The intensity of the geomagnetic field at 3.5 Ga: paleointensity results from the Komati Formation, Barberton Mountain Land, South Africa

Paleointensity measurements have been carried out on 3.5 Ga samples from the Komati Formation type locality using both the Thellier and Van Zijl methods. These samples contain a single steeply-directed negative TRM component acquired during metamorphism of the Komati lavas. Thellier experiments yielded values ranging from 12 to 37 μT but an average paleofield intensity for the four best determinations is 20 ± 3 μT. A slightly lower average paleointensity of 15 ± 3 μT was obtained using Van Zijl experiments. Preheating was used to chemically stabilize seven samples used in Van Zijl determinations and these produced nearly ideal plots with an average paleointensity of 13 ± 2 μT. A single basaltic komatiite sample gave a nearly ideal Van Zijl plot indicating about 21 μT, nearly the same paleointensity as the peridotitic komatiite samples even though its NRM intensity was several orders of magnitude lower. Since the Komati characteristic remanence was acquired during a slow cooling, the data must be reduced by a factor of 1.55 to account for the difference between laboratory and natural cooling rates. Calculation of an equivalent equatorial paleointensity using the paleolatitude implied by the steep Komati characteristic remanence then gives value of 5 μT for the intensity of the geomagnetic field at 3.5 Ga, lower than the present value of about 30 μT.     

Experimental and numerical simulation of the acquisition of chemical remanent magnetization and the Thellier procedure

The results of the Thellier–Coe experiments on paleointensity determination on the samples which contain chemical remanent magnetization (CRM) created by thermal annealing of titanomagnetites are reported. The results of the experiments are compared with the theoretical notions. For this purpose, Monte Carlo simulation of the process of CRM acquisition in the system of single-domain interacting particles was carried out; the paleointensity determination method based on the Thellier–Coe procedure was modeled; and the degree of paleointensity underestimation was quantitatively estimated based on the experimental data and on the numerical results. Both the experimental investigations and computer modeling suggest the following main conclusion: all the Arai–Nagata diagrams for CRM in the high-temperature area (in some cases up to the Curie temperature T _c) contain a relatively long quasi-linear interval on which it is possible to estimate the slope coefficient k and, therefore, the paleointensity. Hence, if chemical magnetization (or remagnetization) took place in the course of the magnetomineralogical transformations of titanomagnetite- bearing igneous rocks during long-lasting cooling or during repeated heatings, it can lead to incorrect results in determining the intensity of the geomagnetic field in the geological past.     

Are ARM and TRM analogs? Thellier analysis of ARM and pseudo-Thellier analysis of TRM

We test the possibility of using the pseudo-Thellier method as a means of determining absolute paleointensity. Thellier analysis of anhysteretic remanent magnetization (ARM) and pseudo-Thellier analysis of thermoremanent magnetization (TRM) have been carried out on a large collection of sized synthetic magnetites and natural rocks. In all samples, the intensity of TRM is larger than that of ARM and the ratio R (=TRM/ARM) is strongly grain size dependent. The best-fit slope (b_TA) from pseudo-Thellier analysis of TRM shows a linear correlation with R. The ratio b_TA/R yielded approximately correct paleointensities, although uncertainties are larger than in typical Thellier-type determinations. For single-domain and multidomain magnetites, alternating field and thermal stabilities of ARM and TRM are fairly similar. However, for ∼0.24 μm magnetite, ARM is both much less intense and less resistant to thermal demagnetization than TRM, reflecting different domain states for the two remanences and resulting in severely non-linear Arai plots for Thellier analysis of ARM.     

Determination of the paleointensity in the early proterozoic from granitoids of the Shumikhinskii complex of the Siberian craton

