HR: 15:10h
AN: GP23B-07    [Abstracts]
TI: Magnetic Anisotropy as an aid to Identifying CRM and DRM in Red Sedimentary Rocks
AU: * Kodama, K P
EM: KPK0@lehigh.edu
AF: Department of Earth and Environmental Sciences, Lehigh University, 31 Williams Drive, Bethlehem, PA 18015 United States
AU: Dekkers, M J
EM: dekkers@geo.uu.nl
AF: Paleomagnetic Laboratory "Fort Hoofddijk",, Department of Geosciences, Utrecht University, Utrecht, 3584CD Netherlands
AB: To evaluate the usefulness of magnetic anisotropy for determining the origin of the natural remanent magnetization (NRM) in red sedimentary rocks, several new remanence anisotropy measurement techniques were investigated. The goal of the work was an accurate separation of the remanence anisotropy of magnetite and hematite in the same sedimentary rock sample. In one technique, Tertiary red and grey sedimentary rock samples from the Orera section of Spain were exposed to 13 T fields in 9 different orientations. This work was done at the High Field Magnet Laboratory of Radboud University, Nijmegen, The Netherlands. In each orientation, alternating field (af) demagnetization was used to separate the magnetite and hematite contributions to the high field isothermal remanent magnetization (IRM). Tensor subtraction was used to separate the magnetite and hematite magnetic anisotropies. Geologically interpretable fabrics did not result, probably because of the presence of goethite. In the second technique, also applied to samples from Orera, an anisotropy of anhysteretic remanence (AAR) was applied in af fields up to 240 mT to directly measure the fabric of the magnetite in the sample. IRMs applied in 2T fields followed by 240 mT af demagnetization, and thermal demagnetization at $90\deg$C to remove the goethite contribution, were used to independently measure the hematite fabric in the same samples. This approach gave magnetic fabrics with minimum principal axes perpendicular to bedding, suggesting that the hematite and magnetite grains in the Orera samples both carry a depositional remanent magnetization (DRM). In a third experiment, IRMs applied in 13 T fields were used to measure the magnetic fabric of samples from the Dome de Barrot area in France. These samples had been demonstrated to have hematite as their only magnetic mineral. The fabrics that resulted were geologically interpretable, showing a strong NW-SE horizontal lineation consistent with AMS fabrics measured in the same samples. These fabrics suggest that the rock's remanence has may have been affected by strain and could have originated as a DRM or a CRM. Our work shows that it is important to account for goethite when using high field IRMs to measure the remanence anisotropy of hematite-bearing sedimentary rocks. It also shows that very high magnetic fields ($>$10 T) may be used to measure the magnetic fabric of sedimentary rocks with highly coercive magnetic minerals without complete demagnetization between each position, provided that the field magnetically saturates the rock.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting