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Christian Koeberl - One of the best experts on this subject based on the ideXlab platform.
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volatile loss under a diffusion limited regime in Tektites evidence from tin stable isotopes
Chemical Geology, 2019Co-Authors: Christian Koeberl, Frederic Moynier, John CreechAbstract:Abstract Tektites are glasses derived from near-surface continental crustal rocks that were molten and ejected from the Earth's surface during hypervelocity meteorite impacts. They are among the driest terrestrial samples, although the exact mechanism of water loss and the behaviour of other volatile species during these processes are debated. Based on the difference in magnitude of the Cu and Zn isotopic fractionations in Tektites, and the difference of diffusivity between these elements, it was suggested that volatile loss was diffusion-limited. Tin is potentially well suited to testing this model, as it has a lower diffusivity in silicate melts than both Cu and Zn, but a similar volatility to Zn. Here, we analysed the Sn stable isotopic composition in a suite of seven Tektites, representing three of the four known Tektite strewn fields, and for which Zn and Cu isotopes were previously reported. Tin is enriched in the heavier isotopes (≥2.5‰ on the 122Sn/118Sn ratio) in Tektites, correlated with the degree of Sn elemental depletion in their respective samples as well as with Cu and Zn isotope ratios, implying a common control. While the isotope fractionation of Sn, Cu and Zn is a result of volatility, the magnitude of isotope fractionation is strongly moderated by their relative rates of diffusion in the molten Tektite droplets. An Australasian Muong Nong-type Tektite analysed has the least Sn depletion and Sn isotope fractionation, consistent with these samples being more proximal to the source and experiencing a shorter time at high temperatures.
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isotopic fractionation of zinc in Tektites
Earth and Planetary Science Letters, 2009Co-Authors: Uwe Reimold, Fred Jourdan, Frederic Moynier, Pierre Beck, Christian KoeberlAbstract:Abstract Tektites are terrestrial natural glasses produced during a hypervelocity impact of an extraterrestrial projectile onto the Earth's surface. The similarity between the chemical and isotopic compositions of Tektites and terrestrial upper continental crust implies that the Tektites formed by fusion of such target rock. Tektites are among the driest rocks on Earth. Although volatilization at high temperature may have caused this extreme dryness, the exact mechanism of the water loss and the behavior of other volatile species during Tektite formation are still debated. Volatilization can fractionate isotopes, therefore, comparing the isotope composition of volatile elements in Tektites with that of their source rocks may help to understand the physical conditions during Tektite formation. For this study, we have measured the Zn isotopic composition of 20 Tektites from four different strewn fields. Almost all samples are enriched in heavy isotopes of Zn compared to the upper continental crust. On average, the different groups of Tektites are isotopically distinct (listed from the isotopically lightest to the heaviest): Muong-Nong type indochinites ( δ 66/64 Zn = 0.61 ± 0.30‰); North American bediasites ( δ 66/64 Zn = 1.61 ± 0.49‰); Ivory Coast Tektites ( δ 66/64 Zn = 1.66 ± 0.18‰); the Australasian Tektites (others than the Muong Nong-type indochinites) ( δ 66/64 Zn = 1.84 ± 0.42‰); and Central European moldavites ( δ 66/64 Zn = 2.04 ± 0.19‰). These results are contrasted with a narrow range of δ 66/64 Zn = 0–0.7‰ for a diverse spectrum of upper continental crust materials. The elemental abundance of Zn is negatively correlated with δ 66/64 Zn, which may reflect that isotopic fractionation occurred by evaporation during the heating event upon Tektite formation. Simple Rayleigh distillation predicts isotopic fractionations much larger than what is actually observed, therefore, such a model cannot account for the observed Zn isotope fractionation in Tektites. We have developed a more realistic model of evaporation of Zn from a molten sphere: during its hypervelocity trajectory, the molten surface of the Tektite will be entrained by viscous coupling with air that will then induce a velocity field inside the molten sphere. This velocity field induces significant radial chemical mixing within the Tektite that accelerates the evaporation process. Our model, albeit parameter dependent, shows that both the isotopic composition and the chemical abundances measured in Tektites can be produced by evaporation in a diffusion-limited regime.