The extreme scarcity of data on the behavior of the paleointensity H _an in the geological past from rocks older than 400 Ma significantly hinders the development of our ideas of the geomagnetic field evolution and the geological history of the Earth as a planet. This work presents H _an determinations for the Early Proterozoic using the Thellier method and meeting modern requirements for their reliability. The data are obtained from 1850-Ma rocks of granite intrusions sampled in the south of the Siberian platform. The rocks are virtually unaltered granites and granitoids. The paleointensity was determined on 15 samples; results from 11 samples were found to be suitable for the calculation of H _an, which is good for experiments of this type. The common feature in the behavior of the natural remanent magnetization (NRM) is a very narrow interval of blocking temperatures: destruction of (60–90)% NRM often took place between 500 and 550°C. Because of the large thickness of the sampled magmatic body, the paleointensity estimates were corrected for its slow cooling rate. With regard for this correction, the probable value of the virtual dipole moment (VDM) from the given collection amounts to 5 × 10²² A m². Analysis of all published data obtained by the Thellier method for the Precambrian and satisfying the well-known minimal criteria of reliability showed that the average VDM value is about 2 × 10²² A m², which is four times smaller than the VDM value of the last million years. This phenomenon can be interpreted in terms of the hypothesis that the solid inner core formed only in Proterozoic and, in its absence, the generation of the geomagnetic field was relatively weak, which yielded a small intensity value of the geomagnetic field at early stages of the Earth’s evolution.     

On correction of loss of mass during Thellier experiments

Determination of geomagnetic paleointensity by the Thellier method compares the decay of natural remanent magnetisation with the gain of a laboratory induced thermoremanent magnetisation. If fragile samples lose some amount of their material after each heating step, the Thellier experiment will be systematically disturbed and paleointensity will be over-estimated. For a lost of 5% of the sample’s mass an over-estimate of 10% in paleointensity is observed. This can easily be corrected by a normalisation to the initial mass of the unheated sample. This is necessary for any fragile materials such as baked clays or when a specimen breaks into pieces during the Thellier experiment.     

Test determinations of paleointensity in historical lavas of Kamchatka

The reliability of the Thellier method for determining the paleointensity of a geomagnetic field is explored on recent igneous rocks of Kamchatka. The main magnetic mineral in the studied rocks is titanomagnetite with different degree of oxidation. It is obtained that the reliability of the results can be assessed based on the deviations of the check points of the partial thermoremanent magnetization (pTRM) during the Thellier experiment. Besides, for different rocks, it is found that the stability of titanomagnetites to heating during the experiments can be insufficient for validating the reliability of the results of paleointensity determination; however, at the same time, the reliability may depend on the initial (oxidation) state of the magnetic minerals of the studied rocks.     

The effect of magnetite particle size on paleointensity determinations of the geomagnetic field

The Thellier method for paleointensity determinations has been applied to prepared samples containing magnetites whose mean particle sizes range from single domain, SD, to multidomain, MD. Linear (ideal) PNRM-PTRM curves are obtained for samples containing SD and submicron magnetite particles. However, for MD particles non-linear (concave-up) PNRM-PTRM curves are observed such that a linear approximation to the lower blocking-temperature data leads to apparent paleointensities that are higher than the actual paleofield; however, the ratio of the end-points, NRM/TRM, yields the correct (laboratory) intensity. The non-linear (concave-up) PNRM-PTRM curves for the MD particles are explained in terms of the lack of symmetry of the domain-wall movements during the two heatings of the Thellier experiment. Low stabilities with respect to alternating fields and with respect to temperature cycles below magnetite's isotropic temperature are diagnostic in detecting samples most likely to exhibit non-linearities due to the MD effect.     

Absolute paleointensities recorded during the Brunhes chron at La Guadeloupe Island

Ten absolute paleointensity determinations within the Brunhes chron have been obtained on andesitic lava flows from La Guadeloupe Island, French West Indies (F.W.I.). The Thellier and Thellier method performed on 124 specimens under either ambient or argon atmosphere allows reliable determinations from high temperature steps. A satisfactory within flow reproducibility has been observed and at least two samples for each flow have been used for calculation of the weighted-mean paleointensity. The average virtual axial dipole moment (VADM=7.1±1.8×10²² A m²) for normal polarity flows from the Brunhes chron recorded at La Guadeloupe is compatible with the historical field. The 10 VADM values obtained show a relatively good agreement with the deep-sea composite record (SINT800) of relative paleointensity. Focusing on the 100–75 ka interval, a 20-ka oscillation observed from a compilation of available volcanic data and present in high-resolution sedimentary records has been tentatively related to the axial dipole moment variations. Finally, a paleointensity decrease of a factor of 10 is observed for flows emitted during the Matuyama–Brunhes (M–B) transition, as already observed in other records of this reversal. Because a large area of the Globe around the Caribbean Islands was previously devoid of data, the present dataset fills a gap towards construction of a global paleointensity database for the Brunhes chron.     

