Ar-40 to Ar-39 ages of the large impact structures Kara and Manicouagan and their relevance to the Cretaceous-Tertiary and the Triassic-Jurassic boundary

Since the discovery of the Ir enrichment in Cretaceous-Tertiary boundary clays in 1980, the effects of a 10-km asteroid impacting on the Earth 65 Ma ago have been discussed as the possible reason for the mass extinction--including the extinction of the dinosaurs--at the end of the Cretaceous. But up to now no crater of this age that is large enough (ca. 200 km in diameter) has been found. One candidate is the Kara Crater in northern Siberia. Kolesnikov et al. determined a K-Ar isochron of 65.6 +/- 0.5 Ma, indistinguishable from the age of the K-T boundary and interpreted this as confirmation of earlier proposals that the Kara bolide would have been at least one of the K-T impactors. Koeberl et al. determined Ar-40 to Ar-39 ages ranging from 70 to 82 Ma and suggested an association to the Campanian-Maastrichtian boundary, another important extinction horizon 73 Ma ago. We dated four impact melts, KA2-306, KA2-305, SA1-302, and AN9-182. Results from the investigation are discussed

Early Thermal Evolution of Planetesimals and its Impact on Processing and Dating of Meteoritic Material

Radioisotopic ages for meteorites and their components provide constraints on the evolution of small bodies: timescales of accretion, thermal and aqueous metamorphism, differentiation, cooling and impact metamorphism. Realising that the decay heat of short-lived nuclides (e.g. 26Al, 60Fe), was the main heat source driving differentiation and metamorphism, thermal modeling of small bodies is of utmost importance to set individual meteorite age data into the general context of the thermal evolution of their parent bodies, and to derive general conclusions about the nature of planetary building blocks in the early solar system. As a general result, modelling easily explains that iron meteorites are older than chondrites, as early formed planetesimals experienced a higher concentration of short-lived nuclides and more severe heating. However, core formation processes may also extend to 10 Ma after formation of Calcium-Aluminum-rich inclusions (CAIs). A general effect of the porous nature of the starting material is that relatively small bodies (< few km) will also differentiate if they form within 2 Ma after CAIs. A particular interesting feature to be explored is the possibility that some chondrites may derive from the outer undifferentiated layers of asteroids that are differentiated in their interiors. This could explain the presence of remnant magnetization in some chondrites due to a planetary magnetic field.Comment: 24 pages, 9 figures, Accepted for publication as a chapter in Protostars and Planets VI, University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C. Dullemond, Th. Hennin

Isheyevo Meteorite: Genetic link between CH and CB chondrites?

Based on the mineralogy, petrography, bulk chemical, oxygen, and nintrogen isotopic compositions and 40Ar-39Ar age, Isheyevo is genetically related to CH and CB carbonaceous chondrites and provides a link between these group of pristine meteorites

Ar-40 to Ar-39 dating of pseudotachylites from the Witwatersrand basin, South Africa, with implications for the formation of the Vredefort Dome

The formation of the Vredefort Dome, a structure in excess of 100 km in diameter and located in the approximate center of the Witwatersrand basin, is still the subject of lively geological controversy. It is widely accepted that its formation seems to have taken place in a single sudden event, herein referred to as the Vredefort event, accompanied by the release of gigantic amounts of energy. It is debated, however, whether this central event was an internal one, i.e., a cryptoexplosion triggered by volcanic or tectonic processes, or the impact of an extraterrestrial body. The results of this debate are presented

The formation of the solar system

The solar system started to form about 4.56 Gyr ago and despite the long intervening time span, there still exist several clues about its formation. The three major sources for this information are meteorites, the present solar system structure and the planet-forming systems around young stars. In this introduction we give an overview of the current understanding of the solar system formation from all these different research fields. This includes the question of the lifetime of the solar protoplanetary disc, the different stages of planet formation, their duration, and their relative importance. We consider whether meteorite evidence and observations of protoplanetary discs point in the same direction. This will tell us whether our solar system had a typical formation history or an exceptional one. There are also many indications that the solar system formed as part of a star cluster. Here we examine the types of cluster the Sun could have formed in, especially whether its stellar density was at any stage high enough to influence the properties of today's solar system. The likelihood of identifying siblings of the Sun is discussed. Finally, the possible dynamical evolution of the solar system since its formation and its future are considered.Comment: 36 pages, 7 figures, invited review in Physica Script

Distribution of mantle and atmospheric argon in mantle xenoliths from the Western Arabian peninsula: Constraints on timing and composition of metasomatizing agents in the lithospheric mantle

