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Impact cratering is an important geological process on Earth. This review summarizes the state of knowledge of the Precambrian (Hadean, Archean, and Proterozoic) impact record on Earth. After an early collision that may have led to the formation of the Moon, both Earth and Moon suffered intense post-accretionary bombardment at about 4.5 and 3.9 billion years ago. Evidence for a "late heavy bombardment" phase at about 3.85 Ga is currently debated because the lunar rock record might be biased, and no relevant impact record has yet been confirmed on Earth. Several 3.5 to 2.5 Ga old spherule layers in South Africa and Australia, and two large impact structures, Vredefort and Sudbury, at about 2 Ga, represent most of the oldest actual terrestrial impact record. The impact record for more than half of the geological history of the Earth is incomplete, and, as a result of the lack of old continental crust on Earth, there is also only limited evidence for impact processes during the first 2.5 billion years of Earth history. Some more (mostly badly dated) impact structures are (partly) preserved for the Proterozoic period, as are a couple of ejecta layers. Given that the rock record preserved on Earth is very restricted for this early time period, the limited impact record is not surprising, but as recent discoveries show, there is still room for more research and new findings. [ABSTRACT FROM AUTHOR]

Lunar meteorite Northwest Africa (NWA) 11962 is a regolith breccia with a diverse range of mineral and lithic clasts. For the present study, major and trace element contents and selected isotopic compositions were determined on a homogenized bulk powder of NWA 11962 by instrumental neutron activation analysis, thermal ionization mass spectrometry, and inductively coupled plasma mass spectrometry. The chemical composition of the sample points toward an origin of the meteorite from within the Procellarum KREEP terrane (PKT). Samarium‐Nd and Rb‐Sr isotopic compositions and concentrations show a similarity to those of Apollo 15 soils and KREEP basalt. Highly siderophile element (HSE) abundances and ratios, as well as the Re/Os isotopic system, are often used as tracers of different impactor types. The 187Os/188Os and 187Re/188Os isotopic ratios are within the range of ordinary chondrites, yet some of the siderophile element and highly siderophile element ratios are comparable to those of iron meteorites. Using Fe, Th, and Ti abundances of lunar surface regolith, measured by the Lunar Prospector gamma‐ray spectrometer, we attempt to constrain potential lunar source regions for NWA 11962. By matching these possible source regions with coordinates of recently (<1 Ma) formed lunar impact craters (so‐called lunar cold spots), we localized a potential source crater of NWA 11962 at the western rim of the PKT close to Sinus Medii. [ABSTRACT FROM AUTHOR]

Constraining the degree of preservation of a meteoritic signature within an impact structure provides vital insights in the complex pathways and processes that occur during and after an impact cratering event, providing information on the fate of the projectile. The IODP-ICDP Expedition 364 drilling recovered a ∼829 m continuous core (M0077A) of impactites and basement rocks within the ∼200-km diameter Chicxulub impact structure peak ring. No highly siderophile element (HSE) data have been reported for any of the impact melt rocks of this drill core to date. Previous work has shown that most Chicxulub impactites contain less than 0.1% of a chondritic component. Only few impact melt rock samples in previous drill cores recovered from the Chicxulub might contain such a signal. Therefore, we analyzed impact melt rock and suevite samples, as well as pre-impact lithologies of the Chicxulub peak ring, with a focus on the HSE concentrations and Re–Os isotopic compositions. Similar to the concentrations of the other major and trace elements, those of the moderately siderophile elements (Cr, Co, Ni) of impact melt rock samples primarily reflect mixing between a mafic (dolerite) and felsic (granite) components, with the incorporation of carbonate material in the upper impact melt rock unit (from 715.60 to 747.02 meters below seafloor). The HSE concentrations of the impact melt rocks and suevites are generally low (<39 ppt Ir, <96 ppt Os, <149 ppt Pt), comparable to the values of the average upper continental crust, yet three impact melt rock samples exhibit an enrichment in Os (125–410 ppt) and two of them also in Ir (250–324 ppt) by one order of magnitude relative to the other investigated samples. The 187Os/188Os ratios of the impact melt rocks are highly variable, ranging from 0.18 to 2.09, probably reflecting heterogenous target rock contributions to the impact melt rocks. The significant amount of mafic dolerite (mainly ∼20–60% and up to 80–90%), which is less radiogenic (187Os/188Os ratio of 0.17), within the impact melt rocks makes an unambiguous identification of an extraterrestrial admixture challenging. Granite samples have unusually low 187Os/188Os ratios (0.16 on average), while impact melt rocks and suevites broadly follow a mixing trend between upper continental crust and chondritic/mantle material. Only one of the investigated samples of the upper impact melt rock unit could also be interpreted in terms of a highly diluted (∼0.01–0.05%) meteoritic component. Importantly, the impact melt rocks and pre-impact lithologies were affected by post-impact hydrothermal alteration processes, probably remobilizing Re and Os. The mafic contribution, explaining the least radiogenic 187Os/188Os values, is rather likely. The low amount of meteoritic material preserved within impactites of the Chicxulub impact structure may result from a combination of the assumed steeply-inclined trajectory of the Chicxulub impactor (enhanced vaporization, and incorporation of projectile material within the expansion plume), the impact velocity, and the volatile-rich target lithologies. [ABSTRACT FROM AUTHOR]

