Low-temperature Demagnetization Research Articles

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

AbstractAnisotropy 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.

Basic properties of transition remanent magnetizations of magnetite in relation to the ambient field using granite samples

We have investigated basic properties of transition remanent magnetization of natural magnetite in granite samples collected from the Minnesota River Valley, North America. Transition remanence was imparted during cooling and/or warming through the Verwey transition around 120 K. Depending on magnetic field conditions during cooling and warming, three types of transition remanences have been categorized: (1) TrRM, acquired during a cycle of field cooling and field warming; (2) TrWRM, acquired during zero-field cooling and field warming and (3) TrCRM, imparted during field cooling and zero-field warming. These remanences fulfil basic laws of remanent magnetization: (1) directions of the transition remanences are parallel to direction of the applied field (the law of parallelism), (2) intensities of the transition remanences are proportional to the applied field intensity (the law of proportionality) and (3) sum of the partial transition remanences is equal to the total transitional remanence, that is, TrRM = TrWRM + TrCRM. In addition, the ratio of TrRM to TRMLTD (the demagnetized component of thermoremanent magnetization by low-temperature demagnetization) shows a nearly constant value of ∼0.34. This relationship might reflect differences in equilibrium magnetic domain state at low and high temperature.

Absolute geomagnetic paleointensity as recorded by ∼1.09 Ga Lake Shore Traps (Keweenaw Peninsula, Michigan)

Absolute geomagnetic paleointensity measurements were made on 255 samples from 38 lava flows of the ∼1.09 Ga Lake Shore Traps exposed on the Keweenaw Peninsula (Michigan, USA). Samples from the lava flows yield a well-defined characteristic remanent magnetization (ChRM) component within a ∼375°C–590°C unblocking temperature range. Detailed rock magnetic analyses indicate that the ChRM is carried by nearly stoichiometric pseudo-single-domain magnetite and/or low-Ti titanomagnetite. Scanning electron microscopy reveals that the (titano)magnetite is present in the form of fine intergrowths with ilmenite, formed by oxyexsolution during initial cooling. Paleointensity values were determined using the Thellier double-heating method supplemented by low-temperature demagnetization in order to reduce the effect of magnetic remanence carried by large pseudosingle-domain and multidomain grains. One hundred and two samples from twenty independent cooling units meet our paleointensity reliability criteria and yield consistent paleofield values with a mean value of 26.3 ± 4.7μT, which corresponds to a virtual dipole moment of 5.9 ± 1.1×1022 Am2. The mean and range of paleofield values are similar to those of the recent Earth’s magnetic field and incompatible with a “Proterozoic dipole low”. These results are consistent with a stable compositionally-driven geodynamo operating by the end of Mesoproterozoic.

Paleointensity variation across the Matuyama-Brunhes polarity transition: Observations from lavas at Punaruu Valley, Tahiti

[1] We have conducted a paleointensity study of the Matuyama-Brunhes (M-B) polarity transition recorded in 34 successive lava flows of Punaruu Valley on Tahiti. A reversed polarity is obtained from the lower part of the record, major directional changes are derived from the middle part of the record, and a normal polarity is recorded in the upper part of the record. These paleomagnetic directions and five 40Ar/39Ar ages yielding a weighted mean of 771 ± 8 (1σ) ka indicate that 30 lava flows recorded the geomagnetic field across the M-B transition. The 215 specimens from 32 flows were subjected to the double-heating technique of the Shaw method combined with low-temperature demagnetization (LTD-DHT Shaw method), yielding 73 successful results from 18 flows. For the reversed polarity period just prior to the major directional changes, paleointensity shows an oscillation-like variation between 3 and 38 μT corresponding to virtual dipole moments (VDMs) between 0.9 × 1022 and 9.6 × 1022 Am2. For the major directional changes, a weak paleointensity of 5 μT is obtained, which gives a VDM of 1.0 × 1022 Am2. For the normal polarity period, paleointensities are 14–21 μT, giving VDMs of 3.5–5.2 × 1022 Am2. For the reversed polarity period just prior to the major directional changes, a linear relationship with a correlation coefficient of 0.96 is recognized on the diagram of VDM versus virtual geomagnetic pole latitude. This linear relationship may be a precursory feature of the geodynamo at the onset of the M-B transition.

