单轴应力作用下两种不同方法研究岩石磁化率变化的结果

本文介绍了对北京——唐山地区八种强磁性岩石标本的压磁实验结果,观察了它们在高压应力下,直至破坏前后的磁化率变化特征。发现磁化率的变化只是其应力值的函数,与体积变化过程、微裂膨胀过程关系甚微。磁化率的压力系数取值范围为——(0.4——3.0)10-3MPa-1。同时,本文还得到了标本中出现破裂后岩石的压磁曲线。 运用铁磁学理论,对上述压磁实验结果作了初步的分析解释。      

溧阳震区岩石磁性的温度和应力效应

本文测试了溧阳地震(M_s=6.0,1979.7.9)震中附近钻井中的岩石在不同温度下磁化率和天然剩余磁化强度随应力的变化。结果表明:1.岩样磁化率为(4-30)×4π×10~4,天然剩余磁化强度为(30-3000)mAm~(-1),居里温度均在580℃左右;2.室温下磁化率的应力灵敏度为(0.4-1.1)×10~(-3)(MPa)~(-1),与其他一些地区同类岩石相比偏低;磁化率应力灵敏度随温度增加,到某一峰值后下降,在邻近居里点处趋于零;3.天然剩余磁化强度的应力灵敏度为(0.2-2.3)×10~(-3)(MPa)~(-1)。分析表明,岩石压磁特性与磁性矿物的磁畴状态有关。 根据实验结果估算的由压磁效应引起的震磁异常仅有1nT的量级,由此推测,该地震前,在溧阳地磁台观测到的近10nT的磁异常很可能主要不是由压磁效应引起的。     

洪湖沉积物的磁性特征及其环境意义

以洪湖沉积物为研究对象,通过对Hh01-2(B)孔岩芯磁参数的测量发现在岩芯所代表的时间段内洪湖沉积物磁化率呈现了明显的四个主阶段变化;频率磁化率变化与磁化率变化具有较好的相关性,亚铁磁性矿物的变化主导了沉积物磁化率的变化.同时结合年代、孢粉分析以及参考相关历史记载资料,发现磁化率高值段对应湖水动力强、湖面较宽、气候较湿润,而磁化率低值段对应湖水动力弱、湖面较小、气候偏干旱.从而得出了洪湖沉积物磁参数曲线波峰和波谷分别对应洪湖在历史上曾经历了扩张和收缩、气候温湿和冷干的旋回.     

北京房山岩体的磁组构特征及其 对岩体侵位的约束

岩浆岩的磁组构与岩浆流动、侵位时应力和冷凝后遭受后期构造改造作用有关.本文分析了北京房山岩体东山口-凤凰亭剖面岩石磁化率各向异性(AMS)变化.热磁曲线和磁滞回线分析显示,岩石的主要载磁矿物为多畴磁铁矿颗粒.AMS磁化率椭球体呈压扁状,磁面理发育且产状陡倾,磁化率各向异性度P值平均值高达1.189.自边缘相(东山口)到中心相(凤凰亭),岩石磁化率椭球的扁率E值和形状因子T值逐渐减小,说明磁组构主要反映岩浆流动和岩体侵位时近NNE-SSW方向的挤压作用,在侵位冷凝后并没有遭受到明显的构造改造.     

