AMS in basalts: is there a need for prior demagnetization?

Schöbel, Stefan; Wall, Heinrich de; Rolf, Christian

doi:10.1093/gji/ggt325

Cited by 8 publications

(14 citation statements)

References 41 publications

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“…[] investigated loess, paleosol, diorite, granite, and gneiss, and found k mean increases between 2 and 27%. Basaltic lava flows from the Deccan Traps, India, display a 8.6% or 5.0% increase in k mean after static and tumbling AF demagnetization, respectively [ Schöbel et al ., ]. A five to eightfold increase in mean susceptibility was observed in mafic dykes from the Henties Bay Outjo Dyke swarm, NW Namibia, after tumbling and static AF demagnetization [ Wiegand , ].…”

Section: Discussionsupporting

confidence: 91%

“…AF demagnetization of NRM shows that although most of the NRM is carried by hematite, small amounts of magnetite are present. Most studies attributed field‐induced anisotropy to MD magnetite grains [ Potter and Stephenson , ; Schöbel et al ., ; Violat and Daly , ], and it has also been described in SD maghemite (γ‐Fe 2 O 3 ) [ Potter and Stephenson , ]. At present, no studies have investigated whether AF‐induced anisotropy occurs in hematite (α‐Fe 2 O 3 ).…”

Section: Discussionmentioning

confidence: 99%

“…Some studies have found that AF treatment enhances the AMS [ Bhathal and Stacey , ; Liu et al ., ]. Others have used AF cleaning to remove any secondary stress‐related components from the AMS measurement [ Park et al ., ], or to avoid biases due to strong remanent magnetization [ Lanci , ; Schöbel and de Wall , ; Schöbel et al ., ]. While yet others treat field‐induced anisotropy as an artefact [ Bhathal and Stacey , ; Henry et al ., ; Jordanova et al ., ; Potter and Stephenson , ; Violat and Daly , ].…”

Section: Introductionmentioning

confidence: 99%

“…These observations suggest that grain size strongly affects the size and shape of the impressed ellipsoid, with MD grains producing prolate ellipsoids, i.e., an increase in susceptibility parallel to the field axis, and SD grains oblate ellipsoids, i.e., a decrease in susceptibility in the direction of the field [ Potter and Stephenson , ; Stephenson et al ., ]. For the same reason, increases and decreases in susceptibility parallel to the last AF direction have been described for samples exhibiting normal (MD grains) or inverse magnetic fabrics (SD grains) [ Schöbel and de Wall , ; Schöbel et al ., ]. Jordanova et al .…”

Section: Introductionmentioning

confidence: 99%

“…While yet others treat field‐induced anisotropy as an artefact [ Bhathal and Stacey , ; Henry et al ., ; Jordanova et al ., ; Potter and Stephenson , ; Violat and Daly , ]. In line with these different interpretations, contradicting recommendations have been put forward as to whether or not samples should be AF demagnetized prior to AMS measurements: either (1) AMS should be measured before AF treatment, in order to avoid effects of artificial field‐induced anisotropy [ Jordanova et al ., ; Potter and Stephenson , ] or (2) samples should be demagnetized before the AMS is determined, to remove the effects of stress‐induced domain wall pinning, or to remove the effects of a strong natural remanent magnetization (NRM) [ Lanci , ; Liu et al ., ; Park et al ., ; Schöbel and de Wall , ; Schöbel et al ., ].…”

Section: Introductionmentioning

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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Section: Discussionsupporting

confidence: 91%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

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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AMS–NRM interferences in the Deccan basalts: Toward an improved understanding of magnetic fabrics in flood basalts

Schöbel

Wall

2014

JGR Solid Earth

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The evaluation of flow direction in volcanic rocks is among the most important applications of magnetic fabrics studies. A statistically significant sample set of titanomagnetite-bearing lava flows from the Malwa Plateau, the northern part of the Deccan traps in India, has been investigated for a possible interference of induced and natural remanent magnetization (NRM). The NRM alters the scalar anisotropy of magnetic susceptibility (AMS) parameter and the orientations of the AMS principal magnetic axes, which are crucial for the evaluation of the flow direction. For cleaning of the NRM component, the lava samples have been demagnetized by use of an alternating field (AF) tumbling demagnetizer (peak fields of 100 mT) as previous studies have shown that static AF demagnetization can bias the results. Samples with normal magnetic fabrics demonstrate a redistribution of their principal axes after the demagnetization. The evaluated flow directions show a more differentiated flow pattern of the Malwa area, which seems to fit better into the regional geological setting. In samples with inverse magnetic fabrics, carrying a higher portion of single-domain particles, AMS principal axes remain unchanged after the demagnetization, indicating that these samples with high coercivity of magnetic carriers are not suitable for geological interpretations. According to these results, we propose that the AMS measurements after tumbling demagnetization give a better reflection of the intrinsic anisotropy of magnetic carriers (at least for samples with normal magnetic fabrics) and therefore a more precise and better reflection of the "actual" mineral fabric.

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Anisotropy of magnetic susceptibility impressed during rock magnetic procedures (AF, IRM) and information on the domain state of the magnetic carriers

Roperch¹,

Kissel²,

Lagroix³

et al. 2023

Physics of the Earth and Planetary Interiors

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AMS in basalts: is there a need for prior demagnetization?

Cited by 8 publications

References 41 publications

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

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

AMS–NRM interferences in the Deccan basalts: Toward an improved understanding of magnetic fabrics in flood basalts

Anisotropy of magnetic susceptibility impressed during rock magnetic procedures (AF, IRM) and information on the domain state of the magnetic carriers

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