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abstract late eocene impact ejecta the north american Tektite strewn field and its sourcee the chesapeake bay impact structure update on the icdp us gs deep drilling projects
2007Co-Authors: Christian Koeberl, Uwe W Reimold, Gregory S Gohn, Kenneth G MillerAbstract:LATE EOCENE IMPACT EJECTA: THE NORTH AMERICAN Tektite STREWN FIELD AND ITS SOURCEE, THE CHESAPEAKE BAY IMPACT STRUCTURE: UPDATE ON THE ICDP-US GS DEEP DRILLING PROJECTS Christian Koeberl1, W. Uwe Reimold2, Gregory S. Gohn3, Kenneth G. Miller. Center of Earth Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria (christian.koeberl@univie.ac.at); Museum of Natural History, Humboldt-University, Invalidenstrasse 43, D10115 Berlin, Germany (uwe.reimold@museum.hu-berlin.de); U.S. Geological Survey, Reston, VA 20192, USA (ggohn@usgs.gov); Rutgers University, Department of Geological Sciences, Piscataway, NJ 08854, USA (kgm@rci.rutgers.edu). Late Eocene Impacts and the North American Tektite Strewn Field. Upper Eocene marine sediments around the world contain evidence for at least two closely spaced impactoclastic layers – i.e., layers containing impact debris, such as Tektites and microTektites and shocked minerals and rock fragments (for references see e.g., Montanari and Koeberl [1]). Initially, it was thought that there is only one layer known from the eastern U.S. coast, the Caribbean, and the Gulf of Mexico, which is correlated with the North American Tektite strewn field. This layer contains microTektites (i.e., glassy, not recrystallized spherules), shocked minerals, and highpressure phases (e.g., coesite), but no marked siderophile element anomaly. The presence of crystalline spherules composed mostly of clinopyroxene (cpx) was detected in the same deep sea sediments and initially it was considered that these spherules also belong to the North American Tektite strewn field. The cpx spherules were found not only in the Caribbean and the Gulf of Mexico but also in the Pacific Ocean. Fig. 1. The two large late Eocene impact craters (Chesapeake Bay and Popigai) and location of coeval impact ejecta deposits. The North American Tektite strewn field is also outlined (from [2]). Impact-produced, crystallite-bearing spherules are called "microkrystites". More detailed work showed that the microTektite layer and the microkrystite (cpx) layer (both in middle to lower magnetic chron C15) are in fact separated from each other by about 25 cm, with the cpx layer being the lower (i.e., older) one. The separation between the two layers amounts to about 10 to 20 k.y. MicroTektites and Tektite fragments at DSDP Site 612 show chemical and isotopic similarities with other North American Tektites (e.g., [1, 2]). The microkrystite layer has now been found at numerous other locations, indicating that it has a more or less global distribution, Koeberl et al.: North American Tektites and Chesapeake Bay Impact Structure and it seems to be associated at several locations with enhanced Ir abundances. Impact ejecta interpreted to be part of the cpx layer also have been found in rocks from the marine Umbria-Marche succession in Italy at the Global Stratotype Section and Point for the EoceneOligocene boundary at Massignano near Ancona. At this location two impactoclastic layers – the other possibly related to Chesapeake Bay – are present. As mentioned above, the source of the North American Tektite strewn field has now been linked, with a certain degree of confidence, to the Chesapeake Bay impact structure [3]. An impact event that created a crater of this size would be capable of globally distributing its distal ejecta (e.g., [1]). There is a second large crater with an age indistinguishable from that of the Chesapeake Bay structure and the two ejecta layers, namely the 100-km-diameter Popigai impact structure in Siberia, which has been dated at 35.7±0.8 Ma. The Popigai structure is exposed in Archean crystalline rocks of the Anabar Shield, with overlying Proterozoic to Mesozoic sedimentary sequences (e.g., [1, 3]), and is the largest Cenozoic crater on Earth. It is now commonly assumed that the global upper Eocene microkrystite layer originated from the Popigai impact event, but this link has yet to be confirmed, probably by using isotope geochemical methods, because radiometric age determinations do not allow to resolve an age difference of 10 or 20 k.y. It is also interesting to note that much enhanced levels of He were found to coincide with the two upper Eocene impactoclastic layers. This isotope is a proxy for the influx of extraterrestrial dust and is interpreted as indicating that, during the late Eocene, there was a time of enhanced comet activity in the inner solar system, probably resulting in a higher impact rate than usual. There are numerous questions associated with the link between the Chesapeake Bay crater and the North American Tektites. Chesapeake Bay impact structure: The late Eocene Chesapeake Bay impact structure is among the largest and best preserved of the known impact craters on Earth [3]. It has a diameter of 85 km (Fig. 2) with what has been called an “inverted sombrero”-.shape cross section. The crystalline basement beneath the impact structure, and beneath the Mid-Atlantic Coastal Plain in general, is one of the most poorly understood geologic provinces in the United States. Granite is the only basement rock type encountered