Natural Remanent Magnetization Directions Research Articles

Paleomagnetism of Tarkhanian sediments of the section Skelya (Kerch Peninsula)

With the goal to correlate the magnetostratigraphic data of the Eastern Paratethys obtained by researchers in different years a rockmagnetic and paleomagnetic investigations of the Tarkhanian sediments of the Skelya section was carried out. The main carrier of the natural remanent magnetization is titanomagnetite in the Argunian beds of the Tarkhanian regional stage; in addition to titanomagnetite there is iron sulfide (monoclinic pyrrhotite and/or greigite) in the deposits of the Terskian and Kuvinian beds. The paleomagnetic fold test of natural remanent magnetization directions, recorded in the rocks of the Argunian beds, is positive, which indicates a pre-folding age of the magnetization formation. A comparison with the magnetochronological scale GTS 2020 shows that the normal polarity interval in the lower part of the Argunian beds can presumably be compared with the С5Bn chron. The reversal polarity interval in the upper part of the Argunian beds is with C5ADr. The age of the С5Bn/C5ADr boundary can be estimated as ~14.7 Ma.

Volcanic Record of the Last Geomagnetic Reversal in a Lava Flow Sequence From the Azores

We present paleomagnetic and geochronological data of twenty-five stacked lava flows from São Miguel Island (Azores) that were emplaced during the last geomagnetic reversal (Matuyama-Brunhes, M-B). The characteristic direction of natural remanent magnetization was isolated through alternating field and/or thermal demagnetization. From bottom to top, the directional changes display a first phase with reverse, transitional and normal directions which is followed by a second phase with transitional and normal directions. Dating by Potassium-Argon (K-Ar) of fresh separated groundmass provide ages of 768 ± 5 ka and 765 ± 5 ka (1σ) for the first and second transitional episodes, respectively. We infer that this sequence recorded the last reversal and reveal the presence of a rebound that occurred soon after the transition. The age of the transition is 5-15 kyr younger than at other volcanic localities, but in agreement with astronomical ages that were recently proposed for the transitional interval. The reversal is characterized by weak field intensity during the transition, a recovery phase and a subsequent decrease during the rebound. The two transitional virtual geomagnetic poles (VGPs) that characterize the transition are found above the South Atlantic and the North Pacific. When compared to other M-B records, no VGP is found above preferred locations.

Magnetic assessment of OSL and radiocarbon ages of sediments beneath a lava in Jeju Island, Korea: Implication of possible resetting of OSL signals and age constraint of the late Quaternary lava

Abstract Better chronological constraining of Late Pleistocene-Holocene volcanic eruptions is essential for figuring out spatio-temporal frequency distribution of ancient volcanism and assessing volcanic eruptions probability in a volcanic field. For this chronological frame work, radiocarbon (14C) and luminescence dating methods are being attempted alternatively, instead of K-Ar and 40Ar/39Ar techniques with lower resolutions and age limits of applicability. For the sediments capped by a lava flow in a Quaternary basaltic volcanic field of Jeju Island, Korea, we here introduce an interesting and probably frequently-encountered case of the discrepancy in age estimates between 14C and optically stimulated luminescence (OSL) chronologies. For samples with different positions in the sediments, the 14C ages of organic material extracts have a stratigraphically consistent but widely dispersed values between 2.25 ± 0.09 and 27.2 ± 0.5 calibrated (cal.) kyr BP, whereas the OSL ages of quartz extracts have a narrow range between 13.4 ± 1.3 and 16.1 ± 0.7 ka. Multiple magnetic parameters including Koenigsberger ratio, ratio of natural remanent magnetization (NRM) to saturation isothermal remanent magnetization intensities, and similarity in NRM direction compared with paleomagnetic direction for the upper lava comprehensively infer the paleo-heating of the sediments by the lava. Further, paleo-heating temperatures estimation by two independent magnetic methods, based on progressive thermal demagnetizations of NRM and repeated magnetic susceptibility versus temperature measurements with increasing peak temperatures, undoubtedly reveals that the sample positions where the OSL ages were yielded had paleo-temperatures between 200 and 250 °C and much higher than 550 °C, which are sufficiently high to reset their luminescence signals. Therefore, we interpret that a weighted mean OSL age of 14.8 ± 0.7 ka is the most probable age for the lava. Accordingly, the 14C ages older than ∼15.5 cal kyr BP are interpreted to be the deposition ages for their horizons, whereas the three exceptionally young 14C ages (∼2.3–∼7.7 cal kyr BP) for the upper sediments were contaminated by modern carbon. We suggest that magnetic applications, when combined with 14C and luminescence dating techniques, can help greatly improve the reliance of chronologies for late Quaternary volcanism of the world.

