has also been produced in shock recovery experiments reidite is brittle with an irregular fracture a hardness of 7 5 a calculated density of 5 2 g cm3 a white streak adamantine luster and it does not fluoresce in index oil in transmitted light shocked zircon grains consisting almost entirely of reidite are transparent pleochroism was not observed reidite appears to have parallel extinction and is length slow the maximum birefringence is roughly 0 015 reidite appears to be uniaxial positive it is tetragonal space group i 41 a a 4 738 1 a c 10 506 2 a v 235 84 2 a3 previous shock loading experiments on zircons indicate that the transition to reidite starts at about 30 gpa and is completed around 53 gpa reidite should be a useful indicator of peak pressure in shock metamorphosed rocks reidite is named after alan reid who first produced this phase in the laboratory

comparison of structures in zircons from lunar and terrestrial impactites there have been a number of reports of the presence of shock features such as planar fracturing and granulation the transformation of zircon to reidite and the breakdown of zircon to baddeleyite

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

nuclear blast induced nanotextures in quartz and zircon within trinitite the intense heat and pressure resulting from the detonation of the world s first nuclear device in the new mexico desert july 16 1945 severely altered the arkosic sand producing the fused glassy material referred to as trinitite the study of trinitite is key to the development of nuclear forensic techniques that can provide crucial information about a nuclear event such as device composition and radionuclide distribution moreover nuclear blasts are often considered analogs to catastrophic natural events such as meteorite impacts and it is well documented that with increasing impact severity zircon and quartz grains deform systematically in trinitite a sufficient number of primary quartz and zircon grains remain identifiable here a multi technique approach focused ion beam scanning electron microscopy transmission electron microscopy and micro raman spectroscopy is employed to study the micrometer to nanometer scale deformation features in altered grains of zircon and quartz to constrain blast pressure and temperature conditions trinitite zircon grains consistently show an outer halo of fibrous baddeleyite radiating from a relatively unaltered core hrtem images show complex twinning likely originating from an intermediate tetragonal zirconia precursor trinitite quartz grains show various states of melting that appear to vary predictably with depth below the surface of the desert sand grains occurring deeper than 1 5 cm are crystalline with occasional planar fractures at the optical scale at shallower depths a systematic increase in quartz vitrification is observed considered together these data suggest maximal temperatures in excess of 1500 c and pressures of 10 gpa the latter being considerably less than for any natural impact event taken in a broader context the implications of this work extend toward exploiting the use of advanced imaging techniques to improve our understanding of mineral processes in extreme non equilibrium environments at the near atomic scale

the barotropic electromagnetic and pressure experiment 1 barotropic current response to atmospheric forcing during 1986 1987 an array of horizontal electrometers which measure the subinertial barotropic velocity was deployed for 11 months in the central north pacific to study atmospherically forced motions in a low eddy kinetic energy region significant coherence is found to be ubiquitous between the barotropic velocity and one or more surface atmospheric variables over the north pacific the maximum squared coherence between currents and wind stress curl at all periods greater than 3 days is typically 0 6 rarely below 0 4 and occasionally above 0 8 and is dominantly nonlocal although local coherence is seen at some sites especially at the shorter periods both the coherence and the intersite pattern similarity are strongest at the periods of relative maxima in the barotropic current autospectra e g at 6 8 10 14 and 20 50 days the locations at which the wind stress curl forces the motions at each site and for each period have been determined while there is substantial intersite variability some of the observed patterns of squared coherence over the north pacific are very similar to those predicted by simple linear smooth bottomed quasi geostrophic models of atmospherically forced barotropic currents the forcing location is occasionally found to be the same spot for all oceanic instruments especially at the autospectral maxima in barotropic current more commonly distinct forcing locations are found for each instrument even when closely spaced suggesting topographic influence in other cases multiple forcing locations are implicated for the motions at a single station all of this site and period dependent variability of the forcing location is incompatible with the simple models and is due either to the known spatial inhomogeneity of the curl forcing field or to topographic effects the importance of the former is emphasized using simple arguments based on analytic transfer functions between the atmospheric and oceanic variables and on observed wavenumber spectra for wind stress curl quantitative assessment of topographic effects must await suitable numerical models in addition maximum likelihood wavenumber spectra of the barotropic currents have been estimated in those bands with an adequate number of coherent sensors the wavenumbers at the spectral peaks are compatible with the dispersion relation for rossby waves modified by the observed large scale linearly sloping topography which shoals to the northeast the observed group velocities are in general quite consistent with the inferred forcing locations based on the ocean atmosphere coherences

infrasound associated with 2004 2005 large sumatra earthquakes and tsunami 1 large earthquakes that occurred in the sumatra region in 2004 and 2005 generated acoustic waves recorded by the diego garcia infrasound array the progressive multi channel correlation pmcc analysis is performed to detect the seismic and infrasound signals associated with these events the study is completed by an inverse location procedure that permitted reconstruction of the source location of the infrasonic waves the results show that ground motion near the epicenter and vibrations of nearby land masses efficiently produced infrasound the analysis also reveals unique evidence of long period pressure waves from the tsunami earthquake m9 0 of december 26 2004

when the sky fell on our heads identification and interpretation of impact products in the sedimentary record research on impact products mechanisms and consequences has benefited greatly in the last ten years from the highly publicized cretaceous tertiary kt boundary debate this debate has led to an increasing awareness among earth scientists of the importance and potential effects of impact events in earth history with more and more researchers realizing the role played by impacts and becoming familiar with the characteristics of impact materials it is likely that in the coming years an increasing number of impact events will be identified in agreement with the earth cratering rate estimations grieve 1984 shoemaker et al 1990 the increasing recognition of impact event is demonstrated by the discovery or identification in recent years of several impact craters some of large diameter chicxulub 180 km in yucatan mjolnir 40 km in the barents sea ames 16 km in oklahoma avak 12 km in alaska pretoria saltpan 1 1 km and kalkkop 640 m both in south africa sediments record impact events in the form of impact glass shocked minerals ir anomalies and perhaps also polymic chaotic breccias and or tsunami deposits identification of impact products particularly in ancient rocks is hampered by poor preservation diagenetic alteration metamorphism misinterpretation or even lack of interest or awareness of their significance impact products represent very short term events most likely geographically localized and stratigraphically concentrated over a thin interval of sediments and often diluted by normal background deposition finding these impact signatures requires very detailed petrographic and geochemical investigations with closely spaced sampling a few cm such detail is not commonly used when studying ancient mesozoic and paleozoic sediments

