Search

In the early days of paleomagnetism, David Strangway was interested in understanding why igneous rocks are faithful recorders of the Earth's magnetic field. He recognized that ferromagnetic (s.1.) grains that could be discerned by optical microscopy were too large to carry a stable remanent magnetization, and speculated whether fine-grained, ferromagnetic (s.1.) inclusions or exsolutions in silicate minerals are responsible. When these inclusions or exsolutions are randomly oriented, or the silicate hosts are randomly oriented in a rock, they can be a good recorder of the field. If these minerals, however, show an alignment within the silicate host, and the host is preferentially aligned due to flow structures or deformation, then the paleomagnetic direction and paleointensity could be biased. We examine the magnetic anisotropy arising from the ferromagnetic (s.1.) phases in silicate-host minerals. Single crystals of phyllosilicate, clinopyroxene, and calcite show most consistent ferrimagnetic fabric with relation to the minerals' crystallographic axes, whereas olivine and feldspar display only a weak relationship. No discernable relationship is found between the ferrimagnetic anisotropy and crystallographic axes for amphibole minerals. Our results have implications when single crystals are being used for either studies of field direction or paleointensity or in cases where silicate minerals have a preferential orientation. Phyllosilicate minerals and pyroxene should be screened for significant magnetic anisotropy. Aux debuts de la science du paleomagnetisme, David Strangway voulait comprendre pourquoi les roches ignees conservent un portrait fidele du champ magnetique de la terre. Ainsi, il note que les grains ferromagnetiques (sensu lato) qui pouvaient etre observes par microscopie optique etaient trop grands pour porter une magnetisation remanente stable et se demande si des inclusions ferromagnetiques (sensu lato) ou exsolutions dans des silicates en etaient plutot la cause. Quand l'orientation de ces inclusions ou exsolutions est aleatoire ou que les silicates hotes sont orientes aleatoirement dans la roche, ils peuvent constituer un bon registre du champ. Toutefois, si ces mineraux presentent un alignement dans le silicate hote et que l'hote montre un alignement preferentiel en raison de structures d'ecoulement ou de la deformation, la direction paleomagnetique et la paleointensite peuvent s'en trouver biaisees. Nous examinons l'anisotropie magnetique decoulant des phases ferromagnetiques (sensu lato) presentes dans des silicates hotes. Les monocristaux de phyllosilicate, de clinopyroxene et de calcite presentent les fabriques ferrimagnetiques qui concordent le mieux avec les axes cristallographiques des mineraux, alors que pour l'olivine et le feldspath, cette relation est faible. Aucune relation n'est observee entre l'anisotropie ferrimagnetique et les axes cristallographiques pour les amphiboles. Nos resultats sont importants pour l'utilisation de monocristaux pour des etudes soit de la direction ou de la paleointensite du champ ou dans des cas ou les silicates presentent une orientation preferentielle. Les phyllosilicates et les pyroxenes devraient faire l'objet d'un examen prealable pour determiner s'ils presentent une anisotropie magnetique significative. [Traduit par la Redaction], Introduction Some of the early research that David Strangway conducted was focused on understanding why volcanic rocks are good recorders of the Earth's magnetic field (Larson et al. 1969; Strangway [...]

Lacustrine and marine sediments are one of the main sources of information in constructing Holocene global geomagnetic field models. The use of sediment records, however, leads to the question whether the compaction of sediments leads to a systematic biasing of inclination. We evaluate 78 sedimentary records worldwide for inclination flattening using the Elongation–Inclination (E/I) method; 20 records indicate flattening. The uncorrected and corrected values for inclination are compared to global geomagnetic field models. The results suggest that the uncorrected values agree better with the predictions from global geomagnetic field models based on sediment and archaeomagnetic data, but also with a model independent of sediment data. The 20 sites are located in mid-latitudes where inclination anomalies are predicted both in the Holocene and throughout the Brunhes epoch. Our results demonstrate that shallow inclination may not only result from compaction but may reflect the structure of the geomagnetic field on short timescales. This suggests that secular variation is not averaged out over a time period that covers the Holocene. [ABSTRACT FROM AUTHOR]

We calculate the torque and force generated by an arbitrary magnetic field on an axially symmetric soft-magnetic body. We consider the magnetization of the body as a function of the applied field, using a continuous model that unifies two disparate magnetic models. The continuous torque and force follow. The model is verified experimentally, and captures the often neglected region between weak and saturating fields, where interesting behavior is observed. We provide the field direction to maximize torque for a given field magnitude. We also find an absolute maximum torque, for a given body geometry and material, which can be generated with relatively weak applied fields. This paper is aimed at those interested in systems-level analysis, simulation, and real-time control of soft-magnetic bodies. Index Terms--Ellipsoid, magnetic actuation, shape anisotropy, uniaxial symmetry, wireless microrobot.

