A magnetic coupling occurring in partial self-reversal of magnetism and its association with increased magnetic viscosity in basalts

Summary. Samples of Rauher Kulm basalt containing titanomagnetite were heat treated to exhibit partial self-reversal caused by a CRM acquired by an exsolved or 'daughter' phase reversely coupled with the original or 'mother' phase. Alternation of thermal demagnetization with alternating field (AF) demagnetization from progressively higher values of peak AF showed that the two phases were mainly demagnetized together. The coercivity of the coupled. phases exceeded that of the natural sample.
Screening of the daughter phase by a shell of mother phase was demonstrated using large pieces of synthetic titanomagnetite with segregated magnetite and applying a field during the process of self-reversal. Saturation magnetization was measured to indicate the relative proportion of the phases.
Measurements of viscous magnetization of Rauher Kulm samples confirmed that heat treatment caused increased viscosity, but with anomalous anisotropy. After a certain time the viscosity coefficient changed sharply. Explained in terms of grains present such viscous growth initially involved growth of the two phases in opposition. Owing to its greater volume the mother phase growth, which involved low-energy domain wall barriers, was assumed to be the greater. When there were no more daughter phase nuclei of sub-critical volume available, the mother phase growth continued unopposed. Analysis of available growth curves showed that the magnetic viscosity coefficient of the daughter phase was related to the viscosity coefficient for this unopposed growth, which depended on the sum of the external field and the change of the back field produced by the daughter phase. Small granules within grains could be a factor in increasing viscosity due to domain wall movement.     

Some aspects of magnetic viscosity in subaerial and submarine volcanic rocks

Summary. The magnetic viscosity of 334 Upper Tertiary and pre-Bruhnes Quaternary volcanic rocks from the Massif Central (France) and the Steens Mountain (Oregon), and of 40 basaltic cores from DSDP leg 37 has been investigated. Thellier's viscosity index follows a log normal distribution with mean values equal to 6 and 3.5 per cent for subaerial and submarine rocks respectively. For subaerial rocks, the average intensity of the viscous remanent magnetization (VRM) acquired in the Earth's field since the beginning of the Bruhnes polarity epoch ( t = 0.7 Myr) is estimated to be equal to one-quarter of the average intensity of the primary remanence. Alternating field demagnetization of VRMs acquired in low fields for acquisition times t ranging from 2 day to 32 month indicates the resistance to alternating fields is quite different from sample to sample and increases linearly with log t.
Néel's diagnostic parameter of domain structure of the grains involved in magnetic viscosity shows that hard VRM is carried by single-domain grains and soft VRM carried by multidomain particles. Single domain particles carrying hard VRM in subaerial volcanic rocks are almost equant magnetite intergrowths with size near the superparamagnetism threshold, resulting from high temperature oxidation of titanomagnetite. Soft VRM is carried by low Curie point homogeneous titanomagnetite. Unlike his single domain theory, Néel's multidomain theory of magnetic viscosity does not account quantitatively for the resistance of VRM to alternating fields.     

A comparative study of the ARM and Isr of some natural magnetites of MD and PSD grain size

Summary. ARM and I _sr stability with respect to alternating field and low-temperature treatment were measured for a set of artificial rock specimens. Each of the specimens contained a known amount of a natural magnetic mineral of a known grain size. The following minerals were used: a homogeneous natural magnetite, a natural titanomagnetite with submicroscopic Fe-rich inclusions, and a natural titanomagnetite and a natural titano-maghemite that both show exsolution phenomena. The grain size of the fractions varied from < 5 to 250 μm. It is found that the stability of ARM and I _rs of multi-domain grains with respect to alternating fields and low-temperature treatment is similar, whereas for the smallest grains studied (< 5 μm) ARM is more stable than I _sr. Multi-domain ARM data from this study are in agreement with current multi-domain ARM theory. Low-temperature behaviour of ARM and I _sr can be explained either in terms of a model in which stable moments in the bulk of the grains are screened by the surrounding matrix or in terms of a model in which regeneration of the bulk remanence plays a role and in which stable moments are situated on the surfaces of the grains.     

