Untersuchungen an Riesgläsern

Zusammenfassung Untersuchungen und Überlegungen geologischer und mineralogischer Art erbrachten folgende neuen Erkenntnisse über die Entstehung der Flädle, der Suevite und.damit auch des ganzen Rieskessels:Eine niemals mit der Suevitmatrix verschweißte Oberfläche, Bruchstücke im Anstehenden, das rasche Verschwinden der Schlieren und Kristallfragmente, beim Wiederaufschmelzen beweisen neben dem Auftreten von Stishovit und den Hochdruckgläsern die außerordentlich rasche Abkühlung der Flädle. Weitere Anzeichen dafür können aus ihrem Oxydationsverhalten gewonnen werden. Die Flädle waren nach ihrem zum Teil 20 km langen Flug durch die Luft beim Aufprall bereits erkaltet.Lage und Ausbildung des Randwulstes, des schlierenreichen Außensaumes, die Einregelung der Blasen und die Striemung der Oberfläche zeigen, daß die Fluggeschwindigkeiten so groß waren, daß typische Ablationserscheinungen auftraten. Diese sind bis jetzt von vulkanischen Ereignissen unbekannt.Horizontale Einregelung der, Flädle, gleichbleibende Größenverhältnisse sowie überraschend eintöniger Çhemismus der Gläser stellen einzelne Förderschlote sehr stark in Frage. Die ständige Unterlagerung von Bunter Bresche der heutigen Suevitvorkommen ist ein weiteres Anzeichen für eine einstmals mehr oder weniger zusammenhängende Suevitdecke. Die radialen Flugrichtungen deuten auf ein gemeinsames Förderzentrum hin, das mit dem eigentlichen Explosionszentrum zusammenfällt. Die Anwesenheit von Coesit, Stishovit, isotropisiertem Quarz und Feldspat sowie der neu gefundenen Hochdruckgläser sind Beweise dafür, daß die Suevite keiner vulkanischen Nachphase zugeordnet werden können, sondern unmittelbar zur Rieskatastrophe gestellt werden müssen. Die Suevite flogen unmittelbar nach der Bunten Bresche als letztes Produkt der eigentlichen Rieskatastrophe aus dem Explosionszentrum und vervollständigen im Ries das Bild der inversen Lagerung. Die inverse Lagerung ist zwar kein Beweis für die Impact-Theorie, aber dennoch sehr charakteristisch für Meteoritenkrater.Aus dem Fe/FeVerhältnis der Gläser gelang es, Anhaltspunkte über die hohen Schmelztemperaturen während der Rieskatastrophe zu erhalten (1500°). Die Beobachtungen vonEl Goresy ergaben sogar Temperaturen über 1700° C. WieEl Goresy richtig betonte, sind diese bei Vulkanen nicht zu erwarten und als übergeologisch zu betrachten.Zu den bisher bekannten Hochdruckindikatoren wurden die Hochdruckgläser mit erhöhter. Dichte und Lichtbrechung hinzugefügt. Wie Stishovit, Coesit, isotropisierter Quarz und Feldspat konnten auch sie in vulkanischen Gläsern bisher nicht nachgewiesen werden. Das Auftreten dieser Hochdruck-indikatoren ist nach wie vor das stärkste Argument für die Impact-Theorie.Die tektonische Lage des Rieskessels kann nicht als Gegenargument für die Meteoritentheorie angeführt werden, da selbst eine vollkommen intakte Albtafel für die Drucke, wie sie im Ries eindeutig nachgewiesen sind, ein viel zu schwaches Widerlager abgegeben hätte. Außerdem stellt das Fehlen von Tridymit und Cristobalit eine phreatische Sprengung in Frage.

