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Intensity of NRM in A m −1 (SI) versus anisotropy of the AMS expressed as k 2 /k 3. Grey shading indicates the slightly more restricted range covered by the best 55 samples in groups A and B following quality assessment (see Section 3.5).

Intensity of NRM in A m −1 (SI) versus anisotropy of the AMS expressed as k 2 /k 3. Grey shading indicates the slightly more restricted range covered by the best 55 samples in groups A and B following quality assessment (see Section 3.5).

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New experimental and computational approaches to interpret orientation and intensity of natural remanent magnetization (NRM) carried by lamellar magnetism are applied to historic magnetic measurements on a collection of 82 massive hemo-ilmenite samples from the Allard Lake District in the Grenville Province, Quebec. The anisotropy of magnetic susce...

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... with a small ratio k 1 /k 2 ranging from 1 (a perfect oblate spheroid with nine examples) to only slightly less than 1.5. A line at a ratio of 1:1 is the boundary between oblate and prolate fields, and all but two samples plot in the oblate area. These two samples may be examples of the 'folding' described in connection with Fig. 3(b) above. Fig. 7 shows the NRM intensity versus the anisotropy of suscep- tibility of AMS as expressed by the ratio k 2 /k 3 for all samples. Table 1. Measurements of NRM intensity, declination and inclination, declination and inclination of k 3 -axis of the AMS ellipsoid, k 1 , k 2 , k 3 values of AMS and mean k of the AMS. (0001) plane to ...
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... Group samples, implying strong lamellar magnetism. Our model predicts that for small α, these samples might have acquired NRM's over 200 A m −1 . (f) Sample 36a with (v, GC) = 1 • has low anisotropy k 2 /k 3 = 1.30 and very low susceptibility ¯ k = 0.029. The NRM is ψ = 47.0 • from (0001), consistent with α = 83.3 • and with the low anisotropy (Fig. 7). The NRM at 22.5 A m −1 is at the lower right end of the envelope (Figs 15 a and b) for ordinary samples from the Lac Tio ...
Context 3
... between the NRM vector and field direction upon the angle α between field direction and statistical basal plane. When the scatter is large (K = 2), β stays small, indicating that the NRM can align well with the field. When the distribution becomes narrow (K = 100), the angle β at first increases linearly with α, but then drops sharply to 0 • (see Fig. 7) when the area of c-axis scatter contains sufficiently many individual axes with more than 90 • deviation from the field direction. Then, the residual NRM can align well with the field by inverse magnetization of these ...

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... Synthetic studies on the magnetic implications of lamellae have been conducted because the anisotropic magnetic susceptibility of lamellae will affect both the shape and amplitude of magnetic anomalies (Biedermann & McEnroe, 2017). This is particularly important when oxide lamellae with a strong crystallographic preferred orientation are abundant (Robinson, Heidelbach et al., 2006;Robinson et al., 2002Robinson et al., , 2013Robinson et al., , 2016. Here, magnetism may be strongest when: (a) the proportion of exsolved lamellae material is large, (b) the total area of the exsolution interfaces is large, and (c) host planes are parallel to the magnetizing field (Robinson, Harrison, & McEnroe, 2006;Robinson et al., 2013Robinson et al., , 2021. ...
... This is particularly important when oxide lamellae with a strong crystallographic preferred orientation are abundant (Robinson, Heidelbach et al., 2006;Robinson et al., 2002Robinson et al., , 2013Robinson et al., , 2016. Here, magnetism may be strongest when: (a) the proportion of exsolved lamellae material is large, (b) the total area of the exsolution interfaces is large, and (c) host planes are parallel to the magnetizing field (Robinson, Harrison, & McEnroe, 2006;Robinson et al., 2013Robinson et al., , 2021. Grain size is also a crucial factor influencing magnetization, especially of titanomagnetite-bearing rocks (Clark, 1997;Dunlop, 1981). ...
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... The resultant bulk magnetization of composite magnetic grains could be weakened by internal inhomogeneities due to the different orientations of the magnetization (Robinson et al., 2013;Robinson, Heidelbach, et al., 2006). The variation in magnetization inclination and declination angles are shown on the rose diagram in the stereonet in Figure 5(c). ...
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... Magnetite is a titanomagnetite that contains oxidation-exsolution lamellae of ilmenite (Duchesne 1972;Dunlop and Özdemir 2009), as well as exsolved lenses and lamellae of Al-spinel parallel, respectively, to the {111} and {100} planes of the host ( Fig. 6a and b). Ilmenite is either optically homogeneous (Fig. 6b) or displays thin lamellae of magnetite and subordinate Al-spinel along its (0001) basal plane (Fig. 6d), formed, respectively, through reduction-exsolution (Robinson et al. 2013) and exsolution. Small grains of Al-spinel are found in ilmenite at the contact with magnetite ( Fig. 6a; subsolidus reaction rims; Duchesne 1972). ...
