zircon xenocrysts from the kambalda volcanics age constraints and direct evidence for older continental crust below the kambalda norseman greenstones abstract the hangingwall basalt at kambalda western australia contains zircons that have been shown by ion microprobe analyses to have very high u and th contents and a wide variety of crystallization ages nearly all of these zircons certainly are xenocrysts a few might relate to intrusive veinlets the age of the youngest xenocrysts 2693 50ma 2 shows that the eruptive age of the basalt cannot exceed 2743 ma this confirms that the apparent sm nd isochron giving 3200 ma 1 2 for kambalda mafic and ultramafic rocks is a mixing line 2 between unrelated components enriched and depleted in light rare earth elements mixing probably occurred at depth by erosion of 3200 3500 ma old felsic crust from the walls of the hwb conduits the zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern yilgarn block the latter implies that the kalgoorlie norseman greenstone sequences were formed in a continental rather than an oceanic environment

winding down the chicxulub impact the transition between impact and normal marine sedimentation near ground zero abstract the chicxulub impact led to the formation of a 200 km wide by 1 km deep crater on mexico s yucatan peninsula over a period of hours after the impact the ocean re entered and covered the impact basin beneath several hundred meters of water a suite of impactites were deposited across the crater during crater formation and by the resurge tsunami and seiche events that followed international ocean discovery program international continental scientific drilling program expedition 364 drilled into the peak ring of the chicxulub crater and recovered 130 m of impact deposits and a 75 cm thick fine grained carbonate rich transitional unit above which normal marine sedimentation resumed here we describe the results of analyses of the uppermost impact breccia suevite and the transitional unit which suggests a gradual waning of energy recorded by this local k pg boundary sequence the dominant depositional motif in the upper suevite and the transitional unit is of rapid sedimentation characterized by graded bedding local cross bedding and evidence of oscillatory currents the lower transitional unit records the change from deposition of dominantly sand sized to mainly silt to clay sized material with impact debris that decreases in both grain size and abundance upward the middle part of the transitional unit is interrupted by a 20 cm thick soft sediment slump overlain by graded and oscillatory current cross laminated beds the uppermost transitional unit is also soft sediment deformed contains trace fossils and an increasing abundance of planktic foraminifer and calcareous nannoplankton survivors the transitional unit as with similar deposits in other marine target impact craters records the final phases of impact related sedimentation prior to resumption of normal marine conditions petrographic and stable isotopic analyses of carbon from organic matter provide insight into post impact processes 13corg values are between terrestrial and marine end members with fluctuations of 1 3 timing of deposition of the transitional unit is complicated to ascertain the repetitive normally graded laminae both below and above the soft sediment deformed interval record rapid deposition from currents driven by tsunami and seiches processes that likely operated for weeks to potentially years post impact due to subsequent continental margin collapse events highly siderophile element enrichment at the top of the unit is likely from fine grained ejecta that circulated in the atmosphere for several years prior to settling the transitional unit is thus an exquisite record of the final phases of impact related sedimentation related to one of the most consequential events in earth history

characterization provenance and tectonic setting of fig tree greywackes from the archaean barberton greenstone belt south africa abstract in this study mineralogical and geochemical data as well as sr and nd isotopic compositions of early archaean greywackes from the fig tree group are presented to provide further information about the evolution of the barberton greenstone belt bgb of the kaapvaal craton south africa the chemical data on the collected greywackes from the bgb suggest limited chemical modification of the source rocks the eu eu anomaly is negative with an average value of 0 76 which is slightly higher than that of eu depleted crustal rocks 0 65 high cr and ni concentrations and a cr ni ratio of about 1 6 indicate derivation from ultramafic sources without fractionation although rounded zircons occur the combination of ciw cia chemical index of weathering chemical index of alteration values and eu eu ratios suggests little transportation until final deposition whereas the juvenile crustal material comes from local sources the siliciclastic material seems to be of mixed felsic and mafic to ultramafic origin the tectonic setting of the source areas appears to reflect a continental island arc of andean type supporting the existence of an active plate tectonic geodynamic system in early archaean time sr isotopic data indicate an alteration during a post depositional event most probably at 2 6 2 7 ga which disturbed the sr isotopic system the sm nd isotopic system must also have been altered since deposition of the rocks as suggested by e nd values both lower and higher than expected the calculated depleted mantle model ages of 3 0 to 3 8 ga are in the range of bgb sedimentary rocks the higher model ages appear to reflect the underlying belt which is represented by the onverwacht group and the surrounding intrusives as being the major source of the greywackes

the cretaceous tertiary boundary cocktail chicxulub impact triggers margin collapse and extensive sediment gravity flows a distinctive mixture of reworked microfossils impact derived materials and lithic fragments occurs in sediments at the cretaceous tertiary boundary in the basinal gulf of mexico and caribbean we have named this mixture the cretaceous tertiary boundary cocktail lithologic and paleontologic evidence suggests that the cocktail was deposited by giant sediment gravity flows apparently triggered by the collapse of continental margins around the gulf of mexico as a result of the chicxulub impact as most microfossils in the gravity flow units are reworked biostratigraphy provides only maximum ages recognition of the cocktail is a reliable way to identify cretaceous tertiary boundary deposits in the basinal gulf of mexico and caribbean

chicxulub impact the origin of reservoir and seal facies in the southeastern mexico oil fields stratigraphic and mineralogic studies of cretaceous tertiary k t boundary sections demonstrate that the offshore oil producing breccias and seals from oil fields in the campeche marine platform are of k t boundary age and that their mode of formation is probably related to the k t impact event at chicxulub the oil producing carbonate breccia and the overlying dolomitized ejecta layer seal found in several wells on the campeche marine platform contain typical chicxulub impact products such as shocked quartz and plagioclase and altered glass these offshore units are correlated with thick 50 300 m onshore breccia and impact ejecta layers found at the k t boundary in the guayal tabasco and bochil chiapas sections regionally the characteristic sequence is composed of from base to top coarse grained carbonate breccia covered by an ejecta bed and typical k t boundary clay the onshore and offshore breccia sequences are likely to have resulted from major slumping of the carbonate platform margin triggered by the chicxulub impact successive arrival times in this area 350 600 km from the crater of seismic shaking ballistic ejecta and tsunami waves fit the observed stratigraphic sequence the k t breccia reservoir and seal ejecta layer of the cantarell oil field with a current daily production of 1 3 million barrels of oil are probably the most important known oil producing units related to an impact event

sm nd age of kambalda and kanowna greenstones and heterogeneity in the archaean mantle a precise sm nd age of 2 790 30 myr and an initial ratio corresponding to nd 3 4 0 6 has been determined for the kambalda sequence from the eastern yilgarn block of western australia the high initial 143nd 144nd implies a long lived light rare earth element depleted high sm nd source for the kambalda volcanics and therefore provides definitive evidence for ancient heterogeneities in the archaean mantle

impact ejecta at the paleocene eocene boundary extraterrestrial impacts have left a substantial imprint on the climate and evolutionary history of earth a rapid carbon cycle perturbation and global warming event about 56 million years ago at the paleocene eocene p e boundary the paleocene eocene thermal maximum was accompanied by rapid expansions of mammals and terrestrial plants and extinctions of deep sea benthic organisms here we report the discovery of silicate glass spherules in a discrete stratigraphic layer from three marine p e boundary sections on the atlantic margin distinct characteristics identify the spherules as microtektites and microkrystites indicating that an extraterrestrial impact occurred during the carbon isotope excursion at the p e boundary

sedimentary signatures and processes during marine bolide impacts a review abstract studies of submarine impact craters resulting from impacts of comets or asteroids demonstrate that the presence of water and the physical properties of target rocks have a major influence on sedimentary processes associated with meteorite impacts this results in difference in sedimentary signature of bolide impacts in marine environments compared to subaerial impact craters in subaerial impacts the targets are commonly hard rocks frequently of igneous and or metamorphic origin whereas in submarine impacts the targets are mostly unconsolidated or poorly lithified sediments or sedimentary rocks with high volumes of pore water such differences result in variability in crater morphology and in sedimentary processes inside and outside the impact area impacts in shallow water marine neritic environments produced craters with low or absent rims and wide and shallow brims as characterize by both the montagnais on the scotian shelf the mjolnir in the barents sea 45 and 40 km in diameter respectively and the chesapeake bay 90 km in diameter lack of elevated rims is thought to be the result of current reworking and resurge of the water back into the excavated cavity as the water in the crater is vaporized during this process resurge gullies can be cut across the rim while mass and debris flows turbidites and other gravity deposits are produced as results of tsunami and crater wall and central high collapse during and after the crater excavation stage such deposits are found both within and outside the crater structure the only difference between gravity deposits triggered by an impact or other rare events such as earthquakes is the admixture of various melt particles and possible enrichments in iridium in the former impacts near the shelf edge may cause partial collapse of the continental margin as shown by the montagnais and chicxulub impacts some of the gravity and debris flows generated by margin collapse may be channelized with final deposits up to several hundred meters thick extending for hundreds of kilometers from the impact site other impact features such as shatter cones tektites spherules and ni spinels shocked quartz isotropication and partial melts are common to both submarine and subaerial impacts theoretical calculations of the destructive forces of mega tsunami waves triggered by meteorite impact in the ocean greatly exceed those based on geologic evidence a dearth of gigantic tsunami evidence for the chicxulub impact outside of the gulf of mexico where from theoretical modelling the maximum near bottom orbital velocity of water flow crossing the deep north atlantic basin should have been 1 m s could be the result of the mitigating effect of the bathymetry of surrounding area causing wave diffraction and interference calculated maximum horizontal orbital velocity near the seafloor at the shelf for the montagnais impact is 22 m s for a 200 m high wave 5 5 m s for a 50 m high wave at 500 km decreasing to 0 5 m s at 1000 km strong enough to scour the deep ocean bottom and produce distinct erosion surfaces and disconformities in marine sedimentary record however lack of cores across impact horizon prevents confirmation of occurrence of such bottom water flows