甘肃敦煌—阿克塞地区岩(矿)石磁化率特征分析及应用

各类岩(矿)石的磁性差异是进行航磁资料地质解释的物理基础.为全面、系统了解甘肃敦煌—阿克塞地区岩(矿)石的磁性特征,2015—2017年在该区开展了系统的岩(矿)石的磁化率调查工作,实测物性点320处,获得有效磁化率数据10154个.对实测资料进行了详细的分类统计,分析总结了区内沉积岩、侵入岩、火山岩、变质岩、矿石及围岩等各类岩矿(石)的磁性特征,沉积岩一般呈弱或无磁性;火山岩具有中等-强磁性,可引起一定走向的磁异常;基性-超基性岩多具有强磁性,可以引起一定强度、尖锋状或带状的磁异常;中、酸性侵入岩和变质岩磁化率变化范围较大,弱磁性的一般在磁场上无异常显示,中等-强磁性的一般在磁场上表现为带状或团块状异常区;磁铁矿、磁黄铁矿等磁性矿物含量的增多,不同类型矿石磁化率由中等磁性逐渐变为极强磁性.在此基础上将实测磁化率应用于磁性地层、侵入岩圈定,探讨了磁化率与矿产之间的联系.研究结果为该地区高精度航磁资料解释提供了基础资料和参考依据.     

甘肃第三系红粘土磁学性质初步研究及古气候意义

研究表明红粘土与黄土-古土壤具有共同的磁性矿物, 即磁铁矿、磁赤铁矿、赤铁矿(可能还有褐铁矿); 红粘土的磁化率大小与超细顺磁性颗粒的含量正相关; 它与第四纪的黄土-古土壤相似, 在湿润气候条件下由成壤作用形成的超细铁磁性矿物对于红粘土中古土壤层的磁化率增大有重要的贡献, 并得到了Rb/Sr比值的证实. 尽管红粘土中也发现了磁化率与其成土强度不对应的现象,但总体上看红粘土的磁化率仍不失为一个简便的古气候替代性指标. 根据磁化率曲线, 从红粘土发展到黄土沉积,其间的古气候变化幅度大致等同于阶段5(S1)到阶段2 - 4(L1)的变化幅度, 可能表明第四纪期间风场加强、从源区携带比上新世更多(大约两倍)较粗的弱风化风积物(不含超顺磁性颗粒)到黄土高原. 7.5 Ma以来, 中国北方中部风积红粘土的出现记录了从下伏白垩纪红砂岩到晚第三纪的重要气候转变. 红粘土的发展与白垩纪以来全球干旱化和变冷密切相关, 可能由于当时青藏高原的强烈隆升导致了东亚季风开始或加强, 从而有了红粘土的堆积.     

强磁性体ΔT异常计算的误差分析研究

本文主要针对当前磁法勘探中高精度处理解释的需求,对强磁性体ΔT异常计算存在的误差进行分析研究.我们首先通过理论模型计算试验,证明常规计算采用的投影关系的ΔT与实际测量的模量差ΔT之间的误差E在磁异常幅值大时是明显存在的,其影响不容忽视.其次,当磁性强且剩磁存在时,投影ΔT曲线及其误差曲线在磁化方向与地磁场方向改变时具有一定的对称性;地磁场T0、磁性体形态(如二度水平圆柱体模型的半径r、柱体埋深R)和磁性参数(如磁化率κ)等参数确定的情况下,最大误差值出现在磁性体正上方,且其大小与磁性参数(κ)和模型体规模(如r/R)之间皆是指数关系;另外,研究还发现ΔT的计算误差曲线的一些其他规律特点,如在各纬度带上,ΔT计算误差的最大值Emax曲线的极值主要分布在中纬度地区;磁异常矢量Ta与地磁场T0的夹角θ逐渐变化时,随θ变化Emax曲线的极值分布在θ=90°~120°范围内;当磁异常幅值小于10000nT时,最大误差近似为磁异常矢量垂直于地磁场方向的测点附近的误差值;另外,磁性体(圆柱体为例)的半径(即尺度)与埋深的比值r/R超过0.5,且磁化率超过0.1SI时误差已达到3.9nT,磁化率增大与对应的Emax的值呈指数增长特点.因此,我们的研究表明,在强磁性体、磁异常幅值大的数据处理、反演及解释时,现有方法会产生较大的误差,应该基于严格的模量差ΔT,完善相应的处理以及反演方法.     

