Iridium Enrichment Research Articles

The K/Pg event at high southern latitudes: New evidence from continental deposits in the Magallanes/Austral Basin, Patagonia, South America

The Cretaceous/Paleogene boundary (K/Pg) event, associated with a meteorite impact at Chicxulub, Mexico, is globally recognized as one of the largest mass extinctions in natural history, marking the end of the Mesozoic Era. However, most of the outcrops with records and geochemical evidence of this boundary are distributed in the Northern Hemisphere and at mid-latitudes. Here, aiming to contribute to the knowledge of this event at high southern latitudes, we characterize a single iridium anomaly correlated with the K/Pg boundary, present within a carbonaceous mudstone level in a continental depositional environment in the Río de las Chinas Valley, Chilean Patagonia. High-resolution geochemical and palynofacies analyses were performed on a stratigraphic section from the top of the Dorotea Formation. Results showed that the iridium enrichment coincides with an anomaly of other platinum group elements. In addition, the palynofacies analysis showed a disturbance in the depositional environment, marked by an abrupt change from non-degraded phytoclasts (e.g., cuticles) to pseudoamorphous and degraded cuticles, and by an increase in the abundance of spores at the same level. The Río de las Chinas Valley locality provides new evidence for the comprehensive study of the end-Cretaceous event, from the poorly represented continental environments of high southern latitudes.

Platinum-group element (PGE) geochemistry of Deccan orangeites, Bastar craton, central India: Implication for a non-terrestrial origin for iridium enrichment at the K–Pg boundary

We report platinum group element (PGE) concentrations of twelve bulk-rock samples from the Behradih and Kodomali orangeite intrusions in the Mainpur field, Bastar craton, central India, which are emplaced synchronously with the Deccan flood basalts. Their palladium-group PGE (PPGE) (1.8–5.2ppb Pt, 1.2–6.4ppb Pd) contents are distinctly higher compared to their iridium-group PGE (IPGE) concentrations (0.8–2ppb Os, 0.8–1.2ppb Ir, 3.2–4.2ppb Ru, and 0.2–0.8ppb Rh). Their PGE contents as well as Pd/Ir ratios are either similar or even lower than those from the Mesoproterozoic and Cretaceous kimberlites and orangeites from the Kaapvaal craton (southern Africa), Cretaceous kimberlites from the Sao Fransisco craton (Brazil), Ordovician kimberlites from the North China craton and the Mesoproterozoic southern Indian kimberlites from the Eastern Dharwar craton. Anomalously elevated iridium (and other PGE) contents in sediments at the Cretaceous–Paleogene (K–Pg) boundary are commonly attributed either to a large bolide impact triggering the K–Pg mass extinction or to terrestrial causes such as volcanic eruptions (Deccan flood basalts) or even to mantle-plume derived lithospheric gaseous explosions (Verneshots). Lack of unusually high abundances of PGE in the Mainpur orangeties as well as in the co-eval Deccan flood basalts and associated alkaline rocks implies that the anomalous iridium enrichment reported at the K–Pg boundary sections was not sourced from the mantle and likely originated from an extraterrestrial source.

Iridium enrichment and poor fractionation from gold, platinum and palladium in clausthalite (PbSe), Tilkerode, eastern Harz, Germany

Enrichments in platinum-group elements were found in clausthalite, PbSe, which occurs in carbonate–hematite veins hosted in black shale of Upper Wenlockian age in the eastern Harz, Germany. Spot measurements by laser ablation–quadrupole inductively coupled plasma mass spectrometry (LA–QICPMS) show that the clausthalite has Pt contents that average 0.459 ppm and range from 0.003 to 2.83 ppm. Other noble metals quantified in the present work include Au (0.226 ppm, 0.109–0.451 ppm), Ru (0.061 ppm, 0.006–0.264 ppm), Ir (0.071 ppm, 0.010–0.185 ppm), Pd (0.046 ppm, 0.017–0.107 ppm) and Os (0.010 ppm, <0.003–0.024 ppm). Elemental ratios of Au/Ir, Pd/Ir and Pt/Ir, the average values of which are respectively 4.6, 0.8, and 4.9, suggest that Au, Pd and Pt are poorly fractionated from Ir. The resulting Ir enrichment is unusual and should reflect a particularly soluble (and stable) aqueous complex of Ir in a highly oxidizing, low-temperature, Se-rich solution.