collisions with ice volatile objects geological implications a qualitative treatment abstract an aperiodic collision of the earth with extra terrestrial ice volatile bodies is proposed as a mechanism to produce rapid changes in the geologic record due to the volatile nature of these bodies evidence for their impacts particularly in the ocean might be subtle and best seen as spikes in the geochemical or fossil record against normal background differing effects would result depending on the site of the major break up of the object in the atmosphere on land or in the ocean this paper focuses on the effects of adding material to the seas oceans and atmosphere the treatment is largely qualitative however mass balance calculations were used to estimate the relative mass needed to affect changes in a variety of reservoirs although actual impactors probably have a variable composition the effects of water c n and s containing objects are discussed in the atmosphere effects could include increased rain acidity increased levels of nutrients and enhanced greenhouse warming cooling oceanic effects might include increased oceanic productivity nitrogen containing objects as a result of increased chemical weathering and or greenhouse effects increased temperatures coupled with enhanced productivity could result in wider spread oceanic anoxia or altered calcite aragonite stability possible examples of such impacts from the geologic record and potential biotic effects are given

conflicting sm nd whole rock and u pb zircon ages for archean lavas from newton township abitibi belt ontario abstract archean komatiitic and tholeiitic lavas from newton township ontario have a sufficient range of sm nd ratios to define a well constrained line on the normal 143 nd 144 nd vs 147 sm 144 nd isochron plot the data give an isochron age of 2826 64ma and an initial e nd of 2 65 0 26 however u pb analysis of zircons from a dacitic volcaniclastic that underlies the komatiite tholeiite suite give an age of 2697 1 1ma there is strong evident that the zircon age is the eruption age suggesting that the older sm nd age is incorrect and probably results from mixing between isotopically distinct mantle sources at the time of eruption the sources had e nd values from about 4 2 to 1 6 indicating that the archean mantle in this area was markedly heterogeneous and not uniformly depleted

single zircon dating constraining the maximum age of the barberton greenstone belt southern africa one of the central issues in early precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high grade gneiss terranes the crucial question in most cases is which of the two is older we report zircon ages using the new single grain evaporation technique from a metaquartzite and a felsic volcanic flow of the onverwacht group in the barberton greenstone belt bgb south africa and swaziland that constrain the age of this basal part of the greenstone sequence between 3451 15 and 3438 6 ma this is almost 100 ma younger than previous age estimates assigned to the onverwacht and agrees well with recent sm nd pb pb and zircon ion microprobe dating tonalitic orthogneisses of the ancient gneiss complex agc adjacent to the bgb have ages up to 3644 4 ma and are thus demonstrably older than the greenstones also they cannot be derived from them by partial melting as has been proposed previously this age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the bgb and suggest that continental crust was either below or nearby

high spatial resolution geochemistry and textural characteristics of microtektite glass spherules in proximal cretaceous paleogene sections insights into glass alteration patterns and precursor melt lithologies abstract using laser ablation inductively coupled plasma mass spectrometry la icp ms we have conducted spatially resolved trace element analysis on fresh unaltered microtektite glasses linked to the cretaceous paleogene k pg boundary chicxulub crater and on their surrounding alteration phases this unique approach offers the opportunity to study in situ and at high spatial resolution both the mixing of different target lithologies and the variation of the major and trace element budget during the alteration process in addition two dimensional element distribution maps reveal important geochemical information beyond the capabilities of single spot laser drilling glasses from two localities in opposite quadrants from the source crater were studied at the beloc locality haiti the glass population is dominated by the presence of yellow high ca glass and black andesitic glass formed by admixture of carbonate dolomite anhydrite platform lithologies with crystalline basement these glasses alter according to the well established hydration palagonitization model postulated for mafic volcanic glasses rees become progressively leached from the glass to below the detection limit for the applied spot size while immobile zr hf nb and ta passively accumulate in the process exhibiting both inter element ratios and absolute concentrations similar to those for the original glass in contrast the arroyo el mimbral locality ne mexico is characterized by abundant green glass fragments high in si al and alkalis and low in mg ca fe low si black glass is less abundant though similar in composition to the black glass variety at beloc the alteration pattern of high si al green glass at the mimbral locality is more complex including numerous competing reaction processes ion exchange hydration dissolution and secondary mineral precipitation generally controlled by the ph and composition of the surrounding fluid all green high si al glasses are hydrated and variably enriched in sr ba and cs indicating preferred adsorption from seawater during hydration despite the onset of ion exchange reactions which only seem to have affected the alkalis the trace element composition of the green high si al glass is still largely representative of the original melt composition refining the geochemical signature of altered melt lithologies may advance our current understanding of glass stability in the natural environment and provide insight into the origin and emplacement of ejecta material during crater formation

advances in the study of mega tsunamis in the geological record abstract extreme geophysical events such as asteroid impacts and giant landslides can generate mega tsunamis with wave heights considerably higher than those observed for other forms of tsunamis in this paper we review recent advances in the study of mega tsunamis in the geological record focusing on well documented examples that have captured particular attention over the past decade we provide up to date background on the source mechanisms of tsunami generation during asteroid impacts and ocean island landslides which are the largest landslides on earth we also discuss the main sources of uncertainty for modelling such mega tsunamis and for addressing associated hazards

reading of the trigger records of sedimentary events a problem for future studies abstract sedimentary records of various three to four dimensional patterns of nature have recently attracted the special attention of many geologists the dramatic development of rhythmic sequence stratigraphy is a conspicuous recent instance the significance of event sedimentation in geohistory however has not been examined sufficiently the problem to how to read the records of triggers or thresholds of sedimentary events has been ignored the difficulty of solving this problem concerns the difference in patterns between natural sedimentary processes and their resulting sediments terminological confusion of event deposits is another cause which hinders our taking note of this problem a few examples of endeavours to discriminate the threshold of sedimentary events are shown by the analysis of two kinds of turbidites of the mid guatemala trench the investigation of slump derived turbidites of the suruga trough and the study of the seismoturbidites in lake biwa well focused investigations of features of trigger known sediments is the best strategy to find the criteria for discriminating the triggers or thresholds based on sedimentary records of various sedimentary events from this point of view more detailed studies of the santorini eruption induced homogenites the k t boundary impact induced tsunamiites together with the lake biwa seismic turbidites are very important further efforts should be made to find and investigate as many trigger identifiable sediments as possible

multiple zircon growth within early archaean tonalitic gneiss from the ancient gneiss complex swaziland abstract u pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the ancient gneiss complex the oldest episode at3644 4ma 2 produced the dominant type of zircon characteristically purplish brown and massive in texture this probably constitutes unaltered zircon precipitated from the original magma recrystallization accompanied and obscured by early pb loss took place within the oldest grains at3504 6ma and3433 8ma whole new grains grew at these times also we interpret the post 3644 ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss in addition many grains are visibly overgrown by two layers of younger zircon of different colour and texture dated at2986 20ma and2867 30ma euhedral finely zoned whole grains having the 2986 ma age are present also evidently contributed by very thin felsic veins associated with the nearby lochiel granite the age of3644 4ma combined with precise zircon u pb dating of volcanics from the onverwacht group reported elsewhere demonstrates that at least part of the ancient gneiss complex is older than the barberton greenstone belt

cretaceous paleogene boundary exposed campeche escarpment gulf of mexico abstract we present the first multibeam bathymetric maps of the campeche escarpment a mesozoic carbonate platform in the gulf of mexico which represents the closest cretaceous paleogene k pg boundary outcrops to the chicxulub impact structure the impact of an extraterrestrial body 65 million years ago on top of this platform is implicated in the end of the cretaceous mass extinction and caused the largest debris flow yet described on earth which is found across the floor of the gulf of mexico and the caribbean sea the location of the k pg boundary has been identified in the escarpment face by combining the new multibeam data with existing information from boreholes the boundary is represented by an abrupt change in gradient on the escarpment face the morphology of the escarpment combined with seismic data reveals that a significant amount of material is missing from the face which failed catastrophically due to seismic shaking produced by the impact the escarpment face is inferred to be an important source for the extensive debris flows triggered by the impact whose deposits are found throughout much of the gulf of mexico

