A theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals

Summary. Susceptibility, thermo-remanent magnetization (TRM) and isothermal remanent magnetization (IRM) anisotropy ellipsoids have been determined for several rock samples. The results indicate that the ellipsoid of initial susceptibility is less anisotropic than the TRM and low field IRM ellipsoids which are found experimentally to be of identical shape. This suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non-destructive low field IRM measurements. Such IRM measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured. The results for a series of artificial anisotropic samples containing magnetite particles of different sizes (in the range 0.2–90 μm) were very similar to those for the rocks. In contrast, a comparison of the susceptibility and IRM ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory. Such results imply that susceptibility and IRM ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single-domain particles (magnetization confined to the easy axis) or whether the particles are essentially multidomain.     

The detection of single-domain greigite (Fe3S4) using rotational remanent magnetization (RRM) and the effective gyro field (Bg): mineral magnetic and palaeomagnetic applications

The intensity of rotational remanent magnetization (RRM) acquired by single-domain greigite at a rotation frequency of 5 rps was combined with measurements of anhysteretic remanent magnetization (ARM) to calculate the effective biasing field ( Bg ) that produced the RRM. Samples of single-domain greigite had Bg values between -137 and -84 μT, and a MDF_RRM of c. 80 mT. By contrast, a suite of natural and synthetic ferrimagnetic iron oxide samples, including single-domain magnetite and y Fe₃O₄ tape particles, acquired Bg values between -3 and -14 μT, and MDF_RRM ranged between 43 and 68 mT (when RRM was acquired). Multidomain magnetite did not acquire a RRM. Bg values at 5 rps were calculated from previously published data for magnetite particles of different grain sizes, which revealed a minimum Bg value of -24 μT and a MDF_RRM of 57 mT for the finest fraction (0.2-0.8 μm in diameter). In a geological example, measurements of Bg and MDF_RRM were used to detect the presence of greigite in a 4 m long Late Weichselian sediment core. Variations in inclination, declination and the intensity of the natural remanent magnetization (NRM) correlate with changes in magnetic mineralogy.     

Gyroremanent magnetization and the magnetic properties of greigite-bearing clays in southern Sweden

The acquisition of a gyroremanent magnetization (GRM) by single-domain (SD) greigite particles during alternating-field (AF) demagnetization is demonstrated. Previous palaeomagnetic studies failed to identify the presence of authigenic greigite in the glacio-marine clays studied. These clays formed the subject of an earlier debate about the validity of a Late Weichselian geomagnetic excursion (the Gothenburg Flip) in southern Sweden. The greigite carries a stable chemical remanent magnetization (CRM), which coexists with a detrital remanent magnetization (DRM) carried by magnetite. AF demagnetization could not isolate the primary remanence in the sediments where magnetite and greigite coexist, due to the overlapping coercivity spectra of the two minerals and the inability to determine the time lag between sediment deposition and CRM formation. Thermal demagnetization removed the CRM at temperatures below 400 C, but this method was hindered by the unconsolidated nature of the sediments and the formation of secondary magnetic minerals at higher temperatures. The results suggest that the low-coercivity DRM carried by magnetite was mistaken for a 'viscous' component in the earlier studies. Hence the former debate about the record of the Gothenburg Flip may have been based on erroneous palaeomagnetic interpretations or non-reproducible results. AF demagnetization procedures applied to samples suspected of bearing SD magnetic particles (such as greigite) should be carefully selected to recognize and account for GRM acquisition.     

The palaeomagnetism and magnetic fabric of cave sediments from Grønligrotta and Jordbrugrotta, Norway

Summary. Twenty-four oriented specimens of laminated clay were obtained from vertical sections in two caves in northern Norway. Studies of the magnetic remanence and susceptibility anisotropy show that the magnetization is depositional in origin and due to magnetite. The palaeomagnetic record in one section is correlated with results from a Swiss lake sediment core to suggest an age of 9600–6800 yr bp for the cave clay. It is shown that the susceptibility lineation produced on gently sloping surfaces was predominantly controlled by gravity and cannot therefore be used directly to estimate the palaeoflow or palaeofield directions.     

