u pb zircon age for a tuff in the campbell group griqualand west sequence south africa implications for early proterozoic rock accumulation rates an ion microprobe u pb age of 2552 11 ma has been obtained on zircon separated from a regional banded tuff horizon in the nauga formation beukes 1980b upper campbell group griqualand west sequence south africa this age permits time constraints to be placed on lithologically correlated units within the adjacent transvaal sequence and correlations to be made with the further removed hamersley group in australia calculated rock accumulation rates of 2 to 4 m m y for these strata of mainly shale and banded iron formation suggest that sedimentation rates were significantly slower in the late archean early proterozoic than is generally assumed

high noble metal concentrations in a late pliocene sediment a 2 3 myr old layer in a sediment from the antarctic ocean contains ir and au at levels comparable with those at the cretaceous tertiary boundary a sizable fraction of the noble metals is contained in vesicular millimetre sized poly mineralic grains that closely resemble ablation debris from chondritic meteorites and there is little doubt that the noble metals resulted from the accretion of a large extraterrestrial object no massive extinctions or other evidence of environmental stress seem to be associated with this accretionary event

pt group metal anomalies in the lower mississippian of southern oklahoma four iridium pt group elements abundance anomalies have been found within a stratigraphic span of 3 m in the lower mississippian of oklahoma in ascending order the first anomalies occur at the top of the woodford shale ir 0 25 ppb pt 48 ppb os 7 5 ppb and au 18 ppb the anomalies occur just below a redox boundary and we suspect that the enriched elements were precipitated from sea water that contacted the organic and sulfide rick black shale two more anomalies occur in the welden limestone the lower one weak and the upper one strong ir 0 42 ppb pt 50 ppb os 0 075 ppb and au 0 14 ppb the excess ir and pt also co as and ni might have been enriched from sea water by bacteria at these two horizons a 70 cm thick interval of excess heavy siderophiles occurs in the overlying caney shale the interval contains the following peak concentrations ir 0 56 ppb pt 150 ppb os 0 51 ppb co 725 ppm and ni 1450 ppm these elements vary in proportion to the al clay content and we suspect that they were carried in with detrital material from erosion of ultramafic source rocks we found no evidence of microspherules or shocked mineral grains in any of these anomaly zones

proximal cretaceous tertiary boundary impact deposits in the caribbean trace element isotopic and mineralogic studies indicate that the proposed impact at the cretaceous tertiary k t boundary occurred in an ocean basin although a minor component of continental material is required the size and abundance of shocked minerals and the restricted geographic occurrence of the ejecta layer and impact wave deposits suggest an impact between the americas coarse boundary sediments at sites 151 and 153 in the colombian basin and 5 to 450 meter thick boundary sediments in cuba may be deposits of a giant wave produced by a nearby oceanic impact on the southern peninsula of haiti a approximately 50 centimeter thick ejecta layer occurs at the k t boundary this ejecta layer is approximately 25 times as thick as that at any known k t site and suggests an impact site within approximately 1000 kilometers seismic reflection profiles suggest that a buried approximately 300 km diameter candidate structure occurs in the colombian basin

ru ir ratios at the cretaceous tertiary boundary implications for pge source and fractionation within the ejecta cloud abstract ruthenium and ir are the least mobile platinum group elements pges within the cretaceous tertiary k t boundary clay the ru ir ratio is therefore the most useful pge interelement parameter for distinguishing terrestrial and extraterrestrial contributions to the boundary clay the ru ir ratio of european marine k t sections 1 67 0 38 is statistically different from that of the north american continental sections 0 76 0 26 it is unlikely that this difference is due to secondary pge remobilization pge input to the boundary clay during multiple impacts or volcanic emissions the global difference in ru ir ratios in the boundary clay may therefore be primary the positive correlation between distance from the chicxulub impact structure yucatan mexico and ru ir ratio and the more than 1000 c difference in the condensation temperatures of these elements lead us to propose that fractionation of ru from ir during condensation from the ejecta cloud may have occurred resulting in the global difference in ru ir ratios

earliest triassic claystone breccias and soil erosion crisis abstract chemostratigraphic methods 13corg for pinpointing the permian triassic boundary in nonmarine rocks of antarctica australia and south africa have drawn attention to unusual claystone breccias at this horizon of the greatest extinction in the history of life on earth these rocks differ from other breccias in having a high proportion of clasts with birefringence microfabrics sepic plasmic fabrics characteristic of soils and can be called sepic pedoliths in the terminology of soil science at many localities earliest triassic sepic pedoliths are thin 5 15 cm beds but some fill shallow paleochannels and pinch out laterally other non marine breccias at the permian triassic boundary are dominated by fragments of coal pedogenic carbonate nodules and deeply weathered volcanic rock fragments earliest triassic sepic pedoliths record an unusually severe and widespread episode of soil erosion associated with forest dieback at the permian triassic boundary comparable sepic pedoliths are found in debris flows after clear cutting of forests in western oregon today studies of these modern deposits show that sepic soil peds do not withstand weathering for more than a few months such clayey clasts are held together mainly by roots and fungi which decay rapidly in soils and streams these modern analogs demonstrate that terminal permian soil erosion was rapid and profound sepic pedoliths are also known at other times of mass extinction such as the triassic jurassic boundary but are not yet known before the devonian advent of trees and forested clayey soils

organic and carbonate rich soil formation 2 6 billion years ago at schagen east transvaal district south africa abstract a 17 m paleosol sequence at schagen south africa which developed on a serpentinized dunite intrusion in a granite gneiss terrain 2 6 ga ago is characterized by an alternating succession of thick 1 3 m carbonate rich dolomite and calcite zones and silicate rich serpentines talc and quartz zones the upper 8 m section is especially rich in organic c up to 1 4 wt petrologic and geochemical data suggest the upper 8 m section is composed of at least three soil profiles that developed on i silicate rich rock fragments and minerals that were transported from local sources serpentinite and granite by fluvial and or eolian processes and ii dolomite and calcite zones that formed by locally discharged groundwater the mg and fe in the paleosol sequence were largely supplied from local sources mostly serpentinite but the ca sr and hco3 were supplied by groundwater that originated from a surrounding granite gneiss terrain in the uppermost soil profile the fe is retained the fe3 fe2 ratio increases and ferri stilpnomelane is abundant these data suggest the atmospheric po2 was much greater than 10 3 7 atm 0 1 present atmospheric level pal the carbonaceous matter in the soils is intimately associated with clays talc chlorite and ferri stilpnomelane and occurs mostly as seams 20 m to 1 mm thick that parallel the soil horizons these occurrences crystallographic structures h c ratios and 13corg values 17 4 to 14 4 pdb suggest that the carbonaceous matter is a remnant of in situ microbial mats originally 1 to 20 mm thick the microbial mats developed a mostly on soil surfaces during the formation of silicate rich soils and b at the bottom of an evaporating anoxic alkaline pond during the precipitation of the fe rich dolomite these 13corg values are difficult to be explained by a current popular idea of a methane and organic haze rich archean atmosphere pavlov et al 2001 these values however can be easily explained if the microbial mats were composed of cyanobacteria and heterotrophs that utilized the remnants of cyanobacteria in a strongly evaporating environment

