Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmgp31a0073b&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #GP31A-0073
Other
1518 Magnetic Fabrics And Anisotropy, 1519 Magnetic Mineralogy And Petrology, 1525 Paleomagnetism Applied To Tectonics: Regional, Global, 1540 Rock And Mineral Magnetism
Scientific paper
The North American apparent polar wander path (APWP) for the upper Paleozoic is dominated by red beds, which have been reported to suffer from deposition-/ compaction- induced magnetic inclination shallowing. To apply a correction for inclination shallowing, experiments to measure the magnetic fabric of hematite have been designed and performed together with paleo-magnetic investigations on Lower Carboniferous red beds from the Shepody Fm of New Brunswick and Nova Scotia and from the Deer Lake Group of Newfoundland. The corrected poles have been plotted on the existing APWPs for comparison. In a previous anisotropy of magnetic susceptibility (AMS)-based, inclination correction of the Shepody Fm, IRM acquisition, Lowrie and fold tests revealed secondary maghemite, goethite and primary hematite as magnetic carriers, thus indicating that AMS would measure a composite fabric rather than that of hematite only. These results were confirmed by further anisotropy measurements. Anisotropy of Anhysteretic Remanence (AAR) measurements revealed an absence of fabric for maghemite, while high field (5 T) isothermal remanent magnetization (hf-IRM) anisotropy, cleaned of the low coercivity components, revealed a very distinct depositional fabric for primary hematite. An inclination correction performed using the hf-IRM anisotropy gave a similar result to the AMS-based correction (mean corrected inclinations: 29.1° for AMS and 28.8° for hf-IRM indicating respectively 11.1° and 8° of inclination shallowing). These values are consistent with results obtained from three other hematite and magnetite- bearing Lower Carboniferous rocks from North America: the A95 for the four North American paleopoles decreases from an uncorrected value of 12° to a corrected value of 4.6°. These corrections imply a ~ 6° increase in co-latitude for the average Lower Carboniferous paleopole. To determine the corrected inclinations, individual magnetic particle anisotropy of hematite must be measured. In order to obtain a direct measure of the anisotropy of individual hematite grains, a magnetic mineral extraction of the Shepody Fm is being conducted. This is difficult in red beds because of the presence of both pigmentary and detrital hematite grains. Because chemical leaching proved successful in isolating the characteristic remanence-carrying grains (primary hematite) as confirmed by the comparison of the AMS and hf-IRM inclination correction, specimens have been crushed to the smallest grain size possible and leached in 3N HCl on a hot plate, until the red pigment was removed. This was achieved in approximately 3 hours. The extraction is then run using standard techniques. A companion paleomagnetic study conducted on rocks from Newfoundland showed that thermal and chemical demagnetization were unsuccessful in isolating the same components of magnetization, therefore prohibiting a meaningful measure of the magnetic fabric of the characteristic remanence- carrying grains by measuring the AMS of chemically treated specimens. An important observation is that principal component analysis conducted on thermal demagnetograms isolated stable components at average temperatures only up to 550-600°C, indicating that hematite is probably not the dominant carrier as might be assumed from the red color of the rocks. IRM, Lowrie tests and demagnetograms revealed the presence of a suite of magnetic minerals in the specimens, consisting of maghemite, magnetite, hematite and goethite. Classic AAR techniques will be used to measure the fabric of the low coercivity, characteristic remanence- carrying grains and an inclination correction will be applied.
Bilardello Dario
Kodama Kenneth P.
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