Article
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... Ilmenite is also homogeneous, except in some Ttn-subfacies samples in which thin exsolution lamellae of hematite are present along the (0001) basal plane of the ilmenite. Some of these hemo-ilmenite grains contain reductionexsolution lamellae of magnetite parallel to the hematite exsolutions ( Fig. 6d; Robinson et al., 2013). Pyrite grains locally contain tiny chalcopyrite areas. ...
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... This exsolution occurs at high temperatures, and is stable to relatively high temperatures (near their respective T N ). The strength of the NRM is a function of the amount of oxide, microstructures and lattice-preferred orientations (Robinson et al., 2013). The low susceptibility of ilmenite-hematite ss (<0.005 ...
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... At Heskestad the steep foliation and lineation are both quasi-parallel to the early Neo-Proterozoic magnetizing field, fulfilling these requirements […]" to explain the strong NRM in this area. Robinson et al. (2013) investigated NRM and preferred orientation of hemo-ilmenite in Allard Lake, Canada, and show a strong dependence of NRM intensity on the orientation of hematite-ilmenite lamellae with respect to the magnetizing field. Biedermann et al. (2017) observed that the NRM orientation changes along the MCU IVe 0 layer and appears to be tilted toward the direction of k 1 . ...
Article
Modeling of magnetic anomaly data is a powerful technique to gain information on the shape of subsurface rock bodies. Most models are based on the assumption that the magnetization in the source body is parallel to the direction of the Earth's magnetic field. It has long been recognized that remanent magnetization affects the magnetization direction, intensity and shape of the anomaly, and therefore the interpreted structure. The effects of anisotropy, however, have only received little attention so far. This study uses synthetic models and a case study to investigate how anisotropy affects magnetization and anomalies over a thick dipping sheet and determines expected errors in interpreted magnetic properties and geometry of the source body for various anisotropy degrees and field inclinations. Anisotropy affects both the shape and amplitude of anomalies. For an oblate uniaxial fabric with the minimum susceptibility normal to the sheet and P = 1.5, errors in interpreted dip are up to 12°, depending on the field inclination, dip, and profile orientation, and errors in estimated mean susceptibility are up to −30%/+20%, if anisotropy is not taken into account during modeling. These effects are larger for higher degrees of anisotropy. A case study over the megacyclic unit IVe′ layer in the Bjerkreim Sokndal layered intrusion, Norway, investigates the contributions of (an)isotropic induced and remanent magnetizations to the total field anomalies. There, the influence of anisotropy is mainly related to remanence deflection. The results shown here will help to further improve interpretation of magnetic potential field data.
... The magnetic properties of hemo-ilmenite are strongly anisotropic, with the minimum susceptibility normal to the basal plane [Hargraves, 1959;Robinson et al., 2013Robinson et al., , 2006]. Both Hargraves [1959] and Robinson et al. [2006] found that the NRM of hemo-ilmenite is confined to the basal plane, at an angle of at least 80°from the minimum susceptibility axis, in agreement with predictions from lamellar magnetism theory. ...
... Both Hargraves [1959] and Robinson et al. [2006] found that the NRM of hemo-ilmenite is confined to the basal plane, at an angle of at least 80°from the minimum susceptibility axis, in agreement with predictions from lamellar magnetism theory. A recent study by Robinson et al. [2013] reports that the NRM in hemo-ilmenite samples from Allard Lake is deflected from the Proterozoic magnetizing field as a result of being confined to the basal plane of hemo-ilmenite. They also show how the NRM intensity is expected to vary with the angle between the preferred hemo-ilmenite orientation and the geomagnetic field at the time the rocks were magnetized and compare it to measurements on massive hemo-ilmenite ore deposits from Allard Lake. ...
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A strong negative magnetic anomaly, caused by an intense natural remanent magnetization (NRM) ca. opposite today’s geomagnetic field, is observed above the MCU IVe’ unit of the Bjerkreim Sokndal layered intrusion. The anomaly is strongest in the east, close to Heskestad, and decreases when following the layer towards the north and west. This study investigates how the NRM changes along the layer, and how its direction and intensity are affected by magnetic fabrics in the intrusion. NRM, low-field anisotropy of magnetic susceptibility and anisotropy of anhysteretic remanence have been measured on 371 specimens from 46 sites. The orientation of both the magnetic fabrics and the NRM change for different locations along the layer, and it appears that the NRM is tilted away from the mean paleofield and towards the direction of maximum susceptibility and maximum anhysteretic remanence. When NRM directions are corrected for magnetic fabrics, the angle between the NRM and mean paleofield direction generally decreases for specimens with a single-component NRM. No correlation was found between the NRM intensity and the directional relationship between NRM, magnetic fabric and mean paleofield.
