干旱区泥炭样品加热过程中磁学特征的变化研究

Magnetic properties during heating of the peat sediments in arid regions

利用采自干旱区高山～新疆阿尔泰山的表层泥炭样品,以K-T曲线为主,结合J-T曲线、磁滞参数以及XRD等相关实验,探讨了应用热磁实验鉴别富含有机质的弱磁性物质磁性特征。结果表明:样品中磁化率的主要贡献者为低矫顽力的强磁性矿物-磁铁矿,磁铁矿的粒径为PSD(准单畴)。加热过程中,含铁粘土矿物绿泥石分解生成新的磁铁矿,磁畴转变成MD(多畴),逐步加热的K-T曲线表明,利用K-T曲线分析含有大量有机质成分弱磁性物质的磁性特征具有一定的指导作用,泥炭样品受热生成的磁性矿物的类型与数量不仅受加热的最高温度控制,同时在加热到不同温度时由于有机质对磁性矿物的还原作用表现出差异性,使得泥炭样品加热到不同温度后的转化、改造及最终生成物亦变得异常复杂。

Sedimentary rock, some metamorphic rock, acidicrock and the sample dominated by peat or organic matter are all located in the weak magnetic measurement range. How to explain the low magnetic susceptibility in the point of magnetic mineral？ As the reliable atmospheric archive of the present and past, peat sediments have high organic matter and low magnetic susceptibility, What is the dominant magnetic properties of the mineral. Whether K -T curve can help to recognize the magnetic minerals and magnetic properties in the peat, whether it al- so helps to understand the sources of magnetic minerals and how organic matter affects magnetic minerals under the influence of the temperature. To understand these, the paper conducted magnetic investigation to a surface peat sample on the Altay Mountains, Xinjiang Uygur Autonomous Region, China. Mainly based on the K -T curve, combined with the J - T curve, hysteresis parameter and XRD experiment, the paper made thermomagnetic experi- ment to distinguish magnetic property of weak magnetic substance which contain plenty of organic matter. The heat- ing and cooling K -T curves are reversible, indicating that crystalloid structure and magnetic domain do not change before the samples are heated to 200 ℃. The cooling lines of magnetic susceptibility all locate above the heating line after being heated to 300℃, 400 ℃, 500 ℃, 600 ℃separately and the changing extent escalates, which shows that more and more magnetic minerals come into being during the heating. When being heated from 0 ℃ to the maximum temperature of 700 ℃, the most distinctive decreasing of magnetic susceptibility occurs at around 580 ℃ and then quickly decreases to 0 ℃, which indicates the presence of magnetite and it demonstrates the unblock- ing temperature of magnetite. During the cooling process from 700 ℃ to 500℃, magnetic susceptibility slowly de- creases, denoting the magnetite probably transforms. For the J - T curve, when the sample starts to be heated, the magnetization slowly decreases. When being heated to around 380 ℃, the magnetization suddenly increases and ar- rives at its max at around 460 ℃, then it starts to decrease, the distinct turn is 580 ℃ and decreases to zero, which means that this mineral is magnetite. In Day - plot, the plotted scatter of the raw sample is mostly located in the PSD area, when being heated from 0 ℃ directly to 700 ℃, the samples are mostly located in the MD area, indi- cating that magnetic particulate becomes coarse during the heating, which corresponds to the magnetic domain re- flected by hysteresis loops. The results of X-ray diffraction show that the main mineral in original peat is quartz, al- bite, illite, there are also iron-containing chlorite and magnetite. The results show that the low-coercivity magnetite contributes most to the magnetic susceptibility. Its grain size is PSD. During the heating, the chlorite break up into new magnetite, magnetic domain transformed into MD. The gradual heating K -T curve indicates that K -T curve is an indicator to analyse the magnetic properties of the weak magnetic substance which contain mass organic mat- ter. The type and quantity of the magnetic mineral created during the heating are not only controlled by the highest temperature, they also present diversity when the peat samples are heated to the different temperature because or- ganic matter has reducing action to the magnetic minerals, which make the transformation, reform and the final product complicated when the samples are heated to different temperature.