Effect of magnetic anisotropy on the natural remanent magnetization in the MCU IVe' layer of the Bjerkreim Sokndal Layered Intrusion, Rogaland, Southern Norway

AbstractA strong negative magnetic anomaly, caused by an intense natural remanent magnetization (NRM) approximately 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 toward 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 changes for different locations along the layer, and it appears that the NRM is tilted away from the mean paleofield and toward 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.

Thellier-type paleointensity data from multidomain specimens

Identifying and excluding inaccurate paleointensity estimates that can arise from the effects of multidomain (MD) particles remains a key challenge for paleointensity studies. Here we approach this problem by incorporating realistic experimental noise into a phenomenological MD model, systematically exploring the parameter space of paleointensity experiments (the choice of protocol, laboratory field strengths and directions), applying optimization algorithms to the model predictions, and testing these against experimental datasets. The results allow us to conclude that: (1) In some cases, applying the lab field in a direction ≲60° from the NRM direction and with a magnitude ∼2 times larger than the ancient field may help to produce more linear and accurate Arai plots by allowing partial thermoremanent magnetization (pTRM) tails to compensate from excess natural remanent magnetization (NRM) loss, the mechanism behind typical MD Arai plot curvature. (2) The recently proposed FRAC criterion is consistently better at isolating accurate results than the traditionally used f parameter and should ideally have minimum values of 0.65, 0.45, 0.65, and 0.55 for the Thellier, Coe, Aitken, and IZZI protocols, respectively. (3) Newly developed sets of selection criteria (MC-CRIT) should maximize the likelihood of accepting accurate estimates from a suite of specimens influenced by MD behavior. The models, methods, and data presented here provide the means to objectively define data selection and demonstrate the effectiveness of a given set of criteria. Such approaches are vital, if we wish to remove the arbitrariness that hinders the identification of reliable paleointensity data.

Petro- and paleomagnetism of the Malyi Kut Lower Neo-Pleistocene marine deposits (Krasnodar Krai)

Petro- and paleomagnetic studies of the Quaternary marine deposits in the Malyi Kut section (the Krasnodar Krai, the western Taman Peninsula) were performed in the 425-375 ka time interval. A complex of rock magnetism methods was used to determine the composition, domain structure, and concentration of ferromagnetic fraction grains that are present in the studied rocks. The directions and reliability of the distinguished directions of natural remanent magnetization were studied. A horizon with an anomalous magnetization behavior (∼395-375 ka) was detected at the end of the Lower Pleistocene. Such a magnetization behavior was observed at the top of the transgressive deposits in the Urundzhik horizon of the Karadzha section (oxygen isotope stage 11) and was assigned to ∼400-375 ka. The Biwa-III excursion corresponds to this interval of an anomalous magnetization behavior.