... Here we show that this is incorrect in experiments on samples with an existing NRM dominated by lamellar moment, and these effects are further enhanced when the sample possesses an LPO. Titanohematite is a phase with a strong magnetocrystalline anisotropy (Robinson et al. 2004(Robinson et al. , 2012 in which the NRM is constrained with respect to the (001) basal plane. If a sample has a strong LPO, consists of several crystals with a similar orientation, or, in the extreme case, consists of a single crystal, then the direction and intensity of magnetization measured will depend heavily on exactly how the sample is placed in the instrument, both with respect to the NRM, and with respect to the LPO. ...
... The same is true for variously oriented collections of crystals within rock samples. The origin of this bimodality comes in part from the nature of lamellar magnetism in a titanohematite host and the orientation of the (001) basal plane with respect to the ancient magnetizing field (Robinson et al. 2002(Robinson et al. , 2004(Robinson et al. , 2012. When the basal plane is not parallel to the magnetizing field, then not all ilmenite lamellae and contact layers will be forced to be magnetically 'in-phase', that is they will not necessarily be separated by odd numbers of hematite layers, which would ensure that the contact layers all had mutually parallel magnetizations. ...
Article
This is the second of three papers investigating properties of titanohematite-bearing quartzofeldspathic rocks that create a significant remanent magnetic anomaly in the Modum District, South Norway. The first paper provided initial magnetic results, mineralogical characterization and evidence for the presence of lamellar magnetism. In this paper, knowledge of lamellar magnetic properties is explored through experimentswhere ilmenite lamellaewere magnetized below 57 K, and interact magnetically along interfaces with the titanohematite host. Samples with known NRM directions were placed in specific orientations in an MPMS then cooled in zero field to 5 K, where hysteresis loops were measured in fields up to 5 Tesla. This assured that results were ultimately related to the natural lamellar magnetism produced during cooling ~1 billion years ago. In a second set of experiments the same oriented samples, were subjected to a +5 Tesla (T) field then field cooled to 5 K before hysteresis experiments. The first experiments consistently produced asymmetric shifted hysteresis loops with two loop separations, one in a positive field and one in a negative field.Without exception,when theNRMwas oriented toward the negative field end of the MPMS, the bimodal loop showed a dominant loop separation in a positive field. By contrast, when the NRM was oriented toward the positive field end of the MPMS, the bimodal loop showed a dominant loop separation in a negative field. Both observations are consistent with antiferromagnetic coupling between the hard magnetization of ilmenite and the more easily shifted lamellar magnetism of the hematite. The bimodal nature of the loops indicates that the NRMs are vector sums of natural lamellar moments, which are oriented both positively and negatively, and that these opposite moments control the orientations of ilmenite magnetizations when cooling through 57 K. Here, extreme exchange biases up to 1.68 Twere measured. The second set of experiments produced asymmetric shifted hysteresis loops with one opening always in the negative field. These observations indicate that the +5 T field applied at room temperature rotated the hematite lamellar magnetism in a positive direction, so that upon cooling all the ilmenite lamellae acquired negative magnetic moments, thus causing unimodal negatively shifted loops. Here, the largest exchange bias among the unimodal loops was only 0.7 T. These results will be used in paper III to build a better understanding of lamellar magnetism at the atomic layer scale. © The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society.
... The NRM due to lamellar magnetism is characterized by (1) moderate to strong intensity, (2) large coercivity particularly for hematite-hosted lamellae, and (3) high thermal stability, and the rocks still carry the signature they acquired in the Proterozoic (Brown and McEnroe, 2015). The intensity of the lamellar magnetism NRM depends on the volume-concentration of the lamellae (McCammon et al., 2009;McEnroe et al., 2009a), and on the orientation of the lamellae with respect to the magnetizing field (Robinson et al., 2013). ...
... Thus, pyroxene CPO results in preferred orientation of hemo-ilmenite and magnetite within the pyroxenes. Hemoilmenite exhibits an oblate AMS, with the minimum susceptibility parallel to the (0001) direction (Robinson et al., 2013). The dominant orientation relationship of hemo-ilmenite in orthopyroxene, (0001)// (100), will cause an AMS carried by hemo-ilmenite with minimum susceptibility parallel to (100) of the orthopyroxene. ...
