Components Of Natural Remanent Magnetization Research Articles

Linear and planar structure in ordered multivariate data as applied to progressive demagnetization of palaeomagnetic remanence

Summary. Consider a sequence of, say, 10 to 20 vector observations in threedimensional space. It is suspected that a few subsets of consecutive observations are made up of collinear points. The purpose of this paper is to construct a statistically based algorithm to find such linear segments and to assess their accuracy. A similar assessment is made for coplanar sets of points. This algorithm is applied here to palaeomagnetic data and is claimed to be superior to previous methods of palaeomagnetic analysis in terms of completeness and balance of analysis, treatment of measurement errors and other sources of scatter, criteria for identification of linear and planar sets of points, and statistical rigour. Stability spectra, with statistically based confidence limits, are obtained as a by-product. 1 Palaeomagnetic analysis: background to the geometrical problem The natural remanent magnetization (NRM) in rocks is known in general to have originated in one or more relatively short time intervals during their formation and subsequent history, i.e. during rock-forming or metamorphic events during which NRM is frozen-in (stabilized) by falling temperature, grain growth etc. The NRM acquired during each such event is a single vector magnetization parallel to the then-prevailing geomagnetic field and is called a -component of NRM’. Rocks which have suffered more than one event of this kind are therefore liable to carry ‘multi-component NRM’. In the laboratory these components can be identified by demagnetizing the rock step-by-step, usually by thermal, alternating field or chemical demagnetization (which involves the analysis of the distribution of blocking rzmperature, coercivity and chemical leaching respectively). Resistance to these treatments is called ‘stability’ of remanence, and the range of treatment over which a particular component is removed is called the stability range of the component. The various discrete

High magnetic coercivity of meteorites containing the ordered FeNi (Tetrataenite) as the major ferromagnetic constituent

Three chondrites, Yamato‐74160 (LL6), ALH‐77260 (L3), and St. Séverin (LL6), contain a considerable amount of the ordered FeNi, tetrataenite, in their metallic components. Natural remanent magnetization (NRM) of the three chondrites contains a highly stable component which can hardly be demagnetized by alternating magnetic fields up to 1800 Oe peak. The highly stable NRM component is identified to the NRM of the tetrataenite phase which has a large magnetic and optical anisotropy. By heating up to about 800°C, however, the tetrataenite phase is broken down to the ordinary disordered taenite which has a much weaker magnetic coercivity.

Magnetic reversal history of Pleistocene sediments at Old Crow, northwestern Yukon Territory

The banks of the Porcupine River near Old Crow in the unglaciated part of the northern Yukon consist of about 55 m of interbedded fluvial and lacustrine gravels, sands, silts, and clays. This sequence was sampled closely for palaeomagnetic analysis and the sediments were found to possess a natural remanent magnetization (NRM) that is stable in direction with mean destructive fields of 200–300 Oe (16–24 kA/m). Therefore, good palaeomagnetic data can be obtained from sediments that have been subjected to permafrost conditions, although some samples show disturbance of NRM directions by cryoturbation.Magnetic polarity transitions are centred at 17 and 26 m above river level and the 15 samples between them define an interval of reversed polarity of considerable duration. The behaviour of NRM during the transition periods is complex with declination switching abruptly by 180° but with the inclination displaying very erratic variation over a considerable time. This may be caused by mixing in single samples of components of NRM with ages that straddle the transition—a situation that would be possible if NRM were post-depositional in origin and so could be locked in at times long after sedimentation.Sedimentological, palaeobotanical, and palaeomagnetic data all support the presence of a major unconformity in the middle portion of the sedimentary sequence. This unconformity lies at the top of the transition that closes the reversed episode so that the sediments below it are likely older than 700 000 years.These results demonstrate that palaeomagnetic studies on the sedimentary sequences exposed in the many isolated sites within the Old Crow region should greatly facilitate their correlation and indirect dating, effecting thereby an improved understanding of the recent geological history of this anthropologically important area.

Palaeointensities from sediments: normalization by laboratory redepositions

Deep-sea sediments, comprising small magnetic grains in coarse and fine fabrics, were reconstituted and deposited in laboratory fields. Both the magnitude and the direction of the natural remanent magnetization (NRM) were accurately reproduced. Only the middle coercivity fraction, however, gave a faithful representation of the laboratory field. This same fraction originally held the stable NRM component. The results were interpreted on a model of post-depositional realignment based on the physical characteristics of the sediment. Laboratory redeposition was found to be a closer analogue to the NRM than were anhysteretic or other laboratory-induced magnetizations. Guidelines are given by which the techniques could be utilised to estimate the intensity of the ancient geomagnetic field.

