Effects of internal mineral structures on the magnetic remanence of silicate-hosted titanomagnetite inclusions: An electron holography study

Titanomagnetite inclusions in pyroxene and plagioclase are carriers of stable magnetic remanence in some slowly cooled rocks such as gabbros, anorthosites, granulites, and diorites. Needle-shaped inclusions with average dimensions of 1 × 1 × >25 μm form epitaxially by exsolution from their host silicate. Close examination of clinopyroxene-hosted inclusions reveals an internal microstructure, which consists of magnetite (Fe₃O₄) prisms and ulvöspinel (Fe₂TiO₄) lamellae that formed as a result of phase unmixing during initial cooling. This internal structure exerts a profound influence on the magnetic remanence properties of each inclusion, primarily by transforming it from a multidomain grain into an assemblage of magnetostatically interacting single-domain prisms. Here we use off-axis electron holography to image the magnetization states of individual prisms and the magnetostatic interactions between them. We show that the inclusions exhibit both single-domain and collective magnetic states that depend primarily on the shape anisotropies of individual magnetite prisms, magnetostatic interactions between closely spaced prism stacks, and the shape anisotropy of the needle itself. Prisms that are separated by thick ulvöspinel lamellae show uniformly magnetized and/or vortex states. In contrast, closely spaced magnetite prisms behave as multipart vortices or as long composite columns, whose strong net magnetization may not be related directly to the orientation and shape of either the needle or the constituent prisms. The overall remanence direction recorded by clinopyroxene crystals containing finely exsolved inclusions is a reflection of both the inclusions' elongation directions and the prism arrangements within them.