塔里木盆地南缘8.5 ka以来的黄土岩石磁学特征及其磁化率变化机制

黄土岩石磁学参数是古气候研究中的重要指标,其中磁化率应用最为广泛,并在黄土高原地区取得重大进展,其受控于成壤作用的变化机制也被普遍接受.然而在黄土高原外缘的新疆地区,磁化率的变化机制仍不明确,导致磁化率的古气候意义在该区存在较大争议.本文选取塔里木盆地南缘具有精确年代控制的典型黄土剖面（羊场剖面）开展岩石磁学和高分辨率磁化率研究,利用交叉小波分析方法并结合剖面粒度、矿物及元素特征对该地区磁化率变化机制进行初步探讨.结果显示,羊场剖面的岩石磁学性质主要由粗颗粒软磁性矿物所控制,同时也表现出一定的顺磁性特征.根据载磁矿物和磁化率变化特征可将剖面进一步划分为两个阶段：阶段Ⅰ（8.5~2.5 ka）,载磁矿物以亚铁磁性的磁铁矿为主,磁化率值整体较高;阶段Ⅱ（2.5~0.2 ka）,亚铁磁性矿物依然占据主导地位,但硬磁性矿物和以黄铁矿为代表的顺磁性矿物相对增多,磁化率值显著降低.相关性研究和交叉小波分析表明：阶段Ⅰ磁化率与粗颗粒组分的变化具有一致性,符合"风速论"模式;阶段Ⅱ磁化率不仅与粗颗粒组分具有明显的正相关关系,而且与指示成壤作用强度的频率磁化率百分含量呈现出显著的负相关关系,暗示了阶段Ⅱ的磁化率变化可能受到"风速论"和"还原性成壤"模式的共同影响.本文拓宽了对新疆地区黄土岩石磁学特征及其磁化率变化机制的深入理解,也为利用磁化率恢复新疆及中亚地区全新世以来的古气候变化历史提供了新的线索.     

巴基斯坦Bahawalpur黄土岩石磁学特征及磁化率变化机制研究

风成黄土是陆地上分布最广泛的沉积物之一,记载了各种古气候演化信息.目前巴基斯坦的黄土研究甚少,磁化率与气候对应的变化机制研究尚未开展.本文对位于巴基斯坦印度河平原Bahawalpur地区新发现的黄土-古土壤剖面进行系统的岩石磁学研究,结合粒度和漫反射光谱(DRS)数据,讨论巴基斯坦黄土的磁化率变化机制.实验结果显示:Bahawalpur(BH)剖面黄土层主要的载磁矿物为磁铁矿,同时含有少量磁赤铁矿和针铁矿,磁性颗粒以原生的MD和PSD颗粒为主.相对于黄土层,古土壤层则是以针铁矿为主,含有顺磁性矿物和少量磁铁矿.BH剖面磁化率与成土作用关系和中国黄土高原典型剖面相反,磁化率的变化可能存在一个阈值12.8×10^-8 m³·kg^-1,在阈值之上,强磁性矿物(磁铁矿、磁赤铁矿)占主导;阈值之下,以弱磁性矿物(主要是针铁矿)为主,这种磁性矿物的转变可能导致磁化率降低.本文可为今后利用磁化率解读该地区地层蕴含的古气候信息提供新线索.     

含不同随机缺陷数目岩样的破坏过程、前兆及全部变形特征

利用拉格朗日元法模拟了平面应变单轴压缩条件下具有初始随机材料缺陷的岩石试样的破坏过程、前兆和宏观力学响应.利用若干FISH函数于岩样内部规定初始缺陷并计算全部变形特征.比密实岩石弱的缺陷在破坏之后经历理想塑性行为.密实岩石在破坏之后先是经历线性应变软化行为,然后是理想塑性行为.随着缺陷数目的增加,剪切带间距缩小;岩样的强度下降.在应变软化阶段,最大不平衡力的明显的突增是由于被剪切带切割的试样块体发生了沿剪切带方向的运动.试样内部的缺陷数目越少,应变软化阶段最大不平衡力的峰值越高.无论从单元破坏角度,还是从侧向应变-轴向应变曲线、体积应变-轴向应变曲线、计算得到的泊松比-轴向应变曲线角度,都得到了相同的结论（即,缺陷越多,破坏前兆越明显）.采用两条倾斜的扫描线对岩样最终的破坏形态进行了扫描,发现当扫描线间距等于半个至1个单元长度时,初始随机缺陷位置与岩样最终破坏形态具有很好的相关性.     