Geomagnetic paleointensity deduced for the last 300 kyr from Unzen Volcano, Japan, and the dipolar nature of the Iceland Basin excursion

We have conducted paleointensity measurements of lavas and pyroclastic flows from Unzen Volcano, Japan. They have been shown in a previous study to contain a record of Brunhes Chron secular variation, with one flow recording a transitional direction (Senbongi geomagnetic excursion; 191±17ka). Various rock magnetic analyses indicate that the main magnetic carriers of samples are Ti-poor titanomagnetites, which have mixtures of single domain (SD) (and/or pseudo-SD) and multi-domain grain sizes. The low-temperature demagnetization (LTD)-double heating test (DHT) Shaw method yielded 209 successful results from 56 sites, giving 27 acceptable site-mean paleointensities for the past 5–300kyr. When excluding the Senbongi excursion data and two other data points, the data give an average virtual dipole moment (VDM) of 7.69×1022Am2 with a standard deviation of 2.51×1022Am2 (N=24). This is consistent with the selected 80 site-mean Thellier paleointensity data from the latest paleointensity database, with an average VDM of 7.64×1022Am2 and a standard deviation of 2.56×1022Am2. Paleointensity measurements on samples that record the Senbongi excursion, which is claimed to correlate with the Iceland Basin excursion, provide a value of 7.74–9.37μT using the LTD-DHT Shaw method. This corresponds to an average (standard deviation) VDM of 2.19×1022Am2 (0.21×1022Am2). When calibrating sedimentary records that capture the Iceland Basin excursion by averaging the LTD-DHT Shaw VDMs obtained from the pre- and post-excursional period, the records give consistent VDM values with the volcanic (Senbongi) record during the excursion. This indicates that variation of the geomagnetic field direction and intensity during the Iceland Basin excursion can be well represented by a simple dipolar variation.

Continuous and stepwise thermal demagnetization: are they equivalent?

SUMMARY Continuous thermal demagnetization, in which measurements of magnetization are made at high temperatureT during heating, is considerably faster than the conventional palaeomagnetic method of stepwise demagnetization, in which measurements are made at room temperature T 0 in a series of cooling-reheating cycles. In the case of single-domain (SD) grains, the two methods give equivalent results after the continuous measurements are converted to equivalent room-temperature values by correcting for the reversible decrease of spontaneous magnetization M S between T 0 and T. To test for equivalence of the two methods in larger pseudo-SD and multidomain grains, three different samples containing magnetite of different grain sizes and origins were heated in zero magnetic field and measurements taken either continuously at T during heating or at T 0 after a set of cooling steps from T. Two samples contained 100–125 μm (mean 110 μm) and 125–150 μm (mean 135 μm) sieve fractions from a crushed natural crystal of magnetite, while the third sample is a natural diabase core sample containing coarse magnetite in the dark minerals and groundmass and fine magnetite in the plagioclase. Two different vibrating-sample magnetometers were used for continuous demagnetization and a mini-furnace and SQUID magnetometer were used for stepwise measurements. Both total thermoremanent magnetization (TRM) and partial TRMs with non-overlapping blocking temperatures (T C, T I) and (T I, T 0) were demagnetized. The Thellier laws of partial TRMs are approximately although not exactly obeyed. In M S(T)-corrected continuous data, there is little overlap of unblocking temperatures: pTRM (T C, T I) demagnetizes almost entirely above T I and pTRM (T I, T 0) almost entirely below T I, demonstrating reciprocity and independence. The stepwise measurements decrease more rapidly in intensity with increasing T than the corrected continuous results, some of which increase slightly with heating up to T ≈ T I. Some additional decay of magnetization must occur during cooling from T, where the continuous measurement is made, to T 0, where the stepwise result is measured. There are no high-temperature measurements of subsidiary cooling-heating cycles to confirm this deduction, but continuously recorded heating-cooling cycles below room temperature in inverse thermal demagnetization (or low-temperature demagnetization) show both reversible and irreversible features, depending on the remanence tested. The most important conclusion of this study is that M S(T)-corrected continuous demagnetization results do not exactly reproduce measured stepwise demagnetization results except for very fine grains, of SD size or close to it. Continuous thermal demagnetization cannot be used in general as a time-saving alternative to stepwise demagnetization if exact equivalence is required.

