Origin Of Natural Remanent Magnetization Research Articles

Remagnetization of magnetite-bearing rocks in the Eastern Qiangtang Terrane, Tibetan Plateau (China): Mechanism and diagnosis

Remagnetization is a common yet notorious phenomenon that interferes with paleogeographic reconstruction. Classical paleomagnetic field tests are helpful in detecting remagnetization but their diagnostic power is limited: remagnetization may occur before folding, the tilting age may be ambiguous, or protracted remagnetization may yield dual polarities. Rock magnetic information can provide other constraints on our understanding of the origin of natural remanent magnetization (NRM). Here we focus on the rock magnetic properties of acknowledged remagnetized limestones and unremagnetized rocks of the Zaduo area in the Eastern Qiangtang Terrane, Tibetan Plateau (China). Chemical remanent magnetization is suggested as a more frequent mechanism than the thermoviscous resetting of the NRM. The secondary NRM resides in authigenic magnetite of stable single domain and superparamagnetic (SP) size which grew during post-depositional burial processes. Both high-field and low-field thermomagnetic runs reveal the alteration of existing iron sulfides to magnetite in the remagnetized limestones. NRM decay curves show that the maximum unblocking temperature of the remagnetized samples is significantly lower than that of the unremagnetized samples. Component analysis of acquisition curves of the isothermal remanent magnetization (IRM) reveals a hard component that represents SP magnetite in remagnetized limestones. This component is absent in unremagnetized rocks. End-member modelling reveals a convex curve in the coefficient of determination versus the number of end-members plot for the unremagnetized limestones, whereas the remagnetized rocks exhibit both near-linear and convex shapes. In addition, quantitative analysis of the hysteresis loop shape for different lithologies indicates its validity in detecting remagnetization. Furthermore, we show the differences in the hysteresis data distributions of the two rock types on the Day plot, the Néel diagram, the Borradaile diagram, and the Fabian diagram. Our research emphasizes that rock magnetic properties can serve as tools to diagnose remagnetization in magnetite-dominated rocks. We recommend a comprehensive rock magnetic study to discriminate remagnetization, involving the Day plot, Fabian diagram, thermal demagnetization curves, IRM component analysis and end member modelling.

A review of the studies results of paleomagnetic applicability of Precambrian crystalline rocks of the Ukrainian Shield.

The review of the data has been conducted of the long-term laboratory methods of studies of stability and origin of natural remanent magnetization (NRM) of Precambrian crystalline rocks of the Ukrainian Shield (USh). Generalization of the data gives weighty arguments for considering the Precambrian rocks of the Ukrainian Shield as promising objects for paleomagnetic studies. Statistical analysis has been made as to directions of NRM in exposures different by their age, composition and origin of crystalline rocks of the USh. It has been found that everywhere within the limits of the Ukrainian Shield in the exposures with magnetically stabile single-component magnetization of thermoremanent origin (according to laboratory and mineralogical criteria) NRM directions in contemporary coordinate system are distributed heterogeneously and are characterized by considerable dispersion. The most probable source of dispersion of NRM vectors within the limits of considered exposures of crystalline rocks of the USh is local tectonics, which produced reorientation within the area of the surface of initial magnetization of rocks.

Measuring remanence anisotropy of hematite in red beds: anisotropy of high-field isothermal remanence magnetization (hf-AIR)

SUMMARY The potential of using high-field anisotropy of isothermal remanence magnetization (hf-AIR) measurements for determining the origin of natural remanent magnetization in red beds and for identifying and correcting possible red-bed inclination shallowing was investigated for specimens of the Carboniferous Shepody Formation of New Brunswick and Nova Scotia, Canada. The technique makes it possible for a typical paleomagnetic laboratory to measure the remanence anisotropy of high-coercivity hematite. High-field (hf) AIR was used in conjunction with 100 mT alternating field (af) and 120 °C thermal demagnetization to separate the contribution of hematite to the remanence anisotropy from that of magnetite/maghemite and goethite, respectively. A 5-T impulse DC magnetic field was used for the hf-AIR to reset the magnetic moment of high-coercivity hematite so that demagnetization between AIR orientations was not necessary. The ability of a 5-T field to reset the magnetization was tested by generating an isothermal remanent magnetization acquisition curve for hematite by using impulse DC magnetic fields up to 5 T in one orientation and followed by applying a field in the opposite direction at each step. Each field application was treated by 120 °C heating and 100 mT af demagnetization before measurement. At 5 T, the difference between the magnetizations applied in opposite directions disappeared indicating that no magnetic memory persisted at this field strength. We performed a validity and reproducibility test of our hf-AIR measurement technique by measuring three specimens multiple times along two orthogonal coordinate systems. The method yielded highly reproducible results and, on rotating the specimen's coordinates, the fabric rotated by 90° as expected, showing that it is not an artifact of the technique. We also measured hf-AIR on samples that had previously been chemically demagnetized in 3N HCl to remove the secondary, chemically grown pigmentary hematite. The hf-AIR fabric of leached samples is similar to that of untreated samples, but shows a better-defined magnetic lineation and imbrication. We interpret the fabric observed for the Shepody Formation to be a compactional fabric that has been reoriented by strain during folding following a flexural-slip model.