New Paleomagnetic results and evidence for a geomagnetic field excursion during the pleistocene-holocene transition at Pichincha province, Ecuador

Paleomagnetic data from three sedimentary sections in Pichincha province -Quito City (QC), Mullimica (Mu) and El Tingo (ET)- Ecuador (northwestern South America) are reported. Analysis of natural remanent magnetization directions obtained from 109 oriented samples taken at 4 sites, shows that some samples recorded a magnetic component different from the normal present geomagnetic field (GMF). The characteristic remanent magnetization (ChRM) was determined by progressive AF demagnetization. The analysis shows that the sections recorded ChRM of normal, intermediate and reverse polarities during the Pleistocene-Holocene transition and Holocene. Normal directions were recorded in QC, while normal and intermediate polarity directions at Mu and, reverse VGPs at ET. QC and the upper portion of Mu correspond to the paleosecular variation Holocene record for Ecuador during the ∼≤4.7 ka BP. On the other hand, the lower portion of Mu logs represents the transition from normal to intermediate directions occurring at ∼≥5.6 ka BP. Sites from ET recorded two stable oblique reverse records with a large fluctuation far from the present GMF at ∼10.5 ka BP. The transitional virtual geomagnetic poles generally agree with those registered during the possible Pleistocene-Holocene excursion observed in other places of the planet. When plotted in a present world map, VGPs calculated from normal samples at QC are very well clustered in Northern North America, Greenland and Northern Europe; most VGP's calculated from Mu are situated between 30° and 60° northern latitude in Northern North America, Greenland, western Europe, Africa and North Pacific Ocean. Interestingly, the majority of the reverse directions from ET conforms a patch located in southern Africa, and a few ones are situated in central Africa, eastern Australia and Antarctica. An Ecuadorian paleopole was calculated with data resulting from QC and Mu. Also other paleopoles of the same age were processed from other North and South American sites. Remarkably they agree well, although they do not agree with the geographical pole showing ∼15o angular difference in relation to the rotation's axis of the Earth. Finally, is discussed the hypothesis of the global excursional state of the GMF during the last ∼11.0 ka BP and the potential use as dating tool the excursion dated at 10.5 ka BP.

Remanent magnetization in fresh xenoliths derived from combined demagnetization experiments: Magnetic mineralogy, origin and implications for mantle sources of magnetic anomalies

Abstract Fresh mantle xenoliths represent an exceptional opportunity to directly access Earth's interior. In particular, magnetic signals carried by upper mantle xenoliths may provide the only opportunity to determine the depth of remanent magnetization below the Moho boundary. In recent years, the nature, intensity and magnetic properties of the magnetic signal have been characterized by measurements of the 0020 induced magnetization in hysteresis loops. The natural remanent magnetization (NRM) intensity, despite providing the first indication of the magnetic nature of magnetite inclusions, has not been studied in detail. This study focusses attention on the number of recorded NRM directions and characteristics of the anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM) demagnetization spectra. A collection of 17 extremely fresh mantle xenoliths has been subjected to AF demagnetization, characterizing samples with one, two or three populations of remanence carriers, with differing coercivities, that are potential carriers of magnetization at mantle depths.

A review of the studies results of paleomagnetic applicability of Precambrian crystalline rocks of the Ukrainian Shield.

The review of the data has been conducted of the long-term laboratory methods of studies of stability and origin of natural remanent magnetization (NRM) of Precambrian crystalline rocks of the Ukrainian Shield (USh). Generalization of the data gives weighty arguments for considering the Precambrian rocks of the Ukrainian Shield as promising objects for paleomagnetic studies. Statistical analysis has been made as to directions of NRM in exposures different by their age, composition and origin of crystalline rocks of the USh. It has been found that everywhere within the limits of the Ukrainian Shield in the exposures with magnetically stabile single-component magnetization of thermoremanent origin (according to laboratory and mineralogical criteria) NRM directions in contemporary coordinate system are distributed heterogeneously and are characterized by considerable dispersion. The most probable source of dispersion of NRM vectors within the limits of considered exposures of crystalline rocks of the USh is local tectonics, which produced reorientation within the area of the surface of initial magnetization of rocks.

