HR: 16:15h
AN: GP34A-02    [Abstracts]
TI: Determining the Anisotropy of Remanence Tensor Using a Conventional Electromagnet: Overcoming the Hematite Problem and Correcting for Inclination Error in Redbeds
AU: * Schmidt, P W
EM: phil.schmidt@csiro.au
AF: CSIRO Exploration & Mining, PO Box 136 North Ryde, Sydney, NSW 1670, Australia
AB: The origin of the natural remanent magnetization of sediments is often investigated by studying their anisotropy, either of magnetic susceptibility or some form of remanence. An oblate magnitude ellipsoid with a minimum principal axis aligned with the bedding pole is necessary (almost, though not sufficient) evidence for a primary depositional fabric and therefore a primary characteristic remanence. The anisotropy of isothermal remanent magnetization (IRM) can also be used to correct for inclination error (Jackson et al., GJI, 104, 95-103, 1991). While this is straightforward for magnetite bearing sediments, the experimental procedure to determine the anisotropy of remanence for hematite bearing sediments is problematical (Tauxe et al., JGR, 95, 4391-4404, 1990). The problem arises from ‘magnetic memory', which for magnetite can be easily erased (reset) by alternating field (AF) demagnetization. The coercivity of hematite is prohibitively high for AF demagnetization and therefore only a fraction of the coercivity spectrum is accessible leaving a significant memory from previous exposures to high magnetic fields. By using a 14 T superconducting magnet Kodama and Dekkers (Stud. Geophys. Geod., 48, 747-766, 2004) have demonstrated that it is possible to derive the full anisotropy tensor for hematite bearing samples by imparting saturation IRMs, which activate almost the whole coercivity spectrum. While this is undoubtedly the ultimate solution to the hematite problem, such machines are not commonly available and are very expensive to operate. An alternative procedure that allows the use of more common and cheaper ~1 T electromagnets that obviates the AF demagnetization stage is to rotate the sample in the field in a manner that erases, or resets, the magnetic history so each IRM is equivalent. Admittedly, not all the coercivity spectrum is accessed, but typically something >50 percent is available compared to perhaps <10 percent for anhysteretic remanence. There is no unique set of rotations that resets the magnetic history but the least requirement, not unlike that for AF demagnetization, is that the sample needs to be exposed to a steady magnetic field along many axes. Instead of slowly reducing the field to zero as for AF demagnetization, the field is held constant with the rotation action converging on the desired axis in a spiral fashion. When the axis and field are aligned the field can be removed and the remanence measured before repeating the exercise for another axis. The effects of magnetite and goethite were eliminated by routine AF and thermal demagnetization to 150 mT and 120°C respectively after imparting the IRMs. Results from ‘rotating in the field' to date suggest they are comparable to the expensive superconducting magnetic results. Results from the simple oblique single IRM method will also be compared.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 1527 Paleomagnetism applied to geologic processes
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting