secondary remanent magnetization carried by magnetite inclusions in silicates a comparative study of unremagnetized and remagnetized granites开题资料.pdf

Earth and Planetary Science Letters 180 (2000) 271^285

Secondary remanent magnetization carried by magnetite
inclusions in silicates: a comparative study of unremagnetized
and remagnetized granites
Yo-ichiro Otofuji a;*, Koji Uno a, Takahiro Higashi a, Tomomichi Ichikawa a,
Tsuyoshi Ueno a, Toshiaki Mishima a, Takaaki Matsuda b
a Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Kobe 657, Japan
b Department of Geology, Faculty of Science, Himeji Institute of Technology, Himeji 671-22, Japan
Received 6 January 2000; received in revised form 6 April 2000; accepted 22 May 2000
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 (^ A/m) and
shows a northwesterly direction with moderate inclination (NW remanence), whereas the unremagnetized rocks
preserve weak NRM ( 6 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 6 median destructive field) than the unremagnetized rocks
( 6 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