Grain-size dependence of initial susceptibility and saturation magnetization-related parameters of four natural magnetites in the PSD—MD range

Summary. Initial susceptibility, coercive force and several types of remanent coercive forces were measured on a set of artificial rock specimens containing grain-size fractions of a natural magnetite and titanomagnetite, both of which are optically homogeneous and a natural titanomagnetite and titanomaghemite, both of which show exsolution phenomena. It was found subsequently that the optically homogeneous titanomagnetite contains some submicroscopic Fe-rich inclusions. These inclusions are magnetically screened in coarse grains; only for the lower grain sizes studied they have an appreciable influence on the data. The size of the fractions varied from < 5 to 250 μm. The parameters and several parameter ratios are plotted as a function of grain size. Several of these parameters can be used as mineralogical and grain-size indicators. The varying degree of exsolution of the minerals is reflected in the gradients of the plots. From plots of initial susceptibility versus grain size it is concluded that the shape of the grains does not play a dominant role in determining the change of this parameter with grain size. Variation of the various parameters with grain size is explained in terms of a gradual transition from large PSD grains to MD grains without there being any sharp transition. For titanomagnetite ( x = 0.55) the transition takes place at grain sizes that are about 15 μm larger than for magnetite.     

Relationship between median destructive field and remanent coercive forces for dispersed natural magnetite, titanomagnetite and hematite

Summary. Remanent acquisition curves, remanent hysteresis curves and alternating field demagnetization curves were determined for a number of artificial rock specimens containing well-defined grain-size fractions between 5 and 250 μm of natural magnetite, titanomagnetite and hematite. From these curves, the remanent acquisition coercive force H '_cr, the remanent coercive force H _cr and the median destructive field of IRM H _½I were determined. Theoretically these parameters should be the same for an assembly of non-interacting, homogeneously distributed, randomly oriented single-domain grains. For a given hematite specimen H '_cr, H _cr and H _½I have about the same value in spite of the strong grain-size dependence of these parameters. For each specimen of magnetite and titanomagnetite the value of H '_cr is larger than H _cr which again is larger than H _½I. However, the ratios H '_cr/ H _cr and H _½I/ H _cr appear to have a (different) constant value. An interesting relationship which appears to hold for dispersed magnetite, titanomagnetite or hematite grains between 5 and 250 μm, independently of grain-size, quantity and packing density of the magnetic material, is:      

Magnetic mineralogy of pelagic limestones

Summary. Due to their extremely small size and low concentration, the carriers of magnetization in limestones are usually identified through indirect rock magnetic methods. This paper describes a method for extracting tiny ferromagnetic grains from consolidated calcareous rocks and summarizes a scanning electron microscope (SEM) investigation of magnetic phases from several palaeomagnetically important pelagic limestones. Details of surface characteristics and elemental composition are readily accessible with the SEM and complement normal rock magnetic experiments in giving a more complete assessment of the major ferromagnetic phases, their origin, and the timing and acquisition of magnetization. The results confirm many indications of the indirect methods and introduce several important new features. In addition to detrital titanomagnetite, diagenetic titanohaematite and iron sulphides, there is a significant contribution by authigenic microcrystalline haematite, detrital cosmic spherules, chromian spinels, and skeletal titanomaghaemite.     