Paleomagnetic studies of late pleistocene rocks from the Malyi Kut section (Taman Peninsula, Krasnodar krai)

Paleomagnetic studies of Quaternary deposits from the Malyi Kut rock sequence (Krasnodar krai, western part of the Taman Peninsula) in two time intervals of Late Pleistocene are carried out. The Malyi Kut sequence is a marine terrace of the Baku age, which is embedded in the disturbed marine Sarmatian deposits. The terrace of the Bakinian age nests the Karangat marine terrace. The presence of the marine molluscan fauna in the both terraces enables reliable dating of the studied deposits. The composition, grain size, and concentration of ferromagnetic fraction present in the studied rocks are investigated by a set of rock magnetic methods. The directions of natural remanent magnetization (NRM) are studied, and the reliability of their isolation is estimated. The results are compared with the paleomagnetic records of NRM in the rocks of the parallel coeval sections of the Tuzla (Taman Peninsula) and Roxolany (Ukraine). This correlation suggests that the studied rock sequences recorded the Mono Lake geomagnetic excursion.

Paleomagnetic record in the Late Pleistocene loess-soil deposits of the Pekla section in the time interval 425–50 Ka

The composition, granulometry data, and concentration of grains that carry the natural remanent magnetization (NRM) are studied in the bottom 6.5 meters of the loess-soil deposits of the Pekla section (Azov coast, Krasnodar region). It was shown that these strata, which correspond to the 9th–11th marine oxygen isotope stages (MIS) and cover the time interval ∼425–300 ka, are suitable for further paleomagnetic investigation. The deposits in the upper portion of the Inzhavino paleosoils (Likhvin Interstadial) contain the records of anomalous deviations of the direction of magnetization from the dipole field at the sampling site. The studied interval was sampled by taking two hand blocks from four sampling levels, which minimizes the errors due to the specimen cutting. This anomaly dated ∼300 ka possibly corresponds to the Biva-II geomagnetic excursion. However, the studies of implications of anisotropy in magnetic susceptibility (AMS) for the direction of natural remanent magnetization (NMR) have shown that parts of the samples from the Inzhavino paleosoils and the underlying loess horizon are magnetically anisotropic, which is characteristic for biogenic magnetite grains, while other parts of the samples exhibit plane anisotropy typical for natural sedimentary structures. A weak correlation between the time series of averaged curves of relative paleointensity, NRM20/ARM20 (and NRM20/K) for the loess horizons of the Pekla section and the global composite reference curve of relative paleointensity, Sint-800, in the time intervals 200–130 ka and 370–320 ka indicates that the paleomagnetic records have been imprinted not only on the detritic magnetic grains but also on the grains produced by chemical reactions and the life processes of bacteria.

Testing Miocene Remagnetization of Bey Dağları: Timing and Amount of Neogene Rotations in SW Turkey

Here we reassess the timing and amount of rotation of the eastern limb of the Aegean orocline, located in SW Turkey. The current model for this orocline involves a 25-30° counterclockwise (CCW) rotation of the Bey Dağları region, which exposes upper Cretaceous to Eocene platform carbonates and lower Miocene flysch deposits. In this model the rotation has to postdate a Miocene remagnetization, which would not exclude non-synchroneity between rotations of the western and eastern limbs of the Aegean orocline. To test this model a detailed palaeomagnetic study was conducted on lower Miocene strata in the Bey Dağları area. Two (composite) sections were sampled near Korkuteli and Doğantaş, spanning the lower Miocene foreland basin stratigraphy from the Aquitanian unconformity with the Bey Dağları limestones to the uppermost Burdigalian-lowermost Langhian, and a wide array of palaeomagnetic analyses were obtained. The natural remanent magnetization (NRM) in the Aquitanian limestones and Burdigalian blue clays resides dominantly in magnetite with minor greigite and haematite. A positive reversals test, a correlation of obtained polarity patterns to the geomagnetic polarity timescale, showing significant compaction following acquisition of the NRM, a scatter in NRM directions that can be confidently attributed to palaeosecular variation and a positive recently-proposed end-member model test of the acquisition curves of isothermal remanent magnetization (IRM) all show that the Miocene sediments on the Bey Dağları platform have not been remagnetized. Our new results, combined with a partial reassessment of existing data, imply that the Bey Dağları region underwent no rotation between the late Cretaceous and late Burdigalian, and 20° counterclockwise rotation between 16 and 5 Ma, i.e. during the middle to late Miocene and prior to deposition of previously reported non-rotated Pliocene sediments north of Antalya. The two limbs of the Aegean orocline thus rotated simultaneously: our new age constraints of 16-5 Ma in the east, compare with (largely) 15-8 Ma in the west, as published previously. The rotation of the Bey Dağları was probably bounded in the south at the plate boundary with Africa, and in the east by the Aksu thrust and Kırkkavak dextral strike-slip fault, which together partitioned dextral transpression induced by the rotating block. Accommodation of the western Anatolian rotations to the north and west of the Bey Dağları will be the subject of a future publication.

