Clues to the nature of the impacting bodies from platinum-group elements (rhenium and gold) in borehole samples from the Clearwater East crater (Canada) and the Boltysh impact crater (Ukraine)

Abstract— Seven large (10 g) impact melt rock samples from boreholes from the Boltysh impact crater (Ukraine) and six samples from the East Clearwater crater (Canada) were analyzed for Os, Ir, Ru, Rh, Pd, Re and Au by the nickel sulfide technique in combination with neutron activation. Earlier analyses of Clearwater East impact melt rocks have shown that they are strongly enriched in Ir, Os, Pd and Re. In this work, I confirm earlier findings and demonstrate similarly high enrichments of Rh and Ru. The average Os/Ir, Ru/Ir, Pd/Ir, Rh/Ir and Ru/Rh ratios of the melt rock samples from Clearwater East are CI-chondritic and yield an average Ir content of 25.2 ± 6.5 ng/g relative to an average upper crust concentration of 0.03 ± 0.02 ng/g Ir. The amount of meteoritic component corresponds to 4 to 7% of a nominal CI component for Clearwater East. The impact melt rock samples from a bore hole from Boltysh are low in Ir with an average of 0.2 ± 0.1 ng/g. The CI-normalized abundances increase from the refractory to the more volatile siderophile elements (Os < Ir < Ru < Rh ～ Pd ～ Au ～ Ni ～ Co). Because of the low Ir anomaly and uncertainties in making corrections (correlations are weak) for indigenous siderophile elements, no clear projectile assignment can be made.     

A database of chondrite analyses including platinum group elements,Ni, Co,Au, and Cr: Implications for the identification of chondritic projectiles

Abstract— Siderophile elements have been used to constrain projectile compositions in terrestrial and lunar impact melt rocks. To obtain a better knowledge of compositional differences between potential chondritic projectile types, meteorite analyses of the elements Ru, Rh, Pd, Os, Ir, Pt, Cr, Co, Ni, and Au were gathered into a database. The presented compilation comprises 806 analyses of 278 chondrites including new ICP‐MS analyses of Allende and two ordinary chondrites. Each data set was evaluated by comparing element ratios of meteorites from the same chondrite group. Characteristic element abundances and ratios were determined for each group. Features observed in the element abundance patterns can be linked directly to the presence of certain components, such as the abundance of refractory elements Os, Ir, and Ru correlating with the occurrence of refractory inclusions in CV, CO, CK, and CM chondrites. The refined characteristic element ratios appear to be representative not only for meteorites, but also for related asteroidal bodies. Chondrite element ratios were compared to previously published values from impact melt rocks of the Popigai and Morokweng impact structures confirming that an identification of the specific type of projectile (L and LL chondrite, respectively) is possible. The assessment for Morokweng is supported by the recent discovery of an LL chondrite fragment in the impact melt rocks. Ultimately, the database provides valuable information for understanding processes in the solar nebula as they are recorded in chondrites. A new type of complementarity between element patterns of CK and EH chondrites is suggested to be the result of condensation, redox, and transportation processes in the solar nebula.     

Lunar meteorite,Dhofar 1428: Feldspathic breccia containing KREEP and meteoritic components

We have studied the feldspathic lunar meteorite Dhofar 1428 chemically and petrologically to better understand the evolution of the lunar surface. Dhofar 1428 is a feldspathic regolith breccia derived from the lunar highland. Bulk chemical and mineral compositions of Dhofar 1428 are similar to those of other feldspathic lunar meteorites. We found a few clasts of evolved lithologies, such as K‐rich plagioclases and quartz monzogabbro. Dhofar 1428 contains approximately 1 wt% of chondritic materials like CM chondrite on the basis of abundances of platinum group elements (Ru, Rh, Pd, Os, Ir, and Pt).     

