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We have conducted a whole-rock type magnetic and absolute paleointensity determination of the red dacite of the Duffer Formation from the Pilbara Craton, Australia. The age of the dated rock unit is 3467 ± 5 Ma (95% confidence). Vector analyses results of the step-wise alternating field demagnetization (NRM up to 100 mT) and thermal demagnetization (from NRM up to 650 °C) yield three components of magnetization. Curie point determinations indicate three characteristic temperatures, one at 150–200 °C, a second one at ∼450 °C and a third one at ∼580 °C. Magnetic grain-size experiments were performed on small specimens with a variable field translation balance (VFTB). The coercivity of remanence (Hcr) suggests that the NRM is carried by low-coercivity grains that are associated with a magnetite fraction as is shown by the high-temperature component with blocking temperatures above 450 °C and up to at least 580 °C. The ratios of the hysteresis parameters plotted as a modified Day diagram show that most grain sizes are scattered within the Single Domain (SD) and the Superparamagnetic and Single Domain SP-SD domain ranges. In addition to the rock magnetic experiments we have performed absolute paleointensity experiments on the samples using the modified Thellier-Coe double heating method to determine the paleointensities. Partial-TRM (p-TRM) checks were performed systematically to document magnetomineralogical changes during heating. The temperature was incremented by steps of 50 °C between room temperature and 590 °C. The paleointensity determinations were obtained from the slope of Arai diagrams. Our paleointensity results indicate that the paleofield obtained was ∼6.4 ± 0.68 (N = 11) micro-Teslas with a Virtual Dipole Moment (VDM) of 1.51 ± 0.81 × 1022 Am2, from a medium-to high-temperature component ranging from 300 to 590 °C that has been interpreted to be the oldest magnetization yet recorded in paleomagnetic studies of the Duffer Formation. The absolute paleointensity is relatively low and we interpret this low-paleofield bias a result of a thermochemical remanent magnetization (TCRM) process that indicates a possible underestimate of the paleofield by a factor of four for the red dacite of the Duffer Fm.

It is widely accepted that cooling rate can strongly influence the intensity of the thermal remanent magnetization (TRM) acquired by rocks during cooling to ambient temperatures. If ignored, this effect might lead to underestimates or overestimates of the ancient magnetic field intensity. To date, however, the cooling rate dependence of TRM acquired by particles with different domain states has never been systematically analyzed from the theoretical or experimental point of view. In this study, we present measurements of the TRM of synthetic magnetites with well-defined grain sizes that were quenched with constant cooling rates of 0.05, 0.1, 1, 3, 10, and 15 K/min. While single domain (SD) and small pseudo-single domain (PSD) samples are found to show larger TRMs after slow cooling, the TRMs of larger PSD and multidomain (MD) magnetites are not affected by an increase or decrease of the cooling rate. Overall, our results suggest that only smallest magnetite grains acquire a cooling rate-dependent TRM. Therefore, cooling rate corrections of paleointensity determinations are only necessary for samples dominated by SD remanence carriers, while rocks dominated by PSD and MD carriers, such as basalts, which are most commonly used for paleointensity studies, do not require such corrections.

SUMMARY Palaeomagnetic observations are being used in increasingly sophisticated geological and geophysical interpretations. It is therefore important to test the theories behind palaeomagnetic recording by rocks, and this can only be achieved using samples containing precisely controlled magnetic mineralogy, grain size and interparticle spacing, the last of which controls the degree of magnetostatic interactions within the samples. Here we report the room- and low temperature magnetic behaviour of a set of samples produced by the nano-scale patterning technique electron beam lithography. The samples consist of 2-D arrays of near-identical magnetite dots of various sizes, geometries and spatial configurations, with dot sizes from ranging from near the single domain threshold of 74–333 nm. We have made a series of magnetic measurements including hysteresis, first-order-reversal curve measurements and remanence acquisition, many as a function of temperature between 20 and 300 K, to quantify the samples’ behaviour to routine palaeomagnetic measurement procedures. We have also examined the behaviour of saturation isothermal remanences (SIRM) to cooling and warming cycling of the sample below room temperature. In addition, we investigated the samples’ responses to alternating-field demagnetization of room temperature induced SIRM, anhysteretic remanent magnetization (ARM) and partial ARM. ARM was used as a non-heating analogue for natural thermoremanence. Given the 2-D spatial distribution of the samples, in all the experiments we conducted both in-plane and out-of-plane measurements. Generally, the samples were found to display pseudo-single-domain hysteresis characteristics, but were found to be reliable recorders of weak-field remanences like ARM. For the closely packed samples, the samples’ magnetic response was highly dependent on measurement orientation.

