Аннотация:Magnetite (MT) micro-inclusions in rock-forming silicates may cause stable bulk remanent magnetization. Given the magnetocrystalline and shape anisotropy of MT, the orientation relationships between MT and silicate host are of interest in the context of potential bulk magnetic anisotropy of the MT-bearing silicate grains. We investigated plagioclase (PL) from oceanic gabbros, which contains needle-shaped MT inclusions more than 95% of which pertain to 8 shape orientation types. For seven types the MT needles are elongated perpendicular to low-index PL lattice planes and along one of the MT[111] directions, which is the easy axis of magnetization; for the eighth type, the MT needles are elongated parallel to the PL[001] and the MT[110] directions. 80-100% of all inclusions are statistically elongated parallel or sub-parallel to PL(010) giving rise to bulk shape orientation anisotropy. PL twinning increases the degree of orientation dispersion sub-parallel to PL(010) but does not change the deviation of the needle orientations from PL(010), and thus leaves the bulk shape orientation anisotropy of the MT inclusions unchanged. In foliated gabbros PL(010) is sub-parallel to the foliation plane, which may lead to both, single-grain and bulk-rock magnetic anisotropy.The MT inclusions are ascribed to sub-solidus exsolution from Fe-rich PL and most likely carry a primary remanent magnetization. Inclusions elongated parallel to the low-index PL lattice planes typically occur in unaltered oceanic gabbro. The inclusions elongated parallel to PL[001] occur in unaltered gabbro, and they dominate in metamorphically or metasomatically altered gabbro. They form by another mechanism than the MT needles of the other orientation types and they occur over a larger range of conditions including hydrothermal alteration.We conclude that, depending on formation route, needle-shaped MT inclusions in PL may attain different shape orientations and induce bulk magnetic anisotropy of PL grains. In unaltered oceanic gabbro, most of the orientation types are sub-parallel to PL(010) leading to planar bulk magnetic anisotropy. In contrast, one single orientation type dominates after metasomatic or metamorphic recrystallization leading to linear type of magnetic anisotropy. In foliated gabbro the shape orientation anisotropy of the PL-hosted MT inclusions contributes to the bulk-rock magnetic anisotropy.
