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

作者：Wang Ronghua^*; Lovlie Reidar

来源：Geophysical Journal International, 2008, 172(2): 504-512.

DOI：10.1111/j.1365-246X.2007.03638.x

• rock and mineral magnetism

Progressive thermal demagnetization of natural samples to above 300 degrees 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 (MS(293K)) and at liquid Nitrogen temperature (MS(77K)) between each demagnetization step.
We have used the difference between MS(77K) and MS(293K) (temperature dependent susceptibility, k(TD) = MS(293K)- MS(77K)) to monitor production of superparamagnetic (SP) magnetic grains. k(TD) is a function of; (1) paramagnetic susceptibility according to the Curie law (k similar to 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 MS(77K). (3) magnetite grains exhibiting magnetic changes when passing through the Verwey transition at 120 K will also reduce MS(77K).
We observe a linear relationship between frequency-dependent MS (k(FD)) and temperature dependent susceptibility (k(TD)) indicating that SP-SD blocking of magnetite may be the dominant factor of MS change during cooling. High values of k(TD) indicates high SP concentrations, and an increase of k(TD) 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 degrees C in air.
This interpretation is also based on results from Zero Field Heating IRM(77K)(1T), Zero Field Cooling of IRM(77K)(1T) and short-time (100 s) viscous decay of IRM(500mT). 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 degrees C.

• 出版日期2008-2