Ferromagnetism of magnetite-bearing plagioclase from oceanic gabbro

Oriented needle shaped magnetite inclusions in plagioclase may give rise to magnetic anisotropy of individual plagioclase grains and, in case of preferred orientation of the plagioclase grains, contribute to magnetic anisotropy of the bulk-rock. Understanding how oriented magnetite inclusions generate magnetic fabrics in single grains of plagioclase is important for interpreting rock magnetic fabrics and for correcting paleomagnetic data. Plagioclase grains from oceanic gabbro dredged at the Mid Atlantic Ridge (11-17°N) were analyzed using optical microscopy, electron backscatter diffraction (EBSD), as well as alternating field demagnetization and anisotropy of magnetic remanence (AMR) measurements to investigate the influence of the shape orientation distribution of acicular magnetite inclusions on the magnetic properties of magnetite bearing plagioclase grains.

In pristine magmatic plagioclase, the needle elongation directions form a 30° wide girdle distribution parallel to the pl(010) plane. This girdle distribution is insensitive to twinning after the Albite, Pericline, Carlsbad and Manebach laws. The statistical maximum in the inclusion orientation lays in the pl(010) plane, closely parallel to the pl[001] direction. The overall shape orientation distribution of the magnetite inclusions produces a triaxial magnetic anisotropy ellipsoid with the minimum axis direction sub-perpendicular to the pl(010) plane and the maximum axis sub-parallel to the pl[001] direction.

The vector of natural remanent magnetization parallels the maximum AMR axis direction indicating that the magnetic anisotropy caused by the magnetite inclusion fabric controls the paleomagnetic signature. In hydrothermally modified plagioclase, most or all magnetite needles are oriented parallel to the pl[001] direction and a prolate rotational ellipsoid of remanent magnetization with the maximum remanent magnetization parallel to pl[001] should occur.

Plagioclase-hosted magnetite inclusions are particularly stable recorders of the paleomagnetic filed. The magnetic anisotropy arising from the anisotropic shape orientation distribution of the magnetite inclusions may, however, bias the magnetic record, an effect that needs to be accounted for in paleomagnetic reconstructions or for absolute paleointensity experiments.

This work was supported by the Austrian Science Fund (FWF): I 3998-N29, and by the Russian Foundation for Basic Research, Grant no. 18-55-14003.