by the five boreholes that reached pre-Cretaceous rocks below the impact structure [3]). Clasts of basement rock found as reworked ejecta within the impact structure consist primarily of cataclastic granites and porphyritic felsites. Atlantic Coastal Plain sediments overlie this basement and constitute a seaward-thickening wedge of dominantly unconsolidated to poorly consolidated siliciclastic sands, silts, and clays of marine and nonmarine origin. This succession was deposited between the Early Cretaceous and the Holocene Chesapeake Bay is distinctive among impact craters on Earth because it is a relatively young and well-preserved structure, which is the source of the North American Tektites – one of only 4 known Tektite strewn fields on Earth. The Chesapeake Bay structure is unique among subaerial and submarine impact craters on Earth because: (1) it is a relatively young structure and, in comparison to other known impact structures of such size, very wellpreserved; (2) its location on a passive continental margin has prevented tectonic or orogenic disruption or distortion that is typical of many large terrestrial craters; (3) its original location on a relatively deep continental shelf allowed marine deposition to resume immediately following the impact, which buried it rapidly and completely, thereby preventing subsequent erosion; (4) the upper part of the breccia section inside the crater was derived from resurge currents and impact-generated tsunami waves; (5) the breccia body contains a large volume of impactgenerated brine; (6) the crater underlies a densely populated urban corridor, whose two million citizens are still affected by crater-related phenomena, such as freshwater availability. Previous studies have raised many questions that can only be answered by a deep drilling project. The ICDP drilling project also included a deep biosphere research opportunity. Drilling: The International Continental Scientific Drilling Program (ICDP) and the U.S. Geological Survey (USGS) completed two deep coreholes to a composite depth of almost 1.8 km into the Chesapeake Bay impact structure during September-December 2005. Post-impact sediments were cored from land surface to a depth of 140 m in a third corehole during April and May 2006. Field operations began in July 2005 with site preparations at Eyreville Farm in Northampton County, Virginia; subsequently, Koeberl et al.: North American Tektites and Chesapeake Bay Impact Structure three coreholes were drilled at the Eyreville site. Eyreville hole A was cored between depths of 125 m and 941 m from September through early October 2005. Problems with lost mud circulation and swelling clays in Eyreville A led to a lengthy period of reaming and ultimately to deviation of the bit from Eyreville A to a new hole, Eyreville B, at a depth of 738 m. Eyreville B was cored from that depth to a final depth of 1,766 m from October through early December 2005. Post-impact sediments were cored from land surface to a depth of 140 m in the Eyreville C hole during April and May 2006. Fig. 2. Schematic cross section through half of the Chesapeake Bay impact structure, showing the deep central part of the crater (including the central uplift), and the location of the ICDP-USGS deep corehole
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establishing the link between the chesapeake bay impact structure and the north american Tektite strewn field the sr nd isotopic evidence
Meteoritics & Planetary Science, 2006Co-Authors: A Deutsch, Christian KoeberlAbstract:The Chesapeake Bay impact structure, which is about 35 Ma old, has previously been proposed as the possible source crater of the North American Tektites (NAT). Here we report major and trace element data as well as the first Sr-Nd isotope data for drill core and outcrop samples of target lithologies, crater fill breccias, and post-impact sediments of the Chesapeake Bay impact structure. The unconsolidated sediments, Cretaceous to middle Eocene in age, have ɛSrt = 35.7 Ma of +54 to +272, and ɛNdt = 35.7 Ma ranging from -6.5 to -10.8; one sample from the granitic basement with a TNdCHUR model age of 1.36 Ga yielded an ɛSrt = 35.7 Ma of +188 and an ɛNdt = 35.7 Ma of -5.7. The Exmore breccia (crater fill) can be explained as a mix of the measured target sediments and the granite, plus an as-yet undetermined component. The post-impact sediments of the Chickahominy formation have slightly higher TNdCHUR model ages of about 1.55 Ga, indicating a contribution of some older materials. Newly analyzed bediasites have the following isotope parameters: +104 to +119 (ɛSrt = 35.7 Ma), -5.7 (ɛNdt = 35.7 Ma), 0.47 Ga (TSrUR), and 1.15 Ga (TNdCHUR), which is in excellent agreement with previously published data for samples of the NAT strewn field. Target rocks with highly radiogenic Sr isotopic composition, as required for explaining the isotopic characteristics of Deep Sea Drilling Project (DSDP) site 612 Tektites, were not among the analyzed sample suite. Based on the new isotope data, we exclude any relation between the NA Tektites and the Popigai impact crater, although they have identical ages within 2σ errors. The Chesapeake Bay structure, however, is now clearly constrained as the source crater for the North American Tektites, although the present data set obviously does not include all target lithologies that have contributed to the composition of the Tektites.