Late Proterozoic reconstructions of North-West Scotland and Central Canada: Magnetic fabrics, paleomagnetism and tectonics

Red-beds dominate the Stoer Group and the unconformably overlying Torridon Group, having accumulated between ∼1400 and ∼1000 Ma, in a rift of the Hebridean foreland. The sequences were weakly strained and shortened E–W, and each underwent successive diagenetic changes which altered their magnetic properties and caused their different characteristic post-depositional magnetizations. Anisotropy of magnetic susceptibility (AMS) reveals an N–S vertical tectonic foliation superimposed on the bedding-planar AMS sub-fabric. The tectonic AMS sub-fabric was isolated by comparing normalized and non-normalized mean tensors of multiple specimens. The N–S vertical AMS tectonic foliation postdates warps of bedding; thus the high-susceptibility minerals, including those carrying palaeomagnetic signals re-oriented or recrystallized after deposition and diagenesis. Thus characteristic remanence vectors (ChRM) were acquired long after deposition, first in the Stoer Group, then in the Torridon. ChRMs were isolated for 143 Stoer specimens and 94 Torridon specimens using two cycles of low-temperature demagnetization followed by at least 12 steps of thermal demagnetization. For the Stoer Group, two structurally integral groups of sites yield mean magnetizations of 317.0/+43.1 α 95 = 11.3 ( n = 49) and 309.9/52.1 α 95 = 10.6 ( n = 23). Some Stoer specimens bear a Torridon age overprint 133.3/+525 α 95 = 9.6 ( n = 19). The younger Torridon Group yields 138.4/+52.2 α 95 = 7.4 ( n = 42); 134.9/+45.5 α 95 = 16.0 ( n = 15) and 134.4/+55.1 α 95 = 11.3 ( n = 19) from three structurally integral clusters of sites. These late chemical magnetizations postdate bedding warps, faults and tectonic AMS fabrics; they do not warrant local “rigid-body” tilt corrections. However, we restored the ChRM directions for the regional post-Cambrian eastwards tilt which affected the entire foreland from the Lewisian basement to the overlying Proterozoic strata. Paleopoles calculated from the strata's restored vectors were rotated into Laurentian coordinates for comparison with Canadian Proterozoic palaeopoles. The Stoer Group was magnetized in a normal polarity geomagnetic field whereas the Torridon Group was magnetized in a reversed polarity epoch. However, both sequences include at least one polarity transition, indicating long duration chemical magnetizations successfully averaging secular variation. At a minimum, red-bed magnetization processes exceeded 0.1 Ma and may have encompassed several million years. During Stoer Group magnetization, the Hebridean craton was at paleolatitudes of 26–44°N; when the Torridon Group magnetized it was at 23–27°S. Accepting published ages, this defines a minimum southward latitudinal displacement rate of 27–40 km/Ma. Restored Stoer Group paleopoles lie in the present-day north-central Pacific Ocean (∼190E/25N); corresponding to the Laurentian APWP dated at between 1090 and 1109 Ma. Torridon Group paleopoles lie in the south-central Pacific (∼210E/25S) near the poorly documented (<900 Ma) southernmost part of the Laurentian APWP.

Development of the microwave LTD-DHT Shaw method for absolute paleointensity determination

In the last decade, the microwave demagnetization and remagnetization techniques have been successfully incorporated into the Thellier-type method for absolute paleointensity determination. As for the Shaw-type method, similar efforts should be made and thus we have tried to incorporate the microwave techniques into the low temperature demagnetization (LTD)-double heating test (DHT) Shaw method. A series of preliminary experiments have been conducted on (1) four samples with artificial microwave thermoremanent magnetization (T mRM) given in 15.0 μ T, (2) 10 samples with artificial thermoremanent magnetization (TRM) given in 25.0 μ T and (3) 10 samples from the Kilauea 1970 lava. During the experiment, not laboratory TRMs but laboratory T mRMs are imparted by the applications of 80 W of microwave power for 5–10 s using the 14 GHz microwave demagnetizing–remagnetizing system. The ‘microwave LTD-DHT Shaw method’ results were 15.2 ± 0.6 μ T ( N = 4 ), 24.6 ± 1.9 μ T ( N = 5 ) and 39.1 ± 1.0 μ T ( N = 4 ) for the experiments of (1), (2) and (3), respectively. Although the result of (3) is statistically different from the expected field of 35.8 μ T (DGRF 1970), there is a reasonable explanation for this. We conclude that the microwave LTD-DHT Shaw method has been successful, though experimental conditions should be further investigated in future studies in order to increase success rates.