Banded iron formations (BIFs) were deposited before and concurrent with the Great Oxidation Event at ∼2.33 Ga. They provide useful archives that document the transformation of the Precambrian hydrosphere from anoxic to progressively oxygenated conditions. Their formation involves removal of oceanic Fe by either inorganic or biologically promoted Fe2+ oxidation, or both. To evaluate depositional settings, elemental sources that affect seawater chemistry, and oxidation pathways, we present the first combined highly siderophile element (HSE) and Fe-Os isotope study for the ∼2.7 Ga Temagami BIF, Abitibi Greenstone Belt, Ontario (Canada). HSE abundances and 187Os/188Os ratios show no systematic variation between alternating magnetite and (meta)chert bands of the Temagami BIF. Whereas HSE concentrations mostly resemble modern crustal values, present-day 187Os/188Os ratios range from ∼0.17 to ∼10.8. Magnetite samples define a regression line corresponding to an age of 2661 ± 126 Ma. A chondrite-like 187Os/188Os initial value is in agreement with earlier studies on Neoarchean marine sediments and is thought to reflect seawater composition, which, unlike modern oceans, is dominated by mantle-like 187Os inventory most likely derived from deep-sea hydrothermal vents. Our δ56Fe data vary from about +0.6‰ to +0.9‰ and define a sawtooth-like pattern between alternating magnetite and (meta)chert layers. Partial oxidation of hydrothermally sourced Fe(II) and a lack of microbially mediated dissimilatory iron reduction provide the most plausible explanation for the positive δ56Fe values. Notably, our δ56Fe data for Temagami are in accord with trends defined by literature results for other Algoma-type BIFs that were deposited throughout the Archean. [ABSTRACT FROM AUTHOR]