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an ordinary chondrite impactor composition for the bosumtwi impact structure ghana west africa discussion of siderophile element contents and os and cr isotope data
LPI, 2004Co-Authors: Christian Koeberl, Alex Shukolyukov, Guenter W LugmairAbstract:Osmium isotope data had shown that Ivory Coast Tektites contain an extraterrestrial component, but do not allow distinction between chondritic and iron meteorite contamination. PGE abundances of Ivory Coast Tektites and impactites and target rocks from the Bosumtwi crater, the source crater of the Ivory Coast Tektites, were all relatively high and did not allow to resolve the presence, or identify the nature, of the meteoritic component. However, Cr isotope analyses of an Ivory Coast Tektite yielded a distinct 53Cr excess of 0.30+/-0.06, which indicates that the Bosumtwi impactor was an ordinary chondrite.
Frederic Moynier - One of the best experts on this subject based on the ideXlab platform.
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volatile loss under a diffusion limited regime in Tektites evidence from tin stable isotopes
Chemical Geology, 2019Co-Authors: Christian Koeberl, Frederic Moynier, John CreechAbstract:Abstract Tektites are glasses derived from near-surface continental crustal rocks that were molten and ejected from the Earth's surface during hypervelocity meteorite impacts. They are among the driest terrestrial samples, although the exact mechanism of water loss and the behaviour of other volatile species during these processes are debated. Based on the difference in magnitude of the Cu and Zn isotopic fractionations in Tektites, and the difference of diffusivity between these elements, it was suggested that volatile loss was diffusion-limited. Tin is potentially well suited to testing this model, as it has a lower diffusivity in silicate melts than both Cu and Zn, but a similar volatility to Zn. Here, we analysed the Sn stable isotopic composition in a suite of seven Tektites, representing three of the four known Tektite strewn fields, and for which Zn and Cu isotopes were previously reported. Tin is enriched in the heavier isotopes (≥2.5‰ on the 122Sn/118Sn ratio) in Tektites, correlated with the degree of Sn elemental depletion in their respective samples as well as with Cu and Zn isotope ratios, implying a common control. While the isotope fractionation of Sn, Cu and Zn is a result of volatility, the magnitude of isotope fractionation is strongly moderated by their relative rates of diffusion in the molten Tektite droplets. An Australasian Muong Nong-type Tektite analysed has the least Sn depletion and Sn isotope fractionation, consistent with these samples being more proximal to the source and experiencing a shorter time at high temperatures.
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isotopic fractionation of zinc in Tektites
Earth and Planetary Science Letters, 2009Co-Authors: Uwe Reimold, Fred Jourdan, Frederic Moynier, Pierre Beck, Christian KoeberlAbstract:Abstract Tektites are terrestrial natural glasses produced during a hypervelocity impact of an extraterrestrial projectile onto the Earth's surface. The similarity between the chemical and isotopic compositions of Tektites and terrestrial upper continental crust implies that the Tektites formed by fusion of such target rock. Tektites are among the driest rocks on Earth. Although volatilization at high temperature may have caused this extreme dryness, the exact mechanism of the water loss and the behavior of other volatile species during Tektite formation are still debated. Volatilization can fractionate isotopes, therefore, comparing the isotope composition of volatile elements in Tektites with that of their source rocks may help to understand the physical conditions during Tektite formation. For this study, we have measured the Zn isotopic composition of 20 Tektites from four different strewn fields. Almost all samples are enriched in heavy isotopes of Zn compared to the upper continental crust. On average, the different groups of Tektites are isotopically distinct (listed from the isotopically lightest to the heaviest): Muong-Nong type indochinites ( δ 66/64 Zn = 0.61 ± 0.30‰); North American bediasites ( δ 66/64 Zn = 1.61 ± 0.49‰); Ivory Coast Tektites ( δ 66/64 Zn = 1.66 ± 0.18‰); the Australasian Tektites (others than the Muong Nong-type indochinites) ( δ 66/64 Zn = 1.84 ± 0.42‰); and Central European moldavites ( δ 66/64 Zn = 2.04 ± 0.19‰). These results are contrasted with a narrow range of δ 66/64 Zn = 0–0.7‰ for a diverse spectrum of upper continental crust materials. The elemental abundance of Zn is negatively correlated with δ 66/64 Zn, which may reflect that isotopic fractionation occurred by evaporation during the heating event upon Tektite formation. Simple Rayleigh distillation predicts isotopic fractionations much larger than what is actually observed, therefore, such a model cannot account for the observed Zn isotope fractionation in Tektites. We have developed a more realistic model of evaporation of Zn from a molten sphere: during its hypervelocity trajectory, the molten surface of the Tektite will be entrained by viscous coupling with air that will then induce a velocity field inside the molten sphere. This velocity field induces significant radial chemical mixing within the Tektite that accelerates the evaporation process. Our model, albeit parameter dependent, shows that both the isotopic composition and the chemical abundances measured in Tektites can be produced by evaporation in a diffusion-limited regime.
