747 research outputs found
Zirconium and hafnium in meteorites
No full textThe ratio of the two refractory trace elements zirconium (Zr) and hafnium (Hf) in meteorites has been proposed to be uniform. The most precise value available is 34.3 +/- 0.2 (1 sigma). It was obtained by isotope dilution ICP-MS applied to 15 chondrites, most of which were carbonaceous chondrites, and six achondrites. We reinvestigated the case and determined Zr/Hf ratios of a broad spectrum of meteoritic samples via laser ablation ICP-MS. Our sample suite comprised 29 chondrites and five achondrites. The main objective of the study was two-fold: we intended to verify the accuracy and precision of a relatively fast and inexpensive sample preparation method combined with expeditious laser ablation ICP-MS techniques. Furthermore, we were looking into the possibility of systematic fine-scale Zr/Hf variations among bulk meteoritic matter of different classes. The applied fusion technique together with laser ablation ICP-MS turned out to be well suited to determine relative refractory trace element abundances. Absolute Zr/Hf ratios yield uncertainties of approximately 4% (1 sigma). As opposed to the most recent findings, we observed variable Zr/Hf ratios in different meteorites ranging from approximately 28 to approximately 38. Our value for Orgueil (CI1) is 34.0 +/- 0.3 (1 sigma). Including literature data, we propose a solar system value of 34.1 +/- 0.3. Our data also suggest that H chondrites tend to exhibit higher Zr/Hf ratios (average of 35.6 +/- 0.5 [1 sigma]) while EL6 chondrites rather show low values (average of 30.8 +/- 0.6 [1 sigma]). In addition to examining Zr/Hf ratios, we also explored the content of refractory major elements in different meteorite groups. Here, we found that EL6 chondrites often show very low Ca/Al ratios. The CI1 value for CaO/Al(2)O(3) is 0.804. EL6 chondrites, however, display ratios as low as approximately 0.3. While the variation in Zr/Hf can be explained by fractional condensation processes in the early solar nebula, the observed low Ca/Al ratios in EL6 chondrites are probably attributable to deficits in oldhamite (CaS).[PA909/2-1]; [PA909/2-2
Age and geochemistry of metabasites of the Raabs Unit in the Moldanubian Zone of Lower Austria (Bohemian Massif)
No full textIn dieser Arbeit werden die Analysenergebnisse der Beprobungskampagnen von 2003 und 2006 an basischen Gesteinen des Waldviertels präsentiert. Insgesamt wurden elf Proben an der Universität Frankfurt von Biljana Starijaš Mayer und Axel Gerdes einer Zirkondatierung unterzogen, während viele weitere Proben an der Universität Salzburg geochemisch mittels Röntgenfluoreszenzmethodik analysiert wurden. Ziel ist es, die bisher unpublizierten Daten einer breiteren Öffentlichkeit zugänglich zu machen und somit zu einem besseren Verständnis der geologischen Rahmenbedingungen der Raabs-Einheit beizutragen. Eine Interpretation der Ergebnisse ist im Online-Katalog der GeoSphere Austria verfügbar mit dem Titel "Alter und Geochemie der Metabasite der Raabs Einheit im niederösterreichischen Moldanubikum (Böhmische Masse)" veröffentlicht von Andreas Mayer, Biljana Starijaš Mayer, Axel Gerdes und Fritz Finger.This paper presents the analytical results of the 2003 and 2006 sampling campaigns on basic rocks of the Waldviertel. A total of eleven samples were subjected to zircon dating by Biljana Starijaš Mayer and Axel Gerdes at the University of Frankfurt, while many other samples were analysed geochemically at the University of Salzburg using X-ray fluorescence methods.The aim of this report is to make the previously unpublished data available to a wider public and thus contribute to a better understanding of the geological framework of the Raabs unit. An interpretation of the results is available in the online catalogue of GeoSphere Austria with the title "Alter und Geochemie der Metabasite der Raabs Einheit im niederösterreichischen Moldanubikum (Böhmische Masse)" publishied from Andreas Mayer, Biljana Starijaš Mayer, Axel Gerdes und Fritz Finger.Die U-Pb Isotopenanalyse der in Harz eingegossenen und angeschliffenen Zirkone erfolgte mittels Laser und einem Sektorfeld-ICPMassenspektrometer an der Universität Frankfurt (Gerät Finnigan Element 2). Mit dieser Methode ließen sich ca. 30 µm große Domänen in