TRM deviations in anisotropic assemblages of multidomain magnetite

Anisotropic assemblages of multidomain magnetite particles develop an anisotropy of magnetic susceptibility (AMS), which in turn induces deviations of thermo-remanent magnetization (TRM) from the field direction. From the theories of multidomain TRM acquisition, it is shown that the TRM anisotropy tensor has its eigenvalue ratios ( T _i) related to the principal weak-field susceptibility ratios ( P _i) by the order of magnitude T _i≃ P ²_i. This relation has been experimentally verified on two sets of highly anisotropic rock samples. The exponent has been determined to be 1.94 in the samples from a Peruvian gabbro, and 1.81 in those from the granite of Flamanville (NW France). Accounting for experimental difficulties in determining the TRM anisotropy tensors, these exponents are judged to agree well with the expected one. It is therefore stressed that AMS measurements provide a good means of evaluating the magnetic field direction from deviated TRM directions, providing magnetic carriers are mainly multidomain magnetites.     

沙漠、戈壁和黄土表土岩石磁学特征及其对黄土磁化率机制的意义

黄土高原黄土来自于其北部或西北部的沙漠和戈壁被广泛认可。本文选取西北地区典型的沙漠、戈壁和黄土表土样品,进行系统的岩石磁学研究。结果显示：沙漠和戈壁磁学特征相似,以磁铁矿占绝对优势,磁颗粒为多畴和假单畴,几乎不含超顺磁颗粒,磁性矿物总体含量高于黄土表土。黄土表土以磁铁矿为主,同时含有磁赤铁矿,磁颗粒以成壤过程中形成的超顺磁和单畴颗粒为主。虽然沙漠和戈壁细粒中亚铁磁性矿物相对比例明显高于粗粒,但粗粒组分含量大,对磁性稀释作用显著,全样总体表现为粗粒磁性特征。影响沙漠、戈壁和黄土磁化率的主导因素不同,沙漠和戈壁为磁性矿物含量主导型,磁性矿物总含量高,黄土表土为磁性矿物粒度主导型,超顺磁对磁化率贡献占主导。合理区分风速论和成土论两种磁化率机制,需准确判断成壤程度和氧化还原状态。因此,不同环境条件下磁化率的环境意义及古气候重建,需首先准确区分影响磁化率的主导因素。     

干旱区泥炭样品加热过程中磁学特征的变化研究

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.     

The angular dependence of rotational and anhysteretic remanent magnetizations in rotating rock samples

Summary. Recent experimental work by Edwards has demonstrated that rotational remanent magnetization (RRM) is not a maximum when the alternating field is normal to the rotation axis of the sample (a rock) but is greatest when the angle is about 75°. Experiments involving the production of ARM during sample rotation gave a similar result with a maximum at about 60°. These results are explained here in terms of the response of an isotropic assembly of identical single-domain particles to a strong alternating magnetic field.     

Rotational remanent magnetization (RRM) and its high temporal and thermal stability

The time and temperature stability of various types of magnetic remanence has been measured in pottery samples containing magnetite and in a clay sample containing manganese ferrite. The time decay of rotational remanent magnetization (RRM), anhysteretic remanent magnetization (ARM) and a low-field isothermal remanent magnetization (IRM) has been measured. While the decay of the last two remanences is easily measurable at about 2 and 19 per cent per decade of time, respectively, the decay of RRM is too small to be measured, being less than about 0.1 per cent per decade of time. Thermal demagnetization of thermoremanent magnetization (TRM), ARM and RRM indicates that RRM is also the most thermally stable. The implications of these experiments are that rocks which exhibit gyromagnetic effects such as RRM contain highly stable particles and therefore are likely to be most suitable for palaeomagnetism.     

