Авторы: Berndt T., Chang L., Paterson G., Cao C.

2020 г.

Geoph.J.Int.

Upon cooling, most rocks acquire a thermoremanent magnetisation (TRM); the cool¬ing rate at which this happens not only affects palaeointensity estimates, but also their unblocking temperatures in stepwise thermal demagnetisation experiments, which is im-portant, for example, to estimate volcanic emplacement temperatures. Traditional single-domain (SD) theory of magnetic remanence relates relaxation times to blocking tem¬peratures - the blocking temperature is the temperature at which the relaxation time becomes shorter than the experimental timescale - and therefore strictly only applies to remanence acquisition mechanisms at constant temperatures (i.e., viscous remanent magnetisations, VRMs). A theoretical framework to relate (constant) blocking temperatures to (time-varying) cooling rates exists, but this theory has very limited experimental verification - partly due to the difficulty of accurately knowing the cooling rates of ge¬ological materials. Here we present an experimental test of this "cooling rate effect on blocking temperatures" through a series of demagnetisation experiments of laboratory-induced TRMs with controlled cooling rates. The tested cooling rates span about 1 order of magnitude and are made possible through (1) extremely accurate demagnetisation ex¬periments using a low-temperature magnetic properties measurement system (MPMS), and (2) the employment of a "1-step-only" stepwise thermal demagnetisation protocol where the relaxation process is measured over time. In this way the relaxation time cor¬responding to the blocking temperature is measured, which can be done to much higher accuracy than measuring the blocking temperature directly as done in traditional stepwise thermal demagnetisation experiments. Our experiments confirm that the cooling rate re¬lationship holds to high accuracy for ideal magnetic recorders, as shown for a synthetic weakly interacting SD magnetoferritin sample. A SD-dominated low-Ti titanomagnetite Tiva Canyon Tuff sample, however, showed that natural samples are unlikely to be suf¬ficiently "ideal" to meet the theoretical predictions to high accuracy - the experimental data agrees only approximately with the theoretical predictions, which may potentially affect blocking temperature estimates in stepwise thermal demagnetisation experiments. Moreover, we find a strongly enhanced cooling rate effect on palaeointensities for even marginally non-ideal samples (up to 43 % increase in pTRM for a halving of the cooling rate).

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