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.

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Shape and lattice orientation relations as well as chemical compositions of Fe-Ti-oxide micro-inclusions and plagioclase host crystals in rocks of a gabbroplagiogranite assemblage from the Mid-Atlantic ridge at 13°34' N were studied using electron back scatter diffraction, transmission electron microscopy and field-emission gun-electron microprobe analyzer. Several evolutionary stages of the micro-inclusionhost assemblages were discerned, starting with precipitation of Fe-Ti-oxides from a super-saturated plagioclase in otherwise unaltered gabbro, followed by transformation and re-crystallization of the micro-inclusions as well as chemical alteration of both inclusions and host during plagiogranite intrusion and subsequent hydrothermal alteration. A detailed sequence of petrogenetic processes could be reconstructed. Fe-Ti-oxide micro-inclusions are the main carriers of the paleo-magnetic record of these rocks, and understanding the transformations affecting Fe-Ti-oxide microinclusions in the highly dynamic mid-ocean ridge environment is crucial for interpreting paleo-magnetic data.

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Participation of seawater in large scale convective circulation in the global system of mid ocean ridges is an important factor of heat and mass transfer resulting in cooling and hydrothermal alteration of the newly formed basic crust and precipitation of sulfide-poly metallic ores on the oceanic floor [1,2]. The deep zones of hydrothermal systems and the areas of hydro thermal-magmatic interaction are the least studied. It is suggested that the local partial melting of the basic crust under the influence of hydrothermal seawater derived fluids and the appearance of oceanic plagiog ranite (OPG), i.e., plutonic quartz-feldspar rocks with occasional pyroxene and hornblende, may be one of the extreme manifestations of such interaction [3,4]. However, there are other models of OPG origin: extreme differentiation of basaltic magma, phase sep aration of differentiated basaltic magma, magmatic assimilation, and partial melting of the hydrated crust [3]. Although the phenomenon of acid magmatism in the modern oceanic crust and paleoanalogs is of spe cial interest, the likely combinations of various petro genetic factors in the generation of OPG melts are poorly studied. In particular, the role of the basic crust composition is not clear.In this paper, we report the results of the study of an occasional biotite bearing gabbro-plagiogranite assemblage discovered in the Mid Atlantic Ridge (MAR), where the formation of the oceanic crust is significantly controlled by mantle magmas of the E MORB type enriched in incompatible elements. The composition of the granitoid melt was estimated by the study of melt inclusions in zircon, the bulk chemistry of OPG veinlets, and clinopyroxene/melt REE parti tioning.The gabbro-peridotite (strongly serpentinized) massif exposed in the footwall of the large offset low angle detachment fault in the range of 13°28′-13°35′ N on the western slope of the MAR was studied and sampled in three cruises of R/V Professor Logachev in 2007-2011 (Fig. 1). It is established that the structure of the massif is complicated by volcanic formations and a series of hydrothermal sulfide fields. In addition, samples of OPG veinlets in amphibolite and gabbro, as well as massive OPG rocks, were dredged at 8 sites ( Fig. 1) [5]. The analysis of fresh lavas with quench glasses from 21 sampling sites showed that within the massif the composition of magmas of volcanic struc tures varied from normal (Nb N /Zr N = 1.2; H 2 O (8) = 0.15 wt %; K 2 O = 0.04 wt %; 87 Sr/ 86 Sr = 0.7024) to sig nificantly enriched (Nb N /Zr N = 3.5; H 2 O (8) = 0.45 wt %; K 2 O = 0.7 wt %; 87 Sr/ 86 Sr = 0.7029) oceanic basalt.Biotite bearing OPGs were found at Sites 97 and 101, on the northern margin of the massif, at a dis tance of ~3 km from each other (Fig. 1). They occur as fine grained biotite-quartz-plagioclase veinlets in coarse graned gabbro. Gabbroids and hydrothermally altered mafic rocks prevailed among the dredged sam ples. Mafic rocks contain quartz-sulfide mineraliza tion. There are samples of massive biotite free OPG as well.S...

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.