composition of impact melt particles and the effects of post impact alteration in suevitic rocks at the yaxcopoil 1 drill core chicxulub crater mexico petrographical and chemical analysis of melt particles and alteration minerals of the about 100 m thick suevitic sequence at the chicxulub yax 1 drill core was performed the aim of this study is to determine the composition of the impact melt the variation between different types of melt particles and the effects of post impact hydrothermal alteration we demonstrate that the compositional variation between melt particles of the suevitic rocks is the result of both incomplete homogenization of the target lithologies during impact and subsequent post impact hydrothermal alteration most melt particles are andesitic in composition clinopyroxene rich melt particles possess lower sio2 and higher cao contents these are interpreted by mixing of melts from the silicate basement with overlying carbonate rocks multi stage post impact hydrothermal alteration involved significant mass transfer of most major elements and caused further compositional heterogeneity between melt particles following backwash of seawater into the crater palagonitization of glassy melt particles likely caused depletion of sio2 al2o3 cao na2o and enrichment of k2o and feotot during an early alteration stage since glass is very susceptible to fluid rock interaction the state of primary crystallization of the melt particles had a significant influence on the intensity of the postimpact hydrothermal mass transfer and was more pronounced in glassy melt particles than in wellcrystallized particles in contrast to other occurrences of chicxulub impactites the yax 1 suevitic rocks show strong potassium metasomatism with hydrothermal k feldspar formation and whole rock k20 enrichment especially in the lower unit of the suevitic sequence a late stage of hydrothermal alteration is characterized by precipitation of silica analcime and na bearing mg rich smectite among other minerals this indicates a general evolution from a silica undersaturated fluid at relatively high potassium activities at an early stage toward a silica oversaturated fluid at relatively high sodium activities at later stages in the course of fluid rock interaction

chicxulub impactites geochemical clues to the precursor rocks the 65 ma chicxulub impact structure mexico with a diameter of 180 km is the focus of geoscientific research because of its link to the mass extinction event at the cretaceous tertiary k t boundary chicxulub now buried beneath thick post impact sediments is probably one of the best preserved terrestrial impact structures known because of its inaccessibility only limited samples on the impact lithologies from a few drill cores are available we report major element and sr nd o and c isotopic data for chicxulub impact melt lithologies and basement clasts in impact breccias of drill cores c 1 and y 6 and for melt particles in the chicxulub ejecta horizon at the k t boundary in beloc haiti the melt lithologies with sio2 ranging from 58 to 63 wt show significant variations in the content of al ca and the alkalies in the melt matrix samples 13c of the calcite is about 3 o the 18o values for the siliceous melt matrices of y 6 samples range from 9 9 to 12 4 o melt lithologies and the black haitian glass have rather uniform 87sr 86sr ratios 0 7079 to 0 7094 only one lithic fragment displays 87sr 86sr of 0 7141 the sr model ages tsrur for most lithologies range from 830 to 1833 ma unrealistic negative model ages point to an open rb sr system with loss of rb in a hydrothermal process the 143nd 144nd ratios for all samples except one basement clast with 143nd 144nd of 0 5121 cluster at 0 5123 to 0 5124 in an nd sr diagram impactites plot in a field delimited by nd of 2 to 6 and sr of 55 to 69 this field is not defined by the basement lithologies described to occur as lithic clasts in impact breccias and cretaceous sediments at least one additional intermediate to mafic precursor component is required to explain the data

