红粘土的磁学性质研究

本文对陕西宝鸡剖面红粘土上部进行了详细的岩石磁学研究，确定了该地区红粘土的主要磁性矿物是磁铁矿。赤铁矿相对含量很少，对剩余磁性贡献较小。磁性质不稳定的磁赤铁矿存在于红粘土中，但不影响剩磁稳定性。特征剩磁载体主要是磁铁矿。磁性矿物的粒度是以准单畴为主。红粘土的短时间弛豫粘滞剩磁很强，因此在零磁空间进行退磁和剩磁测量是非常重要的。

MAGNETIC PROPERTY OF RED CLAY

In order to determine the magnetic properties of the red clay sampled from Baoji section in Shaanxi province, a whole series of rock-magnetic investigations in laboratary have been completed. The thermomagnetic analyses have been carried out with a horizontal Curie Balance, in a nitrogen atmosphere to minimize oxidation. A typical thermomagnetic curve indicates that the induced magnetization progressively decreases with a significant change of the slope in the interval 300～450℃. The Curie temperature is very close to 580℃ and identifies magnetite as the main magnetic mineral. The heating and cooling curves are different, which show thatmineralogical changes have occurred during the heating. The acquisition of the isothermal remanent magnetization (IRM) up to a maximum DC fields of 2.70T reveals that about 90% of the saturation IRM is acquired below 300mT. The remanent coercivity (Hrc) of the saturation IRM is about 33.9 mT. Both values provide the evidence that most of the magnetization is carried by a low-coercivity mineral such as magnetite, although some magnetic phase with higher coercivity may be present. Thermal demagnetization of multi-component IRM, imparted following the method of Lowrie, confirms that soft (<0.05T), medium (0.05～0.50T) and hard (0.50～2.70T) coercivity components have distinct unblocking temperatures.No significant discontinuities indicative of maghenite or goethite have been shown,the low and medium coercivity components are demagnetized smoothly to zero at 580℃; exhibiting evidence for unblocking of magnetite; the hard coercivity component shows a monatomic decay of hemaetite. Hysteresis loops were obtained from small chips of the red clay analyzed with a Micromag Medel 2900 alternating gradient force magnetometer. The saturation magnetization (Ms), the saturation remanence (Mr), the coercive field (He) and the coercivity of the remanence (Hrc) were determined. The ratio of these parameters indicate that the magnetic properties of the samples are mainly carried by pseudo-single domain magnetite.On the basis of the rock magnetic properties, we conclude that in spite of the very small amounts of a higher coercivity mineral, presumably hematite, being present, the remanent magnetization of the red clay is mainly carried by magnetite.The low-field susceptibility of all the samples was also used to monitor possible changes of the magnetic minerals during thermal demagnetization, after each step of the thermal treatment. Samples show a relatively stable susceptibility for temperatures up to 680℃. Thermal demagnetizations were carried out with 15 to 18 steps from room temperature to the limit of reproducible results (maximum 685℃.Measurements of the remanent magnetization were made using a 2G three-axis cryogenic magnetometer in the μ-metal shielded room of the Chuter des Faibles Radioactivites (CFR). Almost all the samples show a secondary magnetization directed along the present geomagnetic, and the characteristic remanent magnetization (CHRM) of the samples was isolated at temperatures between 340 to 560℃ Large viscous remanence with short relax can be obtained by the red clay, therefore, it is important to measure remanence in a zero-field space for obtaining reliable paleomagnetic results from the red clay.