Beyond the second-order magnetic anisotropy tensor: Higher-order components due to oriented magnetite exsolutions in pyroxenes, and implications for palaeomagnetic and structural interpretations

Andrea R. Biedermann, Mike Jackson, Martin Chadima, Ann M. Hirt, Joshua M. Feinberg

Research output: Contribution to journalArticlepeer-review

1 Scopus citations

Abstract

Exsolved iron oxides in silicate minerals can be nearly ideal palaeomagnetic recorders, due to their single-domain-like behaviour and the protection from chemical alteration by their surrounding silicate host. Because their geometry is crystallographically controlled by the host silicate, these exsolutions possess a shape preferred orientation that is ultimately controlled by the mineral fabric of the silicates. This leads to potentially significant anisotropic acquisition of remanence, which necessitates correction to make accurate interpretations in palaeodirectional and palaeointensity studies. Here, we investigate the magnetic shape anisotropy carried by magnetite exsolutions in pyroxene single crystals, and in pyroxene-bearing rocks based on torque measurements and rotational hysteresis data. Image analysis is used to characterize the orientation distribution of oxides, from which the observed anisotropy can be modelled. Both the high-field torque signal and corresponding models contain components of higher order, which cannot be accurately described by second-order tensors usually used to describe magnetic fabrics. Conversely, low-field anisotropy data do not show this complexity and can be adequately described with second-order tensors. Hence, magnetic anisotropy of silicate-hosted exsolutions is field-dependent and this should be taken into account when interpreting isolated ferromagnetic fabrics, and in anisotropy corrections.

Original languageEnglish (US)
Pages (from-to)915-933
Number of pages19
JournalGeophysical Journal International
Volume223
Issue number2
DOIs
StatePublished - Nov 1 2020

Bibliographical note

Funding Information:
Anette von der Handt is thanked for valuable help with the microprobe and imaging. Eric Ferr?, Fatima Martin-Hernandez and an anonymous reviewer as well as editor Eduard Petrovsky are gratefully acknowledged for their detailed and critical evaluation of the manuscript. This study was funded by the Swiss National Science Foundation, projects 167608/167609 and 176917.

Publisher Copyright:
© 2020 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society.

Keywords

  • Fourier analysis
  • Image processing
  • Magnetic fabrics and anisotropy
  • Magnetic properties
  • Numerical modelling
  • Palaeomagnetism

Fingerprint

Dive into the research topics of 'Beyond the second-order magnetic anisotropy tensor: Higher-order components due to oriented magnetite exsolutions in pyroxenes, and implications for palaeomagnetic and structural interpretations'. Together they form a unique fingerprint.

Cite this