Oriented Magnetite Inclusions in Plagioclase: Implications for the Anisotropy of Magnetic Remanence

Abstract

Micron to sub‐micron sized ferromagnetic inclusions in rock forming silicate minerals may give rise to particularly stable remanent magnetizations. When a population of inclusions have a preferred crystallographic or shape orientation in a rock, the recorded paleomagnetic direction and intensity may be biased by magnetic anisotropy. To better understand this effect, we investigated plagioclase grains from oceanic gabbro dredged from the Mid‐Atlantic Ridge at 11°–17°N. The plagioclase grains contain abundant needle and lath shaped magnetite inclusions aligned along specific directions of the plagioclase lattice. Electron back scatter diffraction and anisotropy of magnetic remanence measurements are used to correlate the orientation distribution of the magnetite inclusions in the host plagioclase that contains multiple twin types (Manebach, Carlsbad, Albite, and Pericline) with the bulk magnetic anisotropy of the inclusion‐host assembly. In non‐modified plagioclase, the anisotropy ellipsoid of magnetic remanence has oblate shapes that parallels the plagioclase (010) plane. It is suggested that recrystallization of magnetite inclusions during hydrothermal overprint shifts the relative abundance of the inclusions pertaining to the different orientation classes. We show that the maximum axis of the anisotropy ellipsoid of magnetic remanence parallels the plagioclase [001] direction, which in turn controls the recorded remanent magnetization direction. Our results are relevant for paleointensity and paleodirection determinations and for the interpretation of magnetic fabrics.

Oriented secondary magnetite micro-inclusions in plagioclase from oceanic gabbro

“…The stage was at 70° tilt, and the working distance was in the range of 14-14.5 mm. Details of the analytical parameters during EBSD analysis are described in Ageeva et al (2022) Section 2.4. SE imaging was performed at 70° stage tilt and tilt-corrected.…”

“…For needle-and lath shaped magnetite micro-inclusions, seven SORs with respect to plagioclase are discerned that define the plane-normal type inclusions according to the terminology of Ageeva et al (2022). These inclusions are elongated parallel to one of their MT<111> directions, and they are aligned close to parallel to the normal direction of one of seven specific plagioclase lattice planes including PL(112)n, PL(15 0)n, PL(3 12)n, PL(150)n, PL(100)n, PL(11 2)n, and PL(3 1 2)n, where PL(hkl)n is the direction normal to the PL(hkl) lattice plane.…”

“…The PL[001] type micro-inclusions typically occur in the outermost regions of the plagioclase grains, and they are the dominant micro-inclusion type in samples that experienced hydrothermal overprint at sub-solidus conditions (Pertsev et al 2015). The PL[001] type magnetite micro-inclusions are thus ascribed to hydrothermal processes (Ageeva et al 2022). PL[001] type magnetite micro-inclusions have also been described from metamorphic rocks (Feinberg et al 2004;Wenk et al 2011).…”

“…http://www.minsocam.org/ vector of natural remanent magnetization obtained from a magnetite bearing plagioclase grain may deviate from the direction of the magnetic field prevailing at the time, when the rock cooled through the Curie temperature (Usui et al 2015;Nikolaisen et al 2022), an effect that needs to be accounted for during paleomagnetic reconstructions. It was argued by Ageeva et al (2022) that the orientation distribution of the needle and lath shaped magnetite micro-inclusions undergoes an evolution from an initial dominance of the plane-normal types, which prevail in pristine magmatic plagioclase, towards a dominance of the PL[001] type inclusions in hydrothermally overprinted feldspar. Such a shift in inclusion populations has important implications for the magnetic memory of magnetite bearing plagioclase grains.…”

Micromagnetic Modes of Anisotropy of Magnetic Susceptibility in Natural Magnetite Particles

“…Our AMS calculations use MERRILL 1.3.5 (Conbhuí et al., 2018; Fabian & Shcherbakov, 2018) with magnetite material parameters at 20°C. The cubic anisotropy axes were oriented along the FIB coordinate axes, because we do not have sufficient control over the detailed orientation of the host minerals (Ageeva et al., 2020, 2022; Bown & Gay, 1959; Nikolaisen, Harrison, et al., 2022) and cubic magnetocrystalline anisotropy does not contribute to individual particle AMS. We start by minimizing 25 or 50 different random initial states and choosing the lowest local energy minimum (LLEM) as approximation to the absolute energy minimum.…”