Remagnetization (∼1100 Ma) of Voosges lake tuff (1542 Ma) and other mid-proterozoic rocks, Nipigon graben, northern Ontario

Recently discovered horizons of non-deformed felsic welded crystal tuff in the Proterozoic rift system of northern Ontario yield a geologically stable paleomagnetic vector. Using alternating field (AF), thermal (TD) and low-temperature demagnetization (LTD) techniques, a single stable characteristic remanence (ChRM) vector is found in all specimens (declination/inclination = 115°/−73.5°; α 95 = 2.1°, n = 58). It is overprinted by a Bruhnes’ epoch VRM. The tuffs paleopole position coincides with that of ubiquitous sub-horizontal diabase sheets which intruded at ∼1110 Ma during Late Proterozoic Keewanawan Volcanism. The latter produced numerous volcanic and intrusive rocks along the margins of the Lake Superior paleo-rift system, during the interval 1087–1120 Ma, but the sills are most extensive. Although the Voosges Lake Tuff shares the same paleopole location as the diabase sheets (1109 Ma), the nearby Seagull pluton (1112 Ma) and the Keweenawan volcanic rocks, its primary zircons yielded a Mesoproterozoic crystallization age of 1542 ± 3 Ma. This is similar to the nearby English Bay felsic intrusions dated at 1537 − 2 + 10 Ma [Davis, D.W., Sutcliffe, R.H., 1985. U-Pb ages from the Nipigon “Plate” (sic) and northern Lake Superior. Geol. Soc. Am. Bull. 96, 1572–1579]. Whereas the paleomagnetic magnetic signal appears primary in the 1542 Ma Voosges Tuff, it was completely reset by the Late Proterozoic Keewanawan paleofield (∼1100 Ma). The Tuff units are separated from the diabase sills by stratigraphic thicknesses of ≤50 m and show no traditional petrographic evidence of annealing or thermal metamorphism. Since it is improbable that the 1542 Ma tuff returned to the same paleolatitude at 1109 Ma, we postulate that metasomatic processes caused a ∼1109 Ma remagnetization. A similar event could explain unusual ChRMs reported for certain other pre-Keweenawan Proterozoic sites with adjacent Keewanawan diabase sheets. For example, some paleopoles reported for the Gunflint Formation (1878 Ma) and Sibley Formation (>1339 Ma, perhaps ∼1537 Ma by association with the English Bay intrusions) might have been acquired due to secondary remagnetization.

Low-temperature partial magnetic self-reversal in marine sediments by magnetostatic interaction of titanomagnetite and titanohematite intergrowths

With various low-temperature experiments performed on magnetic mineral extracts of marine sedimentary deposits from the Argentine continental slope near the Rio de la Plata estuary, a so far unreported style of partial magnetic self-reversal has been detected. In these sediments the sulphate-methane transition (SMT) zone is situated at depths between 4 and 8 m, where reductive diagenesis severely alters the magnetic mineral assemblage. Throughout the sediment column magnetite and ilmenite are present together with titanomagnetite and titanohematite of varying compositions. In the SMT zone (titano-)magnetite only occurs as inclusions in a siliceous matrix and as intergrowths with lamellar ilmenite and titanium-rich titanohematite, originating from high temperature deuteric oxidation within the volcanic host rocks. These abundant structures were visualized by scanning electron microscopy and analysed by energy dispersive spectroscopy. Warming of field-cooled and zero-field-cooled low-temperature saturation remanence displays magnetic phase transitions of titanium-rich titanohematite below 50 K and the Verwey transition of magnetite. A prominent irreversible decline characterizes zero-field cooling of room temperature saturation remanence. It typically sets out at ∼210 K and is most clearly developed in the lower part of the SMT zone, where low-temperature hysteresis measurements identified ∼210 K as the blocking temperature range of a titanohematite phase with a Curie temperature of around 240 K. The mechanism responsible for the marked loss of remanence is, therefore, sought in partial magnetic self-reversal by magnetostatic interaction of (titano-)magnetite and titanohematite. When titanohematite becomes ferrimagnetic upon cooling, its spontaneous magnetic moments order antiparallel to the (titano-)magnetite remanence causing an drastic initial decrease of global magnetization. The loss of remanence during subsequent further cooling appears to result from two combined effects (1) magnetic interaction between the two phases by which the (titano-)magnetite domain structure is substantially modified and (2) low-temperature demagnetization of (titano-)magnetite due to decreasing magnetocrystalline anisotropy. The depletion of titanomagnetite and superior preservation of titanohematite is characteristic for strongly reducing sedimentary environments. Typical residuals of magnetic mineral assemblages derived from basaltic volcanics will be intergrowths of titanohematite lamellae with titanomagnetite relics. Low-temperature remanence cycling is, therefore, proposed as a diagnostic method to magnetically characterize such alteration (palaeo-)environments. © 2007 The Authors Journal compilation © 2007 RAS.