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

terrestrial carbon and nitrogen isotopic ratios from cretaceous tertiary boundary nanodiamonds one hypothesis for the origin of the nanometer size diamonds found at the cretaceous tertiary k t boundary is that they are relict interstellar diamond grains carried by a postulated asteroid the 13 c 12 c and 15 n 14 n ratios of the diamonds from two sites in north america however show that the diamonds are two component mixtures differing in carbon and nitrogen isotopic composition and nitrogen abundance samples from a site from italy show no evidence for either diamond component all the isotopic signatures obtained from the k t boundary are material well distinguished from known meteoritic diamonds particularly the fine grain interstellar diamonds that are abundant in primitive chondrites the k t diamonds were most likely produced during the impact of the asteroid with earth or in a plasma resulting from the associated fireball

silicate perovskite and majorite signature komatiites from the archean abitibi greenstone belt implications for early mantle differentiation and stratification three komatiite tholeiite sequences from the archean abitibi southern volcanic zone svz which are separated by major terrane boundaries show systematic differences in composition rare earth element ree patterns and high field strength element hfse ree fractionation komatiites from tisdale township are al undepleted with mgo 12 24 al2o3 tio2 13 17 cao al2o3 1 1 3 and high ni 722 1275 ppm and cr 1875 2820 ppm contents mg tholeiites and fe tholeiites are spatially associated with komatiites both komatiites and mg tholeiites have flat ree patterns la yb n 0 6 1 2 and nb nb zr zr and hf hf 1 calculated using primitive mantle normalized values and adjacent rees in the incompatibility sequence of elements in mid ocean ridge basalt morb suggesting that they may have formed by high degree partial melting of primitive mantle materials with olivine as the main liquidus phase at shallow depths 1 consistent with having been derived from mg perovskite enriched sources at depths 700 km below the transition zone whereas spatially associated tholeiites have la yb n 1 2 and nb nb zr zr and hf hf 1 suggesting a shallow undepleted mantle source distinct from the komatiites komatiites and tholeiites from boston township are characterized by mgo 11 29 al2o3 tio2 4 5 5 4 cao al2o3 l 4 2 5 high ni 203 3420 ppm and cr 194 1965 ppm contents al sc y and heavy rare earth element hree depletion and pronounced negative normalized zr and hf anomalies zr zr and hf hf 1 in accord with either partial melting of a majorite depleted mantle source or majorite fractionation during magma formation in the depth range of 300 600 km the fractionated hfses and rees of munro and boston area komatiites indicate that these archean komatiites may have preserved some signatures of early mantle differentiation which so far has only been inferred based on high pressure experimental studies and theoretical predictions further geochemical studies of archean komatiites especially precise multiple trace element analysis combined with improved measurements of partition coefficients over a wide pressure range will provide better understanding on early mantle differentiation stratification and the transition zone

archean crustal evolution of the aldan shield siberia geochemical and isotopic constraints abstract the relatively unknown aldan shield in eastern siberia is made up of archean granite greenstone and high grade gneiss terrains as commonly observed in many precambrian shield areas new results of geochemical and isotopic study on two terrains of contrasting metamorphic grades olekma granite greenstone and west aldan granulite gneiss terrains are present with the following principal conclusions being reached 1 the crustal evolution of the aldan shield started as early as 3 5 ga as suggested by the tdm model ages of a few high grade gneisses and a retrograded granulite enclave but the most important crust forming and tectonothermal event is firmly established at ca 3 0 ga 2 new shrimp and pb evaporation analyses on zircons have confirmed the presence of crustal rocks older than 3 ga however the zircon u pb systems appear to have been disturbed by later thermal effects and no components of 3 5 ga have been identified 3 komatiitic and basaltic amphibolites form important supracrustal sequences in the olekma terrain both group i and ii komatiites occur and garnet or majorite fractionation in mantle melting processes is likely to have taken place during the genesis of group ii komatiites overall depleted mantle sources were involved in the geneses of basic and ultrabasic magmas as evidenced from lree depletion and nd isotopic compositions nd t 2 crustal contamination was not significant as suggested by the low la nb ratios 1 in komatiites and metabasalts 4 as common to most archean tonalite trondhjemite granodiorite ttg rocks the aldan granitic gneisses also show highly fractionated rare earth element ree patterns with heavy ree depletions suggesting that separation of garnet and amphibole has occurred during melting of their mafic sources 5 although the olekma and west aldan terrains have undergone different metamorphic evolution there is no substantial difference in the time of major crust forming events at 3 0 ga and a strong proterozoic thermal disturbance at 2 ga furthermore from the geochemical comparison of the two terrains it is concluded that the nature of mantle sources for basic ultrabasic magmas and the process of ttg magma generation are similar in both terrains and that the granulite facies rocks cannot be considered as the restites of intracrustal melting

sm nd studies of archaean metasediments and metavolcanics from west greenland and their implications for the earth s early history metasedimentary and metavolcanic rocks from the archaean of west greenland have been examined for evidence of crustal components greater than 3 8 ga in age and for their compatibility with the presently adopted bulk earth sm nd parameters sm nd isotopic data have been obtained for the garbenschiefer metagabbro unit metasediments from the isua supracrustal belt gneisses interior to the isua belt and metasediments from the malene supracrustal belt using estimates of emplacement age t of between 3 77 and 3 67 ga for the parental volcanics to the garbenschiefer unit initial143nd 144nd ratios yield positiveendt values between 1 0 and 3 1 relative to the chur parameters for seven out of eight samples model sm nd ages for the isua gneisses and metasediments are only compatible with their estimated stratigraphic ages if their sources were ca 2end relative to chur at those times similarly model sm nd ages for the malene samples are only compatible with stratigraphic age constraints when based on a source evolution with positiveendt implications of these results for the early development of the earth s mantle are discussed

the influence of alteration on the trace element and nd isotopic compositions of komatiites abstract to investigate the effects of hydrothermal alteration and metamorphism on the chemical and isotopic compositions of komatiites we studied samples from the alexo and texmont regions in the 2 7 ga abitibi belt of canada and from the weltevreden mendon and komati formations of the 3 2 3 5 ga barberton belt of south africa particular emphasis was placed on multiple samples from individual layered spinifex olivine cumulate flows the argument being that if these flows showed variations in the ratios of elements incompatible with olivine then these variations were most likely due to chemical mobility during alteration data for rare earth ree and high field strength elements hfse reveal anomalies non chondritic hfse ree ratios particularly in the most altered samples after taking into account the limits of analytical precision these anomalies are attributed to element mobility not to fractionation of high pressure minerals as suggested by other authors for abitibi komatiites in barberton komatiites three separate processes produced hfse anomalies element mobility during alteration crust assimilation in some samples and majorite fractionation the effects of the latter process were recognized from systematic relationships between hfse ree and al ti that coincided with calculated majorite fractionation trends initial nd isotopic compositions of the komatiites were determined by analysis of magmatic pyroxenes the initial nd of pyroxene from the 2 7 ga alexo komatiite is 3 8 slightly higher than in whole rocks 0 6 to 3 5 average 2 5 in the carbonatized texmont samples the range in initial nd is far greater 6 2 to 8 8 the variable initial nd values in the rocks are attributed to isotopic exchange of nd with surrounding rocks during early alteration and fractionation of sm nd during later events pyroxene from 3 3 ga barberton komatiitic basalt had initial nd of 2 3

petrogenetic evolution of metabasalts and metakomatiites of the lower onverwacht group barberton greenstone belt south africa abstract a well preserved sequence by archean standards of mantle derived metabasalts and metakomatiites forms large parts of the lower onverwacht group of the barberton greenstone belt south africa to elucidate the origin of mafic and ultramafic rocks from this 3 55 to 3 45 ga sequence we present a comprehensive geochemical dataset including major and trace elements as well as lu hf and sm nd isotope compositions for a variety of metavolcanic rocks these include metabasalts of the amphibolite facies sandspruit and theespruit formations as well as metabasalts and metakomatiites of the lower greenschist facies komati formation based on their incompatible trace element patterns the basalts of the sandspruit and theespruit formations can be subdivided into a light rare earth element lree depleted group a lree undepleted group and a lree enriched group positive ehf t and end t values of ca 3 to 4 and 0 to 2 respectively together with depletions in th and lacn ybcn indicate derivation of the lree depleted basalts from a depleted mantle source however chondritic ehf t and end t values combined with positive th and lacn ybcn of the lree enriched samples indicate a contribution from older granitoid crust in the petrogenesis of these samples trace element patterns of komatiites and basalts of the komati formation are generally flat relative to primitive mantle with slight depletions in heavy rare earth elements and th and overall positive ehf t of 2 5 3 5 2 s d and end t of 0 5 2 2 2 s d the coherence in trace element characteristics suggests a common magmatic origin for basalts and komatiites this study reveals that the two lavas were derived from the same mantle plume i e komatiites were formed by high degrees of melting of a depleted mantle source containing residual garnet and the basalts were formed by moderate degrees of partial melting in shallower regions of the mantle based on the current dataset combined with published data we propose a geodynamic model for the oldest units of the barberton greenstone belt that describes the development from a submerged continental setting for the sandspruit and theespruit formations to a submarine plateau setting for the komati formation as a consequence of continental rifting

tsunami induced conglomerates in miocene upper bathyal deposits chita peninsula central japan abstract the tsubutegaura conglomeratic tsunamiites occur in the middle section of the miocene storm related sand silt alternation system in the chita peninsula central japan deposition of the system in the upper bathyal environment in a bay and synsedimentary seismic activity has been elucidated by palaeontological palaeogeographical and sedimentological studies two coupled units each built up by a conglomerate layer with an overlying sandstone layer and the alignment of lenticular sedimentary bodies are exceptional in this sequence some typical lenticular bodies confined within sedimentary troughs consist of a boulder bearing conglomerate layer and calcareous sandstone layer of the lower couple and another conglomerate layer of the upper couple the tuffaceous sandstone layer of the upper couple is distributed more widely than the other lithologies the framework gravels in the coupled conglomerates form a clast supported fabric and are quite notably monomictic the clasts are angular and imbricated partly forming peculiar gravel clusters due to traction current transportation conspicuous laminations develop in the coupled sandstone layers antidunes with chute and pool structure in the upper couple sandstone layer indicate deposition from an upper flow regime current on the other hand the calcareous composition of the lower couple sandstone layer reveals shallow water provenance thixotropic deformations and diastasis cracks in the siltstone bed immediately beneath the layers of the couples show that a severe earthquake tremor and a rapid change of water pressure occurred just before the deposition of the tsunamiites submarine debris flows due to collapse of a fault scarp in a shallow bank and the ensuing wash by two pulses of the tsunami induced ebb current comprise the scenario for the formation of the tsubutegaura conglomeratic tsunamiites