Correlation of magnetitic susceptibility with18O/16O data in late orogenic granites of the southern Appalachian Piedmont

Initial magnetic susceptibility (generally indicative of magnetite content) has been determined for 445 samples from 17 granites located in the southern Appalachian Piedmont of Georgia and South Carolina. These values have been correlated with whole rock δ¹⁸O data from the same plutons, yielding a pronounced inverse relationship. It has previously been shown for the southern Piedmont that low oxygen isotopic (¹⁸O-enriched) values usually occur in S-type granites (Wenner [1], this issue). It follows, then, that I-type granites are characterized by high susceptibilities (χ > 1 × 10^?4 G/Oe), and S-type granites by low susceptibilities (χ < 1 × 10^?4 G/Oe). An interesting result of this work has been the observation that some S-type granites exhibit good within-site clusters of remanent magnetic directions while I-type granites generally do not.     

Low-field variation of magnetic susceptibility measured by the KLY-4S Kappabridge and KLF-4A magnetic susceptibility meter: Accuracy and interpretational programme

The KLY-4S Kappabridge and KLF-4A Magnetic Susceptibility Meter enable automated measurement of susceptibility variation with field in the ranges of 2–450 A/m and 5–300 A/m (in effective values), respectively. Unfortunately, the measurement accuracy decreases with decreasing field and it is not easy to decide whether the susceptibility variation at the lowest fields is natural phenomenon or results from measuring errors. To overcome this problem, the accuracies of both the above instruments were investigated experimentally using artificial specimens (mixture of pure magnetite and plaster of Paris) with variable susceptibilities ranging from 1 × 10⁻⁵ to 5 × 10⁻². The complete curve of the field variation of susceptibility of each specimen was measured 10 times and the relative error was calculated for each field. In the KLY-4S Kappabridge, in specimens with susceptibilities higher than 100 × 10⁻⁶, the relative errors are lower than 3% in all fields and lower than 1% in the fields stronger than 10 A/m. In the KLF-4A Magnetic Susceptibility Meter, in relatively strongly magnetic specimens with susceptibilities 5 × 10⁻⁴ to 5 × 10⁻², the relative error is less than 1.5% in the entire field range. While the former instrument is convenient for investigating almost all rock types, the latter instrument is convenient for measuring moderately and strongly magnetic specimens. To facilitate work with field variation of susceptibility curves, showing variable accuracies with field, the programme FieldVar was written. One of its options is plotting the measured data with corresponding field-variable error bars. In this way, a tool is offered for interpreting such susceptibility changes that are sound and reasonable from the point of view of measuring accuracy.     

A combined rock magnetic and geochemical investigation of Upper Cretaceous volcanic rocks in the Pontides,Turkey

