INFERRING DOMAIN STATE FROM MAGNETIC HYSTERESIS IN HIGH COERCIVITY DOLERITES BEARING MAGNETITE WITH ILMENITE LAMELLAE

Abstract

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 of saturation remanence to saturation magnetization ranges from 0.10 to 0.45. 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.