Basis for Paleoenvironmental Interpretation of Magnetic Properties of Sediment from Upper Klamath Lake (Oregon): Effects of Weathering and Mineralogical Sorting

Studies of magnetic properties enable reconstruction of environmental conditions that affected magnetic minerals incorporated in sediments from Upper Klamath Lake. Analyses of stream sediment samples from throughout the catchment of Upper Klamath Lake show that alteration of Fe-oxide minerals during subaerial chemical weathering of basic volcanic rocks has significantly changed magnetic properties of surficial deposits. Titanomagnetite, which is abundant both as phenocrysts and as microcrystals in fresh volcanic rocks, is progressively destroyed during weathering. Because fine-grained magnetite is readily altered due to large surface-to-volume ratios, weathering causes an increase in average magnetic grain size as well as reduction in the quantity of titanomagnetite both absolutely and relative to hematite. Hydrodynamic mineralogical sorting also produces differences in magnetic properties among rock and mineral grains of differing sizes. Importantly, removal of coarse silicate and Fe-oxide grains by sorting concentrated extremely fine-grained magnetite in the resulting sediment. The effects of weathering and sorting of minerals cannot be completely separated. These processes combine to produce the magnetic properties of a non-glacial lithic component of Upper Klamath Lake sediments, which is characterized by relatively low magnetite content and coarse magnetic grain size. Hydrodynamic sorting alone causes significant differences between the magnetic properties of glacial flour in lake sediments and of fresh volcanic rocks in the catchment. In comparison to source volcanic rocks, glacial flour in the lake sediment is highly enriched in extremely fine-grained magnetite.     

Oceanic basalt continuous thermal demagnetization curves

The palaeomagnetic standard technique of stepwise thermal demagnetization (STD), long regarded as unreliable for oceanic basalts that have undergone low temperature alteration, has recently been applied in a number of studies to characterize the natural remanent magnetization (NRM) of such rocks. In order to better understand STD data of oceanic basalts, and to possibly identify the magnetominerals that are carrying the NRM, we have carried out a number of continuous and STD experiments on seven oceanic basalt samples. During continuous thermal demagnetization (CTD), a sample is heated to a certain temperature and its NRM is measured during heating and subsequent cooling. Even when CTD reveals only titanomaghemite unblocking at 400°C as the remanence carrier, STD behaviour can be very complex and unblocking is observed at temperatures of up to 500°C and higher. CTD also allowed to identify a partial or full self-reversal of NRM due to interaction between two types of magnetominerals in one sample. The higher degree of maghemitization of smaller titanomaghemite grains with respect to larger ones, which are less efficient in carrying the remanence, was seen for three samples by a shift of 80°C between the strong field thermomagnetic curve and the NRM measured at elevated temperature. In several cases, the identification of the NRM-carrying magnetomineral was not possible from CTD data due to the ambiguity of Curie temperatures in the titanomagnetite/titanomaghemite system.     

A catalogue of Spanish archaeomagnetic data

A total of 58 new archaeomagnetic directions has been determined from archaeological structures in Spain. Together with five previous results they allow the compilation of the first archaeomagnetic catalogue for Spain, which includes 63 directions with ages ranging between the 2nd century BC and the 20th century AD. Characteristic remanence directions have been obtained from stepwise thermal and alternating field demagnetization. The hierarchical structure has been respected in the calculation of the mean site directions. Rock magnetic experiments reveal that the main magnetic carrier is magnetite or titanomagnetite with different titanium contents. The age estimate of the studied structures is generally well justified by archaeological constraints. For six structures the proposed date is also supported by physical methods. The data are in close agreement with the French secular variation (SV) curve. This catalogue represents the first step in the construction of a SV curve for the Iberian Peninsula, which will be of much use in archaeomagnetic dating and in modelling of the Earth's magnetic field in Western Europe.     

Explosive volcanic eruptions - VI. Ejecta dispersal in plinian eruptions: the control of eruption conditions and atmospheric properties