B.p. Glass - One of the best experts on this subject based on the ideXlab platform.
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3d x ray tomographic analysis reveals how coesite is preserved in muong nong type Tektites
Scientific Reports, 2020Co-Authors: Matteo Masotta, B.p. Glass, Stefano Peres, Luigi Folco, Lucia Mancini, P Rochette, F Campanale, Nicolas GueninchaultAbstract:Muong Nong-type (MN) Tektites are a layered type of Tektite associated to the Australasian strewn field, the youngest (790 kyr) and largest on Earth. In some MN Tektites, coesite is observed in association with relict quartz and silica glass within inclusions surrounded by a froth layer. The formation of coesite-bearing frothy inclusions is here investigated through a 3D textural multiscale analysis of the vesicles contained in a MN Tektite sample, combined with compositional and spectroscopic data. The vesicle size distribution testifies to a post-shock decompression that induced melting and extensive vesiculation in the Tektite melt. Compared to free vesicles, nucleated homogeneously in the Tektite melt, froth vesicles nucleated heterogeneously on relict quartz surfaces at the margins of coesite-bearing inclusions. The rapid detachment of the froth vesicles and prompt reactivation of the nucleation site favoured the packing of vesicles and the formation of the froth structure. Vesicle relaxation time scales suggest that the vesiculation process lasted few seconds. The formation of the froth layer was instrumental for the preservation of coesite, promoting quenching of the inclusion core through the subtraction of heat during froth expansion, thereby physically insulating the inclusion until the final quench of the Tektite melt.
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North American microTektites are more oxidized than Tektites
'Mineralogical Society of America', 2013Co-Authors: G. Giuli, B.p. Glass, M. R. Cicconi, S. G. Eeckhout, C. Koeberl, G. Pratesi, M. Cestelli-guidi, E. ParisAbstract:Iron oxidation states and coordination numbers have been determined by micro-X-ray absorption near edge spectroscopy (XANES) on the cores of a large group of microTektites from the Australasian, Ivory Coast, and North American (NA) Tektite strewn field. The North American microTektites used in this study have been collected from five sites at different distances from the source crater; most have SiO2 content between 70 and 80 wt%. Accurate analysis of the pre-edge peak energy position and integrated area allowed determination of Fe3+/(Fe2++Fe3+) ratios on all samples with an estimated error of +/- 0.05. MicroTektites from the Australasian and Ivory Coast strewn fields show low values of the Fe3+/(Fe2++Fe3+) ratios, in fair agreement with Tektites from the same strewn field. In contrast, microTektites from the North American strewn fields show a wide range of Fe3+/(Fe2++Fe3+) ratios from 0.02 to ca. 0.61. Comparison of Fe oxidation state data with chemical composition do not show any relation between Fe3+/(Fe2++Fe3+) ratios and Na, Ca, or K contents, thus suggesting that the high-Fe oxidation states are not the consequence of sea-water alteration. The difference between the Fe oxidation state of Tektites and microTektites from the North American strewn fields suggests that some factors in the formation of the North American microTektites were different than for the North American Tektites and for microTektites in the other strewn fields. Previous Fe oxidation state data on NA Tektites strongly suggest that the wide range in Fe oxidation state we found on NA microTektites is not related to lateral heterogeneity of the target rocks. Despite a correlation between