Asteroidal source of L chondrite meteorites

Establishing connections between meteorites and their parent asteroids is an important goal of planetary science. Several links have been proposed in the past, including a spectroscopic match between basaltic meteorites and (4) Vesta, that are helping scientists understand the formation and evolution of the Solar System bodies. Here we show that the shocked L chondrite meteorites, which represent about two thirds of all L chondrite falls, may be fragments of a disrupted asteroid with orbital semimajor axis a = 2.8 AU . This breakup left behind thousands of identified 1–15 km asteroid fragments known as the Gefion family. Fossil L chondrite meteorites and iridium enrichment found in an ≈467 Ma old marine limestone quarry in southern Sweden, and perhaps also ∼5 large terrestrial craters with corresponding radiometric ages, may be tracing the immediate aftermath of the family-forming collision when numerous Gefion fragments evolved into the Earth-crossing orbits by the 5:2 resonance with Jupiter. This work has major implications for our understanding of the source regions of ordinary chondrite meteorites because it implies that they can sample more distant asteroid material than was previously thought possible.

Anomalous iridium enrichment at the Triassic–Jurassic boundary, Blomidon Formation, Fundy basin, Canada

We present new analyses that confirm Ir enrichment (up to 0.31 ng/g) in close proximity to the palynological Triassic-Jurassic boundary in strata near the top of the Blomidon Formation at Partridge Island, Nova Scotia. High Ir concentrations have been found in at least two samples within the uppermost 70 cm of the formation. Ratios of other PGEs and Au to Ir are generally higher by an order of magnitude than in ordinary chondrites. No impact-related materials have been identified at #is horizon in the Blomidon Formation, therefore we cannot confirm an extraterrestrial source for the anomalous Ir levels. We consider, however, the possibility that regional basaltic volcanism is a potential source for the Ir in these sediments. The elevated Ir concentrations are found in reduced, grey colored mudstones, so redox concentration is a possible explanation for the distribution of Ir in these strata.