Reliability of geomagnetic paleointensity data: the effects of the NRM fraction and concave-up behavior on paleointensity determinations by the Thellier method

To test the reliability of the Thellier method for paleointensity determinations, we studied six historic lavas from Hawaii and two Gauss-age lava flows from Raiatea Island (French Polynesia). Our aim is to investigate the effects of the NRM fraction and concave-up behavior of NRM–thermal remanent magnetization (TRM) diagrams on paleointensity determinations. For the Hawaiian samples, the paleointensity results were investigated at both sample and site levels. For consistency and confidence in the paleointensity results, it is important to measure multiple samples from each cooling unit. The results from the Raiatea Island samples confirm that reliable paleointensities can be obtained from NRM–TRM diagrams with concave-up curvature, provided the data are accompanied by successful partial TRM (pTRM) checks and no significant chemical remanent magnetization (CRM) production. We conclude that reliable determinations of the paleofield strength require analyses of linear segments representing at least 40–50% of the total NRM. This new criterion has to be considered for future studies and for evaluating published paleointensities for calculating average geomagnetic field models. Using this condition together with other commonly employed selection criteria, the observed mean site paleointensities are typically within 10% of the Definitive Geomagnetic Reference Field (DGRF). Our new results for the Hawaii 1960 lava flow are in excellent agreement with the expected value, in contrast to significant discrepancies observed in some earlier studies.

Overestimates of paleointensity determinations can arise from cooling-rate dependence of TRM acquisition, viscous remanent magnetization (VRM) at elevated temperatures, and TRM properties of multidomain (MD) particles. These outcomes are exaggerated at lower temperature ranges. Therefore, we suggest that, provided the pTRM checks are successful and there is no significant CRM production, it is better to increase the NRM fraction used in paleointensity analyses rather than to maximize correlation coefficients of line segments on the NRM–TRM diagrams.

We introduce the factor, Q = Nq, to assess the quality of the weighted mean paleointensity, H_w, for each cooling unit.     

Experimental modeling of the chemical remanent magnetization and Thellier procedure on titanomagnetite-bearing basalts

The results of the experimental studies on creating chemical and partial thermal remanent magnetizations (or their combination), which are imparted at the initial stage of the laboratory process of the oxidation of primary magmatic titanomagnetites (Tmts) contained in the rock, are presented. For creating chemical remanent magnetization, the samples of recently erupted Kamchatka basalts were subjected to 200-h annealing in air in the temperature interval from 400 to 500°С under the action of the magnetic field on the order of the Earth’s magnetic field. After creation of this magnetization, the laboratory modeling of the Thellier–Coe and Wilson–Burakov paleointensity determination procedures was conducted on these samples. It is shown that when the primary magnetization is chemical, created at the initial stage of oxidation, and the paleointensity determined by these techniques is underestimated by 15–20% relative to its true values.     

Geomagnetic field intensity from Kilauea 1955 and 1960 lava flows: Towards a better understanding of paleointensity

The record of the Earth’s magnetic field intensity during the past (paleointensity) carries important information about the geodynamo and the state of the Earth’s interior that is not contained in the record of its paleodirection. To determine what the critical factors in obtaining reliable estimate of paleointensity are, we present new results of a paleointensity study of the 1955 and 1960 Kilauea volcano lava flows, from the Big Island of Hawaii. Rock magnetic measurements on representative hand samples from each flow in conjunction with reflected light microscopy observations show the primary carriers of remanence to be pseudo-single domain titanomagnetite with various titanium contents. Paleointensity samples (small fragments previously embedded into salt pellets) were subjected to the Thellier-Coe experimental procedure. Fourteen temperature steps were distributed over the entire temperature range used (ambient temperature to 570°C). Control heating steps (commonly referred to as partial thermo-remanent magnetization — pTRM checks) were also conducted each third double heating step. Mean field intensity value (36.6 ± 0.7 μT) retrieved from 3 reliable site mean determinations reproduces the expected value within 1.1%. With the knowledge of the rock magnetic characteristics of the samples and the strength of the geomagnetic field during cooling of the lava, our investigation suggests that the Hawaiian lavas can faithfully record the local geomagnetic field and confirms that the Thellier-Coe type techniques are suitable on historical lava flows to yield reliable absolute paleointensity determinations. The variations in direct field measurements and in lab paleofield determinations may reflect local heterogeneities of the lava or influence of very local field anomalies due to the volcanic underlying terrain. These results underscore the importance that a better understanding of intensity results of historical lava flows is still required if reliable paleointensity determinations of older periods, for which we do not know the answer, are sought.     