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

thermal seismic and infrasonic evidences of variable degassing rates at stromboli volcano abstract during june 1999 we measured the amplitude and rate number of events per second at which gas exited the vent at stromboli volcano as discrete gas bursts or puffs this allowed us to identify two styles of gas burst puffing activity the first is characterized by frequent rapidly rising puffs the second by less frequent slowly rising puffs each style persisted over 5 40 min long durations and was associated with a high and low number of strombolian explosions per hour respectively each period was also associated with characteristic delay times between the arrival of the infrasonic and thermal signals during strombolian explosions the delays were longer during vigorous puffing periods to explain our observations we propose a model in which the degassing process cycles between vigorous and weak degassing phases during vigorous degassing phases bubble layers ascend the conduit at a frequency of 0 5 1 0 s 1 this high degassing level reflects a gas rich magma column and leads to an increased rate in the formation of shallow foams and hence an increase in puffing and explosive activity as well as a higher free surface level and or gas jet velocity during weak phases bubble layers ascend the conduit at a reduced frequency of 0 2 0 3 s 1 during such times the magma column is poor in gas this leads to a decreased rate of foam layer formation and hence a reduction in puffing and explosive activity as well as a lower free surface level and or gas jet velocity variations in puffing activity can thus be used to track changes in the rate at which the shallow system is supplied by fresh gas rich magma our observations indicate that the two degassing styles last from 5 to 40 min and that the switch from one to the other occurs over a matter of minutes

half a century of progress in research on terrestrial impact structures a review abstract the author who investigated the wolfe creek australia in 1962 and edited two benchmark sets of readings on meteorite craters and possible astroblemes in 1977 and 1979 reviews the state of knowledge at the present time the text is concerned with terrestrial impact structures geological features without any consideration of extraterrestrial analogues a handful of definitive publications are drawn on to present the story of terrestrial impact in a single article the text covers historical aspects briefly the effect of target variations the paucity of human observation of such large scale events distinction from volcanic endogenous structures modification by geological processes the transience of the crater initially formed on the target and its subsequent modifications the global geographic distribution of the 174 structures now listed of which a number are dubious attributions their distribution in geological time many ages being known only known to wide limits maximum or minimum values their size distribution calculations of impact frequencies shock effects processes on impact the stages of formation impact into shallow marine and deep sea targets impacts on ice about which little is known and finally the input of impact into biotic extinctions in this last lengthy section the summaries of the conclusions of scientists researching impact on earth and palaeontologists researching biotic impact are set side by side it is concluded that if the recent foraminiferal evidence obtained by gerta keller and associates is taken at its face value the case of impact as a sole agent in extinction is non existent biotic extinction is clearly a complex process involving a number of causes in some cases it was staggered in time and different sets of organisms responded quite differently and surprisingly even in the same extinction event extraterrestrial impact may have been one of the causes in some cases but it may have been regional rather than global in its effects we may never know how much input it had into the record of biotic extinction on earth an enormous amount of new knowledge has arisen from detailed studies of this new family of remarkable geological structures

analyses of shocked quartz at the global k p boundary indicate an origin from a single high angle oblique impact at chicxulub abstract the precise cause and timing of the cretaceous paleocene k p mass extinction 65 ma ago remains a matter of debate many advocate that the extinction was caused by a meteorite impact at chicxulub mexico and a number of potential kill mechanisms have been proposed for this although we now have good constraints on the size of this impact and chemistry of the target rocks estimates of its environmental consequences are hindered by a lack of knowledge about the obliquity of this impact an oblique impact is likely to have been far more catastrophic than a sub vertical one because greater volumes of volatiles would have been released into the atmosphere the principal purpose of this study was to characterize shocked quartz within distal k p ejecta to investigate whether the quartz distribution carried a signature of the direction and angle of impact our analyses show that the total number maximum and average size of shocked quartz grains all decrease gradually with paleodistance from chicxulub we do not find particularly high abundances in pacific sites relative to atlantic and european sites as has been previously reported and the size distribution around chicxulub is relatively symmetric ejecta samples at any one site display features that are indicative of a wide range of shock pressures but the mean degree of shock increases with paleodistance these shock and size distributions are both consistent with the k p layer having been formed by a single impact at chicxulub one site in the south atlantic contains quartz indicating an anomalously high average shock degree that may be indicative of an oblique impact with an uprange direction to the southeast 45 the apparent continuous coverage of proximal ejecta in this quadrant of the crater however suggests a relatively high impact angle of 45 we conclude that some of the more extreme predictions of the environmental consequences of a low angle impact at chicxulub are probably not applicable

infrasonic tremor observed at kīlauea volcano hawai i 1 infrasonic array data collected at k lauea volcano hawai i during november 12 21 2002 indicate that the active vents and lava tube system near the pu u vent complex emit almost continuous infrasound in the 0 3 10 hz frequency band the spectral content of these infrasonic signals matches well that of synchronous seismic tremor in sites protected from wind noise significant signal to noise ratios were recorded as far as 13 km from the crater of pu u the infrasonic recordings suggest that one or more tremor sources may be close to the surface in addition these results demonstrate that adequate site and instrument selections for infrasonic arrays are essential in order to obtain consistent and reliable infrasonic detections

source location variability and volcanic vent mapping with a small aperture infrasound array at stromboli volcano italy stromboli volcano in italy is a persistently active complex volcanic system in may 2002 activity was confined to 3 major summit craters within which several active vents hosted multiple explosions each hour during a 5 day field campaign an array of 3 low frequency microphones was installed to investigate the coherent infrasound produced by degassing from these vents consistent phase lags across the 3 stations indicate distinct sources that are subsequently investigated to determine the associated vent location apparent depth and origin time the cross correlation routine allows for variations in comparison window length waveform filtering bandwidth and correlation and consistency thresholds allowing for improved detection of certain types of degassing sources identification of activity at the various vents could be subsequently corroborated with 3 channels of synchronously acquired thermal data and video during the may 2002 experiment persistent energetic infrasound was observed from a passive degassing source within the central crater cc and transient infrasound produced by discrete strombolian explosions was identified at 4 additional vents the continuous infrasound produced by the cc exhibits variable frequency dependent correlation lag times that are interpreted as a diffraction effect due to the acoustic radiator s recessed location within a steep walled crater such dispersion has important implications for accurate eruption source modeling because it indicates that infrasonic waveforms may be significantly filtered during propagation transient explosion signals from the northeast crater nec and southwest crater swc vents also exhibit dynamic correlation lag times but this scatter may be more reasonably attributed to variable epicentral locations explosions from the nec west vent for instance appear to emanate from a diffuse zone with a lateral extent in excess of 10 m

interpreting u pb data from primary and secondary features in lunar zircon in this paper we describe primary and secondary microstructures and textural characteristics found in lunar zircon and discuss the relationships between these features and the zircon u pb isotopic systems and the significance of these features for understanding lunar processes lunar zircons can be classified according to i textural relationships between zircon and surrounding minerals in the host breccias ii the internal microstructures of the zircon grains as identified by optical microscopy cathodoluminescence cl imaging and electron backscattered diffraction ebsd mapping and iii results of in situ ion microprobe analyses of the th u pb isotopic systems primary zircon can occur as part of a cogenetic mineral assemblage lithic clast or as an individual mineral clast and is unzoned or has sector and or oscillatory zoning the age of primary zircon is obtained when multiple ion microprobe analyses across the polished surface of the grain give reproducible and essentially concordant data a secondary set of microstructures superimposed on primary zircon include localised recrystallised domains localised amorphous domains crystal plastic deformation planar deformation features and fractures and are associated with impact processes the first two secondary microstructures often yield internally consistent and close to concordant u pb ages that we interpret as dating impact events others secondary microstructures such as planar deformation features crystal plastic deformation and micro fractures can provide channels for pb diffusion and result in partial resetting of the u pb isotopic systems