Is Bedout an Impact Crater? Take 1

L. Becker et al. 's recent proposal that the Bedout structure off northwestern Australia is a giant bolide impact crater of Permian-Triassic (P-Tr) boundary age (“Bedout: a possible end-Permian impact crater offshore of northwestern Australia,” Research Article, 4 June, p. 1469; published online 13 May; 10.1126/science.1093925) provides many readily testable hypotheses, not least of which is that there should be some evidence of the impact in the sedimentary record of the surrounding area. The Bedout structure is located in the Roebuck Basin, which is part of a 2000-km extent of Late Palaeozoic-Mesozoic rift basins developed from Perth to Darwin. Marine conditions during the latest Permian and Early Triassic are recorded by the Kockatea Shale in the Perth Basin and equivalents in other basins. We have examined core material from the Kockatea Shale in the Hovea-3 borehole, located around 1000 km south of the Bedout structure ([1][1], [2][2]). This reveals a P-Tr transition in which bioturbated mudstones with a diverse latest Permian fauna, dominated by brachiopods, are replaced by laminated, anoxic shales with earliest Triassic bivalves. At no level in the core, which spans a Wuchiapingian-to-Dienerian interval, is there evidence for a layer of impact ejecta or a tsunamite. A trace metal assay also failed to find evidence for iridium enrichment. Core material is not available from sites nearer to Bedout, but mudlog data and wireline logs from boreholes as close as 400 to 500 km from Bedout also indicate a shale-on-shale transition across the P-Tr boundary. It is significant that the Hovea-3 core is substantially closer to the proposed impact site than the celebrated K-T impact sites in northeastern Mexico are to the Chicxulub Crater. For example, the famous Mimbral site, with its spherule layers and thick tsunamite record, is over 1500 km distant from Chicxulub. We suggest that either the impact did not occur in the late Permian to Early Triassic interval (and therefore it has no relevance to the P-Tr mass extinction event), or it is not an impact crater and is more likely to be a volcanic structure. 1. 1.[↵][3]1. B. M. Thomas 2. et al. , Austr. J. Earth Sci. , in press. 2. 2.[↵][4]1. B. M. Thomas, 2. C. J. Barber , APPEA J. 44, 59 (2004). [OpenUrl][5] # Response {#article-title-2} We agree with Wignall et al. that the study of cores from Permian-Triassic (P-Tr) sections in western Australia potentially provides a valuable test of the hypothesis that Bedout is a large P-Tr boundary crater. What is at issue is whether any of the existing cores or well logs, including Hovea-3, contain a complete record across the boundary. In many of the onshore basins that are proximal to Bedout, much of the Permian and Early Triassic section is missing (up to 2 km in some basins). Wignall et al. also state that Hovea-3 is closer to Bedout than some K-T impact sites (e.g., northeastern Mexico) are to the Chicxulub crater, and, thus, Hovea-3 should have some preservation of an impact ejecta layer. The distance between Hovea-3 and Bedout (1000 km) is about the same (not closer) as that between Chicxulub and the northeastern Mexico sections such as El Mimbral. These Mexican sections, and even closer (to Chicxulub) ones in Haiti, contain less than 1 m of ejecta; thus, even minor erosion at the boundary could erase the record. Wignall et al. rely on biostratrigraphy to determine the P-Tr boundary in Hovea-3. As shown elsewhere for the P-Tr, biostratigraphy alone is not a reliable indicator of the completeness of the boundary layer ([1][6], [2][7]). However, if it can be further demonstrated (e.g., by isotope dating) that Hovea-3 does indeed represent a record of continuous sedimentation across the P-Tr boundary, and it contains no ejecta, then this would argue against Bedout being a large P-Tr boundary crater. Finally, the absence of iridium in Hovea-3 is consistent with other P-Tr sections worldwide. This may reflect, again, upon the completeness of the P-Tr boundary, the type of section (marine versus continental), or the actual impacting body (e.g., asteroid versus comet). Several impact tracers (see our Research Article) occur in P-Tr boundary sections worldwide, none of which contains elevated iridium. Thus, we await full documentation and future investigations of impact tracers in Hovea-3 and other onshore cores that may include the P-Tr boundary. 1. 1.[↵][8]1. S. A. Bowring 2. et al. , Science 280, 1039 (1998). [OpenUrl][9][Abstract/FREE Full Text][10] 2. 2.[↵][11]1. Y. G. Jin 2. et al. , Science 289, 432 (2000). [OpenUrl][12][Abstract/FREE Full Text][13] [1]: #ref-1 [2]: #ref-2 [3]: #xref-ref-1-1 View reference 1. in text [4]: #xref-ref-2-1 View reference 2. in text [5]: {openurl}?query=rft.jtitle%253DAPPEA%2BJ.%26rft.volume%253D44%26rft.spage%253D59%26rft.atitle%253DAPPEA%2BJ%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [6]: #ref-3 [7]: #ref-4 [8]: #xref-ref-3-1 View reference 1. in text [9]: {openurl}?query=rft.jtitle%253DScience%26rft.stitle%253DScience%26rft.aulast%253DBowring%26rft.auinit1%253DS.%2BA.%26rft.volume%253D280%26rft.issue%253D5366%26rft.spage%253D1039%26rft.epage%253D1045%26rft.atitle%253DU%252FPb%2BZircon%2BGeochronology%2Band%2BTempo%2Bof%2Bthe%2BEnd-Permian%2BMass%2BExtinction%26rft_id%253Dinfo%253Adoi%252F10.1126%252Fscience.280.5366.1039%26rft_id%253Dinfo%253Apmid%252F9582110%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [10]: /lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6Mzoic2NpIjtzOjU6InJlc2lkIjtzOjEzOiIyODAvNTM2Ni8xMDM5IjtzOjQ6ImF0b20iO3M6MjQ6Ii9zY2kvMzA2LzU2OTYvNjA5LjQuYXRvbSI7fXM6ODoiZnJhZ21lbnQiO3M6MDoiIjt9 [11]: #xref-ref-4-1 View reference 2. in text [12]: {openurl}?query=rft.jtitle%253DScience%26rft.stitle%253DScience%26rft.aulast%253DJin%26rft.auinit1%253DY.%2BG.%26rft.volume%253D289%26rft.issue%253D5478%26rft.spage%253D432%26rft.epage%253D436%26rft.atitle%253DPattern%2Bof%2BMarine%2BMass%2BExtinction%2BNear%2Bthe%2BPermian-Triassic%2BBoundary%2Bin%2BSouth%2BChina%26rft_id%253Dinfo%253Adoi%252F10.1126%252Fscience.289.5478.432%26rft_id%253Dinfo%253Apmid%252F10903200%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [13]: /lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6Mzoic2NpIjtzOjU6InJlc2lkIjtzOjEyOiIyODkvNTQ3OC80MzIiO3M6NDoiYXRvbSI7czoyNDoiL3NjaS8zMDYvNTY5Ni82MDkuNC5hdG9tIjt9czo4OiJmcmFnbWVudCI7czowOiIiO30=

