Secondary remanent magnetization carried by magnetite inclusions in silicates: a comparative study of unremagnetized and remagnetized granites

Abstract

Magnetic carriers in remagnetized Cretaceous granitic rocks of northeast Japan were studied using paleomagnetism, rock magnetism, optical microscopy and scanning electron microscopy (SEM) by comparison with unremagnetized granitic rocks. The natural remanent magnetization (NRM) of the remagnetized rocks is strong (0.3–1.7 A/m) and shows a northwesterly direction with moderate inclination (NW remanence), whereas the unremagnetized rocks preserve weak NRM (<0.5 A/m) with westerly and shallow direction (W remanence). Although thermal demagnetization shows that both NRMs are carried by magnetite, the remagnetized rocks reveal a higher coercivity with respect to alternating field demagnetization (20 mT<median destructive field) than the unremagnetized rocks (<10 mT). On the basis of the modified Lowrie–Fuller test and hysteresis parameters, the unremagnetized rocks carry multi-domain grains whereas the magnetic carrier of the remagnetized rocks is described as a mixture of multi- and single-domain grains. Optical examination reveals that although common magnetic particles in the unremagnetized and remagnetized rocks are titanomagnetite grains with ilmenite lamellae larger than 30 μm, numerous pure-magnetite grains finer than 10 μm are characteristic of the remagnetized ones. Studies by SEM equipped with an energy-dispersive analytical system show that these fine pure-magnetite grains occur as discrete particles within actinolitized parts where the Fe/Mg mole ratio is one third of primary hornblende. These observations suggest that the NW remanence in the remagnetized rocks resides in the fine pure-magnetite inclusions secondarily formed from released Fe ions during the alteration of hornblende to actinolite. The NW remanence is a chemical remanent magnetization which was acquired during the alteration of rocks at some time between 62 and 15 Ma. The originally formed coarse-grained titanomagnetite observed in both unremagnetized and remagnetized rocks carry the W remanence of a thermoremanent magnetization acquired in the Cretaceous time.