Secondary Magnetization Components Research Articles

Magnetic properties of deep-sea drilling project basalts from the North Pacific Ocean

Magnetic properties of oceanic basalts from Deep-Sea Drilling Project (DSDP) sites 36, 54, 57, 77, 83, and 84 are described. Secondary components of magnetization were found to be significant at many of these sites. The basalts were placed in deuteric oxidation class 1 on the basis of opaque mineralogy observations. These, together with the analysis of irreversible high-field thermomagnetic curves measured in air, indicated that the dominant magnetic minerals were nonstoichiometric titanomagnetites. The remanent magnetizations were stable against af demagnetization, having median destructive fields (mdf) of 250–550 oe and Königsberger ratios of 10–50. These magnetic properties are compatible with some of the basic assumptions of sea floor spreading. However, the remanent inclinations, although stable, did not give interpretable paleolatitudes for the sites, as has also been observed in other studies of partially oriented submarine basalts. Class 1 basalt from hole 57 had a coarse grain size and lower Curie temperatures than the other basalts. Its remanence was very unstable (mdf of 40 oe) and could be accounted for in direction, intensity, and stability by a viscous remanent magnetization (VRM) acquired in situ during the Brunhes normal polarity epoch. Similar unstable magnetizations have been observed in several other DSDP basalts. The VRM of the oceanic crust is proposed as an explanation of some magnetic ‘quiet zones’.

Pole Positions from the Mamainse Point Lavas and their Bearing on a Keweenawan Pole Path and Polarity Sequence

Thermal and alternating field studies of the remanent magnetization of the Mamainse Point lavas, on the east shore of Lake Superior, indicate stable magnetization. The flows form two groups of normally magnetized (N) flows, each overlying reversely magnetized (R) flows. It is suggested that the sequence consists of a single sequence of R flows overlain by N flows, the sequence being repeated by faulting. A mean pole position calculated from 15 N sites is 30 °N, 183 °E, (dp = 5, dm = 8), and the pole position from 12 R sites is 49 °N, 232 °E (dp = 14, dm = 16). Reasons are given for believing that this difference in pole position between the older (R) and younger (N) flows was caused by relative movement of the Keweenawan area and the rotation axis of the earth, and not to secondary components of magnetization acquired since the rocks cooled. Review of the Keweenawan paleomagnetic data reveals a consistent picture of an older reversely magnetized and younger normally magnetized magnetic field (if allowance is made for radiogenic errors). The time at which the field switched from reversed to normal polarity may provide a valuable time datum traceable in the rocks from continent to continent.

TIME‐DEPENDENT ELECTROMAGNETIC FIELDS OF AN INFINITE, CONDUCTING CYLINDER EXCITED BY A LONG CURRENT‐CARRYING CABLE

Theoretical and numerical computations have been made for the quasi‐static, time‐domain electromagnetic response of an infinite, conducting horizontal cylinder stimulated by long cable‐carrying step and ramp‐function type pulses. The effect of higher‐order induced multipoles on the secondary electric and magnetic field components is analyzed in detail, and the “threshold distances” at which individual multipoles become effective (contributing more than 5 percent of the secondary field) are presented. Also, the field fall‐off directly above the body and the variations in different induced‐field components along a traverse perpendicular to the strike of the ore body are examined.

Paleomagnetism of Some Neogene Sedimentary Rocks on Oga Peninsula, Japan

The Oga Peninsula Neogene section in Japan was sampled for paleomagnetic studies. The results show that the upper Wakimoto formation is normally magnetized and the lower Wakimoto, Kitaura and uppermost Funakawa formations are reversely magnetized. The remainder of the Funakawa and the Onnagawa formation are interpreted as being remagnetized and unreliable for magnetic stratigraphy. The saturation IRM intensity was found to be an effective method of normalizing the effects of magnetic mineral content variation on the NRM intensity. Variations in the NRM intensity in these samples were paralleled by variations in SIRM intensity indicating that they are mineralogically rather than field controlled. The secondary component of magnetization in these rocks is believed to be a VRM acquired over the Brunhes normal epoch. Intensities and stabilities of the secondary component are compatible with the results from the VRM experiments conducted in the laboratory. Due to the fact that only one reliable reversal was found in the section, age interpretations based on the magnetic stratigraphy were difficult to make. Tentative age assignments were postulated based on the interpretations of the diatom evidence by BURCKLE (1971).

