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1.
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, Hw, for each cooling unit.  相似文献   


2.
Based on palaeomagnetic and rock magnetic results, 108 samples from 27 flows from Palaeogene flood basalts of Faroe Islands were chosen for whole-rock Thellier palaeointensity experiments. Altogether 90 samples were rejected due to either chemical alterations or typical multidomain (MD) behaviour evidenced by pTRM-tails. AF pre-treatment was used to reduce the effect of MD grains on Thellier experiments. Only five flows (18 samples) yielded acceptable palaeointensity estimates, with flow mean VDMs ranging from 3.5 to 7.4×1022 A m2. Modest selection criteria imposed on all published 5-160 Ma palaeointensity data left only 15 palaeomagnetic dipole moments: eight from whole-rock samples, six from submarine basaltic glass and one from single plagioclase crystals. More data are needed before the intriguing differences between results from different materials can be put into a geomagnetic context.  相似文献   

3.
We have investigated the low-temperature behavior of a suite of ‘grown’ synthetic and natural magnetites that span single-domain (SD) and multidomain (MD) behavior. Synthetic samples had been grown in the laboratory either in an aqueous medium or in glass. Natural samples included SD magnetites occurring in plagioclase and truly MD magnetites in the form of large octahedra. In all experiments a sample was first given a saturation remanence at room temperature; next, moment was measured continuously during cooling and warming between 230 K and 60 K. Similar to results reported earlier by other workers, magnetic memory is large in SD samples, whereas truly MD samples are almost completely demagnetized by cycling between room temperature and 60 K. Pseudo-single-domain samples exhibit behavior that is intermediate with respect to that of the SD and truly MD states. When data from this study are combined with data obtained by Hartstra [10] from sized, natural magnetites, it is found that the percentage of total remanence that survives cycling between room temperature and 60 K decreases linearly with the logarithm of grain size and, thus, with increasing number of domains. This relation suggests that memory can provide a reasonable estimate of grain size in those magnetite-bearing rocks for which these samples provide good analogues. Remarkably, some of the large natural octahedra provide a magnified view of MD response to low temperatures and thus reveal two surprising and intriguing types of behavior. First, below approximately 180 K these octahedra demagnetize through a series of large Barkhausen jumps. Second, near 117 K these same octahedra exhibit a ‘wild zone’, where magnetic moment executes large, random excursions. We interpret these two phenomena as direct evidence for the unpinning and irreversible displacement of domain walls in response to the drop in coercivity and, possibly, the broadening of domain walls as temperatures drop toward the isotropic point. One implication of this behavior is that cooling to progressively lower temperatures could provide an effective method for stepwise removal of paleomagnetic components carried by MD grains, even without passage through the isotropic point of magnetite.  相似文献   

4.
Iron ore and host rocks have been sampled (90 oriented samples from 19 sites) from the Las Truchas mine, western Mexico. A broad range of magnetic parameters have been studied to characterize the samples: saturation magnetization, Curie temperature, density, susceptibility, remanence intensity, Koenigsberger ratio, and hysteresis parameters. Magnetic properties are controlled by variations in titanomagnetite content, deuteric oxidation, and hydrothermal alteration. Las Truchas deposit formed by contact metasomatism in a Mesozoic volcano-sedimentary sequence intruded by a batholith, and titanomagnetites underwent intermediate degrees of deuteric oxidation. Post-mineralization hydrothermal alteration, evidenced by pyrite, epidote, sericite, and kaolin, seems to be the major event that affected the minerals and magnetic properties. Magnetite grain sizes in iron ores range from 5 to >200 μm, which suggest dominance of multidomain (MD) states. Curie temperatures are 580±5°C, characteristic of magnetite. Hysteresis parameters indicate that most samples have MD magnetite, some samples pseudo-single domain (PSD), and just a few single domain (SD) particles. AF demagnetization and IRM acquisition indicate that NRM and laboratory remanences are carried by MD magnetite in iron ores and PSD–SD magnetite in host rocks. The Koenigsberger ratio falls in a narrow range between 0.1 and 10, indicating the significance of MD and PSD magnetites.  相似文献   

