Extraterrestrial Component Research Articles

Aouelloul impact crater, Mauritania: New structural, lithological, and petrographic data

Aouelloul impact crater, Mauritania: New structural, lithological, and petrographic data

Impact melt rocks from the Late Paleocene Hiawatha impact structure, northwest Greenland

AbstractImpact melt rocks formed during hypervelocity impact events are ideal for studying impact structures. Here, we describe impact melt rock samples collected proximal to the 31 km wide 58 Ma Hiawatha impact structure, northwest Greenland, which is completely covered by the Greenland Ice Sheet. The melt rocks contain diagnostic shock indicators (e.g., planar deformation features [PDF] in quartz and shocked zircon) and form three groups based on melt textures and chemistry: (i) hypocrystalline, (ii) glassy, and (iii) carbonate‐based melt rocks. The exposed foreland directly in front of the structure consists of metasedimentary successions and igneous plutons; however, the carbonate‐based impactites indicate a mixed target sequence with a significant carbonate‐rich component. Well‐preserved organic material in some melt rocks indicates that North Greenland at the time of impact was host to abundant organic material, likely a dense high‐latitude temperate forest. Geochemical signatures of platinum‐group elements in selected samples indicate an extraterrestrial component and support previous identification of a highly fractionated iron impactor in glaciofluvial sand. Our results illustrate the possibility to study impact structures hidden beneath a thick ice sheet based on transported samples and this opens a new avenue for identifying other potential impact craters in Greenland and Antarctica.

Chromium isotopes identify the extraterrestrial component in impactites from Dhala impact structure, India

AbstractThe Dhala structure in north‐central India is a confirmed complex impact structure of Paleoproterozoic age. The presence of an extraterrestrial component in impactites from the Dhala structure was recognized by geochemical analyses of highly siderophile elements and Os isotopic compositions; however, the impactor type has remained unidentified. This study uses Cr isotope systematics to identify the type of projectile involved in the formation of the Dhala structure. Unlike the composition of siderophile elements (e.g., Ni, Cr, Co, and platinum group elements) and their inter‐element ratios that may get compromised due to the extreme energy generated during an impact, Cr isotopes retain the distinct composition of the impactor. The distinct ε54Cr value of −0.31 ± 0.09 for a Dhala impact melt breccia sample (D6‐57) indicates inheritance from an impactor originating within the non‐carbonaceous reservoir, that is, the inner Solar System. Based on the Ni/Cr ratio, Os abundance, and Cr isotopic composition of the samples, the impactor is constrained to be of ureilite type. Binary mixing calculations also indicate contamination of the target rock by 0.1–0.3 wt% of material from a ureilite‐like impactor. Together with the previously identified impactors that formed El'gygytgyn, Zhamanshin, and Lonar impact structures, the Cr isotopic compositions of the Dhala impactites argue for a much more diverse source of the objects that collided with the Earth over its geological history than has been supposed previously.

The Zhamanshin impact structure, Kazakhstan: A comparative geochemical study of target rocks and impact glasses

Abstract The Zhamanshin impact structure, which is about 1 Myr old, has a diameter of 14 km and is situated in the semi-arid region of Kazakhstan (48°24′N, 60°58′E). It has a heterogeneous suite of target rocks, including predominantly crustal lithologies (e.g., clays and siltstones) with minor ultramafics. Zhamanshin is known for its unique association of impact glasses, including basic and acidic varieties of zhamanshinites and (tektite-like and a few aerodynamically-shaped) irghizites. The origin of both of these impact glasses has long been debated, which is complicated by incomplete sampling of target lithologies at the Zhamanshin site and a limited number of isotopic analyses. However, such studies are a prerequisite for a comprehensive discussion of the mechanisms that formed the unique association of different impact glasses in one impact event. We present major and trace element contents, as well as combined Sr-Nd isotope data for target rocks and impact glasses from the Zhamanshin impact structure. These data, for the first time, include Paleogene clays and siltstones from a core drilled in the vicinity of the crater and cutting through all major target lithologies. The core samples contain important source lithologies for the impactites from the Zhamanshin area. Mixing calculations, based on geochemical data and Sr-Nd isotope signatures, indicate that irghizites and Si-rich zhamanshinites can be produced from variously homogenized mixtures of mainly clays and siltstones with minor additions of ultrabasic rocks. Based on highly siderophile element (HSE) and Os isotope data (including the first analyses of the clay and siltstone lithologies) we calculated a hypothetical Os composition of the irghizite precursor, allowing us to approximate a chondritic admixture to the irghizites of roughly 1% of a chondritic component. This confirms previous suggestions about the amount of extraterrestrial components. A new HSE and Os isotope dataset for five zhamanshinites reveals, on average, crust-like HSE concentrations and Os isotope compositions, confirming earlier suggestions of a lack of meteoritic admixtures to these impact glasses.