microtektite like impact glass associated with the frasnian famennian boundary mass extinction abstract glass spherules of probable impact origin occur in the late devonian hony section in belgium they are found at the stratigraphic level of the frasnian famennian boundary which is marked by one of the largest mass extinctions in the phanerozoic the hony glass spherules are chemically identical to those previously described from the stratigraphically equivalent senzeille section located 90 km to the west southwest in contrast to the senzeille layer the age of the hony spherule layer is well constrained by conodont biostratigraphy the spherules have morphological and chemical characteristics similar to tektites and microtektites they have a broad range of chemical composition with generally high k2o na2o and al2o3 na2o k2o their water content is in the range measured for tektites and impact glasses small inclusions of lechatelierite may be present in the spherules these glass spherules are likely to be the product of an impact event that occurred at or very near the frasnian famennian boundary the target rock was probably a clay rich pelitic sediment intermixed with carbonates the 368 1 ma siljan ring crater sweden is the most likely impact site candidate the hony and senzeille glass spherules represent the first substantial evidence that an impact event may be linked to the frasnian famennian mass extinction

chicxulub multiring impact basin size and other characteristics derived from gravity analysis the buried chicxulub impact structure in mexico which is linked to the cretaceous tertiary k t boundary layer may be significantly larger than previously suspected reprocessed gravity data over northern yucatan reveal three major rings and parts of a fourth ring spaced similarly to those observed at multiring impact basins on other planets the outer ring probably corresponding to the basin s topographic rim is almost 300 kilometers in diameter indicating that chicxulub may be one of the largest impact structures produced in the inner solar system since the period of early bombardment ended nearly 4 billion years ago

terminal ordovician extinction geochemical analysis of the ordovician silurian boundary anticosti island quebec elemental abundances including ir carbon and oxygen ratios in carbonates mineral content and thin sections have been measured in samples collected across the conodont defined ordovician silurian o s boundary exposed on anticosti island quebec the ir concentrations ranged from 5 to a maximum at the boundary of 58 parts per trillion ppt however there is no evidence on the basis of these ir results for the association of a large body earth impact with the o s extinction because the ir concentrations like those of most other trace elements are simply proportional to the clay al content in the carbonate sequence the sup 13 c sup 12 c and sup 18 o sup 16 o ratios decrease abruptly at the boundary then just as abruptly increase to a long period of higher than preboundary ratios these patterns are probably related to the salinity in the seaway which was shallowing up to boundary time and then deepened and developed patch reefs fresh water input from rivers would have been most influential during the shallow water conditions 32 references 4 figures

spinel from archean impact spherules abstract large meteorite impacts are recorded in distal fall deposits preserved in two early archean greenstone belt sequences 3 2 3 5 ga mapping stratigraphie and geochemistry studies have been used to identify at least four impact layers in the barberton greenstone belt of southern africa and one in the pilbara greenstone belt of western australia spherules that represent impact melt droplets distinguish these impact layers from associated tuffs spinel with unusual compositional traits occurs within spherules in two of these impact layers spinel is the only preserved primary phase and is found within up to 10 of the spherules in the best preserved layers some spherules contain up to 40 spinel which is often the only primary phase that crystallized within the spherules spinel morphologies include dendrites and skeletal octahedra typical of those found in high temperature melts spinel compositions are similar to those of cenozoic impacts including very high ni and ferric iron contents but the archean spinel is distinctive in being composed dominantly of the chalcophile elements fe ni cr and v this spinel is not metamorphic or metasomatic in origin closely associated komatiites in the underlying onverwacht group both fresh and highly altered have unaltered spinel with igneous compositions and morphologies quite distinct from the impact spinel characteristics of spinel preserved in these archean impact spherules suggest a rather complex series of steps following a large bolide impact initial high temperature reduction produced melt droplets some of which were immiscible silicate metal sulfide density differences allowed separation of some of these immiscible melts which subsequently evolved independently and formed spherules with highly variable chalcophile element contents oxidation during atmospheric reentry increased ni fe and fe 3 fe 2 in the chalcophile siderophile melts archean impact spinel has higher ni fe than cenozoic impact spinel though distinctly lower fe 3 fe 2 which probably reflects a substantially less oxidizing atmosphere during the archean the archean atmospheric oxidation may in fact be a transient phenomenon associated with a large impact into the terrestrial ocean resulting in very high atmospheric h 2 o h 2 bolide size is estimated to be 24 km in diameter based on the average diameter of 0 85 mm for the spinel bearing spherules this estimate is consistent with the observed ir fluence in spinel bearing impact layers of approximately 5 3 mg cm2 which would be produced by a 30 km diameter chondritic bolide