LTD‐Thellier paleointensity of 1.2 Ga Nova Floresta mafic rocks (Amazon craton)

Absolute paleointensity at 1.2 Ga was obtained on basalts and gabbros from Nova Floresta Formation (Amazon craton, Brazil). Magnetic carriers in these rocks are single‐domain to multi‐domain pure (to nearly pure) magnetite. They present a strong decrease in remanence after low‐temperature demagnetization (LTD) accompanied by an increase in median destructive fields. Incorporation of LTD to Thellier double‐heating measurements improved paleointensity estimates and enabled paleofield recovery for otherwise sterile rock samples, giving twenty seven successful determinations for six sites (45% success rate). Site‐mean paleofield values are within 4.90 μT and 15.55 μT (mean at 8.37 ± 4.07 μT). These results correspond to a mean VDM of 2.08 ± 0.94 × 1022 Am2, thus increasing the number of low‐field determinations that dominate the (still scarce) Precambrian database. Our study exemplifies the need for alternative approaches in the study of ancient rocks.

Inverse thermoremanent magnetization

Inverse thermoremanent magnetization (ITRM) is reversed to the thermoremanent magnetization (TRM) process: ITRM results from warming from low temperature T in a magnetic field, while TRM results from field cooling from high T. The development of ITRM was studied in magnetites of grain sizes from submicron to 135 μm, in pyrrhotites and in hematite crystals. All three minerals acquired ITRM after warming through their magnetic transitions (35 K for pyrrhotite, 120 and 130 K for magnetite, 250 K for hematite). However, when an impacting meteorite's cold interior warms to ambient T in the geomagnetic field, magnetite is the most likely candidate for acquiring ITRM. The magnetite ITRM blocking temperature distribution was determined from 12 neighboring partial ITRMs in nested field‐on warming plus field‐off cooling cycles (300–20 K). The largest partial ITRMs are produced in T intervals around magnetite's Verwey transition (TV = 110–120 K) and isotropic point (TK = 130 K). Both transitions involve large changes in crystalline anisotropy and renucleation of magnetic domains. ITRM is blocked when initially broad domain walls narrow and are pinned by dislocations. ITRM has contrasting properties to TRM, which is mainly due to blocked single‐domain moments. ITRM is strongest for 3‐ to 20‐μm grains, whereas TRM peaks for submicron magnetites. Only 10–20% of ITRM survives low‐temperature demagnetization (LTD) at 77 K or AF demagnetization to 10–15 mT, compared to 30–90% for TRM. ITRM decreases quasi‐linearly with T in thermal demagnetization. The median unblocking temperature TUB is ≈300°C and 20–25% survives at 550°C. The low‐TUB part of ITRM could mimic extraterrestrial NRM of low TUB, cited as evidence of negligible heating of meteorites in their transfer to Earth. The high‐TUB ITRM would contaminate paleointensity determinations up to the highest T steps. The best cure for ITRM contamination is AF or LTD pretreatment.

Testing for the presence of magnetite in the upper-beak skin of homing pigeons

We carried out magnetic and nonmagnetic experiments on fresh, upper-beak skin tissue samples isolated from six pairs of homing pigeons to test whether the tissue contains magnetite particles. Results of (1) room-temperature isothermal remanent magnetization (IRM) acquisition and alternating field (AF) demagnetization, (2) low-temperature demagnetization of saturation IRM acquired at 5 K in a field of 5 tesla (T) (SIRM(5 K)) after zero-field cooled (ZFC) and field cooled (FC) treatments, and (3) cycling of the saturation IRM acquired at 300 K in a field of 5 T (SIRM(300 K)) between 5 and 300 K, indicate the presence of magnetite in the measured samples. A significant loss of SIRM(5 K) below 20 K suggests the dominance of superparamagnetic (SPM) particles. The SIRM acquisition capacity of the female pigeon is stronger than that of the male pigeon in all four measured pairs, suggesting for the first time that the magnetite concentration is probably sex dependent. Light microscopic observation on the histological sections stained with Prussian Blue detected the presence of some tiny, dotted, dark-blue staining Fe3+ aggregates (size 1-4 microm) located directly beneath the subcutis within strands of connective tissue, nearby the rim of the regions full of red nuclei. The results of this study support the idea that homing pigeons may have a magnetite-based receptor, which potentially could be used for sensing the Earth's magnetic field during navigation.