Reevaluation of formation of metal nodules in ordinary chondrites

Abstract— Compositions of four metal nodules from two ordinary chondrites, WIS91627 (H3.7) and Juin (H5), were determined by instrumental and radiochemical neutron activation analyses. Compared with bulk metal fractions, the metal nodules are characterized by strong and variable depletion of the refractory siderophile elements Re, Os, Ir, Ru, Pt and Rh but normal W and Mo, some fractionation of Co from Ni, and low Cu concentrations. These characteristics are difficult to explain by shock-induced vaporization followed by fractional condensation, a mechanism suggested by Widom et al. (1986). We propose formation of metal nodules during metamorphism in the parent body. Refractory siderophile elements, such as Ir, Os, Rh, etc., are partly locked up in noble metal nuggets and cannot participate in kamacite formation. The occurrence of metal nodules in both equilibrated and unequilibrated ordinary chondrites suggests that diffusion along grain boundaries was important in the development of kamacite and taenite in ordinary chondrites.     

Differentiation of metal‐rich meteoritic parent bodies: I. Measurements of PGEs,Re, Mo,W, and Au in meteoritic Fe‐Ni metal

Abstract— We describe an analytical technique for measurements of Fe, Ni, Co, Mo, Ru, Rh, W, Re, Os, Ir, Pt, and Au in bulk samples of iron meteorites. The technique involves EPMA (Fe, Ni, Co) and LA‐ICP‐MS analyses of individual phases of iron meteorites, followed by calculation of bulk compositions based on the abundances of these phases. We report, for the first time, a consistent set of concentrations of Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in the iron meteorites Arispe, Bennett County, Grant, Cape of Good Hope, Cape York, Carbo, Chinga, Coahuila, Duchesne, Gibeon, Henbury, Mundrabilla, Negrillos, Odessa, Sikhote‐Alin, and Toluca and the Divnoe primitive achondrite. The comparison of our LA‐ICP‐MS data for a number of iron meteorites with high‐precision isotope dilution and INAA data demonstrates the good precision and accuracy of our technique. The narrow ranges of variations of Mo and Pd concentrations within individual groups of iron meteorites suggest that these elements can provide important insights into the evolution of parent bodies of iron meteorites. Under certain assumptions, the Mo concentrations can be used to estimate mass fractions of the metal‐sulfide cores in the parent bodies of iron meteorites. It appears that a range of Pd variations within a group of iron meteorites can serve as a useful indicator of S content in the core of its parent body.     

Micrometer‐ and nanometer‐sized platinum group nuggets in micrometeorites from deep‐sea sediments of the Indian Ocean

Abstract– We examined 378 micrometeorites collected from deep‐sea sediments of the Indian Ocean of which 175, 180, and 23 are I‐type, S‐type, and G‐type, respectively. Of the 175 I‐type spherules, 13 contained platinum group element nuggets (PGNs). The nuggets occur in two distinct sizes and have distinctly different elemental compositions: micrometer (μm)‐sized nuggets that are >3 μm contain dominantly Ir, Os, and Ru (iridium‐platinum group element or IPGE) and sub‐μm (or nanometer)‐sized (<1 μm) nuggets, which contain dominantly Pt, Rh, and Pd (palladium—PGE or PPGE). The μm‐sized nuggets are found only one per spherule in the cross section observed and are usually found at the edge of the spherule. By contrast, there are hundreds of nanometer‐sized nuggets distributed dominantly in the magnetite phases of the spherules, and rarely in the wüstite phases. Both the nugget types are found as separate entities in the same spherule and apparently, nugget formation is a common phenomenon among I‐type micrometeorites. However, the μm‐sized nuggets are seen in fewer specimens (～2.5% of the observed I‐type spherules). In all, we analyzed four nuggets of μm size and 213 nanometer‐sized nuggets from 13 I‐type spherules for platinum group elements. Chemically, the μm‐sized PGNs contain chondritic ratios of Os/Ir, but are depleted in the more volatile PGE (Pt, Rh, and Pd) relative to chondritic ratios. On the other hand, the nanometer‐sized nuggets contain dominantly Pt and Rh. Importantly, the refractory PGEs are conspicuous by their absence in these nanometer nuggets. Palladium, the most volatile PGE is highly depleted (<1.1%) with respect to chondritic ratios in the μm‐sized PGNs, and is observed in only 17 of 213 nanometer nuggets with concentrations that are just above the detection limit (≥0.2%). Distinct fractionation of the PGE into IPGE (Ir, Os, Ru) and PPGE seems to take place during the short span of atmospheric entry. These observations suggest several implications: (1) The observation of fractionated PGE in an Fe‐Ni system gives rise to the possibility that Earth’s core could contain fractionated PGE. (2) The present data support the processes suggested for the fractionated PGE patterns observed in the ejecta of ancient meteorite impacts. (3) Meteoric metals released in the troposphere could contain fractionated PGNs in large numbers.     