The quest for an extraterrestrial component in Muong Nong-type and splash-form Australasian tektites from Laos using highly siderophile elements and Re-Os isotope systematics

Abstract Extremely low and variable concentrations of osmium (Os) and other highly siderophile elements (HSE) in most tektites make it challenging to establish direct links between these impact-related materials and their possible extraterrestrial contribution. New Os concentrations (2–43 ppt) and 187Os/188Os ratios (0.131–0.68) in a suite of fifteen well-characterized Australasian tektites from Laos (Muong Nong and splash-form types) with variable Ni enrichment indicate a maximum of ∼0.005% addition of a chondritic impactor. This is similar to some Australasian tektites from Vietnam with similarly low siderophile contents, but significantly lower than found in previous studies of more Ni-rich Australasian splash-form tektites and microtektites from different parts of the Australasian strewn field (e.g., Indonesia, South China Sea). The contents of HSE and Re–Os isotopic compositions of layered Muong Nong-type Australasian tektites are highly variable, suggesting mingling of crustal-derived (siderophile element-poor) and extraterrestrial (siderophile element-rich) materials. The absence of a direct correlation between HSE and Ni contents is interpreted to result from a fractionation process related to their different vaporization/condensation temperatures. The low Os abundance in most of the analyzed Australasian tektites, combined with non-radiogenic 187Os/188Os far below average upper continental crust, may provide a direct test to distinguish continental versus seawater impact scenario. In the absence of any specific low-Os target, a particular process of Os loss following impact is required. We envisage a scenario where evaporative loss of >>90% Os in the form of Os oxides from the overheated tektite melt is aided by volatile species derived from dissociated seawater and/or saline pore water embedded in sediments off-shore Indochina, consistent with elevated contents of halogens in Australasian tektites. This water-assisted Os loss could also play significant role for Central European tektites, while the continental surface with limited amount of water would prevent from more efficient HSE loss as could be the case for Ivory Coast tektites.

Volcanology and a New Geophysics. Reality and Prospects: Yellowstone Volcano

This paper presents a dynamic image of activity on Yellowstone Volcano, providing a detailed analysis of the key parameters in real time. Some preliminary inferences show that active volcanoes are energetically open systems that are strongly affected by extraterrestrial astrophysical components, in the first place, solar fluxes of muons and neutrinos. This should be borne in mind in forecasts of their activity. The astrophysical component may well prove to be an essential mechanism for synchronization of the states of supervolcanoes. The astrophysical component of volcanic activity can also be present in volcanoes on planets of the solar system, which are also experiencing synchronizing excitations of solar neutrino fluxes; that is to say, planets of the solar system can be shown to possess common rhythms of volcanic activity.

Transmission electron microscopy of impact‐generated platinum group element alloys from Barberton spherule layers: New clues to their formation

