Crystallographic and shape orientations of magnetite micro-inclusions in plagioclase

Abstract

Plagioclase hosted, oriented magnetite micro-inclusions are a frequently observed phenomenon in magmatic and metamorphic rocks. Understanding the orientation relationships between these inclusions and the plagioclase host is highly relevant for interpreting paleomagnetic measurements. The systematics of the shape and crystallographic orientation relationships between needle- and lath-shaped magnetite micro-inclusions and their plagioclase host from oceanic gabbro were investigated using optical microscopy including universal stage measurements, scanning electron microscopy, and crystal orientation analysis by electron backscatter diffraction. The magnetite inclusions show preferred shape orientations following six well-defined directions and with specific crystallographic orientation relationships to the plagioclase host. These relationships are rationalized based on angular and dimensional similarities between the crystal structures of magnetite and plagioclase, which favor the parallel alignment of oxygen layers with similar lattice spacing in both phases. The parallel alignment of oxygen layers in plagioclase and magnetite can be traced back to the oriented nucleation of magnetite, which occurs by the accommodation of FeO6 octahedra in six-membered rings of SiO4 and AlO4 tetrahedra of the plagioclase structure. The orientation systematics of the magnetite micro-inclusions is related to four orientation variants for placing the FeO6 octahedra into the plagioclase structure.

Formation pathways of oriented magnetite micro-inclusions in plagioclase from oceanic gabbro

“…Sup-2, Sup-3). It has been shown by Ageeva et al (2020) that the elongation directions of the so-called plane-normal type inclusions follow the normal directions of specific plagioclase lattice planes including the PL(112) n, PL(150)n, PL(312)n, PL(150)n and PL(100)n directions, where PL(hkl)n designates the direction that is normal to the PL(hkl) plane. One additional inclusion type has its elongation direction parallel to the PL[001] direction.…”

“…According to the classification scheme of Ageeva et al (2020), the main orientation of the PL(112)n-MT inclusions is defined as MT{111} || PL(112) and MT{110} || PL(150).…”

“…The crystallographic orientation relationships (CORs) and shape orientation relationships (SORs) between magnetite micro-inclusions and plagioclase host crystals have been rationalized earlier. Six frequently observed as well as two less common orientation classes of magnetite microinclusions in plagioclase host have been discerned (Sobolev 1990;Wenk et al 2011;Ageeva et al 2016Ageeva et al , 2020. While the CORs and SORs between the magnetite micro-inclusions and the plagioclase host are well documented (Ageeva et al 2020), the formation pathways of plagioclase-hosted magnetite micro-inclusions have not been fully revealed yet.…”

“…Six frequently observed as well as two less common orientation classes of magnetite microinclusions in plagioclase host have been discerned (Sobolev 1990;Wenk et al 2011;Ageeva et al 2016Ageeva et al , 2020. While the CORs and SORs between the magnetite micro-inclusions and the plagioclase host are well documented (Ageeva et al 2020), the formation pathways of plagioclase-hosted magnetite micro-inclusions have not been fully revealed yet. Knowing the formation conditions of these inclusions is of pivotal importance for paleomagnetic reconstructions.…”

“…We employed correlated optical microscopy including universal stage, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). In particular, high-resolution TEM imaging and selected area electron diffraction (SAED) were used to test the hypothesis on the CORs and the SORs between the magnetite micro-inclusions and the plagioclase host that have previously been put forward based on electron backscatter diffraction crystal orientation data (Ageeva et al 2016(Ageeva et al , 2020. The combined evidence is used to construct a genetic model for the plagioclase-hosted magnetite microinclusions, and the implications for paleomagnetic reconstructions are discussed.…”