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。     

Rock-magnetic properties of topsoils and urban dust from Morelia (>800,000 inhabitants), Mexico: Implications for anthropogenic pollution monitoring in Mexico’s medium size cities

In this work, we investigate the correlation between some magnetic parameters and the level of contamination by heavy metals in urban soils from Morelia city, western Mexico. The magnetic study was carried out on 98 urban soils samples belonging to distinct land uses. Most of analyzed samples contain ferrimagnetic minerals as the responsible for magnetization, most probably corresponding to the titanomagnetites/ titanomaghemites solid solutions. This is inferred from the susceptibility vs. temperature measurements and the isothermal remanent magnetization (IRM) experiments. These measurements also indicate that most of samples are almost completely saturated before 300 mT. Additionally, the S_-200 values (S_-200 = IRM_-200/ SIRM, where IRM_-200= Back-field of 200 mT after magnetic saturation) are between 0.7 and 1.0, characteristic of low coercivity magnetic minerals. The averaged saturation isothermal remanent magnetization (SIRM) curves can be used as an indicator of pollution level, as these curves show different saturation values according to the level of contamination by heavy metals: Cu, Ni, Cr and Sr. These associations of (titano)magnetite with heavy metals were observed by Scanning Electron Microscope revealing some complex aggregates rather than commonly detected spherules.     

Thermomagnetic monitoring of lithic clasts burned under controlled temperature and field conditions. Implications for archaeomagnetism

We carried out a combined thermal and magnetic evaluation on experimentally knapped clasts of different lithologies (chert, quartzite, limestone, sandstone and obsidian) heated under controlled field and temperature conditions. The main aim of this study is to estimate the feasibility of use of these raw materials, which are commonly found in prehistoric archaeological sites for archaeomagnetic purposes. Rock magnetic analysis included measurements of low-field magnetic susceptibility, isothermal remanent magnetisation (IRM) acquisition curves, hysteresis loops and thermomagnetic curves of lithic clasts both before and after experimental heating. All lithologies, except the obsidian, recorded an increase of up two orders of magnitude in their magnetic concentration-dependent parameters revealing the formation of new ferrimagnetic minerals. Obsidian and sandstone are the most reliable magnetic carriers, followed by limestone, chert and quartzite. Magnetic susceptibility values show significant differences among lithologies. Isothermal remanent magnetisation proved also to be highly discriminatory as well as the room temperature hysteresis parameters. The main macroscopic alterations resulted in colour changes, rubefactions, potlids in cherts and the massive formation of internal fissures in obsidian specimens. The multispecimen palaeointensity technique was applied on selected samples yielding satisfactory results for heated obsidian and sandstone samples. The archaeological applicability of the results is discussed as well as their geomagnetic significance.     

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。