did the european dinosaurs disappear before the k t event magnetostratigraphic evidence abstract debate on the non catastrophic or catastrophic extinction of the dinosaurs by an asteroid impact k t event remains a controversy and is mainly based on the interpretation of the sedimentary continental sequences in the north american western interior the global aspect of this event needs to be tested in sedimentary record from all continents where continuous terrestrial deposits through the cretaceous paleogene are well preserved in the western mediterranean realm recognition of the cretaceous paleogene boundary is limited by the lack of biostratigraphic data in the upper cretaceous lower tertiary continental sedimentary sequences new magnetostratigraphic results were obtained from the analysis of two sections in southern france and compared to previous results in northern spain the last occurrence of in situ dinosaurs eggshells the only dinosaur remains found is located in chron 30n southeast france or 31n southwest france and northern spain this last occurrence could demonstrate that the extinction of the european dinosaurs occurred prior to the cretaceous paleogene boundary and would therefore support the idea of a gradual or stepwise extinction unlinked to the k t event

investigating complex isochron data using mixture models the isochron technique for dating geological events is widely used in many fields of isotope geochemistry however data sets can be complex and many samples may not behave as closed systems resulting in excess scatter around a regression line in certain data sets however excess scatter may result when geological samples are partially reset or when samples are comprised of multiple components recording different geological processes separated in time and or initial isotope composition here we introduce a new approach for investigating evidence of such multicomponent mixtures within isochron space we provide a mathematical framework to resolve the number of mixture components without requiring any prior knowledge of this number crucially the model reports the proportion of each individual sample that belongs to each mixture component allowing the user to return to the samples and independently test model outputs with other techniques it is important to distinguish accurate and meaningful ages and initial daughter isotope compositions from the meaningless values that are caused by mismatch between geological processes and the models that we use to describe them first we demonstrate our new model on a synthetic data set to show that it can successfully separate distinct isochrons in a complex mixture we further validate our approach with a previously published data set using the 187re 187os system in bitumen and 147sm 144nd system in magmatic rocks we identify discrete components in these data sets and using complementary geological observations discuss the implications of treating these scattered data as recording multiple geological processes

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

sm nd age of the garhwal bhowali volcanics western himalayas vestiges of the late archaean rampur flood basalt province of the northern indian craton abstract whole rock sm nd isotope dating of the garhwal and bhowali mafic volcanics of the kumaun lesser himalayan sector yield an age of 2 51 0 08 ga and an initial nd 5 1 0 5 this age and initial nd are the same as that of the rampur mandi volcanics which occur to the west in the lesser himalayas in addition to confirming the widespread presence of the late archaean crustal elements in the himalayas these data constrain the age of the oldest rifting event and deposition of supracrustal sediments in the himalayas and set at rest the long standing controversy about the age and inter relationship of these mafic volcanics despite their present day disparate occurrence as a result of the post eruption deformation the area covered by these mafic volcanics is large enough to classify them as flood basalts the rampur flood basalt rfb province rift related volcanism of the same age is known from the contiguous aravalli region of the nw indian shield the orientation of the rifts in the two regions suggests the possibility of a mantle plume setting for the rfb

extensional tectonics on sardinia italy insights into the arc back arc transitional regime although the tectonic features and stress regime typical for accretionary complexes and back arc domains have been widely documented so far few are known on the transitional zone separating these two systems here we report on structural analysis and anisotropy of magnetic susceptibility ams results from eocene pliocene sediments exposed in western sardinia from late oligocene to middle miocene the studied area was located between the alpine apennine wedge to the east which was undergoing shortening and accretion and the liguro provencal basin undergoing extension and spreading we find that prior to the formation of the liguro provencal basin the middle eocene lower oligocene sediments cropping out at the southwesternmost edge of sardinia were subjected to ne sw shortening in present day coordinates in agreement with recently reported geological information conversely the upper oligocene pliocene sedimentary sequences record a different evolutionary stage of extensional processes upper oligocene middle upper burdigalian sediments clearly show a n s oriented magnetic lineation that can be related to extensional direction along the prevalent e w oriented normal faults on the other hand no magnetic lineation has been detected in upper burdigalian serravallian sediments which mark the end of the first rifting process in sardinia which likely coincides with the rift to drift transition at the core of the liguro provencal basin finally a ne sw extension is observed in two tortonian pliocene sites at the northwestern margin of the nnw sse oriented campidano graben our study confirms that ams may represent a valuable strain trajectory proxy and significantly help to unravel the characters of temporally superimposed tectonic events

chicxulub impact event is cretaceous paleogene boundary in age new micropaleontological evidence abstract high resolution and quantitative planktic foraminiferal biostratigraphy from two se mexico stratigraphic sections bochil guayal shows that the chicxulub related complex clastic unit ccu is synchronous with the ejecta rich airfall layer and the cretaceous paleogene k pg catastrophic mass extinction horizon in the el kef tunisia and caravaca spain sections the lowermost danian h holmdelensis subzone biozone p0 was identified in both sections in a thin dark clay bed just above the ccu proving that such bed is chronostratigraphically equivalent to the k pg boundary clay of the el kef stratotype these new micropaleontogical data confirm that the k pg impact event and the chicxulub impact event are the same one this contradicts the suggestion by others that the chicxulub impact predated the k pg boundary by about 300 ka

nondestructive spectroscopic and petrochemical investigations of paleoarchean spherule layers from the icdp drill core barb5 barberton mountain land south africa a paleoarchean impact spherule bearing interval of the 763 m long international continental scientific drilling program icdp drill core barb5 from the lower mapepe formation of the fig tree group barberton mountain land south africa was investigated using nondestructive analytical techniques the results of visual observation infrared ir spectroscopic imaging and micro x ray fluorescence xrf of drill cores are presented petrographic and sedimentary features as well as major and trace element compositions of lithologies from the micrometer to kilometer scale assisted in the localization and characterization of eight spherule bearing intervals between 512 6 and 510 5 m depth the spherule layers occur in a strongly deformed section between 517 and 503 m and the rocks in the core above and below are clearly less disturbed the xrf element maps show that spherule layers have similar petrographic and geochemical characteristics but differences in 1 sorting of two types of spherules and 2 occurrence of primary minerals ni cr spinel and zircon we favor a single impact scenario followed by postimpact reworking and subsequent alteration the spherule layers are al2o3 rich and can be distinguished from the al2o3 poor marine sediments by distinct al oh absorption features in the short wave infrared swir region of the electromagnetic spectrum infrared images can cover tens to hundreds of square meters of lithologies and thus may be used to search for al oh rich spherule layers in al2o3 poor sediments such as eoarchean metasediments where the textural characteristics of the spherule layers are obscured by metamorphism

gneiss greenstone relationships in the ancient gneiss complex of southwestern swaziland southern africa and implications for early crustal evolution the ancient gneiss complex agc of swaziland is made up of a variety of archaean gneisses derived from granitoid intrusions of ttg composition emplaced between 3 6 and 3 2 ga ago and infolded amphibolite grade greenstone remnants locally referred to as dwalile supracrustal suite dss the greenstones were considered to postdate the oldest phases of the agc known as ngwane gneiss and this is now confirmed by conventional u pb zircon geochronology and shrimp single zircon ages on detrital grains from dwalile clastic metasediments suggesting that these rocks were derived from a granitoid source some 3 42 to 3 57 ga in age trondhjemitic ngwane gneiss structurally underlying the greenstones has a zircon age of 3521 23 ma and contains a 3683 10 ma old xenocryst this together with nd tdm model ages between 3 7 and 3 9 ga and a pronounced negative euanomaly for the metasediments strongly argues for the presence of very old crustal components in the agc nd isotopic systematics of dwalile metavolcanics suggest these rocks to be contaminated with older crustal material of ngwane type and resulting nd model ages 3 60 3 99 ga are likely to be too old the major and trace element chemistry of the dwalile metavolcanics is remarkably similar to that of onverwacht group rocks in the nearby barberton greenstone belt bgb and this together with a similar age for these two suites points to a genetic link and a similar mechanism of formation our data as well as field relationships do not favour an intra oceanic origin for the dss as has been inferred for the onverwacht group nor do they support a juvenile origin for the exposed ngane gneisses of the agc the ngwane precursors were apparently derived from older crustal protoliths whose subsequent modification particularly through remelting and deformation gave rise to the various compositional varieties now seen in the ngwane suite

late maastrichtian age of spherule deposits in northeastern mexico implication for chicxulub scenario in the la sierrita area of nuevo leon mexico three spherule layers are present and separated from the overlying siliciclastic deposits by up to 6 m of pelagic marls the marls are of latest maastrichtian age plummerita hantkeninoides cf1 zone micula prinsii zone and deposited under normal pelagic conditions with no significant evidence of reworking or slumping original deposition of the spherule layers occurred during the last 300 ka of the maastrichtian and well prior to the cretaceous tertiary k t boundary event thus if the spherules in northeastern mexico provide critical evidence of an impact at chicxulub this impact predates the k t boundary

lead lead age of komatiitic lavas and limitations on the structure and evolution of the precambrian mantle abstract using lead isotopes we have studied komatiites and associated sulfides from three complexes at barberton the lead lead isochron gives an age of 3 46 0 07gy in good agreement with the sm nd age at munro township three distinct komatiitic flows yield significantly different ages pyke hill is 2 72 0 02gy theo s flow is 2 47 0 13gy and fred s flow is 2 58 0 02gy old 2 72 gy is the emplacement age while the alignment obtained on theo s flow and fred s flow clearly points out the effect of a metamorphism posterior to 2 7 gy at cape smith the age obtained by the lead lead method is 1 6 0 13gy this age is younger than that obtained by other methods in contrast the isochron obtained on sulfide gives an age of 1 88 0 17gy in agreement with sm nd methods these results together with others from the literature are used to describe the evolution of pb in the mantle the important result inferred is that the mantle 2 70 gy ago was very different from the primitive mantle according to both th u and u pb values indeed the latter are lower than the planetary values this evolution is the result of the formation of the continental crust

