The laboratory simulation of deuteric oxidation of titanomagnetites: effect on magnetic properties and stability of thermoremanence

Synthetic single crystals of titanomagnetite of nominal composition Fe_2.4Ti_0.6O₄ have been oxidized at 1275°C in controlled gas atmospheres, producing multiphase intergrowths to simulate the natural process of deuteric oxidation. The evolution of the intergrowths was monitored using the conventional techniques of petrology: optical and electron microscopy and X-ray and electron microprobe analyses. In addition, the measured magnetic properties — particularly the temperature-dependence of hysteresis properties — provided further information about the composition and concentrations of magnetic phases, and their domain state, as oxidation proceeded. The evolution of a trellis pattern of ilmenite lamellae, characteristic of the “exsolution” stages of deuteric oxidation, was observed in the oxidized crystals. The interlamellar spinel region consisted of two iron-enriched titanomagnetites, one thought to occur along the lamellar boundaries. The magnetic hardness of both phases was found to be greater than the original homogeneous multidomain titanomagnetite crystals, although neither phase achieved monodomain characteristics, and the stability of thermoremanence (TRM) remained quite low (median destructive fields (MDF) of the order of a few thousend A m^?1). The lamellae made little contribution to the total remanence. A sharp rise in magnetic hardness, observed during the post-exsolution stages of oxidation, was due to the presence of fine-grain monodomain magnetite, thought to be distributed within the haemoilmenite matrix, but probably not having been formed by lamellar subdivision. The crystals could now carry an intense and stable TRM with MDFs of many tens of thousands of A m^?1.     

Intensity of the geomagnetic field from recent Italian lavas using a new paleointensity method

A new method has been tested on Etna historic lavas for determining the geomagnetic field intensity (F) using the thermoremanent magnetization of volcanic rocks. The procedure involves a number of very short duration heatings above the Curie point, to produce successive laboratory TRM. Thus, it is possible to check the variations in the TRM-acquiring capacity of the samples with the time of heating (t). The curve J = f(t) is then extrapolated towards t = 0, leading to a virtual value of TRM without any laboratory heating, i.e., without the changes that currently occur when the lavas are heated to produce the TRM. Using such virtual values of TRM, satisfactory results of F had been derived from the majority of the samples studied. These results are consistent with Thellier's archeomagnetic data: they show a nearly constant intensity of the geomagnetic field during the last three centuries and, further back into the past, a significant increase of this.     

A comparison of ARM and TRM in magnetite

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, χ_ARM/χ_TRM, 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 χ_ARM/χ_TRM is highly variable, 0.11 ≤ χ_ARM/χ_TRM ≤ 0.50. Therefore, ARM paleointensity methods which do not take into account the large variability in and the particle size dependence of χ_ARM/χ_TRM are subject to order-of-magnitude uncertainties.     

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.     

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.     

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.     

Palaeomagnetic investigations on the Deccan Traps from Chincholi,Mysore State,India

Summary 38 oriented samples of Deccan Traps have been collected from the neighbourhood of Chincholi, Mysore State, India. The Natural Remanent Magnetisation of these rocks has been studied using an astatic magnetometer. It has been found that these rocks are magnetically reversed, the mean magnetic direction being N154°E in declination and 61° down in inclination. Thermoremanance studies conducted on four specimens showed that two specimens with weak NRM and a high secondary magnetisation have Curie temperatures around 560°C for the NRM and exhibited partial reversal of TRM at room temperature, while two specimens with high NRM and with little secondary magnetisation have Curie temperatures much lower than 560°C for the NRM.     

Paleomagnetic field intensity derived from meteorite magnetization

The paleomagnetic field intensity is estimated with the aid of the Koenigsberger-Thellier method for four ordinary chondrites and one carbonaceous chondrite by assuming that the stable NRM component of these meteorites is attributable to the TRM acquired in a low-temperature range (lower than about 400°C) during their extremely-slow cooling process. The results are summarized in Table IV, where the paleomagnetic field intensity ranges from 0.10 to 0.97 Oe.Possible effects of the extremely-slow cooling rate of meteorites and the secondary TRM acquisition of the surface fusion crust upon the original NRM of the meteorite interior are discussed.     

Revisiting the Lowrie–Fuller test: alternating field demagnetization characteristics of single-domain through multidomain glass–ceramic magnetite

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 (J_rs). 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 (J_rs). 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 (J_rs) 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.     

