Chapter

The Geochemistry and Cosmochemistry of Impacts

December 2007

In book: Treatise on Geochemistry (pp.1-52)

Authors:

This chapter is divided into three general areas. At the beginning, to set the stage and introduce the relevant background information and terminology, a brief introduction to impact craters and processes is provided. Then the main geochemical methods employed in the study of impact craters and processes are described. Finally, a number of examples are given in which geochemical methods in the study of impacts were used. It should be noted that the present chapter is almost exclusively concerned with the study of terrestrial impacts (with the exception of a few mentions of lunar examples), mainly due to accessibility of rocks for study.

Meteoritic component in impact breccias of the Jeokjung-Chogye structure, South Korea: Evidence from the HSE abundances and ReOs isotopic systematics

Article

Oct 2022
CHEM GEOL

Geochemical compositions, including highly siderophile element (HSE) abundances and ReOs isotope compositions, of 9 polymict impact breccias in two drill cores and 3 target rocks from the Jeokjung-Chogye structure, South Korea, have been investigated to test the impact origin of the structure, and to infer the potential projectile type for the formation of the crater. The impactites have around 5 times higher abundances of platinum group element (PGE) compared to target rocks, and two orders of magnitude higher Re abundances. The weighted mean for ¹⁸⁷Os/¹⁸⁸Os ratios of impactites is 0.806, which is lower than this value for the target rocks (1.049), suggesting the presence of a meteoritic contribution. The PGE abundances and a broad negative correlation between Os contents and ¹⁸⁷Os/¹⁸⁸Os ratios show that the impactites incorporated ~0.05 wt% of a chondritic component. The endogenic or chondritic relative abundances of the HSE argue against iron meteorites as impactor for the Jeokjung-Chogye structure. Yet, the present data do not allow to unambiguously identify the projectile type (i.e., chondrite class) in the impactites because of the high proportion of the indigenous component.

Non-Magmatic Glasses

Article

May 2022
REV MINERAL GEOCHEM

OVERVIEW On Earth, natural glasses are typically produced by rapid cooling of melts, and as in the case of minerals and rocks, natural glasses can provide key information on the evolution of the Earth. However, natural glasses are products not solely terrestrial, and different formation mechanisms give rise to a variety of natural amorphous materials. In this chapter, we provide an overview of the different natural glasses of non-magmatic origin and on their formation mechanisms. We focus on natural glasses formed by mechanisms other than magmatic activity and included are metamorphic glasses and glasses produced from highly energetic events (shock metamorphism). The study of these materials has strong repercussions on planetary surface processes, paleogeography/paleoecology, and even on the origin of life.

Meteorite impact craters as hotspots for mineral resources and energy fuels: A global review

Article

Full-text available

Dec 2021

The ever-increasing recovery rate of natural resources from terrestrial impact craters over the last few decades across the globe offers new avenues for further exploration of mineral and hydrocarbon resources in such settings. As of today, 60 of the 208 terrestrial craters have been identified to host diverse resources such as hydrocarbons, metals and construction materials. The potential of craters as plausible resource contributors to the energy sector is therefore, worthy of consideration, as 42 (70%) of the 60 craters host energy resources such as oil, gas, coal, uranium, mercury, critical and major minerals as well as hydropower resources. Among others, 19 craters are of well-developed hydrocarbon reserves. Mineral deposits associated with craters are also classified similar to other mineral sources such as progenetic, syngenetic and epigenetic sources. Of these, the progenetic and syngenetic mineralization are confined to the early and late excavation stage of impact crater evolution, respectively, whereas epigenetic deposits are formed during and after the modification stage of crater formation. Thus, progenetic and syngenetic mineral deposits (like Fe, Ni, Pb, Zn and Cu) associated with craters are formed as a direct result of the impact event, whereas epigenetic deposits (e.g. hydrocarbon) are hosted by the impact structure and result from post-impact processes. In the progenetic and syngenetic deposits, the shock-wave induced fracturing and melting aid the formation of deposits, whereas in the epigenetic deposits, the highly fractured lithostratigraphic units of higher porosity and permeability, like the central elevated area (CEA) or the rim, act as the host rock. In this review, we provide a holistic view of the mineral and energy resources associated with impact craters, and use some of the remote sensing techniques to identify the mineral deposits as supplemented by a schematic model of the types of deposits formed during cratering process.

Terrestrial and extraterrestrial chemical components of early Archean impact spherule layers from Fairview Gold Mine, northern Barberton greenstone belt, South Africa

Chapter

Jun 2021

This volume represents the proceedings of the homonymous international conference on all aspects of impact cratering and planetary science, which was held in October 2019 in Brasília, Brazil. This volume contains a sizable suite of contributions dealing with regional impact records (Australia, Sweden), impact craters and impactites, early Archean impacts and geophysical characteristics of impact structures, shock metamorphic investigations, post-impact hydrothermalism, and structural geology and morphometry of impact structures—on Earth and Mars. These contributions are authored by many of the foremost impact cratering researchers. Many contributions report results from state-of-the-art investigations, for example, several that are based on electron backscatter diffraction studies, and deal with new potential chronometers and shock barometers (e.g., apatite). Established impact cratering workers and newcomers to this field will both appreciate this multifaceted, multidisciplinary collection of impact cratering studies.

Sr-Nd-Pb isotope systematics of Australasian tektites: Implications for the nature and composition of target materials and possible volatile loss of Pb

Article

Mar 2020
GEOCHIM COSMOCHIM AC

The Australasian tektite (AAT) strewn field is the largest strewn field on the Earth with about ∼10–30% coverage, both land and ocean, but a clearly identified source impact crater is absent despite the young age of AAT of ca. 790 ka. A genetic link between the Australasian tektites and their unequivocal parental materials is therefore largely impossible to establish. Nevertheless, the nature of the parental materials and the extent of volatilization can be constrained using the splash form tektites, carrying the chemical signatures of high-temperature processes, and the layered (so-called Muong Nong-type) tektites, which are less chemically homogenized and exceptionally abundant in the AAT field. New high-precision Sr, Nd and Pb isotopic measurements were obtained for a chemically and petrographically well-characterized suite of AAT, which included the Muong Nong-type (MN-AAT) with precisely known field locations in Laos and splash forms (SF-AAT) from different parts of the strewn field. In addition, optically dark and light zones of the MN-AAT were also separately analyzed. Homogeneous εNd values from −11.8 to −11.2, combined with a narrow range of two-stage Nd model ages from 1.67 to 1.72 Ga for the entire AAT suite, point to a well-mixed source, in terms of REE, of the crustal segment from which the sedimentary material for tektites was ultimately derived. The Sr isotopic data largely overlap for SF-AAT and MN-AAT (⁸⁷Sr/⁸⁶Sr = 0.71636–0.72021) and indicate Paleozoic to Mesozoic sedimentary parentage. However, late Neogene to early Quaternary re-deposition and formation of a thick silt-sized sedimentary section with vertical stratification is required to comply with ¹⁰Be data. Lead isotope systematics documents at least three different components which can perhaps be represented by different mineral phases, such as feldspar, zircon, organic matter adsorbed on young sediments etc., sorted during fluvial transport and final deposition. In addition, the SF-AAT have systematically lower Pb contents than the MN-AAT, and generally show isotopically heavier Pb isotopic ratios. This is theoretically consistent with a preferential volatilization of lighter Pb isotopes during evaporation and considerably larger Pb loss from SF-AAT when compared to MN-AAT. Nevertheless, further experimental work would be necessary to unambiguously distinguish kinetic fractionation from source mixing.

Impact cratering: The South American record – Part 1

Article

Aug 2018
CHEM ERDE-GEOCHEM

The Earth’s impact record is known to be rather limited in both time and space. There are ca. 190 impact structures currently known on Earth, representing a minor fraction of all the impact events that contributed to the initial formation of our protoplanet, and then to formation and modification of the surface of the planet. Moreover, the distribution of impact structures on Earth is manifestly uneven. One continent that stands out for its relatively small number of confirmed impact structures and impact ejecta occurrences is South America. The limited impact record for this large continent makes a robust case that there is a significant potential for further discoveries. Significant information on the impact record of South America is dispersed in different types of publications (journal articles, books, conferences abstracts, etc.), and in several languages, making it difficult to access and disseminate it among the geoscientific community. We aim to present a summary of the current knowledge of the impact record of this continent, encompassing the existing literature on the subject. It is published in two parts, with the first one covering an up-to-date introduction to impact cratering processes and to the criteria to identify/confirm an impact structure and related deposits. This is followed by a comprehensive analysis of the Brazilian impact structures. The Brazilian impact record accounts for the totality of the large structures of this kind currently confirmed in South America. The second part will examine the impact record of other countries in South America, provide information about a number of proposed impact structures, and review those that already have been discarded as not being formed by impact.

Contrasting meteoritic signatures within the Clearwater East and Clearwater West impact structures: The view from osmium isotopes

Article

Jun 2018
GEOCHIM COSMOCHIM AC

Osmium isotopes provide a powerful tool for identifying meteoritic signatures in impactites. We apply the osmium isotope method to impact melt and country rocks from the Clearwater East and Clearwater West craters located in Quebec, Canada. Impact melts from Clearwater East have ¹⁸⁷Os/¹⁸⁸Os ratios of 0.1281–0.1285. These values indicate a significant meteoritic component, which exceeds that in impact melts at all terrestrial craters studied to date, except Morokweng. Such findings align with earlier results from chromium isotopes and platinum-group elements. In contrast, impact melts from Clearwater West have ¹⁸⁷Os/¹⁸⁸Os ratios between 6.604 and 59.12. These highly radiogenic ratios are indistinguishable from the ¹⁸⁷Os/¹⁸⁸Os ratios in country rocks. Hence, osmium isotopes provide no evidence for a meteoritic component in impact melts at Clearwater West. The Clearwater craters formed in almost identical targets. Therefore, target effects cannot readily explain the stark difference between the two Clearwater craters. If melt sheet heterogeneity is similar at the two craters, the probability that melts at Clearwater West host an undetected chondritic component is < 0.2%. Multiple scenarios may explain the non-detection of a meteoritic signature at West; the possibility of a differentiated achondrite impactor could be tested using chromium isotopes. At Clearwater East, a low impact speed (<10 km s⁻¹) may best explain the unusually strong meteoritic signature. Although the signature (or its nondetection) at each crater may be related to asymmetric preservation of the impactor component, the results presented here provide further evidence that Clearwater East and Clearwater West were temporally separate impact events.

Highly siderophile element budget and osmium isotopes of Paleoarchean spherules-a spherule vs. matrix investigation

Thesis

Full-text available

Sep 2017

Olivier K. A. Heldwein

Distal impact ejecta, consisting of millimeter-sized glassy spherules, comprise the only remnants of the Archean impact record. Studying these spherule layers gives insights into the early meteorite bombardment of the Earth, its effects on the early crust, ocean, atmosphere, and life, and into the complex processes in impact vapor plumes, where these spherules are thought to have formed. Only a few dozens of Archean spherule layers are known so far. Thus, the finding of possibly up to eight new layers in the recent BARB5 drill core from Barberton Greenstone Belt (South Africa) offers a unique opportunity to delve deeper into this subject. This study concentrates on the spatial distribution of the carrier phase(s) of the meteoritic component. For this reason, a sample from the uppermost of the newly discovered layers (layer E) has been separated into spherules and matrix and investigated regarding its highly siderophile element budget (Re, Os, Ir, and Pt) and 187 Os/ 188 Os isotope systematics. Based on spherule-matrix (groundmass) separates, meteoritic components have recently been detected in all BARB5 spherule layers but none of these data were performed on pure spherule separates. Using highly siderophile element abundances and the 187 Os/ 188 Os tool, this study presents confirmation for meteoritic components within both separates. The spherules exhibit Os and Ir contents of 83.78 ppb and 70.29 ppb, respectively, and a 187 Os/ 188 Os ratio of 0.10578 ± 0.00029. These values are comparable to those found in the matrix (171.21 ppb Os; 205.46 ppb Ir; 0.10767 ± 0.00017 187 Os/ 188 Os ratio), providing evidence for the occurrence of non-spherule related carrier phase(s) of the extraterrestrial component. Kurzfassung: Distale Impakt-Auswürfe in Form von millimetergroßen Glas-Sphärulen sind die einzigen Zeugen der Impaktgeschichte des Archaikums. Das Studium dieser Ablagerungen erlaubt Einblicke in das Meteoritenbombardement der jungen Erde und dessen Auswirkungen auf die gerade entstehende Erdkruste, die Ozeane, die Atmosphäre und die Entwicklung des Lebens sowie in die komplexen Prozesse in den Impakt-Gaswolken, in denen diese Sphärulen entstanden sein sollen. Nur wenige Dutzend Sphärulenlagen aus dieser Zeit sind bisher bekannt. Umso wichtiger ist die Entdeckung von bis zu acht neuen Sphärulenlagen in dem kürzlich gezogenen Bohrkern BARB5 aus dem Barberton Grünsteingürtel in Südafrika, da sie eine 2 einzigartige Gelegenheit bietet unser Wissen in diesem Gebiet zu vertiefen. In dieser Studie wird das Hauptaugenmerk auf die räumliche Verteilung der meteoritischen Komponente(n) gelegt. Zu diesem Zweck wurde eine Probe aus der obersten der eben genannten Lagen (Lage E) in Sphärulen und ihre Tonstein-Matrix getrennt und ihr Gehalt an hochsiderophilen Element (Re, Os, Ir and Pt) sowie ihre 187 Os/ 188 Os Isotopensystematik untersucht. In Sphärulen-Matrix-Separaten konnten kürzlich meteoritische Komponenten in allen untersuchten Sphärulenlagen des BARB5 Bohrkerns nachgewiesen werden, es liegen jedoch keine Daten für reine Sphärulenproben vor. Anhand von hochsiderophilen Elementen und 187 Os/ 188 Os Isotopendaten bestätigt diese Studie das Vorhandensein von meteoritischen Spuren in beiden Separaten. Die Sphärulen zeigen Ir-und Os-Konzentrationen von 83.78 ppb beziehungsweise 70.29 ppb und ein 187 Os/ 188 Os-Verhältnis von 0.10578 ± 0.00029. Diese Werte sind vergleichbar mit jenen der Matrix (171.21 ppb Os; 205.46 ppb Ir; 0.10767 ± 0.00017 187 Os/ 188 Os Isotopenverhältnis) und liefern den Beweis, dass zumindest eine jener Phasen, die die meteoritische Komponente in sich tragen auch unabhängig von Sphärulen vorkommt.