The Tissint meteorite fell on July 18, 2011 in Morocco and was quickly recovered, allowing the investigation of a new unaltered sample from Mars. We report new high‐field strength and highly siderophile element (HSE) data, Sr‐Nd‐Hf‐W‐Os isotope analyses, and data for cosmogenic nuclides in order to examine the history of the Tissint meteorite, from its source composition and crystallization to its irradiation history. We present high‐field strength element compositions that are typical for depleted Martian basalts (0.174 ppm Nb, 17.4 ppm Zr, 0.7352 ppm Hf, and 0.0444 ppm W), and, together with an extended literature data set for shergottites, help to reevaluate Mars' tectonic evolution in comparison to that of the early Earth. HSE contents (0.07 ppb Re, 0.92 ppb Os, 2.55 ppb Ir, and 7.87 ppb Pt) vary significantly in comparison to literature data, reflecting significant sample inhomogeneity. Isotope data for Os and W (187Os/188Os = 0.1289 ± 15 and an ε182W = +1.41 ± 0.46) are both indistinguishable from literature data. An internal Lu‐Hf isochron for Tissint defines a crystallization age of 665 ± 74 Ma. Considering only Sm‐Nd and Lu‐Hf chronometry, we obtain, using our and literature values, a best estimate for the age of Tissint of 582 ± 18 Ma (MSWD = 3.2). Cosmogenic radionuclides analyzed in the Tissint meteorite are typical for a recent fall. Tissint's pre‐atmospheric radius was estimated to be 22 ± 2 cm, resulting in an estimated total mass of 130 ± 40 kg. Our cosmic‐ray exposure age of 0.9 ± 0.2 Ma is consistent with earlier estimations and exposure ages for other shergottites in general. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Little is known about the Hadean and the Archean impact record on Earth. In the CT3 drill core from the Fig Tree Group of the northern Barberton Greenstone Belt, 17 spherule layer intersections occur, which, provide an outstanding new opportunity to gain insights into meteorite bombardment of the early Earth. CT3 spherules, as primary features, mostly exhibit textural patterns similar to those of the other Barberton spherule layers, but locally mineralogical and chemical compositional differences are observed, likely as a result of various degrees of alteration. The observed mineralogy of the spherule layers is of secondary origin and comprises K-feldspar, phyllosilicates, carbonates, sulfides, and oxides, with the exception of secondary Ni-Cr spinel that is of primary origin. Our petrographic investigations suggest alteration by K-metasomatism, sericitization, silicification, and carbonatization. Siderophile element contents of bulk samples show significant enrichments in Ni (up to 2 wt%) and Ir (up to ~3 ppm), similar to previously studied Archean spherule layers. These values are indicative of the presence of a meteoritic component. On the other hand, lithophile and chalcophile element abundances indicate hydrothermal overprint on the CT3 samples; this may also have influenced the redistribution of the meteoritic component(s). Last, we group the CT3 spherule layers, which occur in three intervals (A, B, and C), according to their petrographic and geochemical features, which indicate evidence for at least three distinct impact events before tectonic overprint that affected the original deposits. [ABSTRACT FROM AUTHOR]

Archean spherule layers, resulting from impacts by large extraterrestrial objects, to date represent the only remnants of the early meteorite, asteroid, and comet bombardment of the Earth. Only few Archean impact debris layers have been documented, all of them embedded in the 3.23–3.47 billion year old successions of the Barberton Greenstone Belt (BGB) in South Africa and the Pilbara Craton in Western Australia. Some of them might be correlated with each other. Given the scarcity of Archean spherule deposits, four spherule layer intersections from the recently recovered BARB5 drill core from the central Barberton Greenstone Belt, analyzed in this study, provide an opportunity to gain new insight into the early terrestrial impact bombardment. Despite being hydrothermally overprinted, siderophile element abundance signatures of spherule-rich samples from the BARB5 drill core, at least in part, retained a meteoritic fingerprint. The impact hypothesis for the generation of the BARB5 spherule layers is supported by correlations between the abundances of moderately (Cr, Co, Ni) and highly siderophile (Re, Os, Ir, Pt, Ru and Pd) elements, whose peak concentrations and interelement ratios are within the range of those for chondrites. Rhenium-Osmium isotope evidence further support the impact hypothesis. Collectively, this study provides evidence for extraterrestrial admixtures ranging between ∼40 and up to 100% to three of the four analyzed BARB5 spherule layers, and a scenario for their genesis involving (i) impact of a chondritic bolide into a sedimentary target, (ii) varying admixtures of meteoritic components to target materials, (iii) spherule formation via condensation in an impact vapor plume, (iv) transportation of the spherules and sedimentation under submarine conditions, followed by (v) moderate post-impact remobilization of transition metals and highly siderophile elements. [ABSTRACT FROM AUTHOR]

The 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 lower 187Re/188Os and 187Os/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 of HSE. 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. [ABSTRACT FROM AUTHOR]

The Bosumtwi crater is 10.5km in diameter, 1.07Ma old, well preserved impact structure located in Ghana (centered at 06°30′N, 01°25′W). It was excavated in rocks of the Early Proterozoic Birimian Supergroup, part of the West African craton. Here, we present a full and detailed characterization of the three granitoid complexes and one mafic dike in the vicinity of the Bosumtwi crater in terms of petrology, major and trace element geochemistry, geochronology, and isotopic composition. This allows us to characterize magmatic evolution of the West African Craton in this area and better understand the geological framework and target rocks of the impact. This study shows that the similar composition (strongly peraluminous muscovite granites and granodiorites) and age (between 2092±6Ma and 2098±6Ma) of all granitic intrusions in the proximity of the Bosumtwi crater suggest that they are co-genetic. Granitoids were probably formed as a result of anatexis of TTGs (or rocks derived from them) at relatively low pressure and temperature. We propose that the intrusions from the Bosumtwi area are genetically related to the Banso granite occurring to the east of the crater and can be classified as basin-type, late-stage granitoids. Also a mafic dike located to the NE of the Bosumtwi crater seems to be genetically related to those felsic intrusions. Based on those findings a revised version of the geological map of the Bosumtwi crater area is proposed. [ABSTRACT FROM AUTHOR]

El'gygytgyn in northeast Chukotka (Russia) is a 3.6 Ma, 18-km-diameter impact structure. The impact crater was recently drilled in the framework of a project sponsored by the International Continental Scientific Drilling Program (ICDP). Target rocks at the El'gygytgyn area are dominated by the felsic members of the Late Cretaceous Okhotsk-Chukotka Volcanic Belt (OCVB). Such a target lithology is unique among terrestrial impact craters, thereby providing the opportunity to study shock metamorphism in siliceous volcanic rocks. Here, we present a petrographic, geochemical, and isotopic study of the section of the drill core underneath the lacustrine sediments, extending from ∼316 m to 517 m below the lake bottom (blb). The drill core stratigraphy includes ∼80 m of suevite and a cross section through a volcanic suite, which consists of (1) a middle section (∼390-423 mblb) with dominant felsic tuffs and a few mafic members, and (2) a welded rhyolitic-dacitic ignimbrite (∼423-517 mblb). The melt fragments embedded in the suevite are interpreted as being impact-related by comparison with impact glasses from the crater and in opposition to the target rock, which does not include similar melts. A suevitic dyke crosscuts the lower section of the core at the depth 471.40 mblb. Evidence for shock metamorphism is concentrated in the upper 10 m of the drill core and almost limited to the suevitic breccia. The geochemical and isotope (Nd and Hf) composition of samples from the target and the drill core reveals relationships to the 'Berlozhya magmatic assemblage' (BMA) arguing for similar source magmas. The primitive upper mantle (PUM)-normalized trace element plot of rocks investigated here confirms a subduction-related signature, as previously proposed for rocks from both OCVB and BMA. [ABSTRACT FROM AUTHOR]

Abstract: Heavy, proton-rich stable isotopes belong to the least abundant isotopes in the solar system. Their formation mechanisms and their stellar sources are most likely different from those of neutron-capture generated r- and s-process nuclei that comprise the majority of nuclides heavier than iron. Heavy p-nuclide abundances in meteorites are therefore potentially useful in deciphering distinct stellar contributions to the nascent solar system. We therefore conducted the first high-precision measurements of the heavy p-process isotope 180W, achieving a typical precision of ±0.7 ε-units for ca. 300ngW. Measured samples comprise metals from magmatic- and non-magmatic iron meteorites, as well as metal from one H4 chondrite (NWA 926) and two reduced terrestrial basalts (from Disko Island, Greenland and the Dzheltul''ski massif from Eastern-Siberia, Russia). The analyzed iron meteorites show clearly resolvable 180W anomalies of up to +6 ε-units. Conversely, the chondritic metal and both terrestrial samples exhibit 180W abundances indistinguishable from the standard value. As cosmogenic effects during space exposure of the meteoroids may have affected the 180W budget, cosmic-ray exposure of the meteorites has to be critically evaluated. We therefore propose a method to approximate cosmogenic contributions to the 180W signatures in order to unravel nucleosynthetic 180W abundance anomalies. Our study reveals significant cosmogenic effects only for the longest exposed meteorites, shifting 180W anomalies always to lower values (average cosmic-ray correction-factors can be estimated to lie between 0.01 and 0.30 ε 180W-units per 100Myr of exposure). Cosmogenic effects for most of the analyzed meteorites therefore appear to be negligible with respect to the analytical precision achieved for iron meteorites. In addition to cosmic-ray exposure, radiogenic effects can be caused by putative decay of 184Os or by decay of 180Ta in its ground state. Whereas potential alpha decay of 184Os could shift 180W anomalies to higher values (but only up to levels that are within the analytical error of ∼0.5 ε-units for most samples), no significant production of 180W could have occurred from 180Ta decay. Notably, we identified significant and systematic abundance variations in 180W between different iron meteorite groups, indicating that these isotope anomalies are characteristic for their entire parent asteroids. Our finding of decreasing excesses in 180W from early formed magmatic iron meteorites (+3.8±1.2 ɛ-units) towards later formed non-magmatic iron meteorites (+0.6±0.5 ɛ-units), the analyzed chondrite and both terrestrial rocks (−0.3±0.7 ε-units) may thus mirror progressive homogenization of 180W in the early solar nebula. This overall trend is also supported by a co-variation between 180W and metal segregation ages for the different iron meteorite groups as well as by a co-variation between 180W deviations and the respective asteroidal accretion ages. Such an interpretation would suggest progressive homogenization of the solar nebula within about ∼2.5 to ∼6Myr. [Copyright &y& Elsevier]

The presence of tektite-like glasses from a geographically restricted area in Belize (Central America) has been known for several decades. We comprehensively studied 18 such Belize glasses by a variety of petrographic and geochemical methods, including major and trace element analysis, radiogenic isotopic composition (Rb-Sr, Sm-Nd, and Re-Os), water content, oxidation state, and cosmogenic radionuclides. The aims were to determine their compositional variation, their mode of formation and possible source rocks, and their relation to known tektites, and to search for evidence of an extraterrestrial component. In terms of petrography, the samples are similar to tektites from the four "classical" strewn fields, with the presence of lechatelierites, schlieren, and vesicles; these are also widely accepted indicators of an impact origin. No close similarities to volcanic glasses are evident. Water contents are very low, and iron oxidation states are mostly reduced, in both cases similar to observations for other tektites. The geochemical and isotopic data presented, such as Cr, Co and Ni elemental abundances and interelement ratios, as well as trace element patterns are typical for local and regional volcanics from the active Central American Arc. Particular similarities to material comparable to volcanic rocks from Honduras or Guatemala are noted. This is confirmed by Sr-Nd isotope signatures of the Belize glasses, showing close similarities to Central American volcanics in general, and Honduran and Guatemalan volcanic, in particular. Osmium concentrations and 187Os/188Os ratios are comparable to arc volcanics from world-wide locations, but - in a few of the samples - elevated Ir concentrations, near-chondritic Pt/Ir and 187Os/188Os ratios can also be interpreted with the admixture of a minor meteoritic component to some of the Belize samples. 10Be concentrations are consistent with values typical of both, young or deeply buried soils and with values for Central American volcanics, which carry subducted 10Be. Geochemical data clearly indicate a source different from that of the Australasian tektites. Both isotope data sets for the Belize glasses indicate a close relationship to local arc lavas, especially those from Guatemala and Honduras, suggesting that the glasses were not deposited very far from their source. The main evidence that the Belize glasses are of impact origin are their petrographic characteristics and low water content. The evidence from 10Be is consistent with, but does not require, a model of formation for the Belize glasses by an impact on loosely consolidated surface sediments exposed to rain. A probable meteoritic component is low and heterogeneously distributed. [ABSTRACT FROM AUTHOR]