新疆塔城黄土古土壤磁化率特征及其影响因素

 对新疆塔城奴拉厚13 m的黄土-古土壤剖面磁化率、粒度、TOC、CaCO3进行了实验分析。结果表明,奴拉剖面粒度组成以粉砂(63~4 μm)为主,由于该区黄土磁化率受控于原生磁铁矿、磁赤铁矿和赤铁矿等较粗颗粒磁性矿物,而黏粒物质吸附的铁磁性矿物及成土过程中生成的超顺磁颗粒和磁性矿物较少,磁化率与细颗粒物质成负相关,与粗颗粒物质成正相关关系; 受生物成磁、化学作用以及TOC自身磁性的影响,磁化率与TOC表现出很强的正相关性; 对磁化率与CaCO3含量的相关性分析发现,两者关系密切,CaCO3含量对该区磁化率的独特变化有重要影响。     

Palaeomagnetic study of an allochthonous terraine: the Scisti Silicei Formation, Lagonegro Basin, southern Italy

Summary. The Jurassic Scisti Silicei Formation forms part of the Lagonegro superimposed tectonic units I and II that are thought to represent the axial and internal margins of the Mesozoic Lagonegro Basin, prior to nappe formation. Sampling was carried out in the lower (Lagonegro) and upper (Pignola) nappes in two differently oriented anticlines. Single and multi-component magnetizations are present. Isothermal remanence acquisition rates show that magnetite and haematite are present which, in most Lagonegro specimens, show the same direction of magnetization. Comparison of the palaeomagnetic directions with those from Jurassic rocks on the stable African craton indicates a 147° anticlockwise rotation of the lower nappe which is similar to 139° previously reported for the upper nappe at Vietri di Potenza. The same comparisons show a 44° clockwise rotation of the upper nappe at Pignola. These results suggest that the doubled nappe structures, sampled some 50 km apart, resulted from their emplacement by translation with little rotation prior to the opening of the Tyrrhenian Sea and that it was the opening of this Sea that caused the predominantly anticlockwise rotation. This work therefore indicates the way in which palaeomagnetic analyses can be used, even within complex allochthonous areas, as an aid to deciphering their tectonic evolution.     

16 ka以来黄土高原东亚夏季风变化的环境磁学记录

末次冰消期以来的气候变化是第四纪古气候研究的重要内容。本文对黄土高原西部和平镇剖面16 ka以来的黄土沉积序列（厚度为3.5 m）进行了高分辨率环境磁学研究。结果表明：该黄土序列的磁性颗粒主要由细颗粒磁铁矿主导。其磁化率和频率磁化率具有相同的变化特征,与黄土高原中部和东部的黄土磁化率变化机制一致,即黄土高原西部和平镇黄土的磁化率和频率磁化率仍然受成土作用形成的细颗粒磁铁矿控制,因此其磁化率和频率磁化率仍然可以反映季风降雨的变化特征,并且其反映的16 ka以来季风降雨变化与南方石笋氧同位素记录的季风变化特征一致。通过详细的海陆综合对比,本文认为16 ka至11 ka间东亚季风降雨的变化受到北半球太阳辐射和北半球区域温度的共同影响,而11 ka之后的夏季风降雨减少主要受太阳辐射降低的影响。     

Review of the low-temperature magnetic properties of magnetite from a rock magnetic perspective

The low-temperature magnetic properties of magnetite are reviewed, and implications for rock magnetism considered. The behaviour of fundamental properties of magnetite at low temperatures near the Verwey transition ( T _v ) are documented, and attention is given to various Verwey transition theories. The low-temperature behaviour of the magnetic energies that control domain structure is reviewed in detail. For the first time in rock magnetic literature, the low-temperature anomaly in spontaneous magnetization ( M _s ) is documented and the differences between the saturation magnetization and M _s near the Verwey transition are discussed. It is argued that the low-temperature behaviour of the magnetocrystalline anisotropy, and in particular the anomaly at T _v , is most likely to affect multidomain remanence during low-temperature cycling. For multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through T _v is likely to reduce the stability of closure domains.     

Magnetic properties of hydrothermally synthesized greigite (Fe3 S4 )—II. High- and low-temperature characteristics

