Measuring remanence anisotropy of hematite in red beds: anisotropy of high-field isothermal remanence magnetization (hf-AIR)

Abstract

S U M M A R YThe potential of using high-field anisotropy of isothermal remanence magnetization (hf-AIR) measurements for determining the origin of natural remanent magnetization in red beds and for identifying and correcting possible red-bed inclination shallowing was investigated for specimens of the Carboniferous Shepody Formation of New Brunswick and Nova Scotia, Canada. The technique makes it possible for a typical paleomagnetic laboratory to measure the remanence anisotropy of high-coercivity hematite.High-field (hf) AIR was used in conjunction with 100 mT alternating field (af) and 120 • C thermal demagnetization to separate the contribution of hematite to the remanence anisotropy from that of magnetite/maghemite and goethite, respectively. A 5-T impulse DC magnetic field was used for the hf-AIR to reset the magnetic moment of high-coercivity hematite so that demagnetization between AIR orientations was not necessary. The ability of a 5-T field to reset the magnetization was tested by generating an isothermal remanent magnetization acquisition curve for hematite by using impulse DC magnetic fields up to 5 T in one orientation and followed by applying a field in the opposite direction at each step. Each field application was treated by 120 • C heating and 100 mT af demagnetization before measurement. At 5T, the difference between the magnetizations applied in opposite directions disappeared indicating that no magnetic memory persisted at this field strength. We performed a validity and reproducibility test of our hf-AIR measurement technique by measuring three specimens multiple times along two orthogonal coordinate systems. The method yielded highly reproducible results and, on rotating the specimen's coordinates, the fabric rotated by 90 • as expected, showing that it is not an artifact of the technique. We also measured hf-AIR on samples that had previously been chemically demagnetized in 3N HCl to remove the secondary, chemically grown pigmentary hematite. The hf-AIR fabric of leached samples is similar to that of untreated samples, but shows a better-defined magnetic lineation and imbrication. We interpret the fabric observed for the Shepody Formation to be a compactional fabric that has been reoriented by strain during folding following a flexural-slip model.

The tectonic history of the Niğde‐Kırşehir Massif and the Taurides since the Late Mesozoic: Paleomagnetic evidence for two‐phase orogenic curvature in Central Anatolia

“…This bias is known as the shallow bias of inclination angle in sedimentary rocks. [ Jackson et al ., , Tan et al ., , Bilardello and Kodama , , and Kodama , ]. We applied the method proposed by Tan and Kodama [] using anisotropy of anhysteretic remanent magnetization data for the correction of the inclination shallowing.…”

Lower Cretaceous Xigaze ophiolites formed in the Gangdese forearc: Evidence from paleomagnetism, sediment provenance, and stratigraphy

“…1b), and combined datasets yield an inclination that would suggest a paleolatitude of 11.2 • N [7.7 • N, 14.9 • N]. We cannot apply neither the E/I method (Tauxe and Kent, 2004), nor the anisotropy-based correction method (Tan et al, 2003;Bilardello and Kodama, 2009) to assess the magnitude of inclination shallowing in these rocks due to the limited number of isolated ChRM directions and the unavailability of magnetic fabric data. The strongly elongated distribution of the VGPs (Fig.…”

“…Individual magnetic particle anisotropy can be measured directly using specialized laboratory equipment (e.g., Kodama, 1997); it can also be determined by least-squares curve fitting of AMS, AARM (for magnetite) and AIR (for hematite) data (Tan et al, 2003;Bilardello and Kodama, 2009;Kodama, 2009). Our rock magnetic analyses and results of end-member modeling have indicated that magnetite is the dominant magnetic carrier in the Sangsang section.…”

Effect of magnetic anisotropy on the natural remanent magnetization in the MCU IVe' layer of the Bjerkreim Sokndal Layered Intrusion, Rogaland, Southern Norway

“…Under these circumstances, one would ideally isolate the anisotropy due to hemo‐ilmenite and compare the direction and intensity of the NRM to that component of the anisotropy. However, isolating the anisotropy of a high‐coercivity low‐magnetization mineral that coexists with magnetite is difficult, as has been shown for hematite [ Bilardello and Kodama , ; Kodama and Dekkers , ]. Biedermann et al .…”