A thin tephra layer was discovered in a section of cave sediments in the Grotta dei Baffoni Cave (GDB), in the Marche Apennine of central Italy, immediately underlying a clayey layer containing charcoal fragments dated at 12,843 ± 122 years before present. This date indicated that humans were occupying the cave at the very beginning of the Younger Dryas cooling event. Petrographic and geochemical analysis of the volcanic glass contained in the tephra layer suggests that this deposit represents a distal air-fall ash erupted from a volcano in the Campi Flegrei caldera, which produced the Neapolitan Yellow Tuff (NYT). Radiocarbon AMS dating of charcoal particles from this GDB tephra layer yielded an age around 14.4 ± 0.4 ka, which is consistent with the chronology of the NYT eruption, which occurred some 14.1 ± 1.4 thousand years ago. Nevertheless, the sediment succession in this cave deposit actually covers a time interval across the sudden and drastic Younger Dryas cooling event, which occurred around 12.9 ka. There is an ongoing debate about the causes of the Younger Dryas event, which divides the scientific community into a faction sustaining that the Younger Dryas cooling event was triggered by an elusive meteorite impact, and an opposing one that advocates volcanic eruptions, such as the eruption of the Laacher See volcano in central Germany, which was precisely dated at 13 ka. A detailed trace element and 187Os/188Os analysis of the sedimentary succession in the Grotta dei Baffoni Cave did not reveal evidence of any platinum-group element anomalies or an osmium isotope signature that would support an extraterrestrial impact event around the time of the Younger Dryas event. The Grotta dei Baffoni tephra layer turned out to be derived from a large eruption in the Campi Flegrei of southern Italy, which produced the huge Neapolitan Yellow Tuff around 14.1 thousand years ago. This catastrophic event appears to be synchronous with the minor Older Dryas cooling event, which preceded the Younger Dryas by some 1300 years. [ABSTRACT FROM AUTHOR]

The vast majority of excitatory synapses are formed on small dendritic protrusions termed dendritic spines. Dendritic spines vary in size and density that are crucial determinants of excitatory synaptic transmission. Aberrations in spine morphogenesis can compromise brain function and have been associated with neuropsychiatric disorders. Actin filaments (F-actin) are the major structural component of dendritic spines, and therefore, actin-binding proteins (ABP) that control F-actin dis-/assembly moved into the focus as critical regulators of brain function. Studies of the past decade identified the ABP cofilin1 as a key regulator of spine morphology, synaptic transmission, and behavior, and they emphasized the necessity for a tight control of cofilin1 to ensure proper brain function. Here, we report spine enrichment of cyclase-associated protein 1 (CAP1), a conserved multidomain protein with largely unknown physiological functions. Super-resolution microscopy and live cell imaging of CAP1-deficient hippocampal neurons revealed impaired synaptic F-actin organization and dynamics associated with alterations in spine morphology. Mechanistically, we found that CAP1 cooperates with cofilin1 in spines and that its helical folded domain is relevant for this interaction. Moreover, our data proved functional interdependence of CAP1 and cofilin1 in control of spine morphology. In summary, we identified CAP1 as a novel regulator of the postsynaptic actin cytoskeleton that is essential for synaptic cofilin1 activity. [ABSTRACT FROM AUTHOR]

Carbonaceous chondrites are an important meteorite group that closely resembles the bulk composition of the solar system. We report the first elemental and isotope dataset for Hf–W in carbonaceous chondrites that includes chondrules, matrix, magnetic fractions as well as bulk compositions. Our study focuses on the three CV3 chondrites, Allende, Vigarano and Bali. Compared to bulk chondrites, matrix splits have low Hf/W ratios and ε 182 W compositions, whereas chondrule splits are characterized by high, but more variable, Hf/W ratios and ε 182 W compositions. Thus, Hf/W ratios behave complementary between chondrules and matrix in the analysed CV chondrites, supporting the view that both components formed from the same parental reservoir. Strong nucleosynthetic effects were observed in most of the analysed CV3 components, especially in matrices and chondrule splits that were found to have large ε 183 W anomalies of several ε -units. All separates define a rough correlation between initial 182 W/ 184 W and 183 W/ 184 W ratios, in agreement with theoretical model trends based on calculations for stellar nucleosynthesis. Our results, therefore, indicate a heterogeneous distribution of s- and r-process W isotopes among the different CV3 chondrite components, arguing for selective thermal processing of early solar system matter during chondrule formation. After correcting for nucleosynthetic anomalies, chondrules and matrix splits of reduced (Vigarano) as well as oxidised (Allende) CV3 chondrites define a linear correlation in ε 182 W vs. 180 Hf/ 184 W space, which is interpreted as an isochron, covering an age interval within the first ∼2.6 Ma after solar system formation. As peak metamorphic temperatures for CV3 chondrites were well below the 182 Hf– 182 W closure temperature, the resulting isochron within its error most likely defines a common formation interval for all components. The calculated age interval is for the first time based on a combined chondrule-matrix isochron, a marked difference compared to previous studies where only chondrules were analysed. Notably, our formation age interval covers previously reported chondrule formation ages determined using 26 Al and Pb–Pb chronometry, illustrating that chondrule and matrix formation started contemporaneously with CAI formation and lasted over a time interval of about 2–3 Ma. Our results also corroborate previous models from ordinary chondrites, in that chondrite parent bodies were not the first planetesimals to have formed in the early solar system. [ABSTRACT FROM AUTHOR]

