V. The remanent magnetization of unstable Keuper Marls

1957 ◽  
Vol 250 (974) ◽  
pp. 130-143 ◽  
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Laboratory Experiments ◽  
Natural Remanent Magnetization ◽  
Single Domain ◽  
Dipole Field ◽  
Main Field ◽  
Axial Dipole ◽  
Geocentric Axial Dipole ◽  
Three Components

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.

Evidence for geomagnetic field excursions and secular variation from the Wrangell Volcanics of Alaska

1976 ◽  
Vol 13 (4) ◽  
pp. 547-554 ◽  
Author(s):  
D. K. Bingham ◽  
D. B. Stone
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Lava Flows ◽  
Dipole Field ◽  
Common Occurrence ◽  
Late Tertiary ◽  
Field Activity ◽  
Field Excursion ◽  
Geocentric Axial Dipole ◽  
Final Flow

Paleomagnetic studies have been made on 36 late Tertiary lava flows (3–4 m.y.) from the Wrangell Volcanics. Final flow mean remanent magnetization directions show excursions of the geomagnetic field away from a mean corresponding to a geocentric axial dipole field. They also point to the possibility that such excursions may have been a more common occurrence at the time of extrusion of these lavas than appears to have been the case in Quaternary times. These excursions may be due to increased non-dipole field activity. Calculation of the paleosecular variation including field excursion data leads to high values of PSV which do not agree with existing models. Exclusion of field excursion data gives a result that is consistent with current PSV models, but does not allow differentiation between them.

Evidence of Single-Domain Magnetite in the Michikamau Anorthosite

1971 ◽  
Vol 8 (3) ◽  
pp. 361-370 ◽  
Author(s):  
G. S. Murthy ◽  
M. E. Evans ◽  
D. I. Gough
Keyword(s):  
Remanent Magnetization ◽  
Theoretical Models ◽  
Natural Remanent Magnetization ◽  
Single Domain ◽  
Isothermal Remanent Magnetization ◽  
Mineral Separation ◽  
Domain Configuration ◽  
Plagioclase Felspar ◽  
Magnetite Particles ◽  
Theoretical Evidence

The Michikamau anorthosite possesses very stable natural remanent magnetization, some of which resists alternating fields up to 1800 Oe. The rock contains two types of opaque grains, fine opaque needles of order 10 × 0.5 μ in the plagioclase felspar, and large equidimensional magnetite particles. Ore microscope studies suggest, but do not establish, that the needles are composed of magnetite. Saturation isothermal remanence and thermal demagnetization studies indicate magnetite as the carrier of remanent magnetization. In order to distinguish the effects of the large grains from those of the needles, mineral separation was used to show that an artificial specimen of essentially pure plagioclase had very similar isothermal remanent magnetization properties to the whole rock. Both indicated magnetite as the magnetic mineral. Thermoremanent properties of the separated mineral fractions indicated magnetite as the dominant magnetic constituent but showed some evidence of laboratory-produced hematite. Theoretical models of grains elongated along [111] and [110] axes are used to show that magnetite needles can exist in stable single-domain configuration in the size and shape ranges of the needles observed in the Michikamau anorthosite. There is thus considerable experimental and theoretical evidence for the conclusion that the stable remanent magnetization of the Michikamau anorthosite is carried by fine single–domain needles of magnetite in the plagioclase felspar.

The Laschamp excursion

1982 ◽  
Vol 306 (1492) ◽  
pp. 169-177 ◽  
Keyword(s):  
Geomagnetic Field ◽  
Laboratory Experiments ◽  
Northern China ◽  
Natural Remanent Magnetization ◽  
Field Reversal ◽  
Magnetostatic Coupling ◽  
Reversal Mechanism ◽  
Laschamp Excursion ◽  
Optical Examination ◽  
Field Excursion

The reversely magnetized lavas at Laschamp and Olby (Chaine des Puys, Auvergne, France) are commonly believed to document the most recent geomagnetic field excursion in the present Brunhes normal epoch. However, complete or partial self-reversal of natural remanent magnetization (n.r.m.) is observed in many Olby samples and to a lesser extent in the Laschamp rocks during thermal laboratory experiments. Magnetic and optical examination suggests that the self-reversal mechanism is caused by a negative magnetostatic coupling between titanomagnetite phases of widely varying degrees of oxidation. To check independently whether field reversal or self-reversal caused the reversed n.r.m. directions, a contemporaneous loess section at Steinheim (southern Germany) has been studied magnetostratigraphically. No reversed polarities have been detected in this section or in other loess profiles from Czechoslovakia and northern China.

