Anisotropy of Silicate-Hosted Magnetite Inclusions

Anisotropy of magnetic properties is a hallmark of silicate crystals with oriented iron-oxide inclusions. Strongly magnetic magnetite-bearing silicates (10^-1 A m^-1) are common components of gabbros and layered intrusions, contributing to local and regional magnetic anomalies. Additionally, these iron-oxide silicates hold the promise of being exceptional paleomagnetic recorders owing to their features of: physical/chemical isolation from altering fluids, chemical equilibrium with their silicate host, and long relaxation times (enhanced coercivity). However, anisotropy of remanence must be understood before these advantageous features can be utilized. Measurements of single crystals of clinopyroxene and plagioclase (10^-4 g) show anisotropy in direction and intensity that directly reflect the crystallography of the silicate host. The host controls both the crystallographic orientation of the magnetite (magnetocrystalline anisotropy) and the elongation direction of the magnetite inclusion (shape anisotropy). We have found another source of anisotropy that involves an internal exsolution of ulvöspinel within titanomagnetite inclusions. This also reflects a host control as this second exsolution occurs along the magnetite \{100\}. This fixed wall shape anisotropy creates an array of interacting single domain magnetite parallelepipeds, parallel to \{100\}. Each of these anisotropies contributes to enhanced coercivity of remanence, which significantly exceeds the IRM saturation magnetization for magnetite (300 mT). The anisotropy of IRM (aIRM@ 1.1 T) of magnetite-bearing clinopyroxene and plagioclase shows clustering of directions, reflecting the mixture of variables that include: inclusion elongation direction and abundance, orientation of magnetite easy axes relative to the applied field, inclusion aspect ratio and diameter, and pre-existing magnetic domain structure. For pyroxene (monoclinic) with two arrays of needle-shaped magnetite inclusions, the aIRM is strongly planar, showing two sets of antipodal direction clusters. The total intensity is uniform over ∼60% of the remanence plane. The component of magnetization perpendicular to the applied IRM (transverse remanence) exceeds the parallel remanence in some crystallographic directions. For plagioclase (triclinic) with three to five arrays of needle-shaped magnetite inclusions (see Feinberg, et al. this conference), the aIRM is subspherical with numerous minor clusters of directions. Transverse remanence is less important than parallel remanence in plagioclase. Thus, the greater number of inclusion arrays in plagioclase decreases the severity of the IRM anisotropy relative to the two arrays in pyroxene.