Proterozoic magnetic overprinting of Archaean rocks in the Canadian Superior Province

We have measured the various components of natural remanent magnetization (NRM) exhibited by mafic extrusive and intrusive rocks from the Abitibi Subprovince of the Canadian Precambrian Shield. In addition to providing what seems to be a primary Archaean pole for the region, the results testify to three episodes of magnetic ‘overprinting’ at approximately 2,600, 2,100 and 1,800 Myr, respectively. The two earlier episodes coincide with the intrusion of Matachewan and Abitibi diabase dyke swarms, here and elsewhere in the Superior Province. Magnetic overprinting, in these cases, extends far beyond areas of surficial dyke exposure. The third episode postdates magmatic activity in the Abitibi region, but coincides in age with the 1,900–1,700 Myr Hudsonian orogeny in the Churchill and Bear Provinces and the Penokean orogeny in the Southern Province. Although magnetic overprints presumed to be of this age were known previously from Abitibi dykes1 and Nipissing sills2–4, our data are the first evidence of a 1,800 Myr magnetic overprinting on a regional scale in the Superior Province.

Monodomain behaviour in multiphase oxidized titanomagnetite

Large grains, of the order of tens of microns in size, of synthetic titanomagnetite have been oxidized to produce an intergrowth of phases, essentially similar to that produced by the natural process of deuteric oxidation. The scale of the intergrowth is at the limit of the optical range but the electron microscope reveals the characteristic lamella microstructure. The magnetic hysteresis properties of the most highly oxidized material, having probably about 10% of residual spinel phase, are typical of dispersed monodomain magnetite rods. The investigation therefore supports the model in which it is proposed that titanomagnetite grains in a slowly cooled basalt may carry a highly stable component of natural remanent magnetization.

Thermal overprinting of natural remanent magnetization and K/Ar ages in metamorphic rocks.

Burial metamorphism generates secondary components of NRM (natural remanent magnetization). Often two or more remanences are found superimposed in a single geological unit. We trace the steps necessary to resolve and date such magnetic overprints in three studies of intensely metamorphosed rocks from the Grenville Province of the Canadian Precambrian Shield. First, it is shown that the various components may be associated with different rock-forming minerals and can be separated by orienting aggregates of dark or light grains in coarse-grained samples. Next, these magnetizations are shown to have originated at least in part as TRM’s (thermoremanent magnetizations). Their relative ages can then be assigned on the basis of blocking temperatures. Finally, we estimate their absolute ages using the 40Ar/39Ar stepwise heating technique, taking into account the effects of a long heating-cooling history on both the magnetic and radiometric systems.

Natural remanent magnetization studies of a layered Breccia Boulder from the lunar highland region

The average directions of natural remanent magnetization (NRM) of three texturally distinct layers (72215, 72255, and 72275) of a 2 m-sized breccia boulder were found to be the same, while the directions of their stable components of NRM were found to be widely divergent. One clast from 72275 yielded a stable NRM direction which was different from that of the matrix. Approximate paleointensity measurements showed that 72255 and 72275 could have obtained their stable remanence from an ancient magnetic field of the same magnitude. However, 72215 probably was magnetized by a magnetic field of a different intensity. We concluded that the coincident NRM directions owe their origin to a secondary imprint of less stable magnetization imparted during the assembly of the boulder at moderate temperatures (∼ 450°C) on the South Massif. The stable directions, on the other hand, date from the last, higher-temperature (∼ 770°C) magnetizing event experienced by the mineral and lithic components while they were part of the immature pre-Serenitatis regolith.

Paleomagnetic systematics of ordinary chondrites

A pilot systematic study of the remanent magnetic behavior of ordinary chondrites reveals several correlations with their chemical-petrological classification. The intensity of natural remanent magnetization (NRM) decreases in the sequence E→H→L→LL-chondrites. Its relative stability and degree of directional coherence under progressive AF demagnetization increase in the same order. These trends parallel a decrease in average metal content (from ∼20% in E to 2% in LL) and a corresponding Ni enrichment of metal grains (from ∼6%Ni in E to∼35% in LL). Within each chemical group, the stability of NRM is inversely related to petrologic subtype. Shocked meteorites display a distinctive demagnetization curve, an unusually large soft component of NRM, and pronounced directional scatter of NRM components. These characteristics can be reproduced reversibly by brief laboratory heating (to ∼800°C) of unshocked chondrites. Comparative studies of laboratory (saturation isothermal, IRM s, and thermal, TRM) remanence acquisition and behavior, suggest that other magnetic parameters, such as the ratios NRM/TRM and IRM s/NRM, may be useful for classification. Patterns of magnetic behavior are explained in terms of metallography and thermal history. In contrast to the carbonaceous chondrites, very few ordinary chondrites preserve a useful paleomagnetic record. The LL-chondrites hold the best promise in this regard. Paleofield estimates obtained for eight representative chondrites range from 0.01 to 0.3 Oe.

