Impactites Research Articles

Iron-Bearing Microinclusions in Irghizites

The results are presented from studying the structural phase composition of inclusions in irghizites (tektite-like impact glasses from the Zhamanshin astrobleme). The presence of micron and submicron crystalline iron-bearing minerals is detected. These are spheroids, skeletal aggregates, and crystals of unusual structure and complex composition.

Mineralogical Features of Ultrahigh Pressure Impact Glasses of the Kara Astrobleme (Pay-Khoy, Russia)

The behavior of the disordered systems in the conditions of the strong compression attracts particular interest in materials science. Natural impact glasses, being with a metastable state, can keep features of the structure for millions of years, form essential volumes compare to experimentally possible high-pressure products. The complex analysis of geological, mineralogical-petrographic, petrochemical features and degree of crystallinity of tagamites, vein glasses and vitro-clastic formations of the origin type of melt impactites of the Kara astrobleme including their phase state composition have been studied. The melt Kara impactites have dependence on the initial character of the melted sedimentary target and cooling velocity of an impact melt. The lowest order has been described for ultrahigh pressure vein-type glasses which contain silica drops with single crystalline coesite and firstly found melt-crystallized high pressure variety of smectite. The vein melt impactites consist of solidified impact melt presented by totally amorphous alumina silicate glass with augite microcrystallites and ultrahigh pressure silica glass with coesite microcrystals and “drops” of firstly found melt-crystallized high pressure variety of smectite. The boundary between alumina silicate and silica glass is characterized with liquated amorphous Fe-rich drops. The special feature of the studies melt impactites is low alteration in spite of 70 Ma age and high content of water. The large volumes of an impact melt created massive tagamite bodies have longer cooling underwent with the almost complete crystallization. The obtained data indicate the specific nature of melt glasses of vein type and prospects of their further research for fundamental study of amorphous state under ultrahigh pressure and propose new materials.

New lunar meteorite NWA 10986: A mingled impact melt breccia from the highlands—A complete cross section of the lunar crust

AbstractNorthwest Africa (NWA) 10986 is a new mingled lunar meteorite found in 2015 in Western Sahara. This impact melt breccia contains abundant impact melt glass and clasts as large as 0.75 mm. Clasts are predominantly plagioclase and pyroxene‐rich and represent both highland and basalt lithologies. Highland lithologies include troctolites, gabbronorites, anorthositic norites, and troctolitic anorthosites. Basalt lithologies include crystalline clasts with large zoned pyroxenes representing very low titanium to low titanium basalts. In situ geochemical analysis of minerals within clasts indicates that they represent ferroan anorthosite, Mg‐suite, and gabbronorite lithologies as defined by the Apollo sample collection. Clasts representing magnesian anorthosite, or “gap” lithologies, are prevalent in this meteorite. Whole rock and in situ impact glass measurements indicate low incompatible trace element concentrations. Basalt clasts also have low incompatible trace element concentrations and lack evolved KREEP mineralogy although pyroxferroite grains are present. The juxtaposition of evolved, basaltic clasts without KREEP signatures and highland lithologies suggests that these basaltic clasts may represent cryptomare. The lithologies found in NWA 10986 offer a unique and possibly a complete cross section view of the Moon sourced outside of the Procellarum KREEP Terrane.

An Experimental Study on Impact Resistance of Different Layup Configuration of Fiber Metal Laminates

Fiber metal laminates (FMLs) are composed of thin metal sheets and fiber-reinforced composite layers. Compared to monolithic aluminum alloys, FMLs combine lower density, higher fatigue resistance, and improved damage tolerance. The present study aimed to investigate the low-velocity impact induced by drop-weight instrument and the tensile strength on various lay-up configurations of FMLs fabricated. FML samples were composed of two layers of aluminum 2024-T3 and two layers of epoxy resin, which were reinforced with carbon fabric, glass fabric, and Kevlar fabric made in pairs. In addition, another type of FMLs was developed with carbon/Kevlar fabric under the same circumstances. Force-time histories of impact forces were recorded, and the damaged specimens were inspected using optical microscopy in terms of the impact side, nonimpact side, and cross-sectional side. Experimental results indicated that the maximum impact force corresponded to the FMLs that were composed of Kevlar fabric on the impact side and glass fabric on the non-impact side. In addition, the highest tensile strength and Young’s modulus among FMLs belonged to FML with Kevlar fabric and glass fabric.