Petrophysical and paleomagnetic data of drill cores from the Bosumtwi impact structure, Ghana

Abstract— Physical properties from rocks of the Bosumtwi impact structure, Ghana, Central Africa, are essential to understand the formation of the relatively young (1.07 Ma) and small (10.5 km) impact crater and to improve its geophysical modeling. Results of our petrophysical studies of deep drill cores LB‐07A and LB‐08A reveal distinct lithological patterns but no depth dependence. The most conspicuous difference between impactites and target lithologies are the lower bulk densities and significantly higher porosities of the suevite and lithic breccia units compared to meta‐graywacke and metapelites of target lithologies. Magnetic susceptibility shows mostly paramagnetic values (200–500 × 10−6 SI) throughout the core, with an exception of a few metasediment samples, and correlates positively with natural remanent magnetization (NRM) and Q values. These data indicate that magnetic parameters are related to inhomogeneously distributed ferrimagnetic pyrrhotite. The paleomagnetic data reveals that the characteristic direction of NRM has shallow normal (in a few cases shallow reversed) polarity, which is in agreement with the Lower Jaramillo N‐polarity chron direction, and is carried by ferrimagnetic pyrrhotite. However, our study has not revealed the expected high magnetization body required from previous magnetic modeling. Furthermore, the LB‐07A and LB08‐A drill cores did not show the predicted high content of melt in the rocks, requiring a new interpretation model for magnetic data.

Crystallographic-magnetic correlations in single-crystal haemo-ilmenite: new evidence for lamellar magnetism

SUMMARY 17 single crystals were identified by electron backscatter diffraction (EBSD) and isolated from coarse massive haemo-ilmenite ore from South Rogaland, Norway. These were studied using the EBSD results, natural remanent magnetization (NRM), and anisotropy of magnetic susceptibility (AMS), to gain a better understanding of angular relationships between crystallographic axes and magnetic properties of haemo-ilmenite in relation to lamellar magnetism. Electron microprobe analyses gave the following average end-member compositions for ilmenite host: 21.1 per cent MgTiO3, 73.7 FeTiO3, 0.5 MnTiO3, 4.3 Fe2O3, 0.2 Cr2O3 and 0.3 V2O3; and for the coarsest (∼3 μm) haematite exsolution lamellae: 3.5 MgTiO3, 22.4 FeTiO3, 71.4 Fe2O3, 1.6 Cr2O3, 1.0 V2O3 and 0.1 Al2O3, making this sample the most Mg- and Cr-rich haemoilmenite studied in the province, but with similar element fractionations between the coexisting phases. TEM work on similar material suggests the presence of much thinner exsolution down to 1–2 nm. The EBSD, NRM and AMS results from 12 out of 17 crystals indicate a good agreement between the orientation of crystallographic axes, NRM direction and principal axes of the magnetic susceptibility ellipsoid, with the NRM located in the (0001) basal plane [NRM ∧ (0001) < 6.5 ◦ ] and the crystallographic c axis quasi-parallel to the minimum axis of the susceptibility ellipsoid [c ∧ k3 < 13.5 ◦ ]. In addition, in 10 of these 12 crystals, the remanent magnetization vector is parallel or nearly parallel to the positive direction of a crystallographic a axis [NRM ∧ a < 20 ◦ ], hence parallel to a principal magnetic moment direction in haematite as determined by Besser et al., and not parallel to the spin-canted direction of end-member haematite. This is consistent with a basic property of lamellar magnetism, where the magnetic moment is parallel to the principal moments (sublattice magnetization directions) in haematite. Relationships in three additional crystals with NRM ∧ a = 22 ◦ –33 ◦ , only two with good agreement, can be interpreted as consistent with having a magnetic vector quasi-parallel to the spin-canted direction of haematite.