5.
The recently developed first-order reversal curve (FORC) technique for rapidly examining magnetic domain state has great potential for paleomagnetic and environmental magnetic investigations. However, there are still some gaps in the basic understanding of FORC diagrams, in particular the behavior of pseudo-single-domain (PSD) grains and the contribution of magnetostatic interactions. In this paper we address some of these problems. We report the first FORC diagrams measurements on narrowly sized and well-characterized synthetic PSD through multidomain (MD) magnetite samples. The FORC diagrams evolve with grain size from single-domain (SD)-like to MD-like through the PSD grain size range. Since each sample contains grains of essentially a single size, individual PSD grains evidently contain contributions from both SD-like and MD-like magnetic moments, in proportions that vary with grain size; the evolving FORC diagrams cannot be due to physical mixtures of SD and MD grains of widely different sizes. The FORC diagrams were all asymmetric. Small PSD samples have FORC diagrams with a distinctive closed-contour structure. The distributions of the larger MD grains display no peak, and lie closer to the interaction-field axis. To assess the effect of magnetostatic interactions, we measured FORC diagrams between room temperature and the Curie temperature. On heating the FORC distributions contract without changing shape until ∼500°C. Above this temperature the diagrams become more MD-like, and in addition become more symmetric. The temperature dependence of the interaction-field parameter is proportional to that of the saturation magnetization, in accordance with Néel’s interpretation of the Preisach diagram. The decrease in asymmetry with heating suggests that the origin of the asymmetry lies in magnetostatic interactions. The magnetic hysteresis parameters as a function of temperature were determined from the FORC curves. As the grain size decreased the normalized coercive force was found to decrease more rapidly with temperature.  相似文献   

6.
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.  相似文献   

7.
Cooling rate (CR) effects on the intensity of thermoremanent magnetization has been documented for archaeomagnetic materials, where cooling in laboratory conditions is generally much faster compared to natural cooling rates. Since the latter condition also applies to many volcanic rocks, we have investigated in this study the influences of the CR on the determination of absolute paleointensity using recent basaltic rocks. We used magnetically and thermally stable samples mainly containing Ti-poor pseudo-singledomain titanomagnetites (the most widely used material for Thellier paleointensity experiments). These samples previously succeed in retrieving the strength of laboratory field intensities with the Coe’s version of the Thellier method in a simulated paleointensity experiment using similar cooling rates. Our experimental results indicate that the cooling rate effects produce systematic and significant overestimates of the absolute intensity up to 70%. The effect can be much larger than predicted by Neél theory for non-interacting single-domain grains.  相似文献   

8.
We present new archeointensity results obtained from 127 potsherds and baked brick fragments dated from the last four millennia BC which were collected from different Syrian archeological excavations. High temperature magnetization measurements were carried out using a laboratory-built triaxial vibrating sample magnetometer (Triaxe), and ancient field intensity determinations were derived from the experimental procedure described by Le Goff and Gallet [Le Goff and Gallet. Earth Planet. Sci. Lett. 229 (2004) 31–43]. As some of the studied samples were previously analyzed using the classical Thellier and Thellier [Thellier and Thellier . Ann. Geophys. 15 (1959) 285–376] method revised by Coe [Coe. J. Geophys. Res. 72 (1967) 3247–3262], a comparison of the results is made from the two methods. The differences both at the fragment and site levels are mostly within ± 5%, which strengthens the validity of the experimental procedure developed for the Triaxe. The new data help to better constrain the geomagnetic field intensity variations in Mesopotamia during archeological times, with the probable occurrence of an archeomagnetic jerk around 2800–2600 BC.  相似文献   

9.
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 (bTA) from pseudo-Thellier analysis of TRM shows a linear correlation with R. The ratio bTA/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.  相似文献   