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

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.

Initial 40Ar‐39Ar Ages of the Paleocene‐Eocene Boundary Impact Spherules

AbstractWe report 40Ar‐39Ar step‐heating ages of Paleocene‐Eocene (P‐E) boundary impact spherules from Atlantic Margin coastal plain and open ocean sites. We test the hypothesis that the P‐E spherules are reworked from an earlier event (e.g., K‐Pg impact at ~66 Ma), which predicts a cooling age discordant from their depositional age of 55.93 ± 0.05 Ma at the P‐E boundary. Isochrons from the step‐heating analysis yield 40Ar‐36Ar intercepts in excess of the modern in most cases, indicating that the spherules have excess radiogenic Ar (40Ar*), typical of impact glasses incompletely degassed before solidification. The weighted mean of the isochron‐corrected plateau age is 54.2 ± 2.5 Ma (1σ), and their isochron age is 55.4 ± 4.0 Ma, both indistinguishable from their P‐E depositional age, not supporting the K‐Pg reworking hypothesis. This is consistent with all other stratigraphic and geochemical evidence for an impact at the P‐E boundary and ejecta distribution by air fall.

Deposition and age of Chicxulub impact spherules on Gorgonilla Island, Colombia

AbstractThe end-Cretaceous mass extinction (66 Ma) has long been associated with the Chicxulub impact on the Yucatan Peninsula. However, consensus on the age of this impact has remained controversial because of differing interpretations on the stratigraphic position of Chicxulub impact spherules relative to the mass extinction horizon. One side argues that the impact occurred precisely at the Cretaceous-Paleogene boundary, thus coinciding with the mass extinction; the other side argues that the impact predated the Cretaceous-Paleogene boundary, based on the discovery of primary impact spherules deposits in NE Mexico and Texas near the base of planktic foraminiferal zone CF1, dated at 170 k.y. before the Cretaceous-Paleogene boundary. A recent study of the most pristine Chicxulub impact spherules discovered on Gorgonilla Island, Colombia, suggested that they represent a primary impact deposit with an absolute age indistinguishable from the Cretaceous-Paleogene boundary. Here, we report on the Gorgonilla section with the main objective of evaluating the nature of deposition and age of the spherule-rich layer relative to the Cretaceous-Paleogene boundary.The Gorgonilla section consists of light gray-yellow calcareous siliceous mudstones (pelagic deposits) alternating with dark olive-brown litharenites (turbidites). A 3-cm-thick dark olive-green spherule-rich layer overlies an erosional surface separating Maastrichtian and Danian sediments. This layer consists of a clast-supported, normally graded litharenite, with abundant Chicxulub impact glass spherules, lithics (mostly volcanic), and Maastrichtian as well as Danian microfossils, which transitions to a calcareous mudstone as particle size decreases. Mineralogical analysis shows that this layer is dominated by phyllosilicates, similar to the litharenites (turbidites) that characterize the section. Based on these results, the spherule-rich layer is interpreted as a reworked early Danian deposit associated with turbiditic currents. A major hiatus (>250 k.y.) spanning the Cretaceous-Paleogene boundary and the earliest Danian is recorded at the base of the spherule-rich layer, based on planktic foraminiferal and radiolarian biostratigraphy and carbon stable isotopes. Erosion across the Cretaceous-Paleogene boundary has been recorded worldwide and is generally attributed to rapid climate changes, enhanced bottom-water circulation during global cooling, sea-level fluctuations, and/or intensified tectonic activity. Chicxulub impact spherules are commonly reworked and redeposited into younger sediments overlying a Cretaceous-Paleogene boundary hiatus of variable extent in the Caribbean, Central America, and North Atlantic, while primary deposits are rare and only known from NE Mexico and Texas. Because of their reworked nature, Gorgonilla spherules provide no stratigraphic evidence from which the timing of the impact can be inferred.