Small-scale mélange fabric between serpentinite block and matrix: Magnetic evidence from the Mitsuishi ultramafic rock body, Hokkaido, Japan

The Mitsuishi ultramafic rock body in Hokkaido, Japan, consists mainly of serpentinized peridotites that originated from a depleted mantle. This study aims to show new evidence of small-scale mélange fabric of serpentinite matrix in the rock body. Each serpentinite block in the serpentine matrix shows large and stable intensities of natural remanent magnetization (NRM). However, the directions of serpentinite blocks' NRM in the matrix are randomly scattered. A Curie temperature (Tc) of 580 °C corresponding to pure magnetite was also observed. Additionally, there is no evidence of heating over 580 °C after serpentinization. The blocks in the matrix must have obtained crystallization remanent magnetization (CRM) during serpentinization. The directions of the blocks' characteristic remanent magnetization (ChRM) are also scattered. It shows that serpentinite blocks were magnetized prior to uplifting. The results of the study indicate that the magnetic carrier of the serpentinite blocks in the matrix is mainly composed of magnetite, and it can keep original magnetization before uplifting. The results also imply that the scattering directions of NRM indicate the presence of small-scale mélange fabric of serpentinite matrix.

テリエ法実験における強磁性粒子の非理想挙動の検出と補正―新たな古地磁気強度推定方法について―

We present a new method of paleointensity determination based on comparing the thermal demagnetization of natural remanent magnetization (NRM) with that of an artificial total thermoremanent magnetization (TRM). Igneous rocks often contain pseudo-single domain (PSD), multidomain (MD), and/or single domain (SD) particles as magnetic remanence carriers under strong magnetic grain (domain) interactions. The magnetic grain interactions have particular disastrous effects on paleointensity experiments, which make determination of paleointensity unreliable. We have critically examined how magnetic grain interactions affect the Thellier experiment, and have developed a new technique for correcting grain-interaction effects in the experiment of paleointensity estimation. The essential point of our experimental method is that by comparing the thermal demagnetization of natural remanent magnetization (δ NRM_loss) with that of an artificial total TRM (δ TRM_loss) for estimating its paleointensity, rather than that by comparing the remaining of NRM during thermal demagnetization (NRM_remaining) with a progressive TRM_gain in the traditional Thellier-Coe method, which essentially requires the additivity of partial TRM and independence of pTRMs. Using our new method, a mild alternating field (AF) demagnetization pre-treatment is applied to destroy most of the low coercivity remanence, which makes the samples behave more suitebly for a paleointensity study. We also make an apparent paleointensity estimation with pTRM, which is acquired in the perpendicular direction of NRM in a narrow non-overlapping temperature interval and cooled slowly in air. In this way, the non-ideal behavior of samples is detected most sensitively by the discrepancy between NRM loss and pTRM gain. Finally, we employ an artificial total TRM test to elucidate the relation between TRM_loss and pTRM_gain, and to correct interference caused by the non-ideal behavior. We have applied our new method to several representative suites of historical lava flows of known geomagnetic field intensity, and successfully extracted reliable paleointensity with a precision higher than 95% from samples even containing PSD and MD grains.

