Shock metamorphism and geochemistry of impactites from the Bosumtwi impact structure

Im Zuge des Bohrprojektes "2004 International Continental Scientific Drilling Programs (ICDP)" wurden zwei Bohrkerne im zentralen Bereich des Bosumtwi-Impaktkraters (Ghana; 1.07 Ma; 10.5 km Durchmesser) erbohrt: LB-08A in die äußeren Flanke der Zentralerhebung und LB-07A im tiefen Kratergraben. Diese Dissertation präsentiert petrographische und geochemische Untersuchungen von 121 Impaktiten des Bohrkerns LB-08A, hauptsächlich Suevite und Metasedimente. Im Vergleich zwischen Bohrkern LB-07A und mit Suevitproben außerhalb des Kraterrands haben die Suevite innerhalb des Kraters einen geringeren Anteil an Schmelzteilchen, diaplektischem Glas und Kalzitklasten, aber keine Granitklasten. Quarzkörner der Bosumtwi-Impaktite enthalten bis zu vier Gruppen an planaren Deformationsstrukturen (PDFs). Die PDF-Ebenen sind durch Dislokationen und winzige Flüssigkeitseinschlüsse definiert, was auf eine intensive (post-impakte) hydrothermale Alteration der Gesteine der Zentralerhebung hinweist. Mikrosonden-Analysen machen deutlich, dass sogenannte "geröstete" Quarze reichliche kleine Bläschen enthalten, welche die Streuung des Durchlichts erhöhen, und dass einige Spurenelemente aus der Quarzkristallstruktur hinausgedrängt wurden. "Geröstete" Quarze könnten daher durch Bläschenbildung nach der Druckentlastung gebildet worden sein. Untersuchungen mit dem Universal-Drehtisch zeigen, dass der Anteil der geschockten Quarze und die durchschnittliche Anzahl der PDF-Gruppen pro Mineralkorn empfindlichere Indikatoren für geringe Aenderungen in den Schockdrucken darstellen als die PDF-Orientierungen. Weiterhin ist die Präzision der U-Tisch Messungen vorwiegend von der Anzahl der untersuchten PDF-Sets abhängig. Die Kombination von Mikrobeobachtungen und nummerischer Modellierung unelastischer Gesteinsdeformation und Modifikationsprozessen während des Aufstiegs der Zentralerhebung erlauben die Rekonstruktion der Prä-Einschlagsposition der erbohrten Gesteine und lassen eine Schockabschwächung von ~5 GPa innerhalb der obersten 200 Meter der Zentralerhebung, welche von sprödem Bruchverhalten dominiert ist, erkennen. Untersuchungen an Ballensilikaten aus mehreren Impaktstrukturen führten zur Unterscheidung von fünf Ballen-Typen und zur Präsenz von Coesit in Ballen-alpha-Cristobalit, und sie zeigen, dass Ballenquarz/-cristobalit das Ergebnis einer Rücktransformation von schockinduzierten Zuständen ist, und stellt daher einen indirekten Hinweis auf Schockmetamorphose dar.Two boreholes were drilled in the central part of the Bosumtwi impact crater (Ghana; 1.07 Ma; 10.5-km-diameter) during the 2004 International Continental Scientific Drilling Program (ICDP) drilling project: LB-08A from the outer flank of the central uplift and LB-07A from the deep crater moat. This thesis presents petrographic and chemical investigations of 121 impactites, mainly suevite and metasediments, from core LB-08A. Comparing core LB-07A and samples from outside the crater rim show that the crater-fill suevite contains a lower proportion of melt particles and diaplectic glass, as well as calcite clasts but no granite clasts, in contrast to fallout suevite. Bosumtwi impactites contain quartz grains with up to 4 sets of planar deformation features (PDFs). PDFs planes are defined by dislocations and tiny fluid inclusions, indicating that impactites from the central uplift were subjected to intense hydrothermal alteration. Microprobe investigations show that toasted quartz contains abundant small vesicles that enhance scattering of transmitted light, and that some trace elements were expelled from the quartz's structure. Therefore, toasted quartz formed by vesiculation after pressure release. Universal-stage investigations show that the proportion of shocked quartz grains and the average number of PDF sets per grain provide a more sensitive indication of minor changes in shock pressure than PDF orientations, and that the precision of U-stage measurements depends mainly of the number of PDF sets investigated. Combining micro-scale observations with numerical modeling of inelastic rock deformation and modification processes during uplift allowed reconstruction of the pre-impact position of the drilled rocks and revealed a shock attenuation by ~5 GPa in the uppermost 200-meters of the central uplift, and that central uplift formation is dominated by brittle failure. Investigation of ballen silica from several impact structures distinguished five types of ballen, the presence of coesite within ballen alpha-cristobalite, and indicated that ballen quartz/cristobalite result from back-transformation of shock-induced states, thus representing indirect evidence of shock metamorphism

