Drilling‐induced isothermal remanent magnetization

Pinto, Michael J.; McWilliams, Michael

doi:10.1190/1.1442765

Cited by 27 publications

(12 citation statements)

References 10 publications

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“…It shows a distinct upwards vertical remanent magnetisation orientation *58 from the sample axis and is easily demagnetised with a peak field 520 mT. This behaviour is observed in many samples regardless of geographic position or orientation and is inferred to represent an isothermal remanent magnetisation (IRM) overprint acquired from a spinning magnetised drillcore barrel (Pinto & McWilliams 1990;de Wall & Worm 2001;Musgrave et al 2006). Such IRM overprints can completely replace any in situ NRM directions, and all samples in this study that have measured remanence directions within 208 of the sample axis and peak destructive fields 520 mT are treated as drilling IRM overprints (Figure 19).…”

Section: Natural Remanent Magnetisation Directionsmentioning

confidence: 95%

Mass and magnetic properties for 3D geological and geophysical modelling of the southern Agnew–Wiluna Greenstone Belt and Leinster nickel deposits, Western Australia

Williams¹

2009

Australian Journal of Earth Sciences

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Physical property measurements provide a critical link between geological observations and geophysical measurements and modelling. To enhance the reliability of gravity and magnetic modelling in the Yilgarn Craton's Agnew-Wiluna Greenstone Belt, mass and magnetic properties were analysed on 157 new rock samples and combined with an existing corporate database of field measurements. The new samples include sulfide ore, serpentinised and olivine-bearing ultramafic hostrocks, granitoid, and felsic and mafic volcanic and volcaniclastic country rock. Synthesis of the data provides a useful resource for future geophysical modelling in the region. Several rock types in the region have sufficiently distinct physical properties that a discriminant diagram is proposed to facilitate a basic classification of rock types based on physical properties. However, the accumulation of emplacement, metamorphic, hydrothermal and structural processes has complicated the physical properties of the rocks by imposing duplicate and sometimes opposing physical property trends. The data confirm that massive sulfide and ultramafic rocks have the most distinctive mass and magnetic properties but with variability imposed by their complex history. Sulfide content imposes the strongest control on densities, but can only be identified when comprising 410 vol% of the rock. The pyrrhotiterich Ni-sulfide assemblages generally have similar magnetic properties to the host ultramafic rocks, but can have much lower susceptibilities where the thermal history of the rocks has favoured development of hexagonal pyrrhotite over monoclinic pyrrhotite. In ultramafic rocks that contain 510 vol% sulfides, density and susceptibility are primarily controlled by serpentinisation, with olivine breaking down to serpentine and magnetite in the presence of water. Serpentinisation dramatically lowered densities and increased susceptibilities, but had limited influence on the intensity of remanent magnetisation. All ultramafic rocks contain multidomain magnetite, and most contain low coercivity grains prone to overprinting by in situ viscous remanent magnetisation or drilling-induced isothermal remanent magnetisation during extraction. Despite the low coercivities, Koenigsberger ratios of 1-20 are observed indicating that viscous remanent magnetisation aligned parallel to the present Earth field must be considered in any magnetic modelling. It is also noted that coarser-grained intrusive varieties of all rock types (e.g. granite, gabbro) show remanent magnetisation intensities 1-2 orders of magnitude greater than their extrusive equivalents (felsic and basaltic volcanics).

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Section: Natural Remanent Magnetisation Directionsmentioning

confidence: 95%

Mass and magnetic properties for 3D geological and geophysical modelling of the southern Agnew–Wiluna Greenstone Belt and Leinster nickel deposits, Western Australia

Williams¹

2009

Australian Journal of Earth Sciences

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“…There is a clear disposition toward high inclinations indicating that the remanence is directed close to the drill core axes. Apart from samples 20/97.0 and 20/97.2 these remanent magnetizations are far too stable to be drilling‐induced remanences (DIR) that are typically aligned with drill core axes as discussed by Pinto and McWilliams [1990]. Furthermore, the stable components are not parallel to the drill hole axis, but lie about 20° from it.…”

Section: Rock Magnetismmentioning

confidence: 99%

Magnetic properties and potential field modeling of the Peculiar Knob metamorphosed iron formation, South Australia: An analog for the source of the intense Martian magnetic anomalies?

et al. 2007

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[1] Magnetic property measurements show that the strongly metamorphosed Peculiar Knob iron formation (IF), South Australia, is coarse-grained, high-grade hematite with variable amounts of magnetite and maghemite. This body exhibits a relatively low magnetic susceptibility (<0.3 SI) that cannot explain the associated intense magnetic anomaly, 30,000 nT, in terms of induced magnetization alone. Peculiar Knob IF possesses an extremely intense ($120 A m À1 ) remanence, directed steeply upward. This ancient remanence reinforces the local Earth's field (inclination À63°). A simple geological model, constrained by drilling and physical property measurements, explains both the observed magnetic and gravity anomalies, consistent with the Poisson theorem. Koenigsberger ratios (Qs) of 10 and greater, as found here, are rare in nature. We postulate that acquisition of a thermoremanent magnetization (TRM) by the ore during postmetamorphic cooling from above the Curie/Néel temperature accounts for the intense remanence and high Qs. Although the hematite is in the multidomain size range, the coercivity is higher than expected. Also, the natural remanent magnetization (NRM) values are less than 10% of the expected value for a saturated TRM of hematite. On the basis of reflected light, scanning electron microscope observations, and rock magnetism, we propose that the common fine intergrowths of a very small amount of magnetite and/or maghemite within the hematite host are responsible for the relatively high coercivity and contribute to the NRM. These intergrowths are not normal exsolution lamellae and were likely present at high temperature. This study suggests that coarse-grained hematiterich bodies that carry TRM and have been subjected to high-grade (>680°C) metamorphism may be possible sources for some of the prominent Martian anomalies.

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“…3b). The remaining segments contain the VRMdi component, having almost vertical inclinations (N80°), a likely result from coring (Pinto and McWilliams 1990) (Fig. 2b).…”

Section: Results and Analysismentioning

confidence: 98%

Late Paleozoic remagnetization of the Trenton Formation in Ordovician petroleum reservoirs of southwestern Ontario

Garner

Cioppa

2006

Journal of Geochemical Exploration

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Drilling‐induced isothermal remanent magnetization

Cited by 27 publications

References 10 publications

Mass and magnetic properties for 3D geological and geophysical modelling of the southern Agnew–Wiluna Greenstone Belt and Leinster nickel deposits, Western Australia

Mass and magnetic properties for 3D geological and geophysical modelling of the southern Agnew–Wiluna Greenstone Belt and Leinster nickel deposits, Western Australia

Magnetic properties and potential field modeling of the Peculiar Knob metamorphosed iron formation, South Australia: An analog for the source of the intense Martian magnetic anomalies?

Late Paleozoic remagnetization of the Trenton Formation in Ordovician petroleum reservoirs of southwestern Ontario

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