The magnetic behaviour of hydrothermally synthesized greigite was analysed in the temperature range from 4 K to 700 °C. Below room temperature, hysteresis parameters were determined as a function of temperature, with emphasis on the temperature range below 50 K. Saturation magnetization and initial susceptibility were studied above room temperature, along with X-ray diffraction analysis of material heated to various temperatures. The magnetic behaviour of synthetic greigite on heating is determined by chemical alteration rather than by magnetic unblocking. Heating in air yields more discriminative behaviour than heating in argon. When heated in air, the amount of oxygen available for reaction with greigite determines the products and magnetic behaviour. In systems open to contact with air, haematite is the final reaction product. When the contact with air is restricted, magnetite is the final reaction product. When air is excluded, pyrrhotite and magnetite are the final reaction products; the amount of magnetite formed is determined by the purity of the starting greigite and the degree of its surficial oxidation. The saturation magnetization of synthetic greigite is virtually independent of temperature from room temperature down to 4 K. The saturation remanent magnetization increases slowly by 20–30 per cent on cooling from room temperature to 4 K. A broad maximum is observed at ~10 K which may be diagnostic of greigite. The coercive and remanent coercive force both increase smoothly with decreasing temperature to 4 K. The coercive force increases from ~50 mT at room temperature to approximately 100–120 mT at 4 K, and the remanent coercive force increases from approximately 50–80 mT at room temperature to approximately 110–180 mT at 4 K.     

The palaeomagnetism and magnetic fabric of sediments from Peak Cavern, Derbyshire

Summary. This paper describes the magnetic remanence and susceptibility anisotropy of 42 samples of laminated clay from Peak Cavern. The sediments are shown to have acquired a stable remanent magnetization due to the depositional alignment of magnetite grains. The magnetic fabric data confirm the existence of alteration within a reddened surface layer. A record of geomagnetic secular variation in the underlying clay is compared to a dated lake sediment magnetostratigraphy to suggest that deposition occurred around 7000 yr bp . A method for estimating water current directions based on the remanence and magnetic fabric data is critically assessed before deducing the sense of palaeoflow in the cave.     

Magnetostratigraphy of Early Cretaceous Maiolica sections of the Southern Alps (Cismon and Pra da Stua, Italy)

The bottom part of the Cretaceous Cismon section in the Southern Alps was sampled for high-resolution magnetostratigraphy. Although the almost pure pelagic nannofossil limestones ( c. 90 per cent CaCO₃) of the Maiolica/Biancone Formation are extremely weakly magnetized, stepwise thermal and alternating-field demagnetization removed overprints and isolated a characteristic remanent magnetization which is interpreted as a primary magnetization. The dominant magnetic carrier mineral is magnetite; a small fraction of haematite may be present. A clear reversal pattern can be correlated unambiguously with Mesozoic polarity chrons CM10N to CM8. A less well-constrained magnetostratigraphy from the Pra da Stua section could not be directly correlated with the global polarity scale, but biostratigraphic information allows its assignment to the interval CM10-CM5. A counterclockwise rotation of 56 and a northward translation of 28 latitude for the Cismon locality since the Early Cretaceous are derived from the palaeomagnetic data, consistent with previous results from the Southern Alps. The high-resolution magnetostratigraphy of the Cismon section is used in an effort to refine the Cretaceous timescale by the combination of magnetostratigraphic and cyclostratigraphic results from the same section. The cyclostratigraphic duration estimates of chrons CM10N to CM8 are compared to their equivalents in a number of traditional timescales and found to be shorter by a factor of 1.26-2.58.     

Effects of weathering on single-domain magnetite in Early Pliocene marine marls

Rock magnetic parameters are often used to recognize variations in the original magnetic mineralogy and for normalizing purposes in palaeointensity studies. Incipient weathering, however, is shown to have a profound but partly reversible influence on the rock magnetic properties of the marls of the Early Pliocene Trubi formation in southern Sicily (Italy). The remanence in the marls resides in single-domain (SD) magnetite grains, but the remanent coercive force (H_cr) shows a strong variation and most values observed are anomalously high ( H_cr) range 36–188 mT).
The enhanced coercivities are attributed to stress in the magnetite grains induced by surface oxidation at low temperature. Upon heating to 150 °C a reduction of coercivities occurs that can be explained by a stress reduction as a result of a reduction of Fe^2- gradient due to a higher diffusion rate at elevated temperature. After heating to 150 °C, coercivities are quite uniform throughout the outcrop and the values are characteristic of SD magnetite (H_cr range 30–38 mT). The bulk susceptibility increases by 4–24 per cent, and the isothermal remanent magnetization (IRM) decreases by 5–11 per cent. The increase in anhysteretic remanent magnetization (ARM) is large: 20–242 per cent. The magnitude of the changes is related to the degree of weathering.
Another effect of heating the marl samples to 150 °C is a substantial reduction of the coercivities of the secondary overprint in the natural remanent magnetization. After heating. separation of the secondary and primary components by alternating-field demagnetization is more efficient. The usual difficulties of thermal demagnetization above 300 °C may thus be avoided by a combination of moderate heating to 150 °C and subsequent alternating-field demagnetization.     

