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.

Shock-diminished paleomagnetic remanence at the Charlevoix impact structure, Quebec

1979 ◽  
Vol 16 (9) ◽  
pp. 1842-1856 ◽  
Author(s):  
P. Blyth Robertson ◽  
J. L. Roy
Keyword(s):  
Remanent Magnetization ◽  
Marginal Zone ◽  
Natural Remanent Magnetization ◽  
Significant Fraction ◽  
Magnetic Characteristics ◽  
Thermal Treatments ◽  
Magnetic Carrier ◽  
Impact Structure ◽  
Late Precambrian ◽  
Dual Polarity

The magnetic characteristics of shocked and unshocked anorthosites of the Charlevoix impact structure have been reexamined to assess the effects of high thermal and alternating field (af) cleaning treatments, and to evaluate any shock-induced features of the remanence in terms of mineralogy of the shocked rocks.From 16 sites, 197 specimens were subjected to stepwise thermal treatments to 710 °C, and a further 46 to incremental af cleaning to 290 mT. The 10 sites from the unshocked St-Urbain anorthosite carry a magnetization with unblocking temperatures (TUB) generally between 600–625 °C, with a significant fraction of remanence with TUB > 670 °C. Similarly, most of the remanence has a resistive coercive force (rcf) of 100–175 mT, with a significant fraction exceeding 290 mT. The remanence is carried by two exsolution phases of titano-hematite, with the later and more hematite-rich lamellae having the higher TUB and rcf. Although only one direction exists, at two sites where intensities are somewhat lower and some natural remanent magnetization (NRM) directions scattered, treatment reveals a dual polarity remanence. The pole (154°E, 02°S) obtained from these unshocked St-Urbain anorthosite sites falls on a well established segment of the late Precambrian apparent polar path of suitable age (≈950 Ma). Significant results could not be obtained from three additional sites in the marginal zone due to their ready acquisition of a viscous magnetization following thermal treatments.Data from two sites which display no shock effects and lie near the margin of the central uplift, and from one highly shocked sample from the crater centre show some possible effects of the shock event. All three have intensities substantially lower than in the St-Urbain sites, which is interpreted in the case of at least one of these sites as a shock-diminished remanence. A puzzling feature of the highly shocked samples is that the phase with a texture reminiscent of the ilmenite–hematite exsolution in the unshocked anorthosites, contains no iron. The remaining remanence, whose direction is parallel with that of the unshocked sites, is interpreted as pre-crater rather than shock-produced. Although many instances have been reported of a new shock-induced remanence imparted to rocks of equivalent and even lesser shock grade than those examined at Charlevoix, in all cases the magnetic carrier seems to have been magnetite. The lack of a shock-induced remanence at Charlevoix is attributed to the high TUB and rcf of hematite in these rocks.

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.

Paleomagnetism of Late Archean metavolcanics and metasediments, Abitibi orogen, Canada: tholeiites of the Kinojevis Group

1983 ◽  
Vol 20 (3) ◽  
pp. 436-461 ◽  
Author(s):  
John Wm. Geissman ◽  
David W. Strangway ◽  
Ann M. Tasillo-Hirt ◽  
Larry S. Jensen
Keyword(s):  
Geomagnetic Field ◽  
Regional Metamorphism ◽  
Natural Remanent Magnetization ◽  
Blocking Temperature ◽  
High Coercivity ◽  
Polar Wander ◽  
Magnetic Viscosity ◽  
Secular Variations ◽  
Temperature Time

Iron-rich and magnesium-rich basaltic tholeiites of the latest Archean Kinojevis Group, central Abitibi Belt, northeastern Ontario, contain a natural remanent magnetization that is generally multivectorial. The components in all units reside in essentially pure magnetite, not the original titanomagnetites (where x for Fe3−xTixO4 probably was 0.60–0.65). Any original TRM was lost by chemical reconstitution of the magnetic oxides in response to primary deuteric conditions, long-term burial, regional metamorphism to prehnite–pumpellyite facies, and possibly intrusion by Matachewan dikes. Data from contact tests with Matachewan dikes indicate that the units are indeed capable of retaining a very latest Archean – earliest Proterozoic field (e.g., D = 194.9°, I = −14.3°, k = 8.1, α95 = 7.9°; n = 45 vectors, 38 samples). High-coercivity, high-blocking-temperature directions from samples from other flows, corrected for nearly penecontemporaneous downwarping, are in only crude agreement with those of Matachewan dikes, possibly suggesting that these components reflect a general Late Archean – Early Proterozoic field for the Superior Province. The paleomagnetic data from Kinojevis tholeiites indicate the emplacement, burial, and tight downwarping of the 10 km or so of Kinojevis stratigraphy were nearly synchronous with Matachewan intrusion. The Kinojevis data by themselves cannot be taken as statistically reliable indicators of the pre- (or immediately post-) Matachewan geomagnetic field nor can temperature–time relations for magnetic viscosity be used to predict the preservation of a statistically reliable TRM in any of these units. Individual magnetization components were blocked over geologically short periods of time, whereas the ensemble of data from discrete flows must record secular variations, field excursions, and possibly long-term polar wander.

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.

MAGNETIC ORIENTATION OF BOREHOLE CORES

Geophysics ◽  
1969 ◽  
Vol 34 (5) ◽  
pp. 772-774 ◽  
Author(s):  
M. Fuller
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Standard Procedure ◽  
Natural Remanent Magnetization ◽  
Core Sample ◽  
The Core ◽  
Viscous Component ◽  
Late Tertiary ◽  
Geomagnetic Pole ◽  
Orientation Method

The use of natural remanent magnetization (NRM) to orient boreholes was reported at least thirty years ago (e.g., Lynton, 1938). The method depends upon determining the direction of remanent magnetization of the sample and relating it to the geomagnetic field in which the rock was presumed to have been magnetized. If the NRM faithfully records the relevant geomagnetic field and the field is known, the orientation of the core sample is available. Unfortunately, the ancient geomagnetic pole positions are not, in general, sufficiently well known to make this method particularly successful for rocks that are older than late Tertiary. Moreover, the presence of a weak, viscous component of magnetization parallel to the present geomagnetic field at a given site may produce erroneous results unless it is recognized and eliminated. However, the existence of this component provides another means of orienting the core sample. Isolation of this component might initially appear to be difficult, but it is actually a standard procedure of paleomagnetism; and, in fact, many studies have implicitly demonstrated that the direction of the present geomagnetic field at the sample site is recoverable (e.g., As and Zijderveld, 1958; Zijderveld, 1967). Indeed a number of people have recognized the possibility of using this component to orient borehole samples (e.g., Hargraves, 1969—private communication). The use of this soft viscous component has been advocated recently to distinguish between normal and reversely magnetized rocks in connection with tests of the sea floor spreading hypothesis (Irving and Roy, 1968). Nevertheless, no explicit demonstration of the technique of orienting borehole cores has been published. In the course of paleomagnetic surveys, our demagnetization studies have revealed a number of examples of behavior which makes the orientation method possible. This note describes such behavior and explains how the orientation might be recovered.

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.

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