Chemical fingerprints of impacts are usually compromised by extreme conditions in the impact plume, and the contribution of projectile matter to impactites does not often exceed a fraction of per ...cent. Here we use chromium and oxygen isotopes to identify the impactor and impact-plume processes for Zhamanshin astrobleme, Kazakhstan. ε
Cr values up to 1.54 in irghizites, part of the fallback ejecta, represent the
Cr-rich extremity of the Solar System range and suggest a CI-like chondrite impactor. Δ
O values as low as -0.22‰ in irghizites, however, are incompatible with a CI-like impactor. We suggest that the observed
O depletion in irghizites relative to the terrestrial range is caused by partial isotope exchange with atmospheric oxygen (Δ
O = -0.47‰) following material ejection. In contrast, combined Δ
O-ε
Cr data for central European tektites (distal ejecta) fall into the terrestrial range and neither impactor fingerprint nor oxygen isotope exchange with the atmosphere are indicated.Identifying the original impactor from craters remains challenging. Here, the authors use chromium and oxygen isotopes to indicate that the Zhamanshin astrobleme impactor was a carbonaceous chrondrite by demonstrating that depleted 17O values are due to exchange with atmospheric oxygen.
Trachyandesites, trachytes and phonolites represent the most evolved rock types within the České Středohoří Volcanic Complex (CSVC) in the Ohře/Eger Rift. The K–Ar ages of the suite range from ~33.8 ...to ~25.8Ma. Major and trace element variation in the basanite – trachybasalt – trachyandesite series can be explained by several stages of modification of parental magmas by assimilation-fractional crystallization (AFC) involving fractionation of olivine, clinopyroxene, apatite, amphibole and Ti-oxide and bulk continental crust (BCC) as an assimilate. Relative to plausible basanitic starting compositions, the trachytes are moderately depleted in Sr, exhibit more pronounced depletions in P and Ti and some of them also show mild MREE depletion. Such composition requires variable amphibole, clinopyroxene, plagioclase±apatite, titanite and/or Ti-magnetite fractionation and BCC assimilation. Two types of phonolites (type A and B phonolite) can be distinguished on the basis of overall REE patterns, Gd/Gd* ratios and Ba and Sr contents. Type B phonolites are depleted in Ba, Sr and MREE as a result of extensive alkali feldspar, plagioclase and amphibole fractionation. Modelling of trace element distributions implies basanitic magmas as the most likely parental composition of the basanite – trachybasalt – trachyandesite – trachyte – phonolite suite formed through magmatic differentiation. The Sr–Nd isotopic compositions in the samples can be explained with the assimilation of continental crust by such parental magmas. The highly radiogenic 87Sr/86Sr found in some phonolites are contrasted by uniform Nd isotopic signature; this feature may be explained by contamination and/or overprint of source magmas by Na–Rb-rich material with radiogenic Sr signature formed due to high-Rb (>200ppm) character of these melts/fluids. The nature of such contaminant is further evidenced by elevated Li (and Cs in some cases) abundances in type B phonolites although at least two distinct fluids are implicated from the Li–Cs correlations. The derivation of these melts/liquids from sedimentary and/or meta-sedimentary crustal sources is underscored by variable but overall light Li isotopic compositions. Some phonolites exhibit enrichments in high-field-strength elements coupled with increased Zr/Nb ratios. In contrast to previous studies, we show that this feature, apparent in many volcanic rocks from the Bohemian Massif, can be explained with progressive melt fractionation of parental magmas involving amphibole and plagioclase.