A Palaeomagnetic Investigation of the Lundy Dyke Swarm

1957 ◽  
Vol 94 (3) ◽  
pp. 187-193 ◽  
Author(s):  
D. J. Blundell
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Natural Remanent Magnetization ◽  
Lava Flows ◽  
Dyke Swarm ◽  
Upper Oligocene

AbstractThe directions of the natural remanent magnetization of samples collected from dykes on Lundy have been measured and related to those of Tertiary lava flows in Northern Britain. Evidence is given suggesting that the dykes are Tertiary and pre-Upper Oligocene in age, and that the geomagnetic field was reversed at the time of their intrusion.

IV. The natural remanent magnetization of certain stable rocks from Great Britain

1957 ◽  
Vol 250 (974) ◽  
pp. 111-129 ◽  
Keyword(s):  
Great Britain ◽  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Preceding Paper ◽  
Natural Remanent Magnetization ◽  
Red Sandstone ◽  
North West

Certain Permian lavas, Devonian, Cambrian and Pre-Cambrian sediments are shown to be permanently magnetized in directions different from that of the present geomagnetic field. All the Palaeozoic rocks examined possess southward natural remanent magnetizations. Whilst it is not suggested that the geomagnetic field did not reverse during the whole of the Palaeozoic, it is believed that the collection from the Lower and Upper Old Red Sandstone is sufficiently representative to make reversals during these times most improbable. The Wentnor Series of Pre-Cambrian rocks of the Longmynd shows reversal of magnetization about an axis which is not significantly different from that determined for the Torridonian series of north-west Scotland in a preceding paper. This is taken to support the view that they are roughly contemporaneous.

Geophysical Studies of North African Cenozoic Volcanic Areas: III. Garian, Libya

1975 ◽  
Vol 12 (8) ◽  
pp. 1264-1271 ◽  
Author(s):  
J. M. Ade-Hall ◽  
Susann Gerstein ◽  
Robert E. Gerstein ◽  
Peter H. Reynolds ◽  
P. Dagley ◽  
...  
Keyword(s):  
Geomagnetic Field ◽  
Field Direction ◽  
Dipole Field ◽  
Plate Motion ◽  
Paleomagnetic Data ◽  
North African ◽  
Axial Dipole ◽  
Repeated Sampling ◽  
Absolute Age ◽  
E 32

Paleomagnetic and K/Ar whole rock absolute age data are described for material from the Garian area of Libya, centered at 13°E, 32°N. Within-unit cleaned paleomagnetic directions from the essentially unaltered lavas are very well defined and can almost certainly be taken as reliable measurements of the geomagnetic field direction during the initial cooling of each flow. However, the distributions of mean direction, from which the effect of repeated sampling of the field at one time has been removed, does not suggest that a reversing axial dipole field has been recorded in a representative manner. Both N and R groups of directions are azimuthally elongated, and the average poles for the N and R groups differ by 21°, or four times the 95% level uncertainty for each average pole. A number of possible physical explanations for the paleomagnetic results are discussed. The conventional overall average pole at 88°N, 123°E, δp: 3°, δm: 7 °does not differ significantly from the geographic pole, a result which agrees closely with that of Schult and Soffel (1973). However, the value of these overall average poles in estimating absolute plate motion must await an understanding of the sources of the asymmetries in the paleomagnetic data.

scholarly journals A Feasibility Study of Microbialites as Paleomagnetic Recorders

2021 ◽  
Vol 9 ◽  
Author(s):  
Ji Jung ◽  
Julie A. Bowles
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Inner Core ◽  
Salt Lake ◽  
Natural Remanent Magnetization ◽  
High Coercivity ◽  
Magnetic Carrier ◽  
Deep Time ◽  
Geologic Record ◽  
Magnetic Field Variations

Microbialites–layered, organosedimentary deposits–exist in the geologic record and extend back in deep time, including all estimated times of inner core nucleation. Microbialites may preserve magnetic field variations at high-resolution based on their estimated growth rates. Previous studies have shown that microbialites can have a stable magnetization. However, the timing and origin of microbialite magnetization were not well determined, and no study has attempted to evaluate whether actively growing microbialites record the geomagnetic field. Here, we present centimeter-scale magnetization and magnetic property variations within the structure of modern microbialites from Great Salt Lake (GSL), United States, and Laguna Bacalar, Mexico, Pleistocene microbialites from GSL, and a Cambrian microbialite from Mongolia. All samples record field directions close to the expected value. The dominant magnetic carrier has a coercivity of 35–50 mT and unblocking temperatures are consistent with magnetite. A small proportion of additional high coercivity minerals such as hematite are also present, but do not appear to appreciably contribute to the natural remanent magnetization (NRM). Magnetization is broadly consistent along microbialite layers, and directional variations correlate with the internal slope of the layers. These observations suggest that the documented NRM may be primarily detrital in origin and that the timing of magnetization acquisition can be close to that of sediment deposition.

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