TRM, ARM and Isr of two natural magnetites of MD and PSD grain size

Summary. TRM, ARM and Isr stabilities with respect to alternating fields and high- and low-temperature treatment are compared for a set of artificial rock specimens. The specimens contain grain-size fractions of a homogeneous natural magnetite and a natural magnetite showing exsolution lamellae. The grain-size fractions are in the PSD-MD range and vary between < 5 and 250 μm. For large MD grains the coercivity spectra of ARM are similar to those of Isr but differ from those of TRM particularly in the lower part of the spectrum. For small (< 5μm) grains the coercivity spectrum of ARM is similar to that of TRM, and both spectra are different from the coercivity spectrum of Isr. The use of ARM instead of TRM to determine palaeo-intensities is discussed in relation to the results obtained. During high- and low-temperature treatment ARM behaves similarly to Isr but unlike TRM for all grain sizes studied. The results of this study are discussed in terms of a remanence composed of PSD surface moments and MD and PSD bulk moments.