Two methods for high resolution dry etching of permalloy (NiFe) and iron (Fe) nanostructures are presented and discussed. The first involves the use of carbon monoxide (CO) and ammonia (NH"3) as etching gases, the second uses methane (CH"4), hydrogen (H"2) and oxygen (O"2). In both etching processes volatile metallo-organic compounds are the resulting reaction products. The patterned and dry etched thin films were observed with SEM and TEM to study the quality of each of the two processes. It is found that the CO/NH"3 process yields higher etch rates, higher selectively with respect to the SiN mask used, and less redeposition than the CH"4/H"2/O"2 process.

We studied 93 samples from 8 basaltic dykes of the Koolau volcanic range on the island of Oahu,Hawaii,USA,to determine the influence of hydrothermal alteration on the magnetic fabric as determined by anisotropy of magnetic susceptibility (AMS) measurements. Rock magnetic as well as microscopic investigations show that only ≈25% of the samples have retained their original magnetomineralogical composition of unaltered Ti-poor titanomagnetite. The remaining samples have undergone hydrothermal alteration which transformed the primary magnetic phase into a granular intergrowth of titanomagnetite, titanomaghemite and hematite. In both sample groups,this magnetic phase occurs in coarse (tens of microns),irregularly shaped particles as well as interstitial clusters of smaller (

SUMMARY A range of basaltic samples from Olby (France) and Vogelsberg (Germany) displaying the phenomenon of partial and complete self-reversal was studied in order to decipher the physical mechanism responsible for self-reversed magnetic remanence in basalts. Microscopic observations and rock magnetic measurements show titanomagnetite to be the carrier of remanence. Due to low-temperature oxidation the titanomagnetites are present in two magnetic phases forming close side-by-side phase assemblages: The ore grains consist of a non-oxidized magnetically soft titanomagnetite part with Curie temperatures between 140 and 300 °C and a magnetically hard low-temperature oxidized part with Curie temperatures between 410 and 590 °C. Magnetic force microscopy observations present evidence that the oxidation process does not only influence the Curie temperatures, and thus the blocking temperature spectrum, but also the domain configuration. During acquisition of a thermoremanence the two phases are magnetically coupled, leading to a remanent magnetization of the low Curie temperature phase which is antiparallel to the applied external magnetic field. Computational modelling of the remanence acquisition process explains the coupling of the two phases by magnetostatic interaction. As the observed partial oxidation of ore grains is not uncommon in subaerial basalts, partial and complete self-reversal is probably a frequently occurring phenomenon. Nevertheless, it remains unnoticed in most cases as it cannot be detected by the standard stepwise thermal demagnetization technique. Despite the complex remanence properties, our investigations indicate that the upper part of the blocking temperature spectrum still carries reliable palaeodirectional information.

Paleointensity experiments using the Thellier method on samples of the 1915 Mt. Lassen dacite flow yielded concave and S-shaped NRM–TRM curves on Arai plots. Departure of these curves from the ideal straight line corresponding to the known geomagnetic intensity of 54 μT for the region in 1915 is significant. The magnetic properties of these rocks change very little during heating, and hysteresis experiments that suggest pseudo-single domain to multidomain (MD) magnetic grains are confirmed by microscopic observations. Thus, the curvature appears to be due to MD-like behavior rather than to irreversible alteration of magnetic minerals. In support of this interpretation, the slope of the line through just the initial and the final point yields an average value of 52 μT that is in good agreement with the known field intensity. Using the slope of only the lower or higher temperature points, which is never advisable for persistently curved Arai plots such as these, would lead to estimates that are 30–80% too high or too low. S-shaped NRM–TRM curves are atypical in paleointensity studies, and the usual explanation in terms of MD unblocking temperatures less than blocking temperatures cannot account for the convex curvature of the higher-temperature part. Rather, the convex curvature can be explained by the predominance of remanence with unblocking temperatures greater than blocking temperatures, the so-called tail of pTRM * , acquired in the higher-temperature part of the pTRM spectrum during the Thellier experiment.