Palaeomagnetic studies of the Kapiti phonolite of Kenya

Specimens of Kapiti phonolite from sixteen sites were treated in an alternating field up to 900 Oe peak. Four sites were discarded on the basis of Watson's Criterion for randomness. Variations in various magnetic parameters along a vertical section of the rock indicate that a hard secondary component of magnetization of high coercive force is present in the surface samples of the rock — a possible cause for the poor grouping of directions of the remaining sites. Weathering may have introduced this component in the surface samples. The Kapiti phonolite possesses both normal and reverse polarities. The Pole position for the Upper Miocene is calculated at 81°N, 118°E with a circle of confidence of 17°.

Palaeomagnetic Results from Palaeozoic Sediments of Northern Australia

Summary A palaeomagnetic study was made of 52 samples from an Upper Devonian formation and 97 samples from three Cambrian formations. In most cases, the application of demagnetization procedures failed to reveal other than a recent magnetization, presumably acquired by chemical means during weathering processes. Seventeen samples, four of which had been collected a decade earlier, appeared to yield the direction of original magnetization of the Middle Cambrian Hudson Formation. Assigning unit weight to sample mean results, the palaeomagnetic pole position for the period of deposition is calculated to be 19E, 18 N with A,, = 13. The data for the Jinduckin Formation have been re-assessed using sample results, and these give a slightly revised pole position for the Lower Ordovician at 25 E, 13 S with Ags = 11. The first results from a recent palaeomagnetic survey of the Palaeozoic of northern Australia indicated the widespread occurrence of a sometimes dominant secondary magnetization (Luck 1970; McElhinny & Luck 1970). The importance of this effect was revealed during measurements on 149 samples from four other sedimentary formations collected in the same general region (Table 1). The natural remanent magnetization (NRM) of most of these samples was diiected along the Earth's present field. A similarly directed secondary component, presumably representing a viscous remanent magnetization (VRM), had been readily removed from most samples of the Jinduckin Formation by the application of partial demagnetization techniques (Luck 1970). In contrast, the present paper reports that recently acquired magnetization could not be cleaned from the Blatchford Formation, the Hart Spring Sandstone, the Cockatoo Formation or from most samples of the Hudson Formation. This hard, secondary component was taken to represent a chemical remanent magnetization (CRM) imposed by weathering processes. In favourable circumstances, the effect might be neutralized by a chemical method such as devised by Collinson (1965) for separating the red pigment from the specularite of red beds. This technique may be regarded as a means of partial demagnetization (Collinson 1967) if the secondary magnetic component resides in the pigment. Disc-shaped (3.5 cm x 1.0 cm) and cylindrical (2.8 cm x 2-8 cm) specimens were cut from most samples. The discs were investigated using an astatic magnetometer

Palaeomagnetism of Permian Redbeds from the South-western United States

Samples were collected from Permian redbeds of Guadalupian to Ochoan age at 61 sites in the south-western U.S.A. Upon thermal demagnetization, a stable primary component of magnetization could be isolated in most of the samples. Because of the elimination of most of the secondary component of magnetization, the grouping of the virtual pole positions is better than previous results have suggested. There is no apparent chronological migration of the pole position through Permian time. Measurement has shown that although most of the Permian rocks are reversely magnetized, the rocks from four formations appear to have stable primary components approximately anti-parallel to the directions of magnetization of the reversed rocks. These rocks are of middle Leonardian, late Guadalupian, and middle and late Ochoan ages and it is suggested that they represent three, and possibly four, normal polarity events during the Permian.

