Paleomagnetic data provide information on the evolution of the Earth's magnetic field, and are used to reconstruct plate motions. One fundamental assumption underlying these interpretations is that the magnetization of a rock reliably records the direction and intensity of the magnetizing field, i.e. that the magnetization is parallel to the field direction, and the intensity of magnetization is proportional to the field strength. Preferred alignment or anisotropic distribution of magnetic grains can affect both the direction and the intensity of magnetization. Therefore, correction techniques, employing the anisotropy of magnetic susceptibility (AMS), thermal remanence (ATRM), or anhysteretic remanence (AARM) are used to account for these effects. We find that AARM within the same rock can vary dramatically with coercivity/grain size, so that anisotropy corrections can also depend on how AARM was measured. A consequence of the dependence of AARM on coercivity is that although a specimen may have been magnetized in a single direction, different grain size fractions may record magnetizations in different orientations. These directional variations, as revealed during progressive alternating field (AF) demagnetization, could erroneously be interpreted as changes in field or reorientation of the rock unit, when in reality they are related to grain-size-dependent remanence anisotropy. Similarly, intensity variations caused by grain-size-dependent anisotropy may bias paleointensity estimates. These observations have important consequences for studies on the evolution of the Earth's magnetic field, magnetic overprinting, and paleogeographic reconstructions.
Bibliographical noteFunding Information:
This project was funded by the Swiss National Science Foundation (SNSF), project 167608 . The IRM is a US National Multi-user Facility supported through the Instrumentation and Facilities program of the National Science Foundation , Earth Sciences Division (grants 1642268 and 1339505 ), and by funding from the University of Minnesota . Ken Kodama is thanked for his detailed and thoughtful review. Collection of the specimens from the Bushveld Complex was supported by NSF-EAR 0309686 awarded to Paul Renne and Gary Scott. LT acknowledges support from the National Science Foundation under Grant No. 1547263 . This is IRM publication #1802. Data can be obtained from the Supplementary Material of this paper.
© 2019 Elsevier B.V.
- Bushveld Complex
- anisotropy correction
- remanence anisotropy