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Mineralogical and geological investigations concerning the glasses of the Ries-Basin, S-Germany, were made in order to clear the question, whether this large circular depression is caused by volcanic explosion or meteorite impact.The Ries-glasses consist, without a doubt, of molten. crystalline basement rocks, which are in S-Germany covered by 600 m of mesozoik sediments. These glasses are embedded in the so called Suevit, an uncommon breccia of molten material, crystalline and sedimentary rocks. This Suevit is considered to be a proof for volcanic events in the Ries-Basin.The Ries-glasses (Riesbombs, Flädle) show typical aerodynamic shapes and influences of ablation, caused by such high flying velocity as is unknown for volcanic bombs.Through field studies and chemical analysis the old theory, that nearly each Suevit-occurrence belongs to a single pipe, seems to be refuted. The genesis of the Suevit is presumably the following: As the last product of the Ries-explosion, the Suevit flew out of the crater and covered., as a more or less coherent veil, the sedimentary Bunte Bresche. What we see today are only remains of this veil. The inverse layering of the sedimentary breccias is an old fact. In this way, the Suevit seems to complete the inverse layering of all Ries-breccias.Melting experiments show, that the Ries basement rocks were heated rapidly and then cooled instantly. The quenching of the Ries-melt has been confirmed by different observations.The monoton chemism of the Ries-glasses from different localities demonstrates, that a homogeneous basement complex was molten, presumably a gneisic one. Through the Fe/Fe. ratio of glasses and crystalline inclusions we get a hint about the temperatures during the Ries-catastrophe (about 1500°C). Ni- and Co-contents arc neither pro nor Contra an impact origin.By remelting the Ries-glasses, real high-presure-glasses were found, which are a further proof for the very high pressures during the Ries-explosion. They arc together with coesite, stishovite, isotropic quartz and felspar the strongest argument for the impact origin.These investigations and some considerations to volcanic arguments lead us to the conclusion, that the Ries-basin was caused by meteorite impact.

     

Biochemie und Feinstruktur Fossiler Knochen aus Bituminösen Schichten

Zusammenfassung Eingangs wird kurz das Untersuchungsmaterial, seine stratigraphische Eingliederung und seine lokale Herkunft skizziert. Sämtliches Wirbelmaterial wurde aus bituminösen Schichten verschiedenen Alters entnommen. Bei der Bestimmung des Gehalts an organischer Substanz dor Wirbelknochen trennte ich zwischen ursprünglicher, d. h. knocheneigener organischer Substanz und aus dem Sediment eingewanderter organischer Substanz während der Einbettung der Knochen bzw. später.Chemismus und Feinstruktur der fossilen Wirbelknochen zeigen keine wesentlichen Unterschiede gegenüber den rezenten. Lediglich die Aminosäuren Histidin und Methionin fehlen unter denjenigen, die das rezente Kollagen enthält. Auf Grund ihres einerseits mengenmäßigen geringen Vorhandenseins im Kollagen und andererseits wesentlich unbeständigeren Charakters ist ihr Fehlen verständlich. Die röntgenographische Untersuchung der Knochen mit dem Zählrohrgoniometer und die polarisationsmikroskopische Betrachtung der Knochendünnschliffe zeigt, daß keine knochenfremden Mineralien in die Knochen eingewandert bzw. dort auskristallisiert sind. Elektronenmikroskopische Untersuchungen wurden gleichfalls an den Knochen durchgeführt. Die bisherigen vorläufigen Ergebnisse lassen noch nicht klar genug erkennen, ob die Veränderung der Knollagenstruktur auf das absolute geologische Alter der Knochen oder auf die nicht völlig identische Einbettung derselben in die bituminösen Schichten oder den Chemismus bzw. Bitumengehalt des Sediments zurückzuführen ist.Der unterschiedliche Feinbau der Wirbelknochen läßt eine verschieden starke Adsorptionsfähigkeit für Bitumen erkennen. Den höchsten Gehalt an Bitumen zeigt die Zone der inneren Generallamellen und das umgewandelte Gewebe der Chordascheide und des Chordagewebes bei den Ichthyosaurierwirbeln.

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The author analysed vertebrae taken from strata of different geological age. For some specified reasons only bones of bituminous layers have been analysed. These layers and the localities are: the bituminous schist of the Middle Alpine Trias (Anisian/Ladinian) of the Monte San Giorgio (Kanton Tessin, Switzerland), the Lias epsilon (Posidonomya Shales, Toarcium) of Holzmaden (Baden-Württemberg, Germany), and the bituminous swamps of Rancho La Brea (USA). It has turned out that under these special conditions of bedding the collagenous structure has been preserved to a great extent, and no migration of minerals out of the sediments into the bones has taken place. This could be proved by the use of an X-ray-counting goniometer (Zählrohrgoniometer).Comparing analyses of vertebrae taken from non-bituminous layers showed that in the those cases the collagenous structure had been destroyed, and thatThis means that the bitumen must have acted as a protecting film on the bones.