shock behavior of zircon phase transition to scheelite structure and decomposition abstract both single crystal and powdered specimens of zircon zrsio4 were shocked to peak pressures between 30 and 94 gpa using the gun method and specimens recovered were studied by means of x ray diffraction analysis transmission electron microscopy and infrared spectroscopy transformation to the scheelite structure started above 30 gpa and was completed above 53 gpa in the case of single crystal specimens tetragonal unit cell parameters of the scheelite type zrsio4 at room condition are measured to bea 4 7341 1 a c 10 51 1 a c a 2 219 2 andv 235 5 2 a3 which is smaller than that of the zircon type by 9 9 the recovered scheelite type zrsio4 reverts to the zircon type after rapid heating to 1200 c at room pressure this transformation from the zircon type to the scheelite type is unique in that it is fast displacive like but does not reverse tetragonal zro2 was detected as decomposition product in the single crystal specimen shocked to 94 gpa and further confirmed in a powdered specimen shocked to 53 gpa where enhancement of temperature is expected because of high porosity decomposition behavior of zircon observed in natural shock events is discussed on the basis of present experimental results

mineralogic evidence for an impact event at the cretaceous tertiary boundary a thin claystone layer found in nonmarine rocks at the palynological cretaceous tertiary boundary in eastern montana contains an anomalously high value of iridium the nonclay fraction is mostly quartz with minor feldspar and some of these grains display planar features these planar features are related to specific crystallographic directions in the quartz lattice the shocked quartz grains also exhibit asterism and have lowered refractive indices all these mineralogical features are characteristic of shock metamorphism and are compelling evidence that the shocked grains are the product of a high velocity impact between a large extraterrestrial body and the earth the shocked minerals represent silicic target material injected into the stratosphere by the impact of the projectile

deterministic frequency wavenumber methods and direct measurements of rupture propagation during earthquakes using a dense array data analysis data recorded by the smart 1 array in taiwan are used to make the first direct measurements of two dimensional earthquake rupture propagation using frequency wavenumber techniques and ray theory we obtain estimates of the spatial extent duration and directions of rupture propagation during two earthquakes located offshore taiwan our estimates include uncertainties due to errors in fault plane location and orientation velocity structure and slowness ray parameter measured at the array we find that the january 29 1981 ml 6 3 taiwan earthquake ruptured unilaterally updip and toward the west on a 60 dipping 109 striking reverse fault using p waves we find its fault length and duration to be 25 18 km and 7 4 3 4 s respectively s waves indicate a fault length and duration of 27 15 km and 9 4 3 6 s respectively we suggest that the november 14 1986 ml 7 0 hualien taiwan earthquake was triggered by a shallow h 14 km foreshock ml 5 4 which occurred on a subparallel or splay fault approximately 5 s before the mainshock based on p waves rupture propagation during the foreshock is unilateral towards the northeast and slightly downdip on a 58 dipping 38 striking reverse fault its spatial extent and duration are approximately 23 km and 5 4 s the mainshock initiated at greater depth h 34 km than the foreshock with its primary direction of rupture unilateral toward the northeast and slightly updip on a 57 dipping 43 striking reverse fault using p waves we found a spatial extent and duration of 68 15 km and 12 4 2 2 s s waves indicate a similar extent duration and direction of propagation for the mainshock however we found it difficult to distinguish the foreshock and mainshock 5 waves we show by example that these results represent an improvement over what can be obtained using long period t 5 s teleseismic methods comparison of our results with those obtained using teleseismic data suggests either a short duration slip time function or that most slip occurs near the rupture front our results indicate that much of the complexity in the seismograms recorded during these events by the smart 1 array is due to spatial and temporal variations of the source this suggests that at least in some cases predictions of both the phase and amplitude of strong ground motion as a function of time would require a priori knowledge of source properties finally we recommend siting small 10 station dense arrays on land above offshore subduction zones which are considered seismic gaps because they can provide more detailed measurements of rupture propagation than is presently possible

complex history of a zircon aggregate from lunar breccia 73235 abstract results are reported of an investigation of the age and origin of the exceptional zircon aggregate in an anorthositic clast from lunar breccia 73235 cathodoluminescence and birefringence images show that the aggregate consists of numerous angular fragments of sector zoned primary zircon in a matrix of secondary zircon with an overall texture that resembles a pseudotachylite sims u pb analyses of the primary fragments and the matrix yielded two clearly defined ages an age of 4 315 0 015 ga and initial th u ratio of 0 21 0 35 for the primary zircon and an age of 4 187 0 011 ga and th u of 0 04 0 17 for the secondary zircon matrix a raman spectroscopic study the secondary matrix zircon was undertaken to investigate its structure results showed that the matrix has a zircon structure but there is also evidence for the presence of an amorphous component implications of the structural and u pb age data are discussed in terms of the origin and evolution of the aggregate and the history of lunar events it is proposed that an original single millimetre sized sector zoned zircon formed at 4 31 ga was subjected to a severe shock event at 4 18 ga this event resulted in the fracturing of the zircon the displacement and rotation of fragments the compression of the aggregate to a lensoid shape and the shock reduction of zircon to sub micron sized and amorphous granules in crush zones in the mosaic of fractures volatilisation loss of pb and the addition of u to the secondary zircon is attributed to processes activated by the extreme thermal pulse which accompanied the 4 18 ga shock event shock effects are seen in some of the primary fragments but raman spectra of the primary and secondary zircon show no evidence for pressure induced transformation of zircon to a scheelite structure the zircon u pb system has not been affected by the ca 3 95 ga thermal pulse that accompanied formation of the host breccia although this event has largely reset the k ar systems

the effect of 1 9 and 1 4 ga impact events on 4 3 ga zircon and phosphate from an apollo 15 melt breccia 1 zircon and phosphate grains from matrix and quartz monzodiorite qmd clasts in two thin sections of apollo 15 impact melt breccia 15405 were investigated using optical microscopy scanning electron microscopy raman spectroscopy and ion microprobe u pb analyses u pb results for zircon grains with well defined cathodoluminescence zoning define the primary i e magmatic crystallization age as 4330 6 ma 2 one zircon consists of a preserved inner part surrounded by a porous polycrystalline granular mixture of zircon and baddeleyite indicating incomplete reaction of the zircon with melt previous work showed that this microstructure could form at pressures above 60 gpa and a temperature close to 1700 c and is evidence of an impact related melting event the u pb system of this grain indicates a resetting event at 1940 10 ma interpreted as the age of this impact impact 1 other zircon and phosphate grains also have disturbed u pb systems showing an even younger reset event impact 2 at 1407 57 ma evidence of impact is supported by microstructures of zircon and baddeleyite such as secondary rims these impacts are tentatively identified as those having formed autolycus and aristillius craters