The Sensitivity of the Conglomerate Test in Palaeomagnetism

Summary Samples of 25, 74, 100 and 200 directions have been drawn from a population of uniformly distributed directions. The directions were summed vectorially and the direction and length, R, of the resultant vector was calculated. Bias was applied to the random samples by adding a common vector to each random direction in varying proportions. This corresponds to the addition of a secondary component of magnetization of varying magnitude to randomly oriented primary magnetic directions, as might be present in the pebbles of a conglomerate. A comparison between the values of R calculated from the biased and unbiased samples permits estimates to be made of the sensitivity of conglomerate tests based on different sized samples in detecting secondary components of magnetization.

Paleomagnetism and correlation of some Middle Keweenawan rocks, Lake Superior

Paleomagnetic results from the Lake Superior region appear to indicate that Middle Keweenawan formations or magnetic divisions within formations can be correlated on the basis of magnetic polarity. The Logan sills, the volcanics of the Osler Group, the lava section at Alona Bay, the lower 6000 ft (1829 m) of North Shore volcanics, the lower 1000 ft (305 m) of Cape Gargantua volcanics and the lower 3000 ft (914 m) at Mamainse Point are of reverse polarity. Normal directions of magnetization, which differ from the reverse directions of magnetization by amounts ranging from 140° to 163° occur throughout the Michipicoten Island section, and in the upper parts of the North Shore, Cape Gargantua, and Mamainse Point sections. Provided part of the section at Mamainse Point has been repeated by faulting, these new results, together with data from the literature, are compatible with a single polarity reversal in Middle Keweenawan time separating older reverse polarity rocks from stratigraphically younger normal polarity rocks. The mean direction of magnetization from a conglomerate test is defined with higher precision than would be expected from a randomly distributed population. It is believed that this mean direction reflects a secondary component of magnetization which cannot be preferentially removed by alternating field demagnetization and which has caused the non anti-parallel alignment of normal and reverse populations of magnetization. An estimate of the pole position during Middle Keweenawan time can, however, be made by calculating a pole given equal weight to the mean normal and mean reverse pole. This is calculated to be 159° W, 42° N.

Device for chemical demagnetization of red beds

Chemical demagnetization of red sandstones by means of high pressure HCl is an effective way of removing secondary magnetic components acquired in the modern weathering environments.

The Intensity of the Ancient Geomagnetic Field: A Review and Analysis

Summary In this review all palaeomagnetic determinations of the intensity of the ancient geomagnetic field known to the author at 1966 June are brought together and analysed. The lists of data cover determinations from both historic (archaeomagnetic) and geological specimens. The problems encountered in the determination of ancient field intensities from rocks, especially that of rock alteration during the formation of artificial remanence and that of the presence of secondary components of magnetization, are discussed, and methods used to overcome them are described. Experimental techniques used hitherto are briefly summarized. Methods of treating data are reviewed; and suggestions for comparing data from different latitudes are made. The main conclusions drawn from an analysis of both the field intensity data and the problems involved in obtaining this data are: (i) that the problem of rock alteration during the heating necessary to induce an artificial TRM may be overcome by heating selected naturally highly-oxidized specimens in air; (ii) that the effects of secondary components of magnetization may be avoided by utilizing that part of the primary moment which remains after the removal of secondary moments by thermal or a.c. demagnetization; (iii) that the geomagnetic dipole is not constant within any given polarity but fluctuates in strength, possibly with a period of the order of lo4 years; (iv) that because of these fluctuations and non-dipole field variations no conclusions regarding the mean strength of the geomagnetic dipole at any given time may be made if only a few rock samples are measured, and hence that future workers should use sufficient samples to average out these effects; (v) that the limited data from transition zones suggest that during a field reversal the dipole moment reduces to zero but the non-dipole field remains.

