陇西第三纪红土磁学性质初步研究

A PRELIMINARY STUDY ON THE MAGNETIC PROPERTIES OF THE TERTIARY RED EARTH IN THE LONGXI AREA

在六盘山以西广泛分布着一套第三纪红色土状沉积，对这套沉积的成因及性质还没有系统的研究。本文对陇西红土磁学性质的研究发现，该地红土的剩磁载体由主到次依次为磁铁矿、磁赤铁矿和赤铁矿，赤铁矿对剩磁有比较显著的贡献；特征剩磁载体主要是磁铁矿，一些古土壤中的赤铁矿也携带了部分特征剩磁，磁赤铁矿的存在不影响特征剩磁的稳定性。与六盘山以东宝鸡红土最显著的差异是陇西红土中的赤铁矿对磁学性质有明显的影响。与第四纪黄土－古土壤序列的热磁学性质的差异在于陇西红土在高温下仅产生少量或不产生强磁性矿物。这可能暗示着红土中含铁硅酸盐和粘土等矿物（可能在高温下产生磁铁矿）处于与黄土－古土壤的相应矿物不同的演化阶段。

The Longxi Tertiary red earth section consists of altemating paleosols (RS) and interbeded weakly weathered layers (RL). field inveshgation and thin section analyses suggest an eolian origin. In this study, magnetic parameters of selected samples along the section have been measured. The samples of RS and RL acquire over 70% and 80% of the saturation isothermal remanence magnetizahon (SIRM) below 300mT,respectively. The remnant coercivity (Hrc) of the SIRM of RS and RL is respectively 48~ 50 mT and 33~ 60 mT. Both values suggest that the magnetization is mainly carried by low-coercivity minerals, such as magnetite. Thermal demagnetization of multi-component isothermal remanence magnetization (IPM), imparted following the method of Lowrie, confirms that soft (< 0.05T),medium (0.05~0. 5T) and hard (0.5~2.7T) coercivity components have distinct unblocking temperature. The intensity of soft and hard coercivity cornponents is demagnetized to zero at 580 and 675℃, which exhibits evidence for unblocking of magnetite and hematite, respectively. The intensity curve of medium coercivity components shows discontinuity between 300~ 400℃which may result from maghemite. On the basis of the rock magnehc properties, we conclude that remnant magnetization of the Longxi red earth is carried by magneite,maghemite and hematite. These results show no qualitative difference in RS and RL formation, while the hard coercivity component of RS is more variable than that of RL. The temperature dependent susceptibility from room temperature to 700℃ and back to room temperature was measured continuously with a fumace - eqtupped KLY-3s Kappabridge. The powder whole-rock samples (approximately 0.4g per sample) were heated and cooled in an argon atmosphere to prevent possible oxidation.The heating cycles of all sample are characterized by a single Curie point of 580℃,indicahng magnetite is one of the major contributors to the susceptibility. All samples show obviously reduction of suscephbility in the temperature interval between 300 and 540 / 555℃ on heating curves, probably indicating that some maghemite is being converted into hemahte. These results suggest that magnetite and maghemite are the major contributors to suscephbility. For samples from RS formation, the cooling curves are below tLhe heating ones, indicating that the laboratory heahng created hardly any magnetite. While for samples from RL formation, the cooling curves are above the heating ones, indicating that heating created some magnetite.Thermal demagnetization was conducted in Magnetic Measurements MMTD600 Thermal Demagnetizer. Measurements of remnant magnetization were made using a 2G fore-axis cryogenic magnetometer. Both the demagnetizer and natural remanent magnetization (NRM) measurement systems were mounted in a low field cage. The charateristic remnant magnetization (Cny of the samples in RL formahon was isolated in a temperature range between 300 and 585℃. The CHRM of samples in RS formahon was isolated betWeen 350/ 400 and 585℃ or betWeen 300/ 400 and 650℃,which suggests that hemahte also carries CHRM in some RS formation.The rock-magnetic properties of the Longxi red earth are significantly different from those of the Tertiary red earth in the Baoji section, and also from those of the Quatemary loess-soil sequence. Four differences exist between the red earth in the Longxi area and that in the Baoji area. Firstly, the red earth in Longxi section contains more hematite. Secondly, hematite is one of the major contributors to the remnant magnetization in Longxi section, while its contribution to remnant magnetization is much less for the Baoji section. Thirdly, the relative intensity of CHRM is higher. Lastly, the CHRM in some RS formahon is party carried by the hematite in the section. The difference may be explained by the spahal changes of the red earth deposits in the Chinese Loess Plateau or by some other factors, such as the variahons of dust sources and age. The understanding of the mechanism needs to be further investigated. Significant difference also exists