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Résumé Au départ seront brièvement esquissés le matériel de recherche, son enchaînement stratigraphique et son origine locale. L'ensemble du matériel vertébral a été retiré de couches bitumineuses d'âges différents. Par la détermination du contenu de la substance organique des vertèbres, j'ai séparé la substance primitive, c'est-à-dire organique propre aux os, du sédiment, de la substance organique implantée pendant le gisement des os ou après.Par rapport aux vertèbres actuelles, le chimisme et la configuration intime (constitution intime) des vertèbres fossiles n'offrent pas de différences essentielles. Simplement, parmi les acides aminés, l'histidine et la méthionine font défaut parmi ceux que contient le collagène récent. En raison de leur existence en faible quantité d'une part, et de leur caractère essentiellement labile d'autre part, leur absence est compréhensible.L'exploration radiographique des os au compteur goniomètrique et l'examen micropolarographique des coupes minces osseuses montrent qu'aucune substance minérale étrangère n'a été implantée dans les os, ou fixée. Des recherches au microscope électronique ont également été menées sur les os. Les résultats jusque là provisoires ne font pas encore assez clairement ressortir si la transformation de la structure du collagène est à rapporter à l'âge géologique absolu des os ou à leur localisation (Einbettung) non parfaitement identique dans les couches bitumineuses, ou encore au chimisme, disons plutôt au contenu bitumineux du sédiment.La constitution intime différente des vertèbres fait apparaître un potentiel d'adsorption variable pour les bitumes. C'est dans la zone des lamelles générales intérieures (inneren Generallamellen) et dans le tissu transformé (umgewandelte) de la gaine et des tissus chordaux des vertèbres d'Ichthyosaure que l'on trouve la plus grande teneur en bitumes.

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Redistribution of HFSE elements during rutile replacement by titanite

Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromsø eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry.     

Complete recycling of a magmatic arc: evidence from chemical and isotopic composition of Quaternary trench sediments in Chile (36°–40°S)

Marine Quaternary trench and slope sediments were sampled along the margin of the Southern Andes, Chile between 36° and 40°S. Major and trace element contents indicate only minor influence of weathering and transport fractionation. The whole rock composition of the sediments is similar to the average rock of the Cretaceous to Holocene magmatic arc of this section of the southern volcanic zone. Sr, Nd, and Pb isotope composition of the sediments also resembles closely the average composition of the magmatic arc. The contribution of compositionally distinct Palaeozoic crust, which makes up most of the volume of the forearc, is ~0–20% crustal Sr, Nd, and Pb according to the isotope record of the trench and slope sediments. Input of sediments from the continent into the subduction system was dominated by detritus from the magmatic arc at least for the last 20 My on the basis of the Oligocene to Holocene exhumation history of the margin.     

XMM–Newton observations of bright ROSAT selected active galactic nuclei with low intrinsic absorption

We present a sample of 21 ROSAT bright active galactic nuclei (AGNs), representing a range of spectral classes, and selected for follow-up snapshot observations with XMM–Newton . The typical exposure was between 5 and 10 ks. The objects were primarily selected on the bases of X-ray brightness and not on hardness ratio; thus the sample cannot be strictly defined as a 'soft'sample. One of the main outcomes from the XMM–Newton observations was that all of the AGN, including 11 type 1.8–2 objects, required low levels of intrinsic absorption  ( N _H≲ 10²¹ cm⁻²)  . The low absorption in type 2 systems is a challenge to account for in the standard orientation-based unification model, and we discuss possible physical and geometrical models which could elucidate the problem. Moreover, there does not appear to be any relation between the strength and shape of the soft excess, and the spectral classification of the AGN in this sample. We further identify a number of AGN which deserve deeper observations or further analysis: for example, the low-ionization nuclear emission regions (LINERs) NGC 5005 and NGC 7331, where optically thin thermal and extended emission is detected, and the narrow-line Seyfert 1 II Zw 177, which shows a broad emission feature at ∼ 5.8 keV.     

Cenozoic basin development and its indication of plateau growth in the Xunhua-Guide district