•First comprehensive dataset on phonolites and trachytic rocks from the Bohemian Massif•Origin of the whole subalkaline-alkaline volcanic suite is discussed•Modelling of assimilation-fractional crystallization processes
Moldavites, tektites of the Central European strewn field, have been traditionally linked with the Ries impact structure in Germany. They are supposed to be derived mainly from the near-surface ...sediments of the Upper Freshwater Molasse of Miocene age that probably covered the target area before the impact. Comparison of the chemical composition of moldavites with that of inferred source materials requires recalculation of the composition of sediments to their water-, organic carbon- and carbon dioxide-free residuum. This recalculation reflects the fact that these compounds were lost almost completely from the target materials during their transformation to moldavites. Strong depletions in concentrations of many elements in moldavites relative to the source sediments (e.g., Mo, Cu, Ag, Sb, As, Fe) contrast with enrichments of several elements in moldavites (e.g., Cs, Ba, K, Rb). These discrepancies can be generally solved using two different approaches, either by involvement of a component of specific chemical composition, or by considering elemental fractionation during tektite formation. The proposed conceptual model of moldavite formation combines both approaches and is based on several steps: (i) the parent mixture (Upper Freshwater Molasse sediments as the dominant source) contained also a minor admixture of organic matter and soils; (ii) the most energetic part of the ejected matter was converted to vapor (plasma) and another part produced melt directly upon decompression; (iii) following further adiabatic decompression, the expanding vapor phase disintegrated the melt into small melt droplets and some elements were partially lost from the melt because of their volatility, or because of the volatility of their compounds, such as carbonyls of Fe and other transition metals (e.g., Ni, Co, Mo, Cr, and Cu); (iv) large positively charged ions such as Cs+, Ba2+, K+, Rb+ from the plasma portion were enriched in the late-stage condensation spherules or condensed directly onto negatively charged melt droplets; (v) simultaneously, the melt droplets coalesced into larger tektite bodies. Steps (iii)–(v) may have overlapped in time. The still melted moldavite bodies reaching their final size were reshaped by further melt flow. This melt flow was related to moldavite rotation and escape (bubbling off) of the last portion of gaseous volatiles during their flight in a low-pressure region above the dense layer of the atmosphere.
Tektites are natural glasses formed by the melting of Earth's surface materials during the impact of a large extraterrestrial body. At present, they are found in four main strewn fields associated ...with separate impacts: Central European (moldavites), Australasian (australites, indochinites, philippinites, javaites, etc.), North American (georgiaites and bediasites), and West African tektites from the Ivory Coast (ivorites). The location of the impact site and parent crater for Australasian tektites (AAT) has remained an unanswered question for decades. The current review discusses possible locations of the AAT parent crater on the basis of published geochemical, mineralogical, petrographic and isotopic data on both AAT and potential source materials. This was aided by new geochemical data for a representative set of AAT covering the main morphological types and parts of the AAT strewn field, and also assumes some ballistic, palaeogeographic, palaeoclimatic and palaeoecological constraints. The review focuses particularly on three parallel tasks: 1) collecting comprehensive data (geochemical composition, Sr, Nd, Rb, Li, and B isotopes, and cosmogenic 10Be) for revisiting Chinese loess or its precursors as the most suitable source materials for AAT; 2) criticisms of the generally accepted location of the impact in Southeast Asia (Indochina) based mainly on a lack of suitable targets and a mismatch between the geochemical and isotopic compositions of AAT and prevailing sedimentary targets in Indochina; 3) supporting evidence, in addition to 1), for a hypothesis locating the AAT parent crater to the arid area of Northwest China, which could provide a sufficient supply of suitable source materials as well as ideal conditions for burial of the crater under the tallest megadunes on Earth.
This paper introduces instrumental photon activation analysis (IPAA) utilizing short-lived products of photonuclear reactions, mainly (γ, n) and (γ, p), initiated by bremsstrahlung from the MT-25 ...microtron. A rapid nondestructive IPAA method for geochemical major element analysis is introduced as a tool for the basic geochemical characterization of rocks. Procedures were developed and parameters such as beam energy and irradiation-decay-counting times optimized with a representative set of geochemical reference materials, and an optimized scheme was applied in analysis of various geological samples. A complete analytical scheme combined with long-time irradiation IPAA and the possibility of utilization of photoexcitation reactions (γ, γ′) are briefly outlined.
Concentrations of uranium, thorium and rare earth elements (REE) in 36 species of ectomycorrhizal (26 samples) and saprobic (25 samples) macrofungi from unpolluted sites with differing bedrock ...geochemistry were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Analytical results are supported by use of certified reference materials (BCR-670, BCR-667, NIST-1575a) and the reliability of the determination of uranium was verified by epithermal neutron activation analysis (ENAA). It appears that data recently published on these elements are erroneous, in part because of use of an inappropriate analytical method; and in part because of apparent contamination by soil particles resulting in elevated levels of thorium and REE. Macrofungi from unpolluted areas, in general, did not accumulate high levels of the investigated metals. Concentrations of uranium and thorium were generally below 30 and 125 μg kg
−1
(dry weight), respectively. Concentrations of REE in macrofungi did not exceed 360 μg kg
−1
(dry weight) and their distribution more or less followed the trend observed in post-Archean shales and loess.
