TRM, ARM and Isr of two natural magnetites of MD and PSD grain size

Summary. TRM, ARM and Isr stabilities with respect to alternating fields and high- and low-temperature treatment are compared for a set of artificial rock specimens. The specimens contain grain-size fractions of a homogeneous natural magnetite and a natural magnetite showing exsolution lamellae. The grain-size fractions are in the PSD-MD range and vary between < 5 and 250 μm. For large MD grains the coercivity spectra of ARM are similar to those of Isr but differ from those of TRM particularly in the lower part of the spectrum. For small (< 5μm) grains the coercivity spectrum of ARM is similar to that of TRM, and both spectra are different from the coercivity spectrum of Isr. The use of ARM instead of TRM to determine palaeo-intensities is discussed in relation to the results obtained. During high- and low-temperature treatment ARM behaves similarly to Isr but unlike TRM for all grain sizes studied. The results of this study are discussed in terms of a remanence composed of PSD surface moments and MD and PSD bulk moments.     

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.     

Changes in direction of the remanence of rocks produced by stationary alternating field demagnetization

Summary. A theoretical investigation of the way in which an isotropic rock containing single-domain particles acquires both IRM and ARM (or TRM) has indicated that stationary single-axis alternating field (af) demagnetization with the af axis at an angle to the remanence vector should produce progressive angular changes in a single-component remanence as demagnetization proceeds. Just before the remanence is completely removed it should lie at 90° to the af axis irrespective of the original orientation of the remanence (apart from 0°). Experimental observations on a rock sample support these deductions.
This analysis has been extended to investigate the way in which ARM (or TRM) and IRM are demagnetized by static three-axis demagnetization methods which are used by some workers in palaeomagnetism. Theory, in conjunction with the use of a numerical model, predicts that an ARM or TRM should not undergo significant direction changes when these methods are applied but an IRM should undergo progressive direction changes as demagnetization proceeds, usually moving until it makes an angle of cos⁻¹ (1/3) with each of the three af axes just before it is removed. Confirmation that such changes do occur have been obtained by experiments on a rock sample. The relative merits of static and tumbling af demagnetization methods are also discussed.     

The acquisition of thermoremanent magnetization by multidomain single-crystal titanomagnetite

Summary. Large, well-characterized single-crystals of titanomagnetite of approximate composition Fe_2.4Ti_0.6O₄ were ground into spheres and orientated along specific crystallographic axes. The field dependences of the thermoremanent magnetization (TRM) and the temperature dependences of the magnetic hysteresis properties were measured and evaluated against the available theoretical models. The models, of necessity simplifications of real materials, were unable to account for many features of the experimental curves. The discrepancies were partly resolved by consideration of the mode of blocking of each individual coercivity fraction. It was deduced that the domain-walls were pinned by both dislocation lines and by inclusions or voids. A fraction of the walls were not thermally blocked. They could however be thought of as being blocked isothermally as the applied field was relaxed. The proportion of remanence blocked in by each process was found to be dependent on the strength of the inducing field through the field dependence of the blocking temperatures. A simplified model, taking these effects into account, was developed. This predicts the main features of the TRM acquisition curve and of the alternating field and thermal demagnetization spectra.     

Reliability of palaeointensity methods using alternating field demagnetization and anhysteretic remanence

Summary. Using natural volcanic rocks which acquired thermoremanence (TRM) in known fields, reliability of various palaeointensity methods using alternating field (AF) demagnetization were evaluated. Natural remanence (NRM), TRM and anhysteretic remanences (ARM's) before and after heating were stepwisely AF demagnetized following Shaw's method.
It was found that the coercivity spectra of TRM and ARM in these samples are very similar, and that even when changes occurred during heating, the changes for two remanences are quite similar in many samples. Therefore, Shaw's method of palaeointensity determination, which incorporates ARM checks to the conventional comparison of NRM and TRM coercivity spectra, gives results as reliable as those obtained by the Thellier method. Many examples were demonstrated in which TRM and ARM intensities changed substantially by heating, but without changes in the shape of their coercivity spectra. Such changes cannot normally be detected and erroneous palaeointensities with apparent internal consistency would be obtained by usual AF demagnetization methods.
Although ARM is quite similar to TRM, the rate of acquisition of ARM and TRM in weak fields varies by a factor of five among the present samples. To determine palaeointensities from the linear relation between ARM and TRM, it is necessary to determine experimentally the relative acquisition rate of these remanences. Therefore, methods based only on NRM-ARM relations would not give palaeointensities with acceptable errors.     

The relationship between the magnetic properties of terrestrial igneous rocks and the composition and internal structure of their component Fe-oxide grains

Summary. Hysteresis loops and the demagnetization of saturation remanence (SIRM), before and after heating, are used to determine the high field magnetic behaviour and the coercivity spectra for common igneous rock types containing discrete types of magnetic carriers. These incude homogeneous to highly exsolved titanomagnetite grains, oxidized titanomagnetite grains (martite), exsolved hemo-ilmenite grains, and oxidized ilmenite grains. Specific internal structures and changes in composition due to oxidation are found to have characteristic effects on both high and low field magnetic properties.     

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.     

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.     

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.     

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:      

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.     

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.     

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.     