Magnetic studies of magma-supply and sea-floor metamorphism: Troodos ophiolite dikes

Dikes of the eastern Troodos ophiolite of Cyprus intruded at slow ocean-spreading axes with dips ranging up to 15° from vertical and with bimodal strikes (now NE–SW and N–S due to post-88 Ma sinistral microplate rotation). Varied dike orientations may represent local stress fields during dike-crack propagation but do not influence the spatial-distributions or orientation-distributions of dikes' magnetic fabrics, nor of their palaeomagnetic signals. Anisotropy of magnetic susceptibility (AMS) integrates mineral orientation-distributions from each of 1289 specimens sampled from dikes at 356 sites over ∼400 km 2 in the eastern Troodos ophiolite of Cyprus. In 90% of dikes, AMS fabrics define a foliation ( k MAX– k INT) parallel to dike walls and a lineation ( k MAX) that varies regionally and systematically. Magma-flow alignment of accessory magnetite controls the AMS with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea-floor metamorphism. Titanomagnetite has less influence on anisotropy. Occasionally, intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility ( k MAX) still defines the magmatic flow axis. Such blended subfabrics of kinematically compatible mafic-silicate and misaligned multidomain magnetite subfabrics; are rare. Areas of steep magma flow ( k MAX plunge ≥ 70°) and of shallow magma-flow alternate in a systematic and gradual spatial pattern. Foci of steep flow were spaced ∼4 km parallel to the spreading axes and ∼6 km perpendicular to the spreading axes. Ridge-parallel separation of steep flow suggest the spacing of magma-feeders to the dikes whereas ridge-perpendicular spacing of 6 km at a spreading rate of 50 mm/a implies the magma sources may have been active for ∼240 Ka. The magma feeders feeding dikes may have been ≤ 2 km in diameter. Stable paleomagnetic vectors, in some cases verified by reversal tests, are retained by magnetite and titanomagnetite. In all specimens, the stable components were isolated by three cycles of low-temperature demagnetization (LTD) followed by ≥ 10 steps of incremental thermal demagnetization (TD). 47% of primary A-components [338.2 /+ 57.2 n = 207, α 95 = 3.9; mean T UB = 397 ± 8 °C] are overprinted by a B-component [341.4 /+ 63.5, n = 96, α 95 = 8.7; mean T UB = 182 ± 11 °C]. A- and B-components are ubiquitous and shared equally by the N–S and NE–SW striking dikes. A-component unblocking temperatures ( T UB) are zoned subparallel to the fossil spreading axis. Their spatial pattern is consistent with chemical remagnetization at some certain off-axis distance determined by sea-floor spreading. A-components indicate less microplate rotation and more northerly palaeolatitudes that are consistent with metamorphic remagnetization after some spreading from the ridge-axis. Thus, their magnetizations are younger than those of the overlying volcanic sequence for which ChRMs are commonly reported as ∼274 /+ 33 (88 Ma).

Proterozoic paleomagnetism in the Nipigon Embayment of northern Ontario: Pillar Lake Lava, Waweig Troctolite and Gunflint Formation tuffs

By using unusual combinations of demagnetization techniques, Proterozoic paleomagnetic vectors and paleopoles are provided for two recently discovered post-tectonic Proterozoic units near Armstrong, northern Ontario, and also for well-dated Gunflint Formation, which by previous techniques yielded problematical paleomagnetic data. The first paleomagnetic data are provided also for the Seagull Pluton and Inspiration Sills. Characteristic remanent magnetizations (ChRM) for the Pillar Lake Lavas indicate a Keweenawan age, more specifically ∼1000–1040 Ma by comparison with the well-established APWP for the Late Proterozoic Superior craton. Four combinations of demagnetization techniques yield declinations in the range 108–133° and inclinations in the range −65 to −70° ( n = 100), which define paleopoles near 200 W/48 N corresponding to a location on the Keweenawan APWP near ∼1040 Ma. In the underlying basement a recently discovered Proterozoic igneous complex, the Waweig Troctolitic Complex, yields new paleomagnetic data with declination and inclination 42/−54 ( n = 14) defining a paleopole at 238 W/09 N. Its ages may be 1400–1600 or ∼2000 Ma by comparison with the presently available, ambiguous and sparsely populated APWP. The first paleomagnetic results for the Seagull Pluton (U–Pb age 1113 Ma) yield a mean declination of 87.4/−75.7 ( n = 32) corresponding to a Keweenawan paleopole near 233/42 N, consistent with other paleopoles near ∼1200 Ma. Tuffs of the oft studied but problematical Gunflint Formation (U–Pb age1878 Ma) yielded stable and presumably primary vectors using several different demagnetization techniques on the same specimens. Their mean primary declination and inclination ∼303/+48.8 ( n = 17) yields a paleopole now located near 178 W/42 N, comparable with the published locations of paleopoles of ∼2000 Ma. Of broader interest, we recognized that low temperature demagnetization preceding conventional demagnetization techniques enhanced the isolation of characteristic vectors. Combining the conventional techniques (thermal and AF demagnetization) also improved the resolution of characteristic vectors not achieved by other means. Low grade metamorphism affected the non-tectonized Proterozoic cover to the Canadian shield, due to burial or hydrothermal effects, obfuscating or erasing primary vectors in some lithologies and especially at certain sites.

