Studies of partial rotational remanent magnetization and rotational remanent magnetization at slow speeds of rotation

Summary. Rotational remanent magnetizations and partial rotational remanent magnetizations have been induced in four specimens using alternating magnetic fields of 55 mT maximum peak strength and 128 Hz, and speeds of rotation between 0.0016 and 0.4 rev s⁻¹. Each partial rotational remanent magnetization ( PRRM ), was produced by rotating the specimen only at the maximum setting of the alternating field. The variation of PRRM with (a) speed of rotation, ω, and (b) total angle of rotation, θ, was investigated. In (a), PRRM fell slowly but steadily as ω increased; for (b) it rose sharply as θ increased up to 60° and reached a maximum for θ between 90° and 120°. Alternating field demagnetizations of PRRMs were performed with the specimen (a) at rest, and (b) rotating about an axis perpendicular to the field. Rotation significantly enhanced the demagnetization process. Variation of the time T , taken to remove the inducing alternating field produced no detectable effect in the case of PRRM , but affected the value of ω at which a given feature of the RRM —ω curve appeared, and the product θ_F(=ω T ) appears to be more important than either ω or T separately. Current theories on RRM can be used to explain some of the new experimental data on PRRM .     

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.     

Are hydrodynamic shape effects important when modelling the formation of depositional remanent magnetization?

Recent conceptual models have attributed the weak depositional remanent magnetizations observed in natural sediments to flocculation processes in the water column. Magnetic particles included into flocs have not only to rotate themselves into alignment with the geomagnetic field but also the larger particles to which they are attached, making remanence acquisition an inefficient process. Alignment is hindered further when the magnetization vectors of the particles in any given floc partially cancel, reducing the overall magnetic torque. Existing numerical simulations of flocculation effects on depositional remanence formation have been limited to spherical bodies with translational and rotational motion acting independently of each other. In the case of non-spherical flocs, the translational and rotational motion are coupled and such bodies will describe a complex trajectory through the water column. Calculations will be presented that show the torque exerted on a non-spherical floc by the surrounding water can be orders of magnitude greater than the magnetic torque. Non-spherical flocs will, therefore, align less efficiently with the geomagnetic field and hydrodynamic effects may play an important role in controlling the magnitude of sedimentary remanence.     

Using geomagnetic secular variation to separate remanent and induced sources of the crustal magnetic field

Magnetic fields originating from magnetized crustal rocks dominate the geomagnetic spectrum at wavelengths of 0.1–100 km. It is not known whether the magnetization is predominantly induced or remanent, and static surveys cannot discriminate between the two. Long‐running magnetic observatories offer a chance, in principle, of separating the two sources because secular variation leads to a change in the main inducing field, which in turn causes a change in the induced part of the short‐wavelength crustal field. We first argue that the induced crustal field, b ^I( t ), is linearly related to the local core field, B ( t ), through a symmetric, trace‐free matrix A : b ^I( t )= A B ( t ). We then subtract a core field model from the observatory annual means and invert the residuals for three components of the remanent field, b ^R( t ), and the five independent elements of A . Applying the method to 20 European observatories, all of which have recorded for more than 50 years, shows that the most difficult task is to distinguish b ^R from the steady part of b ^I. However, for nine observatories a time‐dependent induced field fits the data better than a steady remanent field at the 99 per cent confidence level, suggesting the presence of a significant induced component to the magnetization.     

Gyroremanent magnetization produced by specimen rotation between successive alternating field treatments

Summary. On the basis of existing theories of rotational remanent magnetization, it is predicted that a gyroremanent magnetization (GRM) might be induced in an isotropic specimen whose orientation is changed during the interval between successive alternating field treatments. An expression is derived, relating the relative magnitude of GRM to the angular displacement, θ of a specimen about an axis perpendicular to the alternating field axis. The predicted effects were observed for four of the five specimens investigated, and in each case GRM was of comparable magnitude to the corresponding partial rotational remanent magnetization. Hysteresis loop curves and characteristics are all consistent with a main magnetic component comprising magnetite grains. A possible connection between GRM and another form of GRM reported in the case of an anisotropic specimen is suggested. Demagnetization of isotropic (and anisotropic) samples by means of the multi-axis technique might be adversely affected by production of GRM.     

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.     

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.     

Rotational remanent magnetization and the torque exerted on a rotating rock in an alternating magnetic field

Summary. Using an air turbine at rotation frequencies of between 1.5 and 275 revolutions per second (rps), the dependence of rotational remanent magnetization (RRM) on rotation frequency has been investigated for two igneous samples in and alternating field of 51 mT peak at 50 Hz. The same experimental arrangement has also been used to measure the dependence on rotation frequency of the torque exerted by the alternating field on the rock samples. The dependence of torque and RRM on peak field has also been measured at a rotation frequency of 112 rps and a linear relationship between RRM and torque has been demonstrated.
In an attempt to elucidate the way in which RRM arises, analytical and numerical models of the rock have been developed in order to calculate the torque curves and these agree quite closely with those observed experimentally. While the precise factor responsible for RRM has not yet been identified from the numerical model it is suggested that RRM may arise as a result of particle moments suddenly flipping into the field direction, and thus by virtue of their intrinsic angular momentum acquiring a transient component of magnetic moment antiparallel to the rotation vector describing the flip. This component, due to the hysteresis of the assembly of particles, will not then entirely disappear when the alternating field is removed. An estimate of the transient axial field which can be considered to deflect each moment towards the rotation axis during the flip yields a value of the order of 1 mT.     

