Influence of cooling rate on thermoremanence of magnetite grains: Identifying the role of different magnetic domain states

It is widely accepted that cooling rate can strongly influence the intensity of the thermal remanent magnetization (TRM) acquired by rocks during cooling to ambient temperatures. If ignored, this effect might lead to underestimates or overestimates of the ancient magnetic field intensity. To date, however, the cooling rate dependence of TRM acquired by particles with different domain states has never been systematically analyzed from the theoretical or experimental point of view. In this study, we present measurements of the TRM of synthetic magnetites with well-defined grain sizes that were quenched with constant cooling rates of 0.05, 0.1, 1, 3, 10, and 15 K/min. While single domain (SD) and small pseudo-single domain (PSD) samples are found to show larger TRMs after slow cooling, the TRMs of larger PSD and multidomain (MD) magnetites are not affected by an increase or decrease of the cooling rate. Overall, our results suggest that only smallest magnetite grains acquire a cooling rate-dependent TRM. Therefore, cooling rate corrections of paleointensity determinations are only necessary for samples dominated by SD remanence carriers, while rocks dominated by PSD and MD carriers, such as basalts, which are most commonly used for paleointensity studies, do not require such corrections.