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

age and isotope geochemistry of the archaean pongola and usushwana suites in swaziland southern africa a case for crustal contamination of mantle derived magma the igneous rocks of the pongola supergroup ps and usushwana intrusive suite uis represent a case of late archaean continental magmatism in the southeastern part of the kaapvaal craton of south africa and swaziland u pb dating on zircons from felsic volcanic rocks of the ps yields a concordia intercept age of 2940 22ma that is consistent with a sm nd whole rock age of 2934 114ma determined on the ps basalt rhyolite suite the initial end of 2 6 0 9 is the lowest value so far reported for archaean mantle derived rocks rb sr whole rock dating of the ps yields a younger isochron age of 2883 69ma which is not significantly different form the accepted u pb zircon age an internal cpx opx plag whole rock isochron for a pyroxenite from the younger uis yields an age of 2871 30 ma and initial 143nd 144nd that lies off the chur growth curve by end 2 9 0 2 however sm nd whole rock data for the uis yield an excessively high age of 3 1 ga that conflicts with firm geological evidence showing the uis to be intrusive into the ps the negative deviations of initialend from the chondritic nd evolution curve suggest significant contamination of the ps and uis melts by older continental crust a mixing process with continental crust after magma segregation is supported by a high initial 87sr 86sr ratio of0 703024 24 for a clinopyroxene sample from a uis pyroxenite compared with an expected value of 0 701 for the 2 9 ga mantle we therefore interpret the linear array of data points for the uis gabbros as a mixing line between 2 87 ga old magma and older continental crust parallel lree enriched ree patterns negative nb ti anomalies a distinctive and uniform ratio of ti zr 46 and a narrow span of initial nd indicate a common source for both the ps and uis suites which is different from primitive mantle

sm nd age and petrogenesis of rampur metavolcanic rocks nw himalayas late archaean relics in the himalayan belt abstract the supposedly late proterozoic rampur metavolcanic rv rocks forming a succession of metabasaltic flows interbedded with phyllites and quartzites in the northwestern himalayas yield a whole rock sm nd isochron age of 2 51 0 09 ga and an i nd 0 509655 88 nd 5 3 1 7 this new date is substantially older than the presumed age and it has an important consequence on the stratigraphic correlation in the region geochemical data characterise rv as continental tholeiites with chemical features that are usually attributed to fractional crystallysation and assimilation of crustal material by magma e g dominantly oversaturated low mg numbers ni and sr contents and negative eu nb p and ti anomalies fe mg relations and large variations in the ratios of most incompatible elements suggest derivation of magma from a variably enriched mantle source with fe mg ratio greater than the ideal model mantle pyrolite progressive increase in the degree of source enrichment during the course of magma extraction is marked by a concomittant decrease in nb la and ti y ratios contrarily highly positive and uniform initial nd values not only reflect derivation of magma from a mantle source with long term lree depletion but also strongly discount any significant crustal contamination in order to account for the apparently conflicting chemical and isotopic constraints we suggest that the source enrichment was through hydrous silicate melt phase and contemporaneous with melting melt generation from an enriched source under the implied hydrous conditions followed by olivine and plagioclase fractionation appears consistent with the observed geochemical and isotopic features

proterozoic crustal evolution in the nw himalaya india as recorded by circa 1 80 ga mafic and 1 84 ga granitic magmatism abstract single zircon dating of the rampur metabasalts of the larji kullu rampur window in the lesser himalayas yielded an evaporation age of 1800 13 ma the zircon age is considerably younger than the previously published whole rock sm nd age of 2510 90 ma suggesting that the sm nd age may be geologically meaningless and that the sm nd whole rock array may have resulted from mixing in the nw himalaya there is also evidence for extensive silicic melt generation in the paleoproterozoic zircons from a metarhyodacite in the larji kullu rampur window yielded an evaporation age of 1840 16 ma which we interpret as the minimum age of magmatism the main central thrust granitic mylonites are interpreted as the basement of the neoproterozoic haimanta group metasediments together with the granitic rocks from the lesser himalaya they were derived from pre existing continental crust prior to 1 84 ga the nd depleted mantle model ages are in the range of 2 6 2 4 ga suggesting a contribution of archean crust a recycled archean component is also documented by a 2 9 ga domain in one of the zircons

the cretaceous paleogene boundary deposit in the gulf of mexico large scale oceanic basin response to the chicxulub impact hydrocarbon exploration in the last decade has yielded sufficient data to evaluate the gulf of mexico basin response to the chicxulub asteroid impact given its passive marine setting and proximity to the impact structure on the yucatan peninsula the gulf is the premier locale in which to study the near field geologic effect of a bolide impact we mapped a thick decimeter to hectometer scale deposit of carbonate debris at the cretaceous paleogene boundary that is ubiquitous in the gulf and readily identifiable on borehole and seismic data we interpret deposits seen in seismic and borehole data in the deepwater gulf to be predominately muddy debrites with minor turbidites based on cores in the southeastern gulf mapping of the deposit in the northern gulf of mexico reveals that the impact redistributed roughly 1 05 105 km3 of sediment therein and over 1 98 105 km3 gulfwide deposit distribution suggests that the majority of sediment derived from coastal and shallow water environments throughout the gulf via seismic and megatsunamic processes initiated by the impact the texas shelf and northern margin of the florida platform were significant sources of sediment while the central and southern florida platform underwent more localized platform collapse the crustal structure of the ancestral gulf influenced postimpact deposition both directly and indirectly through its control on salt distribution in the louann salt basin nevertheless impact generated deposition overwhelmed virtually all topography and depositional systems at the start of the cenozoic blanketing the gulf with carbonate debris within days

proximal impact deposits at the cretaceous tertiary boundary in the gulf of mexico a restudy of dsdp leg 77 sites 536 and 540 restudy of deep sea drilling project sites 536 and 540 in the southeast gulf of mexico gives evidence for a giant wave at cretaceous tertiary boundary time five units are recognized 1 cenomanian limestone underlies a hiatus in which the five highest cretaceous stages are missing possibly because of catastrophic k t erosion 2 pebbly mudstone 45 m thick represents a submarine landslide possibly of k t age 3 current bedded sandstone more than 2 5 m thick contains anomalous iridium tektite glass and shocked quartz it is interpreted as ejecta from a nearby impact crater reworked on the deep sea floor by the resulting tsunami 4 a 50 cm interval of calcareous mudstone containing small cretaceous planktic foraminifera and the ir peak is interpreted as the silt size fraction of the cretaceous material suspended by the impact generated wave 5 calcareous mudstone with basal tertiary forams and the uppermost tail of the ir anomaly overlies the disturbed interval dating the impact and wave event as k t boundary age like beloc in haiti and mimbral in mexico sites 536 and 540 are consistent with a large k t age impact at the nearby chicxulub crater

pdf orientations in shocked quartz grains around the chicxulub crater we measured 852 sets of planar deformation features pdfs in shocked quartz grains in impactite samples of the yaxcopoil yax 1 core and from 4 cretaceous tertiary k t boundary deposits the monaca the cacarajcara and the pealver formations in cuba and dsdp site 536 within 800 km of the chicxulub crater in order to investigate variations of pdf orientations in the proximity of the crater orientations of pdfs show a broad distribution with peaks at 1013 1012 and 1112 plus r z 1011 orientations with minor c 0001 s 1121 t 2241 plus x 5161 and m 1010 plus a 11 20 orientations planar deformation features with c 0001 orientation are relatively more abundant in the proximity of the chicxulub crater than in distal sitessuch as north america the pacific ocean and europe this feature indicates that in the proximity ofthe crater part of the shocked quartz grains in the k t boundary deposits were derived from the lowshock pressure zones moreover the orientations of pdfs with 1122 plus r z 1011 are high in our studied sites and frequencies of these orientations decrease with increasing distance from the crater on the other hand absence of c 0001 and the rare occurrence of pdfs with 1122 plus r z 1011 orientations in the sample from the yax 1 core that was taken at the top of the impactite layer of the chicxulub crater suggests that the sampling horizon that reflects a certain cratering stage is also an important factor for variations in shocked quartz

origin of negative ce anomalies in barberton sedimentary rocks deduced from la ce and sm nd isotope systematics abstract the origin of sedimentary rocks with negative ce anomalies among several samples derived from the barberton greenstone belt 3250 ma south africa was investigated using la ce and sm nd isotopic systematics together with light rare earth element lree abundances negative ce anomalies were recognized in two sedimentary rocks a banded ferruginous chert and a shale their ce ce values are 0 40 and 0 81 respectively initial ce isotopic ratio calculated for the ferruginous chert 138 ce 142 ce 3250 ma 0 0225059 is far lower than chur value at 3250 ma and even the value at earth formation 138 ce 142 ce chur 4560 ma 0 0225409 the calculated initial ce isotopic ratio for the shale with a ce anomaly is also lower than that of other rocks without ce anomalies these results indicate that the negative ce anomalies were not inherited from an archean redox change but occurred following later episodes of alteration two possible formation models were examined to estimate a plausible occurrence age for the negative ce anomalies one is characterized by ce extraction and the other is by an addition of la nd and sm both models indicate that negative ce anomalies arose before 1100 ma this study suggests that la ce isotopic systematics provides a unique tool to constrain the occurrence age of negative ce anomalies observed in archean sedimentary rocks and gives some hints about the redox condition in the archean ocean and the evolution of the earth