Rock-magnetism and ore microscopy of the magnetite-apatite ore deposit from Cerro de Mercado, Mexico

Rock-magnetic and microscopic studies of the iron ores and associated igneous rocks in the Cerro de Mercado, Mexico, were carried out to determine the magnetic mineralogy and origin of natural remanent magnetization (NRM), related to the thermo-chemical processes due to hydrothermalism. Chemical remanent magnetization (CRM) seems to be present in most of investigated ore and wall rock samples, replacing completely or partially an original thermoremanent magnetization (TRM). Magnetite (or Ti-poor titanomagnetite) and hematite are commonly found in the ores. Although hematite may carry a stable CRM, no secondary components are detected above 580°, which probably attests that oxidation occurred soon enough after the extrusion and cooling of the ore-bearing magma. NRM polarities for most of the studied units are reverse. There is some scatter in the cleaned remanence directions of the ores, which may result from physical movement of the ores during faulting or mining, or from perturbation of the ambient field during remanence acquisition by inhomogeneous internal fields within these strongly magnetic ore deposits. The microscopy study under reflected light shows that the magnetic carriers are mainly titanomagnetite, with significant amounts of ilmenite-hematite minerals, and goethite-limonite resulting from alteration processes. Magmatic titanomagnetites, which are found in igneous rocks, show trellis, sandwich, and composite textures, which are compatible with high temperature (deuteric) oxy-exsolution processes. Hydrothermal alteration in ore deposits is mainly indicated by martitization in oxide minerals. Grain sizes range from a few microns to >100 μm, and possible magnetic state from single to multidomain, in agreement with hysteresis measurements. Thermal spectra, continuous susceptibility measurements, and IRM (isothermal remanent magnetization) acquisition suggest a predominance of spinels as magnetic carriers, most probably titanomagnetites with low-Ti content. For quantitative modeling of the aeromagnetic anomalies, we used data on bulk susceptibility and natural remanent intensity for quantifying the relative contributions of induced and remanent magnetization components and allow a better control of the geometry of source bodies. The position and geometry of this magnetic source are shown as an ENE-striking tabular body, steeply inclined (75°) to the south.

Origin of natural remanent magnetization of tephra from the 1979 Surtsey drill hole, Iceland

Origin of natural remanent magnetization of tephra from the 1979 Surtsey drill hole, Iceland

Magnetic properties and remanent magnetization of stony meteorites.

Hysteresis parameters and remanence properties of 22 stony meteorites were measured. Coercive force varies systematically in the sequence E→H→L→LL→C chondrites. The change of natural remanent magnetization (NRM) stability follows the same sequence. Intensity of NRM is proportional to ferromagnetic mineral contents in meteorites. The ratio NRM/saturation magnetization is unrelated to the chemical class. Stability of NRM against alternating field demagnetization was compared to anhysteretic remanence (ARM) induced in a low field. The NRM in four meteorites is as stable as the ARM, and chemical remanence (CRM) or thermoremanence (TRM) origin of the NRM is inferred. NRM in eight meteorites is less stable than ARM. Stability and/or intensity of NRM was compared with that of isothermal remanence (IRM) and viscous remanence (VRM) and partial thermoremanence (pTRM). Artificial IRM and terrestrial VRM origin for NRM is ruled out; pTRM origin (acquired in the temperature range 20<T<200-300°C and in a field 0.5≤h≤2Oe) of NRM in some meteorites seems likely, since such pTRM exhibits almost identical behavior with NRM.

Contributions of rock magnetism and paleomagnetism to recent geophysical advances

The origin of natural remanent magnetization (NRM) in rocks is discussed both in terms of types and carriers of NRM. The importance of the concept of pseudo-single domain (PSD) grains as carriers of stable remanences is underscored. Recent advances in rock magnetism and paleomagnetism have helped to understand (a) continental motions which took place in the first 4 billion years of the earth's life, (b) fine details of field fluctuations both during "normal" times as well as during a geomagnetic field reversal, and (c) indicate the magnitudes of the fields present during the formation of the moon and of the early solar system.