The origin of blue coloration in a fulgurite from Marquette, Michigan

Few rocks or minerals display a blue hue; this is especially true for natural glasses. As glasses are already uncommon compared to crystalline rocks due to their instability, the rarity of blue glass makes a fulgurite found in Marquette Michigan worthy of further examination. We study the morphology and the chemical and structural characteristics of the blue fulgurite using Raman spectroscopy, X-Ray Fluorescence analysis, Election Microprobe Analysis, and Transmission Election Microscopy. We use the experimental results to compare the fulgurite with another blue natural glass, zhanmanshinite, an impact glass named after an exposed impact crater in Kazakhstan, and to evaluate the support for several hypotheses of the origin of the blue color in the fulgurite.

Granular zircon from Vredefort granophyre (South Africa) confirms the deep injection model for impact melt in large impact structures

Abstract The Vredefort impact structure, South Africa, is a 2.02 Ga deeply eroded meteorite scar that provides an opportunity to study large impact craters at their lower stratigraphic levels. A series of anomalous granophyre dikes in the core of the structure are believed to be composed of an impact melt, which intruded downwards from the crater floor, exploiting fractures in basement rocks. However, the melt emplacement mechanisms and timing are not constrained. The granophyre dikes contain supracrustal xenoliths captured at higher levels, presently eroded. By studying these clasts and shocked minerals within, we can better understand the nature of dikes, magnitude of impact melt movement, conditions that affected target rocks near the impacted surface, and erosional rates. We report “former reidite in granular neoblastic” (FRIGN) zircon within a granite clast enclosed in the granophyre. High-pressure zircon transformation to reidite (ZrSiO4) and reversion to zircon resulted in zircon grains composed of fine neoblasts (∼0.5–3 µm) with two or three orthogonal orientations. Our finding provides new independent constraints on the emplacement history of Vredefort granophyre dikes. Based on the environment, where other FRIGN zircons are found (impact glasses and melts), the clast was possibly captured near the top of the impact melt sheet and transported to the lowermost levels of the structure, traveling some 8–10 km. Our finding not only provides the highest-pressure shock estimates thus far discovered in the Vredefort structure (≥30 GPa), but also shows that microscopic evidence of high shock pressures can be found within large eroded craters at their lowest stratigraphic levels.

Magnetic Properties and Redox State of Impact Glasses: A Review and New Case Studies from Siberia

High velocity impacts produce melts that solidify as ejected or in-situ glasses. We provide a review of their peculiar magnetic properties, as well as a new detailed study of four glasses from Siberia: El’gygytgyn, Popigai, urengoites, and South-Ural glass (on a total of 24 different craters or strewn-fields). Two types of behavior appear: 1) purely paramagnetic with ferromagnetic impurities at most of the order of 10 ppm; this corresponds to the five tektite strewn-fields (including the new one from Belize), urengoites, and Darwin glass. Oxidation state, based in particular on X-ray spectroscopy, is mostly restricted to Fe2+; 2) variable and up to strong ferromagnetic component, up to the 1 wt % range, mostly due to substituted magnetite often in superparamagnetic state. Accordingly, bulk oxidation state is intermediate between Fe2+ and Fe3+, although metallic iron, hematite, and pyrrhotite are sometimes encountered. Various applications of these magnetic properties are reviewed in the field of paleomagnetism, magnetic anomalies, recognition of glass origin, and formation processes.

Comparison of the compositions and microstructures of Terrestrial and Lunar impact glasses: samples from The Zhamanshin Crater and Luna 16, 20, and 24 missionse

The paper presents pioneering data on the comparative study of impact glasses from the Zhamanshin crater and lunar regolith (delivered by the Luna 16, 20, and 24 probes). The data were acquired using analytical techniques of ultrahigh spatial resolution. Many of the melt and condensate impact glasses, both terrestrial and lunar, are similar in inner structure and composition, which were controlled primarily by the physics of the impacts and similar compositions of the targets.