Summary. A simple model is developed to relate the maximum down-wind and cross-wind ranges of pyroclasts forming a plinian airfall deposit to the dynamic processes in the eruption cloud from which they fall and the atmospheric wind conditions in the area. The eruption cloud dynamics are in turn related to the eruptive conditions in the vent (vent radius, exsolved magmatic volatile weight fraction, velocity with which material passes through the vent, and mass eruption rate), some or all of which can be deduced if the appropriate field measurements can be made. Some aspects of the stability of convecting volcanic eruption clouds are investigated, and the effects on eruption cloud height of the local atmospheric temperature profile and the value adopted for the entrainment constant (which relates the horizontal flow speed of atmospheric air entering the column to the vertical rise speed of the column material) are explored. It is confirmed that eruption-cloud rise height and pyroclast dispersal are mainly controlled by the mass eruption rate (per unit length of active fissure in the case of linear vents) and, hence, the heat input rate to the cloud; but a significant subsidiary dependence on the amount of exsolved magma volatiles is also found. The eruption cloud model is validated by application to observed historic eruptions, and its use in the analysis of palaeo-eruptions is discussed.     

Efficiency of heavy liquid separation to concentrate magnetic particles

Low-temperature rock magnetic measurements have distinct diagnostic value. However, in most bulk marine sediments the concentration of ferrimagnetic and antiferromagnetic minerals is extremely low, so even sensitive instrumentation often responds to the paramagnetic contribution of the silicate matrix in the residual field of the magnetometer. Analysis of magnetic extracts is usually performed to solve the problems raised by low magnetic concentrations. Additionally magnetic extracts can be used for several other analyses, for example electron microscopy or X-ray diffraction. The magnetic extraction technique is generally sufficient for sediments dominated by magnetite. In this study however, we show that high-coercivity components are rather underrepresented in magnetic extracts of sediments with a more complex magnetic mineralogy. We test heavy liquid separation, using hydrophilic sodium polytungstenate solution Na₆[H₂W₁₂O₄₀], to demonstrate the efficiencies of both concentration techniques. Low-temperature cycling of zero-field-cooled, field-cooled and saturation isothermal remanent magnetization acquired at room temperature was performed on dry bulk sediments, magnetic extracts, and heavy liquid separates of clay-rich pelagic sediments originating from the Equatorial Atlantic. The results of the thermomagnetic measurements clarify that magnetic extraction favours components with high spontaneous magnetization, such as magnetite and titanomagnetite. The heavy liquid separation is unbiased with respect to high- and low-coercive minerals, thus it represents the entire magnetic assemblage.     

Dating of thaw depths in permafrost terrain by the palaeomagnetic method: experimental acquisition of a freezing remanent magnetization

The acquisition of a freezing remanent magnetization (FRM) has been studied in controlled magnetic and thermal environments by successive freezing and thawing (−18 to +20°C) of samples of natural sediments from a frost polygon near Ny Ålesund, Spitsbergen. Successive freeze-thaw cycles cause a significant decrease in the intensity of the initially induced shock remanent magnetization (SRM), associated with directional trends towards the ambient magnetic field direction during the freezing phase. A slow increase in intensity commences after seven to 10 freeze-thaw cycles. The acquisition of a FRM in samples carrying an isothermal remanent magnetization shows a significantly smaller reduction in intensity and only minor directional variations. This result indicates that only a fraction of the magnetic grains in a natural sediment contributes to the natural remanent magnetization. Insignificant changes in lengths and directions of the principal susceptibility ellipsoid axes also indicate that magnetic fabric and remanent magnetization are carried by partly different populations of magnetic grains.
The acquisition of a FRM in nature has yet to be explored. If such a process is confirmed, however, it has the potential for obtaining age estimates of ancient thaw depths and for providing insights into material transport processes in frost polygons.     

Magnetic viscosity, diffusion after-effect, and disaccommodation in natural and synthetic samples