The behaviour of ulvöspinel samples containing magnetized inclusions

Composite samples containing either magnetized iron or a ferrite magnet embedded in ulvöspinel have been cooled through the ulvöspinel Curie point. The observed changes in magnetic moment which occur are shown to be a result of TRM acquisition in the ulvöspinel rather than an effect of high permeability. Nevertheless by defining an effective permeability of about (1 + TRM/H), magnetostatic theory relevant to magnetized inclusions in a permeable medium can be applied to explain the main features observed in this system.     

Review of archaeointensity methods

Most of the traditional methods of determining the intensity of the ancient geomagnetic field from archaeological materials utilized thermal demagnetization of the natural remanent magnetization (NRM) and of the laboratory induced thermoremanent magnetization (TRM). When applied rigorously these methods are foolproof. They are, however, very time consuming and the number of samples with which they can be used is limited. Attempts to speed up these traditional methods have generally led to the use of subjective criteria in assessing the reliability of the results and archaeomagnetic research has recently been concentrated on extending the range of samples to which the method can be applied. Through the use of alternating field, rather than thermal, demagnetization of NRM and TRM it has become possible to apply corrections for alteration occurring during laboratory firing of the archaeological samples and develop objective criteria of reliability. Recent research has shown that it may be possible to determine archaeointensities the laboratory redeposition of lake sediments.     

Author index 1984 (volumes 67–71)

Magnetic properties of samples from Bell Island sedimentary rocks have been studied. X-ray analysis indicates that the main magnetic mineral is hematite in all samples. The other iron-bearing minerals identified are siderite and chamosite. Microscope observations of thin sections suggest that the rocks consist of oolitic hematite in a matrix of siderite or calcite. The intensity of natural remanent magnetization (NRM) varies in the range of (0.03–0.4 A m^?1), depending on the percentage of hematite. The thermal demagnetization curves of NRM show in some cases a sharp increase in magnetization at temperatures in the range 500–600°C. The peaks that occur in these demagnetization curves are due to a chemical change of siderite during repeated laboratory heating. X-ray analysis confirmed that the newly formed material is magnetite. Since the original NRM has been masked by the new intergrown material, this would result in a serious error in the determination of paleomagnetic pole positions. The samples showing this behaviour were not considered for paleomagnetic study. The samples containing oolitic hematite in a calcite matrix exhibit very high stability of NRM, including directional stability until almost 670°C. For these samples, a virtual pole position based on N = 6 samples (32 specimens) demagnetized to 665°C is 34°N, 114°E, not far from published Ordovician poles for the North American craton.     

Composite titanomagnetite-ferrian ilmenite grains and correlative magnetic components in a dacite with self-reversed TRM

Electron microprobe and reflected light microscopic examinations confirm the presence of composite grains of ferrian ilmenite with X_ilm = 0.53 and titanomagnetite with X_usp = 0.13 in a dacite with self-reversed TRM. A parallel TRM component associated with titanomagnetite and a reversed component associated with self-reversing ferrian ilmenite are the principal NRM components. A subordinate, parallel component is associated with ferrian ilmenite which is not magnetically coupled to an “χ-phase”. The natural self-reversing properties are mainly a consequence of the dacite's high quenching temperature, calculated at 862–864°C using the Fe—Ti oxide geothermometer, and are most consistent with the self-reversal mechanism proposed by Lawson et al. [9].The conduction of thermal demagnetization and TRM induction tests in air had a much greater effect on the Fe—Ti oxides than did natural cooling, and resulted in significant oxidation with the consequent modification of some magnetic properties and the formation of another reversed TRM component. The subdivision of titanomagnetite grains by oxidation along fractures decreased its effective grain size and caused an apparent increase in its magnetic intensity, in addition to a slight increase in its resistance to alternating field demagnetization. The χ-phase associated with the reversed NRM component, with 0.42 > X_ilm 0.31, became Fe-enriched during the earlier stages of heat treatment. It is suggested that after heating at 600°C for two hours or more, this χ-phase exsolves as titanohematite with X_ilm < 0.33. The ferrian ilmenite host is consequently enriched in Ti, and another χ-phase much closer in composition to the host generates a reversed TRM component with T_b < 200°C.     

Influence of partial pressure of oxygen on thermoremanent magnetization of basalts

An electric furnace with oxygen-fugacity control was constructed. Oxygen partial pressure inside the furnace is controlled by flowing H₂ and CO₂ gas mixtures in different ratios. The system is set up inside a three-layer permalloy shield and a solenoid coil to produce an axial magnetic field. Oxygen fugacities are directly measured by an yttriadoped zirconia probe. The difference between the predicted and measured values of fugacities was small at 1,200°C, but a substantial discrepancy was observed at 780°C. Thermoremanent magnetization (TRM) was produced in various gas mixtures by heating the samples of recent basaltic lavas of Oshima (Japan) and Hawaii to 600 or 800°C for 1 h and cooling in a 0.4-Oe field. In general, the TRM is larger when produced in an oxidizing atmosphere and smaller in a reducing atmosphere. Alternating field demagnetization shows that the coercivity is also increased (decreased) when the TRM is acquired in oxidizing (reducing) conditions. However, these properties depend also on the previous heat treatments of the samples.     

