Importance of cooling rate dependence of thermoremanence in paleointensity determination

Abstract

The practical effect of cooling rate on the magnitude of thermal remanent magnetization (TRM) is experimentally tested, using both natural and synthetic magnetites. For synthetic and natural single-domain (SD) magnetites, TRM intensity increases as the cooling rate decreases because a longer exposure time for slower cooling offers more chances to achieve an equilibrium magnetization. Multidomain (MD) magnetites have the opposite response: TRM decreases as the cooling rate increases because slower cooling allows more time to achieve lower magnetization because of self-demagnetization. For pseudo-single-domain (PSD) magnetite, the effect of cooling rate on the remanence intensity appears to be insignificant, intermediate in behavior between SD and MD. For a SD, TRM differences are restricted to lower temperatures but diminish near the Curie point, strongly indicating a serious problem in a conventional treatment of cooling rate correction. On the practical side, merely applying TRM differences as a correction can overcorrect the true paleointensity. Another important significance of the present study is that a cooling rate difference alone can cause a slight nonlinearity in an Arai plot for a SD. Copyright 2011 by the American Geophysical Union.