yaxcopoil 1 and the chicxulub impact csdp core yaxcopoil 1 was drilled to a depth of 1 511 m within the chicxulub crater an organic rich marly limestone near the base of the hole 1 495 to 1 452 m was deposited in an open marine shelf environment during the latest cenomanian uppermost rotalipora cushmani zone the overlying sequence of limestones dolomites and anhydrites 1 495 to 894 m indicates deposition in various carbonate platform environments e g sabkhas lagoons a 100 m thick suevite breccia 894 794 m identifies the chicxulub impact event above the suevite breccia is a dolomitic limestone with planktic foraminiferal assemblages indicative of plummerita hantkeninoides zone cf1 which spans the last 300 ky of the maastrichtian an erosional surface 50 cm above the breccia dolomite contact marks the k t boundary and a hiatus limestones above this contact contain the first tertiary planktic foraminifera indicative of an upper p eugubina zone p1a 2 age another hiatus 7 cm upsection separates zone p1a 2 and hemipelagic limestones of planktic foraminiferal zone p1c planktic foraminiferal assemblages of zone plc to p3b age are present from a depth of 794 04 up to 775 m the cretaceous carbonate sequence appears to be autochthonous with a stratigraphic sequence comparable to late cretaceous sediments known from outside the chicxulub crater in northern and southern yucatan including the late cenomanian organic rich marly limestone there is no evidence that these sediments represent crater infill due to megablocks sliding into the crater such as major disruption of sediments chaotic changes in lithology overturned or deep dipping megablocks major mechanical fragmentation shock or thermal alteration or ductile deformation breccia units that are intercalated in the carbonate platform sequence are intraformational in origin e g dissolution of evaporites and dykes are rare major disturbances of strata by the impact therefore appear to have been confined to within less than 60 km from the proposed impact center yaxcopoil 1 may be located outside the collapsed transient crater cavity either on the upper end of an elevated and tilted horst of the terrace zone or even outside the annular crater cavity the chicxulub site thus records a large impact that predates the k t boundary impact and mass extinction

petrology of impact melt rocks at the chicxulub multiring basin yucatan mexico compositions and textures of melt rocks from the upper part of the chicxulub structure are typical of melt rocks at other large terrestrial impact structures apart from variably elevated iridium concentrations less than 1 5 to 13 5 0 9 ppb indicating nonuniform dissemination of a meteoritic component bulk rock and phenocryst compositions imply that these melt rocks were derived exclusively from continental crust and platform sediment target lithologies modest differences in bulk chemistry among samples from wells located approximately 40 km apart suggest minor variations in relative contributions of these target lithologies to the melts subtle variations in the compositions of early formed pyroxene and plagioclase also support minor primary differences in chemistry between the melts evidence for pervasive hydrothermal alteration of the porous mesostasis includes albite k feldspar quartz epidote chlorite and other phyllosilicates as well as siderophile element enriched sulfides suggesting the possibility that chicxulub like sudbury may host important ore deposits

interpolating geochemical patterning of activity zones at late neolithic and early copper age settlements in eastern hungary the late neolithic of southeast hungary is known almost exclusively from excavations of large settlement mounds or tells geochemical analyses of sediments collected from boreholes at small flat late neolithic and early copper age settlements in hungary s koros river basin provide data necessary to interpret the spatial organization of small settlements for the first time in this region principal components analysis of multi element data produced a workable number of variables spatial analyses of these components via interpolation in arcgis 9 identified specific task areas and when combined with sediment characterizations phosphate spot tests and ph suggest long term cultural traditions in the location of activity zones within small farmsteads the results demonstrate the usefulness of multi element geochemistry as an intra site prospection method

new links between the chicxulub impact structure and the cretaceous tertiary boundary the 200 km diameter chicxulub structure1 3 in northern yucatan mexico has emerged as the prime candidate for the cretaceous tertiary k t boundary impact crater3 6 concentric geophysical anomalies associated with enigmatic occurrences of upper cretaceous breccias and andesitic rocks led penfield and camargo1 to suspect that this structure was a buried impact basin more recently the discovery of shocked quartz grains in a chicxulub breccia3 and chemical similarities between chicxulub rocks and k t tektite like glasses3 6 have been advanced as evidence that the chicxulub structure is a k t impact site here we present evidence from core samples that chicxulub is indeed a k t source crater and can apparently account for all the evidence of impact distributed globally at the k t boundary without the need for simultaneous multiple impacts or comet showers shocked breccia clasts found in the cores are similar to shocked lithic fragments found worldwide in the k t boundary ejecta layer7 8 the chicxulub melt rocks that we studied contain anomalously high levels of iridium up to 13 5 parts per 109 also consistent with the iridium enriched k t boundary layer9 our best estimate of the crystallization age of these melt rocks as determined by 40ar 39ar analyses is 65 2 0 4 1 myr in good agreement with the mean plateau age of 64 98 0 05 myr recently reported10 furthermore these melt rocks acquired a remanent magnetization indicating that they cooled during an episode of reversed geomagnetic polarity the only such episode consistent with40ar 39ar constraints is chron 29r which includes the k t boundary