Origin of lunar fragmental matrix breccias—Highly siderophile element constraints

Ejecta at North Ray crater (Apollo 16) sampled a unique section of the lunar highlands not accessible at most other landing sites and provide important constraints on the composition of late accreted materials. New data on multiple aliquots of four fragmental matrix breccias and a fragment‐laden melt breccia from this site display a variety of highly siderophile element patterns which may represent the signatures of volatile element‐depleted carbonaceous chondrite‐like material, primitive achondrite, differentiated metal, and an impactor component that cannot be related to known meteoritic material. The latter component is prevalent in these rocks besides characterized by depletions in Re and Os compared to Ir, Ru and Pt, chondritic Re/Os, and a gradual depletion of Pd and Au. The observed characteristics are more consistent with fractionations by nebular processes, like incomplete condensation or evaporation, than with lunar crustal processes, like partial melting or volatilization. The impactor signature preserved in these breccias may stem from primitive meteorites with a refractory element composition moderately different from known chondrites. The presence of distinct impactor components within the North Ray crater breccias together with observed correlations of characteristic element ratios (e.g., Re/Os, Ru/Pt, Pd/Ir) in different impact lithologies of four Apollo landing sites constrains physical mixing processes ranging from the scale of gram‐sized samples to the area covered by the Apollo missions.     

Highly siderophile element and osmium isotope evidence for postcore formation magmatic and impact processes on the aubrite parent body

Abstract– Aubrites exhibit a wide range of highly siderophile element (HSE—Re, Os, Ir, Ru, Rh, Pt, Pd, Au) concentrations and ¹⁸⁷Os/¹⁸⁸Os compositions. Their HSE concentrations are one to three orders of magnitude less than chondrites, with the exception of the Shallowater and Mt. Egerton samples. While most aubrites show chondritic HSE abundance ratios, significant enrichments of Pd and Re relative to Os, Ir, and Ru are observed in 12 of 16 samples. Present‐day ¹⁸⁷Os/¹⁸⁸Os ratios range from subchondritic values of 0.1174 to superchondritic values of up to 0.2263. Half of the samples have ¹⁸⁷Os/¹⁸⁸Os ratios of 0.127 to 0.130, which is in the range of enstatite chondrites. Along with the brecciated nature of aubrites, the HSE and Re‐Os isotope systematics support a history of extensive postaccretion processing, including core formation, late addition of chondritic material and/or core material and potential breakup and reassembly. Highly siderophile element signatures for some aubrites are consistent with a mixing of HSE‐rich chondritic fragments with a HSE‐free aubrite matrix. The enrichments in incompatible HSE such as Pd and Re observed in some aubrites, reminiscent of terrestrial basalts, suggest an extensive magmatic and impact history, which is supported by both the ¹⁸⁷Re‐¹⁸⁷Os isotope system and silicate‐hosted isotope systems (Rb‐Sr, K‐Ar) yielding young formation ages of 1.3–3.9 Ga for a subset of samples. Compared with other differentiated achondrites derived from small planetary bodies, aubrites show a wide range in HSE concentrations and ¹⁸⁷Os/¹⁸⁸Os, most similar to angrites. While similarities exist between the diverse groups of achondrites formed early in solar system history, the aubrite parent body(ies) clearly underwent a distinct evolution, different from angrites, brachinites, ureilites, howardites, eucrites, and diogenites.     