The Determination of Paleo-Intensities of the Earth's Magnetic Field with Emphasis on Mechanisms which Could Cause Non-ideal Behavior in Thellier's Method

95 NRM-TRM curves were determined by Thellier's method from a variety of volcanic rocks. Most of them deviate from a straight line over parts of their length, sometimes so much that not even a crude estimate of paleo-intensity can be made. Some of the many possible causes of such non-ideal behavior include the effects of the sample demagnetizing field, secondary components of magnetization, mechanisms of acquisition of TRM which violate the assumptions of Thellier's method (such as nonlinearity of TRM with field), changes in the TRM spectrum induced by heating in the laboratory, and others. Where possible these mechanisms are discussed from both a theoretical and experimental standpoint, and their effects are identified in the NRM-TRM curves. In addition, diagnostic tests designed to determine quickly the suitability of a rock for intensity studies were sought. Tests tried included the comparison of heating and cooling Js-T curves, measurement of susceptibility before and after heating, and others. None were adequate. Finally, the quicker method of simply comparing the NRM and the total TRM is compared with Thellier's method for determining paleo-intensities. The latter is clearly the more informative and reliable when dealing with individual units, but the former may be useful for deriving average values of the paleo-intensity during geologic periods from large suites of volcanic rocks of varying types.

Stability of magnetization in cainozoic basalts from Victoria, Australia

Abstract Instability of magnetization in basalts is shown to be dependent on the composition of the magnetic mineral, which is a titanomagnetite. Low Curie temperature titanomagnetites tend to acquire secondary components of magnetization while high Curie temperature titanomagnetites of composition near to magnetite do not.

The Thermal Demagnetization of Natural Magnetic Moments in Rocks

Summary This paper describes a palaeomagnetic investigation of some laterites, containing hematite and maghemite, which have been baked by an overlying Tertiary lava in Northern Ireland. The laterite was magnetically unstable, but a technique of thermal demagnetization eliminated a secondary component of magnetization along the Earth’s present field. This technique established that the lava and baked laterite agreed in direction, and it further allowed a determination of the strength of the Earth’s field, which was apparently reversed at the time of baking, and of the temperature of baking. This determination of a geologically ancient field strength is particularly significant, since many such determinations from rocks of different ages might allow estimates of ancient latitudes for the various continents, independent of estimates derived from directions of magnetization.

Demagnetization of igneous rocks by alternating magnetic fields

Abstract The presence of secondary components of magnetization in basalts often limits the usefulness of their directions of natural remanent megnetization for palaeomagnetic work. Them secondary components may be removed by alternating magnetic fields. The procedures for doing this are described end tests based on the internal consistency of the results re devised to judge their reliability. Them are supplied successfully to a series of results from specimens collected from the Tertiary basalts of New South Wales. Australia. and a determination of the geomegnetic field in S.E. Australia during the Lower Tertiary is obtained. A study of the behaviour of these basalts in high alternating fields suggests that most of the iron mineral grains have a multi-domain character, and contain a hard end soft component in the ratio of about 1:10.

V. The remanent magnetization of unstable Keuper Marls

Measurements of the directions and intensities of magnetization of Keuper Marls from Sidmouth are described. The natural remanent magnetization of these rocks is shown to be unstable in the geomagnetic field. Certain laboratory experiments are described which show the natural remanent magnetization to consist of three components, a primary component created on, or soon after, deposition, in the same direction as that of the natural remanent magnetization of Keuper Sandstones and Marls described by Clegg, Almond & Stubbs (1954); a secondary component in the direction of a geocentric axial dipole field in Britain acquired since the last reversal of the main field and a temporary component built up by the geomagnetic field between collection and measurement. The temporary and secondary components are believed to be isothermal remanent magnetizations and to be due to the red haematite cement. Application of Néel’s theory of the magnetization of small single-domain particles shows that haematite grains of less than 0·15 μ in diameter will be magnetically unstable. The temporary and secondary components of magnetization are explained in terms of Néel’s theory. A suggested test of stability is described.