den Zirkonen selektiv analysieren. Die Messpunktauswahl wurde aufgrund der zuvor erstellten CL-Bilder getroffen, wobei das Bestreben bestand, möglichst homogene und nicht alterierte Zirkonbereiche zu erfassen. Bezüglich Messtechnik und Analysegenauigkeit der Methode siehe GERDES & ZEH (2006) und FREI & GERDES (2009). Die Daten beziehen sich in der Regel auf ca. 1 kg Handstücke, welche genetisch homogen erschienen. Das Material wurde in einer Achatmühle gemahlen und anschließend einer Röntgenfluoreszenzanalyse unterzogen. Zur Verwendung kam dabei ein Gerät S4 Pioneer der Firma Bruker AXS. Die erzielte Analysegenauigkeit (2 σ) ist besser als 1% (relativ) für SiO2 and Al2O3 und besser als 5% (relativ) für Elemente im 1–10 Gew. % Konzentrationsniveau. Die Hauptelementkonzentrationen sind wasserfrei gerechnet. Die Nachweisgrenze für Spurenelemente ist zumeist ca. 2-3 ppm (3 σ), die Genauigkeit der Spurenelementanalyse liegt bei 1–2 ppm bei niedrigen Konzentrationen (<10 ppm), 2–5 ppm bei Konzentrationen von 10–100 ppm und bei maximal 5% (relativ) für höhere Spurenelementkonzentrationen
The East Variscan Shear Zone:geochronological constraints from the Capo Ferro area (NE Sardinia, Italy)
No full textThe exhumation of the Variscan High Grade Metamorphic Complex of Sardinia has been driven by a wide dextral shearing which reached middle crustal levels at 325 ±1.3 Ma and caused partial melting. The migmatization process has been followed by the emplacement of syn-tectonic granites dated at 316 ±2 Ma, 318 ±3 Ma and 317 ±2 Ma which produced contact metamorphism at 315 ±1.3 Ma in mylonitic Silurian orthogneiss. The NW-SE dextral shear zone of the Capo Ferro area (NE Sardinia) shows remarkable petro-tectonic analogies and can thus be related to the East Variscan Shear Zone, a major intra-continental shear zone which was active in the southern European Variscides from 325 Ma to 315 Ma and that is precisely dated with the U-Pb monazite system. The emplacement of migmatitic gneisses along large shear zones within this time interval is not restricted to Sardinia, but has also been observed in southern Corsica, in the French Massif Central, in the Western Alps and in central Spain. This pattern of shear zones is related to a complex geodynamic scenario, linked to the oblique convergence between Gondwana and Laurussia plates during the Middle-Upper Carboniferous
Polyphase Brittle Tectonics in North Victoria Land and Gondwana Fragmentation
No full textAntarctica was at a centre position within Gondwana and holds a key position for any plate tectonic reconstruction related to its break-up history. North Victoria Land (NVL) is located at the Pacific end of the Transantarctic Mountains (TAM), which represent the uplifted western shoulder of the West Antarctic Rift System. The basement of the TAM formed during subduction of the Palaeopacific Ocean under E Gondwana during the Ross Orogeny. Major uplift of the TAM commenced around the Eocene-Oligocene boundary following sediment accumulation within the wide “Mesozoic Victoria Basin”. This long-lasting geological history led to highly anisotropic crust that is susceptible to repeated reactivation. We present evidence for a polyphase structural evolution of NVL after initial break-up of Gondwana at ca. 180 Ma coeval with the Ferrar volcanic event. Mainly Neogene NW-SE striking dextral strike-slip tectonics with local transtension and transpression controls the present structural architecture of NVL. It may be interpreted as dynamic response to intra-oceanic fracture zones between Australia and Antarctica extending into NVL and the Ross Sea. Dextral strike-slip overprints two older increments: (i) WNW-ESE striking sinistral tectonics of possibly late Mesozoic-early Paleogene age within a transform margin setting between Australia and Antarctica, (ii) ENE-WSW directed extension of NVL continental crust possibly coeval to Adare Trough spreading in Eocene-Oligocene times
Provenance of the Surveyor Fan and Precursor Sediments in the Gulf of Alaska—Implications of a Combined U-Pb, (U-Th)/He, Hf, and Rare Earth Element Study of Detrital Zircons
No full textThe East Variscan Shear Zone: geochronological constaints from the Capo Ferro area (NE Sardinia, Italy)