Graphical methods for rapid vector analysis of demagnetization data

Summary. Working graphs are presented which, when used in conjunction with a stereographic projection, enable (a) magnitude and direction of the difference between two vectors to be estimated, and (b) a vector to be resolved into two or three components of prescribed directions. Applications of the methods to multicomponent natural remanent magnetizations are illustrated.     

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.     

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 palaeomagnetism of the central zone of the Lewisian foreland, north-west Scotland

Summary. Three principal directions of magnetization are recognized in the central part of the Lewisian metamorphic terrain of north-west Scotland. The first ('A') magnetization is a high blocking temperature component residing in magnetite and imposed during post-Laxfordian uplift and cooling. Fifty sites yield an overall mean D = 285.9°, I = 54.9° and palaeomagnetic pole at 273.2° E, 37.6° N ( dp = 3.7°, dm = 5.2°); this magnetization was probably acquired at crustal depths of 6–10 km and is linked to K—Ar uplift ages averaging 1650–1625 Ma. The second ('B') magnetizations are defined by E—W directions and also reside in high blocking temperature components; they are, however, dipolar, have some properties distinct from the 'A' magnetizations, and are correlated with late stages in the history of the complex at 1400–1200 Ma. The third ('C') NE directed magnetizations reside predominantly in low blocking temperature components in pyrrhotite and possibly maghemite, and were probably acquired at a late stage of the regional uplift; they do not correlate with post-1450 Ma magnetizations from the Laurentian Shield and probably relate to the as yet undefined interval 1600–1450 Ma. The collective palaeomagnetic data and certain geologic data suggest that the Lewisian foreland should be rotated by 30° clockwise about a local axis of rotation on the conventional reconstruction of the North Atlantic continents; this rotation is associated with Lower Palaeozoic trans-current movements and may be related to a fourth ('D') magnetization of viscous origin.
A collective assessment of 1850–1600 Ma palaeomagnetic data for the Laurentian Shield defines a large apw loop; there is widespread agreement between data from the constituent structural provinces of the Shield although different metamorphic regions define complementary segments of the loop related to uplift over different intervals of time.     

Magnetostratigraphy of mid-Cretaceous limestones from the Sierra Madre of northeastern Mexico

We present the results of a palaeomagnetic study of four mid-Cretaceous limestone sections exposed in northeastern Mexico. The limestones are weakly magnetized and exhibit two- to three-component magnetizations. These magnetization components appear to be carried by both a sulphide mineral and a magnetite-titanomagnetite mineral. The sulphide mineral carries a reverse polarity overprint that often makes it difficult to isolate definitively the higher-unblocking-temperature component. The high-unblocking-temperature component is well defined in the upper portion of the Santa Rosa Canyon section and in the Cienega del Toro section and passes the fold test. The characteristic remanent magnetization (ChRM) inclinations agree well with predicted mid-Cretaceous inclinations for these sites, although the declinations differ by more than 100°. The relative rotation between these two sites probably occurred as the thrust sheets were emplaced during Laramide deformation. At two of the sections, namely Cienega del Toro and the overturned Los Chorros sections, only normal polarity directions are observed. The La Boca Canyon and Santa Rosa Canyon sections exhibit zones of both normal and reverse polarity magnetization. Correlation of these polarity zones with the geomagnetic polarity timescale provides a time framework for lithostratigraphic and palaeoceanographic studies of these sections.     

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.     

Lower Cretaceous magnetic stratigraphy in Umbrian pelagic carbonate rocks

Summary. A record of geomagnetic field polarity for the Barremian, Aptian and Albian stages of the Early Cretaceous has been derived in three over-lapping sections of pelagic carbonate rocks in the Umbrian Apennines of northern Italy. The remanence carrier in the greyish-white Majolica limestone and Fucoid Marls is magnetite, with haematite also an important constituent in a zone of 'couches rouges' within the Fucoid Marls. The weak remanent magnetizations were measured with a cryogenic magnetometer. Alternating field or thermal demagnetization was used to isolate the characteristic remanent magnetization (ChRM) in 655 specimens from 248 stratigraphic levels. The samples respond positively to a tectonic fold test, indicating that the ChRM predates the Late Tertiary folding of the Umbrian sequence. The magnetic stratigraphy derived from variations of virtual geomagnetic pole latitude clearly defines the recognizable reversal pattern associated with Mesozoic marine magnetic anomalies M0 to M4. The sections have been zones palaeontologically on the basis of planktonic foraminifera and calcareous nannofossil assemblages. The ages of magnetic anomalies M0 to M4 determined in this way are somewhat older than those in the reversal time scale of Larson & Hilde (1975). Anomaly M0 is located in the Early Aptian, close to the Aptian/Barremian boundary. A long period of normal polarity in the Aptian and Albian corresponds to the early part of the Cretaceous magnetic quiet zone. It is interrupted in the Late Aptian by a reversal which we find in only one of the Fucoid Marl sections, and which has not been reported in oceanic magnetic anomaly investigations.     