nanosims study of seismically deformed zircon evidence of y yb ce and p redistribution and resetting of radiogenic pb lattice defects in zircon can cause trace elements redistribution and disturbance of isotopic systems this study investigates in detail how seismically induced deformation microstructures in zircon correlate with trace element and isotope re equilibration felsic mylonites with pseudotachylyte veins from the ivrea verbano zone in northern italy were studied with special focus on deformed zircon we have revealed the following post growth deformation microstructures planar deformation bands pdbs planar fractures pfs non planar fractures both healed and open and finite strain patterns pdbs are planar portions of crystal lattice that are strictly parallel to 100 crystallographic planes and are rotated to up to 3 with respect to the host grain they are from 0 5 to 1 m wide and have average spacing of 5 m pdbs originate in seismically active environment at elevated differential stress strain rate and temperature several grains in which pdbs are observed were analyzed with ion microprobe ion maps indicate redistribution of radiogenic pb isotopes associated with pdb formation isotopic redistribution preferably occurs in pdbs with larger crystallographic misorientation profiling demonstrated clear spatial correlation of pdbs with variations of ree abundances both gain and loss and possible correlations with increased and decreased hf ti and p abundances trace elements can be depleted or enriched compared to the abundance in surrounding matrix in deformed domains depending on the spacing of pdbs and the proximity of the analyzed volume to grain boundary or to detrital core 207 pb 206 pb ratio demonstrates systematic pb loss in the pdb bearing lattice domains with respect to pdb free domains in some cases pb gain is observed where the pdbs source radiogenic pb from older detrital cores our study has important implications for geochronology and microchemistry of zircon from seismically deformed sections of ivrea verbano zone and from other paleo seismic zones of the world zircon found in seismically deformed rocks near pseudotachylyte veins may demonstrate distorted and even reset isotopic ages and altered trace element abundances enhanced trace element exchange between deformed zircon and host mylonite can influence mass balance calculations for the bulk rock

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

chicxulub crater source for shocked zircon crystals from the cretaceous tertiary boundary layer saskatchewan evidence from new u pb data new u pb data for five of six individual shocked zircon grains from the cretaceous tertiary k t boundary fireball layer rock creek site south central saskatchewan yield a source age of 548 6 ma the collinear data points are variably discordant in proportion to the intensity of shock metamorphism exhibited by each zircon the age and zircon shock features establish a strong temporal and genetic link with k t distal ejecta from the fireball layer at the berwind canyon site in the raton basin colorado where texturally similar zircons from a previous study gave an indistinguishable source age of 544 5 ma these ages are in agreement with independent ages obtained from a previous study for single shocked zircons from the chicxulub crater mexico and from beloc haiti the u pb zircon data unequivocally support a meteorite impact origin for the global k t boundary layer and imply that the chicxulub crater is the sole north american target source

isotope systematics and shock wave metamorphism ii u pb and rb sr in naturally shocked rocks the haughton impact structure canada minerals and whole rocks from crystalline fragments collected in the allochthonous polymict breccia of the 23 ma old haughton impact crater on devon island arctic canada were analysed for u pb k and rb sr in order to constrain degrees of shock wave induced perturbations in these systems the results show that shock wave metamorphism at 50 gpa with onset of total melting does not significantly affect the u pb chronometers in monazite and zircon monazite preserved its age of crystallization during high grade metamorphism at 1928 2 2 1 2 0 ma 2 errors as dated by the upper intercept of 2 9 discordant monazites the same analyses yield a lower intercept age of 277 5 35 1 35 0 ma in the concordia diagram demonstrating that discordancy is not a direct effect of shock wave metamorphism but a product of time integrated pb diffusion prior to the impact event which occurred 23 ma ago this constrains maximal shock induced pb loss to less than 3 including possible alteration related pb diffusion from the shock damaged grains in 1903 1 1 4 1 1 ma old monazite from an unshocked reference gneiss recent pb loss also reaches 3 corroborating the occurrence of weak alteration related upb fractionation the data substantiate that u pb discordancy patterns are not a reliable chronometer to date impact events small rock fragments and a coherent block of shock stage i to iii vi consist of heterogeneous unequilibrated rb sr sub systems on the 10 m scale with drastically varying isr shock related rbsr fractionation is very limited and in the large block stage iii three biotite fractions yield an rb sr isochron age of 1791 34 ma 2 which is undistinguishable from the 1814 8 ma age of biotite in the unshocked basement gneiss in contrast to this m3 sized fragment rb sr systematics in a fist size sample of shock stage iii do not yield any reliable age information due to small scale 1 cm3 isotopic disequilibrium it appears that such small scale disequilibrium is essentially due to a take up of radiogenic 87sr in feldspar glass from biotite during shock wave passage this prevents dating of the impact events by the rb sr internal isochron method and it can be concluded that total rock melting is the first condition necessary to reset the rb sr chronometer in shocked crystalline rocks

integrating puffing and explosions in a general scheme for strombolian style activity strombolian eruptions are among the most common subaerial styles of explosive volcanism worldwide distinctive features of each volcano lead to a correspondingly wide range of variations of magnitude and erupted products but most papers focus on a single type of event at a single volcano here in order to emphasize the common features underlying this diversity of styles we scrutinize a database from 35 different erupting vents including 21 thermal infrared videos from stromboli italy etna italy yasur vanuatu and batu tara indonesia from puffing through rapid explosions to normal explosions with variable ejection parameters and relative abundance of gas ash and bombs using field observations and high speed thermal infrared videos processed by a new algorithm we identify the distinguishing characteristics of each type of activity and how they may relate and interact in particular we record that ash poor normal explosions may be preceded and followed by the onset or the increase of the puffing activity while ash rich explosions are emergent i e with inflation of the free surface followed directly by emission of increasingly large gas pockets overall we see that all strombolian activities form a continuum arising from a common mechanism and are modulated by the combination of two well established controls 1 the length of the bursting gas pocket with respect to the vent diameter and 2 the presence and thickness of a high viscosity layer in the uppermost part of the volcanic conduit

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

formation of planar deformation features pdfs in zircon during coseismic faulting and an evaluation of potential effects on u pb systematics abstract zircons from pseudotachylyte and cataclasite veins in the svarthumlevatnet metagabbro sw norway are faulted brecciated and display multiple sets of planar deformation features pdfs that we assign to coseismic deformation in the middle crust during caledonian continent collision the zircons give a u pb upper intercept age of 1507 4 ma interpreted as a minimum age of the gabbro and a lower intercept age of 968 31 ma likely indicating the time of granulite facies metamorphism but they do not record the caledonian coseismic faulting even when the fault planes are healed by thin veins of low u zircon the work indicates that pdfs are not exclusive to meteorite impacts but may also develop during seismic faulting