No full textThe exhumation of the Variscan High Grade Metamorphic Complex of Sardinia was driven by large-scale asymmetric shear zones which reached middle crustal levels at 325 ± 1.3 (U–Pb, monazite) Ma. Exhumation was
accompanied by heating and partial melting. The migmatization process was followed by the emplacement of
syntectonic granites dated with U–Pb on zircon at 318 ± 3 Ma and 317 ± 2 Ma, which produced contact metamorphism at 315 ± 1.3 Ma (U–Pb, monazite) in mylonitic Silurian orthogneisses. The NW–SE dextral shear zone
of the Capo Ferro area (NE Sardinia) is affected by partial melting, by granite emplacement and shows remarkable
petrotectonic analogies to, and can thus be related to the East Variscan Shear Zone, a major intra-continental
shear zone which was active in the southern European Variscides from 325 Ma to 315 Ma and that has been precisely dated with the U–Pb monazite system. The emplacement of migmatitic gneisses along large shear zones
within this time interval is not restricted to Sardinia, but has also been observed in southern Corsica, in the French
Massif Central, in the Western Alps and in central Spain. This pattern of shear zones is related to a complex
geodynamic scenario, linked to the oblique convergence of the Gondwana and Laurussia plates during the
Middle–Upper Carboniferous
Zircon U-Pb geochronology and heavy mineral composition of the Camana Formation, southern Peru: Constraints on sediment provenance and uplift of the Coastal and Western Cordilleras
No full textIn the forearc of the Central Andes of southern Peru, the Cenozoic Camana Basin (16 degrees 25'S to 17 degrees 15'S) forms a similar to NW-SE elongated depression filled with coarse-grained deltaic and fluvial deposits. These deposits are termed Camana Formation. We have applied for the first time, advanced multi-method analytical techniques to sediments of the Camana Formation in order to define precise sedimentation ages, unravel sediment provenance, and to explain its tectono-sedimentary evolution. Zircon U-Pb geochronology and multiple geological evidences suggest that the Camana Formation ranges in-age from Late Oligocene to Late Miocene, and may even extend into the Pliocene. We propose a provenance model for the Camana Formation based on U-Pb geochronology, heavy mineral analysis, and single-grain mineral chemistry by LA-ICP-MS. This model suggests that sediments of the lower part of the Camana Formation derive from rocks forming the Coastal Cordillera (i.e. the Arequipa Massif and the San Nicolas Batholith) and the widespread ignimbrites of the similar to 24-10-Ma Huaylillas volcanic arc. In contrast, sediments of the upper part of the Camana Formation derive predominantly from rocks forming the Western Cordillera (i.e. the Arequipa Massif, the Tacaza Group, and the Coastal Batholith) and products of the similar to 10-3 Ma Lower Barroso volcanic arc). Accordingly, we infer that uplift of the Coastal Cordillera has strongly influenced deposition of the Camana Formation since Late Oligocene. A marked shift in provenance within the Camana Formation at around Middle to Late Miocene time (14-12 Ma) suggests drastic uplift of the Western Cordillera at that time. This uplift has triggered increased relief and erosion in the Western Cordillera, and subsequent deposition of fluvial conglomerates in the Camana Basin. (C) 2015 Elsevier Ltd. All rights reserved.Deutscher Akademischer Austauschdienst (DAAD) [416/PKZ A/09/98944
Reseña de "From Ethnomathematics to art-design matrices and cyclic matrices" de Paulus Gerdes
No full textIn this review, we present the ethnomathematics investigations undertaken in the last three decades by Paulus Gerdes, a Mozambique mathematics educator. The author presents a variety of discussions with several examples that explains the relationship of mathematics to human activities. We present, among other things, the cyclic matrices and its interrelationships with the African cultural contexts, undoing the erroneous impression that mathematics can be seen disconnected from the feature, which makes the individuals in their socio-cultural environments. Gerdes shows us that there's mathematics in different cultures and this helps us to realize the beauty of the symmetries and patterns, in addition to allowing depth between mathematics and human actions in the art
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