•Uranium is not accumulated in ectomycorrhizal and saprotrophic macrofungi.•Uranium is not accumulated in ectomycorrhizal roots of Picea abies.•Accumulation of metals in macrofungi does not depend on ...their fractionation in soils.
Interactions of macrofungi with U, Th, Pb and Ag were investigated in the former ore mining district of Příbram, Czech Republic. Samples of saprotrophic (34 samples, 24 species) and ectomycorrhizal (38 samples, 26 species) macrofungi were collected from a U-polluted Norway spruce plantation and tailings and analyzed for metal content. In contrast to Ag, which was highly accumulated in fruit-bodies, concentrations of U generally did not exceed 3mg/kg which indicates a very low uptake rate and efficient exclusion of U from macrofungi. In ectomycorrhizal tips (mostly determined to species level by DNA sequencing), U contents were practically identical with those of the non-mycorrhizal fine spruce roots. These findings suggest a very limited role of macrofungi in uptake and biotransformation of U in polluted forest soils. Furthermore, accumulation of U, Th, Pb and Ag in macrofungal fruit-bodies apparently does not depend on total content and chemical fractionation of these metals in soils (tested by the BCR sequential extraction in this study).
•Pb isotopic composition of saprotrophic macrofungi does not reflect that of topsoils.•206Pb/207Pb ratio was a useful indicator for tracking soil-derived Pb in A. bernardii.•Pb isotopes have ...potential as tracers of fungal uptake of metals in soils.
Four saprotrophic species of macrofungi (Leucoagaricus leucothites, Agaricus bernardii,Agaricuscampestris and Agaricusxanthodermus) were collected from 4 sites in the Czech Republic and analyzed for Pb content and Pb isotopic composition. Lead concentrations were relatively high in L. leucothites (up to 130mgkg−1) collected in site heavily polluted by a lead smelter, but much lower (0.2–6.5mgkg−1) in samples of the Agaricus species collected from urban, rural and pristine areas, respectively. The 206Pb/207Pb isotopic ratio in fruit bodies had a wide range of variation, and except for the smelter-polluted site in Příbram, did not reflect that in the organomineral topsoil horizons at particular sites. In the urban area of Prague, a detailed study of Pb uptake was conducted. The 206Pb/207Pb isotopic ratio in 19samples of A.bernardii varied in a surprisingly wide range, from 1.124 to 1.175. In 5 specimens, the majority of “accumulated” Pb was undoubtedly transported from the topsoil layers (0–5cm) characterized by low 206Pb/207Pb isotopic ratios, corresponding with gasoline-derived Pb from traffic emissions. In most samples, however, lead must have been transported from lower depths. Since the mycelium of A. bernardii was not restricted to the topsoil but could be detected both visually and using specific PCR even in a depth of 30cm, such uptake appears to be possible. At suitable sites, Pb isotopes might represent an interesting tool for tracing the fungal uptake and transport of Pb in soils.
Impact processes are natural phenomena that contribute to a variety of physico–chemical mechanisms over an extreme range of shock pressures and temperatures, otherwise seldomly achieved in the ...Earth’s crust through other processes. Under these extreme conditions with transient temperatures and pressures ≥3000K and ≥100GPa, followed by their rapid decrease, the behavior of elements has remained poorly understood. Distal glassy ejecta (tektites) were produced in early phases of contact between the Earth’s surface and an impacting body. Here we provide evidence for a complex behavior of Os and other highly siderophile elements (HSE; Ir, Ru, Pt, Pd, and Re) during tektite production related to a hyper-velocity impact that formed the Ries structure in Germany. Instead of simple mixing between the surface materials, which are thought to form the major source of central European tektites (moldavites), and impactor matter, the patterns of HSE contents and 187Re/188Os – 187Os/188Os ratios in moldavites, target sediments and impact-related breccias (suevites) can be explained by several sequential and/or contemporary processes. These involve (i) evaporative loss of partially oxidized HSE from the overheated tektite melt, (ii) mixing of target-derived and impactor-derived HSE vapor (plasma) phases, and (iii) early (high-temperature) condensation of a part of the mixed vapor phase back to silicate melt droplets. An almost complete loss of terrestrial Os from the tektite melt and its replacement with extra-terrestrial Os are indicated by low 187Os/188Os ratios in tektites (<0.163) relative to precursor materials (>0.69). This is paralleled by a co-variation between Os and Ni contents in tektites but not in suevites formed later in the impact process.
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