Siderophile elements in Earth's upper mantle and lunar breccias: Data synthesis suggests manifestations of the same late influx

Abstract— The platinum group elements (PGE; Ru, Rh, Pd, Os, Ir, Pt), Re and Au comprise the highly siderophile elements (HSE). We reexamine selected isotopic and abundance data sets for HSE in upper mantle peridotites to resolve a longstanding dichotomy. Re‐Os and Pt‐Os isotope systematics, and approximately chondritic proportions of PGE in these rocks, suggest the presence in undepleted mantle of a chondrite‐like component, which is parsimoniously explained by late influx of large planetisimals after formation of the Earth's core and the Moon. But some suites of xenolithic and orogenic spinel lherzolites, and abyssal peridotites, have a CI‐normalized PGE pattern with enhanced Pd that is sometimes termed “non‐chondritic”. We find that this observation is consistent with other evidence of a late influx of material more closely resembling enstatite, rather than ordinary or carbonaceous, chondrites. Regional variations in HSE patterns may be a consequence of a late influx of very large objects of variable composition. Studies of many ancient (>3.8 Ga) lunar breccias show regional variations in Au/Ir and suggest that “graininess” existed during the early bombardment of the Earth and Moon. Reliable Pd values are available only for Apollo 17 breccias 73215 and 73255, however. Differences in HSE patterns between the aphanitic and anorthositic lithologies in these breccias show fractionation between a refractory group (Re, Os and Ir) and a normal (Pd, Ni, and Au) group and may reflect the compositions of the impacting bodies. Similar fractionation is apparent between the EH and EL chondrites, whose PGE patterns resemble those of the aphanitic and anorthositic lithologies, respectively. The striking resemblance of HSE and chalcogen (S, Se) patterns in the Apollo aphanites and high‐Pd terrestrial peridotites suggest that the “non‐chondritic” abundance ratios in the latter may be reflected in the composition of planetisimals striking the Moon in the first 700 Ma of Earth–Moon history. Most notably, high Pd may be part of a general enhancement of HSE more volatile than Fe suggesting that the Au abundance in at least parts of the upper mantle may be 1.5 to 2x higher than previously estimated. The early lunar influx may be estimated from observed basin‐sized craters. Comparison of relative influx to Earth and Moon suggests that the enrichment of HSE is limited to the upper mantle above 670 km. To infer enrichment of the whole mantle would require several large lunar impacts not yet identified.     

Are high‐temperature fractionations in the solar nebula preserved in highly siderophile element systematics of the Earth's mantle?

Abstract— The relative abundances of the highly siderophile elements (HSE) Os, Ir, Ru, Pt, Rh, and Pd in relatively pristine lherzolites differ from solar abundance ratios and are several orders of magnitude higher than predicted for equilibrium distribution between metal/silicate (core‐mantle). The samples are characterized by a mean Ca/Al ratio of 1.18 ± 0.09 σ_M and a mean Ca/Si ratio of 0.10 ± 0.01 σ_M, overlapping with a mean Ca/Al of 1.069 ± 0.044 σ_M and a mean Ca/Si of 0.081 ± 0.023 σ_M found in chondrites (Wasson and Kallemeyn 1988). Interestingly, the CI‐normalized abundance pattern shows decreasing solar system normalized abundances with increasing condensation temperatures. The abundance of the moderately volatile element Pd is about 2x higher than those in the most refractory siderophiles Ir and Os. Thus, the HSE systematics of upper mantle samples suggest that the late bombardment, which added these elements to the accreting Earth, more closely resembles materials of highly reduced EH or EL chondrites than carbonaceous chondrites. In fact, the HSE in the Earth mantle are even more fractionated than the enstatite chondrites—an indication that some inner solar system materials were more highly fractionated than the latter.     

THE COMPOSITION OF IRON METEORITES: A STUDY BY FACTOR ANALYSIS

A factor analysis has been performed on nickel and trace element data for iron meteorites. The technique shows that the present distribution of these elements is the result of three processes. These can be identified from the elements involved:

• 1 Ga, Ge, Sb and Zn (condensation and accretion).
• 2 Ni, Pd, Co and Cu (oxidation and sulphuration).
• 3 Ir, Au, As, Re, Pt, Os, Ru and Cr (an igneous event).

The distribution of Mo, however, is not readily explicable in terms of these processes. Within the groups IAB and IIAB only one process is required for all elements, but in groups IIIAB and IVA the situation for Ga, Ge and Sb is more complex.     