10.
The role of single-domain (SD) magnetic particles in creation of inverse magnetic fabrics is investigated on simple mathematical models using a realistic estimate for SD intrinsic susceptibility. In contrast to the fraction created by multi-domain (MD) particles, in which the anisotropy of magnetic susceptibility (AMS) is controlled by both the grain AMS and intensity of the preferred orientation of the particles, the AMS of the SD fraction is controlled solely by the intensity of the preferred orientation. The degree of AMS of ensemble of SD grains with a preferred orientation is therefore much higher than that of the same ensemble of MD particles implying the existence of frequent inverse magnetic fabrics. However, the occurrence of inverse magnetic fabrics due to SD particles is more the exception than the rule. Consequently, the amounts of SD particles is probably in general low. Nevertheless, the presence of SD particles in amounts insufficient to create inverse magnetic fabrics may diminish the whole rock AMS substantially. This can be one of the reasons for relatively low AMS in volcanic rocks whose magnetic particles may be really small obeying the conditions for the existence of SD particles.  相似文献   

11.
The validity of magnetic granulometric estimates relies heavily on the ability to distinguish ultrafine particles from coarser grains. For example, populations with dominantly superparamagnetic (SP) or multidomain (MD) grains both are characterized by low remanence and coercivity, and distinguishing these endmembers may provide valuable clues to the origin of magnetization in the intervening stable single domain (SD) size range. The natural grain size variations associated with variable cooling rates in submarine lavas provide a rare opportunity for examining progressive changes in average magnetic grain size, from SP–SD mixtures in submarine basaltic glass to SD–MD mixtures in flow interiors. Based on microanalysis and rock magnetic measurements on pillow basalt samples dredged from the flanks of the Mid-Atlantic Ridge (ages <1 Ma to 70 Ma), a model of preferential dissolution with time of the finest-grained titanomagnetites has recently been suggested as the major process contributing to long-term temporal changes in remanent intensity of mid-ocean ridge basalts. We evaluated the local and long-term temporal trends in effective magnetic grain size predicted by this model using hysteresis data from a large number of submarine basalt samples which span a range of ages from 0 to 122 Ma. Specimens were systematically taken along transects perpendicular to the chilled margin of each sample. The large number of data (750 loops) and the inferred progressive change in grain size approaching the chilled margin allow recognition of mixing trends between MD and SD grains and between SD and SP grains on a Day-plot. These trends in hysteresis parameters are crucial to resolving the inherent, but frequently overlooked, ambiguity in inferring grain size from hysteresis parameters. We illustrate that two additional rock magnetic tests (warming of a low-temperature isothermal remanence and hysteresis loop shapes) often used to address these ambiguities are inconclusive, requiring some independent knowledge of whether SP or MD grains are likely to be present. Even with a considerably larger data set the substantial intrasample variability in oceanic basalts precludes recognition of any systematic trend in magnetic grain size with age.  相似文献   

12.
This paper reports the alternating field demagnetization characteristics of glass–ceramic magnetite assemblages carrying weak-field thermoremanent magnetization (TRM), weak-field anhysteretic remanent magnetization (ARM), and saturation remanence (Jrs). Average grain sizes vary from less than 0.1 μm to approximately 100 μm, and hysteresis parameters indicate that these assemblages encompass single-domain (SD) through truly multidomain (MD) behavior. In all assemblages, weak-field TRM and weak-field ARM are more stable to alternating field demagnetization than is (Jrs). This response is especially remarkable in the 100 μm assemblage, which otherwise displays truly MD behavior. Although the SD samples pass the Lowrie–Fuller test for SD behavior, calculations presented here show that populations of noninteracting, uniaxial SD grains should behave in just the opposite sense to that reported originally by Lowrie and Fuller. This discrepancy could indicate that SD, glass–ceramic magnetite populations are more affected by magnetic interactions than would be expected for magnetite crystals that nucleated individually from a silicate matrix. This interpretation is supported by the SD assemblages failing the ‘Cisowski' test: that is, the curves for acquisition and AF demagnetization of (Jrs) intersect well below the 50% mark. However, a second and intriguing explanation of the SD-like results obtained from all samples is that alternating field demagnetization characteristics reflect a strong dependence of local energy minimum domain state, and its associated stability, on the state of magnetization.  相似文献   