Summary. The magnetic viscosity of some oceanic basalts from IPOD site 417D is investigated. The samples are fine-grained pillow basalts and coarse-grained massive flows. Experiments on the viscous behaviour of these samples include (1) acquisition and decay of weak-field viscous remanent magnetization (VRM) and (2) the influence on VRM produced by the zero field storage time following demagnetization by an alternating magnetic field. In all cases the aquisition of VRM is found to be more rapid than its subsequent decay. The time dependence of the intensity of VRM is observed to be significantly non-logarithmic during acquisition, but less so during decay. The length of time a sample remains in zero field following demagnetization produces a decrease in the aquisition and decay coefficients of VRM. These results cannot be reconciled entirely with predictions based on existing models of magnetic viscosity due to thermal fluctuations. Available evidence also suggest that the effect on VRM produced by (1) zero field storage following demagnetization or produced by (2) different initial states of magnetization, is attributed to susceptibility disaccommodation. Unfortunately, the available data are insufficient fully to ascertain the importance of diffusion after-effect and disaccommodation to long-term viscous behaviour. It is suggested, however, that diffusional processes may contribute to VRM in young pillow basalts (< 0.1 Ma) near mid-ocean ridges where the oxidation of titanomagnetite produces an increase in the number of octahedral lattice site vacancies and the elevated temperature of the crust enhances the diffusive reordering of these vacancies and ferrous ions. In order to gain some additional insights into VRM and attempt to explain some of these observations, theoretical and experimental results pertaining to diffusion after-effect and disaccommodation in magnetic materials are reviewed.     

Palaeoclimatic record from a loess–soil profile in northeastern Bulgaria—I. Rock magnetic properties

A detailed record of mineral magnetic properties of a loess–palaeosol profile comprising seven loess horizons, six interbedded palaeosols and recent soil at the top in NE Bulgaria is analysed. A strong contrast between the soil and loess susceptibilities as well as other concentration-dependent hysteresis parameters is present, similar to the well-documented magnetic characteristics of the Chinese loess ( Hus & Han 1992 ; Maher & Thompson 1992 ; Heller & Evans 1995 ; Hunt et al. 1995 ). The magnetic enhancement of the palaeosol units is caused by very fine-grained pedogenic magnetite with superparamagnetic behaviour. Thermomagnetic analyses on bulk material suggest magnetite and maghemite as the main ferrimagnetic carriers in both soil and loess horizons. Their relative proportions are shown to reflect different palaeoclimatic conditions. Chernozem soils, which include recent soil S₀ and first and second palaeosols S₁ and S₂ developed under steppe vegetation, show a high degree of low-temperature oxidation of the pedogenic magnetite to maghemite. This material is characterized by coercive force H _c showing even higher values than those of the parent loess material. The older palaeosols (S₄ to S₆ ) were formed during more humid climatic conditions and therefore probably developed as forest types. Rock magnetic data suggest the existence here of only partly oxidized magnetite grains. The behaviour of the thermomagnetic curves, characterized by a kink at 200 °C, may be due to either a release of internal stress (built up as a result of partial low-temperature oxidation) or interactions between two phases.     

Polarity Reversals and Oxidation State in Rocks

The correlation between reversed polarity in rocks and strong oxidation of magnetic minerals is briefly re-examined, with especial reference to a well-documented Columbia Plateau basalt suite, recently described in this journal. The correlation appears to vary in quality from place to place and some reasons for this are suggested. Attention is particularly drawn to recent studies which suggest that for palaeomagnetic purposes there may be two groups of lavas, divisible on the basis of whether their magnetic minerals became oxidized under high or low temperature conditions.
A sizable body of opinion now holds (not necessarily correctly) that self-reversals are a negligible factor in the correlation between oxidation and reversed polarity, and that some compositional effect is most probably involved. Virtually the only known petrographic criterion which can (statistically) distinguish reversely from normally magnetized rocks is their (statistically) higher oxidation states. An obvious implication is that the water content of magmas, and perhaps of high-grade metamorphic rocks, is somehow increased during episodes of reversed polarity, so that a slightly larger proportion of rocks is more strongly oxidized.
Since motions of the Earth's core, upper mantle processes, and field reversals are apparently all related, compositional changes originating in the upper mantle could well be also related to field reversals. Alternating linear zones of more and less altered baslat recently reported from one place near the mid-Atlantic ridge might have arisen in such a way.
The importance of relating palaeomagnetic measurements to detailed mineralogical studies of magnetic minerals is again stressed.     