lateritic nickel deposits of brazil many nickel deposits are known in brazil accounting for about 350 106 tons of ore with an average of 1 5 ni all are of the lateritic type these deposits are scattered throughout the country being rarer in the northeastern region and in the south below 25 s latitude they are mainly associated with mafic ultramafic massifs of large dimensions and ultramafic alkaline complexes and occur in climatic regions of contrasting seasons the weathering profile developed over the fresh rock consists from bottom to top of the following horizons altered rock coarse saprolite argillaceous saprolite ferruginous saprolite and lateritic overburden the thickness of each horizon varies from one deposit to another the whole profile generally exceeding 20 m the saprolitic horizons with inherited minerals serpentine chlorite or neoformed minerals smectites constitute the silicated nickel ore and are thicker were climatic conditions are drier the ferruginous upper horizons made up of iron oxide hydroxides are more developed in more humid regions in brazil the silicated ore generally prevails over the oxidized ore the main ni bearing minerals are serpentine smectite garnierite and goethite the lateritic nickel deposits of brazil may be correlated with two erosion surfaces corresponding to the sul americano lower tertiary and velhas upper tertiary levelling cycles the degree of dismantling of the higher and more ancient surface and the consequent development of the velhas surface control the position of the nickel accumulation in the landscape thus the deposits may be found either in the lowlands or in the highlands where they are always covered by a silcrete layer the alteration profiles in the brazilian lateritic nickel deposits are broadly similar to those described elsewhere in the world however they present two characteristic features the silicated ore prevails over the oxidized ore and a silicified layer covers the profies developed on the highlands

iridium emissions from kilauea volcano during may 1983 gas and particulate samples were collected at the cooling vents of kilauea volcano in hawaii three vents on the southeast rift zone were sampled using base treated filter packs to absorb acidic gases and teflon filters for particles the samples were analyzed for 40 elements by nondestructive neutron activation analysis as with other volcanoes the chalcophilic and volatile elements were enriched by up to seven orders of magnitude relative to the erupted basalt unlike the case of other volcanoes iridium was observed to be highly enriched 105 fold at the two higher temperature vents and the degree of enrichment appears to be related to both high temperatures and high fluorine content of the gases estimates of the emission rates of ir during eruptions of kilauea based on the measurements reported here and those of the hawaiian volcano observatory yield an emission rate of about 3 g ir per 106 m3 of magma this rate amounts to only about 0 3 of the ir present in the magma that is being released by the volcano this previously unidentified ir source to the atmosphere and ocean may have a significant influence on the geochemical cycle of ir on the earth the uniqueness of kilauea in its emissions of ir is probably related to the deep source of magma which contains significant levels of ir 0 32 ppb and high levels of the halogens f and cl

an iridium anomaly in the middle lower jurassic of the venetian region northern italy a significant iridium enrichment has been found in a jurassic marine sequence about 180 million years old outcropping in the alps of the venetian region the maximum iridium concentration of 3 2 0 2 ng g 1 occurs in a brown crust 2 3 mm thick containing essentially iron hydroxide this crust characterized by a nearly total absence of detritic components lies on top of upper lias limestones and is overlaid with bajocian bathonian limestones the origin of the ir anomaly is not clearly understood yet but could be explained by either a drop in the sedimentation rate resulting in the concentration within a thin sediment layer of the normally infalling cosmic dust and or by an increase of cosmic material infall following an asteroid or comet impact s

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

searching for travertines calcretes and speleothems in deep time processes appearances predictions and the impact of plants common models for modern calcite precipitation in and around caves soils springs and streams involve co 2 supplied by thick high p co 2 biogenic soils which were probably thin or non existent before vascular plants indeed plant influenced chemical weathering might have caused accelerated terrestrial carbonate production from the devonian onwards however terrestrial carbonates have also been documented from the archaean proterozoic cambrian ordovician and silurian mechanisms which could have caused non marine carbonates to precipitate without organic rich soils are described and some geological events likely to have influenced non marine carbonate precipitation up to the origin of vascular plants are highlighted as organisms have evolved so have the petrographic characteristics of non marine carbonates some examples of this are also given here

rhodium distribution at the cretaceous tertiary boundary analysed by ultrasensitive laser photoionization the extraordinarily high concentrations of indium and other siderophiles found in cretaceous tertiary k t boundary deposits have been interpreted1 as a result of a large extraterrestrial body falling upon the earth causing mass extinction of the biota dominant in the cretaceous the existing alternative models of the cretaceous terminal event try to explain the anomaly of siderophile elements by their being concentrated through sedimentation or volcanic eruptions2 3 to understand the nature of the k t event it is important to establish the proportions of siderophiles and their isotopes in boundary deposits of that age especially the proportions of platinum group elements their concentrations in extraterrestrial material being high and occurring in ratios differing from those typical of terrestrial material here we present the first data on rhodium concentrations at the k t boundary in the sumbar sm 4 section turkmen ssr obtained by the ultrasensitive laser photoionization spectroscopy lapis technique4 5 the maximum rh concentration in the samples studied is 24 2 ng g 1the rh ir ratio is 0 34 0 06 which is close to the cosmic ratio of these elements the lapis technique is efficient and can be used for determining the concentrations of all siderophile elements and their isotopes in geological materials

a search for iridium in the deccan traps and inter traps it has been suggested that flood basalts in the deccan india might be associated with events at the cretaceous tertiary boundary ktb a search for iridium in 47 samples from lava flows and inter trap sediments in the deccan yields negative results concentrations are not statistically different from zero with a minimum detection level on the order of 0 1 ng g 1 ppb this does not help to constrain the debate on the internal vs external origin of ktb boundary events since for instance ir could have been outgassed from the magma and original concentrations of only 0 05 ng g 1 prior to outgassing are sufficient to account for the worldwide ir excess mass at the ktb

post depositional mobility of platinum iridium and rhenium in marine sediments the discovery of high concentrations of iridium in cretaceous tertiary boundary sediments engendered the hypothesis1 that a meteorite collided with the earth 65 million years ago coincident with the mass extinction that occurred at that time iridium spikes of various magnitudes have subsequently been reported at more than 10 other extinction horizons2 11 it has been suggested on the other hand that geochemical processes might create or modify many of these spikes5 11 4 but a critical evaluation of these suggestions has been hindered by incomplete understanding of low temperature iridium geochemistry other platinum group elements ru rh pd re os pt au are often found to be associated with ir spikes and inter element ratios have been used to assess the cosmic or terrestrial nature of the enrichments5 7 9 28 29 but the geochemical influences on these relative abundances are also poorly constrained here we describe analyses of recent abyssal marine sediments which allow us to characterize the behaviour of pt re and ir during early diagenesis these elements are redistributed by changes in sedimentary redox conditions such changes can probably account for many of the small platinum group element spikes found in the geological record and may render ambiguous attempts to interpret inter element ratios

archean spherule beds impact or terrestrial origin a discussion of the paper geochemistry and mineralogy of early archean spherule beds barberton mountain land south africa evidence for origin by impact doubtful by c koeberl w u reimold and r h boer

