The Seiland Igneous Province (SIP), Northern Norway, contains >5000 km 2 of mafic, ultramafic intrusions with minor alkaline, carbonatite and felsic rocks that were intruded into the lower continental crust at a depth of 25 to as much as 35 km.The SIP can be geochemically and temporally correlated to numerous dyke swarms throughout Scandinavia at 560-610 Ma, and is linked to magmatic provinces in W-Greenland and NE-America that are collectively known as the Central Iapetus Magmatic Province (CIMP).

A fundamental task in rock magnetism is to identify the magnetic domain states adopted by natural remanence carriers because these control the remanence acquisition process, the stability of the remanent magnetization over geological time, and subsequently the reliability of the stored paleomagnetic information. Most rocks contain a mixture of magnetic minerals covering a broad range of particle sizes and shapes. Rock magnetism broadly classifies these particles into superparamagnetic (SP), stable single-domain (SD), pseudo-single-domain (PSD), and multidomain (MD) states, with the boundaries between states originally based on experimental results (D.

No abstract

Summary Anisotropy of remanent magnetization and magnetic susceptibility are highly sensitive and important indicators of geological processes which are largely controlled by mineralogical parameters of the ferrimagnetic fraction in rocks. To provide new physical insight into the complex interaction between magnetization structure, shape, and crystallographic relations, we here analyse “slice-and-view” focused-ion-beam (FIB) nano-tomography data with micromagnetic modelling and single crystal hysteresis measurements. The data sets consist of 68 magnetite inclusions in orthopyroxene (Mg60) and 234 magnetite inclusions in plagioclase (An63) were obtained on mineral separates from the Rustenburg Layered Suite of the Bushveld Intrusive Complex, South Africa. Electron backscatter diffraction was used to determine the orientation of the magnetite inclusions relative to the crystallographic directions of their silicate hosts. Hysteresis loops were calculated using the finite-element micromagnetics code MERRILL for each particle in 20 equidistributed field directions and compared with corresponding hysteresis loops measured using a vibrating sample magnetometer (VSM) on silicate mineral separates from the same samples. In plagioclase the ratio of remanent magnetization to saturation magnetization (Mrs/Ms) for both model and measurement agree within $1.0\%$, whereas the coercivity (Hc) of the average modeled curve is 20 mT lower than the measured value of 60 mT indicating the presence of additional sources of high coercivity in the bulk sample. The VSM hysteresis measurements of the orthopyroxene were dominated by multi-domain (MD) magnetite, whereas the FIB location was chosen to avoid MD particles and thus contains only particles with diameters <500 nm that are considered to be the most important carriers of paleomagnetic remanence. To correct for this sampling bias, measured MD hysteresis loops from synthetic and natural magnetites were combined with the average hysteresis loop from the MERRILL models of the FIB region. The result shows that while the modeled small-particle fraction only explains $6\%$ of the best fit to the measured VSM hysteresis loop, it contributes $28\%$ of the remanent magnetization. The modelled direction of maximal Mrs/Ms in plagioclase is sub-parallel to [001]plag, whereas Hc does not show a strong orientation dependence. The easy axis of magnetic remanence is in the direction of the magnetite population normal to (150)plag and the maximum calculated susceptibility (χ*) is parallel to [010]plag. For orthopyroxene, the maximum Mrs/Ms, maximum χ* and the easy axis of remanence is strongly correlated to the elongation axes of magnetite in the [001]opx direction. The maximum Hc is oriented along [100]opx and parallel to the minimum χ*, which reflects larger vortex nucleation fields when the applied field direction approaches the short axis. The maximum Hc is therefore orthogonal to the maximum Mrs/Ms, 16 controlled by axis-aligned metastable single-domain states at zero field. The results emphasize that the nature of anisotropy in natural magnetite does not just depend on the particle orientations, but on the presence of different stable and metastable domain states, and the mechanism of magnetic switching between them. Magnetic modelling of natural magnetic particles is therefore a vital method to extract and process anisotropic hysteresis parameters directly from the primary remanence carriers.

No abstract