AbstractThe oldest known large bolide impacts onto Earth are represented by approximately 3.47–3.2 Ga old Archean spherule layers of the Barberton Greenstone Belt (BGB) in South Africa and the Pilbara craton in West Australia. These layers were recognized as impact deposits by their excessively high platinum group element (PGE) contents that are indicative of an extraterrestrial component. This was followed by measurements of extraterrestrial Cr isotopic ratios, in some cases. Recently, the extraterrestrial PGE signature in Archean spherule layers from the BGB was localized and positively associated with the presence of submicrometer PGE alloy micronuggets associated with Ni,Cr‐rich spinel. The actual formation of these platinum group mineral (PGM) phases has, however, not yet been resolved. Primary meteoritic particles from the impacting body, the products of impact melting, or condensation from impact vapor plumes have all been proposed as possible genetic process. Resolving this requires detailed microanalytical investigation of the internal microchemical and microstructural compositions, textural characteristics, and crystallographic relationships between the different phases. Here, we report the results of a first transmission electron microscopy (TEM) study of six such PGE microparticles enclosed in Ni‐Cr spinel or occurring in groundmass of Barberton spherule layers from the BARB5 ICDP drill core and from the CT3 exploration core. Results include a variety of chemical and structural PGM compositions that are difficult to explain by a single process, leading to the conclusion that several processes may have been involved in the formation of PGMs in Archean spherule layers from the BGB. There is evidence supporting formation of these PGMs by exsolution from the spinel host phase, precipitation from a melt phase, and condensation from a gas phase (of the impact vapor plume).

Geochemical evidence of an extraterrestrial component in impact melt breccia from the Paleoproterozoic Dhala impact structure, India

AbstractThe Paleoproterozoic Dhala structure with an estimated diameter of ~11 km is a confirmed complex impact structure located in the central Indian state of Madhya Pradesh in predominantly granitic basement (2.65 Ga), in the northwestern part of the Archean Bundelkhand craton. The target lithology is granitic in composition but includes a variety of meta‐supracrustal rock types. The impactites and target rocks are overlain by ~1.7 Ga sediments of the Dhala Group and the Vindhyan Supergroup. The area was cored in more than 70 locations and the subsurface lithology shows pseudotachylitic breccia, impact melt breccia, suevite, lithic breccias, and postimpact sediments. Despite extensive erosion, the Dhala structure is well preserved and displays nearly all the diagnostic microscopic shock metamorphic features. This study is aimed at identifying the presence of an impactor component in impact melt rock by analyzing the siderophile element concentrations and rhenium‐osmium isotopic compositions of four samples of impactites (three melt breccias and one lithic breccia) and two samples of target rock (a biotite granite and a mafic intrusive rock). The impact melt breccias are of granitic composition. In some samples, the siderophile elements and HREE enrichment observed are comparable to the target rock abundances. The Cr versus Ir concentrations indicate the probable admixture of approximately 0.3 wt.% of an extraterrestrial component to the impact melt breccia. The Re and Os abundances and the 187Os/188Os ratio of 0.133 of one melt breccia specimen confirm the presence of an extraterrestrial component, although the impactor type characterization still remains inconclusive.

Indications for influence of artificial (man-made) activity on radon signals, in simulation experiments

Radon (Rn-222; a radioactive noble gas) is characterized by large temporal variations that differ significantly from variations of (i) other trace elements in geogas (noble gases); (ii) variation patterns of other dynamic geophysical systems (atmospheric, tidal). Consensus exists that there is no simple and straightforward understanding of the phenomena and its behaviour. This lacuna in the understanding of the underlying principles hampers the development of applications—such as radon as a proxy of processes in the seismogenic context. Using results from field investigations and simulation experiments the GSI suggested that an unidentified extraterrestrial component, probably in solar radiation, drives periodic radon signals in the diurnal and annual frequency bands. Recent findings from experimental investigations shed additional perspectives allowing a new evaluation of the issue. Particular transient signals, measured with alpha and gamma detectors, are interpreted to reflect the influence of artificial activity. Criteria are (i) signals lasting several hours that occur around midday on workdays (Sunday–Thursday); (ii) signals composed of a train of around 10 strong pulses, each lasting less than 15 min, occurring within several hours once a week, from Wednesday afternoon/evening to Thursday morning. A first interpretation is that an unidentified artificial activity of some sort (industrial?) generates and emits an unidentified agent that reaches enhanced confined mode experiments at the GSI laboratory, which respond to the incoming agent in the form of radon signals. Developing the capability of identification of such an earth-bound source generating an influencing agent is a key step towards understanding of external influence on radioactivity of radon.