a review of the structure petrology and dynamic deformation characteristics of some enigmatic terrestrial structures abstract we review three categories of enigmatic terrestrial structures that have a variety of macroscopic and microscopic dynamic deformation features the first category includes the midcontinent united states cryptoexplosion structures they are of shallow origin and illustrate the effects that might be expected from a meteorite impact on a sedimentary rock terrane the second category includes the well known vredefort and sudbury plutons or basement uplifts the lesser known precambrian basement uplifts at steen river manson and carswell and the diatreme breccia dikes at the slate islands they are of relatively deep seated origin and illustrate the effects that might be expected from deep crustal or mantle processes of an explosive nature the third category includes the large landslides in areas of crystalline rocks at koefels and langtang the dynamic deformation features at these latter locations appear to be related to high strain rate tectonic processes associated with the landslide itself we conclude that microscopic dynamic deformation features which occur in natural materials may have originated from meteorite impacts from internal explosions or from high strain rate tectonic process

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

iridium bearing sublimates at a hot spot volcano piton de la fournaise indian ocean sublimates and incrustations derived upon the cooling of volcanic gases have been collected on various sites piton de la fournaise poas momotombo etna ardoukoba and erta ale they have been analysed for ir and other volatile elements se as cu au ag pb tl by means of instrumental neutron activation analysis inaa and proton induced x ray emission pixe among the investigated volcanoes only piton de la fournaise is found to release detectable amounts of iridium ir in piton de la fournaise sublimates is associated with f minerals this confirms its gaseous transport as a volatile fluoride compound iridium seems to be preferentialy released by hot spot type volcanoes and its detection in piton de la fournaise sublimates provides a positive argument in favour of a volcanic hypothesis to explain the ktb events

trace element patterns at a non marine cretaceous tertiary boundary at the fossil pollen defined cretaceous tertiary boundary in the raton basin of new mexico and colorado an iridium abundance anomaly and excess scandium titanium and chromium are associated with a thin ash or dust fallout bed now kaolinitic clay that was preserved in freshwater coal swamps

iridium enrichment in volcanic dust from blue ice fields antarctica and possible relevance to the k t boundary event abstract blue ice fields in antarctica well known as stranding zones of antarctic meteorites contain ice that is several tens to hundreds of thousand years old which records numerous events that have taken place since its formation among the most important structures on the ice fields are dust bands which are mostly of volcanic origin and consist of small volcanic glass shards and lithic fragments petrographic and chemical studies of a number of dust bands during earlier investigations have shown that significant ash layers are deposited at distances of up to several thousand kilometers from the source volcanoes samples of dust layers taken in the lewis cliff beardmore glacier area in antarctica analyzed in the course of this work for major and trace elements were identified as consisting of volcanic ash probably originating from the pleiades melbourne volcanic province northern victoria land some of these dust samples were found to contain iridium in concentrations of up to 7 5 ppb a positive correlation between ir and se as well as enrichments in as sb and other volcanogenic elements has been observed the dust samples investigated here have very small grain size consistent with the distance from the source volcanoes the small grain size of the dust suggests that the ir enrichment is a surface effect and was probably caused by condensation from a vapor phase this is the first time that significant ir enrichments have been found in volcanic dust which demonstrates that ir and possibly also other platinum group metals may be present in some volcanic ash deposits although this has been doubted by the proponents of the pure impact theory for the k t boundary event the results of this study suggest that the ir in the k t boundary clays is not necessarily of cosmic origin but alternatively may have originated from mantle reservoirs that were tapped during extensive volcanic eruptions which may been triggered by impact events

noble metal abundances in an early archean impact deposit we report detailed analyses on the concentrations of the noble metals pd os ir pt and au in an early archean spherule bed s4 of probable impact origin from the lower fig tree group barberton greenstone belt south africa compared to other sedimentary deposits of known or suspected impact origin some noble metals are present in exceptionally high concentrations noble metal abundances are fractionated relative to abundances in chondrites with ratios of osir ptir pdir and auir at only 80 80 41 and 2 of these values in ci chondrites although an extraterrestrial source is favored for the noble metal enrichment the most plausible cause of the fractionation is by regional hydrothermal metasomatic alteration

the compositional classification of chondrites ii the enstatite chondrite groups we present new data from a neutron activation analysis of four enstatite chondrites including the taxonomically important st sauveur and discuss the classification of enstatite chondrites the enstatite chondrites can be divided into two compositionally distinct sets in one set abundances of nonrefractory siderophiles and moderately volatile chalcophiles and alkalis are 1 5 2 0 higher than in the other a well resolved compositional hiatus separates these two sets the differences in composition are as great as those between the groups of ordinary chondrites and therefore it appears best to treat these sets as separate groups by analogy with the symbols used for ordinary chondrites we propose to designate the high fe high siderophile group eh and the low fe low siderophile group el known members of the eh group belong to petrologic types 4 and 5 whereas all el members are petrologic type 6 within the eh group no correlation is observed between petrologic type and abundance of nonrefractory siderophiles or moderately volatiles or alkalis two physical properties show only modest overlap between the eh and el groups cosmic ray ages for eh chondrites are 0 5 7 ma while those for el chondrites are 4 18 ma relative to bjurbole i xe formation intervals are 1 3 0 6 ma for eh chondrites and 2 9 0 5 ma for el chondrites the weight of the chemical and physical evidence indicates that the eh and el groups formed separate bodies at similar distances from the sun the available evidence for shallowater and happy canyon two strongly recrystallized silicate rich meteorites containing 40 mg g fe ni indicates that the former is an enstatite clan chondrite altered by loss of sulfide and plagioclase rich melts whereas the latter is intermediate in composition between el chondrites and the chondritic silicates in the pine river iab anomalous meteorite

the distribution and geochemistry of platinum group elements as event markers in the phanerozoic abstract the discovery of the platinum group element pge enrichment in the cretaceous tertiary k t boundary impact layer popularized the use of pges as geochemical event markers and prompted an explosion of interest in these elements whose geochemical behavior is poorly understood instead of concentrating on the more easily analyzed pges e g iridium many studies now include the entire group analytical methods have been improved to the point where small samples 5 g can be analyzed confidently at the sub ppb level allowing the determination of the distribution of pges in the oceans atmosphere and biosphere the presence or absence of a pge anomaly is often used as a criterion for determining whether an event horizon is impact related however to date only the k t boundary yields a distinct pge anomaly with positive interelement correlations and solar interelement ratios terrestrial sources such as volcanic emissions are also being investigated as potential contributors to pge anomalies at other phanerozoic event horizons primary signatures e g interelement ratios characteristic of the pge source may be altered during various high and or low temperature processes for example fractionation during impact melting vaporization condensation remobilization redistribution and diagenesis can obscure original signatures and make determination of the pge source difficult future research might be directed towards establishing the behavior of the pges during such processes