Geomagnetic field paleointensity in the cretaceous from Upper Cretaceous rocks of Georgia

A representative collection of Upper Cretaceous rocks of Georgia (530 samples from 24 sites) is used for the study of magnetic properties of the rocks and the determination of the paleodirection and paleointensity (H _an) of the geomagnetic field. Titanomagnetites with Curie points of 200–350°C are shown to be carriers of natural remanent magnetization (NRM) preserving primary paleomagnetic information during heatings to 300–350°C. The characteristic NRM component of the samples is identified in the interval 120–350°C. The Thellier and Thellier-Coe methods are used for the determination of H _an meeting modern requirements on the reliability of such results. New paleointensity determinations are obtained and virtual dipole magnetic moment (VDM) values are calculated for four sites whose stratigraphic age is the Upper Cretaceous (Cenomanian-Campanian). It is shown that, in the interval 99.6–70.6 Ma, the VDM value was two or more times smaller than the present value, which agrees with the majority of H _an data available for this time period. According to our results, the H _an value did not change at the boundary of the Cretaceous normal superchron.     

An experimental evaluation of Shaw’s paleointensity method and its modifications using Late Quaternary basalts

Absolute paleointensity experiments were carried out using Shaw’s method [Geophys. J. Res. Astr. Soc. 39 (1974) 133] and its modifications [Geophys. J. Res. Astr. Soc. 54 (1978) 241; Geophys. J. Int. 80 (1985) 773; Geophys. J. Int. 118 (1994) 781] on 49 samples belonging to six Late Quaternary basaltic flows from central Mexico. Samples were selected from a large collection because of their low viscosity index, stable remanent magnetization and close to reversible continuous thermomagnetic curves. Moreover, they previously yielded high quality Thellier paleointensity results, which makes them good candidates to assess the reliability of Shaw’s paleointensity method. Only 13 samples yielded acceptable results using Shaw’s original method (ARM₂/ARM₁ ratio varies from 0.95 to 1.05 for accepted determinations) although 6 samples do not pass the validity test proposed by Tsunakawa and Shaw [Geophys. J. Int. 118 (1994) 781] and thus should be rejected for paleointensity analyses. Rolph and Shaw’s [Geophys. J. Int. 80 (1985) 773] method gives reliable determination only in one case and no single determination was obtained by Kono’s [Geophys. J. Res. Astr. Soc. 54 (1978) 241] modification. Our results indicate an extremely low success rate of Shaw’s paleointensity method, which may be due to magneto-chemical changes that occurred during heating of samples above their Curie temperatures.     

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 of the earth's magnetic field during the Laschamp excursion and its geomagnetic implications

The reversed paleomagnetic direction of the Laschamp and Olby flows represents a specific feature of the geomagnetic field. This is supported by paleomagnetic evidence, showing that the same anomalous direction was recorded at several distinct sites, including scoria of the Laschamp volcano. To examine this anomalous geomagnetic fluctuation, we studied the paleointensity of the Laschamp and Olby flows, using the Thellier method. Twenty-five samples were selected for the paleointensity experiments, and from seven we obtained reliable results. Because the paleointensity results of the Olby and Laschamp flows as well as Laschamp scoria are very similar, they can be represented by a single mean paleointensity,F = 7.7 μT. Considering that this low paleointensity is less than 1/6 of the present geomagnetic field and is more characteristic of transitional behavior, our results suggest that the paleomagnetic directions of the Laschamp and Olby flows were not acquired during a stable reversed polarity interval. A more likely explanation is that the Laschamp excursion represents an unsuccessful or aborted reversal.     