Archaeomagnetic studies in central Mexico—dating of Mesoamerican lime-plasters

For the first time results of an archaeomagnetic study of unburned lime-plasters from Teotihuacan and Tenochtitlan in central Mesoamerica are presented. Plasters made of lime, lithic clasts and water, appear during the Formative Period and were used for a variety of purposes in floors, sculptures, ceramics and supporting media for mural paintings in the Oaxaca and Maya area. In Central Mexico, grinded volcanic scoria rich in iron minerals is incorporated into the lime-plasters mixture. Samples were selected from two archaeological excavation projects in the Teopancazco residential compound of Teotihuacan and the large multi-stage structure of Templo Mayor in Tenochtitlan, where chronological information is available. The intensity of remanent magnetization (natural remanent magnetization (NRM)) and low-field susceptibility are weak reflecting low relative content of magnetic minerals. NRM directions are well grouped and alternating field demagnetization shows single or two-component magnetizations. Rockmagnetic experiments point to fine-grained titanomagnetites with pseudo-single domain behavior. Anisotropy of magnetic susceptibility (AMS) measurements document a depositional fabric, with normal to free-surface minimum AMS axes. Characteristic mean site directions were correlated to the paleosecular variation curve for Mesoamerica. Data from Templo Mayor reflect recent tilting of the structures. Teopancazco mean site declinations show good correspondence with the reference curve, in agreement with the radiocarbon dating. Dates for four stages of Teotihuacan occupancy based on the study of lime-plasters range from ad 350 to 550. A date for a possible Mazapa occupation around ad 850 or 950 is also suggested based on the archaeomagnetic correlation. The archaeomagnetic record of a plaster floor in Teopancazco differed from the other nearby sites pointing to a thermoremanent magnetization; comparison with the reference curve suggests dates around ad 1375 or 1415. The burning of the stucco floor likely occurred during a late re-occupation of the site by the Aztecs. Our results suggest that archaeomagnetic dating can be applied to lime-plasters, which are materials widely employed in Mesoamerica.

Palaeomagnetism of flood basalts in the Pilbara Craton, Western Australia: Late Archaean continental drift and the oldest known reversal of the geomagnetic field

A late Archaean (circa 2775–2715 Ma) succession of terrestrial continental flood basalts, mafic tuffs, felsic volcanic rocks, and clastic sedimentary rocks in the Nullagine Synclinorium (and Meentheena Centrocline) of the East Pilbara Basin, Western Australia, has been sampled for a palaeomagnetic study. Over 500 oriented, mostly basalt, drill cores were collected from the supracrustal succession and associated dykes. Thermal and alternating field demagnetization revealed two distinct components. Positive fold, conglomerate, and reversal tests confirm that the primary natural remanent magnetization (NRM) is still preserved. The secondary component is interpreted as the record of remagnetization during a major thermal event, possibly in the Early Proterozoic. Analysis of the primary NRM directions results in a magnetostratigraphy and an apparent polar wander path (APWP) for the 60 Myr interval covered by the sampled succession. Assuming a geocentric axial dipole during this time interval, the APWP shows that the Pilbara Craton was drifting during the late Archaean and that drift rates probably varied significantly. In particular, a mean 27.2° shift in palaeolatitude is recorded across an unconformity that represents a relatively short time period and that marks a significant change in basalt geochemistry. This study suggests that continents moved horizontally during the late Archaean and that the rates of movement were significantly faster than in the Phanerozoic. In addition, a reversed polarity interval, with a positive reversal test, is recorded. We argue that it documents the oldest known geomagnetic reversals of the geomagnetic field.

