Inferring domain state from magnetic hysteresis in high coercivity dolerites bearing magnetite with ilmenite lamellae

Hysteresis loops were measured for 16 dolerite samples from separate Precambrian dykes whose remanence is probably primary. Remanence is carried by grains of magnetite subdivided into fine particles, usually by ilmenite lamellae, as seems common in mafic igneous rocks of high coercivity. Coercive force HC ranges from 9 to 40 × 103A/m (110–500 Oe). The ratio JRSJS of saturation remanence to saturation magnetization ranges from 0.10 to 0.45. JRSJS varies in approximate proportion to HC with a constant of proportionality appropriate for pseudo-single-domain magnetite with one or two domain walls per particle. A cooling cycle to 77K causes large (40% on average) demagnetization of JRS in most samples, supporting the dominance of pseudo-single-domain magnetite. Most samples also show a large (29% on average) decrease in HC on cooling to 140K, suggesting magnetostrictive control of HC, perhaps through internal stresses opposing domain wall motion. Only the three dolerites with HC greater than 30 × 103 A/m (380 Oe) show little change in HC on cooling, suggesting dominance by single-domain particles with shape anisotropy. netite grains subdivided into single-domain particles by ilmenite lamellae, magnetic interaction may lower JRSJS from the expected 0.5. Some theories [1,2] suggests that this is due to a self-demagnetizing field from magnetic poles on the surface of each grain of magnetite subdivided by ilmenite and should be corrected for. Another theory [3] suggests that this correction is not necessary because the self-demagnetizing field is already approximately cancelled, due to magnetic poles inside each subdivided grain, at the ends of magnetite particles. The latter suggestion is favoured by our three highest HC dolerites, whose JRSJS averages 0.42 ± 0.04 before and 0.66 ± 0.06 after correction for the self-demagnetizing field from poles on the surface of magnetite-ilmenite grains. Applying this correction to our other dolerites usually raises JRSJS significantly above 0.5, which is unreasonable for pseudo-single-domain magnetite. Hence, correcting for self-demagnetizing fields from poles on the surface of magnetite-ilmenite grains in rocks is not recommended, whatever the domain state.