The behavior of osmium and other siderophile elements during impacts: Insights from the Ries impact structure and central European tektites

Article

Apr 2017
GEOCHIM COSMOCHIM AC

Impact processes are natural phenomena that contribute to a variety of physico–chemical mechanisms over an extreme range of shock pressures and temperatures, otherwise seldomly achieved in the Earth’s crust through other processes. Under these extreme conditions with transient temperatures and pressures ≥3,000K and ≥100 GPa, followed by their rapid decrease, the behavior of elements has remained poorly understood. Distal glassy ejecta (tektites) were produced in early phases of contact between the Earth’s surface and an impacting body. Here we provide evidence for a complex behavior of Os and other highly siderophile elements (HSE; Ir, Ru, Pt, Pd, and Re) during tektite production related to a hyper-velocity impact that formed the Ries structure in Germany. Instead of simple mixing between the surface materials, which are thought to form the major source of central European tektites (moldavites), and impactor matter, the patterns of HSE contents and ¹⁸⁷Re/¹⁸⁸Os – ¹⁸⁷Os/¹⁸⁸Os ratios in moldavites, target sediments and impact-related breccias (suevites) can be explained by several sequential and/or contemporary processes. These involve (i) evaporative loss of partially oxidized HSE from the overheated tektite melt, (ii) mixing of target-derived and impactor-derived HSE vapor (plasma) phases, and (iii) early (high-temperature) condensation of a part of the mixed vapor phase back to silicate melt droplets. An almost complete loss of terrestrial Os from the tektite melt and its replacement with extra-terrestrial Os are indicated by low ¹⁸⁷Os/¹⁸⁸Os ratios in tektites (<0.163) relative to precursor materials (>0.69). This is paralleled by a co-variation between Os and Ni contents in tektites but not in suevites formed later in the impact process.

Rare metals on shatter cone surfaces from the Steinheim Basin (SW Germany) – Remnants of the impacting body?

Article

Feb 2017

The ~3.8 km Steinheim Basin in SW Germany is a well-preserved complex impact structure characterized by a prominent central uplift and well-developed shatter cones that occur in different shocked target lithologies. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and electron probe microanalysis have revealed, for the first time, the occurrence of rare metals on the Steinheim shatter cone surfaces. Shatter cones produced from the Middle Jurassic (Aalenian) Opalinus Claystone (‘Opalinuston’), temporarily exposed in the central uplift in spring 2010, and shatter cones in Upper Jurassic (Oxfordian) limestones from the southeastern crater rim domain are commonly covered by faint coatings. The Opalinus Claystone shatter cone surfaces carry coatings dominated by Fe, Ca, P, S, and Al, and are covered by abundant small, finely dispersed micro-particles and aggregates of native gold, as well as locally elevated concentrations of Pt. On several surfaces of the claystone shatter cones, additional Fe, Ni, and Co was detected. The Ca-Mn-rich coatings on the limestone shatter cone surfaces locally include patches of Fe, Ni, Co, Cu, and Au in variable amounts and proportions. The intriguing coatings on the Steinheim shatter cones could either stem from the impacted Lower Jurassic to Paleogene sedimentary target rocks; from the crystalline-metamorphic Variscan crater basement; or, alternatively, these coatings might represent altered meteoritic matter from the Steinheim impactor, possibly an iron meteorite, which may have been remobilized during post-impact hydrothermal activity. We here discuss the most plausible source for the rare metals found adherent to the shatter cone surfaces.

Geochemistry of impact glasses and target rocks from the Zhamanshin Impact Structure, Kazakhstan: Implications for mixing of target and impactor matter

Article

Jul 2016
GEOCHIM COSMOCHIM AC

Internal structure and element chemistry including contents of highly siderophile elements (HSE) and Os isotope ratios have been studied in target rocks and several groups of impact glasses of the Zhamanshin impact structure, Kazakhstan. These include larger irregularly-shaped fragments and blocks of impact glass (zhamanshinite), and three types of tektite-like splash-form glasses, part of fallback ejecta. These glassy objects typically are up to 30 mm large and are shaped as teardrops, irregularly bent and curved glass rods and fibers. They can be subdivided into acidic types (irghizites; typically 69–76 wt.% SiO2), basic splash-forms (typically 53–56 wt.% SiO2), and rarely occurring highly inhomogeneous composites with abundant mineral inclusions. A comparison with the target rocks shows that zhamanshinites and basic splash-forms usually have no detectable admixture of the projectile matter, indicated by major and trace elements as well as highly siderophile element contents, with an exception of one sample containing Fe-, Cr-, Ni- and Ti-enriched particles and elevated HSE contents. In contrast, irghizites exhibit clear admixture of the projectile matter, which was incorporated by complex processes accompanied by strong element fractionations. Microscopic investigations confirm that irghizites were formed mainly by coalescence of smaller molten glass droplets sized typically below 1 mm. Irghizites exhibit significant enrichments in Ni, Co and Cr, whose concentrations are locally enriched in the rims of the original small droplets. A portion of these elements and also part of Fe and Mn and other elements were derived from the impactor, most likely a Ni-rich carbonaceous chondrite. The contents of HSE are low and strongly fractionated, with moderate depletions of Pt and Pd and strong depletions of other HSE with respect to chondritic element ratios. Osmium shows the strongest depletion, likely related to the presence of oxygen in the post-impact atmosphere causing strong Os loss through volatilization. One composite splash-form contains Fe–Ni–S inclusions and exhibits a less fractionated HSE pattern suggesting the lowest degree of melting, volatilization and condensation. The observed structural and microchemical features of irghizites are interpreted to reflect variable proportions of the uppermost target sediments and the projectile matter, with HSE element ratios influenced by evaporation and condensation processes, and differences in volatility of individual HSE elements and/or their compounds. Two possible pathways of incorporation of the projectile matter into the irghizites include either re-condensation of evaporated projectile matter on the surface of glass droplets, or incorporation of less chemically fractionated microparticles dispersed by the explosion.

A 650 km2 Miocene strewnfield of splash-form impact glasses in the Atacama Desert, Chile

Article

Sep 2021
EARTH PLANET SC LETT

Glassy ejecta are associated to a limited number of impact craters, and yet hold key information about hypervelocity impact processes. Here we report on the discovery of a ∼650 km² impact glass strewnfield in the Central Depression of the Atacama Desert. These cm-sized splash-form objects, that we refer to as atacamaites, are essentially composed of a dacitic glass formed by high-temperature melting of local magmatic rocks, with the addition of a variable iron meteorite contamination, 5 wt.% on average. The most likely nature for the impactor is the IIAB iron group. The fission-track plateau method, on two samples, yielded a mean formation age of 7.83±0.26 Ma. No associated impact crater has been discovered so far, suggesting it may be a relatively small, km-sized crater. The glassy nature, aerodynamic shapes, elevated formation temperature, and low water content are reminiscent of tektites. However, their small size, heterogeneity, oxidation state, significant contamination by the impactor, and likely more proximal provenance distinguish them from tektites. Atacamaites have no equivalent among the few known terrestrial ejected impact glasses, and increase the intriguing diversity of such products that we propose to name “tektoids”.

Impact glasses from Belize represent tektites from the Pleistocene Pantasma impact crater in Nicaragua

Article

Full-text available

May 2021

Tektites are terrestrial impact-generated glasses that are ejected long distance (up to 11,000 km), share unique characteristics and have a poorly understood formation process. Only four tektite strewn-fields are known, and three of them are sourced from known impact craters. Here we show that the recently discovered Pantasma impact crater (14 km diameter) in Nicaragua is the source of an impact glass strewn-field documented in Belize 530 km away. Their cogenesis is documented by coincidental ages, at 804 ± 9 ka, as well as consistent elemental compositions and isotopic ratios. The Belize impact glass share many characteristics with known tektites but also present several peculiar features. We propose that these glasses represent a previously unrecognized tektite strewn-field. These discoveries shed new light on the tektite formation process, which may be more common than previously claimed, as most known Pleistocene >10 km diameter cratering events have generated tektites.

Fallout melt debris and aerodynamically-shaped glasses in beach sands of Hiroshima Bay, Japan

Article

Feb 2019

A complex association of millimeter-sized, aerodynamically-shaped debris, including glass spherules, glass filaments, and composite-fused melt particles was recovered from beach sands on the shores of the Motoujina Peninsula in Hiroshima Bay, Japan. Based on optical microscopy, this debris comprises six morphological groups ranging from clear glasses to rubber-like constituents. Scanning electron microscopy and synchrotron X-ray microdiffraction revealed dominant aluminum, silicon and calcium (Al-Si-Ca) elemental composition with some iron, mainly in glass, associated with precipitates of mullite and anorthite microcrystals, hematite dendrites and iron-chromium globules, indicative of original temperature conditions >1800 °C. Aerodynamically-shaped fallout debris, including glass spherules described in this study, are generally produced by single high-energy catastrophic events, such as an extraterrestrial body impacting Earth or a nuclear explosion. This study interprets the large volumes of fallout debris generated under extreme temperature conditions as products of the Hiroshima August 6th, 1945 atomic bomb aerial detonation. The chemical composition of the melt debris provides clues to their origin, particularly with regard to city building materials. This study is the first published record and description of fallout resulting from the destruction of an urban environment by atomic bombing.

Impact-crater ages and micrometeorite paleofluxes compared: Evidence for the importance of ordinary chondrites in the flux of meteorites and asteroids to Earth over the past 500 million years

Article

Full-text available

Mar 2022

Although the ~200 impact craters known on Earth represent only a small fraction of the craters originally formed, the available data suggest an excess of craters by one order of magnitude, in number, in the interval ca. 470-440 Ma during the Ordovician. Most of these "excess" craters may be related to the breakup of the L-chondrite parent body (LCPB) in the asteroid belt at 465.8 ± 0.3 Ma. This is the only obvious peak in the crater-age record that can currently be attributed to an asteroid breakup and shower event. Spatial crater densities in regions with high potential for crater preservation (e.g., Canada and Scandinavia) support a one order-of-magnitude increase in the flux of large (>0.1 km) impactors following the LCPB breakup. A similar pattern as seen in the cratering record is emerging in studies of the flux of micrometeoritic chrome spinel through the Phanerozoic, with so far only one major spike in the flux, and associated with the LCPB breakup. Similarly, the record of K-Ar and (U-Th)/He gas retention ages of recently fallen meteorites only locates one major breakup, the LCPB event, during the Phanerozoic. On the other hand, astronomical backtracking studies of the *birger.schmitz@nuclear.lu.se. Schmitz, B., Schmieder, M., Liao, S., Martin, E., and Terfelt, F., 2022, Impact-crater ages and micrometeorite paleofluxes compared: Evidence for the importance of ordinary chondrites in the flux of meteorites and asteroids to Earth over the past 500 million years, in Koeberl, C., Claeys, P., and Montanari, A., eds.,

Branches of Geochemistry

Book

Nov 2021

Osama Rahil Shaltami

The wide field of geochemistry includes: (1) Elemental geochemistry; (2) Mineral geochemistry; (3) Isotope geochemistry; (4) Cosmochemistry; (5) Geochemistry of igneous rocks; (6) Geochemistry of metamorphic rocks; (7) Geochemistry of sedimentary rocks; (8) Ore geochemistry; (9) Soil geochemistry; (10) Chemostratigraphy; (11) Biogeochemistry; (12) Photogeochemistry; (13) Hydrochemistry; (14) Atmospheric geochemistry; (15) Climate geochemistry; (16) Organic geochemistry; (17) Source rock geochemistry; (18) Reservoir geochemistry; (19) Coal geochemistry; (20) Environmental geochemistry; (21) Industrial geochemistry; (22) Medical geochemistry; (23) Analytical geochemistry; (24) Experimental geochemistry; (25) Exploration geochemistry; and (26) Geochemical engineering. Geochemistry has applications in many fields such as medicine, climate, environment, water quality, petroleum, mineral deposits, age dating, etc. This book is an explanation of the basics of the geochemical branches mentioned above.

