结晶剩磁实验研究及其古地磁学意义

Crysstallization remanent magnetization (CRM) could overprint even totally ruined primary remence of rocks. However, we know little about its influence on magnetic signals due to poor understandings for alteration pathways of Fe-bearing minerals and their related effects. To analysis the decomposition rate of siderite and grain-sizes of the corresponding newly formed magnetic minerals during thermal treatments, low-temperature magnetic measurements, including both saturation isothermal remanent magnetization (SIRM) and AC susceptibilities, were conducted on the natural siderite sample and its oxidation products of evaluated thermal environments. Results show that the natural siderite sample consists of a small amount of impurities of multi-domain (MD) magnetite, which can not be detected by high temperature experiments. The natural siderite can significantly decompose even at relatively lower temperature before 250°C. At this stage, the thermal magnetic products are hematite and probably superparamagnetic (SP) maghemite due to the quick oxidization. The difference between the remanence at 20 and 40K on the cooling curves suggests that approximately 50% and 75% of siderite have been decomposed at 250 and 350 oC, respectively. After 400 oC, both susceptibility and SIRM of the thermally treated samples sharply increase, indicating the formation of strong magnetic minerals. Furthermore, the apparently depressed Verwey transitions at 120 K indicate that most of the newly formed pseudo-single domain (PSD) ferrimagnetic minerals by 490-530°C are highly oxidized magnetite and maghemite. It is also shown that magnetic mineral alteration, e.g. transformation of magnetite into hematite, caused by too high or too long time laboratory heating may result in intensity experimental failure.

利用不同控制条件热处理天然结晶菱铁矿，通过现代岩石磁学和矿物物理学的综合方法，研究菱铁矿复杂氧化过程中结晶剩磁获得和磁性特征，重点分析结晶结构改变对岩样结晶剩磁的贡献。研究新结果将具有重要的岩石磁学和古地磁学理论指导意义。