The most commonly used method for paleointensity determination, the Thellier method, requires the absence of multidomain magnetic behavior. The influence of multidomain remanence on paleointensity determination and its recognition during the experiment are, however, only partially understood. Here, we present a phenomenological model of TRM acquisition and its application to the Thellier experiment. This model is an extension of the approach of [Earth Planet. Sci. Lett. 188 (2001) 45] and is used to investigate, beside other multidomain characteristics, the effect of pTRM*-tails. Furthermore, the appropriateness of typically applied checks during paleointensity determination like alteration checks, pTRM*-tail checks and additivity checks is scrutinized. It is shown that pTRM*-tails significantly affect the shape of the Arai diagram. Concave curvature of the Arai plot for samples containing multidomain particles is related to remanences with unblocking temperatures below their respective blocking temperatures. pTRM*-tails, however, which affect unblocking temperatures above the respective blocking temperatures, lead to a convex shape. An overlap of both convex and concave characteristics can produce s-shaped Arai diagrams [Phys. Earth Planet. Int.]. The effect of pTRM*-tails is strongly dependent on the angle and intensity difference between applied field and NRM of the sample. The commonly used in-field alteration check is biased by pTRM*-tails and shows deviations from the expected value even if alteration is absent. pTRM*-tail checks show the presence of tails and thus give evidence for multidomain behavior, only if the laboratory field is applied in a different direction than the NRM. The larger the angle between NRM and laboratory field, the larger the value of the pTRM*-tail check. A new method for calculation of the pTRM*-tail check is proposed which accounts for such angular difference, as well as intensity differences. Conducting the Thellier experiment with alteration checks, pTRM*-tail checks, and additivity checks at an angle between NRM and laboratory field > 0, provides the possibility to unequivocally identify multidomain bias.

An extensive palaeomagnetic study was carried out on an approximately 250 Ma old reversed to normal transition of the Earth’s magnetic field (EMF) recorded in 86 volcanic lava flows of the Siberian trap basalts, North Siberia, Russia. In addition to the investigation of the directional behaviour of the field (≈700 specimens) a total of 298 specimens was subjected to Thellier-type palaeointensity determinations. Adding several modifications to the original Thellier experiment, such as tests for MD tails and the additivity of partial thermoremanent magnetisation yields highly reliable palaeointensity estimates of the Late Permian/Early Triassic EMF. Transitional directions of the EMF were obtained from 20 flows. During the reversal a clustering of the virtual geomagnetic poles (VGPs) is observed (15 flows). Palaeointensity estimates suggest that this feature is not an artifact due to rapid flow emplacement since the directional cluster is associated with a well-defined increase in palaeointensity from 6 to 13 μT. Subsequently, the next VGPs move towards the pole position of normal polarity. Departing in a sudden movement from normal polarity the VGPs form a second directional cluster comprising the results of 14 flows. This feature is interpreted as a post-transitional excursion but lacks the characteristic intensity variation recorded during the first transitional cluster. The rest of the section (41 flows) is of normal polarity. The characteristic features of this reversal, low intensities and directional clustering during the reversal and an excursion shortly after the reversal, were also observed in records of polarity transitions of younger age. This suggests that the underlying reversal processes were similar. The mean virtual dipole moment calculated for the stable normal part of the studied section yields a rather low value of 2.2±0.9×1022 Am2. These findings confirm that the Mesozoic Dipole Low extends at least down to the Late Permian/Early Triassic. The geomagnetic VGP dispersion was calculated for the stable normal part of the section and yields values similar to those for the last 5 Ma. Considering that the intensity of the EMF was considerably higher in this time interval than in the Early Triassic, a direct relationship between intensity and secular variation seems unlikely.

The natural remanent magnetization (NRM) of ocean basalts, giving rise to the pattern of marine magnetic anomalies, is known to be of comparatively low intensity for about 20 Ma old oceanic crust. The aim of this study is to detect possible peculiarities in the rock magnetic properties of ocean basalts of this age, and to establish a link between magnetomineralogy, rock magnetic parameters, and the low NRM intensity. Ocean basalts covering ages from 0.7 to 135 Ma were selected for rock magnetic experiments and their room temperature hysteresis parameters, Curie temperature and temperature dependence of saturation magnetization MS(T) was determined and complemented by reflected light microscopy. The majority of samples is magnetically dominated by titanomagnetite and titanomaghemite with increasing oxidation state with age. For these, a strong dependence of hysteresis parameters on the age of the samples is found. The samples have a minimum in saturation magnetization and a maximum in magnetic stability in the age interval ranging from approximately 10 to 40 Ma, coinciding with the age interval of low NRM intensity. The observed change in saturation magnetization is in the same order as that for the NRM intensity. A further peculiarity of the titanomaghemites from this age interval is the shape of their MS(T) curves, which display a maximum above room temperature (Neel P-type) and, sometimes, a self-reversal of magnetization below room temperature (Neel N-type). These special rock magnetic properties can be explained by titanomagnetite low-temperature oxidation and highly oxidized titanomaghemites in the age interval 10–40 Ma. A corresponding measurement of the NRM at elevated temperature allows to identify a maximum in NRM intensity above room temperature for the samples in that age interval. This provides evidence that the NRM is equally carried by titanomaghemites and that the low NRM intensities for about 20 Ma old ocean basalts are caused consequently by the low saturation magnetization of these titanomaghemites.