High pressure minerals in the Château-Renard (L6) ordinary chondrite: implications for collisions on its parent body

We report the first discoveries of high-pressure minerals in the historical L6 chondrite fall Château-Renard, based on co-located Raman spectroscopy, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy and electron backscatter diffraction, electron microprobe analysis, and transmission electron microscopy (TEM) with selected-area electron diffraction. A single polished section contains a network of melt veins from ~40 to ~200 μm wide, with no cross-cutting features requiring multiple vein generations. We find high-pressure minerals in veins greater than ~50 μm wide, including assemblages of ringwoodite + wadsleyite, ringwoodite + wadsleyite + majorite-pyropess, and ahrensite + wadsleyite. In association with ahrensite + wadsleyite at both SEM and TEM scale, we find a sodic pyroxene whose Raman spectrum is indistinguishable from that of jadeite but whose composition and structure are those of omphacite. We discuss constraints on the impact record of this meteorite and the L-chondrites in general

Shock metamorphism in plagioclase and selective amorphization

Plagioclase feldspar is one of the most common rock‐forming minerals on the surfaces of the Earth and other terrestrial planetary bodies, where it has been exposed to the ubiquitous process of hypervelocity impact. However, the response of plagioclase to shock metamorphism remains poorly understood. In particular, constraining the initiation and progression of shock‐induced amorphization in plagioclase (i.e., conversion to diaplectic glass) would improve our knowledge of how shock progressively deforms plagioclase. In turn, this information would enable plagioclase to be used to evaluate the shock stage of meteorites and terrestrial impactites, whenever they lack traditionally used shock indicator minerals, such as olivine and quartz. Here, we report on an electron backscatter diffraction (EBSD) study of shocked plagioclase grains in a metagranite shatter cone from the central uplift of the Manicouagan impact structure, Canada. Our study suggests that, in plagioclase, shock amorphization is initially localized either within pre‐existing twins or along lamellae, with similar characteristics to planar deformation features (PDFs) but that resemble twins in their periodicity. These lamellae likely represent specific crystallographic planes that undergo preferential structural failure under shock conditions. The orientation of preexisting twin sets that are preferentially amorphized and that of amorphous lamellae is likely favorable with respect to scattering of the local shock wave and corresponds to the “weakest” orientation for a specific shock pressure value. This observation supports a universal formation mechanism for PDFs in silicate minerals

Analysis of CN emission as a marker of organic compounds in meteoroids using laboratory simulated meteors

Fragments of small solar system bodies entering Earth's atmosphere have possibly been important contributors of organic compounds to the early Earth. The cyano radical (CN) emission from meteors is considered as potentially one of the most suitable markers of organic compounds in meteoroids, however, its detection in meteor spectra has been thus far unsuccessful. With the aim to improve our abilities to identify CN emission in meteor observations and use its spectral features to characterize the composition of incoming asteroidal meteoroids, we present a detailed analysis of CN emission from high-resolution spectra of 22 laboratory simulated meteors including ordinary, carbonaceous, and enstatite chondrites, as well as a large diversity of achondrites (i.e., ureilite, aubrite, lunar, martian, howardite, eucrite, and diogenite), mesosiderite, and iron meteorites. We describe the variations of CN emission from different classes of asteroidal meteor analogues, its correlation and time evolution relative to other major meteoroid components. We demonstrate that CN can be used as a diagnostic spectral feature of carbonaceous and carbon-rich meteoroids, while most ordinary chondrites show no signs of CN. Our results point out strong correlation between CN and H emission and suggest both volatile features are suitable to trace contents of organic matter and water molecules present within meteoroids. For the application in lower resolution meteor observations, we demonstrate that CN can be best recognized in the early stages of ablation and for carbon-rich materials by measuring relative intensity ratio of CN band peak to the nearby Fe I-4 lines