Microanalysis of N.R.M. in a granodiorite intrusion

The natural remanent magnetization (NRM) associated with individual oriented grains of the major rock-forming minerals of the Tatoosh granodiorite has been determined to establish the magnetic carriers of stable and unstable components of NRM. In addition, magnetic parameters such as weak field susceptibility, Königsberger ratio, saturation isothermal remanent magnetization and remanent coercivity have been used to characterize the various carriers. The NRM associated with the different rock-forming minerals may be conveniently classified into two types. Type A NRM is carried by fine particles of magnetite, which occur as inclusions in feldspar, especially in plagioclase. It is magnetically hard and has a narrow range of blocking temperatures. Type B NRM is carried by large grains of magnetite associated mostly with biotite and partly with hornblende. It is magnetically soft and contains thermally unstable or viscous components. It appears that in this intrusion the paleomagnetic signal is carried by the fine particles of magnetite, while the coarse particles record poorly and their remanence contributes noise.

Iron Oxides in Red Beds

This study focuses on: (1) grains of black oxides, (2) pigment in matrix and cement, and (3) natural remanent magnetization in red beds. Black oxides in 13 samples of soils, 26 samples of alluvium in transit and Recent nonmarine deposits, 103 red and drab units in red bed sequences ranging in age from Precambrian to Pleistocene, and in 9 ancient nonmarine drab sequences, were identified by X-ray analysis, supplemented by polished-surface microscopy. These data and pertinent published information support the following conclusions: (1) Magnetite and ilmenite and their various intergrowths predominate in igneous and metamorphic source rocks, and they are delivered relatively unaltered to nonmarine and paralic environments. In oxidizing areas, inherited grains of oxide begin to alter to black hematite and leucoxene. Oxidation continues long after burial, so that successively older red beds contain relatively more hematite. In reducing environments, inherited oxide grains begin to dissolve. Loss of more easily destroyed magnetite increases the relative amounts of ilmenite and black hematite. The latter apparently developed en route or in a preceding oxidizing episode. (2) Muddy alluvium commonly contains brown amorphous and hydrated ferric oxide pigment which begins to age and also dehydrated to red hematite in oxidizing environments. Sandy alluvium with little matrix, after having accumulated in desert basins, acquires hematite pigment largely by postdepositional in situ alteration of iron-bearing silicates. (3) Most detrital red beds possess chemical remanent magnetization (CRM) which was imposed on some of the black grains of hematite after their postdepositional oxidation. Any detrital remanent magnetization (DRM) initially lodged in inherited black oxide grains is destroyed by small-scale organic and inorganic disturbance, as well as by the intrastratal hematitization. Red pigment in matrix and cement may also acquire CRM as the developing hematite grows through critical crystal sizes; in hematite-cemented quartz arenites this is the principal component of natural remanent magnetization (NRM).

Thermal Demagnetization of Some Precambrian Igneous Rocks of Missouri

The results of a deliberate search for stable secondary magnetization in Precambrian igneous rocks of St. Francois Mountains, Southeast Missouri, U. S. A., are described. These rocks, at least one billion years old, possess stable components of natural remanent magnetization, which are not always of thermoremanent origin. The results of thermal demagnetization and of some subsidiary experiments show that rhyolite porphyries of Flint Hill have been remagnetized by strong fields due to lightning and are of little use for paleomagnetic investigations. Similar experiments conducted on felsites and bedded tuffs of Johnson Shut-Ins area reveal the presence of stable and significant magnetization of a direction similar to that found by Scharon, Hayes, and Anderson for the whole of St. Francois Mountains. The granite porphyry from the same area carries a stable component of natural remanent magnetization, which is of secondary origin, but is of little significance for the paleomagnetic studies of the whole system of rock units.It is concluded that the rock units investigated carry no stable secondary magnetization which masks their original magnetization and leads to wrong conclusions about the direction of Precambrian geomagnetic field. From this it is inferred that the natural remanent magnetization of all other rock units of St. Francois Mountains is unlikely to be masked by stable secondary components.

Paleomagnetism of some Mesozoic intrusives and tuffs from eastern Australia

Paleomagnetic studies of the Brisbane tuff and the Noosa Heads and Mount Dromedary intrusive complexes show that the stable components of natural remanent magnetization have rather uniform directions (irrespective of sign) with steep inclinations. In each case the time of formation of the rocks is known with precision, either from stratigraphic or from radioisotope evidence, to be Middle Triassic, Upper Jurassic, and late Lower Cretaceous, respectively. Field and laboratory studies suggest that the stable magnetization originated at the time of formation and was parallel to the earth's field. The observations therefore appear to provide a record of the earth's field at known times in the past. A reversal of magnetization in the Noosa Heads complex is related to a radioisotope age of 140 m.y. A detailed comparison of thermal and magnetic cleaning methods, in which rock samples from the Mount Dromedary intrusive complex were used, yields the result that the latter is to be preferred in this instance.