Using Size and Composition to Assess the Quality of Lunar Impact Glass Ages

Determining the impact chronology of the Moon is an important yet challenging problem in planetary science even after decades of lunar samples and other analyses. In addition to crater counting statistics, orbital data, and dynamical models, well-constrained lunar sample ages are critical for proper interpretation of the Moon’s impact chronology. To understand which properties of lunar impact glasses yield well-constrained ages, we evaluated the compositions and sizes of 119 Apollo 14, 15, 16, and 17 impact glass samples whose compositions and 40Ar/39Ar ages have already been published, and we present new data on 43 others. These additional data support previous findings that the composition and size of the glass are good indicators of the quality of the age plateau derived for each sample. We have further constrained those findings: Glasses of ≥200 μm with a fraction of non-bridging oxygens (X(NBO)) of ≥0.23 and a K2O (wt%) of ≥0.07 are prime candidates for argon analyses and more likely to yield well-constrained 40Ar/39Ar ages. As a result, science resulting from impact glass analyses is maximized while analytical costs per glass are minimized. This has direct implications for future analyses of glass samples for both those in the current lunar collection and those that have yet to be collected.

Pantasma: Evidence for a Pleistocene circa 14 km diameter impact crater in Nicaragua

AbstractThe circa 14 km diameter Pantasma circular structure in Oligocene volcanic rocks in Nicaragua is here studied for the first time to understand its origin. Geomorphology, field mapping, and petrographic and geochemical investigations all are consistent with an impact origin for the Pantasma structure. Observations supporting an impact origin include outward‐dipping volcanic flows, the presence of former melt‐bearing polymict breccia, impact glass (with lechatelierite and low H2O, <300 ppm), and also a possible ejecta layer containing Paleozoic rocks which originated from hundreds of meters below the surface. Diagnostic evidence for impact is provided by detection in impact glass of the former presence of reidite in granular zircon as well as coesite, and extraterrestrial ε54Cr value in polymict breccia. Two 40Ar/39Ar plateau ages with a combined weighted mean age of 815 ± 11 ka (2 σ; P = 0.17) were obtained on impact glass. This age is consistent with geomorphological data and erosion modeling, which all suggest a rather young crater. Pantasma is only the fourth exposed crater >10 km found in the Americas south of N30 latitude, and provides further evidence that a significant number of impact craters may remain to be discovered in Central and South America.

Remnants of paleoflora in impact melt rocks of the El'gygytgyn crater (Chukotka, Russia)

AbstractRemnants of paleoflora were discovered in impact melt rocks from the El'gygytgyn crater, Chukotka, Russia. El'gygytgyn is a 3.58 Ma, 18 km diameter impact structure in Chukotka, northeastern Russia. A circular crater basin is surrounded by an uplifted rim. The crater floor is occupied by the El'gygytgyn Lake, 12 km in diameter, surrounded by lacustrine terraces up to 80 m in height. Impactites found at the El'gygytgyn crater include impact melt rocks, glass bombs, and shock metamorphosed volcanic rocks. Most impact melt rocks occur only in redeposited state in the terrace lake deposits. Floral remnants were discovered in impact melt rocks from various locations in the terrace deposits. The floral remnants include fragments of leaves, cell tissue, and undetermined organic matter that occur in vesicles within glassy melt rocks and impact melt breccias. After the discovery of floral remnants in impact melt breccias from upper Miocene strata in Argentina, and the description of floral imprints in the Dakhleh Glass of proposed impact origin in Egypt, the detection of paleoflora remnants in impact melt rocks of the El'gygytgyn structure is the first such occurrence in a confirmed impact crater on Earth.

