LOW-TEMPERATURE DEMAGNETIZATION OF SATURATION REMANENCE IN MAGNETITE-BEARING DOLERITES OF HIGH COERCIVITY

We studied 16 magnetite-bearing dolerite dyke samples of high coercive force (H-c ranging from 11 to 50 mT) that had been used successfully in Precambrian palaeomagnetic studies. Each dolerite was given a satur ation 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 H-c (greater than or equal to 40 m T) show little change on cooling, suggesting that their magnetite is m ostly in elongated single-domain grains. The rest of the dolerites are likely to be dominated by pseudosingle-domain magnetite. Cooling to s imilar to 135K causes their remanence to decrease (by 37 per cent on a verage) in rough proportion to the decrease in saturation magnetostric tion, as expected if internal stresses oppose domain wall motion. Cool ing from similar to 135 K to 77 K causes remanence to decrease further (by 26 per cent on average), probably mostly because of domain reorga nization forced by magnetite's Verwey crystallographic transition. War ming back to room temperature causes some of the remanence loss to be recovered, perhaps because internal stresses act as a bridge between d ifferent easy axes below the Verwey temperature (similar to 122 K) and above the isotropic point (similar to 135 K). This recoverable low-te mperature demagnetization averages 23 +/- 6 per cent of the initial sa turation remanence, while the permanent demagnetization averages 40 +/ - 9 per cent. Recoverable low-temperature demagnetization is even larg er for natural remanence, averaging 46 +/- 9 per cent for the six dole rites measured, while the corresponding permanent demagnetization aver ages 13 +/- 6 per cent. Large recoverable low-temperature demagnetizat ion seems to be characteristic of pseudosingle-domain magnetite in whi ch high internal stresses block domain-wall motion, and may be common in mafic igneous rocks like our dolerite samples, whose magnetite is i ntergrown with ilmenite lamellae. Measuring natural remanence of such rocks before, after and while at 77 K should help separate remanence c arried by multidomain magnetite (mostly permanently demagnetized), by single-domain magnetite (mostly unchanged) and by pseudo-single-domain magnetite (mostly responsible for recoverable demagnetization).