Reliability checks in Thellier-type experiments usually focus on the detection of chemical alteration of the magnetic mineral component or the formation of new magnetic phases during the heating process. However, a major problem in Thellier-type experiments is the presence of multidomain (MD) particles which can lead either to complete failure of palaeointensity determinations or to serious misinterpretation. We present a modification of the Thellier–Thellier experiment that detects the presence of MD particles by verifying the law of additivity of pTRMs. The law of additivity is valid for regular pTRMs (i.e. the upper temperature of pTRM acquisition is reached by cooling from the Curie temperature) for both SD and MD particles, whereas it is not valid for pTRM* (the upper temperature of pTRM acquisition is reached by heating from room temperature) in the case of MD particles. As the partial thermoremanences imparted in Thellier–Thellier experiments are of the pTRM* type, additivity as a prerequisite for the validity of the obtained result is not given if the remanence is carried predominantly by MD particles. The proposed method is applied to seven synthetic samples covering a grain size range of 23 nm to 12.1 μm. The obtained palaeointensity estimates show a significant error for all samples with grain sizes >0.7 μm due to the failure of the law of additivity. Our experiment is able to detect this failure.

Oceanic crust is the carrier of the marine magnetic anomalies and is therefore a valuable archive of geomagnetic information. ODP/IODP Hole 1256D was the first to sample an entire sequence of oceanic crust down to the gabbro. We studied the vertical variation of magnetic remanence carriers by means of scanning electron microscopy, microanalysis and rock magnetic measurements. The extrusive layer contains dendritic, low-temperature oxidized titanomagnetites (TMs), i.e. titanomaghemite, with initial compositions close to values previously reported for mid-ocean ridge basalts (MORB). The degree of low-temperature oxidation (maghemitisation) remains fairly constant across the extrusives. We explain the observed increase in Curie temperature with depth by submicron inversion of titanomaghemite to intergrowths of titanomagnetite and nonmagnetic phases, where the Ti-content of titanomagnetite is decreasing with depth. In the underlying sheeted dikes, TMs are again the primary magnetic mineral. Due to slower cooling, they are in most cases oxy-exsolved into lamellar intergrowths of Ti-poor TMs and ilmenite. The magnetominerals are altered to a much higher degree than in the extrusives. In the gabbroic part of the section, TMs reach sizes up to several mm, although the magnetic grain size remains consistently in the pseudo-single-domain range because of grain subdivision by exsolution lamellae. The extrusives carry a thermoremanent magnetisation (TRM), retaining the primary paleomagnetic direction but with a reduced remanence intensity. The sheeted dikes hold a thermo-chemical remanent magnetization (TCRM) or secondary TRM acquired during hydrothermal alteration, whereas the underlying gabbro acquired a TCRM significantly after emplacement due to slow cooling at this depth.

[1] Micromagnetic calculations relevant to paleomagnetism have generally focused on ideal shapes such as spheres and cubes. However, oxide grains that occur naturally in rocks often have irregular morphologies, highly skeletal dendritic forms being particularly common. To investigate the potential effects of complex grain morphology on magnetic stability, we have carried out two parallel sets of calculations. The first is based on randomly distorted 30–120 nm magnetite spheres. We use a three-dimensional finite element/boundary integral micromagnetic model able to generate suitable morphologies unrestricted by the regular cell structure required by finite difference models. The second model consists of a 300 nm magnetite octahedron from which most of the material has been removed to leave a small octahedral crystallite at each of the six apices connected by an orthogonal framework of thin rods. The results obtained imply that micromagnetic models currently provide no unambiguous evidence that morphological complexities endow magnetite nanoparticles with enhanced coercivity.

[1] Magnetic measurements of samples with precisely controlled magnetic mineralogy, grain size, and interparticle spacing are needed to provide crucial experimental rock magnetic underpinning for paleomagnetic studies. We report a novel nanofabrication method for producing two-dimensional arrays of cylindrical synthetic magnetite particles with well-defined composition, particle size, and interparticle spacing. The samples are fabricated by writing dot arrays with electron beam lithography, transferring these patterns into sputtered Fe thin films by reactive ion etching in a CO/NH3 plasma, and oxidizing the resulting Fe particles in a controlled atmosphere to form magnetite. Scanning electron microscopy and transmission electron microscopy have been used to monitor the fabrication process and to determine the particle geometry. The particle sizes of our samples range between 100 nm and 265 nm with center-to-center spacings between 180 nm and 310 nm. Low-temperature magnetic remanence data confirm the stoichiometry of the magnetite. We present magnetic hysteresis data and first-order reversal curve diagrams for our samples and compare these with previously published data from other synthetic and natural magnetite samples. The ability to independently control particle size and interparticle spacing of magnetite grains makes our synthetic samples ideal for studying the influence of magnetostatic interactions on the paleomagnetic recording fidelity of naturally occurring magnetite in rocks.