Potassium elemental and isotope constraints on the formation of tektites and element loss during impacts

Article

Jul 2021
GEOCHIM COSMOCHIM AC

Potassium elemental and isotope systematics were investigated for a suite of central European tektites from three strewn sub-fields in Czech Republic and possible parent sedimentary materials from the vicinity of the Ries impact structure in SE Germany, supplemented by data for several other impact-related materials (bediasites, Ivory Coast tektites, Libyan Desert Glass). This is paralleled by computation of potential K loss and attendant isotope fractionation for physico–chemical conditions typical for formation of tektite precursor melts. These theoretical calculations indicate a <0.1% loss of K from tektite precursor melts up to 2,500K and <0.002‰ change in the ⁴¹K/³⁹K ratio even for a small sphere of 0.002 m at 2,500K, precluding any significant K loss and isotope fractionation. Numerical modelling also indicates that differential velocities between surrounding gas and liquid are not sufficient to remove the gaseous boundary layer, such that the partial pressure of potassium developed around the molten moldavite beads impedes further evaporation and also contributes to back-condensation of the already evaporated potassium. Central European tektites (moldavites) are enriched in K compared to the assumed sedimentary sources from the wider Ries area whereby the latter materials do not exceed 2.9 wt.% K2O compared to 2.5–4.1 wt.% K2O in moldavites. The apparent K enrichment in moldavites may be explained by a yet unaccounted process during formation of tektite precursor melts and/or unidentified source, such as volcanoclastic deposits that were produced by large Mid-Miocene volcanic centers in the Pannonian Basin. The K isotope compositions of tektites are more variable than those of sediments from the wider Ries area but they largely overlap (δ⁴¹K from −0.78 ± 0.03‰ to −0.13 ± 0.03‰ versus −0.72 ± 0.03‰ to −0.28 ± 0.02‰, respectively). These ranges mimic ⁴¹K/³⁹K variations reported for igneous and sedimentary portions of the upper continental crust (δ⁴¹K roughly between −0.7 and −0.1‰). They show a slight difference among the three investigated strewn sub-fields, depending on their respective distance from the impact. In detail, moldavites from the closest strewn sub-field in the Cheb Basin show predominantly heavy K isotope compositions and those from the farthest strewn sub-field in Western Moravia are uniformly isotopically light. The origin of this difference may reflect lithological heterogeneity of the target area. Potassium contents in bediasites and Ivory Coast tektites range between 1.3 and 1.8 wt.% K2O and their corresponding δ⁴¹K values vary from −0.57 ± 0.02‰ to −0.41 ± 0.03‰. Both ranges are significantly narrower than those observed for moldavites. When compared to data for possible sedimentary precursors in the Chesapeake Bay and Bosumtwi impact structure, respectively, it is apparent that these tektites were neither depleted nor enriched in potassium. The extent of their K isotope fractionation relative to plausible sources remains unconstrained. The Libyan Desert Glass displays invariant δ⁴¹K of ∼ −0.57 ± 0.06‰ at ≤0.01 wt.% K2O. Given the silica-rich nature of LDG and the lack of possible parent materials, no further constraints can be placed at present to further resolve the source material or reveal details of LDG formation process.

Geochemistry and Cosmochemistry of Potassium Stable Isotopes

Article

Jun 2021
CHEM ERDE-GEOCHEM

Stable potassium isotopes are one of the emerging non-traditional isotope systems enabled in recent years by the advance of Multi-Collector Inductively-Coupled-Plasma Mass-Spectrometry (MC-ICP-MS). In this review, we first summarize the geochemical and cosmochemical properties of K, its major reservoirs, and the analytical methods of K isotopes. Following this, we review recent literature on K isotope applications in the fields of geochemistry and cosmochemistry. Geochemically, K is a highly incompatible lithophile element, and a highly soluble, biophile element. The isotopic fractionation of K is relatively small during magmatic processes such as partial melting and fractional crystallization, whereas during low-temperature and biological processes fractionation is considerably larger. This resolvable fractionation has made K isotopes promising tracers for a variety of Earth and environmental processes, including chemical weathering, low-temperature alteration of igneous rocks, reverse weathering, and the recycling of sediments into the mantle during subduction. Sorption and interactions of aqueous K with different clay minerals during cation exchange and clay formation are likely to be of fundamental significance in generating much of the K isotope variability seen in samples from the Earth surface and samples carrying recycled surface materials from the deep Earth. The magnitude of this fractionation is process- and mineral-dependent. Comprehensive quantification of pertinent K isotope fractionation factors is currently lacking and urgently needed. Significant fractionation during biological activities, such as plant uptake, demonstrates the potential utility of K isotopes in the study of the nutrient cycle and its relation to the climate and various ecosystems, enabling new and largely unexplored avenues for future research. Of significant importance to the cosmochemistry community, K is a moderately volatile element with large variations in K/U ratio observed among chondrites and planetary materials. As this indicates different degrees of volatile depletion, it has become a fundamental chemical signature of both chondritic and planetary bodies. This volatile depletion has been attributed to various processes such as solar nebula condensation, mixing of volatile-rich and -poor reservoirs, planetary accretional volatilization via impacts, and/or magma ocean degassing. While K isotopes have the potential to distinguish these different processes, the current results are still highly debated. A good correlation between the K isotope compositions of four differentiated bodies (Earth, Mars, Moon, and Vesta) and their masses suggests a ubiquitous volatile depletion mechanism during the formation of the terrestrial planets. It is still unknown whether any of the K isotopic variation among chondrites and differentiated bodies can be attributed to inherited signatures of mass-independent isotopic anomalies.

An evolutionary system of mineralogy, Part IV: Planetesimal differentiation and impact mineralization (4566 to 4560 Ma)

Article

May 2021
AM MINERAL

The fourth installment of the evolutionary system of mineralogy considers two stages of planetesimal mineralogy that occurred early in the history of the solar nebula, commencing by 4.566 Ga and lasting for at least 5 million years: (1) primary igneous minerals derived from planetesimal melting and differentiation into core, mantle, and basaltic components and (2) impact mineralization resulting in shock-induced deformation, brecciation, melting, and high-pressure phase transformations. We tabulate 90 igneous differentiated asteroidal minerals, including the earliest known occurrences of minerals with Ba, Cl, Cu, F, and V as essential elements, as well as the first appearances of numerous phosphates, quartz, zircon, and amphibole group minerals. We also record 40 minerals formed through high-pressure impact alteration, commencing with the period of asteroid accretion and differentiation. These stages of mineral evolution thus mark the first time that high pressures, both static and dynamic, played a significant role in mineral paragenesis.

Crisis of the meteorite paradigm: craters, tektites, the Tunguska event

Book

Full-text available

Dec 2019

Boris R. German

In the book, questions of Earth's craters exploration and origin of tektites on our planet are brought up. It is shown that our knowledge in this field remains limited so far and demand revision of the data interpretation. In the context of the Tunguska 1908 phenomenon, we suppose correlations between the Perm LLSVPs Anomaly and the explosion of paleovolcano in Siberia on 30 June 1908. We discuss magnetic pulsations in Kiel, Germany observed from 27 till 30 June 1908, a variation of gravitational constant during the solar eclipse on 28 June 1908, and the discrepancy of Moon longitude at the beginning of the 20th century. The non-typical registrations in Tasmania in the period of the Tunguska 1908 phenomenon and his connection with the Perm LLSVPs anomalies provide a highly probable solution to one of the greatest mysteries of the previous century. For astro- and geophysicists and a wide range of readers who are interested in meteorites and planetology.

Whether there is impact meteorite paradigm crisis?

Poster

Full-text available

May 2019

Boris R. German

The reference of craters to astroblems according to the PGEs/siderophiles concentrations in their melted rocks could be unreliable because these elements are present in the earth сore/mantle. Despite the consensus, we doubt that the determination of cosmic components in the craters via the chromium isotope ratio is correct as well. Hence, such structures as the Zhamanshin, the Clearwater East, and so on, can be geoblems. In this case, the origin of dense mineral phases is probably to be connected to explosive mantle plumes since the centers of earthquakes are fixed to a depth of 700 km.

Mysterious long-living ultrahigh-pressure or secondary impact crisis

Article

Full-text available

Feb 2020

High-pressure glass has attracted interest in terms of both its fundamental state under extreme conditions and its possible applications as an advanced material. In this context, natural impact glasses are of considerable interest because they are formed under ultrahigh-pressure and high-temperature (UHPHT) conditions in larger volumes than laboratory fabrication can produce. Studying the UHPHT glasses of the unique giant Kara astrobleme (Russia), we found that the specific geological position of the UHPHT melt glass veins points to an origin from a secondary ultrahigh-pressure (UHP) melt according to the characteristics of the host suevites, which suggest later bottom flow. Here, we propose a fundamentally novel model involving an upward-injected UHP melt complex with complicated multi-level and multi-process differentiation based on observations of the UHP silica glass, single-crystal coesite and related UHP smectite that crystallized from an impact-generated hydrous melt. This model proposes a secondary UHP crisis during the modification stage of the Kara crater formation. The results are very important for addressing fundamental problems in fields as diverse as condensed matter states under extreme pressure and temperature (PT) conditions, material and geological reconstructions of impact structures, water conditions in mineral substances under UHP conditions in the deep Earth, and the duration and magnitude of the catastrophic effects of large asteroid impacts.

Varieties of Impactites and Impact Diamonds of the Kara Meteorite Crater (Pay-Khoy, Russia)

Article

Full-text available

Nov 2019

Impact diamonds are technical material with valuable mechanical properties. Despite of a quite long story from their discovery and huge diamond storages at the Popigai astrobleme (Siberia, Russia) they were not involved into industrial production, first of all because of remoteness of objects, complexity of extraction and economically more favourable synthesis of technical diamonds in the seventies of the past century. However, due to the high hardness of impact diamonds and also to the high demand of new carbon materials, including nanomaterials, the interest towards this type of natural diamonds is significantly increased in the recent years. Although the mentioned Popigai astrobleme is situated in a remote part of Russia it has been studied in more details. At the same time, the less known Kara giant meteorite crater (Pay-Khoy, Russia) is situated essentially closer to the industrial infrastructure of the European part of Russia. This astrobleme, similarly to Popigai, is enriched in impact diamonds as well. But, till recent years it was not deeply studied using modern analytical methods. During our studies in 2015 and 2017 at the territory of the Kara meteorite crater we have distinguished and described 5 varieties of impactites – bulk melt impactites which form cover-like and thick dike bodies; melt ultrahigh-pressure vein bodies and at least 3 types of suevites formed after specific sedimentary target rocks. These varieties have typomorphic features regarding the crystallinity and mineral composition. It was found that all of them have high concentration of microdiamonds formed by high-pressure high temperature pyrolysis mechanism from precursor materials like coal and organic relicts. Using a set of modern mineralogical methods we have found two principal types of diamond morphologies within the Kara impactites – sugar-like after coal diamonds and diamond paramorphs after organic relicts. The Kara diamonds have several accompanying carbon substances including newly formed graphite, glass-like carbon and probably carbyne. The studied diamondiferous Kara impactites provide an essentially novel knowledge of impact processes in sedimentary targets.

Earth's Impact Events Through Geologic Time: A List of Recommended Ages for Terrestrial Impact Structures and Deposits

Article

Full-text available

Jan 2020

This paper presents a current (as of September 2019) list of recommended ages for proven terrestrial impact structures (n=200) and deposits (n=46) sourced from the primary literature. High-precision impact ages can be used to (1) reconstruct and quantify the impact flux in the inner Solar System and, in particular, the Earth-Moon system, thereby placing constraints on the delivery of extraterrestrial mass accreted on Earth through geologic time; (2) utilize impact ejecta as event markers in the stratigraphic record and to refine bio- and magnetostratigraphy; (3) test models and hypotheses of synchronous double or multiple impact events in the terrestrial record; (4) assess the potential link between large impacts, mass extinctions, and diversification events in the biosphere; and (5) constrain the duration of melt sheet crystallization in large impact basins and the lifetime of hydrothermal systems in cooling impact craters, which may have served as habitats for microbial life on the early Earth and, possibly, Mars.

Implications of K, Cu and Zn isotopes for the formation of tektites

Article

Jun 2019
GEOCHIM COSMOCHIM AC

Tektites are mm to cm sized glassy objects generated through high-energy meteoroid impacts on the surface of the Earth under high temperature and pressure, and reducing conditions. They are the products of large-scale catastrophic events in Earth’s history and can be used to understand the behavior of moderately volatile elements (e.g., K and Zn) during impact vaporization events. Here, we report bulk K isotopic compositions of tektites from three different strewn fields and “in-situ” profile analysis of both K and Zn isotopes in one complete tektite. All tektites span a narrow range in their K isotopic compositions (δ⁴¹KBSE: −0.10 ± 0.03‰ to 0.16 ± 0.04‰), revealing no discernible K isotopic fractionation from the Bulk Silicate Earth (BSE) and upper continental crust materials, which is consistent with previous results. In contrast, Zn isotopes show a large variation (δ⁶⁶Zn: −0.39 ± 0.02‰ to 2.38 ± 0.03‰) even within one specimen. In order to provide a coherent explanation for the different behavior of moderately volatile elements (K, Zn and Cu), we have conducted thermochemical calculations to compute the partial vapor pressures of Cu2O, K2O, and ZnO dissolved in silicate melts as a function of temperature, pressure, oxygen and chlorine fugacities. In a large range of the parameter space, the calculations show that Cu and Zn can be vaporized much easier than K and thus produce large isotopic fractionation. In contrast, the lithophile element K is more prone to remain in the silicate melt because of its very low activity coefficient in the melt, and thus the K isotopes remain unfractionated. This study provides new constraints on the formation of tektites and is consistent with a “bubble-stripping” model to explain the extreme water and volatiles depletion in tektites.

Meteoritic highly siderophile element and Re-Os isotope signatures of Archean spherule layers from the CT3 drill core, Barberton Greenstone Belt, South Africa

Article

Full-text available

Jan 2019

Archean spherule layers represent the only currently known remnants of the early impact record on Earth. Based on the lunar cratering record, the small number of spherule layers identified so far contrasts to the high impact flux that can be expected for the Earth at that time. The recent discovery of several Paleoarchean spherule layers in the BARB5 and CT3 drill cores from the Barberton area, South Africa, drastically increases the number of known Archean impact spherule layers and may provide a unique opportunity to improve our knowledge of the impact record on the early Earth. This study is focused on the spherule layers in the CT3 drill core from the northeastern Barberton Greenstone Belt. We present highly siderophile element (HSE: Re, Os, Ir, Pt, Ru, and Pd) concentrations and Re‐Os isotope signatures for spherule layer samples and their host rocks in order to unravel the potential presence of extraterrestrial fingerprints within them. Most spherule layer samples exhibit extreme enrichments in HSE concentrations of up to superchondritic abundances in conjunction with, in some cases, subchondritic present‐day ¹⁸⁷Os/¹⁸⁸Os isotope ratios. This indicates a significant meteoritic contribution to the spherule layers. In contrast to some of the data reported earlier for other Archean spherule layers from the Barberton area, the CT3 core is significantly overprinted by secondary events. However, HSE and Re‐Os isotope signatures presented in this study indicate chondritic admixtures of up to (and even above) 100% chondrite component in some of the analyzed spherule layers. There is no significant correlation between HSE abundances and respective spherule contents. Although strongly supporting the impact origin of these layers and the presence of significant meteoritic admixtures, peak HSE concentrations are difficult to explain without postdepositional enrichment processes.

Impact Structure

Preprint

Full-text available

Jun 2015

Anatomy of impactites and shocked zircon grains from Dhala reveals Paleoproterozoic meteorite impact in the Archean basement rocks of Central India

Article

Full-text available

Oct 2017
GONDWANA RES

The Dhala structure in Central India has been a topic of global interest ever since the report of an ancient meteorite impact event there. Here we present an integrated study of the petrology, geochemistry, and zircon U-Pb zircon geochronology and rare earth element geochemistry from the structure along with and an analysis of the grain morphology and textural features. Our results provide new insight into the nature and timing of the impact event. The zircon grains from the impactites show textures typical of shock deformation which we correlate with the impact event. We also identified the presence of reidite based on Raman spectroscopy and characteristics such as a persistent planar fracture, bright backscattered electron images, and a lack of zoning, which are all diagnostic features of this mineral formed during an impact event. Our zircon U-Pb data from the various rock types in the basement show magma emplacement at ca. 2.5–2.47 Ga, and the Pb loss features suggest that the impact might have occurred between ca. 2.44 Ga and ca. 2.24 Ga. Another minor group of late Paleoproterozoic zircons with concordant ages of 1826 and 1767 Ma in the brecciated quartz reefs along the margins of the impact crater from unfractured grains represent an younger thermal event after the impact. The rare-earth element patterns of the Neoarchean to early Paleoproterozoic zircon population reflect the effects of hydrothermal alteration on a peralkaline host rock. The abnormally high concentration of K2O in the impactite (up to 15.91 wt%), is also consistent with metasomatic alteration associated with the impact event.

The most distal moldavite findings from Lower Silesia, Poland

Article

Oct 2017

Tomasz Brachaniec

The present note reports new findings of moldavites from southwestern Poland. The material was found in the Nowa Wieś Kącka sandpit. To date, it represents the most distal locality where moldavites have been found. These moldavites, like other moldavites previously described from Lower Silesia, are recovered from fluvial sands and gravels of the Gozdnica Formation. Like other Polish moldavites, the moldavites in this study display high SiO2 contents (~77 wt.%). Their dimensions range from 9 to 11 mm in maximum diameter. Their relatively large sizes suggest that the distribution of Polish tektites defines a sub-strewnfield larger than previously expected.

Implications for behavior of volatile elements during impacts—Zinc and copper systematics in sediments from the Ries impact structure and central European tektites

Article

Jul 2017

Moldavites are tektites genetically related to the Ries impact structure, located in Central Europe, but the source materials and the processes related to the chemical fractionation of moldavites are not fully constrained. To further understand moldavite genesis, the Cu and Zn abundances and isotope compositions were measured in a suite of tektites from four different substrewn fields (South Bohemia, Moravia, Cheb Basin, Lusatia) and chemically diverse sediments from the surroundings of the Ries impact structure. Moldavites are slightly depleted in Zn (~10–20%) and distinctly depleted in Cu (>90%) relative to supposed sedimentary precursors. Moreover, the moldavites show a wide range in δ66Zn values between 1.7 and 3.7‰ (relative to JMC 3-0749 Lyon) and δ65Cu values between 1.6 and 12.5‰ (relative to NIST SRM 976) and are thus enriched in heavy isotopes relative to their possible parent sedimentary sources (δ66Zn = −0.07 to +0.64‰; δ65Cu = −0.4 to +0.7‰). In particular, the Cheb Basin moldavites show some of the highest δ65Cu values (up to 12.5‰) ever observed in natural samples. The relative magnitude of isotope fractionation for Cu and Zn seen here is opposite to oxygen-poor environments such as the Moon where Zn is significantly more isotopically fractionated than Cu. One possibility is that monovalent Cu diffuses faster than divalent Zn in the reduced melt and diffusion will not affect the extent of Zn isotope fractionation. These observations imply that the capability of forming a redox environment may aid in volatilizing some elements, accompanied by isotope fractionation, during the impact process. The greater extent of elemental depletion, coupled with isotope fractionation of more refractory Cu relative to Zn, may also hinge on the presence of carbonyl species of transition metals and electromagnetic charge, which could exist in the impact-induced high-velocity jet of vapor and melts.

Reconnaissance survey of the Duolun ring structure in Inner Mongolia: Not an impact structure

Article

Full-text available

Apr 2017
METEORIT PLANET SCI

New constraints on the Paleoarchean meteorite bombardment of the Earth - Geochemistry and Re-Os isotope signatures of spherule layers in the BARB5 ICDP drill core from the Barberton Greenstone Belt, South Africa

Article

Jun 2017
GEOCHIM COSMOCHIM AC

Archean spherule layers, resulting from impacts by large extraterrestrial objects, to date represent the only remnants of the early meteorite, asteroid, and comet bombardment of the Earth. Only few Archean impact debris layers have been documented, all of them embedded in the 3.23–3.47 billion year old successions of the Barberton Greenstone Belt (BGB) in South Africa and the Pilbara Craton in Western Australia. Some of them might be correlated with each other. Given the scarcity of Archean spherule deposits, four spherule layer intersections from the recently recovered BARB5 drill core from the central Barberton Greenstone Belt, analyzed in this study, provide an opportunity to gain new insight into the early terrestrial impact bombardment. Despite being hydrothermally overprinted, siderophile element abundance signatures of spherule-rich samples from the BARB5 drill core, at least in part, retained a meteoritic fingerprint. The impact hypothesis for the generation of the BARB5 spherule layers is supported by correlations between the abundances of moderately (Cr, Co, Ni) and highly siderophile (Re, Os, Ir, Pt, Ru and Pd) elements, whose peak concentrations and interelement ratios are within the range of those for chondrites. Rhenium-Osmium isotope evidence further support the impact hypothesis. Collectively, this study provides evidence for extraterrestrial admixtures ranging between ∼40 and up to 100% to three of the four analyzed BARB5 spherule layers, and a scenario for their genesis involving (i) impact of a chondritic bolide into a sedimentary target, (ii) varying admixtures of meteoritic components to target materials, (iii) spherule formation via condensation in an impact vapor plume, (iv) transportation of the spherules and sedimentation under submarine conditions, followed by (v) moderate post-impact remobilization of transition metals and highly siderophile elements.

Cathodoluminescence of moldavites

Article

Mar 2017
METEORIT PLANET SCI

A systematic study of a large set of moldavites and the application of cathodoluminescence (CL)-spectroscopy with a detailed discussion of spectral features is presented. Optical CL microscopy and spectroscopy (OM-CL) were performed on 57 moldavite samples from different substrewn-fields in Germany and the Czech Republic. The extracted CL data were supported by SEM-EDX analysis. In general, two different kinds of CL colors can be distinguished: different shades of green in the matrix of the tektite glasses and a variation of blue color for lechatelierite inclusions (a pure silica-glass phase). Spectral analysis of these colors shows three CL emission bands for green and five bands for blue c. Most CL activators are structural defects of the local glass network, influenced by the crystal field. The visible green CL emission is caused by defects related to strong local disorder as well as Al-O−-Al defects. The blue CL emission is activated by different types of lattice defects such as nonbridging oxygen-hole center (NBOHC), self-trapped excitons (STE), and oxygen deficiency centers (ODC). Intensity variations of the CL emissions were observed for samples from the different localities, but there is no direct correlation between substrewn-fields and CL characteristics. Nevertheless, CL microscopy is a powerful tool for the high-contrast visualization of internal textures such as streaks and lechatelierite in the tektite matrix due to the luminescence properties of the defect structures in the glassy network.

Chromium isotope evidence in ejecta deposits for the nature of Paleoproterozoic impactors

Article

Dec 2016
EARTH PLANET SC LETT

Non-mass dependent chromium isotopic signatures have been successfully used to determine the presence and identification of extra-terrestrial materials in terrestrial impact rocks. Paleoproterozoic spherule layers from Greenland (Graenseso) and Russia (Zaonega), as well as some distal ejecta deposits (Lake Superior region) from the Sudbury impact (1,849 +/- 0.3 Ma) event, have been analyzed for their Cr isotope compositions. Our results suggest that 1) these distal ejecta deposits are all of impact origin, 2) the Graenseso and Zaonega spherule layers contain a distinct carbonaceous chondrite component, and are possibly related to the same impact event, which could be Vredefort (2,023 +/- 4 Ma) or another not yet identified large impact event from that of similar age, and 3) the Sudbury ejecta record a complex meteoritic signature, which is different from the Graenseso and Zaonega spherule layers, and could indicate the impact of a heterogeneous chondritic body.

The fate of moderately volatile elements in impact events—Lithium connection between the Ries sediments and central European tektites

Article

Oct 2016

Lithium abundances and isotope compositions are presented for a suite of sediments from the surroundings of the Ries Impact structure, paralleled by new Li data for central European tektites (moldavites) from several substrewn fields (South Bohemia, Moravia, Cheb Basin, Lusatia), including a specimen from the newly discovered substrewn field in Poland. The data set was supplemented by three clay fractions isolated from sedimentary samples. Moldavites measured in this study show a very narrow range in δ7Li values (−0.6 to 0.3‰ relative to L-SVEC) and Li contents (23.9–48.1 ppm). This contrasts with sediments from the Ries area which show remarkable range in Li isotope compositions (from −6.9 to 13.4‰) and Li contents (0.6–256 ppm). The OSM sediments which, based on chemical similarity, formed the major part of moldavites, show a range in δ7Li values from −2.0 to 7.9‰ and Li contents from 5.8 to 78.9 ppm. Therefore, the formation of moldavites was apparently accompanied by large-scale mixing, paralleled by chemical and isotope homogenization of their parent matter. The proposed Li mixing model indicates that sands, clayey sediments, and low volumes of carbonates are the major components for tektite formation whereas residual paleokarst sediments could have been a minor but important component for a subset of moldavites. Striking homogenization of Li in tektites, combined with limited Li loss during impacts, may suggest that moderately volatile elements are not scavenged and isotopically fractionated during large-scale collisions, which is consistent with recent models. In general, whether homogenization of bodies with distinct Li isotope systematics takes place, or collision of bodies with similar Li systematics operates cannot be resolved at present stage but Li isotope homogeneity of solar system planets and asteroidal bodies tentatively implies the latter.

