北京密云水库表层沉积物磁性矿物的鉴别

Identification of magnetic minerals in surface sediments of Miyun Lake,Beijing

本文对密云水库表层沉积物中的磁性矿物进行了岩石磁学和透射电子显微学的综合研究.本实验建立的磁选方法实现将70%~85%左右的磁性矿物从沉积物中分离出来.岩石磁学研究表明,密云水库沉积物中的磁性矿物以多畴和单畴磁铁矿为主,还含有少量高矫顽力弱磁性载磁矿物（可能为赤铁矿）.对磁选矿物的透射电镜观测表明,样品中部分单畴磁铁矿具有纳米尺寸和化学纯度高等特点,为拉长的立方-八面体磁铁矿,是趋磁细菌产生的化石磁小体;多畴磁铁矿多数具有微米尺寸,形状不规则,为碎屑成因;超顺磁磁铁矿粒径约为5~20nm,且含硅、铝等元素,可能为自生成因.研究结果表明,岩石磁学和透射电子显微学的综合应用可以更全面、准确地分析沉积物中磁性矿物的成分、含量、粒径和化学成分等信息,为环境磁学、生物地磁学和古地磁学研究提供依据.

Understanding the magnetic properties of natural sediments, which include magnetic concentration, composition and grain size, is fundamental in paleomagnetic and paleoenvironmental studies. Systematic magnetic measurements and non-magnetic methods （e.g.,transmission electron microscopy, TEM） provide detailed information of magnetic minerals. In this paper, we established a high efficient magnetic extraction method to separate magnetic minerals from surface sediments of Miyun lake in Beijing. We analyzed the bulk samples, magnetic extracts and residues. Saturation magnetization （Ms）, volume magnetic susceptibility （κ） and saturation isothermal remanent magnetization （SIRM） of the magnetic extracts is about 85%, 75% and 70% of the bulk samples, respectively. Multiple-parameter rock magnetic measurements indicated that the major magnetic minerals in sediments are multi-domain （MD） and single domain （SD） magnetite. TEM analyses revealed detrital MD titano-magnetite, biogenic SD magnetite （magnetofossils） and authigenic superparamagnetic （SP） magnetite. We propose that the combination of rock magnetism and TEM observations is useful to accurately identify the magnetic minerals in sediments.