Low-temperature demagnetization of saturation remanence in magnetite-bearing dolerites of high coercivity

Abstract

We studied 16 magnetite-bearing dolerite dyke samples of high coercive force (HC ranging from 11 to 50 mT) that had been used successfully in Precambrian palaeomagnetic studies. Each dolerite was given a saturation remanent magnetization, whose change was measured as the sample was cooled to 77 K in zero field and warmed back to room temperature. Only the three dolerites of highest HC (µ40 mT) show little change on cooling, suggesting that their magnetite is mostly in elongated single-domain grains. The rest of the dolerites are likely to be dominated by pseudosingle-domain magnetite. Cooling to ~135 K causes their remanence to decrease (by 37 per cent on average) in rough proportion to the decrease in saturation magnetostriction, as expected if internal stresses oppose domain wall motion. Cooling from~135 K to 77 K causes remanence to decrease further (by 26 per cent on average), probably mostly because of domain reorganization forced by magnetite’s Verwey crystallographic transition. Warming back to room temperature causes some of the remanence loss to be recovered, perhaps because internal stresses act as a bridge between different easy axes below the Verwey temperature (~122 K) and above the isotropic point (~135 K). This recoverable low-temperature demagnetization averages 23±6 per cent of the initial saturation remanence, while the permanent demagnetization averages 40±9 per cent. Recoverable low-temperature demagnetization is even larger for natural remanence, averaging 46±9 per cent for the six dolerites measured, while the corresponding permanent demagnetization averages 13±6 per cent. Large recoverable low-temperature demagnetization seems to be characteristic of pseudosingle-domain magnetite in which high internal stresses block domain-wall motion, and may be common in mafic igneous rocks like our dolerite samples, whose magnetite is intergrown with ilmenite lamellae. Measuring natural remanence of such rocks before, after and while at 77 K should help separate remanence carried by multidomain magnetite (mostly permanently demagnetized), by single-domain magnetite (mostly unchanged) and by pseudo-single-domain magnetite (mostly responsible for recoverable demagnetization).