the cretaceous paleogene k p boundary at brazos texas sequence stratigraphy depositional events and the chicxulub impact abstract two cores from brazos texas spanning the cretaceous paleogene k p boundary are investigated by a multidisciplinary approach aiming at unraveling environmental changes and sequence stratigraphic setting in addition the sedimentology of the k p event deposit and its correlation with the k p boundary is studied foraminifera and nannofossil stratigraphy indicates that both cores include a latest maastrichtian zone cf1 cf2 and earliest danian p0 p and p1a shale sequence with a sandy and chicxulub ejecta bearing event deposit at the k p boundary a hiatus of unknown duration may be present by the unconformable base of the event deposit planktic foraminifera as well as calcareous nannofossil abundance and diversity both decline abruptly above the event deposit k p mass extinction whereas benthic foraminifera show a pronounced faunal change but no mass extinction mineralogical and geochemical proxies suggest that except for the sandwiched k p event deposit no facies change took place across the k p boundary and no evidence for adverse an or dysoxic sedimentary conditions following the chicxulub impact was observed therefore the interval bracketing the k p event deposit is considered as highstand systems tract increased coarse detritus input and low planktic benthic p b foraminifera ratios during the earliest paleocene p0 and p both suggest an increased coastal proximity or relative sea level lowering although the k p mass extinction of planktic foraminifera might have influenced the p b ratios as well consequently the sandy shales of the early paleocene are considered as late regressive highstand or as lowstand deposit during p1a shales assigned as transgressive systems tract overlie a pyrite and glauconite rich bioturbated transgressive surface or type 2 sequence boundary the smectite dominated clay assemblage with minor illite kaolinite and chlorite indicates semiarid humid climates with no obvious shifts across the k p boundary the magnetic susceptibility signature during the maastrichtian reveals a subtle cyclic or rhythmic pattern whereas a high amplitude cyclic pattern is present during the early danian the k p event deposit shows a succession of high energetic debris flows and turbidites derived from multiple source areas followed by a period of decreasing current energy deposition was likely triggered by multiple tsunami or tempestites followed by a prolonged period of reworking and settling the chicxulub ejecta at the base of the k p event deposit consists of mg rich smectite as well as fe mg rich chlorite spherules their mineralogical composition points to target rocks of mafic to intermediate composition presumably situated in the northwestern sector of the chicxulub impact structure besides these silicic phases the most prominent ejecta components are limestone clasts accretionary carbonate clasts and microspar suggesting that the texas area received ejecta also from shallow carbonate rich lithologies at the impact site on the yucatan carbonate platform the excellent correlation of chicxulub ejecta at brazos with ejecta found in the k p boundary layer worldwide along with the associated mass extinction provides no evidence that chicxulub predated the k p boundary and allows for unequivocal positioning of the k p boundary at the event deposit

anomalous sm nd ages for the early archean onverwacht group volcanics new sm nd isotopic data for eight samples of basalt and komatiite from the tjakastad subgroup lower onverwacht group of the barberton greenstone belt bgb of the kaapvaal craton in southern africa are reported they give new constraints on the interpretation of sm nd ages for the subgroup and highlight the petrogenesis of tjakastad volcanics although sm nd isotopic data earlier reported for volcanic rocks from the tjakastad subgroup yielded an isochron age of 3526 48 ma the new results give a much younger isochron date of 3269 84 ma the 3526 48 ma isochron age has been obtained in combining samples ranging in composition from felsic volcanics to ultrabasic komatiites and is thus considered suspect with regard to the pre requisite of geochronology that all the studied rocks must have had identical initial isotopic compositions the new isochron date of 3269 84 ma has been obtained in combining samples solely of basic ultrabasic composition it might thus represent a more correct age for the eruption of the tjakastad subgroup volcanism in fact owing to the potential problem of source heterogeneity and also in the light of geochronological and geochemical arguments we show that this date also has little chance to have any strict chronological meaning most likely the tjakastad volcanics were formed 3450 ma ago also most likely their source rocks were isotopically heterogeneous and the 3530 ma and 3270 ma linear arrays are not true but apparent isochrons based on the calculated nd 3450 values and other geochemical arguments we show that three possible sources might have been involved depleted mantle primitive mantle and older continental crust

upb zircon ages bearing on the nature of early archaean greenstone belt evolution barberton mountainland southern africa abstract u pb zircon systematics for granitic and gneissic clasts from the moodies conglomerate of the barberton greenstone belt south africa exhibit discordancy patterns that suggest variable and multi episodic lead loss and indicate crystallisation ages for the parent granitoids between 3 3 and 3 47 ga metamict zircons in some of these clasts indicate complete resetting of the older material or new growth at about 2 8 2 9 ga ago that is ascribed to a regional thermal event as also shown by published rb sr data for a nearby granitoid pluton since the moodies clasts consist largely of potassic granites our ages suggest the presence of highly evolved continental crust in the source region of the greenstone belt sediments a feature that contradicts the commonly accepted model for the primitive nature of the granitoid crust in early archaean granite greenstone terrains the maximum age of 3 3 ga for the moodies group also shows that greenstone belt evolution in the barberton region took place over a period of at least 150 ma from about 3450 to 3300 ma ago the kaap valley and stentor plutons intrude the lower part of the greenstone succession and their diapiric emplacement caused large scale deformation of earlier structures in the entire greenstone belt sequence at about 3 23 3 25 ga ago detrital zircons in the sediments of the fig tree group date the mean age of their source terrain at c 3520 ma which is also the maximum age of deposition for these strata this age is indistinguishable from the oldest age reported from the barberton greenstones and suggests the possibility that pre greenstone granitoid crust may exist in the eastern kaapvaal craton

a 40ar39ar geochronological study of komatiites and komatiitic basalts from the lower onverwacht volcanics barberton mountain land south africa abstract 40 ar 39 ar step heating analyses have been carried out on domatiites and komatiitic basalts from the komati formation the uppermost formation of the tjakastad subgroup in the barberton greenstone belt the results for the komatiites fall into two groups which have integrated ages either greater than 3 3 ga or less than 3 0 ga the most precise plateau ages in the first group correspond to an age of 3 49 ga which we take to be the age of metamorphism since the whole rocks analysed are composed of metamorphic minerals other samples in the first group appear to preserve a record of a later event at 3 43 ga evidence of this younger age is also seen in the only komatiitic basalt displaying a stable age spectrum the second group of komatiites integrated ages less than 3 0 ga on a 40 ar 39 ar correlation diagram suggest that a 3 3 ga event may have introduced paleoatmospheric argon with a 40 ar 39 ar ratio of 190 into some of the komatiites comparison of the results of detailed electron microprobe analyses with the ca k spectra obtained in the step runs suggests that tremolite is the mineral tenaciously recording the great ages of the komatiites while hornblende is the dominant argon retainer in the komatiitic basalts the argon ages are completely consistent with recently published u pb ages of zircons from the barberton greenstone belt and suggest that the original metamorphism of the komatiites occurred shortly after their eruption

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

generation of early archaean felsic greenstone volcanic rocks through crustal melting in the kaapvaal craton southern africa abstract high potassium felsic volcanic rocks interlayered with basalt and komatiite in early archaean greenstone sequences in the barberton greenstone belt of south africa and swaziland previously considered to be derived from melting of mafic precursors are shown to be the result of melting of significantly older felsic crust this is documented by a combination of shrimp zircon dating with hf in zircon and whole rock lu hf and sm nd isotopic data zircons from felsic rocks of the oldest barberton unit the 3 53 ga theespruit formation yielded predominantly negative e hf t values whereas whole rock e hf t and e nd t data are slightly negative to slightly positive similar results were obtained for ca 3 45 ga felsic rocks in the hoeggenoog and noisy formations higher up in the greenstone stratigraphy these data rule out derivation of the felsic units from melting of basaltic precursors and favor a crustal source most likely of ttg composition the isotopic data are not compatible with an entirely oceanic origin of the barberton greenstone sequences and favor a pre greenstone basement beneath the volcanic rocks crustal melting of eo to paleoarchaean lower crust probably generated the felsic volcanic rocks and is likely to have been responsible for gradual stabilization of the kaapvaal craton

emplacement of cretaceous tertiary boundary shocked quartz from chicxulub crater observations on shocked quartz in cretaceous tertiary k t boundary sediments compellingly tied to chicxulub crater raise three problems first in north america shocked quartz occurs above the main k t ejecta layer second shocked quartz is more abundant west than east of chicxulub third shocked quartz reached distances requiring initial velocities up to 8 kilometers per second corresponding to shock pressures that would produce melt not the moderate pressure shock lamellae observed shock devolatilization and the expansion of carbon dioxide and water from impacted wet carbonate producing a warm accelerating fireball after the initial hot fireball of silicate vapor may explain all three problems

the sm nd age of kambalda volcanics is 500 ma too old although sm nd isotopic analyses of volcanics from kambalda western australia form a 3 2 ga linear array pb pb analyses of the same suite give an isochron age of 2 73 0 03 ga and a 2 7 ga model age for the associated sulphides we suggest that the 3 2 ga age is incorrect and that the sm nd array results from mixing between depleted mantle and either older continental crust or enriched mantle