The 3.6 Ma, 18-km-diameter El'gygytgyn impact structure (Arctic Russia) is unique among the currently known terrestrial impact craters in that it is the only one that was formed in acid volcanic rocks. Previous analyses of impactites from El'gygytgyn showed minor enrichments of the siderophile elements, including Ir, which, together with distinct Cr enrichments, gave rise to speculation that an achondritic projectile was involved. We studied the major and trace element composition in samples from the new ICDP drill core obtained near the center of the structure, as well as the chromium isotopic composition of an impact glass sample collected on the surface. Several suevitic breccias from the upper part of the suevite sequence in the drill core show higher Cr and Ni contents compared with felsic volcanic rocks in the lower part of the core and from surface samples. However, it is difficult to unambiguously establish a meteoritic component from trace element data, as input from (rare) mafic target rocks is a possibility. In contrast, the Cr isotopic composition of the impact glass sample yielded a nonterrestrial ε54Cr value of −0.72 ± 0.31 (2 std. err.). This negative ε54Cr is different from known carbonaceous chondrite values (ε54Cr of +0.95 to +1.65), but is nearly identical to reported values for ureilites (approximately −0.77). The value is, however, also within analytical error of eucrites (approximately −0.38) and ordinary chondrites (approximately −0.42). Given the chemical signatures found in previous analyses of El'gytgytgyn impactites and the similarity of our Cr isotopic data to ureilites, we suggest that the impacting asteroid could have been an F-type asteroid of mixed composition, similar to the recent Almahata Sitta fall in Sudan. [ABSTRACT FROM AUTHOR]

The article offers information on a study that explores globally distributed iridium layer preserved within the Chicxulub impact structure. It mentions the concentration of ultrafine meteoritic matter occurs in the post-impact sediments that cover the crater peak ring. It discusses that iridium layer provides a key temporal horizon precisely linking Chicxulub to K-Pg boundary sections worldwide.

An exceptional 47-m-thick succession of Maastrichtian to Paleocene inner-platform carbonates is exposed on the Dalmatian island of Hvar (Adriatic Sea, Croatia) in a seaside locality called Majerovica. The middle part of this succession is an ∼5-m-thick intraformational massive deposit, which is underlain by well-bedded peritidal innerplatform limestones containing latest Maastrichtian rudists and shallow-water benthic foraminifera. This deposit includes a polygenic, matrix-supported carbonate breccia characterized by ripped-up platform limestone lithoclasts, up to boulder sized, and polygenic microbreccia in a muddy matrix. The microbreccia contains rare small intraclasts of pelagic mudstone containing terminal Maastrichtian planktonic foraminifera. The deposit is overlain in turn by mudstone containing a planktonic foraminiferal association belonging to the P0 and Pα zones of the basal Paleogene, and by shallow-water muddy limestones containing planktonic foraminifera belonging to the P1 zone. While facies suggest that the deposit was emplaced over the inner platform by a single large tsunami, the biostratigraphic assessment of this section and the presence of enhanced concentrations of platinum group elements, such as iridium, in the topmost part of the massive deposit lend support to the hypothesis that this tsunamite is related to the Chicxulub impact in Yucatán. This is potentially the first case of a tropical carbonate platform sedimentary succession recording the Cretaceous- Paleogene boundary event, which provides a new constraint for modeling both the western Tethyan paleogeography and the catastrophic aftermaths of the Chicxulub impactat the end of the Mesozoic Era. [ABSTRACT FROM AUTHOR]