Critical comment on Aneeshkumar, V. et al., 2022, Lithos 426–427, 106,779, Meteorite impact at Ramgarh, India: Petrographic and geochemical evidence, and new geochronological insights

Article

Dec 2022
LITHOS

Search for a meteoritic component within the impact melt rocks of the Chicxulub impact structure peak ring, Mexico

Article

Full-text available

Feb 2022
GEOCHIM COSMOCHIM AC

Constraining the degree of preservation of a meteoritic signature within an impact structure provides vital insights in the complex pathways and processes that occur during and after an impact cratering event, providing information on the fate of the projectile. The IODP-ICDP Expedition 364 drilling recovered a ∼829 m continuous core (M0077A) of impactites and basement rocks within the ∼200-km diameter Chicxulub impact structure peak ring. No highly siderophile element (HSE) data have been reported for any of the impact melt rocks of this drill core to date. Previous work has shown that most Chicxulub impactites contain less than 0.1% of a chondritic component. Only few impact melt rock samples in previous drill cores recovered from the Chicxulub might contain such a signal. Therefore, we analyzed impact melt rock and suevite samples, as well as pre-impact lithologies of the Chicxulub peak ring, with a focus on the HSE concentrations and Re–Os isotopic compositions. Similar to the concentrations of the other major and trace elements, those of the moderately siderophile elements (Cr, Co, Ni) of impact melt rock samples primarily reflect mixing between a mafic (dolerite) and felsic (granite) components, with the incorporation of carbonate material in the upper impact melt rock unit (from 715.60 to 747.02 meters below seafloor). The HSE concentrations of the impact melt rocks and suevites are generally low (<39 ppt Ir, <96 ppt Os, <149 ppt Pt), comparable to the values of the average upper continental crust, yet three impact melt rock samples exhibit an enrichment in Os (125–410 ppt) and two of them also in Ir (250–324 ppt) by one order of magnitude relative to the other investigated samples. The ¹⁸⁷Os/¹⁸⁸Os ratios of the impact melt rocks are highly variable, ranging from 0.18 to 2.09, probably reflecting heterogenous target rock contributions to the impact melt rocks. The significant amount of mafic dolerite (mainly ∼20–60% and up to 80–90%) , which is less radiogenic (¹⁸⁷Os/¹⁸⁸Os ratio of 0.17), within the impact melt rocks makes an unambiguous identification of an extraterrestrial admixture challenging. Granite samples have unusually low ¹⁸⁷Os/¹⁸⁸Os ratios (0.16 on average), while impact melt rocks and suevites broadly follow a mixing trend between upper continental crust and chondritic/mantle material. Only one of the investigated samples of the upper impact melt rock unit could also be interpreted in terms of a highly diluted (∼0.01–0.05%) meteoritic component. Importantly, the impact melt rocks and pre-impact lithologies were affected by post-impact hydrothermal alteration processes, probably remobilizing Re and Os. The mafic contribution, explaining the least radiogenic ¹⁸⁷Os/¹⁸⁸Os values, is rather likely. The low amount of meteoritic material preserved within impactites of the Chicxulub impact structure may result from a combination of the assumed steeply-inclined trajectory of the Chicxulub impactor (enhanced vaporization, and incorporation of projectile material within the expansion plume), the impact velocity, and the volatile-rich target lithologies.

Mineralogical and geochemical specificity of carbonaceous rocks from the area of Ust'-Kara astrobleme (Pay-Khoy)

Article

Full-text available

Jan 2021

We studied Upper Paleozoic (P1pt-ng) carbonaceous shales and siltstones from the area of the Ust'-Kara astrobleme (PayKhoy). We analyzed mineralogical and geochemical features of carbonaceous rocks of the target in the vicinity of the Ust'-Kara astrobleme event using a complex of modern methods to identify possible mobilization, redeposition and concentration of ore substance under intensive post-impact hydrothermal activity. Geochemical features of carbon deposits, altered by post-impact hydrothermal processes in the vicinity of the Ust'-Kara impact structure, have been determined. We found anomalous contents of Ti, Mn, Cr, Zr, Ni, Li, Co, Sc and REE. Inherent rare metal and rare earth minerals (monazite, florensite), sulfides (pyrite, chalcopyrite, marcasite, sphalerite), apatite, barite, anatase, chrome spinels were diagnosed.

Cerro do Jarau, RS, Brazil, is a bona fide impact structure – Not a cryptoexplosion structure as alleged. [Comment on “Resurfaced paleodunes from the Botucatu erg amid Cretaceous Paraná volcanics” by , Geomorphology (2021), doi:10.1016/j.geomorph.2021.107893]

Article

Oct 2021
GEOMORPHOLOGY

Hartmann and Cervo-Alves (2021) recently discussed the Inhandui Paleodune Field in the southern Paraná Basin of southern Brazil. They particularly emphasized the Cerro do Jarau structure of 13 km diameter, which they interpreted as a “compound paleodune”. The established shock deformation in arenites from this structure (10 GPa, Reimold et al., 2019) was interpreted by them as caused by explosive hydrothermalism, that means jets of a hot water-sand mixture. This process had also been responsible for widespread silicification of Cerro do Jarau rocks. These authors alleged that such a process could generate the shock pressures in the order of 10 GPa constrained from observations of bona fide planar deformation features, in contrast to our interpretation that this evidence represents firm evidence for Cerro do Jarau representing a confirmed impact structure. Lithostatic pressures of the order of 10 GPa or more in the uppermost crust cannot be achieved, so that any endogenic degassing/hydrothermal venting model is unrealistic. This apparent contradiction of hypotheses requires a detailed look at the evidence for an impact cratering origin of Cerro do Jarau, which necessarily results in the complete rebuttal of this “explosive” hydrothermalism hypothesis.

Natural Glasses

Book

Jan 2019

Scientific Comment on Klokočník et al. “Support for two subglacial impact craters in northwest Greenland from Earth gravity model EIGEN 6C4 and other data”, Tectonophysics 780 (2020), 228396

Article

Aug 2020
TECTONOPHYSICS

Mineralogical and geochemical features of the black shales surrounding the Kara Astrobleme (Pay-Khoy)

Article

Full-text available

Apr 2020

Research subject . The Late Paleozoic (D3–P1) black shale strata surrounding the Kara Astrobleme (Pay-Khoy) were studied. Materials and methods . The rocks were sampled radially along the profile from the edge of the astrobleme (the at the contact zone with impactites) with access to black shales that were not affected by post-impact transformations. An analysis of the mineralogical and geochemical features of the black shales surrounding the Kara astrobleme was carried out using a complex of modern research methods (Geonauka Centre for Collective Use, IG FRC Komi Scientific Centre, Ural Branch of the Russian Academy of Sciences) in order to identify the possible mobilisation, re-deposition and concentration of ore matter under the conditions of intensive post-impact hydrothermal altering. Results and conclusions. The geochemical features of the black shale deposits altered by post-impact hydrothermal processes in the vicinity of the Kara impact structure were determined. The sharp abnormal contents of Mn, B, Zr, Sr, Ge, Cd, Hf, Se and Eu as well as the abnormal contents of Ti, Ba, Cr, Rb, Li, Ce, La, Ga, Sc, Co, Cs, Gd, Dy and W were revealed. The geochemical concentration specificity of components in different regions of the Kara astrobleme associated with the specialisation of target rocks was established. Raremetal and rare-earth minerals, sulphides and thymannite (HgSe) were diagnosed.

Stability of α-ketoglutaric acid simulating an impact-generated hydrothermal system: implications for prebiotic chemistry studies

Article

Jan 2020

Life originated on Earth possibly as a physicochemical process; thus, geological environments and their hypothetical characteristics on early Earth are essential for chemical evolution studies. Also, it is necessary to consider the energy sources that were available in the past and the components that could have contributed to promote chemical reactions. It has been proposed that the components could have been mineral surfaces. The aim of this work is to determine the possible role of mineral surfaces on chemical evolution, and to study of the stability of relevant molecules for metabolism, such as α-ketoglutaric acid (α-keto acid, Krebs cycle participant), using ionizing radiation and thermal energy as energy sources and mineral surfaces to promote chemical reactions. Preliminary results show α-ketoglutaric acid can be relatively stable at the simulated conditions of an impact-generated hydrothermal system; thus, those systems might have been plausible environments for chemical evolution on Earth.

A review of the Earth history record in the Cretaceous, Paleogene, and Neogene pelagic carbonates of the Umbria-Marche Apennines (Italy): Twenty-five years of the Geological Observatory of Coldigioco

Chapter

Jan 2019

Walter Alvarez

Impact Craters and Meteorites: The Egyptian Record

Chapter

Jan 2020

This chapter provides an account of the present Egyptian impact cratering record as well as an overview of the Egyptian meteorite collection. The 45-m-diameter Kamil crater in the East Uweinat District in southwestern Egypt is so far the only confirmed impact crater in Egypt. Due to its exceptional state of preservation Kamil can be considered a type-structure for small-scale impacts on Earth. Enigmatic types of natural glasses including the Libyan Desert glass found in the Great Sand Sea and the Dakhleh glass found near Dakhla Oasis (note that Dakhla, Dakhleh and Dakhlah are transliterations) may be products of low-altitude airbursts of large and fragile cometary or asteroidal impactors. A number of circular, crater-shaped geological structures superficially resembling impact craters are discussed. To date the Egyptian meteorite collection totals 2 falls, including the ~10 kg Martian meteorite Nakhla that has served as a keystone for the understanding of magmatic differentiation processes on Mars, and 76 finds. With the exception of a minority of incidental findings, most Egyptian meteorite finds (~75%) were recovered over the last ~30 years from three dense meteorite collection areas, namely the El-Shaik Fedl, Great Sand Sea and Marsa Alam fields. The exceptional exposures of the Precambrian basement and Paleozoic to Cenozoic sedimentary covers in Egypt offer a good opportunity for the identification of new impact structures. Likewise, Egypt’s vast rocky desert surfaces are of great potential for the collection of meteorites through systematic searches. These prospects are fundamental ingredients for fostering the ongoing development of meteoritics and planetary science in Egypt as disciplines for future scientific endeavor in Africa.

The Planet Earth: A Review of the Influence of Cratering on the Geological Evolution of our Planet

Chapter

Jun 2019

Karen McBride

In this chapter, we will look at some aspects of the Earth’s geological evolution from the perspective of intense meteoritic (planetesimal) bombardment and the influence that crater formation had on the crustal morphology, the weather, and ultimately the development and evolution of life. This chapter is an elaborated summary of previously published work, as noted in the citations in the text and references therein.

The quest for an extraterrestrial component in Muong Nong-type and splash-form Australasian tektites from Laos using highly siderophile elements and Re-Os isotope systematics

Article

Mar 2019
GEOCHIM COSMOCHIM AC

Extremely low and variable concentrations of osmium (Os) and other highly siderophile elements (HSE) in most tektites make it challenging to establish direct links between these impact-related materials and their possible extraterrestrial contribution. New Os concentrations (2–43 ppt) and ¹⁸⁷ Os/ ¹⁸⁸ Os ratios (0.131–0.68) in a suite of fifteen well-characterized Australasian tektites from Laos (Muong Nong and splash-form types) with variable Ni enrichment indicate a maximum of ∼0.005% addition of a chondritic impactor. This is similar to some Australasian tektites from Vietnam with similarly low siderophile contents, but significantly lower than found in previous studies of more Ni-rich Australasian splash-form tektites and microtektites from different parts of the Australasian strewn field (e.g., Indonesia, South China Sea). The contents of HSE and Re–Os isotopic compositions of layered Muong Nong-type Australasian tektites are highly variable, suggesting mingling of crustal-derived (siderophile element-poor) and extraterrestrial (siderophile element-rich) materials. The absence of a direct correlation between HSE and Ni contents is interpreted to result from a fractionation process related to their different vaporization/condensation temperatures. The low Os abundance in most of the analyzed Australasian tektites, combined with non-radiogenic ¹⁸⁷ Os/ ¹⁸⁸ Os far below average upper continental crust, may provide a direct test to distinguish continental versus seawater impact scenario. In the absence of any specific low-Os target, a particular process of Os loss following impact is required. We envisage a scenario where evaporative loss of >>90% Os in the form of Os oxides from the overheated tektite melt is aided by volatile species derived from dissociated seawater and/or saline pore water embedded in sediments off-shore Indochina, consistent with elevated contents of halogens in Australasian tektites. This water-assisted Os loss could also play significant role for Central European tektites, while the continental surface with limited amount of water would prevent from more efficient HSE loss as could be the case for Ivory Coast tektites.

The Decorah structure, northeastern Iowa: Geology and evidence for formation by meteorite impact

Article

Aug 2018

The Decorah structure, recently discovered in northeastern Iowa, now appears as an almost entirely subsurface, deeply eroded circular basin 5.6 km in diameter and ~200 m deep, that truncates a near-horizontal series of Upper Cambrian to Lower Ordovician platform sediments. Initial analysis of geological and well-drilling data indicated characteristics suggestive of meteorite impact: a circular outline, a shallow basin shape, discordance with the surrounding geology, and a filling of anomalous sediments: (1) the organic-rich Winneshiek Shale, which hosts a distinctive fossil Lagerstätte, (2) an underlying breccia composed of fragments from the surrounding lithologies, and (3) a poorly known series of sediments that includes shale and possible breccia. Quartz grains in drill samples of the breccia unit contain abundant distinctive shock-deformation features in ~1% of the individual quartz grains, chiefly planar fractures (cleavage) and planar deformation features (PDFs). These features provide convincing evidence that the Decorah structure originated by meteorite impact, and current models of meteorite crater formation indicate that it formed as a complex impact crater originally ~6 km in diameter. The subsurface characteristics of the lower portion of the structure are not well known; in particular, there is no evidence for the existence of a central uplift, a feature generally observed in impact structures of comparable size. The current estimated age of the Decorah structure (460-483 Ma) suggests that it may be associated with a group of Middle Ordovician impact craters (a terrestrial "impact spike") triggered by collisions in the asteroid belt at ca. 470 Ma.

Evidence for an impact-induced biosphere from the δ 34 S signature of sulphides in the Rochechouart impact structure, France

Article

Full-text available

Feb 2017
EARTH PLANET SC LETT

The highly eroded 23 km diameter Rochechouart impact structure, France, has extensive evidence for post-impact hydrothermal alteration and sulphide mineralisation. The sulphides can be divided into four types on the basis of their mineralogy and host rock. They range from pyrites and chalcopyrite in the underlying coherent crystalline basement to pyrites hosted in the impactites. Sulphur isotopic results show that δ 34 S values vary over a wide range, from −35.8h to +0.4h. The highest values, δ 34 S −3.7h to +0.4h, are recorded in the coherent basement, and likely represent a primary terrestrial sulphur reservoir. Sulphides with the lowest values, δ 34 S −35.8h to −5.2h, are hosted within locally brecciated and displaced parautochthonous and autochthonous impactites. Intermediate δ 34 S values of −10.7h to −1.2h are recorded in the semi-continuous monomict lithic breccia unit, differing between carbonate-hosted sulphides and intraclastic and clastic matrix-hosted sulphides. Such variable isotope values are consistent with a biological origin, via bacterial sulphate reduction, for sulphides in the parautochthonous and autochthonous units; these minerals formed in the shallow subsurface and are probably related to the post impact hydrothermal system. The source of the sulphate is likely to have been seawater, penecontemporaneous to the impact, as inferred from the marginal marine paleogeography of the structure. In other eroded impact craters that show evidence for impact-induced hydrothermal circulation, indirect evidence for life may be sought isotopically within late-stage (≤120 • C) secondary sulphides and within the shocked and brecciated basement immediately beneath the transient crater floor.

Impact Structure

Chapter

Aug 2014

Siderophile element concentrations in drill core samples from the Manson crater

Article

Full-text available

Jan 1996

Five samples from drill cores M1 and M8 from the Manson structure were analyzed by neutron activation for the contents of the siderophile elements Ru, Pd, Re, Os, Ir, Pt, and Au, as well as for Cr, Ni, and Co abundances. The study was aimed at the detection of a possible meteoritic component in the impactites, as well as the identification of the projectile type. In only one sample of impact melt rock, from the M1 core, was a small but significant enrichment in the abundances of Os and Ir found, compared to the abundances in the other four samples. This enrichment is consistent with admixture of about 0.1% of a chondritic component to the target rocks. No comparable enrichment was detected for the other siderophile elements that were analyzed, but it may be worth noting that all interelement ratios among the siderophile elements, with the exception of Au but including Ni, are chondritic within a factor of three. Thus, the present result can be interpreted as indicating the presence of a small but unambiguous meteoritic signal in Manson impact melt rocks.

Deformation and transformation of rock-forming minerals by natural and experimental shock processes: II. Physical properties of shocked minerals

Article

Full-text available

Jan 1974

Dieter Stöffler

Ascent of dinosaurs linked to an iridium anomaly at the Triassic-Jurassic boundary

Article

Full-text available

May 2002
SCIENCE

Analysis of tetrapod footprints and skeletal material from more than 70 lo- calities in eastern North America shows that large theropod dinosaurs appeared less than 10,000 years after the Triassic-Jurassic boundary and less than 30,000 years after the last Triassic taxa, synchronous with a terrestrial mass extinction. This extraordinary turnover is associated with an iridium anomaly (up to 285 parts per trillion, with an average maximum of 141 parts per trillion) and a fern spore spike, suggesting that a bolide impact was the cause. Eastern North American dinosaurian diversity reached a stable maximum less than 100,000 years after the boundary, marking the establishment of dinosaur-dominated communities that prevailed for the next 135 million years.

The signature of terrestrial impacts

Article

Full-text available

Jan 1996

The high level of endogenic geological activity makes the terrestrial record of impact difficult to read. In their largely uneroded states, terrestrial impact structures have the basic so-called simple and complex forms observed on other planetary bodies, but few of them have morphometric parameters, such as apparent and true depth and stratigraphic uplift, that can be defined. Erosion severely affects such parameters, and can even result in a positive topographic form due to differential erosion. The principal criterion for the recognition of terrestrial impact structures is, therefore, not their form, but the occurrence of shock-metamorphic effects. In addition to a characteristic geological signature, terrestrial impact structures have characteristic geophysical signatures. The most common is a Bouguer gravity low, which extends out to the rim. The magnetic signature can be more varied but generally corresponds to a subdued low. The geophysical, geological, and morphological characteristics at terrestrial impact structures are summarised in tabular form as an aid to the recognition of additional structures.

The Effects of Differential Scaling of Impact Melt and Crater Dimensions on Lunar and Terrestrial Craters: Some Brief Examples

Chapter

Full-text available

Jan 1992

Three of the principal variables in scaling impact-crater dimensions are the impact velocity, the projectile size, and the gravitational acceleration of the target body. The amount of impact melt generated by an impact, however, is independent of gravity, but will grow in direct proportion to the projectile dimensions and as an increasing function of the impact velocity. Thus, if the impact velocity and gravitational acceleration were held constant and projectiles of increasing size were considered, the amount of melt generated relative to the dimensions of the final crater would grow at a steady rate. Using the Earth and the Moon for comparison, this paper examines the effects of differential scaling on the depth of origin of central-peak material, on the amount of stratigraphic uplift associated with the formation of those peaks, and on the clast contents of impact melts. When craters of similar size are compared, central peaks should be derived from greater depths on Earth because of relatively deeper melting. The amount of stratigraphic uplift, however, should be greater on the Moon. A lunar crater will be larger than its terrestrial counterpart formed by an identical projectile, but the terrestrial crater will be accompanied by substantially more impact melt. As a large fraction of the melt would have lined the transient cavity during the excavation stage of the impact event, a greater fraction of the lunar melt will have been in contact with clastic materials on the cavity wall. Thus, the clast contents of lunar impact melts should be higher than in those in terrestrial craters of similar size.

Terrestrial Acidification at the K/T Boundary

Article

Full-text available

Jan 2003

We focus here on geochemically constraining the production of acid during the KT impact. The potential importance of acid formation at the Cretaceous/Tertiary (K/T) boundary has been discussed by many authors (e.g., [2–7]). Fossil evidence (i.e., the survival of some species of calcareous plankton) for the lack of acidification of ocean surface waters has been cited by D’Hondt et al. [6], thus placing an important constraint on the total acid deposited in the oceans. Global surface cooling due to a decrease in solar insolation by an ejectaderived stratospheric dust layer, an idea originally proposed by Alvarez et al. [8], may have occurred on a decadal timescale as stratospheric SO2 was slowly converted to sulfuric acid aerosol [9–11]. Evaporite deposits at Chicxulub (e.g., [12]) provided the SO2 via devolatilization of anhydrite and gypsum. Numerical model estimates of the amount of SO2 liberated, based on laboratory hyper-velocity impact studies of anhydrite [13, 14], predict ~ 1 × 1016−1 × 1017 moles S were released [7]. Assuming complete oxidation of SO2 to sulfuric acid, this corresponds to ~12−20 × 1016 equivalents (eq) of acid. Zahnle [5] has estimated that ∼ 1 × 1015 moles of nitric acid were produced by shock heating of air during bolide and ejecta passage through the atmosphere, making nitric acid only 1–10 % of total strong acid production. Morgan et al. [15], by determining the diameter of the transient impact crater at Chicxulub, estimated that the bolide was the equivalent of the impact of a 12 km asteroid and that ~1 × 1016 mol SO2 were produced by the event.

End-Permian catastrophe by a bolide impact: Evidence of a gigantic release of sulfur from the mantle

Article

Full-text available

Sep 2001
GEOLOGY

Our studies in southern China have revealed a remarkable sulfur and strontium isotope excursion at the end of the Permian, along with a coincident concentration of impact- metamorphosed grains and kaolinite and a significant decrease in manganese, phosphorous, calcium, and microfossils (foraminifera). These data suggest that an asteroid or a comet hit the ocean at the end of Permian time and caused a rapid and massive release of sulfur from the mantle to the ocean-atmosphere system, leading to significant oxygen consumption, acid rain, and the most severe biotic crisis in the history of life on Earth.

Isotopic comparison of K/T boundary impact glass with melt rock from the Chicxulub and Manson impact structures

Article

Full-text available

Jul 1993

Strontium, neodymium, and oxygen isotopic compositions are reported for core samples of impact melt rock recovered from drill holes into the Chicxulub and Manson craters, which are candidate source craters for the catastrophic impact that occurred at the boundary between the Cretaceous and Tertiary periods (K/T boundary). The data are compared with previously published isotopic data from impact glasses from the K/T boundary of the Beloc formation in Haiti. It is found that the Chicxulub melt rocks are isotopically indistinguishable from the K/T impact glass, supporting the hypothesis that Chicxulub is a source crater for the K/T catastrophe. In contrast, the Manson melt rocks have a clearly different isotopic composition.

Chicxulub Crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucat??n Peninsula, Mexico

Article

Full-text available

Sep 1991
GEOLOGY

We suggest that a buried 180-km-diameter circular structure on the Yucatan Peninsula, Mexico, is an impact crater. Its size and shape are revealed by magnetic and gravity-field anomalies, as well as by oil wells drilled inside and near the structure. The stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with, and overlain by, breccias that contain evidence of shock metamorphism. The andesitic rocks have chemical and isotopic compositions similar to those of tektites found in Cretaceous/Tertiary (K/T) ejecta. A 90-m-thick K/T boundary breccia, also containing evidence of shock metamorphism, is present 50 km outside the crater's edge. This breccia probably represents the crater's ejecta blanket. The age of the crater is not precisely known, but a K/T boundary age is indicated. Because the crater is in a thick carbonate sequence, shock-produced CO 2 from the impact may have caused a severe greenhouse warming.

The Cretaceous-Tertiary Event and Other Catastrophes in Earth History

Book

Jan 1996

Catastrophic events and mass extinctions: impacts and beyond

Book

Jan 2002

The early Precambrian stratigraphic record of large extraterrestrial impacts

Article

Jan 2004

Extraterrestrial matter on Earth: evidence from the Cr isotopes. In Catastrophic Events & Mass Extinctions: Impacts and Beyond.

Article

May 2001

Australian crater-forming meteorites

Article

Jan 1996

A. W. R. Bevan

Meteorites are associated with five impact structures in Australia. Three of them are group IIIAB irons (Wolf Creek, Henbury, and Boxhole). Veevers is a group IIAB iron, and material recovered from the crater at Dalgaranga is a mesosiderite stony-iron. The impacts range in age from a few thousand years (Dalgaranga, Henbury, Veevers, and Boxhole) to 300 000 yr (Wolfe Creek Crater). Metallographic studies of the surviving fragments at some of the craters show that impact damage ranges from simple fracturing, through shock-hardening of metal, to plastic and shear deformation, reheating and attendant recrystallisation, and, ultimately, melting.

Potential of 187Os/186Os as a cosmic versus terrestrial indicator in high iridium layers of sedimentary strata

Article

Jan 1982

Karl K. Turekian

The use of the , 8 7Os/186Os ratio as a diagnostic indicator of cosmic osmium in high osmium (and iridium) layers in sedimentary strata, such as those found at the Cretaceous/Tertiary boundary, is explored. The , 8 7Re/1 8 60s ratio in meteorites is 3.2 and in crustal rocks it is 400, resulting in different 1 8 7 0 s / l 8 60s ratios in the two systems after billions of years have passed. (The radioactive decay constant of, 8 7Re is 1.6 x 10"11 y"1.) Meteorites now have a 1 8 70s/1 8 60s ratio of about one, and the earth's crust averages (assuming no crustal recycling through the mantle) between 13 and 30. As a result, Os from the crust concentrated in certain sedimentary strata should be readily distinguished from Os from extraterrestrial projectiles.

