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

Biedermann, Andrea R; Jackson, Mike; Chadima, Martin; Hirt, Ann M; Feinberg, Joshua M (2020). Beyond the second order magnetic anisotropy tensor: Higher-order components due to oriented magnetite exsolutions in pyroxenes, and implications for paleomagnetic and structural interpretations. Geophysical journal international, 223(2), pp. 915-933. Oxford University Press 10.1093/gji/ggaa355

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Exsolved iron oxides in silicate minerals can be nearly ideal paleomagnetic 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 paleodirectional and paleointensity 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 employed 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.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences

UniBE Contributor:

Biedermann, Andrea Regina

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

0956-540X

Publisher:

Oxford University Press

Language:

English

Submitter:

Andrea Regina Biedermann

Date Deposited:

27 Jul 2020 11:56

Last Modified:

21 Aug 2020 01:33

Publisher DOI:

10.1093/gji/ggaa355

BORIS DOI:

10.7892/boris.145395

URI:

https://boris.unibe.ch/id/eprint/145395

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