Bruce F. Bohor

Mineralogic Evidence for an Impact Event at the Cretaceous-Tertiary Boundary

A thin claystone layer found in nonmarine rocks at the palynological Cretaceous-Tertiary boundary in eastern Montana contains an anomalously high value of iridium. The nonclay fraction is mostly quartz with minor feldspar, and some of these grains display planar features. These planar features are related to specific crystallographic directions in the quartz lattice. The shocked quartz grains also exhibit asterism and have lowered refractive indices. All these mineralogical features are characteristic of shock metamorphism and are compelling evidence that the shocked grains are the product of a high velocity impact between a large extraterrestrial body and the earth. The shocked minerals represent silicic target material injected into the stratosphere by the impact of the projectile.

Impact-shocked zircons: discovery of shock-induced textures reflecting increasing degrees of shock metamorphism

Abstract Textural effects specifically characteristic of shock metamorphism in zircons from impact environments have not been reported previously. However, planar deformation features (PDF) due to shock metamorphism are well documented in quartz and other mineral grains from these same environments. An etching technique was developed that allows SEM visualization of PDF and other probable shock-induced textural features, such as granular (polycrystalline) texture, in zircons from a variety of impact shock environments. These textural features in shocked zircons from K/T boundary distal ejecta form a series related to increasing degrees of shock that should correlate with proportionate resetting of the U Pb isotopic system.

Dinosaurs, spherules, and the “magic” layer: A new K-T boundary clay site in Wyoming

A new Cretaceous-Tertiary (K-T) boundary clay site has been found along Dogie Creek in Wyoming in the drainage of Lance Creek—the type area of the Lance Formation of latest Cretaceous age. The boundary clay was discovered in the uppermost part of the Lance Formation, 4–7 cm beneath the lowest lignite in the Paleocene Fort Union Formation and approximately 1 m above a fragmented dinosaur bone. The boundary clay consists of a basal kaolinitic claystone layer as much as 3 cm thick containing hollow goyazite spherules, overlain by a 2–3 mm smectitic layer (the “magic” layer) containing both shock-metamorphosed minerals and an iridium anomaly of 21 ppb. A palynological break coincides with the base of the claystone layer; numerous Late Cretaceous palynomorph species terminate at this boundary. The paleontological significance of this new boundary site lies in its close association with the well-studied assemblage of dinosaurs and other vertebrates and flora within the type area of the Lance Formation. The spherules at the Dogie Creek site are extremely well preserved by virtue of their replacement by the mineral goyazite. This preservation should facilitate the resolution of the origin of the spherules and of their host layer.

Fingerprinting the K/T impact site and determining the time of impact by UPb dating of single shocked zircons from distal ejecta

Abstract U Pb isotopic dating of single 1–3 μg zircons from K/T distal ejecta from a site in the Raton Basin, Colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site. Data for the least shocked zircons are slightly displaced from the 544 ± 5 Ma primary age for a component of the target site, white those for highly shocked and granular grains are strongly displaced towards the time of impact at 65.5 ± 3.0 Ma. Such shocked and granular zircons have never been reported from any source, including explosive volcanic rocks. Zircon is refractory and has one of the highest thermal blocking temperatures; hence, it can record both shock features and primary and secondary ages without modification by post-crystallization processes. Unlike shocked quartz, which can come from almost anywhere on the Earths crust, shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers. With U Pb zircon dating, it is now possible to correlate ejecta layers derived from the same target site, test the single versus multiple impact hypothesis, and identify the target source of impact ejecta. The ages obtained in this study indicate that the Manson impact site, Iowa, which has basement rocks that are mid-Proterozoic in age, cannot be the source of K/T distal ejecta. The K/T distal ejecta probably originated from a single impact site because most grains have the same primary age.

Magnesioferrite from the Cretaceous-Tertiary boundary, Caravaca, Spain

Abstract Magnesioferrite grading toward magnetite has been identified as a very small but meaningful constituent of the basal iron-rich portion of the Cretaceous-Tertiary (K-T) boundary clay at the Barranco del Gredero section, Caravaca, Spain. This spinel-type phase and others of the spinel group, found in K-T boundary clays at many widely separated sites, have been proposed as representing unaltered remnants of ejecta deposited from an earth-girdling dust cloud formed from the impact of an asteroid or other large bolide at the end of the Cretaceous period. The magnesioferrite occurs as euhedral, frequently skeletal, micron-sized octahedral crystals. The magnesioferrite contains29 ± 11 ppb Ir, which accounts for only part of the Ir anomaly at this K-T boundary layer(52 ± 1 ppb Ir). Major element analyses of the magnesioferrite show variable compositions. Some minor solid solution exists toward hercynite-spinel and chromite-magnesiochromite. A trevorite-nichromite (NiFe 2 O 4 -NiCr 2 O 4 ) component is also present. The analyses are very similar to those reported for sites at Furlo and Petriccio, Umbria, Italy. On the basis of the morphology and general composition of the magnesioferrite grains, rapid crystallization at high temperature is indicated, most likely directly from a vapor phase and in an environment of moderate oxygen fugacity. Elemental similarity with metallic alloy injected into rocks beneath two known impact craters suggests that part of the magnesioferrite may be derived from the vaporized chondritic bolide itself, or from the mantle; there is no supporting evidence for its derivation from crustal target rocks.

SURFACE AREA OF MONTMORILLONITE FROM THE DYNAMIC SORPTION OF NITROGEN AND CARBON DIOXIDE

Surface area determinations were made on a montmorillonite with various cations emplaced on the exchangeable sites, utilizing nitrogen and carbon dioxide as adsorbates at 77°K and 195°K, respectively, in a dynamic system. From the fraction of a Mississippi montmorillonite less than about 1 µ in size, samples were prepared by replacing the original exchangeable cations with Li+, Na+, K+, Rb+, Cs+, Mg++, Ca++, Ba++, and NH/, forming a series of homoionic montmorillonite species.Surface areas from 3-point B.E.T. plots (half-hour adsorption points), with nitrogen as the adsor-bate, ranged from 61 m2/g for Li-montmorillonite to 138 m2/g for Cs-montmorillonite, thus reflecting a certain degree of nitrogen penetration between layers. Complete penetration should theoretically resuit in a surface area of over 300 m2/g for this clay with a nitrogen monolayer between each pair of platelets. The experimental data indicate that the extent of penetration is time-dependent and is also a function of the interlayer forces as governed by the size and charge of the replaceable cation. This finding negates the generally accepted concept that nitrogen at 77°K does not penetrate the layers and provides a measure only of the externa! surface of expandable clay minerais.A further measure of the variation of interlayer forces is provided by the adsorption of carbon dioxide at 195°K. Surface area values ranged from 99 m2/g for Li-montmorillonite to 315 m2/g for Cs-montmorillonite. Although the carbon dioxide molecule is larger than the nitrogen molecule, its greater penetration apparently is a resuit of its being kinetically more energetic (with a larger diffusion coefficient) at its higher adsorption temperature. Similar differences have been found with both adsorbates in the study of microporous substances, such as coal, where activated diffusion is of considerable significance.RésuméDes calculs sur la superficie de surface ont été effectuées sur un montmorillonite avec différents cations placés en des lieux d’échange, et en utilisant le nitrogène et le dioxyde de carbone en tant que produits d’adsorption à 77° et 195°K respectivement, dans un système dynamique. Pour la fraction d’un montmorillonite du Mississippi inférieure, en taille, à environ 1 µ, des échantillons ont été préparés en replacant les premiers cations échangeables par Li+, Na+, K+, Rb+, Cs+, Mg++, Ca++, Ba++ et NH4++, formant une série d’espèces homo-ioniques de montmorillonite.Les zones de surface à partir des relevés B.E.T. en 3 points (points d’adsorption d’une demi-heure), avec du nitrogène comme produit d’adsorption variaient de 61 m2/g pour le Li-montmorillonite à 138 m2/g pour le Cs-montmorillonite, reflétant ainsi un certain degré de pénétration de nitrogène entre les feuillets. La pénétration complète devrait théoriquement avoir lieu dans une zone de surface supérieure à 300 m2/g pour cette argile, avec une monocouche de nitrogène entre chaque paire de lamelles. Les données expérimentales indiquent que l’étendue de la pénétration dépend du temps et est aussi une fonction des forces intermédiaires commandées par la taille et la charge du cation remplacable. Cette découverte rend nul le concept généralement accepté, selon lequel le nitrogène à 77°K ne pénètre par les feuillets et fournit seulement une mesure pour la surface externe des minéraux argileux expansibles.Une autre mesure de la variation des forces intermédiaires est fournie par l’adsorption de dioxyde de carbone à 195°K Les valeurs de la zone de surface varient de 99 m2/g pour le Li-montmorillonite à 315 m2/g pour le Cs-montmorillonite. Bien que la molécule de dioxyde de carbone est plus grande que celle du nitrogène, sa plus grande pénétration résulte apparamment, de ce qu’elle est plus énergétique au point de vue cinétique (avec un coefficient de diffusion plus élevé) à sa plus haute température d’adsorption. Des différences similaires ont été trouvées avec les deux produits d’adsorption dans l’étude de substances microporeuses, telles que le charbon, pour lesquelles la diffusion activée prend une plus grande signification.KurzreferatAn einem Montmorillonit mit verschiedenen Kationen an den austauschfähigen Stellen wurden Oberflächenmessungen durchgeführt, und zwar unter Verwendung von Stickstoff und Kohlendioxyd als Adsorbate bei 77°K bzw. 195°K in einem dynamischen System. Aus einem Bruch von weniger als ca. 1 µ Dicke eines Mississippi Montmorillonit wurden Proben hergestellt durch Ersatz der ursprünglichen austauschbaren Kationen durch Li+, Na+, K+, Rb+, Cs+, Mg++, Ca++, Ba++, und NH4++ und Bildung einer Reihe von homo-ionischen Montmorillonitsorten.Oberflächen von 3-Punkt B.E.T. Stellen (halbstündige Adsorptionspunkte) mit Stickstoff als Adsorbat betrugen von 61 m2/g für Li-Montmorillonit bis zu 138 m2/g für Cs-Montmorillonit, was einem gewissen Grad von Stickstoffdurchdringung zwischen den Schichten entspricht. Vollständige Durchdringung sollte theoretisch durch eine Oberfläche von über 300 m2/g für diesen Ton mit einer einmolekularen Stickstoffschicht zwischen jedem Paar von Plättchen zum Ausdruck kommen. Die Versuchsergebnisse deuten darauf hin, dass das Mass der Durchdringung zeitgebunden ist und daneben auch von den durch die Grösse und Ladung des austauschbaren Kations bestimmten Zwischenschichtkräften abhängt. Dieser Befund widerspricht der allgemein vertretenen Ansicht, wonach Stickstoff bei 77°K die Schichten nicht durchdringt und nur ein Mass der äusseren Oberfläche von aufgeblähten Tonmineralen darstellt.Ein weiteres Mass für die Änderung der Zwischenschichtkräfte stellt die Adsorption von Kohlendioxyd bei 195°K dar. Die oberflächenwerte legen zwischen 99 m2/g für Li-Montmorillonit und 315 m2/g für Cs-Montmorillonit. Obwohl das Kohlendioxydmolekül grösser als das Stickstoffmolekül ist, scheint seine stärkere Durchdringung darauf zu beruhen, dass es bei seiner höheren Adsorptionstemperatur mehr kinetische Energie besitzt (mit einem grösseren Diffusionskoeffizient). Ähnliche Unterschiede wurden bei den zwei Adsorbaten in der Untersuchung von mikroporösen Substanzen wie z.B. Kohle festgestellt, wo die aktivierte Diffusion von beträchtlicher Bedeutung ist.РезюмеОпределения поверхностной площади производились на монтмориллоните с различными катионами, размещенными на обмениваемых местах, пользуясь азотом и двуокисью углерода в качестве адсорбатов при 77 °K и 195°K соответственно, в динамической системе. Из фракции монтмориллонита Миссиссипи размером менее 1μ приготовили образцы, подставляя Li+, Na+, K+, Rb+, Сз+ Мg++, Са++, Ва++, NН4+ вместо исходных заменяемых катионов и образуя серии гомоионных монтмориллонитовых форм. Поверхностные площади из трехточечных кривых В.Е.Т. (получасовые адсорбционные точки) с азотом в качестве адсорбата, находятся в пределах от 61 m2/g Для литий монтмориллонита до 138 m/2g для цезиймонтмориллонита, отражая тем самым некоторую степень азотного проникновения между слоями. Полное проникновение в теории должно произойти в поверхностной площади свыше 300 m2/g для этой глины, с азотным мономолекулярным слоем между каждой парой пластинок. Экспериментальные данные указывают, что количество проникновения переменно по времени и является также функцией прослоечных усилий, регулируемых размером и зарядом сменяемого катиона. Результат этот отрицает общепринятое понятие, что аэот при 77 °K не проникает в слои и предоставляет лишь измерение наружной поверхности расширяемых глинистых минералов.Дальнейшее измерение изменений прослоечных усилий предоставляется адсорбцией двуокиси углерода при 195°К. Значения поверхностной площади колеблются от 99 m2/g для лития-монтмориллонита до 315 m2/g для цезия-монтмориллонита. Хотя молекула двуокиси углерода больше чем молекула азота, ее большее проникновение является вероятно следствием того, что кинетически она более активная (с более высоким коэффициентом диффузии) при своей более высокой температуре адсорбции. Сходные разницы обнаружились с обоими адcорбатами при изучении микропористых веществ, как напр. угля, где активированная диффузия обладает крупным значением.

Origin and distribution of tonsteins in late permian coal seams of Southwestern China

Abstract We have surveyed the areal and stratigraphic distribution of tonsteins in Late Permian coalfields of southwestern China over an area of several hundred thousand square kilometers. We studied the relationship between tonstein distribution and sedimentary environment. Based on mineralogical and petrographic data, we have concluded that these tonsteins originated as air-fall volcanic ashes. Following accumulation in the peat swamps, in situ alteration of the vitric and lithic components took place under acidic conditions, leading to the formation of kaolinite. Based on petrologic, mineralogic, and chemical analytical data, we have determined that the application of mineralogic and geochemical criteria for tonsteins may be useful in correlating coal beds, predicting coal qualities and reconstructing related sedimentary paleoenvironmental conditions.

Origin and clay-mineral genesis of the Cretaceous-Tertiary boundary unit, Western Interior of North America

A 3-cm-thick, two-layered clay unit that records mineralogic and textural evidence of a catastrophic event that occurred at a time now marked as the end of the Cretaceous Period was preserved in ancient peat-forming environments of the Western Interior Basin of North America. The two layers of this unit consist of altered distal ejecta and are easily distinguished by their distinctive texture and impact components from other clay beds, mainly tonsteins and detrital shales, occurring within the sequence of rocks enclosing the Cretaceous/Tertiary (K/T) boundary interval.The lower claystone layer of the K/T boundary unit represents melted silicic target rock that has altered mainly to kaolin minerals. Impact components and signatures of this lower layer include a relict imbricate fabric of glass fragments, shards, bubbles, hollow spherules (altered microtektites), small amounts of shocked mineral grains, and a subdued iridium anomaly. These components and textures, combined with the layer’s restricted areal distribution, indicate that this layer, called the “melt ejecta layer,” is the distal part of an ejecta blanket deposit. We interpret the melt ejecta layer to be an altered deposit of mostly impact-derived, shock-melted, silicic target material that traveled through the atmosphere within a detached ejecta curtain and on other ballistic trajectories.The upper laminated layer of the K/T boundary unit consists mostly of altered vitric dust and abundant shocked minerals whose size and amounts decrease away from the putative crater site in the Caribbean area. High-nickel magnesioferrite crystals, high iridium content, geochemical signature, and worldwide distribution all suggest this upper layer originated from a cloud of vaporized bolide and entrained target-rock materials ejected above the atmosphere. The components of this layer, called the “fireball layer,” settled slowly by gravitational processes from an Earth-girdling vapor cloud and were deposited immediately on top of the already-emplaced melt ejecta layer.The clay minerals that formed in the two layers are largely a function of composition and the highly unstable, shock-modified state of the fallout materials altered in acidic, organic-rich waters of ancient peat swamps. The fireball layer is mostly altered to smectitic clay from a mafic glass condensed from the vaporized chondritic bolide, along with some kaolinite formed from blebs of melted silicic target material entrained in the vapor plume cloud during ejection. In contrast, the melt ejecta layer is mainly kaolinitic, derived from silicic glass formed from melted target rocks. In this layer, the glass rapidly altered to mostly disordered, micrometer-sized “cabbage-like” or submicrometer-sized embryonic forms of spherical hal-loysite, probably from an allophane precursor. These crystallization characteristics of the melt ejecta layer are much different than those which formed coarse vermicular aggregates and platy kaolinite crystals in tonsteins from outside the K/T boundary interval throughout the Western Interior. The contrast in the incipient formation of dominantly kaolinitic clay minerals in the basal melt ejecta layer and of smectitic clay minerals in the overlying fireball layer reflect silicic versus mafic starting materials, respectively, and also supports the proposed two-phased meteorite impact ejection and dispersal model.During subsequent burial and diagenesis of the K/T boundary unit, the metastable halloysite and smectite aggraded to kaolinite and mixed-layer illite/smectite, respectively. Both the ordering of kaolinite and illitization of smectite varies locally as a function of the degree of diagenetic grade or maturity, probably in response to local variations in temperature due to maximum burial depth (burial diagenesis).

