Anisotropy of Stable Single Domain Ferrimagnetic Particles in a Rock Sample from Gyroremanent Magnetization and Comparison with Other Anisotropy Methods

The orientation of stable single domain (SSD) ferrimagnetic particles in an igneous rock sample was determined by a sensitive technique utilizing gyroremanent magnetization (GRM). Components of GRM were measured in the sample upon exposure to an alternating field (AF) at various orientations in 3 orthogonal planes. The major components of GRM exhibited a sin(2θ) dependence on AF orientation in the respective perpendicular planes. This was in accordance with theory [1] and contrary to some previously reported experimental results on magnetic recording tape, which produced a distorted sin(2θ) dependence of the GRM [1]. The explanation is likely due to the SSD ferrimagnetic particles in the rock sample being more dispersed (less interacting) compared to the highly interacting SSD particles in the magnetic tape sample of the previous study. The GRM results were consistent with another remanence anisotropy method, anisotropy of isothermal remanent magnetization (AIRM). This method again measures the anisotropy of the remanence carrying ferrimagnetic particles, but the IRM is also acquired by larger multidomain (MD) particles as well as by the SSD particles. The results were also consistent with the visible rock anisotropy (petrofabric), the anisotropy of magnetic susceptibility (AMS), and the shear wave velocity anisotropy. A comparison of all the methods demonstrated that the fine SSD particles, which make up only a small proportion of the rock, were aligned in quite a similar orientation to that of the main rock forming minerals that constituted the bulk of the sample.

岩石磁组构对剩磁稳定性的制约探讨:以印支地块中生代碎屑岩和拉萨林周盆地设兴组红层为例

岩石磁组构因能提供磁性矿物晶体形状、排列方式等赋存信息而被广泛应用于判别岩石剩磁是否受到了后期构造应力的显著影响;但常规岩石磁化率各向异性(AMS)是否能够准确限定岩石剩磁的稳定性,目前尚无深入探讨.本文以印支地块Nakhon Thai盆地中生代Nam Phong、Phu Kradung和Phra Wihan组三套碎屑岩样品及拉萨地块林周盆地设兴组红层样品为例,通过岩石磁化率组构和剩磁组构的对比分析发现,尽管Nam Phong组绝大多数样品和Phu Kradung组全部样品的AMS组构显示其具有铅笔状至强劈理过渡型构造变形组构特征,但高场等温剩磁各向异性(hf-AIR)显示其高矫顽力赤铁矿所携带的特征剩磁组构仍具有典型沉积组构特征,表明其以赤铁矿为主的载磁矿物未遭受后期构造应力的显著影响,仍然能够准确记录岩石形成时期的古地球磁场方向.另一方面发现有且仅有剩磁组构才是判别碎屑沉积岩特征剩磁是否遭受了后期构造应力影响的充分必要条件.也就是说,如果岩石剩磁组构(如以赤铁矿为主要载磁矿物的岩石hf-AIR组构)指示其原始沉积组构已被构造组构显著叠加或取代,则必然说明该岩石剩磁方向已受到构造应力的作用而发生了显著偏转,因而不能直接用于构造演化和古地理重建等块体运动学研究.

四川盆地早白垩世红层磁倾角偏低研究

对四川盆地东北部巴中地区和西部雅安地区早白垩世红层分别采集9块手标本,进行了详细的古地磁学研究.系统退磁获得两地的特征剩磁,其中巴中地区平均方向为,倾斜校正之前:D_g/I_g=25.3°/19.0°,k=18.6,α_(95)=8.8°;倾斜校正之后:D_s/I_s=25.8°/18.9°,k=24.3,α_(95)=7.6°.雅安地区平均方向为,倾斜校正之前:D_g/I_g=24.5°/45.0°,k=15.7,α_(95)=9.0°;倾斜校正之后:D_s/I_s=356.7°/35.6°,k=28.5,α_(95)=6.6°.磁化率各向异性实验结果显示两地均未受到显著的构造应力影响.等温剩磁各向异性(AIR)实验结果,巴中地区IRMz/IRMLx平均值为0.8194,表明18%的压实率;雅安地区IRMz/IRMx平均值为0.8909,表明11%的压实率,计算得到巴中和雅安地区校正后的磁倾角分别为22.7°和38.8°.根据等温剩磁各向异性(AIR)实验和Tauxe and Kent(2004)提出的EI校正法得到的结果表明,四川盆地早白垩世陆相碎屑岩层中存在沉积压实作用造成的磁倾角偏低现象,而且川东北巴中地区的偏低程度强于川西雅安地区.

基于剩磁各向异性方法对华北下三叠统红层磁倾角浅化效应的研究

本文报道利用岩石剩磁组构对华北下三叠统红层进行磁倾角浅化效应的进一步识别与校正研究结果.首先,采用45°等温剩磁各向异性方法,即通过沿与样品原始水平面(即层面)呈45°夹角方向施加磁化场获得等温剩磁,并进行逐步热退磁,获得平行于层面和垂直于层面的等温剩磁分量随外加磁场和热退磁温度的变化趋势,计算获得浅化因子f=0.70.其次,应用高场等温剩磁各向异性方法,结合峰值为100 mT的交变退磁和120℃热退磁处理,分离获得碎屑赤铁矿对剩磁各向异性的贡献;由直接测量获得的单颗粒碎屑赤铁矿的各向异性度(a=1.35),计算获得f=0.59.该结果与前人对刘家沟组红层进行E/I法磁倾角浅化校正的结果(f=0.60)具有很好的一致性;表明华北下三叠统刘家沟组红层磁倾角浅化效应显著,其浅化因子为f=0.59;高场等温剩磁各向异性方法是红层磁倾角浅化校正的最有效方法.同时,如果有足够的独立样品数,且特征剩磁来自于单一剖面或有证据表明多条采样剖面之间未发生显著的相对运动,E/I法对红层磁倾角浅化因子的估计也是可信的.

华南地块中三叠统巴东组红层磁倾角浅化效应

对华南地块中三叠统巴东组红层样品进行系统的岩石磁学和古地磁学研究,采用高场等温剩磁各向异性(hf-AIR)方法,识别出巴东组红层的磁倾角浅化因子f=0.63。岩石磁学研究结果显示,巴东组红层的主要载磁矿物为赤铁矿和少量磁铁矿;磁化率和高场等温剩磁各向异性组构均指示其具有典型沉积组构特征,表明未遭受后期构造应力改造。高温特征剩磁分量为碎屑赤铁矿所携带,具有单一负极性,并在95%置信水平上通过褶皱检验,与前人在同一剖面不同位置获得的以正极性为主的古地磁方向一致。该特征剩磁方向在地层校正后的平均方向为Ds=222.1°,I_(s)=−27.2°(α95=8.7°),对应古地磁极为48.1°N,215.5°E(A_(95)=8.4°)。对包括本文数据在内的华南中三叠世红层高质量古地磁极数据(Q≥5)用f=0.63进行统一校正后,获得平均古地磁极为48.5°N,207.6°E(A_(95)=10.7°)。对比华北地块同样经hf-AIR方法浅化校正后的早三叠世古地磁极,两者在其东部参考点上的古纬度完全一致,验证了前人提出的华南华北中生代剪刀式旋转拼合模型。