The relationship between magnetic properties and chemical composition of 19 natural carbonate single crystals of the calcite, aragonite, and dolomite groups and of azurite were investigated. Magnetic susceptibility was determined in low fields, and magnetic anisotropy was measured at room temperature and at 77 K using a high-field torque magnetometer. The chemical composition was analyzed using LA-ICP MS. Planar arrangement of the C[O.sub.3]-groups generates an oblate diamagnetic anisotropy with the minimum susceptibility [k.sub.3] along the crystallographic c-axis in all investigated anhydrous carbonate minerals. A prolate paramagnetic anisotropy with the maximum susceptibility [k.sub.1] along the c-axis is produced by [Fe.sup.2+] in the trigonal carbonate lattice, which can lead to a transition from oblate to prolate shape of the total anisotropy. The transition occurs in the calcite structure above an Fe concentration of 400 ppm at room temperature and 150 ppm at 77 K. The susceptibility difference [k.sub.1]-[k.sub.3] for pure calcite is 4.06 x [10.sup.-10] [m.sup.3]/kg and increases to 9.4 x [10.sup.-9] [m.sup.3]/kg for 10800 ppm Fe. In the hexagonal aragonite structure, no paramagnetic anisotropy due to [Fe.sup.2+] was detected. Azurite shows a strong anisotropy with [k.sub.1]-[k.sub.3] = 1.5 x [10.sup.-8] [m.sup.3]/kg, which is assigned to [Cu.sup.2+]. The paramagnetic anisotropy due to [Fe.sup.2+] increases at 77 K in minerals of the calcite and dolomite group and in azurite, but not in minerals of the aragonite group. Upon cooling from room temperature to 77 K, [k.sub.1]-[k.sub.3] increases 13.3 times for 500 ppm to 100 000 ppm [Fe.sup.2+] in the trigonal lattice and 7.2 times for siderite. Keywords: Carbonate minerals, magnetic properties, anisotropy, calcite, siderite, rhodochrosite, LA ICP-MS

Comparison of the paleomagnetic pole positions of late Paleozoic secondary magnetizations with a time-averaged reference apparent polar wander path for North America shows that sedimentary rocks within the central Appalachians were remagnetized in the later half of the Permian, between ca. 255 and 275 Ma. Ages of remagnetization do not vary for sites distributed along or across strike of the central Appalachian fold and thrust belt or across the Appalachian Plateau in New York, nor do they vary with stratigraphic position of the host rocks. However, the pattern of folding relative to remagnetization across the Valley and Ridge and Great Valley in Pennsylvania, Maryland, and West Virginia shows a distinct temporal variation. Individual fold tests yield postfolding magnetizations near the hinterland margin of the fold and thrust belt, synfolding magnetizations in the central part of the belt, and prefolding magnetizations near the foreland. Because of the similar age of the late Paleozoic magnetization, we conclude that rocks across the central Appalachians were remagnetized in a relatively short period of time as folding and thrusting propagated more slowly toward the foreland. We interpret this pattern to indicate that older folds near the hinterland grew prior to remagnetization, folds in the central region grew during remagnetization, and folds near the foreland grew after remagnetization. Rapid remagnetization relative to folding is also evident from the physical characteristics and magnetic properties of secondary hematite in remagnetized (synfolding) folds. In such folds, rocks were apparently remagnetized during an increment of fold growth, in which there was partial and possibly negligible fold-limb rotation. Collectively, these results may constrain the mechanism for basinal fluid mobilization during deformation. Current models for remagnetization involve the migration of chemically active brines into the foreland during tectonism. If realistic, our results bear on this model by suggesting that brine migration was not directly related to the successive emplacement of individual thrust sheets, as has been suggested for other fold and thrust belts.