variations in mixed layer illite smectite diagenesis in the rift and post rift sediments of the jeanne d arc basin grand banks offshore newfoundland canada mixed layer illite smectite i s clays were analyzed from 22 deep exploration wells from the jeanne d arc basin on the grand banks offshore newfoundland the host of large commercial hydrocarbon accumulations discovered in the last two and a half decades the fine fraction of the clays 0 1 m consists mainly of mixed layer i s with minor amounts of kaolinite illite and chlorite smectite and or smectite rich i s clays were supplied to the jeanne d arc basin from upper jurassic to tertiary times smectite rich i s clays occur only in shallow samples irrespective of geologic age the proportion of illite in i s mixed layers as well as the degree of ordering increase with depth and temperature indicating that smectite rich i s clays have been progressively illitized in both rift and post rift sediments of the jeanne d arc basin during burial the transition from random to r1 ordered i s occurs between subsurface depths of 1940 and 3720 m and crosses major stratigraphic boundaries the transition fr

alteration of basaltic glasses from north central british columbia canada abstract evidence of palagonitization glass replacement by poorly crystallized clay like material is seen on all glasses studied from three pleistocene subglacial volcanoes in north central british columbia canada samples from foreset breccias of tuya butte are more highly palagonitized than those from the tephra cones of ash mountain and southern tuya extensive palagonitization is generally associated with authigenic mineralization clays zeolites palagonite composition varies widely relative to glass composition and palagonite can be broadly categorized as either high al or low al depending on whether al was retained or lost to aqueous solutions during palagonitization the lowest al palagonites are those from the foreset breccias of tuya butte loss of al during palagonitization is related to closed system alteration including precipitation of aluminosilicate authigenic cements low initial ph is suggested for al depletion consistent with the behavior of ni co and cr which are retained in high al and depleted in low al palagonite microenvironment appears to be more influential than macroenvironment in determining the composition of palagonite palagonite rinds are compositionally zoned generally becoming progressively higher in al and ca and lower in fe and mg towards the innermost later formed portions of the rinds this apparently reflects changing solution composition increasing ph with time compositional zoning does not change the overall stoichiometry of palagonite which resembles smectite clay mineral paragenesis is related to the ca content of the palagonite with partial replacement of palagonite by smectite fe saponite occurring when ca is retained in the rind this replacement phenomenon occurs prior to zeolitization no such replacement clay occurs with low ca palagonite but a late stage nontronite film overgrows zeolite phillipsite is the first zeolite formed followed by chabazite analcime and calcite occur in the most highly palagonitized samples mass balance considerations indicate higher mass loss where palagonitization has not proceeded to the point where zeolite solubility limits were attained in the local solution zeolites occur in closed system conditions low flow rates where little net system mass loss or gain has occurred the colloidal nature of palagonite allows the effective adsorption of rb cs sr ba and rees

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

spinel bearing spherules condensed from the chicxulub impact vapor plume formation of the giant chicxulub crater off mexico s yucatan peninsula coincided with deposition of the global ir rich cretaceous tertiary k t stratigraphic boundary layer at ca 65 ma the boundary is marked most sharply by abundant spherules containing unaltered grains of magnesioferrite spinel here we predict for the first time the sequential condensation of solids and liquids from the plume of vaporized rock expected from oblique k t impacts we predict highly oxidizing plumes that condense silicate liquid droplets bearing spinel grains whose compositions closely match those marking the actual boundary systematic global variations in spinel composition are consistent with higher condensation temperatures for spinels found at atlantic and european sites than for those in the pacific