The Sedimentary Record of Impact Events

Chapter

Jan 2001

Christian Koeberl

Some impact events in the geological record have been recognized because of the discovery of ejecta layers. Ejecta are classified into proximal ejecta, which are found in the immediate vicinity of an impact crater, within <5 crater radii from the rim, and distal ejecta, i.e., those that occur at considerable distances from the source crater (>5 crater radii from the crater rim). Distal ejecta consist of (usually fine-grained) rock and mineral fragments, and/or contain impact glasses. The study of distal ejecta, e.g., at the Cretaceous-Tertiary boundary, has led to the discovery of large impact structures (in this case, the Chicxulub structure in Mexico). Tektites are also distal ejecta. They occur in geographically extended strewn fields, of which four are currently known: the Australasian (0.78 Ma), the Ivory Coast (1.07 Ma), the Central European (15 Ma), and the North American (35 Ma) strewn fields. The extent of three of the four fields is defined by the occurrence of microtektites in deep-sea sediments, where they occur in a stratigraphie position that is identical to the radiometric age of the tektites. Source craters have been identified for three of the four strewn fields; the source crater remains elusive for the Australasian field. Distal ejecta (also called impactoclastic layers) can serve as event markers in the geological record, because they allow immediate determination if there is a correlation between an impact event and a biological response.

Geochemistry of Soils from the Bosumtwi Impact Structure, Ghana, and Relationship to Radiometric Airborne Geophysical Data

Chapter

Jan 2002

In a high-resolution aerogeophysical survey across the Bosumtwi impact structure, a pronounced high radiometric signal (high equivalent concentration of potassium) was found around the crater rim and to the north of the crater rim. Here we report data on major and trace element chemical compositions of soils from the structure, which have been investigated to determine the provenance, influence of source area weathering, and possible cause of the high equivalent concentration of potassium in the airborne gamma radiation data. Bosumtwi, an impact crater of 1.07 Ma age and a diameter of ~ 10.5 km, is located in early Proterozoic Birimian-Tarkwaian rocks of the semi-equatorial climatic region of Ghana, West Africa. The area forms part of the tropical rainforest environment, where warm climate, high rainfall, and high organic activity prevail, and chemical weathering is intense, leading to the formation of lateritic soils, which can be up to tens of meters thick.

Impact crises and mass extinctions: A working hypothesis

Article

Jan 1996

Based on evidence from astronomical observations, impact dynamics, and the geologic record, we explore a general hypothesis linking impacts of large asteroids and comets with mass extinctions of life. The probability of large-body impacts on the Earth derived from the flux of earth-crossing asteroids and comets and the estimated threshold impact size required to cause a global environmental disaster suggest that impacts of objects ≥ a few kilometers in diameter might be sufficient to explain the Phanerozoic record of extinction pulses. A number of extinction boundaries are known to be marked by severe environmental disturbances, including mass mortality and related extinctions (sometimes in steps), impoverished postextinction fauna and flora, and proliferation of stress-tolerant and opportunistic species, followed by gradual ecological recovery and radiation of new taxa. Abrupt negative shifts in δ13C in marine sedimentary rocks at extinction boundaries suggest major biomass losses followed by low-productivity " Strangelove" oceans, and fluctuations in δ18O may be interpreted as evidence of significant climatic oscillations. These biological, isotopic, and geochemical signatures seem to be consistent with the expected after effects of catastrophic impacts. Six of the extinction pulses may be associated with concurrent (in some cases multiple) impact markers (e.g., layers with high iridium, shocked minerals and/or microtektites, and large, dated impact craters). Elevated iridium levels at, or near, other extinction boundaries have characteristics suggesting a terrestrial origin, although they might be explained by collision of relatively low-Ir objects such as comets, and further work is warranted.

Re-Os isotope study of rocks from the Manson impact structure

Chapter

Jan 1996

We used the Re-Os isotopic system to study seven samples from Manson drill cores M-1, M-8, and M-10. The samples represent a range of impact melt rocks, breccias, and (crystalline) basement rocks. This Re-Os isotopic system allows the sensitive and selective determination of the presence of an extraterrestrial component in impact-derived rocks, as well as quantification of such a meteoritic component, because of the large difference in the Re and Os elemental concentrations and Os isotopic compositions between meteorites and old continental crustal target rocks. We found that impact melt rock M-1/429.0 has a very high Os abundance and its 187Re/188Os and 187Os/188Os isotopic ratios are indistinguishable from the values of the meteoritic data array. The 187Re/188Os and 187Os/188Os isotopic ratios for most target rocks and some of the breccias are typical for the values expected for old continental crust. Our results indicate the presence of about 0.5 wt% of a meteoritic (chondritic?) component in the M-1/429.0 impact melt rock.

Neodymium and strontium isotopic study of Australasian tektites: New constraints on the provenance and age of target materials

Article

Feb 1992
GEOCHIM COSMOCHIM AC

Nd and Sr isotopic studies of Australasian tektites provide information on the age and provenance of the target materials and allow us to characterize the target area and the impact process leading to tektite formation. ϵ_(Nd) values of australites, splash-form indochinites, and Muong Nong-type indochinites are indistinguishable within analytical uncertainty and average -11.5 ϵu. Depleted mantle Nd model ages fall within the narrow range of 1490–1620 Ma, indicating that the source material was derived dominantly from a Proterozoic crustal terrane. ϵ_(Sr) values are variable and are correlated with the geographic location of the tektite samples. Analyses of four Muong Nong-type (or layered) indochinites from a single locality in eastern Thailand yield an isochron age of 167 ± 12 Ma (2σ). A correlation of Rb/Sr fractionation with Sr model ages indicates that the last major Rb/Sr fractionation event experienced by the target materials occurred 175 ± 15 Ma ago. We interpret this age as the time of deposition of sedimentary target rocks and consider the compositional layering observed in Muong Nong-type tektites to reflect compositional variability inherited from Jurassic sediments. The Nd and Sr isotopic data provide evidence that all Australasian tektites were derived from a single sedimentary formation with a narrow range of stratigraphic ages close to 170 Ma. We suggest that all of the Australasian tektites were derived from a single impact event, and that the australites represent the upper part of a melt sheet ejected at high velocity, whereas the indochinites represent melts formed at a lower level in the target material which were distributed closer to the area of impact. The impact site is inferred to be within an area of Jurassic sedimentary bedrock, which spans the geopolitical boundaries between northern Cambodia, southern Laos, and southeastern Thailand.

Airborne geophysical survey of the Lake Bosumtwi meteorite impact structure (southern Ghana) - Geophysical maps with descriptions

Article

Jan 2003

Chicxulub - The K-T boundary impact crater: A review of the evidence, and an introduction to impact crater studies

Article

Jan 1996

C. Koeberl

Tektite origin by hypervelocity asteroidal or cometary impact: Target rocks, source craters, and mechanisms

Article

Jan 1992

Christian Koeberl

Tektites are natural glasses that occur on earth in four distinct strewn fields (North American, Central European, Ivory Coast, and Australasian). Geochemical arguments have shown that tektites have been derived by hypervelocity impact melting from terrestrial upper crustal rocks, most likely sediments. The contents of Be-10 in tektites are evidence for a derivation of tektites from surface rocks, thus precluding an origin from greater depth in the crater. For two of the four tektite strewn fields (Ivory Coast, Central European), a possible connection to impact craters (Bosumtwi, and Ries, respectively) has been suggested on the basis of chemical, isotopic, and age data. No clear crater identifications have been made for the North American or Australasian strewn fields, although there are good candidates for both. Even though the geochemistry of tektites is in unequivocal favor of an origin by impact melting of terrestrial rocks, the unambiguous demonstration of the presence of an extraterrestrial contribution to the chemistry of tektites remains a problem. However, recent osmium isotope studies have shown that there is a clear meteoritic signature in at least some tektites. The exact mechanism of tektite formation is still not obv3ious, although some facts become increasingly clear. Tektite production requires specific impact conditions—otherwise there would be many more tektite strewn fields connected to the 150 or so known impact craters. Tektites are produced by nonequilibrium shock melting of surficial rocks, and the superheated melt may be subjected to a plasma phase during which they are subjected to partial reduction. They are then lofted through the atmosphere (probably in the wake of the expanding vapor cloud), quenched, and distributed over a geographically extended area—the strewn field. Some tektites solidify in a near-vacuum and re-enter the atmosphere. During the re-entry they melt again and form ablation-shaped tektites. Larger tektites, from a lower part of the target stratigraphy, are only distributed closer to the source crater. Many of them are more inhomogeneous melts and show a layered structure; they are called Muong Nong–type tektites. The study of tektites and the identification of possible new strewn fields provide important contributions toward the understanding of impact cratering.

The Stratigraphic Record of Impact Events: A Short Overview

Chapter

Jan 2003

In contrast to many other planets (and moons) in the solar system, the recognition of impact craters on the Earth is difficult, because active geological and atmospheric processes on our planet can obscure or erase the impact record in geologically short times. Impact craters are recognized from the study of actual rocks — remote sensing can only provide supporting information. Petrographic studies of rocks at impact craters can lead to the discovery of impact-characteristic shock metamorphic effects, and geochemical studies may yield information on the presence of meteoritic components in these rocks. Apart from studying meteorite impact craters per se, large amounts of information can also be gained from the study of impact ejecta. Such ejecta are found within the normal stratigraphic record, where they can provide excellent time markers, and allow to relate an impact event directly to possible biological effects. Impact ejecta are commonly divided into two groups — proximal ejecta (those that are deposited closer than 5 crater radii from the crater rim), and distal ejecta. In some cases, impact events have been identified solely from the discovery and study of regionally extensive or globally distributed impact ejecta. A well known case in point is the Cretaceous- Tertiary boundary, where the discovery of an extraterrestrial signature, together with the presence of shocked minerals, led not only to the identification of an impact event as the cause of the end-Cretaceous mass extinction, but also to the discovery of a large buried impact structure about 200 km in diameter, the Chicxulub structure. Tektites are another form of distal impact ejecta, the source craters of which have long remained elusive. To date only three of the four known Cenozoic tektite strewn fields have been connected to source craters. Distal impact ejecta allow to gain information about impact processes and the connection to biological events.

Identification of projectiles of large terrestrial impact craters and some implications for the interpretation of Ir-rich Cretaceous/Tertiary boundary layers

Chapter

Jan 1982

H. Palme

Impact-generated lunar highland rocks have high Ir and other noble metal concentrations. This Ir enrichment is produced by the admixture of a small amount of an undifferentiated meteoritic component. Similar high Ir concentrations are found in rocks from several large terrestrial impact craters. A compilation of all Ir analyses of terrestrial crater samples shows that there are now nine craters with a significant Ir anomaly (Ir > 1 ppb). Judging from the relative abundances of siderophile elements and the simultaneous enrichment of Cr, four of these craters were produced by chondritic projectiles. The Clearwater East impact melt, with an 8% chondritic component, is a well-characterized example. There is some evidence for achondrites, stony-irons, and iron meteorites being among projectiles of terrestrial impact craters. When present at any given impact site, Ir enrichment is commonly found in melt rocks and tends to occur in all samples from the same melt sheet. Lack of a meteoritic component may be due to a differentiated projectile, low in Ir and/or a high impact velocity. The excellent correlations among all siderophile elements and Cr in 23 samples from the Cretaceous/Tertiary boundary layer at Stevns Klint indicate that all these elements are contained in one component. The chondrite-normalized patterns are distinctly "terrestrial" in having large differences in concentrations of elements with similar cosmo-chemical volatilities, such as Au and Ge (low) and As and Cu (high), and suggest that the Ir-rich component is a mixture of meteorite and ejecta material from the site of the impact. However, it can be shown that a two-component mixture with various types of meteorites and terrestrial rocks cannot explain the observed element pattern. Some fractionation (e.g., volatilization and recondensation) may have modified the original pattern, making it difficult to identify the meteorite and, therefore, introducing large uncertainties on the fraction of meteorite in a given sample from a C/T boundary layer.

The Early Earth: Physical, Chemical and Biological Development

Book

Jan 2003

This volume contains a series of papers on a wide range of aspects of the early development of the Earth in the first 2000 Ma of Earth’s history, including geophysics, structure and tectonics, atmosphere, origin of life, biosphere, deep mantle geochemistry, early oceans, microbial ecology, the aim being to facilitate discussion and understanding of this area of research. The book is divided into three parts: (1) Geophysical and Petrological Constraints on Archaean Lithosphere: These papers highlight differences in structure and composition of Archaean and post-Archaean lithosphere, with contributions from seismologists, petrologists, and geochemists. Seismic evidence for the formation and evolution of cratons is presented. Shear wave splitting results, petrologic data and crustal deformation fabrics are used to characterize variations of anisotropy in order to investigate the coupling and decoupling of the crust and upper mantle beneath cratons, isotopic data from large cratons suggesting that the lithospheric mantle is the same age as the overlying crust, provide further constraints on the modification of cratonic roots with time. (2) Models of Cratonic Evolution and Modification: These papers focus on global and regional models of cratonic accretion based on evidence from the rock record. Numerical models of cratonic rock both in normal mantle flow and in the presence of plums suggest that cratons decrease in size by lateral erosion rather than by thinning. (3) Constraints on the Archaean Environment: These papers are concerned with the physical, chemical and biological controls on Earth’s surface environment in the late Hadean and the Archaean. Archaean strata record the early biogenic controls on the carbon cycle, and shift to an oxygen-rich atmosphere. The timing of hydrogen loss, carbon-dioxide build-up, the appearance of life, and the origins of photosynthesis are all controversial topics discussed here. Subjects covered include the chemical and biological controls on the atmospheric and oceans, early controls on the carbon cycle and photosynthesis, petrologic, isotopic, tectonic and seismic evidence for the composition and structure of Archaean lithosphere.

