SP-grain production during thermal demagnetization of some Chinese loess/palaeosol

Abstract

SUMMARY Progressive thermal demagnetization of natural samples to above 300 °C may cause significant alterations of magnetic mineral phases. Such changes are commonly monitored by magnetic susceptibility (MS) determinations at every demagnetization step. In an attempt to characterize in more detail alteration products created during thermal demagnetization of some Chinese loess/palaeosols (L1/S1), MS was measured at both room temperature (MS293K) and at liquid Nitrogen temperature (MS77K) between each demagnetization step. We have used the difference between MS77K and MS293K (temperature dependent susceptibility, kTD= MS293K− MS77K) to monitor production of superparamagnetic (SP) magnetic grains. kTD is a function of; (1) paramagnetic susceptibility according to the Curie law (k∼ 1/T) that predicts a theoretical 3.83 times increase in MS at 77K. (2) SP grains at 293 K that passes into the single-domain (SD) state on cooling to 77 K causing a dramatic reduction in MS77K. (3) magnetite grains exhibiting magnetic changes when passing through the Verwey transition at 120 K will also reduce MS77K. We observe a linear relationship between frequency-dependent MS (kFD) and temperature dependent susceptibility (kTD) indicating that SP-SD blocking of magnetite may be the dominant factor of MS change during cooling. High values of kTD indicates high SP concentrations, and an increase of kTD with increasing demagnetization temperature is mainly attributed to the production of SP grains since the amount of magnetite exhibiting changes in MS at the Verwey-transition is unlikely to increase during heating of loess/palaeosol to 600 °C in air. This interpretation is also based on results from Zero Field Heating IRM77K(1T), Zero Field Cooling of IRM77K(1T) and short-time (100 s) viscous decay of IRM500mT. Our findings suggest that SP-grains are produced continuously in both loess and palaeosol with thermal treatment, and that SP-production in palaeosol apparently commence at the ‘surprisingly’ low temperature of around 200 °C.