motional induction effect on the planetary scale geoelectric potential in the eastern north pacific geoelectric potentials induced by seawater motion on a planetary scale were studied using voltage differences measured on the hawaii 1 haw 1 submarine cable between hanauma bay oahu hawaii and point arena california the motionally induced field was extracted from a 3 6 year segment of the haw 1 voltage by cancelling the component that is coherent with the externally produced geomagnetic field at honolulu the corrected voltage shows significant coherence with atmospheric variables wind stress curl wind stress and surface pressure estimated from the european centre for medium range weather forecasts product at points throughout the pacific basin at periods from 5 to 133 days this suggests that the voltage reflects regional to basin wide depth averaged barotropic water currents that are directly wind driven contour maps of the squared coherence reveal that 1 peak coherence as large as 0 6 0 8 is seen at points both near the cable and nonlocally 2 higher coherence is seen at periods where the cable voltage power spectrum peaks and 3 spatial patterns of the coherence vary with period comparison of power spectra or coherence maps for individual years shows that the wind driven flow fluctuates temporally owing to long term variations of the wind field the main features of these observations are consistent with those from point electric field measurements suggesting that planetary scale voltages measured with cables can serve as a useful tool to monitor large scale water currents at long periods

a record of ancient cataclysm in modern sand shock microstructures in detrital minerals from the vaal river vredefort dome south africa the record of terrestrial meteorite impacts is fragmentary because most impact structures and ejecta are removed by erosion or buried discovery of the missing impact record from hadean to present may be advanced through identifi cation of residual shocked detritus to evaluate which shocked minerals survive erosion and sedimentary transport we investigated modern sands from the vaal river in south africa where it crosses the 2 02 ga vredefort dome the largest terrestrial impact structure known to date shocked minerals were identifi ed in all sediment samples including from the vaal channel and tributaries within the structure in transmitted light detrital quartz preserves discontinuous decorated planar features previously identifi ed as brazil twins which are readily visible as bright continuous features in cathodoluminescence images detrital zircons preserve fi ve orientations of planar fractures pfs which can produce dramatically offset growth zoning and apparent rotation of subgrains other zircons contain fi lled fractures that may represent a new shock microstructure detrital monazite preserves four orientations of pfs and many grains contain oscillatoryzoned shocked zircon inclusions which thus represent shocked inclusions within shocked accessory grains zircon and monazite with granular texture were also identifi ed this study is proof of the concept that shocked minerals can be identifi ed in sediments up to 2 billion years after an impact event and it demonstrates their potential for preserving evidence of ancient impacts the recognition of a new geological repository for impact evi dence provides a means for identifying distal shocked detritus from eroded structures of any age and may be particularly relevant to early earth studies

linking shock textures revealed by bse cl and ebsd with u pb data la icp ms and sims from zircon from the araguainha impact structure brazil

interdecadal variations in the alaska gyre abstract climatic dynamic topography variations in the alaska gyre during the period 1968 1990 are described with an objective analysis of more than 12000 std and xbt stations and coads wind stress data interannual dynamic height and sst variations were correlated and were consistent with recently described large scale climatic shifts in the north pacific the gyre was cantered more to the east circulation appeared stronger and sst was lower during the early to mid 1970s than during the 1980s the aleutian low np and pna indices intensified during the interim but the response did not appear as a gyre spinup instead the associated wind stress anomalies forced a slowly varying dynamic height anomaly across the eastern and northern part of the gyre through ekman convergence which had the effect of displacing the gyre s low somewhat to the wsw in the 1980s the wind curl spectrum was white and the slow oceanic response was modelled as stochastic forced climate variability with a simple first order m

infrasonic monitoring at stromboli volcano during the 2003 effusive eruption insights on the explosive and degassing process of an open conduit system 1 stromboli is an open conduit volcano characterized by persistent conduit degassing and explosive dynamics during the 2003 effusive eruption we monitored this activity with a five element array deployed at 450 m from the active craters the array allows us to track in real time changes of explosive activity in terms of source position and excess pressure release infrasonic monitoring located the new active vents in the same preeruptive position revealing that the collapse of the crater terrace has not changed the geometry of the shallow feeding system elevation of the infrasonic source is stable at 750 m above sea level coincident with the elevation of the crater terrace this suggests a shallow position of the magma column or if the source is embedded in the conduit diffraction at the crater rim at the end of the eruption back azimuth analysis indicates that explosive activity was progressively reestablished at all of the craters following a ne toward sw direction along the collapsed trench the array detected significant small amplitude 1 pa infrasonic activity occurring at a rate of approximately one pulse every 2 s and generally located in only one vent at a time this persistent activity is not related to strombolian explosion but we infer it is reflecting the overpressurized degassing of the magma column amplitude distribution of 7 million infrasonic signals shows that this degassing is 1 order of magnitude more energetic than the explosions besides infrasonic amplitude distribution presents a decay with a double slope indicating that explosions and degassing are driven by conduit dynamics acting at two different rates

planar microstructures in zircon from paleo seismic zones pseudotachylytes resulted from frictional melts associated with ultramylonites in high grade metapelitic rocks from the ivrea verbano zone in the southern alps northern italy were studied with focus on the deformation microstructures in zircon the aims were to investigate the characteristics of zircon deformation in seismic zones and to recognize specific microstructures generated in zircon during earthquakes which could be useful for mineral dating of paleo seismic events helps to understand how seismic energy is released at depth and interacts with metamorphic processes the interior of polished zircon grains ranging from 30 to 150 m in length were investigated with optical microscope and scanning electron microscope sem techniques including secondary electron se backscattered electron bse forward scattered electron fse cathodoluminescence cl imaging and crystallographic orientation mapping by electron backscatter diffraction analysis ebsd grains were studied in situ and as separated fractions embedded in epoxy disks among different cataclastic and crystal plastic deformation microstructures in zircon we identified characteristic planar deformation bands pdbs planar fractures pfs and curviplanar fractures cfs planar deformation bands in zircon are crystallographically controlled planar lattice volumes with misorientation from the host grain which varies from 0 4 to 2 7 pdbs are usually parallel to 100 crystallographic planes have width from 0 3 to 1 m and average spacing of 5 m in 2d sections planar deformation bands appear as contrast lamellae in orientation contrast images and in ebsd maps and in rare cases can be observed with the optical microscope pdbs form in specifically oriented grains due to high differential stresses high temperatures and high strain rates generated in seismically active environment and or due to shearing in the vicinity of frictional melts discovered structures represent a result of crystal plastic deformation of zircon grains with operating dislocations having 010 glide system and misorientation axis therefore they can be classified as a new type 4 lattice distortion pattern according to the existing classification for zircon piazolo et al 2012 kovaleva et al 2014 we have demonstrated that formation of planar fractures in zircon takes place not only during impacts but also in seismically active zones we observe at least two cases of formation of pfs with 100 orientation 1 as a result of evolution of pdbs and 2 as micro cleavage this study demonstrates that planar microstructures in terrestrial zircon do not exclusively form during impact events but also as a result of seismic events at depth due to unusually high differential stress strain rate and temperature according to the new findings pdbs in zircon from the deep crust are supposed to represent newly recognized evidence of seismicity

