陇西第三纪红土磁学性质初步研究

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。与第四纪黄土一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)处于与黄土一古土壤的相应矿物不同的演化阶段。

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

The Longxi Tertiary red earth section consists of altemating paleosols (RS) andinterbeded weakly weathered layers (RL). Field investigation and thin section analysessuggest an eolian origin. In this study, magnetic parameters of selected samples alongthe section have been measured. The samples of RS and RL acquire over 70% and80% of the saturation isothermal remanence magnetization (SIRM) below 300mT,respectively. The remnant coercivity (Hrc) of the SIRM of RS and RL is respectively48～50 mT and 33～60 mT. Both values suggest that the magnetization is mainlycarried by low-coercivity minerals, such as magnetite. Thermal demagnetization ofmulti-component isothermal remanence magnetization (IRM), imparted following themethod of Lowrie, confirms that soft (< 0.05T),medimn (0.05～0.5T) and hard (0.5～2.7T) coercivity components have distinct unblocking temperature. The intensity ofsoft and hard coercivity components is demagnetized to zero at 580 and 675℃, whichexhibits evidence for unblocklng of magnetite and hematite, respectively. The intensitycurve of medium coercivity components shows discontinuity between 300～400℃,which may result from maghemite. On the basis of the rock magnetic properties, weconclude that remnant magnetization of the Longxi red earth is carried by magneite,maghemite and hematite. These results show no qualitative difference in RS and RLformation, while the hard coercivity component of RS is more variable than that ofRL. The temperature-dependent susceptibility from room temperature to 700℃ andback to room temperature was measured continuously with a furnace-equippedKLY-3s Kappa-bridge. The powder whole-rock samples (approximately 0.4g persample) 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℃,indicating magnetite is one of the major contributors to the susceptibility. All samplesshow obviously reduction of susceptibility in the temperature interval between 300 and540/ 555℃ on heating curves, probably indicating that some maghemite is beingconverted into hematite. These results suggest that magnetite and maghemite are themajor contributors to susceptibility. For samples from RS formation, the coolingcurves are below the heating ones, indicating that the laboratory heating created hardly any magnetite. While for samples from RL formation, the cooling curves are abovethe heating ones, indicating that heating created some magnetite. Thermal demagnetization was conducted in Magnetic Measurements MMTD600Thermal Demagnetizer. Measurements of remnant magnetization were made using a 2Gthree-axis cryogenic magnetometer. Both the demagnetizer and natural remanentmagnetization (NRM) measurement systems were mounted in a low field cage. Thecharacteristic remnant magnetization (CHRM) of the samples in RL formation wasisolated in a temperature range between 300 and 585℃. The ℃HRM of samples in RSformation was isolated between 350/400 and 585℃ or between 300/400 and 650℃,which suggests that hematite also carries CHRM in some RS formation. The rock-magnetic properties of the Longxi red earth are significantly differentfrom those of the Tertiary red earth in the Baoji section, and also from those of theQuaternary loess-soil sequence. Four differences exist between the red earth in theLongxi area and that in the Baoji area. Firstly, the red earth in Longxi sectioncontains more hematite. Secondly, hematite is one of the major contributors to theremnant magnetization in Longxi section, while its contribution to remnantmagnetization is much less for the Baoji section. Thirdly, the relative intensity ofCHRM is higher. Lastly, the CHRM in some RS formation is partly carried by thehematite in the section. The difference may be explained by the spatial changes ofthe red earth deposits in the Chinese Loess Plateau or by some other factors, such asthe variations of dust sources and age. The understanding of the mechanism needs tobe further investigated. Significant difference also exists between the red earth in theLongxi area and the Quaternary loess-soil sequence. The laboratory heating is able toproduce magnetite in all of the samples in loess-soil sequence, while this phenomenoncan only be observed for the RL layers in the Longxi red earth. The magefiteproduced by the laboratory heating in the loess-soil samples is thought to be linkedwith the transformation form iron-bearing silicate/clay minerals to magnetite, or withdehydration of geothite or reduction of maghemite to magnetite in high temperature.The difference between the red earth and the loess-soil samples suggests that thoseminerals, which may lead to the formation of magnetite during the heating, are in adifferent mineral phase in the RS layers.