what is a mega tsunami abstract no unambiguous and widely accepted definition currently exists for the term mega tsunami this is in spite of the rapidly growing popularity of the expression in the scientific literature especially in recent years following the devastation wrought by the 2004 indian ocean and 2011 tohoku oki tsunamis a comprehensive literature search is revealing we find that although there have been several previous attempts at a definition the term mega tsunami has generally been applied in a rather arbitrary fashion to a number of tsunami characteristics such as wave height or amplitude at both source and distant locations run up height geographical extent and impact this haphazard situation is undesirable in response we propose a stricter definition for mega tsunami that is based solely on initial wave height amplitude at source exceeding 100 m 50 m respectively a source related definition conveniently avoids any difficulties associated with the potential influence of coastal physical attributes e g configuration bathymetry geomorphology on tsunami parameters at affected locations using this definition it becomes apparent that mega tsunamis can only include those rare events on geological timescales generated by large bolide impacts violent volcanic activity or oceanic island flank collapse and possibly extreme tsunamigenic submarine earthquakes most seismically triggered events instead fall into the group of souteigai tsunamis i e unexpected tsunamis which are considered exceptional according to historical experience and local perspectives

physical and chemical evidence of the 1850 ma sudbury impact event in the baraga group michigan an ejecta layer produced by the sudbury impact event ca 1850 ma occurs within the baraga group of northern michigan and provides an excellent record of impact related depositional processes this newly discovered 2 4 m thick horizon accumulated in a peritidal environment during a minor sea level lowstand that punctuated a period of marine transgression common ejecta clasts include shock metamorphosed quartz grains splash form melt spherules and tektites accretionary lapilli and glassy shards suggesting sedimentation near the terminus of the continuous ejecta blanket sedimentologic and geochemical data indicate that primary fallout from a turbulent ejecta cloud was reworked to varying degrees by an impact generated tsunami wave train observed platinum group element anomalies ir rh and ru within the sudbury ejecta horizon are sufficient to suggest that the impactor was a meteorite documenting and interpreting the detailed characteristics of the sudbury ejecta horizon in michigan have yielded a fingerprint to identify this chronostratigraphic marker in other paleoproterozoic basins for the first time a foundation exists to assess the consequences of the sudbury impact on precambrian ocean chemistry and early life

coeval 40ar 39ar ages of 65 0 million years ago from chicxulub crater melt rock and cretaceous tertiary boundary tektites 40 ar 39 ar dating of drill core samples of a glassy melt rock recovered from beneath a massive impact breccia contained within the 180 kilometer subsurface chicxulub crater in yucatan mexico has yielded well behaved incremental heating spectra with a mean plateau age of 64 98 0 05 million years ago ma the glassy melt rock of andesitic composition was obtained from core 9 1390 to 1393 meters in the chicxulub 1 well the age of the melt rock is virtually indistinguishable from 40 ar 39 ar ages obtained on tektite glass from beloc haiti and arroyo el mimbral northeastern mexico of 65 01 0 08 ma mean plateau age for beloc and 65 07 0 10 ma mean total fusion age for both sites the 40 ar 39 ar ages in conjunction with geochemical and petrological similarities strengthen the recent suggestion that the chicxulub structure is the source for the haitian and mexican tektites and is a viable candidate for the cretaceous tertiary boundary impact site

majorite fractionation and genesis of komatiites in the deep mantle abstract melting experiments of the primitive mantle indicate that majorite fractionation is the key process to determine the geochemical nature of magmas generated in the deep upper mantle and the transition zone a model of petrogenesis of komatiite magma is presented by using the melting relations of the primitive mantle and the partitioning data of lithophile elements between majorite and the liquid alumina depleted komatiite generally observed in the early archean can be generated by any degree of partial melting at depths from 450 to 650 km this type of komatiite is also generated by a relatively low degree of partial melting at depths from 180 to 450 km alumina enriched komatiite can be generated by remelting of the residual mantle after separation of alumina depleted komatiite alumina undepleted komatiite generally observed in the late archean can not be formed by a low degree of partial melting of the primitive mantle at any depths this type of komatiite can be generated only by a high degree of partial melting of the mantle at the depths shallower than 450 km as an alternative model the alumina depleted komatiite can be formed by a high degree of partial melting of the archean upper mantle depleted in majorite such heterogeneity in the archean upper mantle was generated by a global melting event succeeding the accretion of the earth and erased by convection by the later archean hf isotope data seem to favour the former model of the petrogenesis of komatiite i e single stage melting of the primitive mantle at various depths

comment on geochemistry of the 2 51 ga old rampur group pelites western himalayas implications for their provenance and weathering by bhat and gosh 2001 precambrian research 108 1 16

deposition and age of chicxulub impact spherules on gorgonilla island colombia the end cretaceous mass extinction 66 ma has long been associated with the chicxulub impact on the yucatan peninsula however consensus on the age of this impact has remained controversial because of differing interpretations on the stratigraphic position of chicxulub impact spherules relative to the mass extinction horizon one side argues that the impact occurred precisely at the cretaceous paleogene boundary thus coinciding with the mass extinction the other side argues that the impact predated the cretaceous paleogene boundary based on the discovery of primary impact spherules deposits in ne mexico and texas near the base of planktic foraminiferal zone cf1 dated at 170 k y before the cretaceous paleogene boundary a recent study of the most pristine chicxulub impact spherules discovered on gorgonilla island colombia suggested that they represent a primary impact deposit with an absolute age indistinguishable from the cretaceous paleogene boundary here we report on the gorgonilla section with the main objective of evaluating the nature of deposition and age of the spherule rich layer relative to the cretaceous paleogene boundary the gorgonilla section consists of light gray yellow calcareous siliceous mudstones pelagic deposits alternating with dark olive brown litharenites turbidites a 3 cm thick dark olive green spherule rich layer overlies an erosional surface separating maastrichtian and danian sediments this layer consists of a clast supported normally graded litharenite with abundant chicxulub impact glass spherules lithics mostly volcanic and maastrichtian as well as danian microfossils which transitions to a calcareous mudstone as particle size decreases mineralogical analysis shows that this layer is dominated by phyllosilicates similar to the litharenites turbidites that characterize the section based on these results the spherule rich layer is interpreted as a reworked early danian deposit associated with turbiditic currents a major hiatus 250 k y spanning the cretaceous paleogene boundary and the earliest danian is recorded at the base of the spherule rich layer based on planktic foraminiferal and radiolarian biostratigraphy and carbon stable isotopes erosion across the cretaceous paleogene boundary has been recorded worldwide and is generally attributed to rapid climate changes enhanced bottom water circulation during global cooling sea level fluctuations and or intensified tectonic activity chicxulub impact spherules are commonly reworked and redeposited into younger sediments overlying a cretaceous paleogene boundary hiatus of variable extent in the caribbean central america and north atlantic while primary deposits are rare and only known from ne mexico and texas because of their reworked nature gorgonilla spherules provide no stratigraphic evidence from which the timing of the impact can be inferred

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

precambrian tidal and glacial clastic deposits implications for precambrian earth moon dynamics and palaeoclimate abstract over the past decade the study of precambrian clastic tidal rhythmites stacked laminae of sandstone siltstone and mudstone that display periodic variations in thickness reflecting a strong tidal influence on sedimentation has provided accurate palaeotidal and palaeorotational data palaeotidal records obtained from tidal rhythmites may be systematically abbreviated however and derived periods and frequencies can be misleading the validity of such values including past length of day can be assessed by testing for internal self consistency through application of the laws of celestial mechanics such a test supports the estimated length of day of 21 9 0 4 h derived from the late neoproterozoic 620 ma elatina reynella rhythmites in south australia and the indicated mean rate of lunar retreat of 2 16 0 31 cm year since 620 ma the validity of estimated lengths of day obtained from other precambrian tidal rhythmites remain unverified because the data sets contain only one primary value directly determined from the rhythmites the elatina reynella data militate against significant earth expansion at least since 620 ma and suggest that the free nutation or tipping of the earth s fluid core has undergone a resonance with the earth s annual forced nutation since the neoproterozoic glaciogenic deposits are readily distinguishable from ejecta resulting from impacts with earth crossing bodies palaeomagnetic data based on the geocentric axial dipole model for the geomagnetic field indicate that neoproterozoic and palaeoproterozoic glaciation and cold climate near sea level occurred in low palaeolatitudes this enigmatic finding may imply global glaciation or an increased obliquity of the ecliptic and is relevant to modelling the effect of ice sheet formation on the earth s obliquity history by obliquity oblateness feedback mechanisms through multidisciplinary studies clastic sedimentology and geophysics together can make substantial contributions to understanding precambrian earth moon dynamics and global palaeoenvironments

iridium and other platinum group elements as geochemical markers in sedimentary environments iridium anomalies are commonly used as indicators of extraterrestrial processes which caused global extinctions however platinum group element pge anomalies may result from several different processes both syn and postdepositional acting separately or jointly the sources and processes of enrichment involved are extraterrestrial asteroid comet cosmic dust impact ejecta volcanogenic pge rich condensates precipitation from seawater low sedimentation rate anoxic conditions microbial concentration dissolution reprecipitation exhalative hydrothermal leaching transport and precipitation at a redox boundary geochemical characteristics of pge and accompanying elements in sediments can be used to identify the contributions of some specific sources and processes 1 extraterrestrial chondritic ru ir and os ir ratios low 187os 186os ratio high ir compared to pt pd and au 2 volcanogenic high iridium and se as sb and in contents 3 exhalative hydrothermal high au pd and pt relatively low ir contents 4 precipitation from seawater high pt and low pd contents also high cd and ag contents under anoxic conditions and 5 reduction from intermediate and low temperature solutions high pge and cu or mo u ni contents