SUMMARY The palaeomagnetic standard technique of stepwise thermal demagnetization (STD), long regarded as unreliable for oceanic basalts that have undergone low temperature alteration, has recently been applied in a number of studies to characterize the natural remanent magnetization (NRM) of such rocks. In order to better understand STD data of oceanic basalts, and to possibly identify the magnetominerals that are carrying the NRM, we have carried out a number of continuous and STD experiments on seven oceanic basalt samples. During continuous thermal demagnetization (CTD), a sample is heated to a certain temperature and its NRM is measured during heating and subsequent cooling. Even when CTD reveals only titanomaghemite unblocking at 400 ◦ C as the remanence carrier, STD behaviour can be very complex and unblocking is observed at temperatures of up to 500 ◦ C and higher. CTD also allowed to identify a partial or full self-reversal of NRM due to interaction between two types of magnetominerals in one sample. The higher degree of maghemitization of smaller titanomaghemite grains with respect to larger ones, which are less efficient in carrying the remanence, was seen for three samples by a shift of 80 ◦ C between the strong field thermomagnetic curve and the NRM measured at elevated temperature. In several cases, the identification of the NRM-carrying magnetomineral was not possible from CTD data due to the ambiguity of Curie temperatures in the titanomagnetite/titanomaghemite system.

[1] The ThellierTool4.0 is an intuitive and easy-to-use software which provides the possibility to analyze a wide range of different modifications of the Thellier absolute paleointensity experiment (available at http://earthref.org/tools/). Besides the Arai plot for paleointensity determination, orthogonal projections of the direction, decay of NRM during thermal demagnetization, and additional plots regarding alteration and multidomain checks enable the user to visualize the quality of individual determinations. Experimental checks for magnetomineralogical changes, either in-field or zero-field pTRM* checks, are evaluated regarding their differences to the corresponding pTRM* acquisition in two most commonly used ways. Furthermore, a measure for the cumulative alteration differences beginning at room temperature is calculated, and the possibility to correct for magnetomineralogical changes is provided. Two different experimental methods to check for multidomain bias are supported and analyzed by the software. Intensity differences recorded by pTRM*-tail checks are calculated. Accounting for the directional difference between applied laboratory field and magnetization of the sample, the effective pTRM*-tail is determined, and thus failures of Thellier's law of independence are monitored. Failures of the law of additivity, experimentally observed by additivity checks, are also evaluated by the software. The vectorial character of individual measurements is fully considered for all calculations. Uniform selection criteria for acceptance and rejection of determinations can be applied, and a set of such criteria with emphasis on minimal bias due to alteration, multidomain remanence, and analysis/experimental inaccuracies is suggested.

We analyze magnetic properties from Ocean Drilling Program (ODP)/Integrated ODP (IODP) Hole 1256D (6°44.1' N, 91°56.1' W) on the Cocos Plate in ∼15.2 Ma oceanic crust generated by superfast seafloor spreading, the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting. Fuzzy c-means cluster analysis and nonlinear mapping are utilized to study down-hole trends in the ratio of the saturation remanent magnetization and the saturation magnetization, the coercive force, the ratio of the remanent coercive force and coercive force, the low-field magnetic susceptibility, and the Curie temperature, to evaluate the effects of magmatic and hydrothermal processes on magnetic properties. A statistically robust five cluster solution separates the data predominantly into three clusters that express increasing hydrothermal alteration of the lavas, which differ from two distinct clusters mainly representing the dikes and gabbros. Extensive alteration can obliterate magnetic property differences between lavas, dikes, and gabbros. The imprint of thermochemical alteration on the iron-titanium oxides is only partially related to the porosity of the rocks. Thus, the analysis complements interpretation based on electrofacies analysis. All clusters display rock magnetic characteristics compatible with an ability to retain a stable natural remanent magnetization suggesting that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies. Paleointensity determination is difficult because of the propensity of oxyexsolution during laboratory heating and/or the presence of intergrowths. The upper part of the extrusive sequence, the granoblastic dikes, and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization.