Methoden zur Abschätzung der Wärmeflußdichte aus dem oberen Mantel

Zusammenfassung Die an der Erdoberfläche gemessene Wärmeflußdichte kann mit Hilfe verschiedener Methoden soweit reduziert werden, daß Aussagen über die Größe der Wärmeflußdichte aus dem oberen Mantel möglich sind.Die lineare Beziehung zwischen Wärmeproduktion und Wärmeflußdichte innerhalb einer Wärmeflußprovinz ergibt den reduzierten Wert direkt, ohne Modelle zum Aufbau der Erdkruste benutzen zu müssen.Für kleinere Gebiete kann die Wärmeproduktion unter der Annahme ihrer exponentiellen Abnahme mit der Tiefe aus der Struktur der Erdkruste abgeschätzt werden, und das Integral der Verteilung der radiogenen Wärmeproduzenten über die Tiefe ergibt die in der Kruste generierte Wärmeflußdichte, die, vom Oberfläcbenwert subtrahiert, die Wärmeflußdichte aus dem oberen Mantel ergibt.Eine weitere Methode wird aus Korrelationen zwischen der Wärmeproduktion sowohl mit der Dichte als auch mit der Kompressionswellengeschwindigkeit abgeleitet. Die Korrelationen ergeben aus gravimetrischen und seismischen Modellen ein Modell der Wärmeproduktionsverteilung in der Kruste, aus deren Integral und der Oberflächen-Wärmeflußdichte der Wert aus dem oberen Mantel abgeschätzt wird.

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The heat flow, measured at the surface can be reduced with the help of various methods, so that conclusions are possible about the quantity from the upper mantle.The linear relationship between heat generation and heat flow within a heat flow province yields directly the reduced heat flow without models of the structure of the crust.The heat generation can also be estimated from the structure of the crust for small areals with the assumption of its exponential depletion with depth. The integral of the distribution of the radiogen heat sources and the thickness of the crust yields the heat flow generated in the crust, so that the heat flow from the upper mantle is deducable from the value at the surface by substraction the value generated in the crust.Another method is deduced from correlations between heat generation as well with the density as with the longitudinal velocity. A model of the distribution of heat generation in the crust results directly from gravity and seismic models by using those correlations. The integral of the distribution of the heat sources in the crust yields the heat flow from the upper mantle by subtraction the value of the integral from the surface value of the heat flow.

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Resume La densité du flux géothermique mesurée à la surface, peut être réduite à l'aide de différentes méthodes, de manière à pouvoir en déduire la grandeur du flux thermique du manteau supérieur.La relation linéaire entre la production thermique et le flux thermique, donne directement la valeur réduite, sans qu'il y ait besoin d'utiliser des modèles de structuraux de la croûte.Dans les régions locales, la production thermique, en admettant qu'elle diminue exponentiellement avec la profondeur, peut être évaluée approximativement à l'aide de la structure de la croûte; l'intégrale de la distribution en profondeur des producteurs de chaleur radiogènes y donne la densité du flux thermique engendré dans la croûte, qui, soustraite de la valeur à la surface, donne la densité du flux thermique du manteau supérieur.Une autre méthode s'obtient à l'aide de corrélations entre la production thermique due à la densité et celle due à la vitesse des ondes de compression. Les corrélations, obtenues à l'aide de modèles gravimétriques et sismiques, donnent un modèle de la distribution de la production thermique dans la croûte, dont l'intégrale d'une part et la densité du flux thermique à la surface d'autre part, permettent d'évaluer approximativement la valeur relative au manteau supérieur.

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Distribution of uranium in intrusive bodies due to combined migration and diffusion

Uranium is enriched in the border zones of magmatic bodies and the enrichment is believed to be caused by the migration of hydrous solutions which carry that element along intergranular paths towards the contact zone with the wall rock. We propose that the contact zone is a geochemical barrier at which the uranium, present in the solution, would be deposited if it were not for diffusion away from the increased concentration at the margins.The two particle flows, the one caused by migration and the other caused by diffusion, can be described by a differential equation, whose solution is the concentration of uranium as a function of time, diffusion coefficient and velocity of migration.The distribution of uranium in two intrusive bodies, the Mont Blanc granite (Swiss Alps) and a pluton in the Dshetui-Oguz massif (U.S.S.R.), gives the following parameters: duration of process 0.3–1 m.y., diffusion coefficient 4 × 10^?4 to 5 × 10^?4 cm²/s, and velocity of migration 0.1–0.3 cm/year.The combined process of migration and diffusion is assumed to be an important mechanism for controlling the distribution of uranium throughout the earth's crust and for its change in geological time.     

Temperature paleogradient estimations in the central Bohemian basin

Summary The coalification data of 12 boreholes in the Central Bohemian Basin are used to evaluate the paleotemperature gradients for the Upper Carboniferous period of the basin's development. Two versions of the burial history considered are supposed to yield an upper and a lower estimate. According to the more probable lower version, the average values of the paleogradient suggest an increasing tendency from west to east in the interval of 45–53K/km. The current geothermal gradients vary in the range of 28–35K/km. By combining the present thermal conductivity and the paleogradients, we have tried to estimate the Upper Carboniferous heat flow. Its values range from 96mW/m ² to 117mW/m ² .The results obtained can be compared with the paleogradient estimates in the Saar-Nahe Basin (F. R. of Germany). This region, which is similar with respect to the time of origin and tectonic pattern to the Central Bohemian Basin, displays on the average a slightly higher Permo-Carboniferous geothermal gradient of 60K/km.     

Heat-generating radioelements in granitic magmas

Spatial variation trends of the radioelements U, Th and K in granitic units show distinct regularities: zonation patterns of U and Th (at fairly constant K contents) are common in individual granitic bodies. It is shown that radioelement abundance and distribution result from a combined migration-diffusion process in the fluid phase during and after solidification of the granite magma. The process is mainly governed by pressure and temperature gradients as well as by chemical potentials. Model calculations yield numerical values for the intrinsic diffusion coefficient (≈ 10⁻⁴ cm²/s) and migration velocity (0.2–2 cm/s) which are in agreement with laboratory experiments on comparable systems. The process distributing the radioelements is of limited duration; it is also clearly related to the cooling history of the granite. Analogies to the overall distribution of crustal radioactivity are discussed.     

The degree of metamorphism of organic matter in sedimentary rocks as a paleo-geothermometer,applied to the Upper Rhine Graben

The subsidence of sedimentary layers implies increasing temperature downwards within the sedimentary column, so that the degree of coalification of organic matter increases continually. Apart from temperature, the slowly reacting chemical compounds of the organic matter strongly depend on time, too.It is shown that the coal rank is proportional to the integral of temperature and time of burial (t) for the Tertiary sedimentary rocks of the Upper Rhine Graben. This relationship is used to calculate paleogeothermal gradients (gradT) for some boreholes in the Upper Rhine Graben, from which the rate of burial during geological history (z(t)) is known. The degree of coalification is measured by its mean optical reflectivity (R _m), so that the relationship between coalification and geothermal history isR _m ² gradT z(t) dt.The results show high heat flow during Lower Tertiary and a decrease during Upper Tertiary at some locations of the Upper Rhine Graben. The recent high heat flow is not detectable in coalification. The young thermal anomaly is perhaps caused by ascending pore fluid and/or by heat conduction from a heat source in the lower crust.