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

near k t age of clastic deposits from texas to brazil impact volcanism and or sea level lowstand near k t boundary clastic deposits from texas mexico haiti guatemala and brazil often described as impact generated tsunami deposits are stratigraphically below well defined k t boundary horizons and appear not to be causally related to the k t boundary event stratigraphic evidence indicates that their deposition began during the last 170 200 kyr of the maastrichtian coincident with a major eustatic sea level lowstand that lowered sea level by as much as 70 100 m clastic deposition ended a few tens of thousands of years before the k t boundary during a rapidly rising sea level the presence of glass in clastic deposits in haiti northeastern mexico and yucatan suggests that the sea level lowstand coincided with a time of major volcanism or pre k t boundary bolide impact

combined use of single zircon ages and sm nd isotopes in the analysis of early archaean crustal growth processes the ancient gneiss complex of swaziland southern africa

rare earth element composition as evidence of the precursor material of cretaceous tertiary boundary sediments at distal sections abstract the chicxulub impact event led to a worldwide deposition of impact materials originated from target rocks and the vaporized bolide relative contributions of both types of material to the k t ejecta deposits vary with distance to the crater site at distal sites e g agost and caravaca in the se of spain a major contribution of extraterrestrial material is indicated by different impact signatures such as os and cr isotope composition abundant microkrystites platinum group elements and other siderophile elements that are typical of extraterrestrial components closer settings to the chicxulub crater for example the blake nose plateau in the north american margin display major continental crustal rock contributions in the ejecta layer ree compositions provide additional evidence for terrestrial vs extraterrestrial rock contributions previous research has not focused specifically on ree concentrations and corresponding c1 and nasc normalized patterns however normalized ree patterns are already generating supplementary insights into the nature of the original material of the k t boundary layer thus blake nose ejecta c1 normalized patterns indicate a derivation from continental crustal target rocks in more distal sections ree compositions point to a probable mafic precursor and confirm that extraterrestrial materials represent a major contribution the ejecta layer

energy volatile production and climatic effects of the chicxulub cretaceous tertiary impact a comprehensive analysis of volatiles in the chicxulub impact strongly supports the hypothesis that impact generated sulfate aerosols caused over a decade of global cooling acid rain and disruption of ocean circulation which contributed to the mass extinction at the cretaceous tertiary k t boundary the crater size meteoritic content of the k t boundary clay and impact models indicate that the chicxulub crater was formed by a short period comet or an asteroid impact that released 0 7 3 4 x 10 31 ergs of energy impact models and experiments combined with estimates of volatiles in the projectile and target rocks predict that over 200 gigatons gt each of so2 and water vapor and over 500 gt of co2 were globally distributed in the stratosphere by the impact additional volatiles may have been produced on a global or regional scale that formed sulfate aerosols rapidly in cooler parts of the vapor plume causing an early intense pulse of sulfuric acid rain estimates of the conversion rate of stratospheric so2 and water vapor to sulfate aerosol based on volcanic production of sulfate aerosols coupled with calculations of diffusion coagulation and sedimentation demonstrate that the 200 gt stratospheric so2 and water vapor reservoir would produce sulfate aerosols for 12 years these sulfate aerosols caused a second pulse of acid rain that was global radiative transfer modeling of the aerosol clouds demonstrates 1 that if the initial rapid pulse of sulfate aerosols was global photosynthesis may have been shut down for 6 months and 2 that for the second prolonged aerosol cloud solar transmission dropped 80 by the end of first year and remained 50 below normal for 9 years as a result global average surface temperatures probably dropped between 5 degrees and 31 degrees k suggesting that global near freezing conditions may have been reached impact generated co2 caused less than 1 degree k greenhouse warming and therefore was insignificant compare to the sulfate cooling the magnitude of sulfate cooling depends largely upon the rate of ocean mixing as surface waters cool sink and are replaced by upwelling of deep ocean water this upwelling apparently drastically altered ocean stratification and circulation which may explain the global collapse of the delta 13c gradient between surface and deep ocean waters at the k t boundary

geochemical investigation of archaean bimodal and dwalile metamorphic suites ancient gneiss complex swaziland the bimodal suite bms comprises leucotonalitic and trondhjemitic gneisses interlayered with amphibolites based on geochemical parameters three main groups of siliceous gneiss are recognized i sio2 14 and fractionated light rare earth element ree and flat heavy ree patterns ii sio2 and al2o3 contents similar to i but with strongly fractionated ree patterns with steep heavy ree slopes iii sio2 73 al2o3 14 zr 500 ppm and high contents of total ree having fractionated light ree and flat heavy ree patterns with large negative eu anomalies the interlayered amphibolites have major element abundances similar to those of basaltic komatiites mg tholeiites and fe rich tholeiites the former have gently sloping ree patterns whereas the mg tholeiites have non uniform ree patterns ranging from flat 10 times chondrite to strongly light ree enriched the fe rich amphibolites have flat ree patterns at 20 30 times chondrite the dwalile metamorphic suite which is preserved in the keels of synforms within the bms includes peridotitic komatiites that have depleted light ree patterns similar to those of compositionally similar volcanics in the onverwacht group barberton basaltic komatiites and tholeiites the basaltic komatiites have ree patterns parallel to those of the bms basaltic komatiites but with lower total ree contents the dwalile tholeiites have flat ree patterns the basic and ultrabasic liquids were derived by partial melting of a mantle source which may have been heterogeneous or the heterogeneity may have resulted from sequential melting of the mantle source the fe rich amphibolites were derived either from liquids generated at shallow levels or from liquids generated at depth which subsequently underwent extensive fractionation

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

high resolution planktonic foraminiferal analysis from the cretaceous tertiary boundary at ain settara tunisia evidence of an extended mass extinction the ain settara section located in the kalaat senan area of north central tunisia spans the cretaceous tertiary k t boundary which is characterized by a red layer and a thin non bioturbated boundary clay sediment accumulation across the k t boundary at ain settara was probably interrupted by three short hiatuses and or condensed sedimentation the first hiatus occurs at the top of the cf1 zone the second hiatus condensation occurred just below the boundary clay and the third hiatus at the p0 p1a boundary in the earliest danian these hiatuses are marked by weak unconformities bioturbation and sudden disappearances appearances of species which are known to disappear evolve sequentially in continuous sections quantitative high resolution planktonic foraminiferal analysis across zones cf1 upper p0 and p1a 1 reveals an extended and selective mass extinction all 41 of the species which disappeared at or below the k t boundary are rare to very rare and primarily ecologically specialized keeled deeper dwelling tropical subtropical forms globotruncana globotruncanita gublerina planoglobulina rosita contusotruncana racemiguembelina their combined relative abundance varies between 10 and 15 of the total population at the end of the maastrichtian the k t crisis thus appears more catastrophic when viewed in tropical subtropical assemblages and based on analysis of larger species 200 m which preferentially includes the more specialized forms though in fact the k t mass extinction actually involved a relatively small part of the foraminiferal population in terms of relative abundance the pattern of extinction and changes in dominant population at ain settara appear to be very similar to the planktonic foraminiferal turnover of the other north central tunisian sections el kef elles the selective mass extinction pattern suggests that the catastrophic effects of the bolide impact superimposed those related to long term environmental changes such as variations in temperature sea level and associated water mass changes

oldest reliable 40ar 39ar ages for terrestrial rocks barberton mountain komatiites we report here the first 40ar 39ar ages for komatiites and komatiitic basalts from the barberton mountain greenstone belt on the transvaal swaziland border in southern africa1 both rock types display remarkable argon retentivity while some variation is found among the samples the best argon age estimate for the time of metamorphism is in the 3 450 3 490 myr range this age is only slightly less than that found in komatiites from the same area by sm nd dating 3 540 15 myr 2 within the uncertainties of the decay constants involved the two age estimates are not different the results show that the principal pervasive greenschist metamorphism1 in the area must have occurred within 100 myr of the eruption of the komatiite analytically precise ages were obtained despite potassium contents as low as 80 p p m in the presence of high calcium concentrations 3 5 ca to our knowledge the results represent by far the oldest reliable ages obtained for terrestrial rocks using the k ar system

3 5 ga old amphibolites from eastern hebei province china field occurrence petrography sm nd isochron age and ree geochemistry amphibolite blocks frequently occur in association with grey gneisses within a small area near the villages of caozhuang and huangbaiyu in eastern hebei they are interpreted as remnants of an earlier bimodal suite now engulfed in late archaean k rich granitic magma and are further cut by pegmatite dykes they are considered together as giant enclaves within late archaean granite plutons u pb zircon ages 2 5 2 6 ga the amphibolites have been dated by the sm nd isochron method the results yielded t 3470 107 ma ind 0 50827 11 and nd t 2 7 0 6 this age represents the time of protolithic basalt eruption and possibly suggests a period of formation of primitive mafic crust this is the oldest reliable age so far obtained for the archaean rocks of china the sm nd results of grey gneiss samples yielded model ages tdm of about 3 24 ga it is not clear whether this younger age in comparison with that of the amphibolites indicates the intrusive nature of these rocks or is a result of contamination by later granite about 2 5 ga ago the rb sr and common pb isotopic systems have been highly disturbed and no isochron ages could be established the common pb data show retarded growth of radiogenic pb and hence imply reduction of u pb ratios in the past this is consistent with the petrographic study which suggests that the amphibolites have undergone an earlier period of granulite facies metamorphism and were subsequently retrogressed to the present amphibolite facies mineral assemblages the positive nd t value of 2 7 for the amphibolites implies that the protolithic basalt magmas were generated from mantle sources which had undergone long term depletion in light rare earth elements lree the depletion event must have taken place prior to 4 ga ago if their source had a chemical nature similar to the modern oceanic mantle it could have occurred as early as 4 5 ga ago if their source was less impoverished in lree if the mantle depletion was related to extraction of continental or sialic material the sm nd data of the caozhuang amphibolites lend support to the existence of sialic crust if not of a continental size before 4 ga ago

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