Upper Cretaceous volcanic rocks were collected at 24 sites along the Pontides, N-NE Turkey, for rock magnetic and geochemical studies. Rock magnetic and petrographic methods showed that the lavas are characterized predominantly by titanomagnetites with a mixture of pseudo-single and multi-domain grains, whereas in tephrite single domain titanohematite was dominant. Measurements of magnetic susceptibility and the geochemical properties on different volcanic rock types provide important knowledge about the magnetic stability of the rocks. The magnetic properties are interpreted in terms of the composition, concentration, magma generation. Tephrite and phonotephrites with the highest intensities (5200 mA/m) and high magnetic susceptibility values (2585 × 10⁻⁵), largest grain sizes and Fe/Ti values, showing minor or no alteration are the most magnetic stable samples in contrast to dacites with the lowest intensity-magnetic susceptibility (520 mA/m − 573 × 10⁻⁵) and high alteration degree. The basanite samples show very low NRM (48–165 mA/m) but very high magnetic susceptibility (2906–3100 × 10⁻⁵) values suggesting the alteration of Fe-Ti minerals. It is shown that the magnetic properties of the basic to acidic rocks show a systematic variation with magma differentiation and could be related to fractional crystallization. Major and trace elements revealed that the lavas are compatible with complex magma evolution, with mineral phases of olivine+magnetite+clinopyroxene in basic series, amphibole+ +clinopyroxene in intermediate rocks and plagioclase+clinopyroxene+biotite in acidic series.     

The piezomagnetic computation of magnetic anomalies due to ground loading by a man-made lake

Summary A method has been developed to determine the stress distribution and resultant piezomagnetic surface anomaly field due to the load imposed on the earth's crust from filling man-made lakes of variable depths and arbitrary geometries but uniform elastic properties. The dam-magnetic effect observed byDavis andStacey [1] on Talbingo reservoir in the Snowy Mountains of SE Australia is adequately explained in terms of the piezomagnetic effect if more highly magnetic rock than that measured on the surface is present at depth.     

Magnetic features of the magmatic intrusion that occurred in the 2007 eruption at Stromboli Island (Italy)

Significant changes in the local magnetic field marked the resumption of eruptive activity at Stromboli volcano on February 27, 2007. After differential magnetic fields were obtained by filtering out external noise using adaptive filters and seasonal thermal noise using temperature data, we identified step-like changes of 1–4 nT coincident with the opening of eruptive fissures in the upper part of the Sciara del Fuoco. The magnetic variations detected at two stations are closely related to the propagation of a shallow NE–SW magmatic intrusion extending beyond the summit craters area. These observations are consistent with those calculated using piezomagnetic models in which stress-induced changes in rock magnetization are produced by the magmatic intrusion. No significant magnetic changes were observed when the first fractures opened along the NE crater rim. Indeed, the stress-induced magnetization caused by this magmatic activity is expected to be too low because of the structural weakness and/or thermal state of the summit area. The continuous long-term decay characterizing the post-eruptive magnetic pattern can be related to a time-dependent relaxation process. A Maxwell rheology was assumed and the temporal evolution of the piezomagnetic field was evaluated. This allowed us to estimate the rheological properties of the medium; in particular, an average viscosity ranging between 10¹⁶ and 10¹⁷ Pa⋅s was a relaxation time τ of about 38 days.     

南海北部陆区岩石磁化率的矿物学研究

基于2517套现场测量资料,245块岩石样品的体积磁化率测量和详细的岩矿鉴定及硅酸盐全分析结果,结合单矿物磁化率特征及各岩石之间的对比研究,发现岩石磁化率主要受组成岩石的矿物磁化率控制.即岩石磁化率(κr)与组成岩石各个矿物磁化率(κ1)及其体积含量(C2)成正比.例如侵入岩磁化率,κr=-5.68×102Cq+2.8...     

Depth calculation of piezomagnetic effect for earthquake prediction

Tectonic stresses acting in the lithosphere cause transitory changes in the magnetic properties of rock and thus of its observed magnetic field. Calculations have been made to estimate this piezomagnetic effect as a function of depth, by considering the relative stress sensitivity of magnetite with increasing temperature and hydrostatic pressure. The response of magnetization to seismotectonic stress is determined by an appropriate balancing of magnetocrystalline and magnetoelastic anisotropy energies. Magnetite becomes progressively more responsive to applied stress as depth increases, with the rate of change depending on the local geothermal gradient. The upper 15 km of the lithosphere is likely to be the most important in yielding observable piezomagnetic field anomalies for earthquake prediction.     