Geomagnetic field intensity between 70 000 and 130 000 years B.P. from a volcanic sequence on La Réunion, Indian Ocean

A detailed paleointensity study was made of a sequence of 70 successive lava flows of the Piton des Neiges volcano on the island of La Réunion (Indian Ocean). Radiometric dating brackets the age of this sequence between 130 ± 3 ka and 72 ± 3 ka. Rock magnetic investigations show that titanomagnetites in the pseudo single domain range are the main magnetic carrier of the Natural Remanent Magnetisation (NRM). Over 350 samples were used for paleointensity determinations carried out with the Thellier method in vacuum or in an argon atmosphere. Of these, 89 samples yielded reliable results, with within-flow scatter often lower than 20%. These results indicate that the geomagnetic field intensity has varied at La Réunion between 13 and 65 μT during the period of time explored. The average value, 42 μT, is higher than the present field at La Réunion (35 μT). The results from the upper part of the section are consistent with previous results obtained for the 82–98 ka period also at La Réunion [1] and document a broad low around 95 ka. not associated with large directional changes. On a larger geographic scale, the paleointensity values from La Réunion are significantly higher than those obtained from Mount Etna [2]. Precise comparison is, however, difficult because of the lack of detail in the Etna results. In the lower part of the section, a marked intensity low, coinciding with significant deviation from the dipole field direction is observed at 115 ka and could correspond to the end of the Blake event.     

Archaeomagnetic investigations in Bulgaria: Field-intensity determinations

Geomagnetic paleointensity determination have been made by the Thellier method using samples from 27 sites in Bulgaria. The samples include bricks, specimens taken from historic kilns, from prehistoric hearths and the sites of ancient fires. The ages of the samples, which range from about 4500 B.C. to the 19th century A.D., have been determined partly by the ¹⁴C method and partly from archaeological evidence. The (residual NMR)-(induced TRM) diagrams tend to be less linear for the prehistoric samples either due to weathering or because the NRM is not a total TRM.     

考古标本岩石磁性的初步研究

讨论了考古标本中所含矿物的磁学性质与古代地球磁场强度测定数据可靠性之间的关系,进行了岩石磁学实验,包括高温、低温磁化率测定、居里温度测定、磁滞回线各参数的测定等．结果表明,文中所用各类标本（陶片、砖、烧土等）的主要载磁矿物为磁铁矿,采用这些标本用Ｔｈｅｌｌｉｅｒ方法进行逐步热退磁实验所测定的古强度数据较为可靠．     

Paleointensity determination by a modified thellier method

It has been frequently observed that the ferromagnetic minerals in volcanic rocks may undergo some physical/chemical changes when they are heated in a laboratory. For paleointensity determinations, it is therefore desirable to use the Thellier method, in which internal consistency of the data assures that such undesirable changes did not take place in a particular sample. The most serious drawback of this method is that it requires a considerable number of heatings and coolings and measurements of the remanent magnetization. Following a suggestion of Kono, a modified version of Thellier's technique was developed and applied to a historical lava flow of Hawaii and Oligocene rhyolites and welded tuffs of San Juan volcanic field, Colorado, U.S.A. Samples were heated only once to each temperature and cooled under a magnetic field applied perpendicular to the NRM directions. Results were compared to those obtained by the conventional Thelliers' technique. It was found that the modified method gives results as satisfactory as the original method. With the selection of appropriate samples, the modified method will cut down the time required for the experiment to almost half of what is required in the original method, without losing the merits of Thelliers' technique.     

Constraints of archaeomagnetic dating and field intensity determinations in three ancient tile kilns in Belgium

The aim of this study is to date by the archaeomagnetic method the last heatingcooling cycle of one Roman and two Medieval tile kilns, discovered in Belgium. The investigation demonstrates the limitations when well-documented local directional secular variation curves of the geomagnetic field in the past are used for dating and the difficulties when trying to determine field intensities from “in situ” baked clays from the kilns. The three kilns yielded very well defined ancient field directions but two possible dating solutions for each of them when no a priori time constraints are taken into account, due to field direction recurrence. As an increase of the dating accuracy and reduction of the number of dating solutions can be expected using the full field vector information, also field intensity determinations on burnt clays from the kilns were attempted. Field intensities from samples of the Roman and of one of the Medieval kilns are quite scattered. On the other hand, results obtained applying the Thellier-Thellier method and the modified method developed by Dekkers and Böhnel on sister samples from the Roman kiln agree fairly well. Rock magnetic properties reveal high variance in the kilns that point to varying spatial heating and cooling conditions in the kilns. Even well burnt material from the kilns shows irreversible changes when heated in air in the laboratory. Reliable field intensities on “in situ” baked materials from kilns themselves can therefore only be obtained when measuring sufficient number of samples from different parts of the kiln, taking into account the spatial-temporal conditions during kiln operation and cooling history. More reference intensity data is needed in our regions in order to improve dating based on directional reference data only.