Gyromagnetic remanence acquired by greigite (Fe3S4) during static three-axis alternating field demagnetization

A magnetic study was carried out on lacustrine sediments from the Zoigê basin, Tibetan Plateau, in order to obtain a better understanding of palaeoclimatic changes there. Gyromagnetic remanence (GRM) acquisition is unexpectedly observed during static three-axis alternating field (AF) demagnetization in about 20 per cent of a large number of samples. X-ray diffraction (XRD) analysis on a magnetic extract clearly shows that greigite is the dominant magnetic mineral carrier. Scanning electron microscopy (SEM) reveals that the greigite particles are in the grain size range of 200–300  nm, possibly in the single-domain state. Greigite clumps of about 3  μm size are sealed by silicates. Fitting of XRD peaks yields a crystalline coherence length of about 15  nm, indicating that the particles seen in the SEM are polycrystalline.
  GRM intensities of most samples are of the same order as the NRM, while others show much stronger GRM although their magnetic properties are similar. Variation of the demagnetization sequence confirms that GRM is mainly produced perpendicular to the AF direction. The anisotropy direction can be derived from GRM, but more systematic studies are needed for detailed conclusions. An attempt to correct for GRM failed due to high GRM intensities and because smaller GRM acquisition was also found along the demagnetization axis. Behaviours of acquisition and AF demagnetization of GRM are comparable with those of NRM, ARM, IRM, indicating fine grain sizes of remanence carriers.     

The causes of low-temperature demagnetization of remanence in multidomain magnetite

In this paper, the mechanisms of low-temperature demagnetization of remanence in multidomain magnetite are considered. New experimental observations of the behaviour of the saturation isothermal remanence, thermoremanence and partial thermoremanence at low temperatures are presented. The results show that there are two main contributions to this low-temperature demagnetization. The first and predominant contribution (type-1 demagnetization) is due to 'kinematic' domain state reorganization and occurs throughout cooling from room temperature to the Verwey transition, T _v , at 120–124  K. The second contribution arises from the change in anisotropy from cubic to monoclinic at T _v , which changes the overall domain structure of the grain. On warming in zero field, some domain walls will not return to their original positions but will take up a position that leads to a lower net remanence (type-2 demagnetization). In stoichiometric magnetite, demagnetization does not occur at 130  K at the isotropic point, T _k , contrary to some previous predictions. In non-stoichiometric magnetite, the influence of the Verwey transition is greatly reduced, and anomalous behaviour is observed at T _k .     

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.     

An investigation of the scattered remanent magnetization of the Dunnet Head sandstone

Summary. The directions of remanent magnetism of samples of the Dunnet Head sandstone from Scotland are very scattered on a scale down to a few millimetres, although an overall mean direction is reasonably well defined. The scattered directions show considerable stability against thermal demagnetization and there is evidence that haematite pigment is an important carrier of the remanence. It is concluded that the origin of the inhomogeneous magnetization is a disturbed ambient field during acquisition of chemical remanence by the pigment.     

Palaeointensity determination and K—Ar dating of the Tertiary north-east Jalisco volcanics (Mexico)

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Sixteen samples representing eight Tertiary volcanic units from north-east Jalisco, Mexico were studied in an attempt to estimate the palaeointensity of the Earth's magnetic field. The experimental technique used was similar to that proposed by Shaw (1974) and an attempt was made to incorporate further criteria using the directional behaviour of the NRM, TRM and two ARMs during the af treatment and measuring the rate of ARM acquisition before and after heating. The directional information was used for checking the stability of all remanent magnetizations involved and for correcting the ARM test values. In addition the TRM directional behaviour could be a valuable yet simple test to detect the problem of insufficient heating. The use of the ARM acquisition test in the palaeointensity determination permits the full investigation of the coercive force spectrum and could lead to a more reliable palaeointensity method. This combined ARM method was applied to one sample. Further work is needed to understand the NRM—ARM1 and TRM—ARM2 relationships if the ARM tests are to be used for correcting TRM alteration effects. Whole rock K—Ar age determinations were carried out on samples from four selected units. Ten samples, representing six of the units, are considered to yield reliable palaeointensity values. Mean values were computed for each unit and reduced to the palaeoequator. The mean palaeoequatorial values and K—Ar ages determined are: I (13 |Mp 2 Myr), 0.504 Oe; II, 0.453 Oe; III, 0.439 Oe; VI (52 |Mp 10 Myr), 0.074 Oe; VII (14 |Mp 2 Myr), 0.187 Oe and VIII (12 |Mp 2 Myr), 0.251 Oe. These results agree reasonably well with those from previous studies. A large number of palaeointensity estimations, many more than currently available, are required to obtain an average estimate of the behaviour of the Earth's magnetic field during the Tertiary.     

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.     

Multivectorial magnetization in late Palaeozoic volcanics from North Sardinia; partial remagnetization and rotation

Summary . Detailed thermal demagnetization results combined with vector analysis and study of the convergence point of remagnetization circles reveal that the late Palaeozoic ignimbrites of North Sardinia possess a multi component remanence in addition to having experienced a tectonic rotation. The degree of palaeomagnetic complexity increases with increasing degree of oxidation of the magnetic mineralogy. It is concluded that the rocks were laid down in late Permian time just before the close of the reversed Kiaman geomagnetic epoch. Subsequent oxidation and partial remagnetization basically occurred in late Permian—Triassic time, during a period characterized by alternating field polarity. In the majority of the sites this remagnetization cycle brought about fairly erratic and relatively stable resultant magnetizations which are generally smeared out towards easterly directions. At a later date Sardinia was subject to an anticlockwise rotation of about 45 degrees, after which a minor chemical magnetization, aligned along the direction of the present axial dipole field, seems to have been acquired by some specimens.