The Xunhua, Guide and Tongren Basins are linked with the Laji Mountain and the northern West Qinling thrust belts in the Xunhua-Guide district. Basin depositional stratigraphy consists of the Oligocene Xining Group, the uppermost Oligocene-Pliocene Guide Group and the Lower Pleistocene. They are divided into three basin phases by unconformities. Basin phase 1 is composed of the Xining Group, and Basin phase 2 of the Zharang, Xiadongshan, Herjia and Ganjia Conglomerate Formations in the Guide Group, and Basin phase 3 of the Gonghe Formation and the Lower Pleistocene. Three basin phases all develop lacustrine deposits at their lower parts, and alluvial-braided channel plain depositional systems at upper parts, which constitute a coarsening-upward and progradational sequence. Basin deposition, paleocurrent and provenance analyses represent that large lacustrine basin across the Laji Mountain was developed and sourced from the West Qinling thrust belt during the stage of the Xining Group (Basin phase 1), and point-dispersed alluvial fan-braided channel plain deposition systems were developed beside the thrust and uplifted Laji Mountain and sourced from it, as thrusting migrated northwards during the stage of the Guide Group (Basin phase 2). Evolution of basin-mountain system in the study area significantly indicates the growth process of the distal Tibetan Plateau. The result shows that the Tibetan Plateau expanded to the northern West-Qinling at Oligocene (29–21.4 Ma) by means of northward folded-and-thrust thickening and uplifting and frontal foreland basin filling, and across the study area to North Qilian and Liupan Mountain at the Miocene-Pliocene (20.8–2.6 Ma) by means of two-sided basement-involved-thrust thickening and uplifting and broken foreland basin filling, and the distant end of Tibetan Plateau behaved as regional erosion and intermontane basin aggradational filling during the Pliocene and early Pleistocene (2.6–1.7 Ma).

     

Shock-implanted noble gases II: Additional experimental studies and recognition in naturally shocked terrestrial materials

Abstract— Several experimentally and naturally shocked silicate samples were analyzed for noble gas contents to further characterize the phenomenon by which ambient gases can be strongly implanted into silicates by shock and to evaluate the possible importance of this process in capturing planetary atmospheres in naturally shocked samples. Gas implantation efficiency is apparently mineral independent, as mono-mineralic powders of oligoclase, labradorite, and diopside and a powdered basalt shocked to 20 GPa show similar efficiencies. The retentivity of shock-implanted gas during stepwise heating in the laboratory is defined in terms of two parameters: activation energy for diffusion as determined from Arrhenius plots, and the extraction temperature at which 50% of the gas is released, both of which correlate with shock pressure. These gas diffusion parameters are essentially identical for radiogenic ⁴⁰Ar and shock-implanted ⁴⁰Ar in oligoclase and labradorite shocked to 20 GPa, suggesting that the two ⁴⁰Ar components occupy analogous lattice sites. Our experiments indicate that gas implantation occurs through an increasing production of microcracks/defects in the lattice with increasing shock pressure. The ease of diffusive loss of implanted gas is controlled by the degree of annealing of these microcracks/defects. Identification of a shock-implanted component requires relatively large concentrations of implanted gas which is strongly retained (i.e., moderate activation energy) in order to separate implanted gas from surface adsorbed gases. Literature data on shocked terrestrial samples indicate only weak evidence for shock-implanted gases, with an upper limit for ⁴⁰Ar of ～ 10^?6 cm³STP/g. New analyses of shocked samples from the Wabar Crater indicate the presence of shock-implanted Ar, having concentrations (～ 10^?4 cm³STP/g) and activation energies for diffusive loss which are essentially that expected from experimental studies. Lack of sufficient target porosity or the presence of ground water may explain the sparse evidence for shock-implanted gas at other terrestrial craters. Although Wabar Crater may represent an unusually favorable environment on Earth for shock-implanting gases, surfaces of other planetary bodies, such as Mars, may frequently provide such environments. Analyses of returned samples from old Martian terraines may document temporal changes in earlier atmospheric composition.     

Lower Jurassic magnetostratigraphy at the Breggia Gorge (Ticino, Switzerland) and Alpe Turati (Como, Italy)

Summary. The Breggia gorge (Ticino, Switzerland) provides a continuous outcrop of Mesozoic limestones covering the time between the Lower Jurassic and Upper Cretaceous. The Jurassic section from the Lower Pliens-bachian to Aalenian is well dated by ammonites. The limestones of these stages are a good quality recorder of the polarity of the geomagnetic field during this time interval. More than 500 cores have been drilled along a 120 m thick section with a median distance between cores of about 20 cm. Detailed thermal and af demagnetization and further rock magnetic studies were used to determine the characteristic NRM direction for each sample. Initial NRM intensities and low field susceptibilities reflect variations in oxidation potential and sedimentation rate during carbonate deposition. The Breggia section covers an estimated 12Myr of Lower to Middle Jurassic time during which more than 60 distinct reversals of the geomagnetic field are recorded. This corresponds to a comparatively high mean reversal frequency of about 5 reversals Myr⁻¹. The Breggia polarity zonation can be correlated with the nearby section at Alpe Turati (Como, Italy) and comparable polarity patterns are recognized in other biostratigraphically well-dated magnetostratigraphic profiles from Hungary and Umbria (Italy).