Breaking the ecosystem services glass ceiling: realising impact

Through changes in policy and practice, the inherent intent of the ecosystem services (ES) concept is to safeguard ecosystems for human wellbeing. While impact is intrinsic to the concept, little is known about how and whether ES science leads to impact. Evidence of impact is needed. Given the lack of consensus on what constitutes impact, we differentiate between attributional impacts (transitional impacts on policy, practice, awareness or other drivers) and consequential impacts (real, on-the-ground impacts on biodiversity, ES, ecosystem functions and human wellbeing) impacts. We conduct rigorous statistical analyses on three extensive databases for evidence of attributional impact (the form most prevalently reported): the IPBES catalogue (n = 102), the Lautenbach systematic review (n = 504) and a 5-year in-depth survey of the OPERAs Exemplars (n = 13). To understand the drivers of impacts, we statistically analyse associations between study characteristics and impacts. Our findings show that there exists much confusion with regard to defining ES science impacts, and that evidence of attributional impact is scarce: only 25% of the IPBES assessments self-reported impact (7% with evidence); in our meta-analysis of Lautenbach’s systematic review, 33% of studies provided recommendations indicating intent of impacts. Systematic impact reporting was imposed by design on the OPERAs Exemplars: 100% reported impacts, suggesting the importance of formal impact reporting. The generalised linear models and correlations between study characteristics and attributional impact dimensions highlight four characteristics as minimum baseline for impact: study robustness, integration of policy instruments into study design, stakeholder involvement and type of stakeholders involved. Further in depth examination of the OPERAs Exemplars showed that study characteristics associated with impact on awareness and practice differ from those associated with impact on policy: to achieve impact along specific dimensions, bespoke study designs are recommended. These results inform targeted recommendations for ES science to break its impact glass ceiling.

Comparison of the Compositions and Microstructures of Terrestrial and Lunar Impact Glasses: Samples from the Zhamanshin Crater and Luna 16, 20, and 24 Missions

Comparison of the Compositions and Microstructures of Terrestrial and Lunar Impact Glasses: Samples from the Zhamanshin Crater and Luna 16, 20, and 24 Missions

Iron oxidation state of impact glasses from the Zhamanshin crater studied by X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) has been used to analyze the change in iron oxidation state and coordination numbers of impact glasses from the Zhamanshin crater. Series of samples, specifically aerodynamically shaped small glassy bodies – irghizites, and silica-rich and silica-poor glasses – zhamanshinites, have been investigated. The Fe2.2+ oxidation state was determined for the irghizites and the highest value Fe2.8+ for the silica-poor zhamanshinite. These values agree with the trend of oxidation processes upon high speed interaction of meteorite parent body and target rocks. The obtained data can be used for future establishment of the formation parameters (P, T, fO2 conditions) of these impact glasses as well as relationships between the rocks.

Structure of natural impact glasses by microscopic data

Newly found impact vein-bodied ultrahigh pressure high temperature (UHPHT) glasses from Kara astrobleme (Russia) and impact drop-shaped glasses from Ries crater suevites (Germany) have been investigated. The nanostructure of the glasses has been analysed using atomic force microscopy (AFM) supported by a complex of optical microscopy, scanning electron microscopy (SEM) combined with EDS analysis, and Raman spectroscopy. The received data demonstrate the different levels of impact melt differentiation through partial crystallization and polymerization; the amorphous substance is presented by similar 60-80 nm globular nanostructural composition.

Nano-heterogeneity of natural impact silica-rich glasses according to atomic force microscopy and spectroscopy data

In nature extreme PT-conditions result in numerous glass-like solids, the study of which allows specifying various structural and chemical features of extreme materials, which have a great application potential in various fields of technology. We studied features of nanostructure of natural impact glasses, including recently discovered ultrahigh-pressure high-temperature impact glasses, compared to low-pressure natural and synthetic standard silica glasses. In this paper we presented complex data of atomic force microscopy, X-ray diffraction, X-ray energy-dispersive spectrometry, infrared and Raman spectroscopy. We described nanostructural characteristics of impact glasses and showed influence of chemical composition on the features of their structure. We discovered that the elemental composition was the most important factor determining the glasses nano-heterogeneity. We noted an essential role of influence of Na impurity on glass nanostructure. The impact glasses with pure SiO2 composition have the smallest sizes of nanostructural elements.

