Eclogite-facies rocks in the Saxonian Erzgebirge,Germany: high pressure metamorphism under contrasting P-T conditions

The metamorphic sequences of the Saxonian Erzgebirge were thoroughly overprinted by a Variscan medium-pressure event under amphibolite facies conditions. However, eclogitic relics documenting an older high-pressure event are widespread. P-T conditions of the eclogite-facies metamorphism systematically decrease, over a distance of 50 km, from about >29 kbar/850°C, in the central part, to 20–24 kbar/650°C, in the westernmost part of the Erzgebirge crystalline complex. A distinct gap in P-T conditions exists between the central and the western Erzgebirge coinciding with the fault zone of the Flöha syncline. Therefore, the eclogitebearing sequences are assumed to represent at least two different nappe units. The lower-grade eclogite assemblages in the western Erzgebirge display a continuous metamorphic zonation with a gradual decrease of peak metamorphic temperatures towards the west. Assemblages formed in the stability field of coesite and thus indicating a regional ultra-high pressure metamorphism, are restricted to the central Erzgebirge, where they are widespread in the eclogites, but also present in metaacidic country rocks. The same high-temperature/high-pressure conditions, testifying to a burial of at least 100 km, were independently recorded for the ultramafic garnet pyroxenites associated with the eclogites of the central Erzgebirge. Mineral relics included in the eclogite phases and mineral assemblages formed by retrograde reactions permit reconstruction of the prograde and retrograde P-T paths in the different parts of the Erzgebirge crystalline complex.     

A cold air outbreak near Spitsbergen in springtime — Boundary-layer modification and cloud development

A moderate cold air outbreak from the Arctic ice over the warm West-Spitsbergen current on 15 and 16 May 1988 during the field experiment ARKTIS '88 is analysed using data from four aircraft and one research vessel.The downstream development of cloud coverage appears to depend sensitively on the moisture content above the inversion. The cloud amount determines the energy balance at the sea surface. Under daytime conditions and little cloud cover, energy is added to the ocean in spite of sensible and latent heat losses.The downstream temperature increase in the boundary layer is controlled by sensible heat flux and by longwave radiation cooling. The entrainment sensible heat flux is the dominating term in the region near the ice edge. The downstream moisture increase is controlled by surface evaporation. Condensation processes play no significant role.On 16 May 1988 cloud streets near the ice edge changed to closed cloud meanders in the downstream direction. The aspect ratio increased from 3 to around 10 over a distance of 200 km. In the cloud street region, the dynamical generation of turbulent kinetic energy due to wind shear at the tilted inversion was larger than the thermal generation.Cloud droplet concentration, mean droplet radius and liquid water content increased linearly with height. The maximum liquid water content was only 0.1 g/kg near the top of a 400 m thick closed cloud and clearly below the adiabatic value. The net longwave radiation flux decreased by 50 W/m² at cloud top and increased by 13 W/m² at cloud base.     

Measurement of total dissolved phosphorus in small volumes of iron rich interstitial water

It is shown that sorption of orthophosphate to iron compounds, formed during persulfate digestion, can cause a significant underestimation of total dissolved phosphorus in interstitial waters rich in iron and poor in phosphorus. Labelling the samples with carrier free³²PO₄ before digestion allows to correct for these losses.     

Geochemistry of basaltic and gabbroic rocks from the West Mariana basin and the Mariana trench

This paper describes the chemistry of 33 basaltic rocks dredged from the West Mariana basin and from the Mariana trench during the R/V “Dmitry Mendeleev” 1976 cruise in the western Pacific.The shipboard investigations were carried out by an international working group of 66 earth scientists under the IGCP Project “Ophiolites” and sponsored by the U.S.S.R. Academy of Sciences, Moscow. The purpose of the expedition was to investigate the structure and composition of the oceanic crust of marginal basins, remnant island arcs and deep-sea trenches. Tholeiitic basalts and gabbros as well as ultramafic rocks in various stages of alteration were dredged from the central part of the West Mariana basin demonstrating the presence of oceanic crust.The Pacific slope of the Mariana trench yielded altered basaltic rocks of tholeiitic and alkalic (? trachybasaltic to shoshonitic) composition. The lower part of the island arc slope contains typical tholeiitic basalts, dolerites and gabbros as well as ultramafics associated with flysch-type sediments. This is strong evidence for the formation of an “ophiolite-schuppenzone”, probably due to subduction of Pacific oceanic crust.Associated with these rocks are amygdaloidal, highly magnesian lavas (similar to boninites), which have not been recognized previously in oceanic ridge basalts.These rocks (together with the dolerites) are interpreted as parts of the Mariana island arc and are thought to be the first stage of island arc development (an immature island arc).     

Transmitted waves in the vicinity of the interface beyond the critical point

Summary The space (time) delay of transmitted waves in the vicinity of the interface beyond the critical point is investigated on two-dimensional seismic models of a low velocity channel; results of laboratory experiments are compared with the conclusions of the theory. The effect of the time delay in field measurements is demonstrated by means of two typical geological formations.     

Altersbestimmung nach der K-Ar-Methode

Zusammenfassung Seit ihrer Entdeckung vor 10 Jahren hat die K-Ar-Methode einen wesentlichen Beitrag zur Altersbestimmung geliefert. In diesem Artikel soll versucht werden, einen Überblick über den Stand und die Zuverlässigkeit der Methode zu vermitteln.Es werden Zerfallsdaten, Meßtechnik und Meßfehler kurz behandelt. An Hand einiger Resultate wird die Hauptfehlerquelle — das Problem der Argondiffusion — diskutiert und auf die Glimmer-Feldspat-Diskrepanz aufmerksam gemacht. Beim Vergleich mit anderen Methoden zeigen sich die Glimmeralter als sehr zuverlässig.Zum Schluß wird über die Ergebnisse der K-Ar-Alter an Meteoriten berichtet.     

Trocken- und Niederschlagsperioden und ihre theoretische Behandlung

Zusammenfassung Die beobachtete Anzahl von Trocken- und Niederschlagsperioden einer gewissen Länge und Mindestlänge sowie die mittlere Perioden-Andauer wird mit den Erwartungswerten nach der Theorie vonL. v. Bortkiewicz verglichen und keine befriedigende Übereinstimmung festgestellt; das gleiche ergibt sich bezüglich einer Poissonschen Verteilung. Berücksichtigt man eine eventuelle Verkettung der Merkmale trocken oder Niederschlag aufeinanderfolgender Tage nach der Theorie der Wahrscheinlichkeits-Ansteckung, so stehen Theorie und Beobachtung in weit besserer Übereinstimmung. Zur Erweiterung der Theorie ist zu berücksichtigen, daß die mittlere Andauer der Perioden bei verschiedenen Typen von Wetterlagen verschieden groß ist und daß eine Verkettung über die Größenordnung der Periodendauer hinaus noch dadurch zustande kommt, daß gewisse Wetterlagen gehäuft auftreten. Die Theorie führt zu zusammengesetzten Poisson-Verteilungen. Zahlenwerte über die mittlere Periodendauer bei verschiedenen Typen von Wetterlagen liegen noch nicht vor.

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Summary The observed number of dry and wet periods of a certain length and minimum length as well as the mean duration of the periods compared with the expectation values of the theory ofL. v. Bortkiewicz. As a result no satisfactory agreement was found. The result was the same with regard to a Poisson distribution. If an eventual concatenation of the characteristics dry or wet of successive days is taken into account according to the theory of probability contagion, theory and observation are in much better agreement. To extend the theory it must be considered that the mean duration of the periods varies with different types of weather conditions and that a concatenation, beyond the order of magnitude of the period duration, is caused by a greater frequency of certain weather situations. The theory leads to composite Poisson distributions. Numerical values of the mean period duration with different types of weather situations are not yet available.

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Résumé L'auteur compare le nombre de périodes sèches et humides d'une longueur déterminée ainsi que la durée moyenne des périodes aux valeurs résultant de l'application de la théorie deL. v. Bortkiewicz et ne trouve pas d'accord satisfaisant; la distribution de Poisson ne fournit pas de meilleur résultat. Si l'on considère un enchaînement éventuel entre jours secs et pluvieux du point de vue de la théorie de contagion probable, l'accord se révèle bien meilleur. Si l'on veut développer la théorie, il faut admettre que la longueur moyenne des périodes diffère selon les types de temps et que certaines périodes peuvent être anormalement longues par suite d'une fréquence accure de certaines situations météorologiques. On aboutit alors à des combinaisons de distributions de Poisson. On ne possède pas encore de données relatives aux périodes sèches et humides classées par types de temps.

     

Lebensspuren als Kennzeichen des Sedimentationsraumes

Zusammenfassung Es ist meist außerordentlich schwierig, bei den Lebensspuren (Bewegungs-, Freß-, Wohn- und Ruhespuren) den Urheber zu ermitteln, was ihren paläozoologisch-systematischen Wert mindert. Doch bestehen, wie in einem Rück- und Ausblick gezeigt wird, Möglichkeiten, die Lebensspuren zusammen mit anderen entsprechenden Kriterien für die Ermittlung des Sedimentationsraumes heranzuziehen. Von Beispielen der Gegenwart ausgehend, wird insbesondere auf die möglichen Fehlerquellen bei solchen paläogeographischen Rückschlüssen hingewiesen. Die weitere Vertiefung neo-ökologischer Studien und neues ichnologisches Beobachtungsmaterial aus allen Biotopen, auch aus der Tiefsee, lassen diejenige Sicherheit in der ökologischen Beurteilung fossiler Spuren und Spuren-Vergesellschaftungen erhoffen, die für die Rekonstruktions-Versuche der Umwelt- und Sedimentations-Bedingungen aus diesen Fossilien erforderlich ist.     

Ergebnisse von Versuchen der EAWAG mit dem Tauchtropfkörper

Ohne Zusammenfassung