Track of fluid paleocirculation in dolomite host rock at regional scale by the Anisotropy of Magnetic Susceptibility (AMS): An example from Aptian carbonates of La Florida,Northern Spain

The present study aims to apply the AMS method (Anisotropy of Magnetic Susceptibility) at a regional scale to track the fluid circulation direction that has produced an iron metasomatism within pre-existing dolomite host rock. The Urgonian formations hosting the Zn–Pb mineralizations in La Florida (Cantabria, northern Spain) have been taken as target for this purpose. Sampling was carried out, in addition to ferroan dolomite host rock enclosing the Zn–Pb mineralizations, in dolomite host rock and limestone to make the comparison possible between magnetic signals from mineralized rocks, where fluid circulation occurred, and their surrounding formations. AMS study was coupled with petrofabric analysis carried out by texture goniometry, Scanning Electron Microscopy (SEM) observations and also Shape Preferred Orientation (SPO) statistics. SEM observations of ferroan dolomite host rock illustrate both bright and dark grey ribbons corresponding respectively to Fe enriched and pure dolomites. SPO statistics applied on four images from ferroan dolomite host rock give a well-defined orientation of ribbons related to the intermediate axis of magnetic susceptibility K₂. For AMS data, two magnetic fabrics are observed. The first one is observed in ferroan dolomite host rock and characterized by a prolate ellipsoid of magnetic susceptibility with a vertical magnetic lineation. The magnetic susceptibility carrier is Fe-rich dolomite. These features are probably acquired during metasomatic fluid circulations. In Fe-rich dolomite host rock, ?c? axes are vertical. As a rule, (0001) planes (i.e. planes perpendicular to ?c? axes) are isotropic with respect to crystallographic properties. So, the magnetic anisotropy measured in this plane should reflect crystallographic modification due to fluid circulation. This is confirmed by the texture observed using the SEM. Consequently, AMS results show a dominant NE–SW elongation interpreted as the global circulation direction and a NW–SE secondary elongation that we have considered as sinuosities of the fluid trajectory. The second type of magnetic fabric is essentially observed in the limestone and characterized by an oblate form of the ellipsoid of magnetic susceptibility, a horizontal magnetic foliation and mixed magnetic susceptibility carriers. It is interpreted as a sedimentary fabric.     

Using magnetic susceptibility to assess soil degradation in the Eastern Rif,Morocco

The soil in the Rif, Morocco, is at serious risk because increasing anthropogenic pressures are gradually transforming large natural areas into farmland. The distribution of magnetic minerals within the soil profile can be used to assess soil development and degradation. The soils in the study area are severely eroded because of a combination of highly erodible soils, intense rainstorms and scarce vegetation cover. To sample of representative soil profiles, lithology, slope gradient and land use were considered. The ranges of magnetic susceptibility in the soil profiles distinguished between two primary soil groups. Magnetic susceptibility varied in the soil profile and along the soil toposequence, and the variations were related to the differences in the original magnetic composition and the influence of main erosion factors. Lithology is the main factor contributing to the variation in magnetic susceptibility. The magnetic susceptibility values in soils on Tertiary marls (χ = 13·5 × 10^?8 m³ kg^?1) differed significantly from those on Quaternary terraces (χ = 122·1 × 10^?8 m³ kg^?1). Slope affected the distribution of magnetic susceptibility because of the continuous loss of topsoil in some parts of the slope and the deposition of eroded soil in others. Elimination of the natural vegetation cover and a shift to cultivated land for cereals has had a negative impact on soil development and, on similar slopes and substrates, magnetic susceptibility decreased significantly in cultivated soils. The soils on steep slopes that had natural vegetation cover retained the magnetic minerals better than did those on gentler slopes that were under cultivation. Grazing, clearing and, especially, tilling has weakened the soil and made it much more vulnerable to erosion. An analysis of the main factors causing erosion will help to promote rational use of the land and to establish conservation strategies in such fragile agroecosystems. Copyright © 2009 John Wiley & Sons, Ltd.