Crystallographic and shape orientations of magnetite micro-inclusions in plagioclase

Olga Ageeva,Rainer Abart,Alexey Pertsev,Gerlinde Habler,Ge Bian

doi:10.1007/s00410-020-01735-8

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.

Highlights

IntroductionOriented micro-inclusions of Fe–Ti oxide minerals such as magnetite, ulvospinel, hematite, and ilmenite hosted in rockforming silicate minerals are frequently observed in igneous and metamorphic rocks (Poldervaart and Gilkey 1954; Divljan 1960; Neumann and Christie 1962; Armbrustmacher and Banks 1974; Sobolev 1990; Usui et al 2006; Wenk et al, 2011; Biedermann, et al 2016; Ageeva et al 2016, 2017).Communicated by Othmar Müntener.The Fe–Ti oxide micro-inclusions most likely precipitate within the silicate minerals that originally crystallized with high Fe and Ti contents from evolved melt (Natland et al 1991) and became supersaturated with respect to Fe–Ti oxide minerals during cooling (Okamura et al 1976) and/ or under changing oxygen fugacity (Doukhan et al 1990; Ashworth and Chambers 2000)
The mineral content of the gabbros is dominated by primary magmatic coarse-grained (1–15 mm) tabular plagioclase and prismatic and xenomorphic pyroxene, where most pyroxenes are clinopyroxene, and orthopyroxene is rare
Oriented sub-micrometer-sized lamellae of ilmenite and/or ulvospinel may be present within individual magnetite inclusion (Fig. 1c, d)

Summary

Introduction

Oriented micro-inclusions of Fe–Ti oxide minerals such as magnetite, ulvospinel, hematite, and ilmenite hosted in rockforming silicate minerals are frequently observed in igneous and metamorphic rocks (Poldervaart and Gilkey 1954; Divljan 1960; Neumann and Christie 1962; Armbrustmacher and Banks 1974; Sobolev 1990; Usui et al 2006; Wenk et al, 2011; Biedermann, et al 2016; Ageeva et al 2016, 2017).Communicated by Othmar Müntener.The Fe–Ti oxide micro-inclusions most likely precipitate within the silicate minerals that originally crystallized with high Fe and Ti contents from evolved melt (Natland et al 1991) and became supersaturated with respect to Fe–Ti oxide minerals during cooling (Okamura et al 1976) and/ or under changing oxygen fugacity (Doukhan et al 1990; Ashworth and Chambers 2000). Igneous rocks may get metasomatized by interaction with evolved Fe- and Ti–rich melts or fluids during the late magmatic stages, which induces precipitation of Fe–Ti oxide micro-inclusions in the pre-existing silicate minerals (Trindade et al 1999, 2001; Diot et al 2003; Bolle et al 2003; Malpas and Robinson 1997; Grimes et al 2008; Hekinian 2014). Magnetite is ferrimagnetic with a Curie temperature of 580 °C, and it is the main carrier of the natural remanent magnetization of rocks (Dunlop and Ozdemir 1997, 2007; Tauxe 2010). In this context, the temperature at which the oriented magnetite inclusions formed is of crucial importance.

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