Thermal evolution and sintering of chondritic planetesimals IV. Temperature dependence of heat conductivity of asteroids and meteorites

Understanding the compaction and differentiation of the planetesimals and protoplanets from the Asteroid Belt and the terrestrial planet region of the Solar System requires a reliable modeling of their internal thermal evolution. An important ingredient for this is a detailed knowledge of the heat conductivity of the chondritic mixture of minerals and metal in planetesimals. The temperature dependence of the heat conductivity is evaluated here from the properties of its mixture components by a theoretical model. This allows to predict the temperature dependent heat conductivity for the full range of observed meteoritic compositions and also for possible other compositions. For this purpose, published results on the temperature dependence of heat conductivity of the mineral components found in chondritic material are fitted to the model of Callaway for heat conductivity in solids by phonons. For the Ni,Fe-alloy published laboratory data are used. The heat conductivity of chondritic material then is calculated by means of mixing-rules. The role of micro-cracks is studied which increase the importance of wall-scattering for phonon-based heat conductivity. The model is applied to published data on heat conductivity of individual chondrites. The experimental data for the dependence of the heat conductivity on temperature can be reproduced rather well by the model if the heat conductivity is calculated for the composition of the meteorites. It is found that micro-cracks have a significant impact on the temperature dependence of the heat conductivity because of their reduction of phonon scattering length.Comment: 18 pages, 7 figures, accepted by Astronomy & Astrophysic

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

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

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

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

Thermal evolution and sintering of chondritic planetesimals III. Modelling the heat conductivity of porous chondrite material

The construction of models for the internal constitution and the temporal evolution of large planetesimals, the parent bodies of chondrites, requires information on the heat conductivity of the complex mixture of minerals and iron metal found in chondrites. It is attempted to evaluate the heat conductivity of a multi-component mineral mixture and granular medium from the heat conductivities of its mixture components. Random mixtures of solids with chondritic composition and packings of spheres are numerically generated. The heat conduction equation is solved in high spatial resolution for a test cube filled with such matter. From the heat flux through the cube the heat conductivity of the mixture is derived. The model results for porous material are consistent with data for compacted sandstone, but are at odds with measurements for H and L chondrites. The discrepancy is traced back to shock modification of the currently available meteoritic material by impacts on the parent body over the last 4.5 Ga. This causes numerous micro-cracks that act as additional barriers for heat transfer. The void structure in meteorites is different from that which probably existed in the pristine material of the parent bodies. The results obtained for the heat conductivity of the pristine material are used for calculating models for the evolution of the H chondrite parent body which are fitted to the cooling data of a number of H chondrites. The fit to the data good.Comment: 19 pages, 8 figures, accepted by Astronomy & Astrophysic