microTektite oxidation state and distance from the source crater, we maintain that Fe oxidation state is not related only to the microTektite droplet flight distance. This is in keeping with the fact that no significant variations in the Fe oxidation state have been found in microTektites from the Australasian strewn field, even for Australasian microTektites recovered in Antarctica. The Fe oxidation state in North American microTektites could be explained by interaction of melt droplets with a H2O-rich vapor plumes generated during the impact. These data point out that some difference must exist between the thermal histories of microTektites and Tektites from the NA strewn field. Moreover, microTektites from the NA strewn field show also distinctively higher oxidation states than those from Ivory Coast or the Australasian strewn fields
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upper eocene impact ejecta spherule layers in marine sediments
Chemie Der Erde-geochemistry, 2002Co-Authors: B.p. GlassAbstract:Abstract MicroTektites with compositions and ages similar to those of the North American Tektites have been found in deep-sea deposits in the Gulf of Mexico, Caribbean Sea, northwestern Atlantic Ocean and on Barbados. Unmelted impact ejecta (including shocked quartz and feldspar with multiple sets of planar deformation features, coesite, stishovite, and reidite, a high-pressure polymorph of zircon) have been found associated with this microTektite layer. These microTektites appear to belong to the North American Tektite strewn field based on their geographic location, age (∼35 Ma), and composition. Clinopyroxene-bearing spherules (cpx spherules) are closely associated with the North American microTektitestherefore, occurrences of cpx spherules in the Indian Ocean and equatorial Pacific were originally interpreted to indicate a major extension of the North American strewn field. However, at a few sites it is clear that the cpx spherules are older than the North American microTektites and it is now accepted that they belong to a different impact event. The major crystalline phase in the cpx spherules is clinopyroxene, as their name implies, but they also contain Cr- and Ni-rich spinels. The cpx spherules generally have lower SiO 2 and Al 2 O 3 and higher FeO, MgO, and CaO contents compared with the North American microTektites. The cpx spherules also have high Ni, Cr, Co, and Ir contents compared to the North American microTektites and to the average upper continental crust. The cpx spherule layer is associated with a positive Ir anomaly and with the extinction of several taxa of Radiolaria. Flattened pancake spherules composed of clay have been found in upper Eocene deposits at Massignano, Italy, associated with a positive Ir anomaly. The pancake spherules are believed to be diagenetically altered cpx spherules. Shocked quartz with multiple sets of planar deformation features have been found associated with the spherule layer at Massignano. The number of upper Eocene microTektite/spherule layers is debated, but most authors agree that there are probably only two: the North American microTektite layer and the cpx spherule layer. Geographic variations in abundance, age, composition, and Sr and Nd isotopic data are consistent with the North American microTektites (and Tektites) being derived from the 90 km-diameter Chesapeake Bay structure. Based on the nature of the shocked quartz found associated with the spherule layer at Massignano and the age and Sr and Nd isotopic composition of the cpx spherules, it has been suggested that the 100 km-diameter Popigai impact structure in northern Siberia may the source crater for the cpx strewn field.