characterization of the 2008 kasatochi and okmok eruptions using remote infrasound arrays 1 the 2008 plinian eruptions of kasatochi and okmok volcanoes were recorded by six remote international monitoring system infrasound arrays high amplitude infrasound at these stations combined with remote sensing permits insight into important volcanic source parameters such as origin times durations and source characteristics infrasound from the 7 8 august kasatochi eruption consists of three well defined eruption pulses with the first two steam rich and the last ash rich pulse 2 is the most energetic and impulsive okmok produced over 9 h of continuous infrasound on 12 13 july acoustic propagation modeling for the okmok eruption and first kasatochi pulse predict thermospheric ducting and origin times consistent with seismic and satellite observations however theoretical acoustic origin times of pulses 2 3 are predicted to occur 15 min earlier than the seismic stratospheric ducting for these later pulses provides more consistent origin times although both volcanoes ejected ash into the stratosphere 15 km kasatochi produced higher amplitude infrasound than okmok previous studies have shown sustained infrasound with frequencies 2000 km and does not seem to be appreciably affected by changes in ash content between the eruption pulses

very high k kreep rich clasts in the impact melt breccia of the lunar meteorite sau 169 new constraints on the last residue of the lunar magma ocean abstract in the impact melt breccia imb of sayh al uhaymir sau 169 the most kreep rich lunar meteorite to date gnos et al 2004 clasts of a new type of lithologies were discovered consisting of ca poor and ca rich pyroxenes 60 8 vol ba rich k feldspar 27 9 vol phosphates 5 6 vol nb rich ilmenite 4 0 vol zircon 1 2 vol and minor sulfide 0 6 vol these mafic lithic clasts are more enriched in kreep component 1500 ci than the host meteorite and are highly enriched in potassium they are referred to as very high k vhk kreep lithology and probably most close to the last residual liquid of the lunar magma ocean without significant dilution by other mg rich magmas the fine grained matrix of the sau 169 imb has very similar mineral chemistry to the vhk kreep lithology but contains abundant plagioclase with trace k feldspar the matrix shows decoupling of k from the reep like component however it cannot be simply interpreted by mixing the vhk kreep lithology with anorthosites which should have diluted the reep like component with the same proportion sims pb pb dating was conducted on zircons in various petrographic settings and with different crystal habits all analyses show a main age peak at 3921 3 ma and a smaller one at 4016 6 ma the main age peak is identical to the previous pb pb age by gnos et al 2006 and u pb age by liu et al 2009 dating the catastrophic shock event contributed to the formation of sau 169 imb the older ages are consistent with the previous report of an older bulk u pb age by kramers et al 2007 suggestive of presence of relict crystals in a few large zircon grains the vhk kreep clasts predated the fine grained matrix but have the same zircon pb pb ages as the latter within the analytical uncertainties plagioclase was converted to maskelynite whereas zircon was shocked to diaplectic glass probably by a second event at 2 8 ga however the identical zircon pb pb ages of the amorphous parts and the remained crystalline areas indicate no resetting of pb pb isotopes by the later shock metamorphism or there was another severe impact event postdated solidification of the fine grained matrix within a few million years

the comparative behavior of apatite zircon u pb systems in apollo 14 breccias implications for the thermal history of the fra mauro formation we report secondary ion mass spectrometry sims u pb analyses of zircon and apatite from four breccia samples from the apollo 14 landing site the zircon and apatite grains occur as cogenetic minerals in lithic clasts in two of the breccias and as unrelated mineral clasts in the matrices of the other two sims u pb analyses show that the ages of zircon grains range from 4023 24 ma to 4342 5 ma whereas all apatite grains define an isochron corresponding to an age of 3926 3 ma the disparity in the ages of cogenetic apatite and zircon demonstrates that the apatite u pb systems have been completely reset at 3926 3 ma whereas the u pb system of zircon has not been noticeably disturbed at this time the apatite u pb age is slightly older than the ages determined by other methods on apollo 14 materials highlighting need to reconcile decay constants used for the u pb ar ar and rb sr systems we interpret the apatite age as a time of formation of the fra mauro formation if the interpretation of this formation as an imbrium ejecta is correct apatite also determines the timing of imbrium impact the contrast in the pb loss behavior of apatite and zircon places constraints on the temperature history of the apollo 14 breccias and we estimate from the experimentally determined pb diffusion constants and an approximation of the original depth of the excavated samples in the fra mauro formation that the breccias experienced an initial temperature of about 1300 1100 c but cooled within the first five to ten years

a dual layer chicxulub ejecta sequence with shocked carbonates from the cretaceous paleogene k pg boundary demerara rise western atlantic abstract an up to 2 cm thick chicxulub ejecta deposit marking the cretaceous paleogene k pg boundary the k t boundary was recovered in six holes drilled during odp leg 207 demerara rise tropical western atlantic stunning features of this deposit are its uniformity over an area of 30 km2 and the total absence of bioturbation allowing documentation of the original sedimentary sequence high resolution mineralogical petrological elemental isotopic sr nd and rock magnetic data reveal a distinct microstratigraphy and a range of ejecta components the deposit is normally graded and composed predominantly of rounded 0 1 to max 1 mm sized spherules spherules are altered to dioctahedral aluminous smectite though occasionally relict si al rich hydrated glass is also present suggesting acidic precursor lithologies spherule textures vary from hollow to vesicle rich to massive some show in situ collapse others include distinct fe mg ca ti rich melt globules and lath shaped al rich quench crystals both altered glass spherules and the clay matrix site 1259b display strongly negative e nd t 65 ma values 17 indicating uptake of nd from contemporaneous ocean water during alteration finally fe mg rich spherules shocked quartz and feldspar grains few lithic clasts as well as abundant accretionary and porous carbonate clasts are concentrated in the uppermost 0 5 0 7 mm of the deposit the carbonate clasts display in part very unusual textures which are interpreted to be of shock metamorphic origin the preservation of delicate spherule textures normal grading with lack of evidence for traction transport and sub millimeter scale compositional trends provide evidence for this spherule deposit representing a primary air fall deposit not affected by significant reworking the odp leg 207 spherule deposit is the first known dual layer k pg boundary in marine settings it incorporates compositional and stratigraphic aspects of both proximal and distal marine sites its stratigraphy strongly resembles the dual layer k pg boundary deposits in the terrestrial western interior of north america although there carbonate phases are not preserved the occurrence of a dual ejecta layer in these quite different sedimentary environments separated by several thousands of kilometers provides additional evidence for an original sedimentary sequence therefore the layered nature of the deposit may document compositional differences between ballistic chicxulub ejecta forming the majority of the spherule deposit and material falling out from the vapor ejecta plume which is concentrated in the uppermost part

volcanic eruption volume flux estimations from very long period infrasound signals we examine very long period infrasonic signals accompanying volcanic eruptions near active vents at lokon empung volcano in indonesia aso kuchinoerabujima and kirishima volcanoes in japan the excitation of the very long period pulse is associated with an explosion the emerging of an eruption column and a pyroclastic density current we model the excitation of the infrasound pulse assuming a monopole source to quantify the volume flux and cumulative volume of erupting material the infrasound derived volume flux and cumulative volume can be less than half of the video derived results a largely positive correlation can be seen between the infrasound derived volume flux and the maximum eruption column height therefore our result suggests that the analysis of very long period volcanic infrasound pulses can be helpful in estimating the maximum eruption column height