Geochemical signals of the late Jurassic, marine Mjolnir-impact

Article

Jan 1999
METEORIT PLANET SCI

Of the only seven submarine impact craters that have been found globally, the Mjolnir crater is one of the best preserved and retains crater and ejecta. Geochemical studies (organic pyrolysis using the Rock Eval technique and XRF analysis for major, minor, and trace elements) of the Institute for Petroleum Research (IKU) core 7430/10-U-01 that was taken from a drillhole located -30 km north-northeast of the crater rim show gradual establishment of anoxic sea floor conditions through the late Jurassic. These poorly ventilated water conditions were overturned due to the Mj01nir impact event. Waves and currents transported impact glass (which is now partly weathered to smectite) into the depositional area where the drillhole is located. The succeeding crater collapse transported impact material (e.g., shocked quartz and Ir) from the crater rim and deeper levels to the core site. Normal marine depositional conditions were established a short time after the crater collapsed.

Heavy bombardment of the Earth at -3.85Ga: the search for petrographic and geochemical evidence

Article

Jan 2000

G. Ryder

Acraman, South Australia: Australia's largest meteorite impact structure

Article

Jan 1994

G.E. Williams

Acraman, located in the Mesoproterozoic Gawler Range Volcanics (1592±2 Ma) in the Gawler Ranges, South Australia, is the largest known impact structure in Australia. The centre of the structure is marked by sparse outcrops of shattered dacite in Lake Acraman within a topographic depression 30 km in diameter. The disrupted bedrock exhibits shatter cones and shock lamellae in quartz grains that indicate shock pressures of up to ~15 GPa. Acraman is eroded below the original crater floor. The transient cavity that formed immediately after the impact was probably ~40 km in diameter and ~4 km deep. The Acraman structure could have been formed by the impact of an Earth-crossing chondritic asteroid 4.7 km in diameter and of density 3500 kg/m3 travelling at 25 km/s. The energy of crater formation was 6 × 1022 J, equivalent to 1.5 × 107 megatons of explosive energy. Ejecta blanketed at least 7 × 105 km2. -from Author

An extraterrestrial impact at the Permian-Triassic boundary?

Article

Sep 2001

Yukio Isozaki

Support for the Lunar Cataclysm Hypothesis from Lunar Meteorite Impact Melt Ages

Article

Dec 2000

Barbara Cohen

Lunar meteorites represent a more random sampling of lunar material than the Apollo or Luna collections and, as such, lunar meteorite impact melt ages are the most important data in nearly 30 years with which to reexamine the lunar cataclysm hypothesis. Within the lunar meteorite breccias MAC 88105, QUE 93069, DaG 262, and DaG 400, seven to nine different impact events are represented with40Ar-39Ar ages between 2.76 and 3.92 billion years ago (Ga). The lack of impact melt older than 3.92 Ga supports the concept of a short, intense period of bombardment in the Earth-moon system at ∼3.9 Ga. This was an anomalous spike of impact activity on the otherwise declining impact- frequency curve.

The record of impact processes on the early Earth: A review of the first 2.5 billion years

Article

Jan 2006

Christian Koeberl

Collisions and impact processes have been important throughout the history of the solar system, including that of the Earth. Small bodies in the early solar system, the planetesimals, grew through collisions, ultimately forming the planets. The Earth started growing ca. 4.56 Ga in this way. Its early history was dominated by violent impacts and collisions, of which we only have circumstantial evidence. The Earth was still growing and had reached ~70%–80% of its present mass when at ca. 4.5 Ga a Mars-sized protoplanet collided with Earth, leading to the formation of the moon—at least according to the currently most popular hypothesis of lunar origin. After its forma-tion, the moon was subjected to intense post-accretionary bombardment between ca. 4.5 and 3.9 Ga. In addition, there is convincing evidence that the Moon experienced an interval of intense bombardment with a maximum at ca. 3.85 ± 0.05 Ga; subsequent mare plains as old as 3.7 or 3.8 Ga are preserved. It is evident that if a late heavy bom-bardment occurred on the Moon, the Earth must have been subjected to an impact flux at least as intense as that recorded on the Moon. The consequences for the Earth must have been devastating, although the exact consequences are the subject of debate (total remelting of the crust versus minimal effects on possibly emerging life forms). So far, no unequivocal record of a late heavy bombardment on the early Earth has been found. The earliest rocks on Earth date back to slightly after the end of the heavy bombardment, although there are relict zircons that have ages of up to 4.4 Ga (in which, however, no impact-characteristic shock features were found so far). In terms of evidence for impact on Earth, the first solid evidence exists in the form of various spherule layers found in South Africa and Australia with ages between ca. 3.4 and 2.5 Ga; these layers represent several (the exact number is still unknown) large-scale impact events. The oldest documented (and preserved) impact craters on Earth have ages of 2.02 and 1.86 Ga. Thus, the impact record for more than half of the geo-logical history of the Earth is incomplete and not well preserved, and we mostly have only indirect evidence regarding the impact record and its effects during the first 2.5 b.y. of Earth history.

The Geochemical Evolution of the Continental Crust

Article

May 1995
REV GEOPHYS

A survey is given of the dimensions and composition of the present continental crust. The abundances of immobile elements in sedimentary rocks are used to establish upper crustal composition. The present upper crustal composition is attributed largely to intracrustal differentiation resulting in the production of granites senso lato. Underplating of the crust by ponded basaltic magmas is probably a major source of heat for intracrustal differentiation. The contrast between the present upper crustal composition and that of the Archean upper crust is emphasized. The nature of the lower crust is examined in the light of evidence from granulites and xenoliths of lower crustal origin. It appears that the protoliths of most granulite facies exposures are more representative of upper or middle crust and that the lower crust has a much more basic composition than the exposed upper crust. There is growing consensus that the crust grows episodically, and it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons, or 2.7 Ga). There appears to be a relationship between episodes of continental growth and differentiation and supercontinental cycles, probably dating back at least to the late Archean. However, such cycles do not explain the contrast in crustal compositions between Archean and post-Archean. Mechanisms for deriving the crust from the mantle are considered, including the role of present-day plate tectonics and subduction zones. It is concluded that a somewhat different tectonic regime operated in the Archean and was responsible for the growth of much of the continental crust. Archean tonalites and trond-hjemites may have resulted from slab melting and/or from melting of the Archean mantle wedge but at low pressures and high temperatures analogous to modern boninites. In contrast, most andesites and subduction-related rocks, now the main contributors to crustal growth, are derived ultimately from the mantle wedge above subduction zones. The cause of the contrast between the processes responsible for Archean and post-Archean crustal growth is attributed to faster subduction of younger, hotter oceanic crust in the Archean (ultimately due to higher heat flow) compared with subduction of older, cooler oceanic crust in more recent times. A brief survey of the causes of continental breakup reveals that neither plume nor lithospheric stretching is a totally satisfactory explanation. Speculations are presented about crustal development before 4000 m.y. ago. The terrestrial continental crust appears to be unique compared with crusts on other planets and satellites in the solar system, ultimately a consequence of the abundant free water on the Earth.

Geochemistry and age of Ivory Coast tektites and microtektites

Article

Apr 1997
GEOCHIM COSMOCHIM AC

Ivory Coast tektites were first reported in 1934 from a geographically restricted area at Ivory Coast, West Africa. Although some additional specimens have been found later, the total number remains small (a few hundred). The Bosumtwi impact crater in Ghana is most likely the source crater for the Ivory Coast tektites, based on the finding that the tektites and the crater have the same age as well as similar isotopic and chemical compositions. In addition to tektites on land, microtektites were found in (so far) eleven deep-sea cores off the West African coast, between about 9°N and 11°S and 0° and 23°W, defining the extent of the Ivory Coast tektite strewn field. In this study we analyzed eleven Ivory Coast tektites for their major and trace element composition, studied their petrographical characteristics, provided major element data for 111 microtektites, and major and trace element data for four microtektites. We determined the 40Ar39Ar step-heating age of five Ivory Coast tektites and four microtektites and obtained fission-track dates for ten tektites and one Bosumtwi impact glass. The tektites have very small intersample and intrasample variations of their major and trace element composition. 111 Ivory Coast microtektites from eleven cores were analyzed for their major element compositional range. Their compositional range is significantly wider than that of the Ivory Coast tektites, but the majority of all microtektites have compositions very similar to those of the tektites (within a factor of 1.2). Trace element compositions of the tektites also show little variation between samples. The samples do not show any distinct Eu anomaly in the REE patterns. This characteristic, as well as the high absolute REE abundances and La NYbN ratios of about 8, indicate that Archean rocks are plausible source rocks. The major and trace element contents of four individually analyzed Ivory Coast microtektites show compositions that are very similar to those of the Ivory Coast tektites. However, the microtektites contain >20 rel% higher abundances of some of the lithophile and siderophile trace elements, such as Sc, Cr, Co, Ni, Sr, Zr, Ba, Hf, Ta, Th, and the REEs. These differences are probably due to incorporation of a higher abundance of accessory trace minerals with the microtektite-forming melt. The Ivory Coast microtektites also have a uniform internal composition. Duplicate 40Ar39Ar step-heating age analyses were performed on five tektites. The best age estimate for the formation age of the tektites was calculated by taking a weighted average of the ages from the plateau portions of the runs, resulting in an age of 1.1 ± 0.05 Ma. We also tried to date four microtektites by 40Ar39Ar age analyses, but their young age and small sample size makes it impossible to assign a reliable age to the microtektites. One run yielded satisfactory results that were similar to the tektite age. In addition, we determined the fission-track ages for ten individual Ivory Coast tektite samples and for one impact glass sample from the Bosumtwi crater. The track-size corrected ages for the Ivory Coast tektites ranged from 0.91 to 1.18 Ma, resulting in an average fission-track age of 1.05 ± 0.11 Ma. This age is, within errors, identical to that of the Bosumtwi impact glass at 1.03 ± 0.11 Ma, and to the 40Ar39Ar age of 1.1 ± 0.05 Ma. The preferred age of the Ivory Coast tektite event is 1.07 Ma.

A meteorite from the Cretaceous/Tertiary boundary

Article

Nov 1998

Frank T. Kyte

Cretaceous/Tertiary boundary sediments are now widely recognized to contain the record of a large asteroid or comet impact event, probably at the site of the Chicxulub crater on the Yucatan peninsula. After nearly two decades of intensive research, however, much remains unknown about the specific nature of the projectile and of the impact event itself. Here we describe a 2.5-mm fossil meteorite found in sediments retrieved from the Cretaceous/Tertiary boundary in the North Pacific Ocean that we infer may be a piece of the projectile responsible for the Chicxulub crater. Geochemical and petrographic analyses of this meteorite indicate that it probably came from a typical metal- and sulphide-rich carbonaceous chondrite rather than the porous aggregate type of interplanetary dust considered typical of cometary materials. The fact that meteorite survival should be enhanced by impacts at low (asteroidal) velocities also implies that this meteorite had an asteroidal rather than a cometary origin.

Spheroids at the Cretaceous-Tertiary Boundary are Altered Impact Droplets of Basaltic Composition

Article

Nov 1983
GEOLOGY

Sand-size spheroids of K-feldspar in the Cretaceous-Tertiary (C-T) boundary clay at Caravaca, S Spain, were interpreted by Smit & Klaver (Nature, 292(5818), 1981, pp 47-49) as having solidified from a melt resulting from the impact of a large extraterrestrial body. Sand-size spheroids of K-feldspar, glauconite and magnetite-quartz have been found in the C-T boundary clay in N Italy, and spheroids of K-feldspar and pyrite were found in the boundary clay at D.S.D.P. site 465A in the central Pacific. These spheroids have textures similar to those of rapidly crystallized feldspar and mafic silicates. They are interpreted as diagenetically altered microcrystalline spherules of basaltic composition produced by the impact of a large asteroid in an ocean basin at the end of the Cretaceous. They are analogous to the glassy microtektites produced by impacts of more siliceous target rocks.-A.P. Dept. of Geology & Geophysics, Univ. of California, Berkeley, CA 94720, USA.

Glass From The Cretaceous Tertiary Boundary In Haiti

Article

Feb 1991

The Geochemistry and Cosmochemistry of Impacts

Abstract

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