strangelove ocean before the cambrian explosion the palaeozoic and mesozoic eras were terminated by faunal changes involving mass extinction of the old and explosive evolution of the new fauna but the fossil record shows only a cambrian explosion at the end of the precambrian stanley speculated that the explosion was only possible after the ubiquitous algae community had been largely eliminated1 ecological niches were thus liberated for explosive evolution if the cambrian explosion were preceded by a mass mortality or by a mass extinction such an event should leave a record in the form of geochemical anomalies we have undertaken a search for geochemical anomalies at the precambrian cambrian contact we report here the discovery of a sharp negative carbon isotope shift in the carbonate of a clay immediately above a marker in the precambrian cambrian boundary the china c marker and interpret this signal as evidence of sudden decrease in fertility before the cambrian explosion of invertebrate evolution the discovery suggests that the precambrian cambrian boundary might be defined by an event marker at a palaeontologically correlative horizon

evidence for a meteoritic component in impact melt rock from the chicxulub structure abstract the chicxulub structure in yucatan mexico has recently been recognized as a 200 km diameter multi ring impact crater of k t boundary age crystalline impact melt rocks and breccias from within the crater which have compositions similar to those of normal continental crustal rocks and which show shock metamorphic effects have been studied for trace element and re os isotope compositions re os isotope systematics allow the sensitive and selective determination of an extraterrestrial component in impact derived rocks a melt rock sample shows elevated indium concentrations an osmium concentration of 25 ppb and a low 187 os 188 os ratio of 0 113 which are incompatible with derivation from the continental crust even though the 187 os 188 os ratio is slightly lower than the range so far measured in meteorites a mantle origin seems unlikely for mass balance reasons and because the cratering event is unlikely to have excavated mantle material the data support the hypothesis of a heterogeneously distributed meteoritic component in the chicxulub melt rock a sample of impact glass from the haitian k t boundary at beloc yielded about 0 1 ppb osmium and an 187 os 188 os ratio of 0 251 indicating the presence of a small meteoritic component in the impact ejecta as well

paleoclimatic significance of widespread proterozoic silcretes in the bear and churchill provinces of the northwestern canadian shield abstract silcrete is widespread in the thelon basin and lower coppermine homocline two early to middle proterozoic cratonic cover sequences in the northwest canadian shield in the thelon basin silcrete occurs as silicified lateritic weathering profiles weathered tops of rhyolite flows and silicified terrestrial volcaniclastic sediments in coppermine homocline silcrete comprises silica cemented rock fans at the basal unconformity silicified evaporitic fracture fills in basement and silicified ephemeral lake sediments petrographic studies of the silcretes indicate that they are strikingly similar to cenozoic silcretes from southern africa and australia formed under arid climates the silcretes in both proterozoic sequences are associated with climate sensitive sediments such as eolian tes and evaporites that also suggest an arid environment of deposition geochronologic data indicate that the silcretes represent part of an arid paleogeographic province that existed in the northwest canadian shield at about 1 85 1 7 ga although the proterozoic silcretes are petrographically similar to cenozoic arid climate analogs the lack of land plants in the precambrian suggests that geochemical criteria used to distinguish cenozoic arid and humid climate silcretes may not be applicable to precambrian examples

cretaceous tertiary boundary of dsdp site 596 south pacific sediments from the cretaceous tertiary k t boundary at deep sea drilling project site 596 record accumulation of pelagic clays at about lat 45 s in the center of the ancestral south pacific the k t boundary in this region is characterized by oxidized thoroughly bioturbated sediments containing large amounts of ir and relict high pressure and high temperature mineral phases the net ir fluence 320 ng cm 2 is among the largest on earth and the number of shocked quartz grains 30 m in size 1800 cm 2 exceeds that in all localities outside of north america

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

preservation of carbon isotopes in kerogen from thermally altered mesoproterozoic lacustrine microbialites stable carbon isotope geochemistry is a well established and reliable tool for studying metabolisms of microbial communities in the precambrian record however the isotopic effects of high tempera

the cretaceous tertiary boundary in the gosau basin austria the cretaceous tertiary boundary has been identified in the gosau beds elendgraben near salzburg austria the undisturbed 2 mm thick boundary clay in the palaeomagnetic g zone differs from the surrounding sediments in having significant colour no biogenic calcite and different contents of rare earth and siderophile elements carbon and magnetic minerals the clay also contains shocked quartz and plagioclase particles and indicates a dramatic change in sedimentation caused by a short lived event

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

iridium abundances across the ordovician silurian stratotype chemostratigraphic analyses in the ordovician silurian boundary stratotype section bracketing a major extinction event in the graptolitic shale section at dob s linn scotland show persistently high iridium concentrations of 0 050 to 0 250 parts per billion there is no iridiumn concentration spike in the boundary interval or elsewhere in the 13 graptolite zones examined encompassing about 20 million years iridium correlated with chromium both elements showing a gradual decrease with time into the middle part of the lower silurian the chromium iridium ratio averages about 10 6 paleogeographic and geologic reconstructions coupled with the occurrence of ophiolites and other deep crustal rocks in the source area suggest that the high iridium and chromium concentrations observed in the shales result from terrestrial erosion of exposed upper mantle ultramafic rocks rather than from a cataclysmic extraterrestrial event

lateritic weathering of pyroxenites at niquelandia goias brazil the supergene behavior of nickel two typical lateritic weathering profiles jacuba ahd angiquinho from the niquelandia ni deposits brazil were studied in order to establish the petrological relationship between the supergene ni products and the parental pyroxenes frbm the base to the top of the profiles pyroxenes are replaced by goethite and kaolinite through a series of transitional ni bearing phyllosilicates the mineralogy and the chemical composition especially the ni content of these clay minerals depends on the degree of fracturing and serpentinization of the pyroxenite and the location of the pyroxenite with respect to neighboring dunite within the jacuba profile smectite and pimelite pseudomorphs after pyroxene are especially ni rich and in fact are the most ni enriched clay minerals now known in lateritic weathering profiles