Microtextures in the Chelyabinsk impact breccia reveal the history of Phosphorus‐Olivine‐Assemblages in chondrites

Funder: European Union‐funded Integrated ActivitiesFunder: SYNTHESYSAbstract: The geochemistry and textures of phosphate minerals can provide insights into the geological histories of parental asteroids, but the processes governing their formation and deformation remain poorly constrained. We assessed phosphorus‐bearing minerals in the three lithologies (light, dark, and melt) of the Chelyabinsk (LL5) ordinary chondrite using scanning electron microscope, electron microprobe, cathodoluminescence, and electron backscatter diffraction techniques. The majority of studied phosphate grains appear intergrown with olivine. However, microtextures of phosphates (apatite [Ca5(PO4)3(OH,Cl,F)] and merrillite [Ca9NaMg(PO4)7]) are extremely variable within and between the differently shocked lithologies investigated. We observe continuously strained as well as recrystallized strain‐free merrillite populations. Grains with strain‐free subdomains are present only in the more intensely shocked dark lithology, indicating that phosphate growth predates the development of primary shock‐metamorphic features. Complete melting of portions of the meteorite is recorded by the shock‐melt lithology, which contains a population of phosphorus‐rich olivine grains. The response of phosphorus‐bearing minerals to shock is therefore hugely variable throughout this monomict impact breccia. We propose a paragenetic history for P‐bearing phases in Chelyabinsk involving initial phosphate growth via P‐rich olivine replacement, followed by phosphate deformation during an early impact event. This event was also responsible for the local development of shock melt that lacks phosphate grains and instead contains P‐enriched olivine. We generalize our findings to propose a new classification scheme for Phosphorus‐Olivine‐Assemblages (Type I–III POAs). We highlight how POAs can be used to trace radiogenic metamorphism and shock metamorphic events that together span the entire geological history of chondritic asteroids

Similarities in element content between comet 67P/Churyumov–Gerasimenko coma dust and selected meteorite samples

We have analysed the element composition and the context of particles collected within the coma of 67P/Churyumov–Gerasimenko with Rosetta’s COmetary Secondary Ion Mass Analyzer (COSIMA). A comparison has been made between on board cometary samples and four meteorite samples measured in the laboratory with the COSIMA reference model. Focusing on the rock-forming elements, we have found similarities with chondrite meteorites for some ion count ratios. The composition of 67P/Churyumov–Gerasimenko particles measured by COSIMA shows an enrichment in volatile elements compared to that of the investigated Renazzo (CR2) carbonaceous meteorite sample.</p

Impact Earth: A review of the terrestrial impact record

Over the past few decades, it has become increasingly clear that the impact of interplanetary bodies on other planetary bodies is one of the most ubiquitous and important geological processes in the Solar System. This impact process has played a fundamental role throughout the history of the Earth and other planetary bodies, resulting in both destructive and beneficial effects. The impact cratering record of Earth is critical to our understanding of the processes, products, and effects of impact events. In this contribution, we provide an up-to-date review and synthesis of the impact cratering record on Earth. Following a brief history of the Impact Earth Database (available online at http://www.impactearth.com), the definition of the main categories of impact features listed in the database, and an overview of the impact cratering process, we review and summarize the required evidence to confirm impact events. Based on these definitions and criteria, we list 188 hypervelocity impact craters and 13 impact craters (i.e., impact sites lacking evidence for shock metamorphism). For each crater, we provide details on key attributes, such as location, date confirmed, erosional level, age, target properties, diameter, and an overview of the shock metamorphic effects and impactites that have been described in the literature. We also list a large number of impact deposits, which we have classified into four main categories: tektites, spherule layers, occurrences of other types of glass, and breccias. We discuss the challenges of recognizing and confirming impact events and highlight weaknesses, contradictions, and inconsistencies in the literature. We then address the morphology and morphometry of hypervelocity impact craters. Based on the Impact Earth Database, it is apparent that the transition diameter from simple to complex craters for craters developed in sedimentary versus crystalline target rocks is less pronounced than previously reported, at approximately 3 km for both. Our analysis also yields an estimate for stratigraphic uplift of 0.0945D0.6862, which is lower than previous estimates. We ascribe this to more accurate diameter estimates plus the variable effects of erosion. It is also clear that central topographic peaks in terrestrial complex impact craters are, in general, more subdued than their lunar counterparts. Furthermore, a number of relatively well-preserved terrestrial complex impact structures lack central peaks entirely. The final section of this review provides an overview of impactites preserved in terrestrial hypervelocity impact craters. While approximately three quarters of hypervelocity impact craters on Earth preserve some portion of their crater-fill impactites, ejecta deposits are known from less than 10%. In summary, the Impact Earth Database provides an important new resource for researchers interested in impact craters and the impact cratering process and we welcome input from the community to ensure that the Impact Earth website (http://www.impactearth.com) is a living resource that is as accurate and as up-to-date, as possible

