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

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

Science Priorities for the Extraction of the Solid MSR Samples from their Sample Tubes NASA-ESA Mars Rock Team

editorial reviewedPreservation of the chemical and structural integrity of samples that will be brought back from Mars is paramount to achieving the scientific objectives of MSR. Given our knowledge of the nature of the samples retrieved at Jezero by Perseverance, at least two options need to be tested for opening the sample tubes: (1) One or two radial cuts at the end of the tube to slide the sample out. (2) Two radial cuts at the ends of the tube and two longitudinal cuts to lift the upper half of the tube and access the sample. Strategy 1 will likely minimize contamination but incurs the risk of affecting the physical integrity of weakly consolidated samples. Strategy 2 will be optimal for preserving the physical integrity of the samples but increases the risk of contamination and mishandling of the sample as more manipulations and additional equipment will be needed. A flexible approach to opening the sample tubes is therefore required, and several options need to be available, depending on the nature of the rock samples returned. Both opening strategies 1 and 2 may need to be available when the samples are returned to handle different sample types (e.g., loosely bound sediments vs. indurated magmatic rocks). This question should be revisited after engineering tests are performed on analogue samples. The MSR sample tubes will have to be opened under stringent BSL4 conditions and this aspect needs to be integrated into the planning

Probing the hydrothermal system of the Chicxulub impact crater

The ~180-km-diameter Chicxulub peak-ring crater and ~240-km multiring basin, produced by the impact that terminated the Cretaceous, is the largest remaining intact impact basin on Earth. International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP) Expedition 364 drilled to a depth of 1335 m below the sea floor into the peak ring, providing a unique opportunity to study the thermal and chemical modification of Earth’s crust caused by the impact. The recovered core shows the crater hosted a spatially extensive hydrothermal system that chemically and mineralogically modified ~1.4 × 105 km3 of Earth’s crust, a volume more than nine times that of the Yellowstone Caldera system. Initially, high temperatures of 300° to 400°C and an independent geomagnetic polarity clock indicate the hydrothermal system was long lived, in excess of 106 years

Globally distributed iridium layer preserved within the Chicxulub impact structure

The Cretaceous-Paleogene (K-Pg) mass extinction is marked globally by elevated concentrations of iridium, emplaced by a hypervelocity impact event 66 million years ago. Here, we report new data from four independent laboratories that reveal a positive iridium anomaly within the peak-ring sequence of the Chicxulub impact structure, in drill core recovered by IODP-ICDP Expedition 364. The highest concentration of ultrafine meteoritic matter occurs in the post-impact sediments that cover the crater peak ring, just below the lowermost Danian pelagic limestone. Within years to decades after the impact event, this part of the Chicxulub impact basin returned to a relatively low-energy depositional environment, recording in unprecedented detail the recovery of life during the succeeding millennia. The iridium layer provides a key temporal horizon precisely linking Chicxulub to K-Pg boundary sections worldwide