hydrothermal alteration in the core of the yaxcopoil 1 borehole chicxulub impact structure mexico petrographic electron microprobe and raman spectrometric analyses of yaxcopoil 1 core samples from the chicxulub crater indicate that the impact generated a hydrothermal system relative textural and vein crosscutting relations and systematic distribution of alteration products reveal a progression of the hydrothermal event in space and time and provide constraints on the nature of the fluids the earliest calcite halite and gaylussite suggest that the impactite sequence was initially permeated by a low temperature saline brine subsequent development of a higher temperature hydrothermal regime is indicated by thermal metamorphic diopside hedenbergite aeg3fs 18 33 en32 11wo 47 53 after primary augite and widespread na k for ca metasomatic alkali exchange in plagioclase hydrothermal sphene apatite magnetite bornite as well as early calcite combined 3 to 8 vol were introduced with metasomatic feldspar a lower temperature regime characterized by smectite after probable primary glass secondary chlorite and other pre existing mafic minerals as well as very abundant calcite veins and open space fillings extensively overprinted the early hydrothermal stage the composition of early and late hydrothermal minerals show that the solution was chlorine rich cl f 10 and that its fe mg ratio and oxidation state increased substantially 4 to 5 logo2 units as temperature decreased through time the most altered zone in the impactite sequence occurs 30 m above the impact melt the lack of mineralogical zoning about the impact melt and convective modeling constraints suggest that this unit was too thin at yaxcopoil 1 to provide the necessary heat to drive fluids and implies that the hydrothermal system resulted from the combined effects of a pre existing saline brine and heat that traveled to the yaxcopoil 1 site from adjacent areas where the melt sheet was thicker limonite after iron oxides is more common toward the top of the sequence and suggests that the impactite section was subjected to weathering before deposition of the tertiary marine cover in addition scarce latest anatase stringers chalcopyrite and barite in vugs francolite after apatite and recrystallized halite are the likely products of limited post ydrothermal ambient temperature diagenesis or ocean and or meteoric water circulation

chicxulub crater yucatan tektites impact glasses and the geochemistry of target rocks and breccias a comparison of breccias and basement rocks from four drill holes near the chicxulub impact structure yucatan a possible cretaceous tertiary k t boundary crater with black and yellow impact glasses from the haitian k t boundary layer shows that mixtures of andesite like rocks with carbonates and evaporites from chicxulub are compositionally different from the yellow glasses at the time of impact the chicxulub area was covered by evaporitic and carbonate deposits several kilometres thick because impact glasses are generally produced by shock melting of surface rocks a link between the haitian glasses and chicxulub is difficult to understand the 34 s isotopic signature of chicxulub breccias and evaporites is incompatible with that of a continental k t boundary we suggest that the pre impact target stratigraphy at chicxulub as well as the interpretation of the carbonate evaporite rocks as breccias should be revised

impactites of the yaxcopoil 1 drilling site chicxulub impact structure petrography geochemistry and depositional environment the impact breccias encountered in drill hole yaxcopoil 1 yax 1 in the chicxulub impact structure have been subdivided into six units the two uppermost units are redeposited suevite and suevite and together are only 28 m thick the two units below are interpreted as a ground surge deposit similar to a pyroclastic flow in a volcanic regime with a fine grained top unit 3 23 m thick nue ardente and a coarse breccia unit 4 15 m thick below as such they consist of a mlange of clastic matrix breccia and melt breccia the pyroclastic ground surge deposit and the two units 5 and 6 below are related to the ejecta curtain unit 5 24 m thick is a silicate impact melt breccia whereas unit 6 10 m thick is largely a carbonate melt breccia with some clastic matrix components unit 5 and 6 reflect an overturning of the target stratigraphy the suevites of units 1 and 2 were deposited after emplacement of the ejecta curtain debris reaction of the super heated breccias with seawater led to explosive activity similar to phreomagmatic steam explosion in volcanic regimes this activity caused further brecciation of melt and melt fragments the fallback suevite deposit of units 1 and 2 is much thinner than suevite deposits at larger distances from the center of the impact structure than the 60 km of the yax 1 drill site this is evidence that the fallback suevite deposit units 1 and 2 originally was much thicker unit 1 exhibits sedimentological features suggestive of suevite redeposition erosion possibly has occurred right after the k t impact due to seawater backsurge but erosion processes spanning thousands of years may also have been active therefore the top of the 100 m thick impactite sequence at yaxcopoil in our opinion is not the k t boundary

trace element mobility during sub seafloor alteration of basaltic glass from ocean drilling program site 953 off gran canaria trace element concentrations of altered basaltic glass shards layer silicates and zeolites in volcaniclastic sediments drilled in the volcanic apron northeast of gran canaria during ocean drilling program odp leg 157 document variable element mobilities during low temperature alteration processes in a marine environment clay minerals saponite montmorillonite smectite replacing volcanic glass particles are enriched in transition metals and rare earth elements ree the degree of retention of ree within the alteration products of the basaltic glass is correlated with the field strength of the cations the high field strength elements are preferentially retained or enriched in the alteration products by sorption through clay minerals most trace elements are enriched in a boundary layer close to the interface mineral altered glass this boundary layer has a key function for the physico chemical conditions of the subsequent alteration process by providing a large reactive surface and by lowering the fluid permeability the release of most elements is buffered by incorporation into secondary precipitates sodium rich zeolites phillipsite fe and mn oxides as shown by calculated distribution coefficients between altered glasses and authigenic minerals chemical fluxes change from an open to a closed system behavior during prograde low temperature alteration of volcaniclastic sediments with no significant trace metal flux from the sediment to the water column