no geochemical evidence for an asteroidal impact at late devonian mass extinction horizon one way to test the proposed causal relationship between mass biotic extinction and asteroidal impact is to search for the geochemical signature of such an event at additional mass extinction horizons other than the cretaceous tertiary k t boundary we report here the results of an extensive search for an indium ir anomaly across the late devonian frasnian famennian f f extinction horizon 367 myr bp over a decade ago mclaren1 proposed that this extinction was caused by an astrophysical catastrophe we have examined three sedimentary sequences in new york state the standard reference section for eastern north america and one in belgium the type locality which cross the f f boundary no ir anomaly was found in any of the analysed stratigraphie sections the highest values of ir detected ranged from only 0 2 to 2 of those reported for the marine and terrestrial ir analyses at the k t boundary further devonian pyrite rich sediments similar to those at the k t boundary do not contain high ir concentrations

distribution of noble metals across the cretaceous tertiary boundary at gubbio italy iridium variation as a constraint on the duration and nature of cretaceous tertiary boundary events iridium pd pt and au were determined in sections from the bottaccione gorge and contessa valley gubbio italy by radiochemical neutron activation shales and limestones were sampled from 2 85 m above to 219 m below the cretaceous tertiary k t boundary metal enrichment was evaluated by comparing the boundary shale region with the lower part of the section background iridium is concentrated by 63 times in the boundary shales in comparison with the background whereas other metals are enriched by no more than 2 2 times the enrichment of ir is not confined to the boundary shales but extends approximately 2 m above and below this horizon within this ir rich region there are four distinct ir maxima in addition to the major ir enrichment in the k t boundary shales iridium maxima are stratigraphically coincident with maxima in abundances of shocked minerals characteristic of explosive volcanism limestones are much lower in noble metals than shales and their ir contents in the k t boundary region are largely accounted for by their minor clay mineral contents the time represented by the 4 m of ir rich section is at least 3 x 10 5 yr if published sedimentation rates are used to sustain an increased ir flux over this period and to account for the ir distribution near the k t boundary intense volcanic activity is a preferred alternative to impact of extraterrestrial material

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

micromorphology and geochemistry of groundwater silcretes in the eastern south downs uk groundwater silcretes have been recognised recently as major terrestrial silicon sinks and yet their origins are poorly understood this paper aims to further understanding of the micro fabric geochemistry and formation of groundwater silcretes through detailed analyses of silcrete boulders from the south downs sussex uk in depth petrological investigations of silcrete blocks identified three varieties of silcrete across the study area saccharoid hard and pebbly conglomeratic or silcrete breccia similar to those elsewhere in england silcrete fabrics were universally simple and preserved host material structures optically continuous quartz overgrowths were the dominant cement and developed on framework grains in the absence of silt and clay sized particles finer grained silica cements occurred in isolated patches and as vein or cap like geopetal features these caps and veins formed through the silicification of illuviated clay rich material that entered the host sediment prior to or in the early stages of overgrowth formation titanium contents were related to the amount of fine grained silica and appear to reflect variations in host material chemistry subsequent to silicification the silcretes were altered by at least two phases of ferrunginisation characterised by the ingress of iron minerals and partial fabric dissolution or replacement the study reveals strong similarities in the micromorphology and geochemistry of groundwater silcretes in the study area and those developed in sandy host sediments in neighbouring parts of northwest europe variations that do occur could be explained by differences in the host sediment geological setting or tectonic history of the respective basins or sub basins suggesting that there may be a common mechanism for silcrete formation across northwest europe silcrete development appears to be related to the release of silica accompanying acid leaching of the host material or adjacent strata in the south downs this probably resulted from oxidisation of pyrite in the argillaceous and lignitic sediments overlying the host sands it is envisaged that palaeoenvironmental conditions were of limited importance for silcrete development and that there was no specific era of formation with phased development accompanying landscape evolution through the neogene into the pleistocene

platinum group elements in tulameen coal british columbia canada discussion sures at the blakeburn opencast pit was due to selective fixation by organic complexing of au and pge carried as solute or colloidal suspensions anomalously high concentrations of pge in fault zones were explained by enhanced ground water flow along more permeable channelways which promoted greater fluid rock coal interactions two sources for the pge were identified an external pt rich source the nearby tulameen ultramafic complex and an internal source comprising volcanic units interbedded with the coal including tonsteins bentonites and basalt on the basis of the relative abundances of pt pd and au in stream sediments coals and ultramafic rocks of the tulameen complex van der flier keller 1991 p 394 further proposed that the precipitation of pge from ground waters could also account for the origin of these metals in alluvial placers of the region it was the latter conclusion requiring wholesale mobility and precipitation of pge in the surficial environment that prompted a close examination of the data that led van der flier keller to favor a low temperature origin for pge in coals and alluvial sediments of the tulameen district previously raicevic and cabri 1976 and nixon et al 1990 had documented a magmatic origin for platinum group minerals in tulameen placers and demonstrated a genetic link with chromitites in the core of the tulameen ultramafic complex of course there are many well documented examples of the mobility and hydromorphic dispersion of noble metals to varying degrees during surface weathering of rocks and ores e g taufen and marchetto 1990 wood and vlassopoulos 1990 in addition experimental and theoretical work has demonstrated the ability of organic acids to complex with pge in aqueous solutions e g wood 1990 plimer and williams 1987 however given our present state of knowledge of the distribution of pge in coal swaine 1990 it seems unwise to invoke some poorly understood process uch as regional hydromorphic dispersion of pge in order to account for subeconomic concentrations of these metals in coal when other more conventional mecha

fractionation of ruthenium from iridium at the cretaceous tertiary boundary new data on ru ir abundance ratios are presented for nonmarine hell creek montana frenchman river saskatchewan and marine cretaceous tertiary boundary sites brazos river texas beloc haiti dsdp 577 and dsdp 596 the ru ir ratio varies from 0 5 to 1 within 4000 km of chicxulub and increases to 2 3 at paleodistances 65 ma of up to 12 000 km from the impact site for ci chondrites ru ir 1 5 a ballistic model of ejecta cloud cooling and expansion which employs the available vapor pressure versus temperature data for ru and ir predicts qualitatively similar global variation in the ru ir ratio but by only a factor of 1 5 we infer that several other factors such as remobilization of pge during diagenesis preferential oxidation of ru condensation kinetics and atmospheric chemical and circulation processes may account for the observed larger ru ir variation