Abstract: The decay of the rare nuclide 184Os by alpha emission to 180W has been theoretically predicted, but was previously never observed in experiments. Variable excesses of 180W were recently observed for iron meteorites, but the contribution to these excesses by 184Os-decay was regarded as insignificant. Here, we present combined 180W and Os–W concentration data for meteorites and terrestrial rocks, now indicating that the 180W heterogeneities can be explained by α-decay of 184Os. A combined 184Os–180W isochron for iron meteorites and chondrites yields a decay constant value of of (half life yr), in good agreement with theoretical estimates. The 184Os–180W decay system may constitute a viable tracer and chronometer for important geological processes like core formation, silicate differentiation or late accretion processes. This is illustrated by a measured 180W-deficit in terrestrial basalts relative to chondrites by parts in 10 000, consistent with core formation ∼4.5 Ga ago. [Copyright &y& Elsevier]

Abstract: Radiogenic isotope compositions of Hf and Nd are typically coupled in Phanerozoic and Proterozoic mafic rocks due to a similar behaviour of Lu–Hf and Sm–Nd during mantle melting. Eoarchean rocks, for instance those from southern West Greenland, exhibit an apparent decoupling of Hf and Nd isotope compositions. This apparent decoupling may either indicate metamorphic disturbance or, alternatively, mirror early differentiation processes in the silicate Earth. To evaluate the issue, we performed combined measurements of Hf–Nd isotope compositions together with major and trace element concentrations for well preserved >3720 to >3800Ma old tholeiitic metabasalts and gabbros from the ∼3700Ma and ∼3800Ma old terranes of the Isua Supracrustal Belt, southern West Greenland. In contrast to younger mafic rocks, calculated initial εHf–εNd values of the Isua tholeiites show similar spreads and are both near chondritic to strongly depleted (−0.7 to +6.3 and −0.8 to +4.4, respectively), also in contrast to previously reported more depleted signatures in nearby boninite-like metabasalts of the Garbenschiefer unit. An evaluation of alteration effects based on preserved major and trace element arrays reveals pristine magmatic trends and therefore the measured isotope compositions indeed in most cases characterize contrasting Eoarchean mantle sources. In accord with this view, compositions of the Isua metabasalts yield Eoarchean regression ages in Sm–Nd and Lu–Hf isochron spaces, overlapping with emplacement ages inferred from crosscutting relationships with tonalites. Lutetium–Hf systematics of the Isua metabasalts studied here, yield clear isochron relationships. For both terranes, there is some scatter in Sm–Nd space, indicating early disturbance of the Sm–Nd system close in time to the extrusion ages, possibly by seafloor alteration. Trace element compositions of the metabasalts indicate an arc setting and a strong source overprint by melt-like subduction components. It is likely, that the source overprint may have caused partial decoupling of the εHf–εNd values, due to selective addition of Nd as observed in modern subduction settings. In this case, the most radiogenic initial εNd and εHf isotope values characterize the most depleted mantle sources, and less radiogenic values would reflect increased contributions of isotopically more enriched subduction components. However, the most depleted samples still exhibit decoupled Hf–Nd compositions, making a case for the presence of even older mantle heterogeneities. A proposed superchondritic composition of the silicate Earth (SCHEM), however, cannot account for the most depleted sample compositions. Conversely, a depleted upper mantle formed by crystallization of perovskite-rich cumulates in the early Hadean may well explain these observed compositions. A literature survey reveals an overlap in initial Hf–Nd compositions between southern West Greenland TTGs and the metabasalts analyzed here. This overlap suggests a genetic relationship between these lithologies, where the TTGs may have inherited their unusual Hf–Nd compositions from mafic precursors isotopically similar in composition to the Isua tholeiites. [Copyright &y& Elsevier]