Ocean Drilling Perspectives on Meteorite Impacts

Extraterrestrial impacts that reshape the surfaces of rocky bodies are ubiquitous in the solar system. On early Earth, impact structures may have nurtured the evolution of life. More recently, a large meteorite impact off the Yucatán Peninsula in Mexico at the end of the Cretaceous caused the disappearance of 75% of species known from the fossil record, including non-avian dinosaurs, and cleared the way for the dominance of mammals and the eventual evolution of humans. Understanding the fundamental processes associated with impact events is critical to understanding the history of life on Earth, and the potential for life in our solar system and beyond. Scientific ocean drilling has generated a large amount of unique data on impact pro- cesses. In particular, the Yucatán Chicxulub impact is the single largest and most sig- nificant impact event that can be studied by sampling in modern ocean basins, and marine sediment cores have been instrumental in quantifying its environmental, cli- matological, and biological effects. Drilling in the Chicxulub crater has significantly advanced our understanding of fundamental impact processes, notably the formation of peak rings in large impact craters, but these data have also raised new questions to be addressed with future drilling. Within the Chicxulub crater, the nature and thickness of the melt sheet in the central basin is unknown, and an expanded Paleocene hemipelagic section would provide insights to both the recovery of life and the climatic changes that followed the impact. Globally, new cores collected from today’s central Pacific could directly sample the downrange ejecta of this northeast-southwest trending impact. Extraterrestrial impacts have been controversially suggested as primary drivers for many important paleoclimatic and environmental events throughout Earth history. However, marine sediment archives collected via scientific ocean drilling and geo- chemical proxies (e.g., osmium isotopes) provide a long-term archive of major impact events in recent Earth history and show that, other than the end-Cretaceous, impacts do not appear to drive significant environmental changes

Sphene Emotional: How Titanite Was Shocked When the Dinosaurs Died

Accessory mineral geochronometers such as zircon, monazite, baddeleyite, and xenotime are increasingly being recognized for their ability to preserve diagnostic microstructural evidence of hypervelocity processes. However, little is known about the response of titanite to shock metamorphism, even though it is a widespread accessory phase and U-Pb geochronometer. Here we report two new mechanical twin modes in titanite within shocked granitoids from the Chicxulub impact structure, Mexico. Titanite grains in the newly acquired International Ocean Discovery Program Site expedition 364 M0077A core preserve multiple sets of polysynthetic twins, most commonly with composition planes (K1), = ~{111}, and shear direction (1) = , and less commonly with the mode K1 = {130}, 1 = ~. In some grains, {130} deformation bands have formed concurrently with shock twins, indicating dislocation glide with Burgers vector b = [341] can be active at shock conditions. Twinning of titanite in these modes, the presence of planar deformation features in shocked quartz, and lack of diagnostic shock microstructures in zircon in the same samples highlights the utility of titanite as a shock indicator for a shock pressure range between ~12 and ~17 GPa. Given the challenges of identifying ancient impact evidence on Earth and other bodies, microstructural analysis of titanite is here demonstrated to be a new avenue for recognizing impact deformation in materials where other impact evidence may be erased, altered, or did not manifest due to low shock pressure