On partial thermoremanent magnetization tail checks in Thellier paleointensity determination

Coarse magnetite grains, of multidomain (MD) or large pseudo‐single‐domain (PSD) size (≥10 μm approximately), cause nonlinearity and other problems in paleomagnetic field intensity determination and can lead to spurious paleointensity values. One suggested method of detecting their presence is to carry out an additional zero‐field heating step following the acquisition of partial thermoremanent magnetization (pTRM) to check for complete demagnetization of the pTRM. Residual undemagnetized pTRMs (“pTRM tails”) are characteristic of MD and large PSD grains and cause nonideal behavior in Thellier‐type paleointensity experiments. We have measured pTRM tails in synthetic and natural magnetites of many sizes and domain states (including PSD and MD), starting from two different initial states. First, a thermally demagnetized sample was given a pTRM by cooling in a field H from the blocking temperature TB. The pTRM tail is the remanence ΔMptr remaining after zero‐field reheating to TB. ΔMptr is most prominent for large grain sizes and when TB ≥ 500°C. We next measured pTRM tails ΔMptr* produced in a Thellier paleointensity experiment. The initial state in this case is a total thermoremanent magnetization gradually reduced by double heatings (zero‐field followed by in‐field) to a set of increasing TB. For all synthetic and natural samples containing large PSD and MD magnetites, regardless of grain size or lithologic differences, ΔMptr is significant but ΔMptr* is negligible. That is, the pTRM tail checks are always zero in our model Thellier experiments. However, in practical paleointensity studies, the pTRM tail check method has been shown to be successful in detecting MD grains. The reason for these contrasting results is that our experiments were carried out with the field Hlab that produced pTRM equal in intensity and parallel to the field H that produced natural remanent magnetization (NRM). In this special situation the pTRM tail is completely masked. For the pTRM tail check method to be most effective, either Hlab should be applied at a large angle to the NRM direction or Hlab should be larger than (twice or more) the paleofield H. This will highlight the pTRM tail; in the first case by deflecting the apparent NRM direction after the tail‐check step, and in the second case by increasing the apparent NRM intensity.

Rock-Magnetic and Oxide Microscopic Studies of the El Laco Iron Ore Deposits, Chilean Andes, and Implications for Magnetic Anomaly Modeling

Microscopic and rock-magnetic studies of the ores and host rocks of the El Laco iron oxide deposits permit us to characterize the magnetic mineralogy and the processes affecting natural remanent magnetization (NRM) during emplacement and evolution of the deposits. Particular attention was devoted to identifying the magnetic mineral composition (magnetite and/or titanomagnetite, and hematite and/or titanohematite, and titanomaghemite) and grain size variations of both ores and host rock. Rock-magnetic data are used to clarify magnetic domain states and remanence acquisition processes, and to assess their significance as a source of magnetic anomalies. Microscopy under reflected light demonstrates that magnetic carriers are mainly magnetite, with significant amounts of ilmenite-hematite minerals. Magmatic titanomagnetites in the andesitic rocks show trellis textures, compatible with high-temperature oxy-exsolution processes. Supergene reactions in ore deposits under eruption conditions are indicated by goethite and hematite oxide minerals. Grain sizes range from a few microns to >100 μm. Hysteresis measurements point to pseudo-single-domain states. Thermal spectra, continuous temperature-dependent susceptibility measurements, and isothermal remanent magnetization (IRM) acquisition suggest predominance of spinels (titanomagnetite or titanomaghemite) with low-Ti contents as magnetic carriers. Although the presence of (titano)hematites is indicated by hysteresis and IRM studies, their contribution to the total remanence seems to be minor. The Fe-oxides in the ore are typically poor in Ti, whereas in the rocks they are Ti-bearing. For the modeling of the magnetic anomalies, we used data on bulk susceptibility and NRM intensity and direction in order to constrain the relative contributions of induced and remanent magnetization components, and to obtain improved control regarding depth and geometry of source bodies. The deep magnetic source corresponds to an ENE-striking tabular body, steeply inclined 65° to the north.