caliche as a geological repository for atmospheric sulfate 1 recent studies demonstrate that important information on sulfur source oxidation pathway transport pattern and reaction kinetics of atmospheric oxidation of sulfur gases can be uncovered by measuring multiple isotope compositions of the oxidation product secondary atmospheric sulfate sas sas from earth s distant past however is rarely preserved in the rock records because of its high solubility and small quantity we report here from the measurement of 47 modern and fossil caliche samples that caliches contain variable amounts of carbonate associated sulfate cas and the 17o values of the cas range from 0 12 to 2 32 with most values higher than the background value of 0 05 for seawater sulfate indicating the presence of an sas component derived from the oxidation of sulfur gases by atmospheric ozone or related oxidants this study demonstrates that caliche a pedogenic carbonate common in the geologic record could serve as a repository for ancient sas

allochthonous surface alteration of ni laterites abstract lateritic weathering sections over ultramafic rocks in burma puerto rico venezuela malagasy and brazil display anomalous chemical variations in their upper parts these variations are not due to weathering of ultramafics but to the addition of varying amounts of weathered material from other rocks this contribution can cause a considerable increase in si al ti and zr and decrease in fe cr and ni in the surficial parts of the limonite zone in other occurrences the admixture of foreign material can only be recognized by strongly elevated ti and zr amounts in surface layers which is atypical for ultramafic rocks and their weathering products in most of the occurrences the foreign material was probably transported by colluvial alluvial processes but an eolian derivation is also possible sedimentary processes may cause large amounts of foreign material to be mixed with ni limonite smaller amounts of foreign clayey material can also be washed downwards into clearly autochthonous limonite the ni laterite deposit on the tagaung taung burma which has been explored in detail by drill holes provides an excellent example for the processes discussed above which can also influence the composition of the weathering mantle above other parent rocks

along strike variations in the composition of sandstones derived from the uplifting western greater caucasus causes and implications for reservoir quality prediction in the eastern black sea oligo miocene outcrops along the southern margin of the western greater caucasus preserve a record of sediments shed from the range into the northern and central parts of the eastern black sea sandstones in the russian western caucasus are significantly more quartz rich than those located farther se in western georgia the latter contain appreciably more mudstone and volcanic rock fragments oligo miocene turbidite systems derived from the russian western caucasus in the tuapse trough and central eastern black sea may therefore form better quality reservoirs at shallow to moderate depths than sediments derived from west georgian volcaniclastic sources in the easternmost part of the basin palynomorph analysis indicates sediment derivation predominantly from jurassic and cretaceous strata in the russian western caucasus and from eocene strata and an increasing proportion of cretaceous strata upsection in western georgia an eocene volcaniclastic source is proposed for the increased rock fragment component in west georgian sandstones eocene volcaniclastic rocks are no longer exposed in the greater caucasus but similar rocks form the inverted fill of the adjara trialet basin farther south in the lesser caucasus the former presence of a northern strand of this basin in the west georgian caucasus is supported by earlier thermochronological work

the cretaceous tertiary extinction a lethal mechanism involving anhydrite target rocks the chicxulub crater yucatan mexico is a leading contender as the site for the impact event that caused the cretaceous tertiary k t extinctions a considerable thickness of anhydrite caso4 forms part of the target rock high temperatures resulting from impact would drive so2 off from the anhydrite hundreds of billions of tonnes of sulfuric acid aerosol would thus enter the stratosphere and cause considerable cooling of the earth s surface decrease photosynthesis by orders of magnitude deplete the ozone layer and permit increased uv radiation to reach the earth s surface finally the aerosol would fall back to earth as acid rain and devastate land and some lacustrine biota and near surface marine creatures the presence of anhydrite in the chicxulub target rock may thus help explain the many extinctions observed at the k t boundary

a late archaean palaeoproterozoic 2 6 ga palaeosol on ultramafics in the eastern transvaal south africa abstract this article describes a palaeosol developed on archaean 3 4 ga ultramafics and overlain by sedimentary rocks of the chuniespoort group 2 55 ga the palaeosol is thick mature and complex forming carapaces on palaeohills from base to top it comprises four lithological units 1 carbonated serpentinite 2 black talc rock 3 silcrete dolocrete 4 silcrete breccia in mudstone matrix petrographic and geochemical studies major elements plus v cr ni zn ga y zr nb pge and th indicate that units 1 to 3 represent former saprolites and silicified carbonated saprolites with bedrock textures still preserved unit 4 has been more intensely leached with some admixture of allochthonous sialic material collapse of the original texture and concentration of the immobile elements an irregular ni mineralization in the bottom part of the profiles is described and compared with equivalent modern lateritic deposits a high carbon content suggests the presence of a surficial mat of primitive micro organisms which could have contributed to the stability of the palaeosol on the hilly topography and greatly enhanced the weathering rate the presence of silcrete is indicative of a semi arid climate possibly with alternating wet and dry seasons or of a dry period having succeeded a wet one the absence of fe oxides and the general deficiency of this element in the upper part of the profiles taking into account that the soil was well drained and aerated support the model of an atmospheric o2 level much lower than at present by an order of two magnitudes