Article
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Magnetic anisotropy can provide important information about mineral fabrics, and thus magmatic processes, particularly when it is known how multiple mineral species contribute to the anisotropy. It may also affect the direction of induced or remanent magnetization, with important consequences for paleomagnetic studies or the interpretation of magnetic anomalies. Here, we aim at describing the magnetic fabrics in the Bjerkreim Sokndal Layered Intrusion and identifying their carriers. Anisotropies of magnetic susceptibility and remanence were measured on samples covering different geographic locations and stratigraphic units within the Bjerkreim Sokndal Layered Intrusion. The intrusion is characterized by magmatic layering and has a synform structure, with strong foliation on the limbs. Detailed comparison between magnetic and mineral fabric shows that they are not necessarily coaxial, but the minimum susceptibility, and minimum anhysteretic remanence are generally normal to the foliation or the magmatic layering. The minimum susceptibility and anhysteretic remanence are associated with pyroxene (100) axes, and the maximum susceptibility and anhysteretic remanence are sub-parallel to the pyroxene [001] axes in layers MCU IVc and MCU IVe for which electron backscatter data are available. However, the paramagnetic anisotropy of pyroxene is too weak to explain the observed anisotropy. We propose that the magnetic anisotropy of magnetite-free specimens is carried by hemo-ilmenite exsolutions within pyroxene, in addition to pyroxene itself. When present, multi-domain magnetite dominates both the anisotropy of magnetic susceptibility and anhysteretic remanence, due to shape-preferred orientation and distribution anisotropy. The orientation of the magnetic fabric appears independent of carrier, due to their common deformation history, but the degree of anisotropy is stronger for magnetite-bearing specimens. The results of this study will facilitate future structural interpretations and may be used to correct for magnetization deflection.
... In this region, as a result of ancient sediment deposition under oxidizing conditions and later Sveconorwegian (∼1092 Ma) amphibolite-facies regional metamorphism (Bingen et al. 2008), the NRMs in these metamorphic rocks are commonly carried by titanohematite with ilmenite and rutile exsolution lamellae, creating a very stable remanence attributed to 'lamellar magnetis' (Robinson et al. , 2004(Robinson et al. , 2013. The extraordinarily high Q values indicate that the nature and geometry of these present-day anomalies are due to a memory acquired nearly 1 billion years ago and retained to the present. ...
... Titanohematite is the densest mineral in these rocks, which are mainly composed of quartz, feldspar and muscovite and is strongly correlated with the NRM. The orientation of the titanohematite crystals will also effect the intensity of NRM (Robinson et al. 2013). Numerous 'large' crystals are actually groups of crystals, likely geometrically related due to deformation-induced lattice-preferred orientation that is demonstrated by EBSD studies to be presented in Paper III. ...
Article
Recent high-resolution aeromagnetic surveys in South Norway have revealed numerous remanent anomalies over Mesoproterozoic metamorphic rocks. Studies on the nature of the minerals that are the remanent carriers has led to discoveries of titanohematite samples with unusual magnetic properties caused by nanoscale exsolution lamellae with their related lamellar magnetism. Here we focus on a rock unit dominated by quartz-plagioclase-biotite granulite containing titanohematite grains with a strong lattice-preferred orientation parallel to regional foliation. When samples with their natural remanent magnetization (NRM), acquired nearly 1 billion years ago, are cooled to 10 K and hysteresis loops measured, these loops show bi-modal exchange bias caused by the magnetism induced within the ilmenite by antiferromagnetic coupling with the adjacent lamellar NRM. By contrast when the samples are cooled in a strong magnetic field (1.5 Tesla), this results in unimodal lamellar magnetism, and, below the T-N of ilmenite it adopts a consistent negative orientation, giving rise to unimodal negative exchange bias of >500 mT. The results presented here cover the chemical and magnetic properties, Mossbauer results and transmission electron microscopy of the titanohematite and ilmenite lamellae. Initial magnetic experiments indicated the shifts found in the exchange-bias experiments were directly related to the orientation of the sample to the applied field and the initial state of the NRM. In most samples with these unusual magnetic properties, ilmenite lamellae could not be seen in an optical or a scanning electron microscope. However magnetic experiments gave proof of the presence of ilmenite, later confirmed by Mossbauer spectroscopy. Several attempts were made to identify ilmenite in TEM studies, finally successful in showing ilmenite lamellae parallel to (001) of hematite with thicknesses similar to 1.2 to 1.7 nmand aspect ratios 7-13. Here we compare new TEM images and the magnetic behaviour of these samples to the MOD2 samples that previously showed extraordinary exchange bias properties, and investigate further the nature of these magnetic minerals.