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upper eocene Tektite and impact ejecta layer on the continental slope off new jersey
Meteoritics & Planetary Science, 1998Co-Authors: B.p. Glass, Christian Koeberl, Joel D Blum, C M MchughAbstract:— During Leg 150 of the Ocean Drilling Project (ODP), two sites (903C and 904A) were cored that have sediments of the same biostratigraphic age as the upper Eocene Tektite-bearing ejecta layer at Deep Sea Drilling Project (DSDP) Site 612. Core 45X from ODP Site 904A (∼4 km north of Site 612) contains a 5 cm thick Tektite-bearing ejecta layer, and Core 56 from Site 903C (∼8 km north-northwest of Site 904) contains a 2 cm thick layer of impact ejecta without any Tektite or impact glass. Shocked quartz and feldspar grains, with multiple sets of planar deformation features (PDFs), and abundant coesite-bearing grains are present at both sites. The major oxide contents, trace element compositions, and rare earth element (REE) patterns of the Site 904 Tektites are similar to those of the Site 612 Tektites and to North American Tektites (especially bediasites). The ɛSr and ɛNd values for one composite Tektite sample from Site 904 fall within the range previously obtained for the Site 612 Tektites, which defines a linear trend that, if extrapolated, would intersect the values obtained for North American Tektites. The water contents of eight Tektite fragments from Site 904 range from 0.017 to 0.098 wt%, and, thus, are somewhat higher than is typical for Tektites. The heavy mineral assemblages of the 63–125 μm size fractions from the ejecta layers at Sites 612, 903, and 904 are all similar. Therefore, we conclude that the ejecta layer at all three sites is from the same impact event and that the Tektites at Sites 904 and 612 belong to the North American Tektite strewn field. Clinopyroxene-bearing (cpx) spherules occur below, or in the lower part of, the main ejecta layer at all three sites. At all three sites, the cpx spherules have been partly or completely replaced with pyrite that preserved the original crystalline textures. Site 612, 903, and 904 cpx spherules are similar to those found in the Caribbean Sea, Gulf of Mexico, central equatorial Pacific, western equatorial Pacific, and eastern Indian Ocean. The cpx event appears to have preceded the North American Tektite event by 10–15 ka or less. The fining-upward sequence at all three sites and concentration of the denser, unmelted impact ejecta at the top of the Tektite layer at Sites 612 and 904 suggest that the Tektite-bearing ejecta layers are not the result of downslope redeposition and that the unmelted ejecta landed after the glass. Geographic variations in thickness of the Tektite-bearing ejecta layer, the lack of carbonate clasts in the ejecta layer, and the low CaO content of the Tektite glass suggest that the ejecta (including the Tektite glass) were derived from the Chesapeake Bay structure rather than from the Toms Canyon structure. A sharp decline in microfossil abundances suggests that local environmental changes caused by the impact may have had adverse effects on benthic foraminifera, radiolaria, sponges, and fish as well as the planktic foraminifera.
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ablated Tektite from the central indian ocean
Meteoritics & Planetary Science, 1996Co-Authors: B.p. Glass, Dean R Chapman, Shyam M PrasadAbstract:A well-preserved ablated (button-shaped) Tektite recovered from the surface sediments of the central Indian Ocean lacks flow ridges and has apparently undergone ablation of 6.9 to 7.9 mm. The lack of flow ridges and amount of ablation indicate that, if it originated in Southeast Asia, it must have had a very shallow trajectory (only a few degrees) and a velocity on the order of 7 km/s as it re-entered the atmosphere. The central Indian Ocean Tektite is compositionally similar to high-magnesium (HMg) australites found at Serpentine Lakes and Lake Wilson, Australia, and to some HMg microTektites found in deep-sea sediments from the central Indian Ocean. This discovery supports a previous conclusion that the Australasian Tektite strewn field covers most of the Indian Ocean.
John Creech - One of the best experts on this subject based on the ideXlab platform.
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volatile loss under a diffusion limited regime in Tektites evidence from tin stable isotopes
Chemical Geology, 2019Co-Authors: Christian Koeberl, Frederic Moynier, John CreechAbstract:Abstract Tektites are glasses derived from near-surface continental crustal rocks that were molten and ejected from the Earth's surface during hypervelocity meteorite impacts. They are among the driest terrestrial samples, although the exact mechanism of water loss and the behaviour of other volatile species during these processes are debated. Based on the difference in magnitude of the Cu and Zn isotopic fractionations in Tektites, and the difference of diffusivity between these elements, it was suggested that volatile loss was diffusion-limited. Tin is potentially well suited to testing this model, as it has a lower diffusivity in silicate melts than both Cu and Zn, but a similar volatility to Zn. Here, we analysed the Sn stable isotopic composition in a suite of seven Tektites, representing three of the four known Tektite strewn fields, and for which Zn and Cu isotopes were previously reported. Tin is enriched in the heavier isotopes (≥2.5‰ on the 122Sn/118Sn ratio) in Tektites, correlated with the degree of Sn elemental depletion in their respective samples as well as with Cu and Zn isotope ratios, implying a common control. While the isotope fractionation of Sn, Cu and Zn is a result of volatility, the magnitude of isotope fractionation is strongly moderated by their relative rates of diffusion in the molten Tektite droplets. An Australasian Muong Nong-type Tektite analysed has the least Sn depletion and Sn isotope fractionation, consistent with these samples being more proximal to the source and experiencing a shorter time at high temperatures.
A Deutsch - One of the best experts on this subject based on the ideXlab platform.
