Some observations of the gyroremanent magnetization acquired by rocks in a rotating magnetic field

Noël, Mark

doi:10.1111/j.1365-246x.1988.tb01125.x

Cited by 5 publications

(2 citation statements)

References 12 publications

(17 reference statements)

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“…1). Some studies have followed the opposite convention Stephenson 1976;Noel 1988;Edwards 1982b) and when discussing these studies, we use a conversion to the convention of Stephenson (1980).…”

Section: Quantification Of Rotational Remanent Magnetisationmentioning

confidence: 99%

Rotational remanent magnetization as a magnetic mineral diagnostic tool at low rotation rates

Hounslow

Horng

Karloukovski

2022

Geophysical Journal International

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Summary Prior work on rotational remanent magnetisation (RRM) and rotational anhysteretic remanent magnetisation (ARMROT) has demonstrated promise for magnetic mineral identification in earth materials. One challenge has been to calibrate the measurements to magnetic mineral types and microstructural controls, since previous studies have used differing spin rates, alternating field (AF) intensities and decay times, which hinders a comparison of datasets. Using a RAPID magnetometer we show that the range of usable practical rotation rates is 0.25 to 3 Hz [rps] which allows a wide range of RRM and ARMROT characteristics to be utilised (at 100 mT AF field, 100μT bias field). Sets of magnetic mineral extracts from sediments, and well characterised rock samples that contain the key magnetic minerals magnetite, pyrrhotite and greigite are used for a calibration of the RRM- ARMROT behaviour. Detrital pyrrhotite and pyrrhotite-bearing phyllites have largely small positive effective field (Bg) values, with differences in Bg and ARMROT ratios at 0.5 and 2.5 Hz [rps] allowing grain-size discrimination. The positive Bg values, and changes in RRM and ARMROT with rotation rates allow distinction of pyrrhotite from magnetite and diagenetic greigite. Diagenetic greigite has Bg values of -83 to -109 μT (at 0.5 Hz [rps]) and unusual RRM variation at low rotation rates caused by anisotropy affects. In contrast to previous work, based on crushed and sized natural magnetite at high spin rates, Bg for single domain magnetite from intact bacterial magnetofossils from Upper Cretaceous Chalk has some of the lowest Bg (0 -1 μT) and displays a steep decline in ARMROT with increasing rotation rates. A simple tool for particle size characterisation of magnetite may be the ratio of ARMROT at spin rates 2.5 and 0.5 Hz [rps]. Stability of RRM is better studied using RRM acquisition with increasing AF field intensity, since static demagnetisation imparts a nuisance gyroremanence along the field axis. Mineral microstructure, dislocations and particle interactions are likely additional effects on RRM behaviour that need more investigation.

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Section: Quantification Of Rotational Remanent Magnetisationmentioning

confidence: 99%

Rotational remanent magnetization as a magnetic mineral diagnostic tool at low rotation rates

Hounslow

Horng

Karloukovski

2022

Geophysical Journal International

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“…3 cannot however be explained by only a change in the g factor. The high rotation rates show a positive g factor behaviour whereas the low rotation rates suggest a negative g factor such as that noted by Noel (1988) in two samples (Stephenson 1988).…”

Section: The Source Of the Positive Rrm At Low Rotation Ratesmentioning

confidence: 87%

The anomalous demagnetization of pyrrhotite

Thomson¹

1990

Geophysical Journal International

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The anomalous behaviour of pyrrhotite-bearing rocks from the Pennine Orefield, northern England on alternating field demagnetization is studied with particular reference to rotational remanent magnetization (RRM). Using a heat treated sample, it is demonstrated that a two-axis tumbling demagnetizer is capable of producing a repeatable RRM along its inner axis of rotation which is comparable in strength to that produced in an instrument which uses single axis rotation and which is specifically designed to produce a RRM. The unheated samples gave a scattered RRM which was of sufficient intensity to disturb the primary magnetization. After heating the RRM become reproducible, much larger and parallel to the rotation vector for rotation rates between 1.5 and 120 rev s-l for an AF frequency of 50 Hz. This behaviour is unlike all previously measured samples, whether oxides or sulphides, by other workers. The source of the anomalous R R M and the change in RRM with heating are discussed with reference to the physical structure and the strong magnetocrystalline anisotropy in pyrrhotite.

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Bibliographie

Soffel

1991

Paläomagnetismus Und Archäomagnetismus

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Some observations of the gyroremanent magnetization acquired by rocks in a rotating magnetic field

Cited by 5 publications

References 12 publications

Rotational remanent magnetization as a magnetic mineral diagnostic tool at low rotation rates

Rotational remanent magnetization as a magnetic mineral diagnostic tool at low rotation rates

The anomalous demagnetization of pyrrhotite

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