Shock-induced microdeformations in quartz and other mineralogical indications of an impact event at the Cretaceous-Tertiary boundary

Abstract The event terminating the Cretaceous period and the Mesozoic era caused massive extinctions of flora and fauna worldwide. Theories of the nature of this event can be classed as endogenic (volcanic, climatic, etc.) or exogenic (extraterrestrial causes). Mineralogical evidence from the boundary clays and claystones strongly favor the impact of an extraterrestrial body as the cause of this event. Nonmarine KT boundary claystones are comprised of two separate layers—an upper layer composed of high-angle ejecta material (shocked quartz, altered glass and spinel) and a basal kaolinitic layer containing spherules, clasts, and altered glass, together with some shocked grains. Recognition of this dual-layered nature of the boundary clay is important for the determination of the timing and processes involved in the impact event and in the assignment and interpretation of geochemical signatures. Multiple sets of shock-induced microdeformations (planar features) in quartz grains separated from KT boundary clays provide compelling evidence of an impact event. This mineralogical manifestation of shock metamorphism is associated worldwide with a large positive anomaly of iridium in these boundary clays, which has also been considered indicative of the impact of a large extraterrestrial body. Global distributions of maximum sizes of shocked quartz grains from the boundary clays and the mineralogy of the ejecta components favor an impact on or near the North American continent. Spinel crystals (magnesioferrite) occur in the boundary clays as micrometer-sized octahedra or skeletal forms. Their composition differs from that of spinels found in terrestrial oceanic basalts. Magnesioferrite crystals are restricted to the high-angle ejecta layer of the boundary clays and their small size and skeletal morphology suggest that they are condensation products of a vaporized bolide. Hollow spherules ranging up to 1 mm in size are ubiquitously associated with the boundary clays. In nonmarine sections, where a high-angle ejecta layer and an underlying kaolinitic layer can be distinguished, the spherules are found only in the kaolinitic layer. The morphologies and surface features of these spherules suggest that they are original forms, and not secondary growths or algal bodies. These impact spherules closely resemble microtektites in size and shape. All of these features of the boundary clay are uniquely associated with impact, and cannot have been formed by volcanic or other terrestrial processes.

Distinguishing shocked from tectonically deformed quartz by the use of the SEM and chemical etching

Abstract Multiple sets of crystallographically-oriented planar deformation features (PDFs) are generated by high-strain-rate shock waves at pressures of > 12 GPa in naturally shocked quartz samples. On surfaces, PDFs appear as narrow (50–500 nm) lamellae filled with amorphosed quartz (diaplectic glass) which can be etched with hydrofluoric acid or with hydrothermal alkaline solutions. In contrast, slow-strain-rate tectonic deformation pressure produces wider, semi-linear and widely spaced arrays of dislocation loops that are not glass filled. Etching samples with HF before examination in a scanning electron microscope (SEM) allows for unambiguous visual distinction between glass-filled PDFs and glass-free tectonic deformation arrays in quartz. This etching also reveals the internal ‘pillaring’ often characteristic of shock-induced PDFs. This technique is useful for easily distinguishing between shock and tectonic deformation in quartz, but does not replace optical techniques for characterizing the shock features.