Magnetic properties,self-reversal remanence and thermal alteration products of smythite

Summary This study is a follow up of the investigation of some magnetic properties and metastability of greigite in samples obtained from Miocene claystones in the Kruné hory (Erzgebirge) Piedmont basins (Bohemia). Three different methods of upgrading the smythite were applied; the magnetic properties of the concentrates are compared. The thermal conversion of smythite sets in at 200°C while greigite converts at 250°C. The first intermediate products to be formed are iron sulphides, marcasite clearly dominating over pyrite and pyrrhotite. Apart from a Fe³⁺ sulphate with a composition of Fe₂(SO₄)₃, oxidation of these sulphides gives rise to -Fe₂O₃. The result of the subsequent decomposition of the mentioned sulphate is the formation of -Fe₂O₃, which retains the sulphate structure. The final product of the thermal decomposition at 800°C is -Fe₂O₃. In the smythite concentrate the conversion to Fe³⁺ sulphate and -Fe₂O₃ is about twice as intensive as in greigite. No direct conversion to -Fe₂O₃ was found. During the thermal process self-reversals of remanence were observed, in various samples as many as four reversals in the temperature interval from 340 to 590°C. The occurrences of self-reversals of remanence were only observed at high degrees of thermal demagnetization, of the order of 10^–2 down to 10^–3 in the temperature interval of sulphide origin (below 400°C), and of the order of 10^–4 down to 10^–6 in the temperature interval of Fe-oxides origin (above 400°C).Presented at the 3rd Conference on New Trends in Geomagnetism, Castle of Smolenice, Czechoslovakia, June 22–29, 1992     

80 Jahre von univ. prof. dr. alois gregor DrSc.

Ohne Zusammenfassung     

An effective method of frequency analyses of time series of geo-physical quantities

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On sweeping frequency impedance measurements

Summary The current spectral density I() dependence on the width of the frequency band and sweep period is studied. The relation between the required accuracy of measurement an the optional parameters of the sweeping process is formulated.     

Evolution microtectonique et paléo-champs de contraintes du Bassin de Vienne

Résumé

Une étude des structures cassantes permet de préciser l’évolution tectonique du Bassin de Vienne, généralement considéré comme un bassin de type « pull apart » typique. Le champ de contrainte ottnangien-carpathien (18,5-16, 5 Ma) est caractérisé par une compression NNW-SSE. Pendant le Badénien-Sannanticn (16,5-11 Ma) une zone cisaillante sénestre de direction NE-SW a été formée dans un régime en compression N-S et extension E-W. En liaison avec cette zone décrochante trois types d’extension locale conduisent à une forte subsidence dans les dépression : extension NE-SW parallèle aux accidents sénestres, extension E-W entre les décrochements et extension NW-SE au-dessus des zones décrochantes profondes. Le champ de contrainte vers la fin du Sannaticn est marqué par une compression ENE-WSW, tandis que celui du Pannonicn- récent est similaire á celui du Miocène moyen.

Une rotation de la paléocontrainte maximale horizontale, de NNW-SSE a ENE-SSW est mise en évidence pendant la période Ottnangien-Sannantien (18,5-11 Ma). Cette rotation est parallèle A celle des directions de mise en place des nappes carpathicnnes externes. L’origine des rotation est l’échappement continental des Alpes orientales et des Carpathes septentrionales vers le NE et son blocage graduel d’Ouest en Est au front des nappes carpathiennes. Après ce blocage, l’échappement a continué avec une faible intensité du Pannonien à l’Actuel. Alors que l’échappement a été initié par une compression N-S due á la collision Europe-promontoire Adriatique, le champ de contrainte à l’intérieur des blocs échappés reflète l’effet de leur blocage au front des nappes carpalhiques.     

Evolution of geochemistry and depositional settings of Early Palaeozoic siliciclastics of the Barrandian (Teplá-Barrandian Unit,Bohemian Massif,Czech Republic)

Cambrian and Ordovician-Middle Devonian sequences of two successive Early Palaeozoic basins of the Barrandian unconformably overlie Cadomian basement in the Bohemian Massif NW interior (Teplá-Barrandian unit) which is the easternmost peri-Gondwanan remnant within the Variscides. Correlation of stratigraphy and geochemistry of the Early Palaeozoic siliciclastic rocks elucidated sediment provenances. Sandstones of the Middle Cambrian Píbram-Jince Basin were derived from a Cadomian Neoproterozoic island arc. The source area of the Ordovician shallow-marine siliciclastics of the successor Prague Basin is a dissected Cadomian orogen. Late Cambrian acid volcanics of the Barrandian and Cambrian (meta)granitoids emplaced in the W part of the Teplá-Barrandian Cadomian basement are also discernible in these sediments. Old sedimentary component increased during the Ordovician. Early Llandovery siliciclastic rocks show characteristics of an abruptly weakened supply of terrigenous material and an elevated proportion of synsedimentary basic volcanics as a result of Silurian transgression. Emsian siliciclastics (intercalated in the Late Silurian to Early Devonian limestone suite) presumably comprise an addition of coeval basic/ultrabasic volcaniclastics. Middle Devonian flysch-like siliciclastics indicate reappearance of Cadomian source near the Barrandian during early Variscan convergences of Armorican microplates that preceeded accretion of the Teplá-Barrandian unit within the Bohemian Massif terrane mosaic.Dr. Patoka deceased in July 2004.     

The effect of quartz on the magnetic anisotropy of quartzite

Summary The magnetic susceptibility of quartz single crystals is diamagnetic (–14×10 ^–6 in SI units) and exhibits only very small anisotropy (mostly less than 1%); thus the susceptibility of the quartz matrix in quartzite can be regarded as virtually isotropic. Owing to the influence of the negative and isotropic susceptibility of the quartz matrix, the degree of anisotropy of quartzite, as inferred from model calculations, is higher than that of the ferrimagnetic fraction. This influence is very strong if the mean susceptibility of quartzite is in the vicinity of zero.

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