直流场和交变场对泥岩磁化率的影响

磁化率值不仅受控于天然岩石和环境物质中磁性颗粒的组合特征,还与实验室内的处理过程有关,包括热处理过程和加场处理过程。热处理过程通常改变磁性矿物的类型,使样品的磁化率值发生明显变化,因而备受关注。但加场处理过程对样品磁化率的影响在于改变其中所含磁性颗粒的磁畴结构,这种变化是微观的,目前尚未受到应有的重视。本文选择8块泥岩样品进行了加场处理对其磁化率影响的研究。样品取自松辽盆地松科1井青山口组一段,主要携磁矿物为准单畴(PSD)或多畴(MD)磁铁矿。在逐步获得等温剩磁直至饱和的过程中,逐步测量样品的磁化率值。随后对其进行单轴系统交变退磁,逐步测量剩磁后,完成磁化率测量。结果发现: 1)直流场使样品的磁化率值增大。 在低于50mT的外加场中,随外加场的增强,样品的磁化率值快速增大; 之后,随外加场的增强,样品的磁化率值基本不变,直到600mT; 600mT~1T之间样品的磁化率值又大幅增大; 最终,样品的磁化率较初始值(X0)增大0.84%~3.40% 。这种磁化率值的增大受控于样品内PSD或MD磁铁矿磁畴结构的改变,与磁铁矿的含量无关。2)样品获得SIRM后进行的逐步交变退磁使其磁化率值有所降低,随交变场的增强,其降低的幅度也增大,但是,交变场达到100mT时,样品的磁化率值仍较X0高。经逐步交变退磁后,样品的磁化率较X0增加0.03%~2.42% 。因而,天然样品在进行磁化率测量之前,不能将其置于任何直流场或交变场中处理。

EFFECTS OF DIRECT FIELD AND ALTERNATING FIELD ON MAGNETIC SUSCEPTIBILITY OF MUDSTONE

Magnetic susceptibility(X) is one of the most basic parameters in the study of environmental magnetism and paleomagnetism. X is controlled not only by magnetic mineral assemblages in natural rocks/sediments,but also by the processes that they have experienced in laboratory,including thermal and field-impressing treatment.Thermal treatment has been paid attention to,because it usually causes the transformation of magnetic minerals,which results in significant variation of X. But the effect of field-impressing treatment on X is occasionally overlooked.In this study,eight mudstone samples,from Member 1 of the Qingshankou Formation of SK-1 south borehole core(45°34'14.42″N,124°40'15.59″E) in Songliao Basin with the depth of 1723.54~1723.68m,are selected to analysis of the effect of field-impressing treatment on X.The study of Li et al.(2012)[20] indicated that Pseud-Single-Domain(PSD)or Multi-Domain(MD)magnetite is the main remanence-carrier of the sediments from Member 1 of the Qingshankou Formation.Isothermal remanent magnetization(IRM) acquisition curves were gained and X was measured step by step.After getting their saturation IRMs(SIRMs),these samples were stepwise alternating field(AF)demagnetized.X was acquired after measuring the remanence of each step.Results indicate that:(1)the impress of direct-field(DF)enhances X of these mudstone samples: when the field is less than 50mT,X increases quickly with the field rising; then X show undetectable variation till the field reaches to 600mT; X enhances again when the field gets to 1 T; at last,X enhances 0.84%~3.40% than the original value(X0).This enhancement of X may be controlled by the change of magnetic domain structure of the PSD or MD magnetite and is independent of the content of the magnetite.(2)the stepwise AF demagnetization,following the acquiring of SIRM,reduces X slightly.This reduce of X strengthens with the AF enhancing.But values of X of these mudstone samples are still higher than X0 after the AF demagnetization is finished,with an enhancement of 0.03%~2.42%.Therefore,it is advised that natural samples must not experience any field-impressing processes before measuring X.