fracture characterization using converted waves we present a method for inversion of fracture compliance matrix components from wide azimuth noisy synthetic ps reflection data and quantitatively show that reflection amplitude variations with offset and azimuth for converted ps waves are more informative than p waves for fracture characterization we consider monoclinic symmetry for fractured reservoir parameters chosen from woodford shale which can be formed by two or more sets of vertical fractures embedded in a vertically transverse isotropic background components of effective fracture compliance matrices for a medium with monoclinic symmetry are related to the characteristics of the fractured medium monte carlo simulation results show that inversion of ps reflection data is more robust than that of pp reflection data to uncertainties in our a priori knowledge vertically transverse isotropic parameters of unfractured rock than pp reflection data we also show that while inversion of pp reflections is sensitive to contrasts in elastic properties of upper and lower media inversion of ps reflections is robust with respect to such contrasts

behaviour of rare earth elements during seawater basalt interactions in the mururoa massif abstract the rare earth elements ree were analysed in the interstitial fluids in the basalts and in the alteration products clays and zeolites of mururoa atoll volcanic rocks french polynesia in order to investigate ree behaviour during seawater basalt interactions at low temperature the ree distribution coefficients between fluids and secondary products were calculated using the eq3 6 geochemical code the reversible irreversible character of the ree entrapment in secondary products was tested the ree concentrations in the clays are close to that of basalt the concentrations in zeolites are 10 times lower comparison of measured and modeled concentrations in solutions favours the irreversible incorporation assumption the affinity of the ree for the secondary products is shown to be high the calculated distribution coefficients range from 3 105 for lu and eu to 107 for ce in clays and from 104 to 7 105 in zeolites the distribution coefficients exhibit a progressive decrease from la to lu this behaviour is related to the greater stability of hree complexes relative to lree complexes in solution under reducing conditions in the interstitial fluids eh down to 500 mv eu undergoes reduction to the mobile eu ii this enhanced mobility is reflected by eu distribution coefficient being lower than those of adjacent ree the distribution coefficient of ce is higher as ce is trivalent in the fluids fractionation from the other trivalent ree was unexpected the calculated distribution coefficients of ree are lower in zeolites than in clays and this can be related to the spatial distribution and paragenesis of the secondary phases the dissolution of the basalt promotes the precipitation of clays at the basalt fluid interface and of zeolites in the bulk solution the ree released by dissolution are incorporated into the clays and further uptake occurs in zeolites for the ree left in solution

size and morphology of the chicxulub impact crater the chicxulub impact in mexico has been linked to the mass extinction of species at the end of the cretaceous period from seismic data collected across the offshore portion of the impact crater the diameter of the transient cavity is determined to be about 100 km this parameter is critical for constraining impact related effects on the cretaceous environment with previous estimates of the cavity diameter spanning an order of magnitude in impact energy the offshore seismic data indicate that the chicxulub crater has a multi ring basin morphology similar to large impact structures observed on other planets such as venus

integrated deep drilling coring downhole logging and data management in the chicxulub scientific drilling project csdp mexico impact structures in the solar system are mainly recognized and explored through remote sensing and on earth through geophysical deep sounding to date a continuous scientific sampling of large impact craters from cover rocks to target material has only seldom been performed the first project to deep drill and core into one of the largest and well preserved terrestrial impact structures was executed in the winter of 2001 2002 in the 65 myr old chicxulub crater in mexico using integrated coring sampling and in situ measurements the combined use of different techniques allows a three dimensional insight and a better understanding of impact processes here we report the integration of conventional rotary drilling techniques with wireline mining coring technology that was applied to drill the 1510 m deep yaxcopoil 1 yax 1 well about 40 km southwest of mrida yucatn mexico during the course of the project we recovered approximately 900 m of intact core samples including the transitions of reworked ejecta to post impact sediments and that one from large blocks of tilted target material to impact generated rocks i e impact melt breccias and suevites coring was complemented by wireline geophysical measurements to obtain a continuous set of in situ petrophysical data of the borehole walls the data acquired is comprised of contents of a natural radioactive element velocities of compressional sonic waves and electrical resistivity values all the digital data sets including technical drilling parameters initial scientific sample descriptions and 360 core pictures were distributed during the course of the operations via internet and were stored in the icdp drilling information system http www icdp online org serving the global community of cooperating scientists as a basic information service