correlating planar microstructures in shocked zircon from the vredefort dome at multiple scales crystallographic modeling external and internal imaging and ebsd structural analysis microstructural and geochronological analysis of shocked zircon has greatly advanced understanding the formation and evolution of impact structures however fundamental aspects of shock produced planar microstructures in zircon remain poorly known such as their deformation mechanisms crystallographic orientations and how planar microstructures visible at the grain scale by scanning electron microscopy correlate to microstructures visible at sub micrometer scales by transmission electron microscopy and electron backscatter diffraction ebsd to unify observations of planar microstructures in zircon made at different scales into a consistent framework we integrate the results of 1 three dimensional crystallographic modeling of planar microstructure orientations with 2 360 external prism backscattered electron imaging at the grain scale and 3 polished section cathodoluminescence and ebsd analysis at the sub micrometer scale for a suite of detrital shocked zircons eroded from the vredefort dome in south africa our combined approach resulted in the documentation of seven planar microstructure orientations that can be correlated from grain to sub micrometer scales of observation 010 100 112 112 112 1 12 and 011 all orientations of planar microstructures exhibit minor variations in style however all are considered to be fractures no amorphous zrsio4 lamellae were identified we therefore favor the usage of planar fracture pf over planar deformation feature pdf for describing the observed planar microstructures in zircon based broadly on the nomenclature developed for shocked quartz some 112 pfs visible at the grain scale contain impact microtwins detectable by ebsd and are the first report of polysynthetic twinning in zircon the microtwins consist of parallel sets of thin lamellae of zircon oriented 65 about and occur in multiple crosscutting 112 orientations within single grains curviplanar fractures and injected melt are additional impact related microstructures associated with pf formation crosscutting relations of shock microstructures reveal the following chronology 1 early development of c axis parallel pfs in 010 and 100 orientations 2 the development of up to four 112 pfs including some with microtwins 3 the development of curviplanar fractures and the injection of impact derived melt 4 the development of 011 pfs associated with compressional deformation and 5 grain scale non discrete crystal plastic deformation experimental constraints for the onset of pfs together with the absence of reidite suggest formation conditions from 20 to 40 gpa for all of the planar microstructures described here

resolution of impact related microstructures in lunar zircon a shock deformation mechanism map abstract the microstructures of lunar zircon grains from breccia samples 72215 73215 73235 and 76295 collected during the apollo 17 mission have been characterized via optical microscopy cathodoluminescence imaging and electron backscatter diffraction mapping these zircon grains preserve deformation microstructures that show a wide range in style and complexity planar deformation features pdfs are documented in lunar zircon for the first time and occur along 001 110 and 112 typically with 0 1 25 m spacing the widest pdfs associated with 112 contain microtwin lamellae with 65 misorientation relationships deformation bands parallel to 100 planes and irregular low angle misorientation axes this geometry is consistent with a dislocation glide system with 010 during dislocation creep nonplanar fractures recrystallized domains with sharp irregular interfaces and localized annealing textures along fractures are also observed no occurrences of reidite were detected shock deformation microstructures in zircon are explained in terms of elastic anisotropy of zircon pdfs form along a limited number of specific hkl planes that are perpendicular to directions of high young s modulus suggesting that pdfs are likely to be planes of longitudinal lattice damage twinned 112 pdfs also contain directions of high shear modulus a conceptual model is proposed for the development of different deformation microstructures during an impact event this shock deformation mechanism map is used to explain the relative timing conditions and complexity relationships between impact related deformation microstructures in zircon

blast waves from violent explosive activity at yasur volcano vanuatu 1 infrasonic and seismic waveforms were collected during violent strombolian activity at yasur volcano vanuatu averaging 3000 seismic events showed stable waveforms evidencing a low frequency 0 1 0 3 hz signal preceding 5 6 s the explosion infrasonic waveforms were mostly asymmetric with a sharp compressive 5 106 pa onset followed by a small long lasting rarefaction phase regardless of the pressure amplitude the ratio between the positive and negative phases was constant these waveform characteristics closely resembled blast waves infrared imagery showed an apparent cold spherical front 20 m thick which moved between 342 and 405 m s before the explosive hot gas fragments cloud we interpret this cold front as that produced by the vapor condensation induced by the passage of the shock front we suggest that violent strombolian activity at yasur was driven by supersonic dynamics with gas expanding at 1 1 mach number inside the conduit

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

underwater records of submarine volcanic activity el hierro canary islands 2011 2012 eruption abstract we present the monitoring of a submarine volcanic eruption that took place near the southernmost emerged land of el hierro island canary islands spain from october 2011 to february 2012 right after the onset of the eruption in mid october 2011 we deployed an offshore geophone array for the purpose of monitoring the submarine eruptive activity signals it acquired continuous data from october 2011 to may 2012 sometime after the end of the eruption the array consisted of 8 high frequency 3 component geophones assembled into a cable string with 6 m of separation the geophone string was installed in la restinga harbor at a distance of less than 2 km from the volcanic edifice the dataset acquired with the array is a unique continuous acoustic record of the activity associated with this eruption we analyzed the continuous signal of the eruptive activity with special interest in those events reflecting the eruption dynamics our results show that the geophone array was recording acoustic waves from a back azimuth source corresponding to the position of the submarine vent traveling at a speed of 1510 m s compatible with the speed of sound in water acoustic data shows a good correlation with the seismic data recorded on land for the case of earthquake occurrence in addition it provides relevant information towards the understanding of the eruptive surface activity therefore this methodology can be successfully used in cases of submarine eruptions

experimental shock deformation in zircon a transmission electron microscopic study abstract in recent years apparently shock induced and thus impact characteristic microdeformations in the form of planar microdeformation features and so called strawberry granular texture have been observed in zircons in rocks from confirmed impact structures and from the k t boundary the nature of the planar microdeformations in this mineral is however still unknown and critical information is needed regarding the shock pressure range in which these deformation effects are produced we experimentally shock deformed two series of thin zircon zrsio 4 target plates cut perpendicular to the c axis at shock pressures of 20 40 and 60 gpa the recovered samples were characterized by optical and scanning electron microscopy in addition one sample series was studied by transmission electron microscopy tem microdeformation effects observed at 20 gpa include pervasive micro cleavage and dislocation patterns plastic deformation is indicated by a high density of straight dislocations in glide configuration the dominant glide systems are 010 micro cleavages induced by shear stresses during the compression stage occur mostly in the 100 planes the large density of dislocations at crack tips shows that plastic deformation was initiated by the micro cracking processs at 40 gpa the sample was partly transformed from the zircon z to a scheelite cawo 4 type s structure planar deformation features pdfs containing an amorphous phase of zircon composition are present in the not yet transformed zircon relics the phase with scheelite structure initiated in the 100 planes of zircon consists of thin 0 1 to several m bands that crosscut the zircon matrix the phase transformation is displacive martensitic and can be related by 100 z 112 s and 001 z s the scheelite structure phase is densely twinned with twins in the 112 plane the 60 gpa sample consists completely of the scheelite structure phase crosscutting and displacing relationships between twins and pdfs demonstrate that pdfs are formed in the zircon structure i e before the phase transformation to the scheelite structure occurred most likely at the shock front crystallographic orientations of optically visible planar features in zircon in comparison with orientations of planar defects at the tem scale suggest that the optically visible features are more likely planar microfractures than pdfs

