Tomasz Werner

Homogeneous magnetic susceptibilities of tektites: Implications for extreme homogenization of source material

Abstract Low field magnetic susceptibility of 151 tektites from the Australasian strewn field is dominated by paramagnetism of the silicate glass. Ferromagnetic contributions are negligible because the formation of oxides was suppressed during rapid quenching. A few samples from the Central European strewn field show similar properties. Also, susceptibilities calculated from the large number of published geochemical analyses yield similar low susceptibilities. To produce low mass susceptibilities in such a narrow range of 50–100×10 −9 m 3 kg −1 , paramagnetic behaviour and negligible remanence, a target source that is both appropriate and widespread over the Earths surface is required. Modern marine sediments would appear to fill these requirements. The uniformity of the material, and hence its magnetic properties would be enhanced by the thorough mixing of molten terrestrial sedimentary during vaporization and ejection and its subsequent rapid quenching.

Differences in paleomagnetic interpretations due to the choice of statistical, demagnetization and correction techniques: Kapuskasing Structural Zone, northern Ontario, Canada

Abstract The Kapuskasing Structural Zone (KSZ) reveals a section through the Archean lower crustal granoblastic gneisses. Our new paleomagnetic data largely agree with previous work but we show that interpretations vary according to the choices of statistical, demagnetization and field-correction techniques. First, where the orientation distribution of characteristic remanence directions on the sphere is not symmetrically circular, the commonly used statistical model is invalid [Fisher, R.A., Proc. R. Soc. A217 (1953) 295]. Any tendency to form an elliptical distribution indicates that the sample is drawn from a Bingham-type population [Bingham, C., 1964. Distributions on the sphere and on the projective plane. PhD thesis, Yale University]. Fisher and Bingham statistics produce different confidence estimates from the same data and the traditionally defined mean vector may differ from the maximum eigenvector of an orthorhombic Bingham distribution. It seems prudent to apply both models wherever a non-Fisher population is suspected and that may be appropriate in any tectonized rocks. Non-Fisher populations require larger sample sizes so that focussing on individual sites may not be the most effective policy in tectonized rocks. More dispersed sampling across tectonic structures may be more productive. Second, from the same specimens, mean vectors isolated by thermal and alternating field (AF) demagnetization differ. Which treatment gives more meaningful results is difficult to decipher, especially in metamorphic rocks where the history of the magnetic minerals is not easily related to the ages of tectonic and petrological events. In this study, thermal demagnetization gave lower inclinations for paleomagnetic vectors and thus more distant paleopoles. Third, of more parochial significance, tilt corrections may be unnecessary in the KSZ because magnetic fabrics and thrust ramp are constant in orientation to the depth at which they level off, at approximately 15-km depth. With Archean geothermal gradients, primary remanences were blocked after the foliation was tilted to rise on the thrust ramp. Therefore, the rocks were probably magnetized in their present orientation; tilting largely or entirely predates magnetization.

Paleomagnetism and magnetic mineralogy of metabasites and granulites from Orlica-Śnieżnik Dome (Central Sudetes)

The results of palaeomagnetic, rock magnetic, and microscopic study of Early Paleozoic metabasites and granulites from the OrlicaŚnieżnik Dome (OSD, Sudetes) have been combined with geochronological data. In the eastern part of the OSD (Śnieżnik Massif, SM) ferrimagnetic pyrrhotite is prevalent, accompanied by various amounts of Fe-oxides. In the western part of the OSD (Orlica-Bystrzyca Massif, OBM) Fe-oxides dominate. All magnetic minerals originated during hydrothermal and weathering processes. The palaeomagnetic study revealed the presence of three secondary components of natural remanence: Late Carboniferous, Late Permian, and Mesozoic. Two Paleozoic components are related to volcanic activity in the Sudetes. They are carried by pyrrhotite and Fe-oxides and were isolated only in SM rocks. The Mesozoic component was determined in both parts of the OSD and is carried by Fe-oxides. It covers a time span, from ∼160 to ∼40 Ma, corresponding to a long period of alteration.

Fifty Years of Palaeomagnetic Studies in the Institute of Geophysics, Polish Academy of Sciences

Palaeo- and archaeomagnetic investigations in the Institute of Geophysics were initiated at the beginning of 1960s. The archaeomagnetic study was performed in the Geomagnetic Observatory in Hel till the mid-1980s and was resumed lately in the Warsaw laboratory. Palaeomagnetic study begun in 1963 but the palaeomagnetic group obtained necessary place for the laboratory in the Central Geophysical Observatory in the newly finished building in Belsk later in 1966. Formally, the Paleomagnetic Laboratory was established in 1972. From the very beginning, palaeomagnetic studies were accompanied by studies of magnetic properties of rocks and minerals-carriers of natural remanence and by measurements of magnetic susceptibility and its anisotropy. Therefore, the laboratory became successively equipped with the apparatus for strictly palaeomagnetic study (astatic and spinner magnetometers and demagnetizing devices) as well as with devices for rock-magnetic investigations (apparatus for thermoanalysis, system for measuring of hysteresis parameters and for study of magnetic remanence at low temperatures, torsion balance, device for study of magnetic properties under mechanical stress and temperature, device for study of alterations of magnetic minerals based on thermally stimulated emission of exo-electrons (TSEE). The last three devices have not been in use for several years. At the end of 1980s, the Institute moved to a new building and the whole laboratory was moved to Warsaw. Moving palaeomagnetic measurements to town was possible due to the replacement of spinner magnetometer by the cryogenic magnetometer SQUID, which is much more sensitive and less affected by the urban disturbances. Computer software packages that are written in the Institute or obtained from their inventors became a great help in the interpretation of experimental results. Palaeomagnetic investigations have been performed in our laboratory on rocks of various origins and age, coming from various regions of Poland and other countries often in cooperation with scientists from foreign laboratories. Numerous palaeomagnetic results that are obtained in our laboratory are in the world data palaeomagnetic basis and on Apparent Polar Wander Paths constructed for various regions. Study of anisotropy of magnetic susceptibility brought interesting results concerning problems of tectonics; study of rock-magnetic properties helped to identify carriers of primary and secondary components of natural magnetic remanence in many cases. In the last 10 years, a group in our laboratory has been studying very important problems connected with environmental magnetism by applying of magnetometric methods for investigations of soil and air pollution.