The origin of rotational remanent magnetization

Summary. Rotational remanent magnetization, RRM, is the magnetization acquired when a sample is rotated during alternating field demagnetization. Although the existence of RRM has been well documented in different laboratories, until now no physical mechanism explaining its origin has been given. We propose that the RRM originates from thermal fluctuations biased by a precessional torque associated with the alternating field. Our theory is consistent with the observation that no directional preference exists in the experimental situation until the sample is rotated relative to the alternating field. Moreover, our theory predicts that the combined sample rotation and precession will produce a RRM that switches direction when the frequency of sample rotation increases from any value below the frequency of the alternating field to any value above that frequency as observed in experiments. Although no precise theory is given for the intensity of RRM, the model presented here can qualitatively explain previous intensity observations.     

An experimental study of the intensity and stability of thermoremanent magnetization acquired by synthetic monodomain titanomagnetite substituted by aluminium

Summary. Titanomagnetites of composition Fe_2.4-δAl_δTi_0.6O₄ and Fe_2.6-δAl_δTi_0–4O₄(δ=0, 0.1 and 0.2 in both cases) were prepared in the monodomain state by pulverization of sintered synthetic material. In low fields, the thermoremanence (TRM) was found to be linear with inducing field and of high enough intensity to account for typical natural remanent magnetizations of fresh submarine basalts. The higher field TRM acquisition curves follow the Néel model curve for an assemblage of non-interacting identical particles in a general way only, the differences being due to interactions, or the range of particle blocking temperatures and volumes or other features of the samples not included in the model. The unblocking temperatures of low field TRM lie in a narrow range below the Curie point. The low field TRM is very resistant to alternating field demagnetization and provides a very striking illustration of the strength of the TRM mechanism in preserving a stable record of a weak magnetic field. The result of a Lowrie-Fuller test on the material is consistent with the monodomain state.     

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.     

Decay of a post-depositional remanent magnetization in wet sediments due to the effect of drying

Summary. The decay of the post-depositional remanent magnetization (post-DRM) during desiccation in magnetic field free space is measured as a function of the loss of water. The decay is ascribed to the drying effect and the time decay of viscous remanent magnetization (VRM). The VRM forms only 10 per cent of the total of loss of remanent magnetization. The decay due to the drying effects depends both on the loss of water and on either the evaporation rate or the period of storage. The percentage of loss of magnetization is independent of its intensity.
A critical drying stage appears (about 60 per cent in water content on a dry basis) which is characterized as a vanishing point of mobile particles or particle units. The mobile particles or units play an important role both in acquisition and demagnetization through physical rotational motion within wet sediments before the critical drying stage. More than 80 per cent of the total loss of the post-DRM is destroyed before the desiccation proceeds to the critical drying stage. The decay of post-DRh4 is concluded to be mainly due to the physically random rotation of the magnetic particles trapped in shallow energy wells which are overcome by the torques caused by the application of the alternating magnetic field less than 200 Oe.     

The additivity of partial thermal remanent magnetization in magnetite

Summary. Experiments were done to test the additivity of partial thermal remanent magnetizations (PTRMs) for prepared samples containing magnetite particles whose sizes range from SD (single domain) to MD (multidomain). The experiments compare the sum of two PTRMs with total-TRM, all produced by the same external field of 0.47 oe. The most significant conclusion of this paper is that, to first order, the additivity of PTRMs is obeyed for the magnetites of this study regardless of particle size. However, small, higher order deviations from additivity occur such that ΣPTRM > TRM by an average of about 1 per cent. Though small, these departures from additivity are significant at the 99 per cent confidence level, and they can be understood in terms of Néel's theory for SD particles. The small departures from additivity are intrinsic to the experimental procedure in which some particles acquire remanence twice, in each of the two PTRM steps. In the limit of small inducing fields additivity should be obeyed exactly for the magnetites of this study and for samples of interest in palaeomagnetism. The deviations from additivity should have no effect on palaeointensity determinations by the Thelliers' version of the Thellier palaeointensity method. For palaeointensity determinations by Coe's version of the Thellier method the effects of deviations from additivity would be very small, less than 4 per cent on the average for a worst-case experimental configuration, and these effects can be minimized by producing PTRMs parallel to the original NRM and by using weak laboratory fields.