Origin of a late Eocene to pre‐Miocene buried crater and breccia lens at Fohn‐1, North Bonaparte Basin,Timor Sea: A probable extraterrestrial connection

Abstract— Seismic reflection data and an at least 350 m thick, PGE‐rich carbonate breccia lens intersected by the Fohn‐1 exploration well in the Timor Sea off northern Australia, are interpreted in terms of a buried 4.8 km‐diameter impact crater of late Eocene to pre‐Miocene age. The crater displays the classic elements of impact structures, including a central uplift, ring syncline, and upraised rims. The presence in the breccia of redeposited Campanian and Maastrichtian microfossils suggests rebound of strata from levels deeper than 1250 m below the pre‐Miocene unconformity. Morphometric modelling suggests an original crater at least 1400 m deep, which is consistent with the excavation of Cretaceous strata. Stratigraphic and palaeontological evidence suggests that the impact occurred between 36 and 24.6 Ma. The breccia contains a pseudotachylite component enriched in the inert Pt group elements (PGE) (Ir, Ru) by factors of 5–12 above the values of common sediments. The more mobile PGE (Os, Pt, Pd) show a wide scatter and terrestrial‐type values. Opposite geochemical/stratigraphic trends pertain to different PGE species—the relatively inert Ir‐Ru group shows an overall concentration at the base of the section, whereas the more mobile Os shows peaks at median levels of the section—suggesting upward diagenetic leaching. The near‐chondritic PGE patterns at the base of the breccia pile are accompanied by near‐chondritic Ni/Cr, Co/Cr, Ni/Ir, Ni/Pt, and Cu/Pd ratios. Departure from these values related to alteration at higher levels in the breccia pile is accompanied with high S levels (～1%).     

Highly siderophile element and 187Re‐187Os isotopic systematics of ungrouped achondrite Northwest Africa 7325: Evidence for complex planetary processes

The abundances of highly siderophile elements (HSE; including Re, Os, Ir, Ru, Pt, and Pd) and ¹⁸⁷Re‐¹⁸⁷Os isotopic systematics were determined for two fragments from ungrouped achondrite NWA 7325. Rhenium‐Os systematics are consistent with closed‐system behavior since formation or soon after. The abundances of the HSE were therefore largely unaffected by late‐stage secondary processes such as shock or terrestrial weathering. As an olivine gabbro cumulate, this meteorite has a bulk composition consistent with derivation from a body that produced a core, mantle, and crust. Also consistent with derivation from a body that produced a core, both fragments of NWA 7325 have HSE abundances that are highly depleted compared to bulk chondrites. One fragment has ~0.002× CI chondrite Ir and relative HSE abundances similar to bulk chondrites. The other fragment has ~0.0002× CI chondrite Ir and relative HSE abundances that are fractionated compared to bulk chondrites. The chondritic relative HSE abundances of the fragment characterized by higher HSE abundances most likely reflect the addition of exogenous chondritic material during or after crystallization by surface impacts. The HSE in the other fragment is likely more representative of the parent body crust. One formation model that can broadly account for the HSE abundances in this fragment is multiple episodes of low‐pressure metal‐silicate equilibration, followed by limited late accretion and mantle homogenization. Given the different HSE compositions of the two adjoining fragments, this meteorite provides an example of the overprint of global processes (differentiation and late accretion) by localized impact contamination.     

Geochemical identification of projectiles in impact rocks

Abstract— The three major geochemical methods for impactor identification are evaluated with respect to their potential and limitations with regards to the precise detection and identification of meteoritic material in impactites. The identification of a projectile component in impactites can be achieved by determining certain isotopic and elemental ratios in contaminated impactites. The isotopic methods are based on Os and Cr isotopic ratios. Osmium isotopes are highly sensitive for the detection of minute amounts of extraterrestrial components of even <<0.05 wt% in impactites. However, this only holds true for target lithologies with almost no chemical signature of mantle material or young mantle‐derived mafic rocks. Furthermore, this method is not currently suitable for the precise identification of the projectile type. The Cr‐isotopic method requires the relatively highest projectile contamination (several wt%) in order to detect an extraterrestrial component, but may allow the identification of three different groups of extraterrestrial materials, ordinary chondrites, an enstatite chondrites, and differentiated achondrites. A significant advantage of this method is its independence of the target lithology and post‐impact alteration. The use of elemental ratios, including platinum group elements (PGE: Os, Ir, Ru, Pt, Rh, Pd), in combination with Ni and Cr represents a very powerful method for the detection and identification of projectiles in terrestrial and lunar impactites. For most projectile types, this method is almost independent of the target composition, especially if PGE ratios are considered. This holds true even in cases of terrestrial target lithologies with a high component of upper mantle material. The identification of the projectile is achieved by comparison of the “projectile elemental ratio” derived from the slope of the mixing line (target‐projectile) with the elemental ratio in the different types of possible projectiles (e.g., chondrites). However, this requires a set of impactite samples of various degree of projectile contamination.     