A possible causal relationship between extinction of dinosaurs and K/T iridium enrichment in the Nanxiong Basin, South China: evidence from dinosaur eggshells

Multiple distinct iridium anomalies have been observed in dinosaur eggshells in the K/T boundary sections of the Nanxiong Basin, Guangdong Province, South China. The eggshells collected at and near the fossil–pollen-defined K/T boundary interval show iridium increases of about 19 and 28 times, respectively, above the background level. The enrichment of other trace elements in the eggshells occurs mostly at and near the interval. The distribution of Ir-bearing levels proves the existence of at least six Ir-delivering events from the latest Cretaceous into the earliest Paleocene. The enrichment of Ir and other trace elements in eggshells may have been caused by the assimilation of these elements into the dinosaur body through food, and then into the eggs laid by them. Two types of pathological development, i.e. variation in eggshell thickness and eggshell microstructure, have been observed from the basin. It seems that they occur frequently during the K/T transition. The physiological mechanisms producing pathologic dinosaur eggshells are evaluated in the light of homologous phenomena occurring in living birds. On this basis, it is concluded that the formation of pathologic dinosaur eggshells was caused by anomalous trace element concentrations. The extinction of the dinosaurs in the Nanxiong Basin did not occur instantaneously, but spread out within 250 ka with major extinction beginning at the boundary interval. The cause may have been environmental poisoning and adverse changes in climatic conditions as indicated by the geochemical analyses of the dinosaur eggshells. A repeating short- and long-term geochemically induced environmental stress adversely affected the reproductive process and contributed to the extinction of the dinosaurs.

Occurrence of noble metals in aluminum‐ and chromium‐ rich objects in ordinary chondrites and of baddeleyite (ZrO2) in a chromite inclusion

Abstract— Platinum group elements (PGE) enrichments are found in Fe‐Ni blebs (&lt;1 μm) in Al‐ and Cr‐rich objects in the ordinary chondrites Ste. Marguerite, Forest Vale, Montferré. In Ste. Marguerite, high ZrO2 concentrations (baddeleyite) are also present in a chromite inclusion. Iridium enrichment in this material compared to its metal content is confirmed by INAA. The widespread occurrence of PGE in Al‐ and Cr‐rich objects, mostly present in H‐group chondrites of nearly equilibrated types, must be taken into account to understand their conditions of formation.

Cretaceous/Tertiary boundary sections in the East Balkan area, Bulgaria

From the investigations in the East Balkan area, south of the trust and nappe of the Balkan Mountains until now only on the coast of the Black Sea there has been found a real Cretaceous/Tertiary (T/K) boundary. Cretaceous/Tertiary boundary sections in Bulgaria on the coast of the Black Sea near Bjala, 35 km south of Varna have been identified by micro- and nannofossils, magnetic reversals and event markers. The hemipelagic sediments of profile Bjala 2b and Bjala 2c show an iridium enrichment in the boundary clay, a minimum of CaCO3, shocked quartzes, a mass extinction of Cretaceous nannoplankton species, as well as a bloom of survivors at the K/T boundary and the first appearance of new nannoplankton species after the K/T event. The comparison of biostratigraphic and magnetostratigraphic results provides a time scale for the evolution of these new nannoplankton species.

