Compositional control of magnetic anisotropy in the Thomson formation, east-central Minnesota

Johns, Mary; Jackson, Michael J.; Hudleston, Peter J.

doi:10.1016/0040-1951(92)90127-r

Cited by 12 publications

(11 citation statements)

References 28 publications

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“…Previous magnetic studies on the Thomson Formation (Johns et al 1992;Sun et al 1995) found AMS fabrics were dominantly oblate, with a strong foliation and weak lineation. A subset of the specimens studied by Sun et al (1995) were used for our study.…”

Section: H a R A C T E R I Z At I O N O F T H E T H O M S O N F O Rmentioning

confidence: 91%

“…For our samples, we chose the Thomson Formation of northern Minnesota. We chose this rock type because previous work (Johns et al 1992; Sun et al 1995) had shown that parts of the Thomson Formation contained primarily a paramagnetic signal. We used this absence of a significant ferromagnetic signal to test whether the high field method was able to separate the natural paramagnetic signal and an added ferromagnetic component of known magnitude and intensity.…”

Section: Characterization Of the Thomson Formationmentioning

confidence: 99%

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A new method for the separation of paramagnetic and ferromagnetic susceptibility anisotropy using low field and high field methods

Kelso¹,

Tikoff

Jackson

et al. 2002

Geophysical Journal International

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S U M M A R YMost rocks contain both ferromagnetic and paramagnetic minerals that contribute to their bulk magnetic susceptibility and the anisotropy of magnetic susceptibility. Anisotropy of magnetic susceptibility techniques typically measure the net susceptibility and are not able to separate paramagnetic and ferromagnetic contributions. Since different minerals may form at various times and/or under different conditions, examination of their individual contributions provides unique information related to the rock's formation and evolution. By subjecting a sample to high magnetic fields, the ferromagnetic minerals become saturated and the contribution of the paramagnetic minerals can be evaluated (the slope of the line at high field values, on a field vs magnetization plot). Using this approach, we developed a new technique that separates the ferromagnetic and paramagnetic components of standard 1 inch cylindrical samples using a Vibrating Sample Magnetometer. This separation is tested by artificially combining separate samples with known paramagnetic-only and ferromagnetic-only behaviour. By comparing the high-field results of a combined paramagnetic and ferromagnetic signal to the classic low field alternating current susceptibility of the paramagnetic-only signal, we demonstrate that the high field anisotropy is the result solely of the paramagnetic fabric even when the low field anisotropy of magnetic susceptibility is dominated by the ferromagnetic minerals. A ferromagnetic-only fabric is calculated for the combined paramagnetic and ferromagnetic rock, by tensor subtraction of the high field (paramagnetic-only) and low field (paramagnetic plus ferromagnetic) measurements on the same sample. Application of this technique to natural samples of combined paramagnetic and ferromagnetic behaviour is discussed.

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Section: H a R A C T E R I Z At I O N O F T H E T H O M S O N F O Rmentioning

confidence: 91%

Section: Characterization Of the Thomson Formationmentioning

confidence: 99%

A new method for the separation of paramagnetic and ferromagnetic susceptibility anisotropy using low field and high field methods

Kelso¹,

Tikoff

Jackson

et al. 2002

Geophysical Journal International

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“…A variety of samples has been used for this study, in order to investigate the influence of AF treatment on AMS for a number of different mineralogies and grain sizes. The sample collection includes (1) metamorphic rocks (n = 8 specimens) from the Thomson Slate, Minnesota, USA, whose AMS is carried by paramagnetic minerals and magnetite or Ti‐magnetite [ Johns et al ., ; Sun et al ., ], (2) sedimentary red beds (n = 6) of the Mauch Chunk Formation, Pennsylvania, containing magnetite and hematite [ Bilardello and Kodama , ; Tan and Kodama , ], (3) igneous rocks (n = 7) from the cumulate series of the Bjerkreim Sokndal Layered Intrusion, Southern Norway, comprising MD and PSD magnetite and hemo‐ilmenite [ Biedermann et al ., ; McEnroe et al ., ; Robinson et al ., ], and (4) basalts (n = 14) from Fogo, Cape Verde, containing a variety of grain sizes and domain states from MD to interacting SD magnetite, with compositions ranging from low‐Ti titanomagnetite to high‐Ti titanomagnetite (TM0 – TM70) [ Brown et al ., ].…”

Section: Methodsmentioning

confidence: 99%

Section: Samplesmentioning

confidence: 99%

“…The Thomson Slate samples display no systematic change in AMS principal directions or the AMS parameters P and U during either AF or low temperature demagnetization. Because the AMS of these samples is dominated by paramagnetic minerals [Johns et al, 1992;Kelso et al, 2002], it is likely that the small amount of ferromagnetic mineral present is not sufficient to have an effect on the AMS, even if domain alignment in magnetite takes place during AF treatment.…”

Section: Changes In Ams During Af Demagnetization and Low-temperaturementioning

confidence: 99%

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Influence of static alternating field demagnetization on anisotropy of magnetic susceptibility: Experiments and implications

Biedermann

Jackson

Bilardello

et al. 2017

Geochem Geophys Geosyst

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Anisotropy of magnetic susceptibility (AMS) indicates the preferred orientation of a rock's constituent minerals. However, other factors can influence the AMS, e.g., domain wall pinning or domain alignment in ferromagnetic minerals. Therefore, it is controversial whether samples should be alternating field (AF) demagnetized prior to AMS characterization. This may remove the influence of natural remanent magnetization (NRM) or domain wall pinning on AMS; however, it may also result in field‐induced anisotropy. This study investigates the influence of stepwise AF and low‐temperature demagnetization on mean susceptibility, principal susceptibility directions, AMS degree and shape for sedimentary, metamorphic, and igneous rocks. Alternating fields up to 200 mT were applied along the sample x, y, and z axes, rotating the order for each step, to characterize the relationship between AMS principal directions and the last AF orientation. The changes in anisotropy, defined by the mean deviatoric susceptibility of the difference tensors, are between <2% and 270% of the AMS in NRM‐state. Variations in AMS parameters range from small changes in shape to complete reorientation of principal susceptibility axes, with the maximum susceptibility becoming parallel to the last AF direction. This is most prevalent in samples with low degrees of anisotropy in the NRM‐state. No clear correlations were found between field‐induced anisotropy and hysteresis properties. Therefore, we propose that future studies check any samples whose AMS is carried by ferromagnetic minerals and low anisotropy degrees for AF‐induced artifacts. These results highlight the need for understanding the AMS sources and carriers prior to any structural interpretation.

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Magnetic and petrofabric studies in the multiply deformed Thomson Formation, east-central Minnesota

1995

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Compositional control of magnetic anisotropy in the Thomson formation, east-central Minnesota

Cited by 12 publications

References 28 publications

A new method for the separation of paramagnetic and ferromagnetic susceptibility anisotropy using low field and high field methods

A new method for the separation of paramagnetic and ferromagnetic susceptibility anisotropy using low field and high field methods

Influence of static alternating field demagnetization on anisotropy of magnetic susceptibility: Experiments and implications

Magnetic and petrofabric studies in the multiply deformed Thomson Formation, east-central Minnesota

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