10Be in Australasian microtektites compared to tektites: Size and geographic controls

Research Article| August 08, 2018 10Be in Australasian microtektites compared to tektites: Size and geographic controls P. Rochette; P. Rochette 1Aix-Marseille Université, CNRS, IRD, INRA, Coll France, UM 34 CEREGE, Technopôle de l’Environnement Arbois-Méditerranée, BP80, 13545 Aix-en-Provence, France Search for other works by this author on: GSW Google Scholar R. Braucher; R. Braucher 1Aix-Marseille Université, CNRS, IRD, INRA, Coll France, UM 34 CEREGE, Technopôle de l’Environnement Arbois-Méditerranée, BP80, 13545 Aix-en-Provence, France Search for other works by this author on: GSW Google Scholar L. Folco; L. Folco 2Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy Search for other works by this author on: GSW Google Scholar C.S. Horng; C.S. Horng 3Institute of Earth Sciences, Academia Sinica, P. O. Box 1-55, Nankang, Taipei 11529, Taiwan Search for other works by this author on: GSW Google Scholar G. Aumaître; G. Aumaître 1Aix-Marseille Université, CNRS, IRD, INRA, Coll France, UM 34 CEREGE, Technopôle de l’Environnement Arbois-Méditerranée, BP80, 13545 Aix-en-Provence, France Search for other works by this author on: GSW Google Scholar D.L. Bourlès; D.L. Bourlès 1Aix-Marseille Université, CNRS, IRD, INRA, Coll France, UM 34 CEREGE, Technopôle de l’Environnement Arbois-Méditerranée, BP80, 13545 Aix-en-Provence, France Search for other works by this author on: GSW Google Scholar K. Keddadouche K. Keddadouche 1Aix-Marseille Université, CNRS, IRD, INRA, Coll France, UM 34 CEREGE, Technopôle de l’Environnement Arbois-Méditerranée, BP80, 13545 Aix-en-Provence, France Search for other works by this author on: GSW Google Scholar Geology (2018) 46 (9): 803–806. https://doi.org/10.1130/G45038.1 Article history received: 16 Apr 2018 rev-recd: 18 Jul 2018 accepted: 23 Jul 2018 first online: 08 Aug 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation P. Rochette, R. Braucher, L. Folco, C.S. Horng, G. Aumaître, D.L. Bourlès, K. Keddadouche; 10Be in Australasian microtektites compared to tektites: Size and geographic controls. Geology 2018;; 46 (9): 803–806. doi: https://doi.org/10.1130/G45038.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract High 10Be contents in tektites reported in literature are taken as evidence of a source material, melted at the impact site, enriched in atmospheric 10Be; i.e., a soil or sediment. In 0.8 Ma Australasian tektites, 10Be content increases with distance from the putative impact location in Indochina, with geographic averages from 69 × 106 atoms/g (Indochina) to 136 × 106 atoms/g (Australia). Here we report, for the first time, 10Be contents in microtektites collected from Antarctica and the South China Sea. We show that microtektites are ∼30 × 106 atoms/g richer in 10Be than tektites from the same geographic areas. Antarctic microtektites, with an average 10Be content of 184 × 106 atoms/g after correction for in situ production, are the richest impact glass ever measured. The simplest explanation for such systematic size and geographic trends is that the source depth of the melt within the target surface decreases with ejection velocity. Indeed, higher initial kinetic energy implies higher launch distances and higher fragmentation of the ejecta. Antarctic microtektite source depth may tentatively be restricted to the upper tens of centimeters at the impact site. Alternative models invoking a marine or loessic sediment source, or a secondary enrichment in the microtektite (either by atmospheric scavenging, selective fractionation by volatilization, or post-depositional contamination) fail to reproduce the observed relationships. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.