Extraterrestrial Components from Deep Sea Sediments of Indian Ocean

Two different types of extraterrestrial particulate matter collections were undertaken during the last few years – by using magnets encased in dredges that were dragged along the seafloor and by sieving massive quantities of deep sea sediments from which cosmic spherules were subsequently isolated. Detailed investigations on close to 1000 sectioned cosmic spherules revealed the presence of Pt-group nuggets in all the three basic types of cosmic spherules – all of whom had carbonaceous chondritic meteorites as precursors, thus in a way unifying them. Further, Fe-Ni beads in all types of the investigated cosmic spherules further confirmed this finding. Oxygen isotopes of relict-grains in silicate cosmic spherules suggested chondrules from carbonaceous chondrites to be major contributors. In addition, relict chromite grains and dusty olivines observed in several cosmic spherules suggested that at least 3% of the cosmic material that rains on the earth is sourced from ordinary chondrites. The Australasian microtektites found in the Indian Ocean retained unique microimpact features generated while they were in flight due to inter-particle collisions in the ejecta. This unique phenomenon outlined the importance of the impact process during estimation of cosmic dust fluxes estimated from lunar materials.

Target rocks, impact glasses, and melt rocks from the Lonar crater, India: Highly siderophile element systematics and Sr‐Nd‐Os isotopic signatures

AbstractThe Lonar crater is a ~0.57‐Myr‐old impact structure located in the Deccan Traps of the Indian peninsula. It probably represents the best‐preserved impact structure hosted in continental flood basalts, providing unique opportunities to study processes of impact cratering in basaltic targets. Here we present highly siderophile element (HSE) abundances and Sr‐Nd and Os isotope data for target basalts and impactites (impact glasses and impact melt rocks) from the Lonar area. These tools may enable us to better constrain the interplay of a variety of impact‐related processes such as mixing, volatilization, and contamination. Strontium and Nd isotopic compositions of impactites confirm and extend earlier suggestions about the incorporation of ancient basement rocks in Lonar impactites. In the Re‐Os isochron plot, target basalts exhibit considerable scatter around a 65.6 Myr Re‐Os reference isochron, most likely reflecting weathering and/or magma replenishment processes. Most impactites plot at distinctly lower187Re/188Os and187Os/188Os ratios compared to the target rocks and exhibit up to two orders of magnitude higher abundances of Ir, Os, and Ru. Moreover, the impactites show near‐chondritic interelement ratios ofHSE. We interpret our results in terms of an addition of up to 0.03% of a chondritc component to most impact glasses and impact melt rocks. The magnitude of the admixture is significantly lower than the earlier reported 12–20 wt% of extraterrestrial component for Lonar impact spherules, reflecting the typical difference in the distribution of projectile component between impact glass spherules and bulk impactites.

Estimation of the extraterrestrial 3He and 20Ne fluxes on Earth from He and Ne systematics in marine sediments