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

accretion rate of extraterrestrial matter iridium deposited 33 to 67 million years ago iridium measured in 149 samples of a continuous 9 meter section of pacific abyssal clay covering the time span 33 to 67 million years ago shows a well defined peak only at the cretaceous tertiary boundary in the rest of the section iridium ranges from a minimum concentration near 0 35 nanograms per gram in the paleocene to a maximum near 1 7 in the eocene between 63 and 33 million years ago the mean iridium accumulation rate is approximately 13 nanograms per square centimeter per million years correction for terrestrial iridium leads to an extraterrestrial flux of9 3 nanograms of iridium per square centimeter per million years and an estimated annual global influx of 78 billion grams of chondritic matter consistent with recent estimates of the influx of dust meteorites and crater producing bodies with masses ranging from 10 13 to 1018 grams combining the recent flux of objects ranging in mass from 106 to 107 grams with the flux of 1014 to 1015 gram objects indicates that the number of objects is equal to 0 54 divided by the radius in kilometers to the 2 1 power periodic comet showers should increase the cometary iridium flux by a factor of 200 to 600 on a time scale of 1 to 3 million years the predicted iridium maxima more than 30 times background are not present in the intervals associated with the cretaceous tertiary boundary or the tektiteproducing late eocene events

some comparative marine chemistries of platinum and iridium abstract iridium has seawater concentrations about one hundred times less than those of platinum as a consequence of its weaker complexing abilities with halides and of a stronger tendency to hydrolyze iridium like platinum is enriched in some ferromanganese minerals compared to palladium platinum and iridium are probably oxidized during mineral formation from the divalent and trivalent states respectively to the tetravalent ones the pelagic sediment contents of irridium as well as those of platinum are related to the manganese contents for higher platinum contents of the authigenic ferromanganese minerals the pt ir ratios approach the seawater ratio suggesting little dilution by platinum and iridium in detrital minerals the marked enrichments of platinum and iridium in the ferromanganese concretions enhance their value as potential ores for heavy metals

role for microorganisms in the formation of iridium anomalies we propose a microbial role in the enhancement erasure dispersal and creation of certain sedimentary iridium anomalies such as those of the cretaceous tertiary boundary the proposal is based on laboratory experiments that have demonstrated the ability of some bacteria and fungi to concentrate iridium from solution and to dissolve iridium from source igneous and meteoritic rocks

iridium and palladium as discriminants of volcanic exhalative hydrothermal and magmatic nickel sulfide mineralization neutron activation of pd au and ir in samples from 1 economically significant ni sulfide deposits of western australia 2 the host rocks of these deposits 3 hydrothermal and or metasomatic ni sulfide mainly millerite mineralization in peridotite at nullagine western australia 4 hydrothermal replacement volcanic exhalative ni cu sulfide mineralization at sherlock bay western australia and 5 volcanic exhalative sulfides in inter flow sediments at kambalda are used to show that ir and pd can be used to discriminate between the various origins magmatic volcanic exhalative hydrothermal metasomatic and metamorphic ascribed to ni sulfide deposits all the economically significant ni sulfide deposits in western australia are strongly enriched in the precious metals relative to other crustal rocks and have similar pd ni and ir ni ratios to each other these ratios persist regardless of whether the nickel sulfides occur at the base of volcanic ultramafic piles e g kambalda or as centrally disposed interstitial sulfides in intrusive dunite bodies the strong similarities of the pd ir ratios of the ni sulfide ores and of the spinilex textured komatiites which represent ultramafic liquids from which the ni sulfide liquids were derived together with other features of the precious metal geochemistry of the ores and host rocks of the ni sulfide deposits are used to show that the deposits had magmatic origins in contrast the pd ir and ir ni ratios of the hydrothermal and or metasomatic ni sulfide mineralization at nullagine are distinctly different from those of the major ni deposits the former contains high au moderate but variable pd and exceptionally low ir similarly the volcanic exhalative sulfides at kambalda and sherlock bay contain negligible quantities of pd and ir all the ir in both the interflow sediments and the hydrothermal metasomatic mineralization is contributed by the host rocks and not by the sulfides the western australian data together with precious metal data for other examples of hydrothermal ni cu sulfide mineralization strongly suggest that although ni au and pd may be transported in aqueous solution ir cannot hence the relative abundance of ir can be used as a discriminant of magmatic sulfides

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

use of platinum group elements for impactor identification terrestrial impact craters and cretaceous tertiary boundary abstract this study presents the abundances of ru ir pt pd rh and au platinum group elements pge in the ejecta layer at the cretaceous tertiary boundary and melt rocks from east clearwater lake brent new quebec and lake wanapitei impact craters all believed to have been formed by the impact of chondritic projectiles the objective was to evaluate the pure application of pge interelement ratios e g ru ir for impactor identification to compare the utility of both melt rocks and ejecta deposits for this purpose and to expand the pge database for these craters the similarity of pge interelement ratios within the chondrite and iron meteorite groups precludes their use in the fine classification of impactors of this composition introduction of indigenous pge from country rock may contaminate the original meteoritic signature in addition the extent of pge fractionation during impact and melt sheet cooling and of post sedimentary pge remobilization must be evaluated in each case for east clearwater the pge chondrite normalized plot is flat indicating a chondritic impactor based on ru ir and cr ir ratios a cl or l chondrite is most likely however variation of ratios with depth in the core makes any fine classification of the projectile tenuous for the brent and new quebec craters the flat chondrite normalized pge plot indicates a chondritic impactor but finer classification is precluded by possible post syn depositional pge fractionation at lake wanapitei ni cr and cr ir ratios indicate a ll chondrite impactor wolf et al 1980 but sparse pge data for this chondrite type does not allow further classification of the projectile a flat chondrite normalized pge plot indicates a chondritic rather than an iron impactor at lake wanapitei the global ejecta layer at the cretaceous tertiary boundary has a chondritic pge signature but only when the integrated flux of pge on a global scale is considered individual sites may not show chondritic ratios due to differential pge remobilization resulting from local differences in depositional environment continental vs marine and to vaporization and or condensation pge fractionation occurring during impact the results for melt rocks and the k t ejecta deposit indicate that pge can be used for impactor identification but primarily in combination with other siderophile chalcophile meteoritic elements and their ratios

microscopic lamellar deformation features in quartz discriminative characteristics of shock generated varieties the appearance and orientation of microscopic lamellar deformation features in quartz have been examined in samples from different geologic environments lamellar deformation features from known shock and or impact environments are sharp parallel closely spaced and tend to extend in a continuous manner across entire grains they have specific well documented orientations and occur in multiple sets per grain lamellar deformation features from other geologic environments including tectonic and explosive volcanic only superficially resemble shock features they are generally less well defined slightly curved wider spaced and tend to extend in a discontinuous manner across parts of grains they have relatively random orientations only some of which correspond to known shock orientations and generally occur as single sets per grain on the basis of these observations we conclude that there is a characteristic set of microscopic deformation features produced in quartz by shock these so called planar features differ from microscopic lamellar deformation features produced by other dynamic processes therefore the case for the formation of characteristic planar features in quartz by natural processes other than impact is still not proven