Chemical species of iridium and other trace elements in the Cretaceous-Tertiary boundary clays and their implication

The geological samples: Cretaceous-Tertiary (K-T) boundary clays, meteorites, volcanic lava, and ultrabasic rock were separated into carbonate, metal, sulfide, oxide, silicate and acid-resistant residue by a selective chemical dissolution procedure developed in our laboratory. Some conclusions were drawn by analyzing the chemical speciation of anomalous iridum in the above samples and especially by studying the characteristics of mineral components in those residue phases. (1) It is impossible that the anomalous Ir was caused by geochemical enrichment; (2) The iridium enrichment in the K-T boundaries did not necessarily associate with kerogen; (3) The volcanic activity has participated in the extinction event at the end of Cretaceous; (4) Extraterrestrial impact was the trigger of K-T event. Then, we proposed a mixed model by which the extant anomalous Ir in K-T boundary clays was interpreted as a combined effect of extraterrestrial impact, volcano eruption and post-depositional redistribution.

Synthesis of macroporous poly(vinyldiethylenetriamine) chelating resin and the enrichment—separation of rhodium and iridium

Poly(vinyldiethylenetriamine) chelating resin was synthesized from macroporous poly(vinyl chloride) and used for the enrichment-separation of rhodium and iridium. The efficiency, rate and equilibrium constant adsorption of rhodium and iridium on the resin were determined and real samples were analysed. The mechanism of rhodium and iridium enrichment is discussed.

ジオカタストロフィー 生物の絶滅と地球磁場の逆転

One of the most interesting paleontological contributions to general knowledge is evidence that once-dominant giant reptiles died out in the end of the Cretaceous period. Recently, on the basis of the iridium enrichment in Cretaceous-Tertiary (K-T) boundary layers, the Berkeley research group suggested that the process of mass extinction of species is extraterrestrial in origin, probably caused by the impact of large meteorites on the Earth. Since this suggestion, it has become fashionable for geochemists to discover an iridium abundance at K-T boundary sites or at other mass-extinction horizons around the world.Many Earth- and planet-scientists have eagerly discussed on the possibilities of a global catastrophe due to bolide impacts. The probability of striking the Earth by an Earth-crossing asteroid is estimated to be not no small. As a matter of fact, in January 18, 1991, astronomical observations found a small asteroid crossing the Earth's orbit within half the distance to the Moon. Earth-crossing comets are even more plentiful. The probability of comet collisions to the Earth is supposed to increase extremely when the Solar System encounters with a dense molecular cloud in the Galaxy.Some volcanologists protest against the impact hypothesis, insisting that a bolide impact is not the only way to cause a global catastrophe. Despite that a large-scale volcanic eruption can produce iridium enrichments, the presence of shock quartz at K-T boundary sites remains a problem for proponents of volcanism. It seems that the extraterrestrial-origin hypothesis has advantage over the volcanic hypothesis. Various hypotheses of mass extinction are reviewed in the first half of this paper.It is a well-known fact that the Earth's magnetic field has repeatedly flip-flopped. The great circulation of molten iron in the Earth' core generates the geomagnetic field. The sophisticated theory of electromagnetic fluid dynamics explains the mechanism of the geomagnetic generation, but there still remain uncertainties in the mechanism of geomagnetic reversal. Some paleomagneticians speculated that the asteroid impact is an immediate cause of geomagnetic reversal, but others rejected the idea of such a linkage from standpoints of microtektite studies.Recently, a Japanese research group has analyzed samples of sedimentary rock, whose sedimentation rate was 3 m/ka during the Matuyama-Brunhes reversal period, and discovered changes in both oxygen and carbon isotope ratios coincident with reversal. The decrease in carbon isotope ratio of 1 permill corresponds supposedly to the 40 % reduction of biomass, which indicates a large amount of CO2 gas flows into the ocean and the atmosphere from the biosphere. The concurrent decrease in oxygen isotope ratio can be explained as the CO2-induced greenhouse effect.The coincidence of a biomass changes with a geomagnetic reversal implies that the reversal may be one of possible causes of mass extinction. The biosphere is protected by the shield effect of magnetic field against solar high-energy particles. During the time of reversal when the shield effect disappears, the biosphere may be directly exposed to harmful ultraviolet ray and further affected by the concurrently-occurred global climate change. On the basis of observational facts that the geomagnetic dipole moment has been rapidly decreasing, many geomagneticians give a warning that the Earth is now standing at the starting point of a magnetic reversal. If the decrease will continue as it is, the geomagnetic field will completely disappear in one thousand years or later.

