Continuous and stepwise thermal demagnetization: are they equivalent?

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SUMMARY Continuous thermal demagnetization, in which measurements of magnetization are made at high temperatureT during heating, is considerably faster than the conventional palaeomagnetic method of stepwise demagnetization, in which measurements are made at room temperature T 0 in a series of cooling-reheating cycles. In the case of single-domain (SD) grains, the two methods give equivalent results after the continuous measurements are converted to equivalent room-temperature values by correcting for the reversible decrease of spontaneous magnetization M S between T 0 and T. To test for equivalence of the two methods in larger pseudo-SD and multidomain grains, three different samples containing magnetite of different grain sizes and origins were heated in zero magnetic field and measurements taken either continuously at T during heating or at T 0 after a set of cooling steps from T. Two samples contained 100–125 μm (mean 110 μm) and 125–150 μm (mean 135 μm) sieve fractions from a crushed natural crystal of magnetite, while the third sample is a natural diabase core sample containing coarse magnetite in the dark minerals and groundmass and fine magnetite in the plagioclase. Two different vibrating-sample magnetometers were used for continuous demagnetization and a mini-furnace and SQUID magnetometer were used for stepwise measurements. Both total thermoremanent magnetization (TRM) and partial TRMs with non-overlapping blocking temperatures (T C, T I) and (T I, T 0) were demagnetized. The Thellier laws of partial TRMs are approximately although not exactly obeyed. In M S(T)-corrected continuous data, there is little overlap of unblocking temperatures: pTRM (T C, T I) demagnetizes almost entirely above T I and pTRM (T I, T 0) almost entirely below T I, demonstrating reciprocity and independence. The stepwise measurements decrease more rapidly in intensity with increasing T than the corrected continuous results, some of which increase slightly with heating up to T ≈ T I. Some additional decay of magnetization must occur during cooling from T, where the continuous measurement is made, to T 0, where the stepwise result is measured. There are no high-temperature measurements of subsidiary cooling-heating cycles to confirm this deduction, but continuously recorded heating-cooling cycles below room temperature in inverse thermal demagnetization (or low-temperature demagnetization) show both reversible and irreversible features, depending on the remanence tested. The most important conclusion of this study is that M S(T)-corrected continuous demagnetization results do not exactly reproduce measured stepwise demagnetization results except for very fine grains, of SD size or close to it. Continuous thermal demagnetization cannot be used in general as a time-saving alternative to stepwise demagnetization if exact equivalence is required.