Importance of cooling rate dependence of thermoremanence in paleointensity determination

Abstract

[1] 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.