Abstract Sediments contain interplanetary dust particles (IDPs) carrying extraterrestrial noble gases, such as 3 He, which have previously been used to estimate the IDP accretion flux over time and the duration of past environmental events. However, due to its high diffusivity, He can be lost by diffusion either due to frictional heating during entry in the atmosphere, or once it has been incorporated in the sediments. Therefore the absolute values of 3 He IDP fluxes cannot be known. Due to its lower diffusivity, Ne is less likely to be lost by diffusion than He and can potentially provide an absolute IDP flux value. Here, we studied the Ne and He isotopic composition of 21 sediments of different ages (3 to 38 Myr, 56 Myr and 183 Myr) in order to better constrain the retention of 3 He in such deposits. The samples are carbonates from 2 sites of the Integrated Ocean Drilling Program (IODP), which previously showed evidence of detectable extraterrestrial 3 He, and from the Sancerre core in the Paris basin. The 3 He/ 4 He, 20 Ne/ 22 Ne and 21 Ne/ 22 Ne ratios of decarbonated residues vary respectively from 0.09 × 10 − 6 to 76.5 × 10 − 6 , 9.54 ± 0.08 to 11.30 ± 0.60 and from 0.0295 ± 0.0001 to 0.0344 ± 0.0003 . These isotopic compositions can be explained by a mixing between two terrestrial components (atmosphere and radiogenic He and nucleogenic Ne present in the terrigenous fractions) and an extraterrestrial component. The linear relationship between 20 Ne/ 22 Ne and 3 He/ 22 Ne ratios shows that the extraterrestrial component has a unique composition and is similar to the He and Ne composition of implanted solar wind. This composition is different from the individual stratospheric IDPs for which the Ne and He isotopic compositions have been measured. We suggest that this difference is due to a bias in the sampling of the individual IDPs previously analyzed toward the largest ones that are more likely to lose He during entry in the atmosphere. Our data further constrains the size of the majority of the IDPs to be less than 10 μ m in diameter. In addition, the constant 3 He/ 22 Ne ratio of the extraterrestrial component present in the samples, which is similar to the implanted solar wind composition, suggests that no diffusive loss of 3 He occurred in the atmosphere or on the seafloor. Thus, neglecting any non-fractionating He and Ne loss by weathering and/or alteration of the host phases on the seafloor, the extraterrestrial 3 He and 20 Ne fluxes between 3 to 38 Myr ago are respectively 0.2 ± 0.1 × 10 − 12 cm 3 cm − 2 kyr − 1 and 0.2 ± 0.1 × 10 − 11 cm 3 cm − 2 kyr − 1 . During the sharp increases of the late Eocene and late Miocene, the IDP 3 He and 20 Ne fluxes reach values up to five times higher.

Discovery of extraterrestrial component carrier phases in Archean spherule layers: Implications for estimation of Archean bolide sizes

In the Barberton greenstone belt (BGB) of South Africa, four distinct spherule horizons (S1–S4) with ages between ca. 3.5 and 3.2 Ga are among the oldest records of large asteroid impacts on Earth. Spherules in these layers are interpreted as molten impact ejecta, condensation products from impact plumes, or impact ejecta melted during atmospheric reentry. Past research has shown that some spherule layer samples from the BGB carry ultrahigh abundances of siderophile elements, including platinum group elements (PGEs), in some cases as much as four times the chondritic abundances. Inferences for very large projectile sizes responsible for these impact layers have been made on the strength of these data. Drilling by the International Continental Scientific Drilling Program has yielded 4 new spherule layer intersections in a 22-cm-long core segment. Scanning electron microscopy–energy dispersive X-ray spectrometry at high spatial resolution identified local areas of PGE enrichment with Ni-rich chromium spinel (Ni-Cr spinel) clusters. They are associated with PGE-rich metal alloy and sulfarsenide phases 600–1400 nm in size. The metal alloys are interpreted as primary objects that formed during the impact process. PGE sulfarsenides are the result of secondary alteration by S- and As-rich fluids. Thus, a micronugget effect caused by PGE phases and Ni-Cr spinel is responsible for the anomalously high extraterrestrial component in some spherule layer samples. This heterogeneous incorporation of meteoritic components due to primary heterogeneous fallout from the vapor plume must be taken into consideration in any attempt to estimate the global fallout of extraterrestrial components and, thus, to constrain projectile sizes.

Geochemistry of Impactites

Geochemical analysis is an essential tool for the confirmation and study of impact structures and the characterization of the various rock types involved (target rocks, impact breccias, melt rocks, etc.). Concentrations and interelement ratios of the platinum-group elements, as well as the osmium and chromium isotope systems, allow quantification of extraterrestrial components and the identification of impactor types in impact deposits. In addition, chemolithostratigraphy can reveal the possible role of impacts in environmental change throughout the geologic record. This article deals predominantly with terrestrial impact structures.

Iridium anomaly in the cretaceous-paleogene boundary at Højerup (Stevns Klint, Denmark) and Woodside Creek (New Zealand): The question of an enormous proportion of extraterrestrial component

The Cretaceous-Paleogene boundary clays at H?jerup and Woodside Creek show anomalous enrichments of iridium compared with the marine sedimentary rocks. For the average iridium content of 465 ppb in CI chondrite the estimate of the carbonaceous chondritic proportions in the decarbonated iridium-rich boundary layers, based on the integrated iridium fluencies, is about 26% at H?jerup and 65% at Woodside Creek. These proportions are most likely too high due to a significant Ir influx from the nearby marine or continental site to these sections.