Structural clues to the origin of refractory metal alloys as condensates of the solar nebula

Abstract– Alloys of the refractory metals Re, Os, W, Ir, Ru, Mo, Pt, and Rh with small amounts of Fe and Ni are predicted to be one of the very first high‐temperature condensates in a cooling gas of solar composition. Recently, such alloy grains were found in acid‐resistant residues of the Murchison CM2 chondrite. We used focused ion beam (FIB) preparation to obtain electron‐transparent sections of 15 submicrometer‐sized refractory metal nuggets (RMNs) from the original Murchison residue. We studied their crystallography, microstructures, and internal compositional variations using transmission electron microscopy (TEM). Our results show that all RMNs studied have hexagonal close‐packed (hcp) crystal structures despite considerable variations of their bulk compositions. Crystallographic superstructures or signs of spinodal decomposition are absent and defect microstructures are scarce. Internally, RMNs are compositionally homogeneous, with no evidence for zoning patterns or heterogeneities due to exsolution. Many RMNs show well‐defined euhedral crystal shapes and all are nearly perfect single crystal. Our findings are consistent with a direct (near‐) equilibrium condensation of refractory metals into a single alloy at high temperature in the solar nebula as predicted by current condensation models. We suggest that this alloy is generally hcp structured due to an extended ε‐phase field in the relevant multicomponent alloy system. The high degree of structural perfection and compositional homogeneity is attributed to high defect energies, high formation temperatures, slow cooling rates, small grain sizes, and rapid internal diffusion.     

Geochemical studies of impact breccias and country rocks from the El'gygytgyn impact structure,Russia

The complex impact structure El'gygytgyn (age 3.6 Ma, diameter 18 km) in northeastern Russia was formed in ~88 Ma old volcanic target rocks of the Ochotsk‐Chukotsky Volcanic Belt (OCVB). In 2009, El'gygytgyn was the target of a drilling project of the International Continental Scientific Drilling Program (ICDP), and in summer 2011 it was investigated further by a Russian–German expedition. Drill core material and surface samples, including volcanic target rocks and impactites, have been investigated by various geochemical techniques in order to improve the record of trace element characteristics for these lithologies and to attempt to detect and constrain a possible meteoritic component. The bedrock units of the ICDP drill core reflect the felsic volcanics that are predominant in the crater vicinity. The overlying suevites comprise a mixture of all currently known target lithologies, dominated by felsic rocks but lacking a discernable meteoritic component based on platinum group element abundances. The reworked suevite, directly overlain by lake sediments, is not only comparatively enriched in shocked minerals and impact glass spherules, but also contains the highest concentrations of Os, Ir, Ru, and Rh compared to other El'gygytgyn impactites. This is—to a lesser extent—the result of admixture of a mafic component, but more likely the signature of a chondritic meteoritic component. However, the highly siderophile element contribution from target material akin to the mafic blocks of the ICDP drill core to the impactites remains poorly constrained.     

Thallium in meteorites

Abstract— Thallium has been quantified in 50 iron meteorites and 6 chondrites using a combination of solvent extraction and graphite furnace atomic absorption spectrometry. The accuracy of the data was checked by analysis of two iron meteorites by laser-excited ICP mass spectrometry. The Tl abundance values for irons appear to be the first recorded and show that the Tl content allows for taxonomic separation of several groups on Tl vs. Ni abundance plots. The Tl content of irons is inversely correlated with abundances of platinum group metals such as Ir, Pt, and Rh and, in this respect, behaves like Pd and As that favour sulphur-rich phases in meteorites. Analysis of carbonaceous chondrites showed a 30-fold enrichment of Tl compared with ordinary chondrites.     