dating the shock wave and thermal imprint of the giant vredefort impact south africa u pb geochronology of single grains of zircon and monazite has been used to date an episode of intense postimpact metamorphism in the core of the deeply eroded vredefort impact structure of south africa results from two basement units exposed in the uplifted central region indicate that the impact and a later pyroxene hornfels metamorphic event were penecontemporaneous at 2020 3 ma discovery of a synimpact to postimpact dike of norite that intruded at 2019 2 ma is the first recognition of mafic igneous activity related to impact the dike is either derived from a sudbury type impact melt layer since eroded or is the product of decompression melting of kaapvaal mantle in response to the ablation of 15 km of crust at the center of the crater the combined heating effects of the shock wave and impact triggered magmas are thought to have created the 300 km 2 thermal imprint of the asteroid collision with kaapvaal craton and account for the nearly coeval timing relationship between core metamorphism and shock revealed by this study

fingerprinting the k t impact site and determining the time of impact by upb dating of single shocked zircons from distal ejecta abstract u pb isotopic dating of single 1 3 g zircons from k t distal ejecta from a site in the raton basin colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site data for the least shocked zircons are slightly displaced from the 544 5 ma primary age for a component of the target site white those for highly shocked and granular grains are strongly displaced towards the time of impact at 65 5 3 0 ma such shocked and granular zircons have never been reported from any source including explosive volcanic rocks zircon is refractory and has one of the highest thermal blocking temperatures hence it can record both shock features and primary and secondary ages without modification by post crystallization processes unlike shocked quartz which can come from almost anywhere on the earth s crust shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers with u pb zircon dating it is now possible to correlate ejecta layers derived from the same target site test the single versus multiple impact hypothesis and identify the target source of impact ejecta the ages obtained in this study indicate that the manson impact site iowa which has basement rocks that are mid proterozoic in age cannot be the source of k t distal ejecta the k t distal ejecta probably originated from a single impact site because most grains have the same primary age

strombolian surface activity regimes at yasur volcano vanuatu as observed by doppler radar infrared camera and infrasound abstract in late 2008 we recorded a continuous multi parameter data set including doppler radar infrared and infrasound data at yasur volcano vanuatu our recordings cover a transition in explosive style from ash rich to ash free explosions followed again by a phase of high ash discharge to assess the present paradigm of strombolian behavior in this study we investigate the geophysical signature of these different explosive episodes and compare our results to observations at stromboli volcano italy to this end we characterize yasur s surface activity in terms of material movement temperature and excess pressure the joint temporal trend in these data reveals smooth variations of surface activity and regime like persistence of individual explosion forms over days analysis of all data types shows ash free and ash rich explosive styles similar to those found at stromboli volcano during ash free activity low echo powers high explosion velocities and high temperatures result from the movement of isolated hot ballistic clasts in contrast ash rich episodes exhibit high echo powers low explosion velocities and low temperatures linked to the presence of colder ash rich plumes furthermore ash free explosions cause high excess pressure signals exhibiting high frequencies opposed to low amplitude low frequency signals accompanying ash rich activity to corroborate these findings we compare fifteen representative explosions of each explosive episode explosion onset velocities derived from doppler radar and infrared camera data are in excellent agreement and consistent with overall observations in each regime examination of infrasound recordings likewise confirms our observations although a weak coupling between explosion velocity and excess pressure indicates changes in wave propagation the overall trend in explosion velocity and excess pressure however demonstrates a general correlation between explosive style and explosion intensity and points to stability of the uppermost conduit on timescales shorter than at stromboli volcano

u th he zircon dating of chesapeake bay distal impact ejecta from odp site 1073 single crystal u th he dating has been undertaken on 21 detrital zircon grains extracted from a core sample from ocean drilling project odp site 1073 which is located 390 km northeast of the center of the chesapeake bay impact structure optical and electron imaging in combination with energy dispersive x ray microanalysis eds of zircon grains from this late eocene sediment shows clear evidence of shock metamorphism in some zircon grains which suggests that these shocked zircon crystals are distal ejecta from the formation of the 40 km diameter chesapeake bay impact structure u th he dates for zircon crystals from this sediment range from 33 49 0 94 to 305 1 8 6 ma 2 implying crystal to crystal variability in the degree of impact related resetting of u th he systematics and a range of different possible sources the two youngest zircon grains yield an inverse variance weighted mean u th he age of 33 99 0 71 ma 2 uncertainties n 2 mean square weighted deviation 2 6 probability p 11 which is interpreted to be the u th he age of formation of the chesapeake bay impact structure this age is in agreement with k ar 40ar 39ar and fission track dates for tektites from the north american strewn field which have been interpreted as associated with the chesapeake bay impact event

age of detrital zircon and titanite in the meguma group southern nova scotia canada clues to the origin of the meguma terrane abstract precise u pb isotopic analysis of seventeen selected high quality laboratory abraded zircon single grains seven multigrain fractions of similar material and four paramagnetic titanite fractions record the ages of the various detrital grain types from two turbidite samples collected from the top and about 5 km below the top of the goldenville formation of the meguma group the results show that the source of both sediments contained 600 ma and 2000 ma old volcanic plutonic components whereas a third 3000 ma old component occurs exclusively in the upper sample and the youngest zircons and titanite are found in the lower sample lead lead ages of bulk fractions at 2300 ma and two nearly concordant single grains at 2040 ma and 2140 ma upper sample together with 2157 ma bulk fraction and 2060 ma and 2080 ma single grain fractions lower sample indicate that the 2000 ma component dominates the pink purple euhedral grain types detrital titanite gives ages of 605 ma upper sample and 552 ma lower sample with the latter sample containing a minor 1975 ma component colourless zircons record this same age difference the lower sample contained the youngest concordant single zircon grain 566 ma and 580 605 ma bulk grain fraction even though two concordant single grains have ages of 632 and 643 ma estimated bulk fraction ages for certain zircon grain types from the upper sample fall in the range of 605 to 675 ma these data suggest that while the youngest rocks in the source region were removed by erosion during the early stages of deposition of the goldenville formation the oldest source rocks were not unroofed until later in the depositional history the 566 8 ma age of the youngest concordant zircon from the stratigraphically lower sample provides a maximum age for the deposition of this grain with the base of the cambrian placed between 545 and 530 ma the goldenville formation may straddle the precambrian cambrian boundary graptolites from the immediately overlying halifax formation are of early ordovician tremadocian age the lower sample contains spherical zircons that may have been derived from mafic granulite with an estimated age of 2000 2200 ma with this one possible exception all of the zircons analyzed appear to be igneous in origin and all are relatively euhedral indicating that rocks of the ages referred to above probably outcrop in the source area anomalous discordance patterns imply that part of the 2960 ma area was metamorphosed by the 2000 ma event and that both the 2960 and 2000 ma regions were metamorphosed during the 600 ma event the complete lack of detritus with ages between 680 and 2040 ma virtually eliminates a northern north american or baltic shield provenance however the available data are not sufficient to distinguish between a west african or guyana shield source