Magnetic investigation of heavy metals contamination in urban topsoils around the East Lake, Wuhan, China

Magnetic measurements and heavy metal analyses were performed on 133 samples from the urban soils around the East Lake in Wuhan, China. Samples were collected from four areas with different environmental settings: a heavy industrial area well known for thermal power generation and steel works; villages located in the downwind area of the industrial area; a main road with heavy traffic and roads around the East Lake. Results show that concentrations of magnetic particle and heavy metals in urban topsoils are significantly elevated due to the input of coarser-grained magnetite from industrial (e.g. power generation and steel production) and other anthropogenic activities (e.g. vehicle emissions). Concentration-related magnetic parameters, for example, magnetic susceptibility, saturation isothermal remanent magnetization and anhysteretic remanent magnetization, significantly correlate with the concentration of heavy metals. Moreover, in terms of grain sizes, the magnetic particles of different origins can be efficiently discriminated at the studied region. Therefore, magnetic measurements provide a basis for discrimination and identification of different contamination sources, and can be used as an economic alternative to chemical analysis when mapping heavy metal contamination in urban soil around the East Lake region, Wuhan, China.     

New Early Cretaceous palaeomagnetic pole from Córdoba Province (Argentina): revision of previous studies and implications for the South American database

A continental sequence of red beds and interbedded basaltic layers crops out in the Sierra Chica of Córdoba Province, Argentina (31.5°S, 64.4°W). This succession was deposited in a half-graben basin during the Early Cretaceous. We have carried out a palaeomagnetic survey on outcrops of this basin (147 sites in seven localities). From an analysis of IRM acquisition curves and detailed demagnetization behaviour, three different magnetic components are identified in the volcanic rocks: components A, B and X are carried by single- or pseudo-single-domain (titano) magnetite, haematite and multidomain magnetite, respectively. Component A is interpreted as a primary component of magnetization because it passes conglomerate, contact, tilt and reversal tests. The carrier of the primary magnetization, fine-grained (titano)magnetite, is present in basalts with a high degree of deuteric oxidation. This kind of oxidation is interpreted to have occurred during cooling. Components B and X are discarded because they are interpreted as recent magnetizations. In the sedimentary rocks, haematite and magnetite are identified as the carriers of remanence. Both minerals carry the same component, which passes a reversal test. The calculated palaeomagnetic pole, based on 55 sites, is Lat. 86.0°S, Long. 75.9°E ( A ₉₅=3.3, K =35). This palaeomagnetic pole supersedes four with anomalous positions reported in previous papers.     

Morphology, mineralogy and magnetic susceptibility of epikarst-Terra Rossa developed in late Quaternary aeolianite deposits of southeastern Saurashtra, India

The nature and development of epikarst and soil development in aeolianites under a monsoonal climatic regime has not yet been described. Late Quaternary aeolianites of the southeastern coast of Saurashtra in western India show a wide array of epikarst and red-soil formation, and serve to typify the character of aeolianite weathering under a monsoonal climate. Three varieties of epikarst are identified that represent down-profile increase in groundwater flow. Five types of Terra Rossae represent a transition from the karstified limestone to soil development. Terra Rossae differ in the content of residual aeolianite and show both simple and complex profiles. The latter at places shows ped development. The results show that an ontogenetic (growth) sequence exists from incipient epikarst to complex palaeosols. This ontogenetic sequence represents an incremental increase in the groundwater budget of the region in response to changing intensification of the Indian monsoon rainfall. Magnetic properties of the Terra Rossae formed under a monsoonal climate are described for the first time. The magnetic susceptibility together with its frequency-dependent and anhysteretic remanent magnetization (ARM) represents both the concentration of single domain and ultrafine superparamagnetic (SP) magnetite. The formation of single domain and superparamagnetic magnetite and hematite are linked genetically to weathering of the aeolianite that leads to the formation of Terra Rossa.