Meteoritic highly siderophile element and Re‐Os isotope signatures of Archean spherule layers from the CT3 drill core,Barberton Greenstone Belt,South Africa

Archean spherule layers represent the only currently known remnants of the early impact record on Earth. Based on the lunar cratering record, the small number of spherule layers identified so far contrasts to the high impact flux that can be expected for the Earth at that time. The recent discovery of several Paleoarchean spherule layers in the BARB5 and CT3 drill cores from the Barberton area, South Africa, drastically increases the number of known Archean impact spherule layers and may provide a unique opportunity to improve our knowledge of the impact record on the early Earth. This study is focused on the spherule layers in the CT3 drill core from the northeastern Barberton Greenstone Belt. We present highly siderophile element (HSE: Re, Os, Ir, Pt, Ru, and Pd) concentrations and Re‐Os isotope signatures for spherule layer samples and their host rocks in order to unravel the potential presence of extraterrestrial fingerprints within them. Most spherule layer samples exhibit extreme enrichments in HSE concentrations of up to superchondritic abundances in conjunction with, in some cases, subchondritic present‐day ¹⁸⁷Os/¹⁸⁸Os isotope ratios. This indicates a significant meteoritic contribution to the spherule layers. In contrast to some of the data reported earlier for other Archean spherule layers from the Barberton area, the CT3 core is significantly overprinted by secondary events. However, HSE and Re‐Os isotope signatures presented in this study indicate chondritic admixtures of up to (and even above) 100% chondrite component in some of the analyzed spherule layers. There is no significant correlation between HSE abundances and respective spherule contents. Although strongly supporting the impact origin of these layers and the presence of significant meteoritic admixtures, peak HSE concentrations are difficult to explain without postdepositional enrichment processes.     

Platinum group element abundances in a peat layer associated with the Tunguska event, further evidence for a cosmic origin

We have measured excesses of Pd, Rh, Ru, REE, Co, Sr, and Y in a peat column from the Northern peat bog of the 1908 Tunguska explosion site. Earlier, in this peat column the presence of an Ir anomaly at the event layers (30- depth) has been found (Planet Space Sci. 48 (1998) 179). In these layers, Pd, Rh, Ru, Co, Sr, and Y show pronounced anomalies of a factor 4-7 higher than the background value. In the event layers there are also good correlations between the siderophile platinum group elements (Pd, Rh, Ru) and Co, indicators of cosmic material, which imply they might have the same source, i.e. the Tunguska explosive body. The patterns of CI-chondrite-normalized REE in the event layers are much flatter than those in normal peat layers and different from those in the nearby traps. Furthermore, in these layers the patterns of CI-chondrite-normalized PGEs and the element ratios (e.g. C/Pd, C/Rh, and between some siderophile elements) give evidence that the Tunguska explosive body was more likely a comet, although we cannot exclude the possibility that the impactor could be a carbonaceous asteroid. We have estimated the total mass of a solid component of the explosive body up to 10³-10⁶ tons.     

Aerial crystallization of hematite in impact bombs from the Araguainha astrobleme,Mato Grosso,central Brazil

Abstract— Optical and scanning electron microscope (SEM) observations of hematite impact bombs from the Araguainha astrobleme in central Brazil reveal an aerodynamically stable microstructure, which is interpreted to reflect the aerial crystallization of hypervelocity melt globules derived from Fe-rich sedimentary target rocks. The bombs are composed of an aggregate of preferentially oriented conic units with different degrees of packing, giving rise to porous and massive hematite varieties. These varieties generally occupy the front and rear of the bombs, respectively. Quartz fragments with planar deformation features are common in the porous domains and represent ejected solid particles captured by the bombs during cooling. The absence of typical contorted shapes in these bombs reflects the lack of spin during their ballistic trajectory. The bombs are enriched in Rh (4×); Ru, Ir, and Pd (5×); and Pt (>30×) relative to the indigenous level of target sediments. This suggests the presence of a projectile-derived component in their constitution. Single composition oxidic bombs (pure hematite and pure silica) predominate in this occurrence, indicating that a mechanism of segregation has operated in the ejected melt volumes.