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establishing the link between the chesapeake bay impact structure and the north american Tektite strewn field the sr nd isotopic evidence
Meteoritics & Planetary Science, 2006Co-Authors: A Deutsch, Christian KoeberlAbstract:The Chesapeake Bay impact structure, which is about 35 Ma old, has previously been proposed as the possible source crater of the North American Tektites (NAT). Here we report major and trace element data as well as the first Sr-Nd isotope data for drill core and outcrop samples of target lithologies, crater fill breccias, and post-impact sediments of the Chesapeake Bay impact structure. The unconsolidated sediments, Cretaceous to middle Eocene in age, have ɛSrt = 35.7 Ma of +54 to +272, and ɛNdt = 35.7 Ma ranging from -6.5 to -10.8; one sample from the granitic basement with a TNdCHUR model age of 1.36 Ga yielded an ɛSrt = 35.7 Ma of +188 and an ɛNdt = 35.7 Ma of -5.7. The Exmore breccia (crater fill) can be explained as a mix of the measured target sediments and the granite, plus an as-yet undetermined component. The post-impact sediments of the Chickahominy formation have slightly higher TNdCHUR model ages of about 1.55 Ga, indicating a contribution of some older materials. Newly analyzed bediasites have the following isotope parameters: +104 to +119 (ɛSrt = 35.7 Ma), -5.7 (ɛNdt = 35.7 Ma), 0.47 Ga (TSrUR), and 1.15 Ga (TNdCHUR), which is in excellent agreement with previously published data for samples of the NAT strewn field. Target rocks with highly radiogenic Sr isotopic composition, as required for explaining the isotopic characteristics of Deep Sea Drilling Project (DSDP) site 612 Tektites, were not among the analyzed sample suite. Based on the new isotope data, we exclude any relation between the NA Tektites and the Popigai impact crater, although they have identical ages within 2σ errors. The Chesapeake Bay structure, however, is now clearly constrained as the source crater for the North American Tektites, although the present data set obviously does not include all target lithologies that have contributed to the composition of the Tektites.
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geochemistry and neodymium strontium isotope signature of Tektite like objects from siberia urengoites south ural glass
Meteoritics & Planetary Science, 1997Co-Authors: A Deutsch, M Ostermann, V L MasaitisAbstract:— We report Sr-Nd isotope parameters, rare earth element (REE), and major element data for isolated findings of Tektite-like objects from western Siberia (urengoites, South-Ural glass), as well as for two indochinites. The latter were recovered in Vietnam and their overall geochemical characteristics equal those of other Tektites from the indochinite subgroup of the Australasian strewn field. The three urengoites (∼24 Ma) are extremely silica-rich (89 to 96 wt% SiC2), and their REE abundances vary between 45 and 76 ppm. With LaN/YbN ranging from 7.6 to 10.4 and EuN/EU* between 0.69 and 0.75, their REE distribution patterns match that of average upper crust. The urengoites have present-day ɛSr of +155 to +174 and ɛNd ranging from −18 to −23. Their model ages in million years are: TSruR = 1200 up to 4060 and TNdcHUR = 1570 up to 2070. Data points for the urengoites plot colinearly in the Rb-Sr evolution diagram. The age corresponding to the slope is 183 ± 30 Ma (2s), which is indistinguishable from the intercept age of 211 Ma in the TSrURvs. l/fRb diagram. Rubidium-strontium and Sm-Nd systematics of the urengoites indicate a heterogeneous precursor material, derived from Paleoproterozoic continental crust, which underwent Rb/Sr fractionation and partial Sr isotope homogenization in Jurassic times. Any relation between the urengoites and the Haughton impact crater, having within 2s errors an identical age, can be excluded on the basis of isotope relationships and geochemical data. The only known South-Ural glass (∼6.2 Ma) is characterized by intermediate SiO2 (65 wt%), high Al2O3 (14 wt%) and CaO (12 wt%), and low FeOTOT (0.4 wt%) contents. This unique Tektite-like object contains 110 ppm REE displaying a steeply negative C1 normalized distribution with LaN/YbN of 17, and EuN/Eu1 of 0.71. The Rb abundance (10 ppm) and Rb/Sr ratio are low, and combined with a “crustal” 87Sr/86Sr ratio of 0.722, yielding an unrealistic TSruR age of 2.5 Ga. The Rb-Sr systematics imply a rather recent parent/daughter element decoupling. The TNdCHUR age of the South-Ural glass is ∼1690 Ma. Geochemical data suggest that urengoites and the South-Ural glass belong to two discrete groups of Tektites, whose source craters remain to be discovered.