To investigate the geochemical behaviour of argon isotopes during mantle metasomatism and to obtain chronological information on the age of metasomatic events under the Arabian Shield, we analyzed mantle xenoliths and hornblende megacrysts from Saudi Arabian volcanic fields (Uwayrid, Al Birk) applying the 40Ar-39Ar dating technique. Two hornblende megacrysts yield plateau ages indicating formation or total resetting of the K/Ar system 1-2 Myr ago. The ultramafic xenoliths trapped mantle-derived and atmospheric argon in different proportions, resulting in variable isotopic compositions: 40Ar/36Ar ratios range from 296 (i.e. atmospheric) to 10 500, reflecting interactions with isotopically and genetically different fluids and/or melts during recent mantle metasomatism. One such episode of metasomatism led to the formation of Ba-rich phlogopite, which yielded a saddle-shaped age spectrum with a maximum age estimate of 18 Ma. Another episode, inducing formation of secondary pargasite in the lithospheric mantle, was dated to 4 Ma. In the mantle xenoliths the concentration of mantle argon is clearly related to the intensity of metasomatism. Argon extraction by high-resolution stepwise heating allowed us to deconvolve various argon components distributed heterogeneously within single xenoliths and ascribe them to specific carrier phases. Pyroxenes generally preserve much higher 40Ar/36Ar ratios than olivine, as they contain up to 100 times higher concentrations of mantle argon, which also correlates with a higher fluid inclusion content in pyroxenes. Hydrous phases (phlogopite/amphibole) have more variable 40Ar/36Ar ratios. K and Cl concentrations and the argon isotope compositions of the Uwayrid xenoliths indicate distinct metasomatic agents, causing elemental and isotopic disequilibrium on a local scale. On the basis of correlations between Ar isotope composition and K and Cl concentration in the samples most strongly affected by the late metasomatic fluids, we suggest that metasomatic processes in the local mantle occurring simultaneously with the opening of the Red Sea were accompanied by the introduction of saline-water saturated fluids into deep lithospheric zones

Thermal history modeling of the H chondrite parent body

The cooling histories of individual meteorites can be empirically reconstructed by using ages from different radioisotopic chronometers with distinct closure temperatures. For a group of meteorites derived from a single parent body such data permit the reconstruction of the cooling history and properties of that body. Particularly suited are H chondrites because precise radiometric ages over a wide range of closure temperatures are available. A thermal evolution model for the H chondrite parent body is constructed by using all H chondrites for which at least three different radiometric ages are available. Several key parameters determining the thermal evolution of the H chondrite parent body and the unknown burial depths of the H chondrites are varied until an optimal fit is obtained. The fit is performed by an 'evolution algorithm'. Empirical data for eight samples are used for which radiometric ages are available for at least three different closure temperatures. A set of parameters for the H chondrite parent body is found that yields excellent agreement (within error bounds) between the thermal evolution model and empirical data of six of the examined eight chondrites. The new thermal model constrains the radius and formation time of the H chondrite parent body (possibly (6) Hebe), the initial burial depths of the individual H chondrites, the average surface temperature of the body, the average initial porosity of the material the body accreted from, and the initial 60Fe content of the H chondrite parent body.Comment: 16 pages, 7 figure

Результаты теоретических и опытных работ по изучению механизма работы буровых компоновок со смещенным центром масс поперечного сечения

Актуальность работы: необходимость повышения эффективности бурения геологоразведочных скважин в сложных горно-геологических условиях, в том числе связанных с естественным искривлением скважин. Цель исследования: разработка методики использования и конструкций буровых компоновок со смещенным центром масс (тяжести) поперечного сечения, позволяющих повысить эффективность бурения геологоразведочных скважин. Методы исследования: аналитические исследования, опытно-конструкторские работы и экспериментальные опытные работы. Результаты. Разработаны теоретические положения, методика применения и конструкции буровых компоновок со смещенным центром тяжести поперечного сечения; проведены производственные испытания различных конструкций буровых компоновок со смещенным центром тяжести поперечного сечения при различных способах бурения. Выводы. На основе разработанной модели движения буровых компоновок со смещенным центром тяжести поперечного сечения получены аналитические зависимости для расчета величин эксцентриситета поперечного сечения буровых компоновок, обеспечивающие их вращение вокруг оси скважины (вид Ф1), а также длину вовлекаемого в режим вращения Ф1 участка колонны, что позволяет создавать компоновки бурильной колонны, способные работать в более благоприятном режиме и обеспечивать повышение эффективности бурового процесса. На основе стандартных снарядов со съемным керноприемником типоразмера HQ разработаны и изготовлены трубы со смещенным центром тяжести, которые испытаны на производственных скважинах в составе компоновки, в которой размещено три трубы со смещенным центром тяжести поперечного сечения. Результаты испытаний показали, что в составе высокосбалансированных бурильных колонн снаряда со съемным керноприемником эффективно применение труб со смещенным центром тяжести: достигается снижение интенсивности естественного искривления скважин, снижается вибрация и затраты мощности на работу бурильной колонны.Relevance of the research is the necessity to increase the efficiency of drilling prospecting wells in difficult mining-and-geological conditions, including those connected with a natural curvature of wells. The aim of the research is to develop a technique of using and designs of boring configurations with the displaced cross section mass center which allow increasing the efficiency of drilling the prospecting wells. Research methods: analytical researches, developmental works and experimental skilled works. Results. The authors have developed the theoretical regulations, a technique of application and a design of boring configurations with the displaced cross section mass center and carried out the production tests of various designs of boring configurations of with the displaced cross section mass center when drilling. Conclusions. Based on the developed model of movement of boring configurations with the displaced center of gravity of cross section the authors obtained the analytical dependences for calculating the sizes of eccentricity of boring configuration cross section providing their rotation round a well axis (Ф1 type), as well as the length of the column part involved in the rotation mode Ф1 that allows developing the configurations of a boring column capable of operating in more favorable mode and providing the increase of boring efficiency. Based on standard shells with the removable core receiver of a standard size of HQ the pipes with the displaced cross section mass center were developed and produced. They were tested on production wells as a part of configuration in which three pipes with the displaced cross section mass center were placed. The results of the tests showed that it is efficient to apply the pipes with the displaced cross section mass center as a part of the high-balanced boring columns as the decrease in intensity of natural curvature of wells is reached, vibration and costs of power for boring column operation decrease