Thermal and Alternating Magnetic Field Demagnetization Experiments on Cainozoic Basalts from Victoria, Australia

i
A re-examination of the palaeomagnetism of the Older Cainozoic Volcanics of Victoria has indicated that these rocks possess temporary unstable components of magnetization. Thermal demagnetization experiments show the magnetic mineral in these basalts is a titanomagnetite. A.c. cleaning experiments to remove the unstable components of magnetization are also described.     

Origin of the Magnetization of the Wichita Mountains Granites, Oklahoma

Summary. The magnetization of the Cambrian Wichita Mountains basement complex consists of several components, two of which are shown to be significant. The residual primary remanence is associated with the original titanomagnetite and the dominant secondary component is carried by minerals of the ilmenite-haematite series. The latter has been acquired at temperatures up to 300°C as a result of the regional hydrothermal alteration during late Palaeozoic. The secondary remanence is of thermochemical origin and carries a memory of a variable ambient field direction which may be modelled as a resultant of the geomagnetic field direction contemporaneous with the hydrothermal process and of the direction of the magneto-static field of the primary titanomagnetite. Although the destruction of the original presumably Cambrian remanence has been extreme, in a few cases its direction appears to have been recovered. The results are complicated by the possibility of a self-reversal of secondary remanence as well as geomagnetic field reversal. An issue unresolved by the investigation relates to the possibility of generation of magnetite by heat treatment of rock material in the laboratory. It is generally concluded that it is difficult to recover the original cooling thermoremanence direction in an igneous intrusion which has been subjected later in its life to extreme conditions of hydrothermal alteration.     

广东韶关市农业土壤磁学性质的空间分布及其影响因素

环境磁学方法具有快速、经济、非破坏性等优点,在农业土壤方面具有较高的研究价值与应用潜力。选取广东省韶关市范围内的32个耕地和65个林地表层土壤样品进行环境磁学分析,测试其高、低频磁化率,非磁滞剩磁与饱和等温剩磁等磁学参数,分析农业土壤磁学性质的空间分布特征及其影响因素,以及磁学指标之间的相关性。结果表明：韶关市农业土壤磁学参数在空间上存在较大变异性,曲江、新丰和乐昌北部的磁性矿物质量分数较高,乳源、武江、始兴南部和翁源北部的磁性矿物粒度较细。土地利用方式是研究区内农业土壤磁学性质的主要影响因素,耕地的磁性颗粒粒径比林地粗,磁性矿物质量分数比林地低。母质类型对农业土壤磁学性质的影响程度不及土地利用类型,可能由于农业土壤受人为因素的影响比母质更显著。人为排放源,包括工业活动和交通运输,对农业表层土壤磁性有明显的增强效应,对磁性物质的粒径分布也有一定的影响。     

High-temperature characteristics of a natural titanomagnetite

Summary. Continuous recordings were made of thermal demagnetizations of Isr, ARM and TRM induced in artificial rock specimens. The artificial rock specimens contained well-defined grain-size fractions of a natural titanomagnetite, which under an optical microscope was homogeneous. Using a Transmission Electron Microscope it was shown that submicroscopic inclusions exist within the titanomagnetite. Screening of the remanence of these inclusions by the surrounding titanomagnetite is thought to be responsible for the occurrence of 'peaks' in the continuous thermal decay curves of Isr and ARM; higher 'peaks' observed in the case of TRM are thought to be caused by the combined effects of screening and an anti-parallel remanence in the titanomagnetite. Further submicroscopic exsolution processes, which occur in the titanomagnetite as a result of repeated heating, can be deduced from the development of new 'peaks', from changes in the temperature at which the 'peak(s)'occur, their shape and their relative height. The use of stepwise demagnetization techniques on the titanomagnetite yielded erroneous directional results due to the screening in the titanomagnetite; the biasing effects due to screening could be avoided by using continuous thermal demagnetization techniques.