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

Abstract

Plagioclase hosted needle- and lath-shaped magnetite micro-inclusions from oceanic gabbro dredged at the mid-Atlantic ridge at 13° 01–02′ N, 44° 52′ W were investigated to constrain their formation pathway. Their genesis is discussed in the light of petrography, mineral chemistry, and new data from transmission electron microscopy (TEM). The magnetite micro-inclusions show systematic crystallographic and shape orientation relationships with the plagioclase host. Direct TEM observation and selected area electron diffraction (SAED) confirm that the systematic orientation relations are due to the alignment of important oxygen layers between the magnetite micro-inclusions and the plagioclase host, a hypothesis made earlier based on electron backscatter diffraction data. Precipitation from Fe-bearing plagioclase, which became supersaturated with respect to magnetite due to interaction with a reducing fluid, is inferred to be the most likely formation pathway. This process probably occurred without the supply of Fe from an external source but required the out-diffusion of oxygen from the plagioclase to facilitate partial reduction of the ferric iron originally contained in the plagioclase. The magnetite micro-inclusions contain oriented lamellae of ilmenite, the abundance, shape and size of which indicate high-temperature exsolution from Ti-rich magnetite constraining the precipitation of the magnetite micro-inclusions to temperatures in excess of ~ 600 °C. This is above the Curie temperature of magnetite, and the magnetic signature of the magnetite-bearing plagioclase grains must, therefore, be considered as the thermoremanent magnetization.

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

“…The plagioclase hosted magnetite inclusions of the studied samples most likely formed by exsolution from iron bearing plagioclase during the sub-solidus evolution of the gabbro (Bian et al, 2021;Usui et al, 2006).…”

“…The albite and pericline twins are polysynthetic and are regarded as transformation or deformation twins. Bian et al (2021) argued that the magnetite inclusions formed during or after transformation twinning. As a consequence, several shape orientation variants of the eight orientation classes may occur within a twinned plagioclase Note.…”

“…It should be noted that the inclusions of the plane-normal orientation classes are elongated parallel to one of their mt [111] directions, which are the easy axes of magnetization (Ageeva, Bian, et al, 2020;Bian et al, 2021). In contrast, the pl[001]-mt inclusions are elongated parallel to one of their mt[011] directions.…”

Predicting and explaining crystallographic orientation relationships of exsolved precipitates in garnet using the edge‐to‐edge matching model

“…Analysis of preserved garnet and precipitate chemistry from UHP gneiss of the Central Maine Terrane (USA) shows that loss of Na and gain of Mg and Al following majoritic garnet substitution vectors during open‐system precipitation results in balanced garnet chemical formulae (Keller & Ague, 2020). Open‐system precipitation of oxide exsolutions from plagioclase is supported by chemical and crystallographic data (Bian et al, 2021). Moreover, open‐system behaviour has been demonstrated unequivocally in sulphide systems during solid‐state exsolution (Adegoke et al, 2021).…”

“…In a similar manner, oxide needles preferentially elongated along directions of similar oxygen packing, and crystallographic plane spacing to host plagioclase have been noted (Ageeva et al, 2020). These characteristics are taken as evidence of an exsolution origin (Bian et al, 2021). Experimental studies of exsolution induced in laboratory settings have shown coherent structural matches parallel to long axes of lath‐ or needle‐shaped precipitates (e.g., Bailey, 1963; Phillips et al, 1980a; Weatherly & Nicholson, 1968; Winkelman et al, 2007).…”