Multiproxy analysis of a new terrestrial and a marine Cretaceous–Paleogene (K–Pg) boundary site from New Zealand

An integrated study of palynology, Mossbauer spectroscopy, mineralogy and osmium isotopes has led to the detection of the first K-Pg boundary clay layer in a Southern Hemisphere terrestrial setting. The K-Pg boundary layer was independently identified at centimetre resolution by all the above mentioned methods at the marine K-Pg boundary site of mid-Waipara and the terrestrial site of Compressor Creek (Greymouth coal field), New Zealand. Mossbauer spectroscopy shows an anomaly of Fe-containing particles in both K-Pg boundary sections: jarosite at mid-Waipara and goethite at Compressor Creek. This anomaly coincides with a turnover in vegetation indicated by an interval dominated by fern spores and extinction of key pollen species in both sections. In addition to the terrestrial floristic changes, the mid-Waipara section reveals a turnover in the dinoflagellate assemblages and the appearance of global earliest Danian index species. Geochemical data reveal relatively small iridium enrichments in the boundary layers of 321 pg/g at mid-Waipara and 176 pg/g at Compressor Creek. Unradiogenic Os-187/Os-188 values of the boundary clay reveal the presence of a significant extraterrestrial component. We interpret the accumulation of Fe nano-phases at the boundary as originating from both the impactor and the crystalline basement target rock. The goethite and jarosite are interpreted as secondary phases formed by weathering and diagenesis. The primary phases were probably controlled by the initial composition of the vapor plume and condensation kinetics rather than condensation thermodynamics. This investigation indicates that identification of Fe in nano-phases by Mossbauer spectroscopy is an accurate and cost-effective method for identifying impact event horizons and it efficiently complements widely used biostratigraphic and geochemical methods. (C) 2010 Elsevier Ltd. All rights reserved. (Less)

Geochemical data reported by Paquay et al. do not refute Younger Dryas impact event

In the 12,900-year-old Younger Dryas boundary layer (YDB), Firestone et al. (1) reported inferred extraterrestrial (ET) iridium peaks in sediments and magnetic separates that coincide with elevated abundances in potential impact markers, including microspherules and nanodiamonds (2). Paquay et al. (3) tested YDB sediments but could not reproduce previous iridium concentrations. However, their results seem problematic because standardization uncertainties ranged up to ±140%, and reproducibility varied up to ≈400% (tables S2 and S1, respectively, in ref. 3). Despite these uncertainties, they documented iridium peaks at Murray Springs, AZ (profile B1) and Lake Hind, AB, Canada (batches 1 + 2) at the same stratigraphic levels as spikes in nanodiamonds, magnetic grains, and microspherules (Fig. 1). Although their iridium values are ≈7% of our median values, theirs rise to ≈33% of lower limits and to >300% above background. Additionally, although they reported sedimentary osmium ratios (187Os/188Os) that seem terrestrial, they failed to analyze YDB magnetic separates, in which previous iridium concentrations were much higher and in which osmium ratios might possibly reveal an ET component, as Sharma et al. (4) reported from YDB-aged Pacific and Atlantic ferromanganese crusts. Paquay et al. (3) also speculated that wildfires created the YDB nanodiamonds, contradicting ≈100 years of research demonstrating that hexagonal and cubic diamonds form only under extreme temperature and/or pressure regimes (5), as occurred during the K/T impact and as proposed for the YDB. Overall, the results of Paquay et al. (3) are useful contributions to the YDB discussion, reinforcing the importance of additional research into the cooccurrence of iridium and osmium anomalies with inferred impact proxies.

Geochemistry of 2.63–2.49 Ga impact spherule layers and implications for stratigraphic correlations and impact processes

Thin layers rich in spherules formed during impacts by large extraterrestrial objects have the potential to shed new light on impact processes and aid in the long-distance stratigraphic correlation of Precambrian successions. Seven formations in Western Australia and South Africa clustered around the Archean-Proterozoic boundary each contain a single spherule layer, all of which were deposited between ca. 2.49 and 2.63 Ga. Analyses of 25 samples from 6 of the 7 spherule layers and 23 samples from closely associated strata free of spherules revealed an extraterrestrial component (ETC) in all six layers, based on PGE ratios and/or Cr isotopic composition. The amount of ETC varies from marginally detectable to clear and obvious; it generally amounts to ca. a few percent. Both PGE ratios and Cr isotopic anomalies indicate most if not all of the impactors were ordinary chondritic in composition. In contrast, all spherule layers older than ca. 3.0 Ga that have been analyzed were produced by carbonaceous chondritic impactors. Normalized rare earth element patterns suggest most target rocks were basaltic in composition and variability in PGE ratios favors ballistic emplacement of melt droplets rather than spherule formation via vapor condensation. The geochemical data also provide a means to test proposed intra- and intercontinental stratigraphic correlations. Cr isotopic compositions are consistent with the formation of the oldest layers on both continents by a single impact event about 2.63 Ga. In contrast, clear geochemical differences between the middle layers, both deposited ca. 2.54 Ga, suggest they were not produced by the same impact event. The youngest, banded iron formation-hosted layers on both continents have also been correlated, but no geochemical data are available on the African layer to test this at present.

Extraterrestrial 3He in Paleocene sediments from Shatsky Rise: Constraints on sedimentation rate variability

article i nfo We attempt to constrain the variability of the flux of extraterrestrial 3 He in the Paleocene by studying sediments from Shatsky Rise (Ocean Drilling Program, ODP Leg 198) that have tight orbital age control. 3 He concentrations in Shatsky Rise sediments vary periodically at high frequency by about a factor of 6 over the 800-ka record analyzed. Virtually all of the sedimentary 3 He (N99.98%) is of extraterrestrial origin. The total helium in the sediments can be explained as a binary mixture of terrestrial and extraterrestrial components. We calculate an average 3 He/ 4 He ratio for the extraterrestrial endmember of 2.41±0.29×10 �4 , which is, remarkably, equal to that measured in present-day interplanetary dust particles. We determine a constant extraterrestrial 3 He flux of 5.9±0.9×10 �13 cm 3 STP

A tungsten isotope approach to search for meteoritic components in terrestrial impact rocks

Abstract The identification of meteorite impact structures on Earth is based on two main criteria: the presence of shock-metamorphic effects in the crater rock ejecta and/or the confirmation of an extraterrestrial (meteoritic) component in breccias or melt rocks. For the latter, both high elemental abundances of siderophile elements (especially the platinum group elements) with corresponding meteoritic inter-element ratios and the osmium (Os) and chromium (Cr) isotopic signatures characteristic of meteorites have been used successfully. Inspired by earlier suggestions of a meteoritic component in Archean rocks based on tungsten (W) isotope anomalies, here we explore the possible use of 182 W, which has been produced by the decay of now extinct 182 Hf ( T 1/2 = 8.9 Ma), as a tracer of meteoritic component in terrestrial material. Each group of meteorites has W isotopic compositions that are distinct from each other and from the terrestrial crust. 182 W has already been used to try to identify the impactor at the K/T boundary by analyzing the sediments and Ni-rich spinel. In the present study, we broaden the field of investigation by choosing a different approach, namely analyzing a variety of known impact rocks. We measured the W isotope composition in four samples from different impact structures (Gardnos, Norway; Morokweng, South Africa; Vredefort, South Africa; Ries, Germany) as well as in two samples from a distal ejecta layer (K–T boundary samples from Gams, Austria, and Berwind Canyon, USA). All these samples are unambiguously impact-produced and in several of those materials a meteoritic component has unequivocally been identified by other geochemical proxies. In all these samples, the isotopic composition of W is identical with analytical error to that of the Earth's continental crust, and no 182 W anomalies are present, even in the samples containing a significant (percent level) meteoritic component. Therefore, we conclude that, in contrast to the Cr or Os isotopes, W isotopes are not suitable for the identification of meteoritic components in terrestrial rocks.