Enhancement of exchange coupling and hard magnetic properties in nanocomposites by magnetic annealing

An enhancement of the exchange coupling and hard magnetic properties of melt-spun Nd 2Fe 14B/α-Fe-type nanocomposites was achieved by optimization of their nanostructured morphology via magnetic annealing. Compared with the Nd 2.4Pr 5.6Dy 1Fe 84Mo 1B 6 sample annealed without a magnetic field, the magnetic annealing results in a noticeable improvement in the coercivity i H c, the remanence 4πM r, and energy product (BH) max. (BH) max at 50 K was enhanced by 43.7% after magnetic annealing in a field of 19 T. The kink in the demagnetization curve disappeared and, additionally, a much better squareness of the demagnetization curves was observed in the magnetically annealed samples. The intergrain exchange coupling was evaluated by Henkel plots. Evidence for stronger intergrain exchange coupling was found in the magnetically annealed sample, which is due to nanostructure refinement promoted by the magnetic annealing. The nanostructure refinement was confirmed by X-ray diffraction, transmission electron microscope, low-temperature demagnetization curves and a modified Brown’s equation analysis. The improvement of hard magnetic properties of the magnetically annealed sample results mainly from the magnetic-field-induced exchange-coupling enhancement.

Geomagnetic field intensity during the last 5 Myr: LTD-DHT Shaw palaeointensities from volcanic rocks of the Society Islands, French Polynesia

SUMMARY On the basis of the Thellier palaeointensities reported so far it has been thought that the time-averaged virtual dipole moment (VDM) and virtual axial dipole moment (VADM) for the last few million years are almost the same as the present geomagnetic dipole moment (∼8 × 1022 A m2). This estimate has been called into question, however, because recent studies have revealed that the Thellier method occasionally overestimates palaeointensities by as much as twice the true values. In contrast, a recently developed palaeointensity technique, namely the double heating technique of the Shaw method combined with low-temperature demagnetization (the LTD-DHT Shaw method), can yield expected field intensities from samples which give unreliable palaeointensities using the Thellier method. Therefore, we have measured absolute palaeointensities from 0.5–4.6 Ma volcanic rocks from the Society Islands, French Polynesia, mainly using the LTD-DHT Shaw method. As a result, 195 out of 361 samples passed the selection criteria, some of which are compared with additional results obtained with Coe's version of the Thellier method. In the Thellier experiments, 18 out of 40 samples passed the criteria, giving palaeointensities both consistent and inconsistent with the LTD-DHT Shaw results. These samples are characterized mostly by two-segmented Arai diagrams. If we take the LTD-DHT Shaw palaeointensities, 24 reliable site means are available and give a mean VADM of (3.64 ± 2.10) × 1022 A m2. This is nearly half of the mean of the 0–5 Ma Thellier data selected from the latest palaeointensity database [(7.46 ± 3.10) × 1022 A m2, N= 458] as well as the present dipole moment. The LTD-DHT Shaw palaeointensities newly determined in this study suggest that the present-day field is so strong that it may not be typical of the past geomagnetic field.