A magnetic study of single-crystal titanomagnetite (Fe2.4Ti0.6O4)

Single crystals of approximate composition Fe_2.4Ti_0.6O₄ were prepared from which spherical samples of diameters 1–2 mm were obtained. The measured values of the Königsberger ratio, the ratio of saturation remanence to saturation magnetization and a Lowrie-Fuller test showed that they were multidomain in character. The temperature variation of the coercive force and saturation magnetization was measured between room temperature and the Curie point. The field dependence of intensity of acquired thermoremanent magnetization (TRM) was determined. The predictions of some of the theoretical models for multidomain TRM, which, of necessity, apply to simplifications of real materials (either natural or synthetic), compare favourably with the results of the present study. The validity of the assumptions made in this comparison is discussed.     

Change of structure,phase composition and magnetic properties with the oxidation of titanomagnetites

Summary Phase, structural and magnetic changes, occurring under oxidation and at increased temperatures, are studied on four samples of magnetic fractions. The samples of magnetic fractions, containing titanomagnetites at different oxidation levels, were oxidized at a temperature of 400°C for 1, 60 and 300 mins. With the aid of X-rays and Mössbauer's spectrometry it has been proved that under oxidation non-stoichiometric titanomagnetites and titanomagnetites plus ilmenite and pseudobrookite are formed.     

ARM,TRM and magnetic interactions: Concentration dependence

ARM (anhysteretic remanence)/SIRM (saturation isothermal remanence) and TRM (thermoremanence)/SIRM were measured as a function of the concentration (volume fraction) of single-domain magnetite (3 × 10^?6 ? C ? 2 × 10^?2), ARM/SIRM increases with decreasing concentration, showing that there is magnetic interaction between fine particles. The role of magnetic interaction in TRM acquisition is also important at higher concentrations of magnetite (C ? 0.1%), where the value of TRM/SIRM increases with decreasing concentration. It is only for concentrations ofC ? 0.1% that the value of TRM/SIRM is fairly constant and interactions among magnetite grains seem to be ignored. The ratio TRM/ARM decreases from seven to almost unity as the concentration of magnetite decreases.     

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.     

Combining paleointensity methods: A dual-valued determination on lunar sample 10017,135

A single-heating procedure is presented which makes possible the determination of two partially independent values of paleofield intensity for a given sample, one serving as a check to the other. The approach combines data required for Shaw-type and “ARM-method” determinations and in so doing furnishes a value of the ratio of TRM to ARM acquisition efficiency (f′) corrected for any physicochemical alteration to the magnetic carriers which may have occurred during laboratory heating.

Applicability of the Shaw-method to Fe-bearing samples is favorably demonstrated through simulated paleointensity determinations conducted on synthetic samples containing multi-domain grains. Moreover, coercivity spectra corresponding to anhysteretic remanent magnetization (ARM) are found to be considerably more sensitive to thermally induced alteration when compared with those corresponding to thermoremanent magnetization (TRM).

The combined Shaw-ARM procedure was successfully applied to lunar basalt sample 10017,135 rendering a paleointensity of 0.82 ± 0.11 Oe. The Thellier-Thellier method, however, was not able to provide a meaningful determination on the neighboring chip (number 136). These apparently conflicting findings may be explained by one or more of the following possible interpretations: (1) multiple step-wise heatings cause considerably more damage to the carriers of remanence than does a single-heating procedure; (2) the rock possesses extreme variability in magnetic properties from one sub-sample to the other; (3) the natural remanent magnetization in this lunar basalt is not a simple TRM.     

A new model for the acquisition of thermoremanence by multidomain magnetite

Isothermal remanence (IRM) induced in multidomain grains at high temperature (T_i) decreases on cooling in zero field. The intensity and stability (against temperature change) of the IRM depend on whether the temperature (T_i) was reached from higher temperature or from lower temperature. These results cannot be explained by the existing multidomain theories. A new model of a thermoremanence (TRM) acquisition mechanism, in which domain wall interactions play the major role, is proposed. This model explains the main features of the experimental results very well, predicting an almost linear relationship between TRM and the inducing field, up to relatively high fields.