ON THE USE OF CONTINUOUS THERMOMAGNETIC CURVES IN PALEOMAGNETISM: A CAUTIONARY NOTE

Abstract

We report the results of a comparative thermomagnetic investigation on natural volcanic samples. Susceptibility vs temperature (k–T curves), induced magnetization vs temperature and remanent (saturated) magnetization vs temperature continuous curves were recorded on the same virgin samples. Continuous susceptibility curves appear more sensitive to the magnetic mineralogy, in agreement with the theoretical suggestions. In addition, some new continuous thermomagnetic curves were obtained using VSTM (vibrating sample thermomagnetometer) apparatus, which allows estimating the domain state of magnetic minerals through the study of partial thermoremanent (pTRM) magnetizations. Alternatively, the domain state estimation on the sister samples was derived using hysteresis parameters at room temperature. The interpretation differs depending on the method used. This disagreement is probably due to the fact that the domain state estimation using room temperature hysteresis parameters [Phys. Earth Planet. In. 13 (1977) 260–267] in terms of the plot of magnetization ratio vs coercivity ratio has no resolution for mixture of grain sizes of a single mineral or an assemblage of different minerals. Complex magnetic mineral assemblage with more than one characteristic domain state likely occurs in most natural rocks.