地磁気逆転開始期の地球磁場変動

We report the paleomagnetic behaviors at the beginning of the latest geomagnetic polarity reversal (Brunhes-Matuyama reversal of 0.78 Ma age) recorded in Tahitian lava flows. We applied an improved paleointensity determination method (double heating technique of the Shaw method with low-temperature demagnetization, LTD-DHT Shaw method), and detected oscillation-like changes in virtual dipole moments (VDMs) with a large amplitude ranging from 1.5 × 1022 to 10.9 × 1022 AM2 just prior to the directional reversal. The VDM changes are almost linearly correlated with the virtual geomagnetic pole (VGP) latitudes (60-90°S). We call this VDM-VGP latitude trend “reversal line”. Although the Thellier data reported for the Brunhes-Matuyama reversal are more scattered than our data, they also show a weak trend in the VDM-VGP latitude diagram, supporting the reversal line from a different locality. The scatter in the Thellier data may be caused by overestimated paleointensities due to methodological problems which are indicated in the recent studies of historical lavas. Therefore it appears that the reversal line from the Tahitian lavas in this study does not show a local change but represents a global-scale phenomenon in the geomagnetic reversal process.

Multi-cycle low-temperature demagnetization (LTD) of multidomain Fe3O4 (magnetite)

An improved multi-cycle low-temperature demagnetization (MC-LTD) method for multidomain (MD) Fe3O4 (magnetite) by repeating LTD cycles between any two temperatures below 300K is proposed to provide more rigorous constraints on the utilization of the LTD mechanism. MC-LTD was performed for four temperature intervals, 300–150K, 150–125K, (cycling through the isotropic point Tk∼130K, where the first magnetocrystalline anisotropy constant K1 changes sign), 125–95K (cycling through the Verwey transition Tv∼120K, where crystal symmetry changes from cubic to monoclinic) and 95–60K. Results show that the remanence loss during MC-LTD depends on the number of LTD cycles. Above Tv, demagnetization of remanence after MC-LTD is caused by the gradual reorganization of magnetic domains. For the 95–125K interval, the remanence loss results from the changes of easy axes between [001] of the cubic phase above Tv and the c-axis of the monoclinic phase below Tv. These two processes approximately follow the Boltzmann-analog relation, but the latter involves more degree of freedom. Below Tv, thermal equilibrium processes control the minor decay (less than 0.5% of the initial remanence intensity) of remanences.

Alternating field demagnetization, single‐domain‐like memory, and the Lowrie‐Fuller test of multidomain magnetite grains (0.6–356 μm)

A fundamental question in paleomagnetism is how magnetite grains much larger than single‐domain size preserve stable remanence over millions of years. In an effort to answer this question we measured alternating field (AF) demagnetization of thermoremanent magnetization (TRM) and saturation isothermal remanent magnetization (SIRM) before and after low‐temperature demagnetization (LTD). LTD (zero‐field cycling through 120 K) unpins or nucleates domain walls, reducing the remanence of multidomain grains. We used two sets of sized crushed magnetites (0.6, 1, 3, 6, 9, 14, 20, 110, and 135 μm), one set unannealed and the other annealed, and a set of hydrothermally grown magnetites (0.8, 6.3, 25, 64, 94, and 356 μm). For all sizes, TRM and SIRM memories after LTD are much harder to AF demagnetize than the original remanences. AF demagnetization curves after LTD are flat for the first ∼10 mT. Such initial plateaus are one hallmark of single‐domain behavior. In high‐stress unannealed grains, after‐LTD response depends on grain size, larger grains demagnetizing more easily than smaller ones. In low‐stress annealed and hydrothermal magnetites, after‐LTD response is almost independent of grain size over nearly 3 decades in grain size. This size‐independent behavior could be due to grains in metastable single‐domain states in the smaller‐sized samples, but in &gt;100 μm grains, there must be a source of single‐domain‐like AF behavior within the multidomain grains themselves. We propose an ad hoc model in which LTD triggers domain wall unpinning and nucleation events up to a coercivity threshold, producing the observed initial plateaus in AF demagnetization curves of TRM and SIRM memories. The size‐independent demagnetization behavior of memory in hydrothermal and annealed magnetites is ascribed to nucleation events above the threshold level, and the additional size‐dependent AF demagnetization of memory in high‐stress unannealed grains is explained by wall unpinning from strong stress centers. In both cases the AF properties merely mimic single‐domain behavior. Although LTD memory has single‐domain‐like AF curves and erased remanence has multidomain‐like curves in grains of all sizes, the Lowrie and Fuller [1971] test still works for the annealed samples. For memory, erased fraction and pre‐LTD remanence alike, TRM is more stable than SIRM in fine grains (1 μm), less stable in large grains (135 μm), and of comparable stability in medium‐size grains (9 and 20 μm).