geochemistry of drill core samples from yaxcopoil 1 chicxulub impact crater mexico the chemical composition of suevites displaced cretaceous target rocks and impactgenerated dikes within these rocks from the yaxcopoil 1 yax 1 drill core chicxulub impact crater mexico is reported and compared with the data from the yucatn 6 y6 samples within the six suevite subunits of yax 1 four units with different chemical compositions can be distinguished a upper lower sorted and upper suevite depth of 795846 m b middle suevite depth of 846861 m c brecciated impact melt rock depth of 861885 m and d lower suevite depth of 885895 m the suevite sequence a b and d display an increase of the cao content and a decrease of the silicate basement component from top to bottom in contrast the suevite of y6 shows an inverse trend the different distances of the yax 1 and y6 drilling sites from the crater center 60 and 47 km respectively lead to different suevite sequences within the cretaceous rocks of yax 1 a suevitic dike depth of 916 m does not display chemical differences when compared with the suevite while an impact melt rock dike depth of 1348 m is significantly enriched in immobile elements a clastic breccia dike depth of 1316 m is dominated by material derived locally from the host rock while the silicate rich component is similar to that found in the suevite significant enrichments of the k2o content were observed in the yax 1 suevite and the impact generated dikes all impactites of yax 1 and y6 are mixtures of a crystalline basement and a carbonate component from the sedimentary cover an anhydrite component in the impactites is missing yax 1 or negligible y6

geochemical and petrographic characteristics of impactites and cretaceous target rocks from the yaxcopoil 1 borehole chicxulub impact structure mexico implications for target composition we present major and trace element data as well as petrographic observations for impactites suevitic groundmass bulk suevite and melt rock particles and target lithologies including cretaceous anhydrite dolomite argillaceous limestone and oil shale from the yaxcopoil 1 borehole chixculub impact structure the suevitic groundmass and bulk suevite have similar compositions largely representing mixtures of carbonate and silicate components the latter are dominated by melt rock particles trace element data indicate that dolomitic rocks represented a significant target component that became incorporated into the suevites in contrast major elements indicate a strong calcitic component in the impactites the siliceous end member requires a mafic component in order to explain the low sio2 content multicomponent mixing of various target rocks the high alteration state and dilution by carbonate complicate the determination of primary melt particle compositions however two overlapping compositional groups can be discerned a high ba low ta group and a high fe high zn and high hf group cretaceous dolomitic rocks argillaceous limestone and shale are typically enriched in u as br and sb whereas anhydrite contains high sr contents the oil shale samples have abundances that are similar to the north american shale composite nasc but with a comparatively high u content clastic sedimentary rocks are characterized by relatively high th hf zr as and sb abundances petrographic observations indicate that the cretaceous rocks in the yaxcopoil 1 drill core likely register a multistage deformation history that spans the period from pre to post impact contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins we have found no evidence in our samples from a depth of 1347 1348 m for the presence of melt breccia we favor that clastic veinlets occur in a sheared and altered zone that underwent intense diagenetic overprint prior to the impact event

isotopic comparison of k t boundary impact glass with melt rock from the chicxulub and manson impact structures the discussion of candidate source craters for the catastrophic impact that occurred at the boundary between the cretaceous and tertiary periods k t boundary has recently centred on two buried craters the chicxulub in mexico 200 km diameter and the manson in iowa 35 km diameter both of which have 40ar 39ar ages of 65 myr indistinguishable from that of impact glass spherules found in k t boundary sediments1 4 here we report the strontium neodymium and oxygen isotopic compositions of core samples of impact melt rock recovered from drill holes into both the chicxulub and manson craters and compare these with previously published isotopic data for impact glasses from the k t boundary of the beloc formation in haiti5 7 the chicxulub melt rocks are isotopically indistinguishable from the k t impact glass strongly supporting the proposition that chicxulub is a source crater for the k t catastrophe the manson melt rocks by contrast have a clearly different isotopic composition strongly suggesting that they are unrelated to the k t impact spherules

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