Dynamic deformation of quartz and feldspar: clues to causes of some natural crises

Indications of natural catastrophes and crises are preserved in some geologic strutures and in the stratigraphic record, emphasizing the dynamic nature of the Earth's evolution. The crises may result either from exogenic or endogenic energy sources. Distinguishing between these sources is a complex, multidisciplinary problem in the Earth sciences. Shock deformation by impact has been well-studied both in shock recovery experiments and in known impactites, but only recently has the possibility of generation of shock waves by high-temperature internal explosions been seriously considered. An optically based reconaissance survey of microstructures in quartz and feldspar in some explosive silicic volcanics suggests that they may have been induced by dynamic deformation at high temperature; in some respects the character of the microstructures differs greatly from those induced by room temperature shock, whereas in others the differences are subtle. Results from recent shock recovery experiments on quartzite preheated to 440 ° and shocked to 28 GPa support this contention. For two classic Cretaceous/Tertiary sections at Gubbio, Italy, it is shown that shock mosaicism and shock lamellae occur over a 4 m interval, bisected by the boundary, in a pattern approximately correlatable with an independently established iridium enrichment. These data are incompatible with any single event and are most plausibly interpreted as reflecting a period (∼ 0.5 my) of intense, probably episodic, volcanism with a culmination at the K/T transition. Multiple dynamic events may also account for macroscopic and microscopic structures developed during the evolution of the Vredefort structure, South Africa.

Iridium enrichment in volcanic dust from blue ice fields, Antarctica, and possible relevance to the K/T boundary event

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.

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).

An iridium rich layer at the Cretaceous/Tertiary boundary in the Bidart Section (southern France)

We have measured an anomalous iridium concentration in the clays of the Cretaceous/Tertiary boundary in the Bidart section, thus confirming the planetary character of this anomaly. The iridium anomaly is synchronous with a very sharp minimum of the 13C/12C ratio and there is no evidence of any other iridium enrichment in marly concentrations away from the boundary. The integrated iridium content is of the same order of magnitude as that calculated from deep sea cores in the Pacific Ocean. In spite of a careful search, we have found no evidence of any exotic material.

Iridium Enrichment in Airborne Particles from Kilauea Volcano: January 1983

Airborne particulate matter from the January 1983 eruption of Kilauea volcano was inadvertently collected on air filters at Mauna Loa Observatory at a sampling station used to observe particles in global circulation. Analyses of affected samples revealed unusually large concentrations of selenium, arsenic, indium, gold, and sulfur, as expected for volcanic emissions. Strikingly large concentrations of iridium were also observed, the ratio of iridium to aluminum being 17,000 times its value in Hawaiian basalt. Since iridium enrichments have not previously been observed in volcanic emissions, the results for Kilauea suggest that it is part of an unusual volcanic system which may be fed by magma from the mantle. The iridium enrichment appears to be linked with the high fluorine content of the volcanic gases, which suggests that the iridium is released as a volatile IrF(6).

Sanidine spherules at the Cretaceous–Tertiary boundary indicate a large impact event

The hypothesis that a catastrophic impact of an extraterrestrial body caused the terminal Cretaceous mass extinctions of dinosaurs, planktonic foraminfera and other species is now accepted as respectable following the discovery of a worldwide iridium enrichment in the Cretaceous–Tertiary (K–T) boundary clay1–5. In the basal lamina of the K–T boundary clay of Caravaca (Spain)7 numerous spherules were discovered composed of finely crystallized, almost pure K-feldspar in the structural state of high sanidine. It is concluded here that these spherules solidified from a melt and were probably derived from the impacting body. This poses problems as high K-values are not reported from bulk analyses of meteorites6. The K-feldspar phenocrysts reported in some iron meteorites23 suggest the body may have been a metal–sulphide–silicate planetesimal. A cometary body is suggested as an alternative.