13.
Stoichiometric titanomagnetites Fe3?xTixO4 with compositions between x = 0 (magnetite) and x = 0.72 (a titanomagnetite having a Curie temperature of 60°C) have been synthesised using the double-sintering technique in controlled atmospheres. The quality of these materials was tested by various mineralogical and magnetic measurements. Isolated small multidomain (MD) and pseudo-single-domain (PSD) particles within pores of the bulk material were investigated with respect to their domain structures, and threshold sizes for the transition from the PSD to the SD stage determined for titanomagnetites of various compositions by extrapolation from the domain state of small MD grains. The threshold size was found to be 0.7 and 0.5 μm, respectively, for TM72 (x = 0.72) and TM62 (x = 0.62). The threshold size decreases slightly for smaller x values; however, the experimental data obtained to date are not sufficiently reliable to yield precise results.Preliminary experiments concerning hysteresis loops and TRM generation are also reported.  相似文献   

14.
AF (alternating field) demagnetization, ARM (anhysteretic remanent magnetization) and strong-field hysteresis properties of a large collection of mostly continental igneous rocks are reported here. The collection included rocks whose magnetic carriers were believed from previous work to be of one of three types: MD (multidomain); SD/PSD (single-domain/pseudo-single-domain); or a bimodal mixture of MD grains (e.g., discrete opaques) and SD/PSD material (e.g., silicate inclusions). Two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour. SD/PSD rocks have relatively hard inflected AF decay curves (decay rate initially increasing, then decreasing), MD rocks have soft, exponential-like decay curves, and bimodal rocks have a combination of these characteristics. Relative hardnesses of normalized decay curves of remanences acquired in weak, intermediate and strong fields (the Lowrie-Fuller test) are also distinctively different for the three classes, and the results support the theory developed in an accompanying paper [1] that Lowrie-Fuller characteristics are an expression of the shapes of decay curves. The Lowrie-Fuller test, although its result can be expressed as a numerical parameter, is not capable of fine-scale classification of domain structure or grain size. The shape of the ARM induction curve does have a quasi-continuous variation with grain size, however. The parameter χar/Jrs (initial anhysteretic susceptibility normalized to saturation remanence), which is easily measured with standard paleomagnetic instrumentation, is potentially useful for magnetic granulometry, although χar itself was not diagnostic of grain size.  相似文献   

15.
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．  相似文献   

16.
We report normalized AF demagnetization curves of anhysteretic remanences (ARM's) produced by 1-, 10- and 40-Oe steady fields and of saturation isothermal remanence (IRMs) in a suite of dispersed, unannealed magnetite powders with median sizes of 2, 4, 6, 10 and 14 μm (pseudo-single-domain or PSD size range) and 100 μm (multidomain or MD size). Interpreted in the light of the domain structure test first proposed by Lowrie and Fuller [12], the relative stability trend of curves for the 2 μm sample is of single-domain (SD) type, the 1-Oe ARM being most resistant to demagnetization followed by the 10-Oe and 40-Oe ARM's and IRMs. For the 100-μm sample, the trend is exactly reversed and is of MD-type. In the 4–14 μm samples, hitherto undescribed transitional trends between SD-type and MD-type occur. At 6 μm, 1-Oe, 10-Oe and 40-Oe ARM's preserve an SD-type trend but for all AF's > 75 Oe, IRMs is more resistant than any of these remanences. At 10 μm, this trend is unmistakable, and only at 14 μm do the 1-Oe, 10-Oe and 40-Oe ARM curves merge. We conclude (1) that the Lowrie-Fuller test distinguishes between small MD grains enhanced by PSD remanence and large MD grains lacking PSD remanence, rather than between SD and MD structures per se, and (2) that in the PSD transition region from 6 to 14 μm in magnetite, IRMs changes over to MD-type relative stability around 6 μm, whereas 10-Oe and 40-Oe ARM's achieve an MD-type trend around 14 μm, in accord with the predicted field dependence of the PSD threshold size.Our theoretical interpretation assumes that the intrinsic (internal field) coercive force spectra of weak-field and strong-field remanences are identical but that the observed (external field) spectrum is shifted to lower fields as a result of the internal demagnetizing field — NJr of the remanence Jr. The effect is slight for weak-field Jr's but substantial for IRMs. Since all coercivities, high as well as low, are shifted, the result of the Lowrie-Fuller test is determined simply by the shape of the intrinsic coercivity spectrum or the corresponding AF demagnetization curve. Depending on the model of self-demagnetization used, either subexponential or sublinear AF decay curves of weak-field remanence will automatically lead to an MD-type trend, whereas by either model the decay curves that characterize SD and PSD remanences (decaying slowly initially and then more rapidly) will always produce and SD-type trend.  相似文献   

17.
A wide range of rock magnetic properties have been determined from two collections of mid-Cretaceous basalts; one from Israel, the other from the Rajmahal traps in northeast India. Deuteric oxidation is rare in both collections, with titanium-rich titanomagnetite being the principal remanence carrier in most cases. There are a number of differences in rock magnetic properties between the two groups. Some of these seem to be primary, whereas others appear to be caused by hydrothermal alteration and weathering, which are more prevalent in the Indian rocks. These rocks are being used in palaeointensity experiments, from which it is hoped to determine the strength of the Earth's magnetic field during the long period of normal polarity in the mid-Cretaceous. Thellier palaeointensity experiments have been performed on two samples from each site. The degree of agreement between the two results is highly variable. The low blocking temperatures and the presence of secondary viscous components in many samples make Thellier palaeointensity experiments very difficult. A further problem is that of thermal alteration, two main types of which are observed. The first manifests itself as a large and sudden increase in partial thermoremanent magnetization (pTRM) capacity, and the second as a steady decrease in the size of pTRM with increasing temperature.  相似文献   

18.
Rock magnetic measurements of Nihewan sediments from Xujiayao section demonstrate that magnetite, hematite and maghemite are dominant remanent magnetization carriers. Monitoring the variations of magnetic susceptibility (MS) and saturating isothermal remanent magnetization (SIRM) at low temperature are the attractive ways of detecting the presence of magnetite, maghemitization and superparamagnetic grain sizes. Low-temperature MS investigations suggest that susceptibility enhancement for Xujiayao samples is mainly due to the remarkable presence of SD/MD magnetite to some degree though some magnetite grains have been partially oxidized at some depths. It is tentatively concluded that both SD/MD magnetite and hematite are of detrital origin and carry a characteristic remanent magnetization (ChRM), whereas maghemite can be attributed to be chemical origin, overprinting a reversed polarity component of Matuyama age.  相似文献   

19.
The rockmagnetic and palaeomagnetic signal in pyrrhotite bearing limestones of different contact-metamorphic settings were investigated related to intrusions ranging from small sills to large magmatic complexes. Magnetic susceptibility, the pyrrhotite/magnetite ratio and thermal modelling serve as an investigative tool to define three metamorphic zonations: a contact zone of a mixed magnetic assemblage and low susceptibilities, a pyrrhotite bearing transitional zone, where full thermoremanent magnetisations (TRMs) are acquired due to temperatures above the Curie temperature of pyrrhotite (Tc,po), and a marginal zone containing pyrrhotite and magnetite generated at temperatures below Tc,po. The fact that TRMs can consist of independent pTRMs is successfully tested by modified Thellier experiments. It is shown that a metamorphic environment with low fluid circulation provides a scenario for the recording of independent pTRMs. Multicomponent behaviour of the NRM residing in samples from the transitional zone can therefore be interpreted as a continuous record of Earth magnetic field variations.  相似文献   

20.
Experiments comparing anhysteretic remanence (ARM) and thermoremanence (TRM) in samples containing natural and synthetic magnetite, whose mean particle sizes range from single domain to multidomain, show that ARM and TRM are very similar (but not identical) in their stabilities with respect to alternating field (AF) demagnetization, temperature cycles in zero field to below magnetite's isotropic temperature near 130°K, and stability with respect to spontaneous decay in zero field. Therefore, for magnetites, ARM can be used to model (with reasonable success) these stability properties of TRM. The field dependence of the acquisition of ARM and TRM shows that the low field susceptibility ratio, χARM/χTRM, has a particle size dependence, increasing from 0.1 for certain submicron particles to 2.0 for large multidomain crystals. Even for samples whose remanence is predominantly carried by submicron particles χARM/χTRM is highly variable, 0.11 ≤ χARM/χTRM ≤ 0.50. Therefore, ARM paleointensity methods which do not take into account the large variability in and the particle size dependence of χARM/χTRM are subject to order-of-magnitude uncertainties.  相似文献   

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